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Review

A Global Comparative Analysis of Drought Responses of Pines and Oaks

by
Surendra P. Singh
1,
Surabhi Gumber
2,
Ripu Daman Singh
1,*,
Tong Li
3 and
Rajiv Pandey
4
1
Centre for Himalayan Studies, Graphic Era (Deemed to be) University, Dehradun 248002, Uttarakhand, India
2
Department of Environmental Science, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar 263145, Uttarakhand, India
3
School of Agriculture and Food Sustainability, The University of Queensland, St. Lucia, QLD 4067, Australia
4
Division of Forestry Statistics, Indian Council for Forest Research and Education (ICFRE), Forest Research Institute (FRI), Dehradun 248006, Uttarakhand, India
*
Author to whom correspondence should be addressed.
Forests 2025, 16(11), 1660; https://doi.org/10.3390/f16111660
Submission received: 19 September 2025 / Revised: 22 October 2025 / Accepted: 28 October 2025 / Published: 30 October 2025
(This article belongs to the Special Issue Soil and Water Conservation in Forestry)
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Abstract

Pinus (~113 species, generally early-seral) and Quercus (~435 species, generally late-seral), currently co-occur over a wide range of climates and biomes in the Northern Hemisphere. Climate change is expected to threaten the coexistence dynamics of pine and oak species. Here, we analyze the responses of Pinus and Quercus to water stress, with the objective of determining how they vary globally in their responses to drought at the genus level. The results show that pines tend to tightly close stomata before stress becomes severe and may deplete their stored carbon; on the other hand, oaks begin stomatal control at a lower water potential and hence do not suffer from carbon depletion. Pines exhibit a wider hydraulic safety margin (average: 3.33 MPa) than oaks (average: 1.41 MPa) because of lower Ψ50 (average: −3.62 MPa) and earlier stomatal closure (average: −2.19 MPa). For oaks, stomatal closure and Ψ50 occur at −2.61 MPa and −3.07 MPa, respectively. We discuss and show that these contrasting drought responses are consistent with their seral roles. While the difference in the basic strategies to drought in the two genera is unmistakable, the species studied are still too few to make convincing generalizations. Research is also needed on other components related to drought adaptations.

1. Introduction

Pinus (~113 species) [1] and Quercus (~435 species) [2] are among the most important forest-forming genera of the planet [3]. Overlapping in distribution, particularly in mid-latitudinal and mid-altitudinal locations, pines and oaks form forests across all continents of the Northern Hemisphere. Pinus and Quercus are widely separated in evolutionary time (pines appeared > 150 million years ago, and oaks ~50–56 million years ago). Oaks have been evolutionarily successful because of their high within-population diversity, rapid migration, high rate of evolution and divergence, and propensity for hybridization, which contribute to adaptive introgression [4]. Even so, pines and oaks often form forests together, and the “pine–oak type” is an ecologically unique system of global significance [5]. Broadly, the linkage between pine and oak distribution and suboptimal environments is apparent [6]. Aided by fire, and anthropogenic disturbances, pines have not only persisted along with oaks, but also co-existed ecologically in complex ways for millions of years to form resilient communities (as an example, in the Mediterranean basin, pine and oak have lived together for 2.3–3.5 million years [7]). For both pines and oaks, Mexico and China are major diversity centers [5,8,9], but they are also important forest-forming genera in North America, much of the Mediterranean region, other parts of Europe, the Himalayan region, Japan, and Vietnam [10,11]. Although only five pines occur in the Himalayas, they occupy large areas in western and central Himalayas [12]. For example, P. roxburghii and P. wallichiana are among the top five tree species of the Indian sub-continent in terms of growing stock [13].
Species of pine and oak are linked successionally in many regions with seasonal drought and fire, the pine being early-seral and oak late-seral [5,14]. In the Mediterranean Basin, pines (e.g., P. nigra) are replaced by oaks (e.g., Q. faginea), as succession progresses [15,16]. The pine–oak mixed forests may represent an early- or mid-successional stage. In the Qinling Mountains in China, where about 25% of the area is covered by P. tabulifromis, P. armandii, and Q. aliena var. acutiserrata [17], pines are pioneer, and mixed pine–oak forests are late-successional [18]. In the central Himalayas, particularly in Uttarakhand State of the Indian Himalayas, P. roxburghii and Q. leucotrichophora co-occur widely over thousands of km2, respectively, as early-seral and late-seral dominants [19,20].
Drought affects succession by weakening trees and making them vulnerable to insect attacks and pathogens [21,22]. Increased rates of drought-induced mortality of trees and changes in hydrologic cycles are being witnessed across the planet [23,24,25]. In Mexico, a major diversity center for both pine and oak, successional changes involve many pine and oak species, driven by causal factors like climate change-induced drought and fire [26]. In southern Europe, where pine and oak coexist over large areas, changes are occurring particularly under dry and warm conditions [27,28]. Severe and prolonged droughts may affect succession as disturbing and filtering factors. Persistent drought may delay succession from pine to oak stage in Himalayas as oak regeneration fails [20]. Pines and evergreen oaks have high leaf mass per unit area (evergreen oaks LMA average of 45 species: 130 ± 6 g m−2 [29]) and sclerophylly as adaptations to stressful environments of high elevation and semi-arid areas. A certain level of seasonal drought and fire has contributed to the worldwide distribution and ecological success of these genera [30,31,32]. Pines and oaks differ in several ways with regard to stomatal control, hydraulic safety margin, and water-use efficiency [33,34,35,36]. Stomatal control affects transpiration, which in turn affects tissue water potential, nutrients supply, leaf temperature regulation, and CO2 entry into the leaf and thus photosynthesis and carbon accumulation [37]. With regard to stomatal conductance in relation to water stress, plants fall into two categories, namely isohydric and anisohydric [38]. Isohydric plants decrease their stomatal conductance in the face of dry conditions (such as a decrease in soil moisture and an increase in atmospheric vapor pressure deficit) so that plant water potential remains above a threshold. In contrast, anisohydric plants keep their stomata open and allow midday water potential (ΨMD) to decline considerably as drought intensifies. Most plants responses to drought lie somewhere between these two opposite poles [39]. The isohydric strategy generally enables plants to escape dangerously high loss of xylem conductivity and avoid hydraulic failure during drought, but early stomatal closure has a heavy cost in terms of reduced carbon uptake [40].
On the other hand, by allowing plant water potential to decline proportionately with soil water potential, anisohydric plants risk exposure to hydraulic failure. Stomatal closures in isohydric plants deprives them of carbon uptake, but plants with high water potential can recover as stomata reopen. In contrast, anisohydric plants are less likely to recover after hydraulic failure, and plant mortality may be caused by hydraulic collapse following irreversible cavitation or extremely low Ψ (water potential) [38]. Drought also affects pine and oak by affecting them in other ways, such as terpene emission and photoinhibition processes [41,42,43].
The objectives of this study are as follows: (i) to analyze genera-level adaptive attributes of pine and oak to drought across various species and areas from the literature. To be more specific, we address the question of the extent to which strong stomatal regulation and high hydraulic safety margin (isohydric) are associated with Pinus, and weak stomatal regulation and a narrow safety margin (anisohydric) with Quercus at the genus level; (ii) to analyze the extent to which the drought responses of pines and oaks are consistent with their successional roles (early and late, respectively); (iii) to identify attributes of the genera of pine and oak, and their subgroups, in the context of the hydraulic safety margin (HSM), a concept that has generated widespread interest in view of the intensification of drought under the influence of global climate change; (iv) to shed light on the implications of drought responses of pine and oak species for related parameters, such as carbon assimilation and reserve, terpene emission, and changes in antioxidative compounds; and (v) to shed light on how global climate change tends to affect the balance between pine and oak distribution in a region. However, the findings are only indicative. We hypothesize that the difference between pine and oak species with regard to their strategies in dealing with water stress could be a major contributor to their co-occurrence at a global scale in the Northern Hemisphere and the resilience of pine–oak systems over millions of years. This study adds to existing knowledge about the drought responses of pine and oak by comparing many species across various habitats. However, our generalizations are likely to be affected by the limited number of species and geographical regions studies and by the scarcity of information on related study components, namely root system, guttation and transpiration, and changes in relation to leaf longevity within the genera. However, we are aware of the fact that the studies include only a small fraction of the existing species and regions; hence, generalizations based on them remain tentative.

