Hydraulic Response of Deciduous and Evergreen Broadleaved Shrubs, Grown on Olympus Mountain in Greece, to Vapour Pressure Deficit

In this study, leaf hydraulic functionality of co-occurring evergreen and deciduous shrubs, grown on Olympus Mountain, has been compared. Four evergreen species (Arbutus andrachne, Arbutus unedo, Quercus ilex and Quercus coccifera) and four deciduous species (Carpinus betulus, Cercis siliquastrum, Coronilla emeroides and Pistacia terebinthus) were selected for this study. Predawn and midday leaf water potential, transpiration, stomatal conductance, leaf temperature and leaf hydraulic conductance were estimated during the summer period. The results demonstrate different hydraulic tactics between the deciduous and evergreen shrubs. Higher hydraulic conductance and lower stomatal conductance were obtained in deciduous plants compared to the evergreens. Additionally, positive correlations were detected between water potential and transpiration in the deciduous shrubs. The seasonal leaf hydraulic conductance declined in both deciduous and evergreens under conditions of elevated vapor pressure deficit during the summer; however, at midday, leaf water potential reached comparable low values, but the deciduous shrubs exhibited higher hydraulic conductance compared to the evergreens. It seems likely that hydraulic traits of the coexisting evergreen and deciduous plants indicate water spending and saving tactics, respectively; this may also represent a limit to drought tolerance of these species grown in a natural environment, which is expected to be affected by global warming.


Introduction
The climate in the Mediterranean region is characterized by a prolonged summer drought period, which according to the majority of climate change scenarios, is expected to become more severe in the future [1][2][3]. Drought is the main environmental stress directly linked to plant survival, growth, competitiveness, persistence, and productivity in Mediterranean habitats that are under an increasing risk of degradation [1,[4][5][6][7]. On the other hand, plants have evolved a variety of morphological and physiological tactics [8] to withstand drought stress [9,10]; these include (a) hydraulic strategies via deep, tap root systems to extract water from deep soil layers, enabling the plants to sustain elevated water potential and xylem pressure during drought and (b) morphological and physiological adaptations at the leaf level in order to reduce water loss [11,12]. The importance of the hydraulic system in water consumption has led to the hypothesis of the functional convergence in the regulation of water use among phylogenetically diverse species [13,14]. Nonetheless, few studies have been performed to characterize the differences in leaf hydraulic conductance and hydraulic status among phylogenetically different plant species, such as deciduous and evergreen shrubs, in Mediterranean habitats [15,16]. Leaf is a substantial organ for the transport of water throughout the plant and hence leaf hydraulic conductance is an important parameter to determine plant water status [15,[17][18][19][20]. To the best of our knowledge, a comparative study among hydraulic characteristics at the leaf level of evergreen and deciduous species co-occurring in the same habitat, with respect to the impact of climatic stimuli on functionality, has not hitherto been published.
In order to comprehensively address differences between evergreen and deciduous plants concerning physiological responses to drought, the hydraulic and stomatal performance should be examined with well-established approaches. The water status for each individual, specific plant depends on the difference between transpiration (E) and water absorption (A), but it is not clear which of these factors is more important for plants' gas exchanges in response to drought stress [12]. If the soil water shortage increases, water stress will increase over time, negatively affecting many physiological and metabolic aspects of the plants [8,21,22]. Plants close the stomatal pores to regulate water loss [12,23] through transpiration when the available soil water decreases and/or an increase in the difference between the saturation (i.e., amount of water vapor that the air can hold, namely the saturation vapor pressure) and actual vapor pressure, i.e., the Vapour Pressure Deficit (VPD), is detected.
VPD has been recognized as an important parameter for plant functionality and survival, which is influenced by the so-called hydraulic failure [24][25][26][27][28][29]. Oren et al. [30] fount in drought tolerant species a regulation of water potential (Ψ) as VPD increases and recommended different sensitivity of stomatal apparatus to VPD among different functional groups. However, E could either increase or decrease in response to an increased VPD [27,31]; the first response is known as "feedback" response while the second as "feed forward" response. The transpiration rate is influenced by atmospheric conditions and, over a short time-scale, is regulated by the function of stomatal apparatus [32]. It has been argued that a declining stomatal conductance (g s ) concomitantly with increasing VPD would rather occur as a feedback response to E and water loss from the leaf, than as a direct response to humidity [33][34][35].
The function of the stomatal apparatus under global climate change is an essential subject in plant ecophysiological research because it affects plant growth, vegetation distribution and ecosystem function [36]. The stomatal response to VPD varies either across and within species, or within the same species [37][38][39][40]. Partial stomatal closure under elevated VPD, especially during midday, will negatively affect CO 2 assimilation rate [7,41].
The movement of water in the soil-plant-atmosphere (SPA) continuum is a function of the difference between hydrostatic pressure and hydraulic conductance along this pathway. Water flows through the SPA continuum driven by a gradient in Ψ, which depends on the water flow rate and the hydraulic conductivity of the different pathways [32]. The amount of water that will be absorbed by the roots of the plants and the amount that will move from the roots to the leaves and then to the atmosphere depends mainly on g s , VPD and hydraulic conductance (K). The differences between plant species in Ψ, E and g s , determine to a greater or lesser extent the plants' response to various water stresses [12,[42][43][44][45]. However, the effects of rising VPD on vegetation and hydraulic dynamics remain poorly studied. It has been reported that significant higher leaf and stem hydraulic conductance [25,46,47] under increased irradiance are due to the regulation of aquaporins [46][47][48]. A midday decline of g s has been obtained in many plant species and has been related with variation in midday stem water status [49,50]; this aspect supports the idea that stomatal response to VPD is substantially related to the hydraulic characteristics at both the whole plant scale and the leaf level [27,51]. There is a fundamental role of hydraulics on stomatal sensitivity to VPD [27,41], while inverse and non-linear relationship between conductance and VPD have been observed [36]. The opening and closing of the stomata seem to be controlled by complex mechanisms, which include chemical and hydraulic signals from roots, to shoots and leaves [52]. In this frame, hydraulic traits could be an important factor buffering the negative impact of drought on plant function [53].
A lot of research has been devoted to understanding how plants' hydraulic systems have evolved to accommodate survival under different environments. However, concerning old-grown, trees and shrubs the relationship between K and the response of g s to VPD has not explicitly evaluated, in situ. Despite the above-mentioned trends, species grown under the same environmental conditions may exhibit entirely different hydraulic properties [54,55]. This interspecific variation sometimes may be ascribed to different functional types, such as deciduous and evergreen [56]. Nevertheless, variation of hydraulic traits cannot only be explained by categorization of species in functional types [56,57]. Forest and shrub communities' response to climate change are most closely related to microclimatic change and not to macroclimatic change [58,59].
In considering that the water balance (W) is expressed by the relationship W = A − E, it is very interesting to study whether: (1) variation of hydraulic conductance in Mediterranean shrubs during the dry summer period is reflected in the leaf phenology (i.e., evergreen vs. deciduous), and (2) changes of VPD and consequently of microclimatic conditions influence the physiological mechanism and the performance of evergreen vs. deciduous shrubs.
The main objective of this study was to compare the ability of co-existing deciduous and evergreen broadleaved shrubs grown on the Olympus Mountain to maintain their water status and control their stomatal conductance throughout the dry season, as well as to evaluate the response of deciduous and evergreen shrubs to enhanced vapor pressure deficit. It is likely that hydraulic responses of co-occurring deciduous and evergreen shrubs, which are to some extent linked to water exploitation and effectiveness of plants' life forms during a period of soil drying, have not been reported.

