1. Introduction
Tree fine roots (roots < 2 mm in diameter) play a crucial role in forest ecosystem functioning, even though they represent only a few percent of tree biomass [
1,
2,
3]. Fine roots serve as the interface between soil and tree and thus control water and nutrient uptake, they closely interact with mycorrhizal fungi and rhizosphere microbiota, and represent a major source of soil organic carbon (C) [
4,
5]. Due to their rapid turnover, it has been estimated that up to a third of the global annual net primary production refers to fine root growth [
2]. The size, morphology, and turnover rate of the fine root system of trees is dependent on many biotic and abiotic factors. Water availability is a key determinant among the soil factors besides nutrient availability, soil acidity, and temperature [
6,
7,
8,
9].
With climate change, forests are exposed to warmer summers and a higher evaporative demand, and, in various regions, reduced and more irregular summer precipitation [
10,
11], likely exposing trees to increased drought and heat stress. Recent reviews of climate change-related decreases in tree vitality and increasing mortality in many forest regions of the earth have predominantly focused on aboveground tree parts [
12,
13,
14,
15], ignoring root responses. This is primarily caused by the fact that the fine root system of mature forests is difficult to observe, and methods are labour-intensive and often quite imprecise [
6].
How tree roots and the root system respond to drought is increasingly a matter of debate, but the empirical data basis is quite limited, especially for mature trees. Optimal partitioning theory (OPT) predicts that trees tend to increase their root-to-shoot ratio (R/S) under conditions of limited water availability to increase their absorptive capacity in relation to the transpiring surface [
6,
16,
17]. Evidence in support of OPT has been obtained by comparing the R/S of different plant functional types from different biomes [
18,
19], by analysing the R/S of trees or forests along precipitation gradients [
20,
21], and by manipulating soil moisture in experiments with tree saplings [
22,
23,
24]. Most studies with increases in R/S found a reduction in aboveground biomass (leaf mass or shoot mass), while fine root biomass (FRB) changed only a little, suggesting that shoot growth may respond more sensitively to drought than root growth [
25]. Yet, it is not well understood how the fine root system of trees responds to a decrease in soil water availability, and existing findings are partly contradictory [
6,
26,
27]. Many sapling studies found a decrease in FRB with decreasing soil moisture [
28,
29,
30,
31], but these responses can hardly be extrapolated to mature trees, and limited rooting space may sometimes have influenced the results. For mature stands, Leuschner and Hertel [
8] found in a meta-analysis of studies from temperate broad-leaved forests no clear trend in FRB in dependence on precipitation. The results of multi-site field studies with a single tree species are inconsistent: Some authors reported a higher FRB at drier sites [
20,
26,
32,
33], others a higher biomass at moister sites [
7,
27,
34,
35]. The controversial results are partly explained by differences in species, in the steepness of the precipitation gradient, or in the severity and timing of drought events at the study sites. Moreover, different combinations of growth-related reductions in overall productivity, of shifts in aboveground/belowground carbohydrate partitioning, and of altered root lifespan may lead to opposing root/shoot ratio responses [
36].
More consistent are the reports about a fine root necromass (FRN) increase or elevated necromass/biomass (N/B) ratios as a consequence of drought exposure [
6,
37], indicating increased fine root mortality. In accordance, Hertel and Leuschner [
38] found a large FRN increase in a mature beech forest after a summer drought, while FRB remained constant over the study period. It appears that certain tree species are capable of compensating elevated drought-induced fine root losses through increased production of new fine roots, thereby avoiding reductions in standing FRB. It is not known which species are capable of this response and under which conditions it occurs.
Root mortality reduces the lifespan of fine roots and, when dying roots are replaced by new ones, a reduction in mean fine root age in the root population is the consequence [
36,
39]. One possible physiological explanation of a shortened fine root lifespan in dry soil is active root shedding, in which fine roots act as ‘hydraulic fuses’ in the tree’s xylem system, uncoupling the rest of the hydraulic system from low water potentials in the dry soil to avoid embolism formation in more expensive or irreplaceable plant organs [
40,
41,
42]. This could happen when fine roots are indeed more sensitive to cavitation in the xylem than the stem and branches in the canopy. An alternative explanation assumes that younger fine roots, that replace the shed ones, are physiologically more active and therefore can support the tree by extracting more water, which increases fitness [
6,
39]. Such a response would also be in agreement with OPT, which predicts a higher investment in the root system. In any case, root mortality will increase the N/B ratio, which must be seen as an integral over the processes of root mortality and the production of new fine roots during the observation period, thus reflecting both the root system’s resistance to drought and its resilience after drought. The ratio has therefore been used in several studies and reviews as an indicator of tree and root system vitality under exposure to drought and chemical stress [
7,
37,
43,
44].