2. Material and Methods

2.1. Data Collection

A comprehensive search strategy was employed to gather articles that discuss the adaptive strategies of pine–oak forests in response to water stress using the Web of Science Core Collection database, focusing specifically on publications indexed in the Science Citation Index and Social Sciences Citation Index. The search included a combination of terms targeting pine and oak species and their physiological responses to water stress and drought conditions. English language was selected, with no restrictions of publication years, i.e., all available information on the topic in the database was considered, allowing a comprehensive review of the literature from the inception of the database to present, i.e., April 2024. Key findings related to hydraulic adaptations and other physiological responses to drought and water stress were categorized and tabulated for further analysis (Table S1).

2.2. Data Analysis

To define drought responses of the two genera, several parameters and units, as given in Table 1, were considered as follows: water-use efficiency (WUE), sap flow, Ψ at stomata closure, daily change in Ψ in plants (generally the difference between Ψpredawn and Ψmidday), and stomatal or hydraulic conductance in relation to Ψpredawn, drought damage such as loss of tissues in buds and leaves, post-drought recovery, carbon depletion, and other adverse effects of stomatal closure.
The extracted data were tabulated and used to characterize physiological responses of species to drought. Various strategies adopted by pines and oaks for either avoiding or tolerating drought were noted, along with a summary of qualitative and quantitative information, such as stomatal conductance, net photosynthesis rates, water potential, leaf water content, and net assimilation rate. The hydraulic safety margin (defined as the difference between plant water potential when it loses 50% of its conductance and the lowest water potential it reaches during drought [34,44,45]) was analyzed. We calculated the summary statistics for the quantitative information available, while in the other cases only trends were indicated for the species of the two genera. Other related summaries were also evaluated and compared as per the availability of qualitative and quantitative information. Thus, data indicating the effect of species adaptation to drought on various parameters, such as carbon depletion due to prolonged stomatal closure and severe photoinhibition resulting from tissue desiccation, were also included.
Responses of pine and oak species to drought (natural or induced) have been analyzed in various ways. The drop in Ψplant due to drought can indicate the tendency of a species to close its stomata. The daily change in water potential (ΨΔ) is an indicator of a species’ ability to tolerate desiccation or avoid it. Ability to resprout or regenerate through other vegetative means after a drought could be used as an indicator of a species’ ability to recover. Roots are broadly considered as shallow or deep. In some species, deep roots may have access to water present in rock fractures and crevices. Change in water-use efficiency in response to water stress could be conveniently inferred from isotope discrimination. Drought decreases stomatal conductance in plants, but the decrease varies and can be used to assess drought responses. We used water potential in monoculture compared with that in mixed culture to determine how species responses are modified by the presence of competitors.
Data on photoinhibition gave an idea of the cost of over-spending water or the advantage of water conservation. We used data on terpene emissions and the concentration of antioxidants to assess how species responses to drought affect their other physiological parameters. Studies on the impact of global climate change on pine–oak types have been few and are generally indicative in nature.