Climatic Conditions
The seasonal pattern of VPD during the experimental period is given in Figure 1. Overall, predawn vapour pressure deficit (VPD p ) was significantly lower (p < 0.05) than midday vapour pressure deficit (VPD m ), except from the first measurement (mid-May) (p ≥ 0.05). Predawn VPD p ranged from 0.769 ± 0.019 kPa to 1.731 ± 0.054 kPa, while VPD m ranged from 0.974 ± 0.044 kPa to 4.043 ± 0.106 kPa. The relative humidity (RH) followed a reverse course relatively to VPD during the experimental period ( Figure 1). The Photosynthetic Photon Flux Density (PPFD) was maintained at a relatively high level (i.e., >1500 µmol m −2 s −1 ) ( Figure 2) from the end of May to the end of August; only during the first measurement (end of spring) PPFD was relatively low, approximately 800 µmol m −2 s −1 .
Data analysis revealed that only the date was a significant predictor for leaf temperature (T l ) (p < 0.001), whereas the shrub life form (deciduous, evergreen) and the interaction between date and shrub life form were not significant (p ≥ 0.05). Leaf temperature (T l ) during the study period almost coincided with the midday ambient air temperature (T m ), ranging from 19.86 • C to 33.04 • C and 19.29 • C to 33.61 • C, respectively ( Figure 2). Predawn air temperature (T p ) ranged from 17.80 • C to 23.21 • C. The differences between T p and T m were statistically significant (p < 0.05) throughout the experimental period, except from the first measurement (mid-May).  Data analysis revealed that only the date was a significant predictor for leaf temperature (Tl) (p < 0.001), whereas the shrub life form (deciduous, evergreen) and the interaction between date and shrub life form were not significant (p ≥ 0.05). Leaf temperature (Tl) during the study period almost coincided with the midday ambient air temperature (Tm), ranging from 19.86 °C to 33.04 °C and 19.29 °C to 33.61 °C , respectively ( Figure 2). Predawn air temperature (Tp) ranged from 17.80 °C to 23.21 °C . The differences between Tp and Tm were statistically significant (p < 0.05) throughout the experimental period, except from the first measurement (mid-May).   Data analysis revealed that only the date was a significant predictor for leaf temperature (Tl) (p < 0.001), whereas the shrub life form (deciduous, evergreen) and the interaction between date and shrub life form were not significant (p ≥ 0.05). Leaf temperature (Tl) during the study period almost coincided with the midday ambient air temperature (Tm), ranging from 19.86 °C to 33.04 °C and 19.29 °C to 33.61 °C , respectively ( Figure 2). Predawn air temperature (Tp) ranged from 17.80 °C to 23.21 °C . The differences between Tp and Tm were statistically significant (p < 0.05) throughout the experimental period, except from the first measurement (mid-May).