Field studies on the drought response of tree fine root systems usually adopt one of two approaches, either investigating temporal (natural or experimental) variation in soil moisture [
45,
46,
47,
48,
49,
50], or examining spatial differences in water availability, often along precipitation or soil moisture gradients [
7,
26,
27,
32,
35,
38,
43,
51]. In central Europe, such studies focused on the economically most important coniferous and broad-leaved tree genera, i.e.,
Picea,
Pinus,
Fagus, and
Quercus.
Only little is known about the fine root system of other common tree species that are minor timber species or of no use in current forestry. Small-leaved lime (
Tilia cordata Mill.), Norway maple (
Acer platanoides L.), European hornbeam (
Carpinus betulus L.), and European ash (
Fraxinus excelsior L.) may be more drought resistant than European beech and Norway spruce according to their distribution ranges and knowledge of aboveground physiological traits [
1,
52]. While these species can be of interest for forestry in a warmer and drier climate in the future, their belowground drought response is unknown. Recent root research in temperate mixed forests with lime, maple, hornbeam, oak, and ash species has focused on species interactions and diversity effects, but drought responses have not been examined [
53,
54,
55]. A better understanding of the belowground drought response of these minor timber species is fundamental for predicting the species’ performance in a drier climate.
This study investigates the response of the fine root biomass and necromass, and fine root morphology of four secondary tree species (T. cordata, A. platanoides, C. betulus, and F. excelsior) to a reduction in water availability, combining a precipitation gradient study with the comparison of a moist and a dry season. The well-studied and relatively drought-resistant sessile oak (Quercus petraea (Matt.) Liebl.) was included in the study for comparison. The sample thus comprised three ECM (ectomycorrhizal; T. cordata, C. betulus, Q. petraea) and two AM tree species (arbuscular mycorrhizal; A. platanoides, F. excelsior), and two ring-porous (Q. petraea, F. excelsior) and three diffuse-porous species (T. cordata, C. betulus, A. platanoides), thereby covering a broad range of tree functional types of the European temperate tree flora. Fine root inventories were carried out in the topsoil of nine study sites along a precipitation gradient (mean annual precipitation (MAP): 918–528 mm year−1), comparing data from a moist (spring 2017) and a subsequent dry period (summer 2018). The precipitation gradient covers most of the MAP range encountered by the species at their natural occurrences in northern central Europe.
We expected that the five species differ in their root system response to water deficits and that the sensitivity of belowground and aboveground organs are linked to each other in these species. The extent of the N/B ratio increase was used as a measure of the species’ belowground sensitivity to water deficits. Based on the existing knowledge about the drought response of tree fine root systems [
6,
26,
27], we hypothesized that, in all species, (i), the fine root N/B ratio increases with decreasing mean annual precipitation due to higher root mortality and thus an increase in necromass, while biomass changes only little, (ii) a severe summer drought increases necromass and the N/B ratio, and (iii) the increase in the N/B ratio upon drought is more pronounced at the moister sites, where trees are assumed to be more sensitive to water shortage.
4. Discussion
4.1. Fine Root Biomass and Belowground C Allocation in Dependence on Long-Term Water Reduction
Our study at nine sites along the steep precipitation gradient in the rain shadow of the Harz mountains (MAP: 920–530 mm year
−1) found only weak support for optimal partitioning theory, when applied to the FRB stocks in the topsoil. FRB in the moist sampling period 2017 did not change in a consistent manner with decreasing mean annual precipitation.