3. Results

3.1. Water Balance in Response to Drought: Generalizations

Without exception, drought responses of pine and oak followed isohydric and anisohydric strategies, respectively, though the degree varied (Table 2). For example, anisohydry in oak was more pronounced in deciduous species than in evergreen species. Pines (P. pinea, P. sylvestris) showed tighter stomatal control than oaks (Q. ilex, Q. subpyrenaica), closing stomata while Ψ was still high (the mean of the lowest ΨMD being −1.96 MPa for pines (n = 15) and −2.61 MPa for oaks (n = 17)) (Table 2). At the end of the drying cycle, ΨPD was −1.5 MPa in pine (P. pinaster) and −3.3 MPa in oak (Q. petrea) (Table S1). Oaks resisted drought and underwent a greater daily ΨΔ (mean = 0.79 MPa, n = 10) than pines (mean = 0.50 MPa, n = 10) (Table 2). The daily change (ΨΔ) was 0.39 MPa for pine (P. pinea) and 1.69 MPa for oak (Q. ilex), showing higher transpiration in oak (Table S1).
As conceptualized in Figure 1, the delay in stomatal closure enables oaks (Q. ilex, Q. faginea) to maintain carbon reserves, while in pines (P. halepensis), carbon reserves are depleted through respiration. Stomatal closure occurred at higher plant water potential in pine (P. ponderosa, −1.9 MPa) than in oak (Q. gambelii, −2.5 MPa) (Table S1). The drought was found to reduce the stomatal conductance of P. halepensis by 42% compared to 28% in Q. ilex, which had a higher net photosynthesis rate than P. halepensis. Similar responses of P. halepensis to drought were observed with Q. coccifera.
Along a gradient in the aridity index, oaks (Q. petrea) showed marked adjustment in hydraulic architecture, while pines (P. sylvestris) remained less sensitive to change in aridity. Xylem formation ceased 1–3 weeks earlier in pine (P. sylvestris) than in oak (Q. pyrenaica) in response to drought (Table S1). In oak (Q. pubescens), stomatal conductance was reduced by drought, but radial lumen diameter increased.
The early closure of stomata enabled pines (P. halepensis) to maintain higher WUE (Table 2). Water-use efficiency ranged from 2.31 to 3.23 in pine (P. nigra) and from 0.40 to 1.14 in oaks (Q. ilex and Q. faginea), indicating conservative water use in pines (Table S1). When exposed to drought, oaks (Q. ilex) shed fewer leaves than pines, resulting in less reduction in leaf area than in pine (P. halepensis). Leaf loss in pines also occurred earlier and at a higher Ψ than in oak. Both Q. pyrenica and P. sylvestris in a mixed forest were found sensitive to water stress, but WUE of the pine was significantly higher than that of the oak (106.7 ± 99 and 83.5 ± 73 µmolCO2 mol H2O−1, respectively). Despite high WUE, both showed decline in growth and depletion in non-structural carbohydrate (NSC) concentration, but it was much more in oak than in pine, with the NSC concentration being 7.8 ± 3.5% in oak and 3.9 ± 1.7% in pine (Table S1). In the face of increasing drought, Q. petrea showed more adjustment in canopy conductance than P. sylvestris.
Furthermore, drought reduced sap flux density more in pine (P. strobus) than in oak (Q. rubra, Q. velutina) (Table 2, Table S1). In oak, sap flux density may even increase (Table S1). Oaks acclimate to water stress more effectively through osmotic adjustment and the plasticity of functional and physiological traits. Oaks maintained leaf turgor even under the lowest midday leaf water potential, while pines experienced a severe drop in hydraulic efficiency. Pines (P. halepensis) avoid chronic and intense photoinhibition, possibly by retaining water in their tissues, whereas oak tissues were vulnerable to photoinhibition (Table S1). Water stress decreased leaf conductance in species of both genera, but the reduction was greater in pine (P. halepensis) than in oak (Q. coccifera).
In pine (P. halepensis), ΨΔ decreased linearly down to zero at ΨPD −3 MPa, while in oak (Q. ilex) it decreased exponentially and became constant at −0.5 MPa when ΨPD was lower than −3 MPa (Table S1). Responses along the soil moisture gradient improved under high irradiance in pine (P. pinaster) and under moderate shade in oak (Q. pyrenaica) (Table S1). Because of the muti-layered epidermis and calcium oxalate on the epidermis, oak (Q. potosina) can explore deep rock fractures and crevices, accessing additional water (Table S1). Pine (P. cembroides) cannot enter deep fractures in rocks and hence it depends on shallow soil water.
At the genus level, pines exhibit stronger isohydric behavior and wider hydraulic safety margins with longer needle longevity (Figure 2), enhancing water regulation, while oaks show more pronounced anisohydric behavior in deciduous species with shorter leaf lifespan, resulting in generally lower safety margins than evergreen oaks (Table 3). Leaf mass per area (LMA) increased with leaf lifespan in both genera but remained higher in evergreen oaks. Pines are typically early-seral, establishing rapidly after disturbance, whereas oaks are mostly late-seral, though some species can resprout and assume early-seral roles. Across diverse regions, these consistent patterns of within-genera variation reflect adaptive strategies influencing drought responses and successional dynamics.
Guttation, the expulsion of water from a plant leaf due to root pressure (when roots absorb water from moist soil faster than it can be released through transpiration), is presumably uncommon in pine and oak species, which generally have tall trees. In them, root pressure is generally not strong enough to force water to the high canopy; however, data are scarce. Transpiration pull is the main force for water movement in pine and oak species. Needle-like leaves of pines with sunken stomata are an adaptation to conserve water, while broad leaves are liable t guttation, but not when trees are tall. Pines rely on resin ducts and cuticular transpiration to manage excess xylem pressure. Oak leaves can show guttation through leaf margins or hydathodes, particularly in seedlings. Between the two genera, oaks seem to transpire at higher rates than pines. The details of stomatal regulation are poorly known and likely vary across species and water status of the plants. For example, the dynamics of water potential, abscisic acid (ABA), and stomatal conductance during drought are important and differ among conifers. P. radiata was found to show ABA-driven stomatal closure during droughts, resulting in strong isohydric regulation of water. In contrast, stomatal closure in Callitris rhomboidea was initiated by high foliar ABA, but sustained water stress saw a considerable decline in ABA and a shift to water potential-driven closure of stomata. The response speed of stomata and related ABA to drought in pines is slow, while in oaks, especially in deciduous oaks, it is high and fast.

3.2. Hydraulic Safety Margin

In response to drought, on average, pine species in comparison to oak species maintain a wider hydraulic safety margin (Table 4). Pines have a higher safety margin both because they close stomata significantly earlier (high Ψmin, −2.19 MPa vs. −3.07 MPa) as water stress mounts and because they have lower Ψ50 values (n = 12; −3.62 ± 0.20 MPa) than oaks (n = 13; average −2.61 ± 0.38). The water potential at which 50% loss of stem hydraulic conductivity occurred was −4.67 MPa in pine (P. halepensis) and −7.13 MPa in oak (Q. ilex).
However, the drop in tree water potential from Ψ50 to Ψ88 is greater in oaks (average, n = 13, −2.16 ± 0.53 MPa) than in pines (average, n = 12, −1.40 ± 0.21 MPa); thus, the values of Ψ88 of pine and oak are more similar than those for Ψ50. The drop in Ψ from Ψ50 to Ψ88 was sharper in deciduous oaks than in evergreen oaks, and in Diploxylon pines than in Haploxylon pines. In other words, the loss in hydraulic conductivity with a drop in Ψ down to Ψ50 is less in oaks than in pines, but subsequently it is more rapid in oaks. Several oak species had HSM less than 1 MPa both at Ψ50 and Ψ88 (Table 4). Both in oak (based on 25 species) and pine (based on 12 species), HSM increases with leaf longevity (Figure 3).