Physiological Parameters
The principal component analysis (PCA, Figure 3) of the physiological parameters showed that the eight species are classified into the groups they belong to, deciduous and evergreen, and characterize the first principal component (Axis x), i.e., the species Coronilla emeroides Boiss. and Spruner, Carpinus betulus L., Pistasia terebinthus L. and Cercis siliquastrum L. are deciduous, while the species Arbutus unedo L., Arbutus adrachnae L., Quercus coccifera L. and Quercus ilex L. are evergreens. Additionally, variables such as predawn leaf water potential (Ψ p ), g s and leaf hydraulic conductance (K Leaf ) are negatively correlated with VPD p , VPD m , the Julian date of the measurements and the second principal component (Axis y). Midday leaf water potential (Ψ m ) characterizes both components, while E does not characterize any of the two components. The deciduous species presented higher values (less negative) of Ψ p , Ψ m , g s and K Leaf relatively to evergreen species (Table 1). cus coccifera L. and Quercus ilex L. are evergreens. Additionally, variables such as predawn leaf water potential (Ψp), gs and leaf hydraulic conductance (KLeaf) are negatively correlated with VPDp, VPDm, the Julian date of the measurements and the second principal component (Axis y). Midday leaf water potential (Ψm) characterizes both components, while E does not characterize any of the two components. The deciduous species presented higher values (less negative) of Ψp, Ψm, gs and KLeaf relatively to evergreen species (Table 1).

Leaf Water Potential
On one hand, data analysis revealed that the date was a significant predictor for Ψ p and Ψ m (p = 0.021 and p < 0.001, respectively). On the other hand, the shrub life form (deciduous, evergreen) significantly affected the Ψ m (p < 0.001). The interaction between date and shrub life form was not significant concerning the two variables (p ≥ 0.05) (Figure 4a). The estimated Ψ m values were significantly lower in the considered evergreen shrubs in comparison to the deciduous shrubs during the experimental period (p < 0.001), (Figure 4b). Ψ p ranged in deciduous shrubs from -0. 87  and shrub life form was not significant concerning the two variables (p ≥ 0.05) (Figure 4a). The estimated Ψm values were significantly lower in the considered evergreen shrubs in comparison to the deciduous shrubs during the experimental period (p < 0.001), ( Figure  4b). Ψp ranged in deciduous shrubs from -0.87 MPa to −1.21 Mpa and in the evergreens from −0.92 Mpa to −1.14 Mpa, while Ψm from −1.4 Mpa to −2.59 Mpa and from −1.88 Mpa to −2.86 Mpa, respectively. The average values of the studied parameters are presented in Table 1.

Transpiration Rate, Stomatal and Leaf Hydraulic Conductance
Data analysis revealed that the date was a significant (p < 0.001) predictor for E, gs and KLeaf. The shrub life form (deciduous, evergreen) significantly affected variable KLeaf (p < 0.001). The interaction between date and shrub life form was significant only for gs (p < 0.05). The seasonal pattern of E (at solar noon) of evergreen and deciduous shrubs did not