A. platanoides was the only species with a significant increase in FRB from the moister to the drier sites in 2017, which could point at increased belowground C allocation to increase water uptake, while
Q. petraea showed a decrease and the other three species no relation of FRB to MAP. The outcome was not different when other precipitation variables (e.g., MGSP, current-year precipitation or climatic water balance) were used instead of the long-term mean. Other studies along precipitation gradients obtained mixed results, either no consistent change in FRB and relative C allocation to roots [
68,
69], a decrease [
7,
27], or an increase with decreasing water availability [
26,
33], suggesting a large influence of species and soil moisture conditions on the drought response of carbon allocation. Clearly, our fine root inventory covers only the topsoil and we hence may have missed preferential biomass partitioning to other parts of the root system. Trees growing at drier sites could allocate more carbon to root growth in deeper soil layers to access the moister subsoil and escape surface drying [
70,
71]. However, a meta-analysis of root biomass data by Schenk and Jackson [
72] and a detailed study of the subsoil root system of
Fagus sylvatica along a precipitation gradient by Meier et al. [
68] showed the opposite response to long-term precipitation reduction, i.e., shallower rooting of trees under water limitation. An alternative explanation for the weak support for OPT in our study could be the only moderate length of the studied precipitation gradient (MAP difference: 390 mm year
−1), which apparently had only a minor effect on total tree productivity. Carbon allocations shifts in support of OPT were mostly found in studies across biomes or in experiments with very different treatments. Poorter et al. [
73] concluded from a meta-analysis that marked increases in allocation to the root system occur only, when drought reduces biomass by 50 percent or more, which is not the case here.
The PCAs with root-related and environmental variables support the conclusion that the average soil moisture regime as indicated by the MAP gradient has only a minor influence on the FRB stocks at our study sites. In both root inventories, the association of FRB with edaphic and climatic variables suggests that topsoil FRB generally increases with organic matter content, but decreases with increasing nitrogen content, silt content and soil bulk density, while the effect of climatic factors (MAP, MAT) and also soil pH and P content is weak. This is in accordance with the observation that fine root density in temperate forest soils is usually highest in the carbon-rich Ah horizon and the organic layer with low bulk density [
7,
74].
Inherited tree species differences in FRB seem also to be more influential on FRB patterns than moisture conditions. In our study,
F. excelsior had up to five times higher FRB densities in the topsoil than
Q. petraea and
T. cordata, with intermediate values in
C. betulus and
A. platanoides. While part of this variation may be due to differences in DBH and small-scale variations in stem density between species in mixed stands, comparison with earlier fine root studies in mixed forests suggests that the high FRB of
F. excelsior and the low values of
Q. petraea may be species-specific. This is indicated by a meta-analysis of fine root studies from temperate forests [
8] and root inventories in mixed forests [
54,
75]. An additional explanation for low FRB values of
Q. petraea might be a different depth distribution of fine roots, as other authors state that Central European oak species are generally deeper rooted than beech and other broadleaf tree species [
76], but precise data on depth-distributions are lacking for our sites. The contrasting FRB patterns of
A. platanoides and
Q. petraea along the precipitation gradient further indicate that co-occurring tree species may differ not only in standing FRB but also in root mortality and the response of their carbon allocation modes to long-term reduction in water availability.
As predicted (hypothesis 1), all five species showed increasing amounts of FRN and an increasing N/B ratio in the topsoil with a decrease in MAP, while FRB remained unchanged (with the exception of
A. platanoides). One possible explanation of this pattern is that fine root mortality increases with a permanent reduction in water availability, as has been observed in many field studies (e.g., [
27]) and concluded from literature reviews (e.g., [
36,
77]), which in turn may trigger increased carbon allocation to root growth in compensation of the FRB loss. This was observed, for example, in Norway spruce roots under mild drought stress (soil matrix potentials of—0.06 MPa, [
45]). Such a response will reduce mean fine root age and likely increase the water and nutrient uptake capacity of the tree and thus its fitness under water shortage [
6,
39]. Another possible explanation is fine root shedding with the assumed function to uncouple the rest of the hydraulic system from very low water potentials in dry soil to avoid embolism formation in more valuable organs (hydraulic fuse theory, [
41,
42]). It could take place during more severe drought events and does not imply the immediate replacement by new fine roots. In support of this idea, McCormack and Guo [
36] predicted on the basis of a conceptual model exponential increases in root mortality at the highest drought stress intensities. This is in accordance with the results of a rainfall exclusion experiment with
Picea abies trees, in which, under mild drought, root mortality increased, while fine root production was also stimulated. Under more severe drought, root mortality was high and no replacement occurred [
45].