3.3. Growth and Carbon Recovery

The delay in stomatal closure enables oaks (Q. ilex, Q. faginea) to maintain carbon reserves, while in pines (P. halepensis), the carbon reserve is depleted through respiration. Non-structural carbon concentration was higher in oak (Q. subpyrenaica, 7.8%) than in pine (P. sylvestris, 4.0%), showing greater carbon storage in oak (Table S1).
Radial increment decreased more in pine (P. sylvestris) than in oak (Q. pubescens). When exposed to drought, oaks (Q. ilex) shed fewer leaves than pines, resulting in a smaller reduction in leaf area than in pine (P. halepensis). Leaf loss began earlier in pine (P. ayacahuite, 13 days, Ψ −4.13 MPa) than in oak (Q. xalapensis, 25 days, Ψ −4.34 MPa), indicating greater drought tolerance in oak (Table S1).
Resprouting after damage due to drought and fire is rare among pines (P. nigra) and is limited to the seedling stage. Oak (Q. ilex) resprouted with vigor after drought-induced disturbances, but not pine (P. halepensis) (Table S1). Recruitment failure occurred widely in pines (P. halepensis, P. pinea, P. pinaster, P. nigra, P. sylvestris, P. uncinate), while oak species (Q. suber, Q. ilex, Q. petraea, Q. robur, Q. faginea, Q. pyrenaica) expanded and increased basal area (Table S1). Drought-induced decline was stronger in pine, while oak showed a faster recovery after a drought. Warming may initially favor pine growth, but water stress offsets this advantage. Drought killed more pine seedlings and trees than oak, showing higher oak survival under warming.
In a mixed stand, the drought resistance of oak (Q. petraea) increased and that of pine (P. sylvestris) decreased compared to their pure stands (Table S1). Mortality of seedlings and trees due to drought is more common in pines (P. sylvestris) than in oaks (Q. pyrenaica), and drought recovery is more rapid in oak than in pine. During prolonged drought, sap flow decreased by 25%–50% in oak (Q. faginea) and 69%–90% in pine (P. nigra), but oak recovered more after drought (Table S1).
Oak (e.g., Q. rubra) can tolerate browsing even under water stress, while pine (e.g., P. strobus) succumbs when both stressors occur simultaneously (Table S1).
Broadly, pines have predominantly shallow root systems, and oaks have deeper root systems (Table S1). A strong tap root develops while trees are young in oaks, while in pines, deep tap roots are uncommon. The shallow pine roots, however, may extend laterally beyond the canopy width. Oaks may also develop vertical ‘sinker’ roots down from laterals. The oak fine roots have a specialized triple-layered epidermis with calcium oxalate crystals (Table S1), which enables them to extend roots deep into the rock fissures and fracture. When growing together, the difference in root systems may allow pine and oak to avoid competition and result in higher resilience to drought in mixed stands (Table S1). The ectomycorrhizal networks further influence their drought resistance in mixed stands. Oaks are well equipped to manage water-spending strategies, which may expand their dominance and distributional range at the expense of pines, resulting in a shift in balance toward oaks in regions under the influence of climate change.

3.4. Terpene Emissions and Antioxidants

The Mediterranean species P. halepensis, P. pinea, and Q. ilex are among the most investigated species with regard to terpene emissions. The most emitted terpene under water stress differs between pine and oak species. Between two typical Mediterranean species, P. pinea and Q. ilex, the oak emitted three times higher monoterpenes than the pine under dry conditions (Table 5). The oak’s response to water stress with regard to terpene emission is faster too.

3.5. Climate Change Effect

Studies on climate change indicate improved regeneration of oak at the expense of pine, an increase in dominance and expansion of oaks in mixed forests, lengthening of the foliated period in deciduous oak species, and more drought-induced decline in pines than oaks in mixed forests. A model-based study showed more increase in gross primary productivity of oak than of pine (Table S1). Data show that root growth is more adversely affected in pine than in oak (Table S1). Pines appear to be more vulnerable to droughts during hot weather than oaks; hence, these pines (e.g., P. sylvestris) are likely to be replaced by oak (Q. pubescens in Switzerland) (Table S1). A comparison between two regions differing by 2 °C in temperature in Germany showed that warming would adversely affect the root growth of Scots pine (P. sylvestris) and Douglas fir (Pseudotsuga menziesii), but not of sessile oak (Q. petraea) (Table S1).

4. Discussion

The increasing breadth of the field of ecology over the past several decades [52] is also evident in studies of water relations in pine and oak species. The increasing breadth of water relation studies involves approaches based on multidisciplinary fields, with an emphasis on applied environmental issues. The collaborative efforts in research, however, largely remain restricted to a few developed countries having similar climates and geography, such as those in the Mediterranean regions [53]. Since several species and geographical areas of pine and oak remain under-researched, generalizations made in this study about the comparative responses of pine and oak should be taken with caution. Furthermore, inadequacy of research is conspicuous in relevant areas like root systems and species behavior with regard to guttation and transpiration at the seedling level. Nevertheless, the available studies show a remarkably consistent association of pine species with isohydry and of oak species with anisohydry. However, the degree may vary within each genus (Figure 1); for example, deciduous oaks are more anisohydric than evergreen oaks. This difference in drought responses between pine and oak species is expected to play a key role in their co-occurrence. Related to this difference in drought responses is the linkage between pine oak species in ecological succession (pine being early-seral and oak late-seral). The severity of drought and varying responses to it could influence the course of succession. Climatic warming is likely to influence pine–oak dynamics by intensifying droughts beyond species’ safety margin thresholds and their capacity to recover after drought [54].

4.1. Contrasting Drought Responses and Hydraulic Safety Margin

Though studies are limited both in number of species and the geographical area of pine and oak, their drought responses are unmistakably different and consistent. Species of the co-occurring genera clearly differ in Ψplant at which they close stomata, water-use efficiency, changes in carbon reserves, hydraulic safety margin, and the ability to resprout and recover after drought and other disturbances.
Almost all the studies broadly concur with the above differences between pine and oak species. On an average, pines have a wider safety margin than oaks because they close stomata at higher Ψplant and lose conductivity at lower water potential Ψ50. Within each genus, species groups also differ. The hydraulic safety margin decreases with decreasing leaf lifespan within a genus (Figure 3). At Ψ50, the hydraulic safety margin is generally wider in Haploxylon than in Diploxylon pines, and in evergreen compared to deciduous oaks (derived from Choat et al. [34]; Skelton et al. [46]). The average leaf lifespan is 6.09 ± 1.14 years for Haploxylon pines and 3.42 ± 0.18 yearr for Diploxylon pines. Generally, evergreen oaks have a leaf lifespan of 1–2 years and deciduous oaks < 1 year. Species with shorter leaf longevity are physiologically more flexible in responding to environmental changes than those with longer leaf lives [55]. The difference in safety margin between pine and oak could be because of the difference between the larger plant groups to which they belong, gymnosperms and angiosperms, respectively. Only 6% of gymnosperms operate at negative safety margins, that is, Ψmin below Ψ50, compared to 42% of all angiosperms [34,46]. However, to validate this observation, far more data is required than currently available.
Seedlings are likely to have narrower hydraulic safety margins than mature trees, as they have more negative ΨMD in the field than mature trees [56,57,58]. In Himalayan oaks (Q. leucotrichophora, Q. floribunda, and Q. lanata), the minimum ΨMD in the field is more negative for seedlings (down to −2.81 MPa) than for mature trees (about −2.0 MPa).
Apart from these basic strategies, the species-rich oak genus employs some interesting responses to drought. A study of oaks in xeric Mediterranean regions, such as Q. frainetto and Q. canariensis, showed that growth responsiveness to drought was higher in declining individuals (trees with defoliated crowns) [59]. Several oaks (such as Q. ilex and Q. cerris) show plasticity in response to drought [60]. Oaks from xeric climate show higher osmolytes and reduced stomata pore index, which allow moderate gas exchange and less tissue loss. Oaks of xeric regions also maintain increased drought tolerance through osmoregulation, allowing continuous conservative growth [60].

4.2. Costs Associated with Drought Responses

Both types of drought responses have costs and consequences. The prolonged stomatal closure in pines may deplete the carbon reserve (Figure 1). On the other hand, delaying stomatal closure reduces tissue water content in oaks. Desiccation of leaf tissues may make the leaves more vulnerable to photoinhibition, reducing photosynthesis.
An extreme drought reduced growth in both Q. faginea and P. nigra in a Mediterranean forest (Table S1). However, P. nigra recovered little after drought. Its sap flow decreased by 69%–90%, compared to 25%–45% in Q. faginea. The oak recovered much better than the pine (Figure 2; [61]). The better recovery of oaks after drought probably results from their higher carbon reserves, as oaks keep their stomata open at lower water potentials. However, the response of oak and pine to drought and warming is modified by irradiance, with pine performing well in microhabitats with high irradiance and oak being more successful in moderate shade (Table S1).
The effect of guttation on the water balance of pine and oak species is presumably low; however, there is hardly any research on this. Guttation is developed in seedlings of some oak species such as Q. robur and Q. macrocarpa, while pines seem to rely on resin ducts and cuticle transpiration to manage xylem pressure in drought conditions. In a subhumid region of Turkey, Q. petrea was found to transpire annually 801.7 mm year−1, which was more than twice that of P. nigra (378.3 mm year−1). The modeled transpiration rate was calculated from sap flow measurements made by the trunk heat balance method [62]. Q. ilex exhibits remarkable resilience to both summer drought and winter cold, with populations showing high resistance to xylem cavitation and temperature extremes [63]. These adaptations allow the species to withstand climatic stresses, making it a potentially important component of Mediterranean ecosystems under future climate change scenarios.

4.3. Drought Response in Relation to Ecological Succession and Co-Occurrence

The drought responses of pine and oak species may be best interpreted in the context of the seral situation of most pine–oak forests. Though among the approximately 435 oak species, some may be early-seral [64,65,66], oaks are largely late-successional and generally follow the early-seral pine species [14,16].
Most natural selection and effective competition occur at the seedling stage, when plants are small and have shallow and small root systems. Pines are usually established in open, harsh, disturbed sites, with no shading and limited soil depth for rooting. Burned slopes, exposed glacial till, and sites with skeletal soil are common habitats which pines colonize. Thus, their early stomatal closure (maintaining high water potential and allowing a wide hydraulic safety margin along with their narrow tracheids) is required for survival. Shoot dieback kills the plant, as few pines sprout new shoots [67,68]. Pines usually have full sunshine year-round, so loss of carbon assimilation during drought can be recovered in other seasons. However, pines may be outcompeted once the canopy closes and competition for light becomes an overriding factor in a late-seral stage.
By keeping stomata open and carbon gain running in the face of declining soil moisture, oak seedlings and saplings perform well as succession progresses. Oaks, which often establish on sites already occupied by pine, can take advantage of shade and a developed forest floor to alleviate the effects of drought and use their large seed mass to establish deep roots. The drought response of P. pinaster is improved at higher irradiance, while that of Q. pyrenaica is more effective under moderate shade [69]. Anisohydry in oaks enables them to drive succession, in which carbon reserves could play a key role in survival under stress and disturbances.
Successional linkage is the core axis of co-occurrence of pine and oak species, however, other factors also contribute to it. For example, microclimatic variations contribute to the stable formation of a mosaic of communities: pines generally occupy ridge tops with high light intensity and shallow soils, and dry south-facing slopes, while oaks occupy hill bases where soils are deep and moist [70,71], as well as north-facing slopes and moist hollows. Between these two, the intermediate areas support pine–oak mixed forests. Variation in soil nutrients and soil moisture in landscapes favor their co-occurrence, with oaks occupying moist and moderately fertile sites, and pines tolerating dry soils with low pH and low fertility. Their species may have common mycorrhizal fungi, which facilitate their coexistence. Frequent disturbances (severe drought, landslides, grazing, and fire) may prevent succession from pine to oak and maintain persistence of both pine and oak [5]. Pine and oak species do well under suboptimal environmental conditions [5,8,72], which are likely to be associated with mid-latitudes and mid-altitudes where the pine–oak type dominates. To conclude, the coexistence of pines and oaks largely represents a balance between the colonization ability of the former and the competitive endurance of the latter, their ability to occupy topographically different sites, and their adaptation to different soil depths and nutrient conditions. As shown below, complementarity between species of the two genera may enable them to form mixed forests.