Our data suggest that the five species differ in specific root mortality rates upon soil desiccation, as the N/B ratio showed a more than tenfold increase with the MAP reduction along the transect in T. cordata, intermediate N/B slopes in C. betulus, A. platanoides, and F. excelsior, and the lowest increase in Q. petraea. We interpret these patterns as a hint that Q. petraea is better able than the other species to produce fine roots capable of tolerating long-term reductions in soil moisture without suffering increased root mortality. Physiological and genetic studies have to show whether this is due to a principally different physiological constitution of the fine roots of this ring-porous species, or is caused by the specific acclimation or adaptation of different oak populations along the precipitation gradient. An alternative explanation for increasing FRN amounts and N/B ratios with a MAP reduction is that the drier and somewhat warmer climate toward the east of the transect reduces root decomposition rate and thus leads to the accumulation of FRN, independent of changes in root mortality rate. In the absence of decomposition data, this possibility cannot be ruled out, but it is not very likely. The gradients in MAT (7.9–9.9 °C), soil pH (4.2–6.5) and C/N ratio (10.7–18.9) along the transect were only moderate and the latter factors did not covary significantly, either with MAP or FRN. Moreover, site differences in decomposition rate should mainly affect the finest root necromass particles, which were not investigated here. We assume that the analyzed larger, less fragmented necromass fractions (>10 mm length) reflect more directly the root mortality processes, where decomposition likely has started only very recently.
4.2. Effects of the 2018 Summer Drought on Fine Root Biomass and Belowground C Allocation
It is noteworthy that none of the species showed a decrease in mean FRB across the transect after the severe 2018 summer drought (in
C. betulus, a non-significant tendency for a decrease existed). The 2018 drought with summer precipitation amounts 55–73 % lower than the long-term average was extreme and resulted in the local dieback of more sensitive tree species (
Picea abies,
Fagus sylvatica) in the region. This indicates that all five species must be relatively tolerant of soil desiccation compared to other major timber species, and that precipitation is playing only a secondary role for the standing FRB of these species. It is possible that our FRB figures are influenced by temperature and other seasonal influences unrelated to summer drought, as fine root production and biomass stocks typically peak between April and July in central European broadleaf tree species, as is visible from studies in beech [
27,
78] and beech-oak mixed forests [
38]. Thus, we cannot exclude with certainty that the biomass figures observed in September 2018 represent reduced values which are influenced by the typical seasonal FRB decrease that should have taken place later in summer. However, we did not observe a FRB reduction. Moreover, if the reduction had occurred, it should have been similar along the transect. In addition, the
A. platanoides data from the moist transect end indicate the opposite, a FRB increase from the 2017 to the 2018 inventory.
Higher FRN amounts in three of the species (A. platanoides, F. excelsior, and Q. petraea) in 2018 (in comparison to 2017) indicate that drought has increased root mortality. Interestingly, the severe drought drove all N/B ratios to converge on a higher level, or, in other words, all species lost the MAP dependence of necromass and N/B ratio after the drought. This suggests that the mortality increase was greater at the moister than the drier sites in all species except for Q. petraea, which largely supports hypothesis 3. We explain this pattern with a generally higher drought sensitivity of the root systems at MAP > ca. 700 mm year−1, which caused higher root mortality and leveled all FRN differences that exist along the transect in normal years. In T. cordata and C. betulus, FRN and N/B ratio were also higher at the moister sites in 2018 than 2017, but this response was compensated by lower FRN amounts at the drier sites. Thus, our second hypothesis is only partly supported.
In a meta-analysis about stand- and soil-related drivers of fine root N/B ratio across biomes, Wang et al. [
37] found elevated N/B ratios at reduced precipitation only in forests dominated by ECM tree species, but not in AM forests. This suggests an influence of mycorrhizal type on the drought response of the root system. Liese et al. [
79] confirmed these findings in a mesocosm experiment for several temperate tree species (including
Acer,
Fraxinus,
Carpinus,
Tilia, and
Quercus taxa), demonstrating a much greater drought-induced root lifespan reduction in ECM than AM species (40–56% vs. 0.5–13%). Our data from three ECM (
C. betulus,
T. cordata, and
Q. petraea) and two AM species (
A. platanoides and
F. excelsior) do not support this conclusion, as
Q. petraea was the species with the smallest N/B response to a MAP reduction at the drier end of the gradient. The response to the 2018 summer drought even revealed the opposite response pattern to that found by Wang et al. [
37] and Liese et al. [
79], with a significant increase in the N/B ratios in the AM species
A. platanoides and
F. excelsior, while all ECM species did not respond. One possible explanation for the discrepancy between the results of the Wang et. [
37] and Liese et al. [
79] studies and our investigation is that largely different spatial scales (comparison of biomes with different climates; sapling experiment; regional gradient study) are considered.