4.4. Recent Changes in Oak and Pine Dominance in an Area Appear to Reflect Climate Change Effect

There are several instances to suggest that the species of oak and pine differ in their responses to global climate change, with consequences for their distribution. The difference includes increased regeneration of oaks at the expanse of pines, gain in carbon because of longer growth period in deciduous oak species, increased dominance of oaks in mixed forests, more recovery of oaks than pines after severe droughts, and more depletion of carbon due to a disproportionately longer closure of stomata under droughts [73].
Increased regeneration of oaks relative to pines in S Europe and that of Fagaceae (125 species including many Quercus spp.) relative to Pinaceae (61 species, largely pines) in Mexico is seen as a climate change effect in which warming-induced drought is a key driving factor [26,74,75]. Expansion of oak species in many more plots than pine [28], through recent recruitment and less mortality [76], has been observed in Spain. The anisohydric traits, particularly the gain in carbon during drought in most situations may enable oaks to outcompete pines. In the changing global climate, deciduous oaks like Q. robur enlarge carbon reserves far more than evergreen oaks because of the marked lengthening of the foliated period. Available studies predict that climate change will lead to the increase in the dominance of oaks at the expense of pines; however, the advantage of oaks may diminish with the frequent occurrence of extreme droughts [77].
Though drought-induced damage and mortality vary, oaks appear to recover more than pines after droughts. Climate change is predicted to cause drought-induced decline in pine than in oak in the Mediterranean Q. pyrenaica–P. sylvestris forest (Mediterranean islands). Though oak is more vulnerable to drought than pine, oak rapidly recovers after drought, while pine takes more time [76]. However, when droughts are extremely severe, oaks also undergo increasing risks of drought-induced tree mortality [78].
Because P. sylvestris exhibits disproportionately strong stomatal closure under increasing temperatures and warm drought conditions compared to neighboring Q. pubescens, it is predicted to be increasingly replaced in Switzerland (Table 2). A model-based study on Q. robur, Q. ilex, P. sylvestris, and P. pinaster showed that gross primary production is predicted to increase from 1960 to 2100; however, the rate of increase is predicted to slow down after 2020 because of water stress (Table 2). The ecosystems are predicted to remain carbon sinks; however, sink strength globally is predicted to decrease in pines and increase in oaks. The increase in carbon sink is predicted to be greater in deciduous species largely because of the lengthening of the foliated period. P. sylvestris may be outcompeted by oak (Q. pyrenaica) under the impact of drought in Spain. This pine was reported to have high mortality even when growth was high [76]. Roots are likely to be more adversely affected in pines than in oaks. In fact, in oaks, root growth may increase, particularly in deeper soil, under the influence of drought [79,80]. In some oaks, the deeper roots with a triple-layered epidermis and calcium oxalate crystals are able to access water from rock fissures and crevices [81].
Like most other species, roots of pine and oak in response to drought have been studied only in a limited number of species. Generally, pines are known for shallow root system and oaks for deep root system. Some oaks may also develop vertical sinker roots down from the laterals [82]. The deep roots with access to subsoil water in some oaks enable them to maintain a water-spending trait. A study on Scots pine (P. sylvestris) and Sessile oak (Q. petrea) shows that fine root production decreases in the pine, while in oaks it may increase [83]. A study in the Himalayas shows that during winter drought, new roots are formed in deeper soil layers in oak (Q. leucotrichophora), while in pine (P. roxburghii) root distribution becomes shallower [80]. From the available studies, it is apparent that climate change is likely to favor oaks [84], and thus may change the patterns of pine and oak occurrence. Future research should include study of root growth phenology, depth distribution, and mycorrhizal association in pines and oaks across different habitats and regions. The available information on root growth seasonality could also be used for plantation purposes. Our data analysis shows that research on several regions and species is scant, rendering ecological generalizations less meaningful.

4.5. Extent of Pine–Oak Complementarity

Complementarity between pine and oak species in using water seems to play a positive role in their occurrence in mixed stands. Liu et al. [85] have shown that, because of soil depth complementarity, Q. accutissima and P. massonia can be grown in a mixed plantation. A similar inference was made in a study evaluating the growth and intrinsic water-use efficiency (iWUE) of P. halepensis, Q. robur, and other hardwood and conifer species [86]. The study showed that the pine and oak had a balanced ratio between basal area increment and iWUE and hence were appropriate for mixed plantations [86]. In a mixture, P. sylvestris and Q. petrea showed less negative ΨPD than in monoculture. Pardos et al. [87] have shown greater resilience of oak–pine mixture than their monocultures. P. pinaster and P. sylvestris, when grown with Quercus spp. (Q. pubescens, Q. petrea, and Q. pyrenaica) were less negatively affected by drought than when occurring alone. To what extent will pine–oak co-occurrence be affected by climate change? Answering this question could lead to an improved understanding of the complex dynamics between climate change and ecosystem functions. Climate change-induced droughts exacerbate fire, to which the species of the two genera respond differently [88,89,90,91].

5. Conclusions

Co-occurring widely in the regions with varying degrees of drought across the continents of the Northern Hemisphere, species of pines and oaks differ with remarkable within-genera consistency in their drought responses. Relying on early stomatal closure during the drought, pines largely exhibit isohydric traits, whereas oaks display primarily anisohydric characteristics, such as delayed stomatal closure and continued carbon exchange in the face of increasing drought. These differences in drought responses can be attributed largely to their successional situation. They owe their co-occurrence and persistence partly to their successional linkage, in which oak normally follows pine, and to disturbances which may reverse this pattern.
Climate change is predicted to favor the dominance of oaks in many parts of the world. Overall, oaks appear to have evolved strategies that prioritize carbon fixation even at the risk of impairment of hydraulic conductivity due to water stress, while pines rely on maintaining a wide hydraulic safety margin that enables them to persist safely in the environment. However, the limitation of data, both in terms of species and geographical area coverage, makes these interpretations less convincing. Data are lacking for several important areas which are relatively moist, like Himalayas, sub-tropical Asia, China, and the species-rich parts like Mexico. Research on them may greatly improve our understanding of species’ hydraulic safety margins in a rapidly warming climate.
The contrasting hydraulic safety margins between pine and oak may apply to the difference between gymnosperms and angiosperms. The evolutionarily advanced angiosperms are better adapted to maintain carbon fixation despite stress.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f16111660/s1, Table S1: A summary of the basic strategies of drought responses of co-occurring pine and oak species. Studies are arranged chronologically within regions [92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145].