4.3. Root Morphological Change in Response to Reduced Water Availability
Trees can adapt to shortages in water or nutrients by increasing the absorptive capacity of the root system in two different ways: by enhancing root production and maintaining larger absorbing surface areas (extensive strategy), or by modifying root morphology and physiology in order to increase uptake efficiency per root mass (intensive strategy, [
80,
81]). In contrast to the other four species,
Q. petraea showed characteristics of an intensive adaptation strategy by increasing SRA and the number of root tips per root mass towards the drier sites according to the 2017 inventory, while FRB remained constant.
Q. petraea differed further from the other species by showing no root tip shedding and no SRA reduction after the 2018 drought.
A. platanoides showed the opposite response with a FRB increase towards the drier sites, while root morphology was not altered. The other three species maintained a constant FRB along the gradient, but the marked FRN increase toward the drier sites points at elevated root turnover and compensatory stimulation of fine root production under desiccation, which can be viewed as attributes of an extensive strategy.
After the 2018 drought, all species except Q. petraea showed a marked dieback of root tips and more distal thin rootlets. This resulted in the observed SRA reduction, which was particularly strong in T. cordata. The losses in the putatively most active finest rootlets had not been replaced until September 2018, when sampling was conducted. We interpret this response in 2018 as an indicator of belowground vulnerability to extreme drought, which must have reduced the vitality and absorptive capacity of the fine root system of F. excelsior, A. platanoides, C. betulus, and T. cordata.
4.4. Species Differences in the Belowground Drought Response
The fine root system of
Q. petraea seems to be more resistant to both permanent moisture reduction and severe drought events than that of
T. cordata,
C. betulus,
F. excelsior, and
A. platanoides due to the following features: (1) Although fine root necromass increased after the 2018 summer drought, the N/B ratio changed only a little and it was roughly constant across the precipitation gradient. (2) Fine root morphology and the number of root tips were not affected by the drought, indicating either low sensitivity or rapid recovery in oak roots. This fits to findings from the dendrochronological analysis of climate sensitivity (e.g., [
82]) and more general comparative assessments of drought resistance of the species based on climate envelopes [
1,
83]. On the other hand,
Q. petraea maintained the lowest fine root density in the topsoil and more generally seems to produce a relatively small fine root system. Due to still-unknown morphological and/or physiological properties, oak can also maintain its fine roots in dry periods instead of shedding and partly replacing them. This rather “conservative” strategy with lower maintenance costs and a more or less constant root biomass during wet and dry periods was also observed in other Central European oak forests by Leuschner et al. [
75], who compared
Q. petraea to
Fagus sylvatica and concluded that this strategy comes with the drawback of inferior interspecific competitive ability.
The other four species have in common that they all show indications of a somewhat greater belowground vulnerability to severe soil drought, but they pursue different strategies.
T. cordata seems to be the most vulnerable species due to large drought-induced reductions in SRA and tip frequency, which is in line with assessments based on leaf and stem level data [
84,
85].
F. excelsior is unique due to its high fine root density, which may secure water uptake in drought periods and increase the species’ competitive ability in mixed stands and on very shallow and dry soils.
5. Conclusions
Our results suggest that co-occurring tree species differ in the drought sensitivity of their fine root systems, which could play an important role with respect to the species’ fitness and drought survival. Yet, much less is known about the belowground growth and stress tolerance strategies of trees than about aboveground responses.
This case study suffers from a number of shortcomings that are introduced with the study design and the methods used, which may bias some of the conclusions. First, the study design is not fully symmetric, as not all tree species occur at all sites, which weakens the power of statistical analysis. Second, our FRN analysis covers only the larger fragments, as the finest particles could not be identified to the species level. Consideration of the complete root necromass pool might have led to somewhat different results. Finally, edaphic inhomogeneity introduces some noise in the climatic signal retrieved from the precipitation gradient, which may weaken some of the conclusions. Fortunately, climate and soil properties did not covary systematically. While the retrieved patterns seem plausible, they need verification by additional gradient studies in other regions and with additional species.
We conclude that the comparative analysis of fine root biomass, necromass, and fine root morphology along precipitation gradients, and in moist and dry periods, has the potential to provide valuable information on the belowground drought sensitivity of tree species, thereby complementing results from canopy- and leaf-level studies.