Author Contributions

Conceptualization, S.P.S.; methodology, S.P.S., S.G., R.D.S., S.G., T.L., and R.P.; software, T.L.; validation, S.P.S., S.G., R.D.S., S.G., T.L., and R.P.; formal analysis, S.P.S., S.G., R.D.S., S.G., T.L., and R.P.; data curation, S.P.S., S.G., R.D.S., S.G., T.L., and R.P.; writing—original draft preparation, S.P.S.; writing—review and editing, S.P.S., S.G., R.D.S., S.G., T.L., and R.P. All authors have read and agreed to the published version of the manuscript.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are thankful to Don Zobel from Oregon State University for taking special interest in editing and providing academic inputs from time to time during the development of the manuscript. S.P.S. acknowledges the generous research facilities provided by the Indian National Science Academy (INSA), Delhi, Graphic Era (Deemed to be) University, Dehradun, and the Central Himalayan Environment Association, Nainital. S.G. expresses heartfelt thanks to G. B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, for their kind and liberal support. R.D.S. is grateful to Graphic Era (Deemed to be) University, Dehradun, for their continued support. R.P. acknowledges the generous support of the Indian Council of Forestry Research and Education (ICFRE).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A schematic representation of stomatal conductance and carbon assimilation in relation to increasing drought stress. Carbon assimilation is approximately ten times that of stomatal conductance.
Figure 1. A schematic representation of stomatal conductance and carbon assimilation in relation to increasing drought stress. Carbon assimilation is approximately ten times that of stomatal conductance.
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Forests 16 01660 g002
Figure 2. A diagrammatic representation of drought responses of pines (Pinus spp.) and oaks (Quercus spp.) and trade-offs associated with these strategies. These are broad patterns. Pines are terpene-storing plants, and oaks are not. * Here, safety margin is the difference between Ψplant when 50% conductivity loss occurs (Ψ50) and the minimum seasonal Ψmin in the field.
Figure 2. A diagrammatic representation of drought responses of pines (Pinus spp.) and oaks (Quercus spp.) and trade-offs associated with these strategies. These are broad patterns. Pines are terpene-storing plants, and oaks are not. * Here, safety margin is the difference between Ψplant when 50% conductivity loss occurs (Ψ50) and the minimum seasonal Ψmin in the field.
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Forests 16 01660 g003
Figure 3. The relationship between leaf longevity and the hydraulic safety margin (HSM, difference between P50 and the lowest Ψplant in the field) is represented for 12 pine species and 25 oak species. Each symbol corresponds to an individual species: Q. alba (al), Q. frainetto (fr), Q. petraea (pe), Q. phellos (ph), Q. robur (ro), Q. sebifera (se), Q. kelloggii (ke), Q. douglasii (do), Q. gambelii (gam), Q. garryana (gar), Q. lobata (lo), Q. pubescens (pu), Q. berberidifolia (be), Q. ilex (il), Q. oleoides (ol), Q. nigra (ni), Q. corneliusmulleri (co), Q. dumosa (dum), Q. durata (dur), Q. pacifica (pa), Q. agrifolia (ag), Q. wislizeni (wi), and Q. chrysolepis (ch). The pines are as follows: P. contorta (con), P. corsicana, P. halepensis (ha), P. mugo (mu), P. nigra (ni), P. pinaster (pina), P. pinea (pine), P. sylvestris (sy), P. taeda (ta), P. albicaulis (al), P. cembra (ce), and P. edulis (ed).
Figure 3. The relationship between leaf longevity and the hydraulic safety margin (HSM, difference between P50 and the lowest Ψplant in the field) is represented for 12 pine species and 25 oak species. Each symbol corresponds to an individual species: Q. alba (al), Q. frainetto (fr), Q. petraea (pe), Q. phellos (ph), Q. robur (ro), Q. sebifera (se), Q. kelloggii (ke), Q. douglasii (do), Q. gambelii (gam), Q. garryana (gar), Q. lobata (lo), Q. pubescens (pu), Q. berberidifolia (be), Q. ilex (il), Q. oleoides (ol), Q. nigra (ni), Q. corneliusmulleri (co), Q. dumosa (dum), Q. durata (dur), Q. pacifica (pa), Q. agrifolia (ag), Q. wislizeni (wi), and Q. chrysolepis (ch). The pines are as follows: P. contorta (con), P. corsicana, P. halepensis (ha), P. mugo (mu), P. nigra (ni), P. pinaster (pina), P. pinea (pine), P. sylvestris (sy), P. taeda (ta), P. albicaulis (al), P. cembra (ce), and P. edulis (ed).
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Table 1. Summary of the parameters and measurement units analyzed in studies assessing drought responses of pine and oak species.
Table 1. Summary of the parameters and measurement units analyzed in studies assessing drought responses of pine and oak species.
Drought Response Mechanism Parameters (Units)
Plant Water Relations and Hydraulic ResponsesPredawn water potential (ΨPD, MPa); midday water potential (ΨMD, MPa); leaf water potential (Ψleaf, MPa); daily change in tree water potential (Ψ∆, MPa); stem water potential (Ψstem, MPa); sap flow rate (%); stomatal conductance (gs, mol m−2s−1); transpiration rate; xylem embolism; percentage loss of conductivity (PLC); and hydraulic safety margin (Ψ50 and Ψ88, MPa)
Carbon Dynamics and MetabolismNon-structural carbon (NSC) concentration (%); carbon reserve depletion; respiration; net photosynthetic rate; water-use efficiency (WUE/iWUE, µmol CO2 mol−1 H2O), gross primary productivity (GPP) variation
Drought and Growth ImpactRadial growth (mm); xylem formation; sap flow recovery; leaf area increment (LAI) change (%); leaf senescence timing (days); post-drought physiological recovery
Root Traits and Belowground AdaptationsRoot depth and distribution; hydraulic lift; mycorrhizal symbiosis; microbial enhancement of water uptake; soil–water access
Climate and Environmental DriversLong-term drought impact; rainfall exclusion; temperature and CO2 effects; species range shift; mortality/recruitment
Terpene Emission and Secondary MetabolitesMonoterpene/sesquiterpene concentration (µg G DM−1h−1); stress-induced emission under drought/temperature stress
External FactorsMycorrhization; rhizobacteria; irradiance, canopy position; browsing pressure; site aspect
Table 2. A summary of comparisons between pine and oak species with regard to drought responses (including global climate change). The details of studies and references are given in Table S1. (n = number of studies showing data; however, where data were few, the number is not given). Values are averages where quantitative information was available, while in other cases, only trends (e.g., lower and higher) were indicated.
Table 2. A summary of comparisons between pine and oak species with regard to drought responses (including global climate change). The details of studies and references are given in Table S1. (n = number of studies showing data; however, where data were few, the number is not given). Values are averages where quantitative information was available, while in other cases, only trends (e.g., lower and higher) were indicated.
PineOak
Plant response to drought in terms of Ψplant when stomatal closure occurs (n = 15)


- Daily change in Ψplant (n = 7)

- Stomatal conductance (n = 12)


- Decrease in sap flow density		Higher (less negative, average ΨMD being −1.96 MPa) Ψplant; early during the drought (in all studies)


Narrow (average 0.50 MPa)

Generally lower; stomata sunken and located in pits


Higher		Lower (more negative, average ΨMD being −2.61 MPa) Ψplant; late during the drought (in all studies)



Wide (average 0.79 MPa)

Generally higher; stomata flush with or slightly below the epidermis


Lower
Water-Use Efficiency (WUE)HigherLower
Transpiration patternPeaked early in the day, but fell rapidly as VPD increased; seasonal rapid decline with drought (in all studies)More uniform in the day, sustained under moderate stress; maintained
longer transpiration during the years (in all studies)
Root-related responses
- Seasonal change in roots in response to soil drying

- Deep soil carbon sequestration

- Access of roots to deep water in rock crevices and fissures (scarce data)


- Aerenchyma tissues in roots
Decrease in root soil depth


Lower

Less common; shallow root domination, strong tap roots only at seedling stage; high fine root and mycorrhizal density in top 20 cm of soil



Less developed
Root growth extended to deeper soil


Higher

More common because oaks have three-layered epidermises with Ca-oxalate crystals; deeper root domination, deep penetration of mycorrhizal hyphae, high fine root density even below 20 cm of soil depth

More developed in some species (e.g., Q. robur, Q. macrocarpa)
Drought effect on carbon reserve (n = 7) More depletion; lower C reserve)Less depletion; higher C reserve
Leaf-specific transpiration rate LowerHigher
Growth-related parameters

- Shoot growth during spring

- Leaf loss

- Cessation of xylem formation

- Change in leaf mass per unit area (LMA)

- Stem circumference increment

- Effect of mixed pine–oak stand on stem circumference increment
More decrease

More and early during the drought

Early in the drought

Increased


More decrease

Decreased from pure stand to mixed stand
Less decrease

Less and late during the drought

Late in the drought

Increased


Less decrease

Increased from pure stand to mixed stand
Decrease in conduit area and radial lumen diameter More Less
Effect of mycorrhization on ΨplantIncreases Increases
High irradiance effect on drought plantsUnaffected Adversely affected
Guttation to manage high xylem pressure (scanty information)Rare, needles lack hydathodes for guttation and rely on resin ducts and cuticular transpiration to manage high xylem pressure Guttation through leaf margins or hydathodes; few species, particularly in seedlings, guttation involves an active root pressure mechanism facilitated by deeper roots
Climate change effect
- Predicted effect on GPP from 1960–2100
- Regeneration

- Post-drought recovery (intensified drought due to climate change)

- Climate change effect on competition

- Hot drought effect
Decrease


More negative or less positive effects

Less


Unaffected


More unfavorable
Increase


Less negative or more positive effects


More, through resprouting


Modulated


Less unfavorable or favorable
Table 3. Within-genera variation with consequences on drought responses.
Table 3. Within-genera variation with consequences on drought responses.
Pine SpeciesOak Species
Isohydry/anisohydryIsohydry, increases with needle longevity, so more in Haploxylon pines than in Diploxylon pinesAnisohydry, more in deciduous (4–6 months leaf longevity) than in evergreen species (1–3 years leaf longevity).
Hydraulic safety marginIncreases with needle longevity, so more in Haploxylon pines than in Diploxylon pinesLess in deciduous (0.53 MPa) than in evergreen species (1.77 MPa) [46].
Leaf mass per area (LMA, gm−2)Increases with leaf longevity from ~170 to 314 gm−2Higher in evergreen species (average 130 ± 6) than in deciduous species (average 92 ± 2) [29].
Seral status (support comes from Mediterranean regions, China, Mexico, USA, and the Himalayas, but not universal)Generally early-seral Generally late-seral; however, some oaks (e.g., Q. faginea) may dominate through resprouting immediately after disturbance (early-seral role).
Table 4. A comparison between Pinus and Quercus for hydraulic safety margin (HSM), minimum seasonal water potential, and the decline in water potential between 50% and 88% loss of xylem conductivity (derived from Choat et al., [34]). Values are averages with standard error, and n is the number of species for which data were available.
Table 4. A comparison between Pinus and Quercus for hydraulic safety margin (HSM), minimum seasonal water potential, and the decline in water potential between 50% and 88% loss of xylem conductivity (derived from Choat et al., [34]). Values are averages with standard error, and n is the number of species for which data were available.
GeneraSpecies GroupΨ50 Safety Margin (MPa)Ψ88 Safety Margin
(MPa)		Drop in Ψ from Safety Margin Ψ50 to Ψ88
(MPa)		Ψmin Seasonal
(MPa)
QuercusDeciduous0.55 ± 0.192.03 ± 0.691.88 ± 0.51−2.86 ± 0.50
(n = 8)(n = 8)(n = 9)(n = 9)
Evergreen1.14 ± 0.471.66 ± 1.122.80 ± 1.36−3.37 ± 0.41
(n = 4)(n = 4)(n = 4)(n = 6)
All oaks0.75 ± 0.20
(n = 12)		1.91 ± 0.56
(n = 12)		2.16 ± 0.53
(n = 13)		−2.19 ± 0.17
(n = 15)
PinusDiploxylon1.22 ± 0.182.73 ± 0.281.51 ± 0.25−2.26 ± 0.21
(n = 9)(n = 9)(n = 9)(n = 10)
Haploxylon2.01 ± 0.373.09 ± 0.7211.08 ± 1.73−1.92 ± 1.73
(n = 3)(n = 3)(n = 3)(n = 3)
All pines1.42 ± 0.18
(n = 12)		2.82 ± 0.26
(n = 12)		1.40 ± 0.21
(n = 12)		−3.07 ± 0.34
(n = 13)
Table 5. A summary of the studies indicating the effect of water stress on terpene emissions in species of Pinus and Quercus. Studies are arranged chronologically.
Table 5. A summary of the studies indicating the effect of water stress on terpene emissions in species of Pinus and Quercus. Studies are arranged chronologically.
Pine and Oak Species/PairSite/Country (Reference)Findings
P. halepensis; Q. ilexCatalonia, NE Spain
[41]		Both pine and oak emitted large and similar amounts of monoterpenes (~20 µg G DM−1h−1).
P. halepensis; Q. coccifera; Q. ilexBarcelona, Spain
[47]		Terpene emission was 5.64% and 1.65% of C fixation in summer in Q. coccifera and Q. ilex, respectively, and 5.39% in pine.
P. pinea; Q. ilexMediterranean Sea Shore, Spain
[48]		The oak (21.1 ± 19.8 µg (g d.w.)−1 h−1) emitted about 3 times more terpene than the pine (6.5 ± 5.4 µg (g d.w.)−1 h−1).
P. halepensis; Q. ilexBellaterra, Barcelona, Spain
[49]		Both oak and pine produced a large and similar amounts of monoterpenes (31.45 and 31.71 mu g g(−1) DM h(−1), respectively). However, the responses of oak were faster and stronger than that of pine.
P. halepensis; Q. ilexBarcelona, Spain
[50]		Terpene concentration is much more in the unstressed pine than in the oak. The drought treatment (reduction to 1/3 of full watering) significantly increased the total terpene concentrations in both pine and oak species but more in oak (119%) than in pine (54%).
P. halepensis; Q. calliprinos; Q. ithaburensis
Israel
[51]		At a mesic site, terpenes increased 2–3 fold in Q. calliprinos and Q. ithaburensis, and 5.8-fold in the pine when stressed.
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Singh, S.P.; Gumber, S.; Singh, R.D.; Li, T.; Pandey, R. A Global Comparative Analysis of Drought Responses of Pines and Oaks. Forests 2025, 16, 1660. https://doi.org/10.3390/f16111660

AMA Style

Singh SP, Gumber S, Singh RD, Li T, Pandey R. A Global Comparative Analysis of Drought Responses of Pines and Oaks. Forests. 2025; 16(11):1660. https://doi.org/10.3390/f16111660

Chicago/Turabian Style

Singh, Surendra P., Surabhi Gumber, Ripu Daman Singh, Tong Li, and Rajiv Pandey. 2025. "A Global Comparative Analysis of Drought Responses of Pines and Oaks" Forests 16, no. 11: 1660. https://doi.org/10.3390/f16111660

APA Style

Singh, S. P., Gumber, S., Singh, R. D., Li, T., & Pandey, R. (2025). A Global Comparative Analysis of Drought Responses of Pines and Oaks. Forests, 16(11), 1660. https://doi.org/10.3390/f16111660

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