Transpiration Rate, Stomatal and Leaf Hydraulic Conductance
Data analysis revealed that the date was a significant (p < 0.001) predictor for E, g s and K Leaf . The shrub life form (deciduous, evergreen) significantly affected variable K Leaf (p < 0.001). The interaction between date and shrub life form was significant only for g s (p < 0.05). The seasonal pattern of E (at solar noon) of evergreen and deciduous shrubs did not fluctuate substantially during the experimental period and significant difference was not observed in E between deciduous and evergreen shrubs (p > 0.05). The average values of E were found 16.12 ± 0.89 and 15.87 ± 0.68 mmol m −2 s −1 for deciduous and evergreen shrubs, respectively (Table 1). In particular during the dry season, E ranged from 11.71 to 20.29 mmol m −2 s −1 in the deciduous and from 13.04 to 17.02 mmol m −2 s −1 in the evergreen shrubs ( Figure 5). fluctuate substantially during the experimental period and significant difference wa observed in E between deciduous and evergreen shrubs (p > 0.05). The average valu E were found 16.12 ± 0.89 and 15.87 ± 0.68 mmol m −2 s −1 for deciduous and everg shrubs, respectively (Table 1). In particular during the dry season, E ranged from 11. 20.29 mmol m −2 s −1 in the deciduous and from 13.04 to 17.02 mmol m −2 s −1 in the everg shrubs ( Figure 5).  observed in E between deciduous and evergreen shrubs (p > 0.05). The average values of E were found 16.12 ± 0.89 and 15.87 ± 0.68 mmol m −2 s −1 for deciduous and evergreen shrubs, respectively (Table 1). In particular during the dry season, E ranged from 11.71 to 20.29 mmol m −2 s −1 in the deciduous and from 13.04 to 17.02 mmol m −2 s −1 in the evergreen shrubs ( Figure 5).  The K Leaf was significantly higher in deciduous compared to evergreen shrubs (p < 0.001), (Figure 7), especially during the dry season. The mean K Leaf was significantly higher in deciduous 17.18 ± 1.45 mmol Mpa −1 m −2 s −1 compared to evergreens 13.15 ± 1.57 mmol Mpa −1 m −2 s −1 (p < 0.001), (Figure 7, Table 1). However, the seasonal changes of K Leaf in both groups of plants revealed a decrease in the K Leaf when VPD m increased during the dry period (Figure 7). The KLeaf was significantly higher in deciduous compared to evergreen shrubs 0.001), (Figure 7), especially during the dry season. The mean KLeaf was significantly h in deciduous 17.18 ± 1.45 mmol Mpa −1 m −2 s −1 compared to evergreens 13.15 ± 1.57 m Mpa −1 m −2 s −1 (p < 0.001), (Figure 7, Table 1). However, the seasonal changes of KLeaf in groups of plants revealed a decrease in the KLeaf when VPDm increased during the period (Figure 7). The relationship between KLeaf and Ψm is presented in Figure 8. It is likely that a growing season proceeded KLeaf decreased and exhibited the lowest values concomit with the lowest Ψm values. It is worth mentioning that for the same low values of Ψm deciduous shrubs exhibited higher KLeaf in relations to the evergreens. The relationship between K Leaf and Ψ m is presented in Figure 8. It is likely that as the growing season proceeded K Leaf decreased and exhibited the lowest values concomitantly with the lowest Ψ m values. It is worth mentioning that for the same low values of Ψ m , the deciduous shrubs exhibited higher K Leaf in relations to the evergreens.  In Table 2, the Pearson correlation between physiological parameters and VPDm is presented. In deciduous species a significant positive correlation was detected between VPDm and E and a negative between Ψm and gs, while in evergreen shrubs VPDm was negatively correlated with Ψm and KLeaf. Table 2. Pearson correlation between predawn (Ψp) and midday leaf water potential (Ψm), transpiration rate (E), stomatal conductance (gs), leaf hydraulic conductance (KLeaf) and midday Vapour Pressure Deficit (VPDm) for deciduous (left) and evergreen (right) shrubs. Values are means ± SE (n = 24). Polynomial regression analysis was used to assess the relationship between K Leaf and Ψ m .
In Table 2, the Pearson correlation between physiological parameters and VPD m is presented. In deciduous species a significant positive correlation was detected between VPD m and E and a negative between Ψ m and g s , while in evergreen shrubs VPD m was negatively correlated with Ψ m and K Leaf .

Discussion
The results of this study demonstrate that the water status of co-occurring evergreen and deciduous broadleaved shrubs in semi-arid Mediterranean conditions under rising VPD m is different. Additionally, it seems likely that deciduous shrubs control more efficiently their water status during the dry season, by exhibiting lower stomatal conductance and higher hydraulic conductance than the evergreens. In other words, the deciduous plants possess a more water spending behaviour than the evergreens.
The increase of air temperature accompanied by decreasing RH and increasing heat load due to incident radiation on the leaf, in combination with air speed, may elevate VPD in the atmosphere [60]. It has been reported that RH concerning future climatic scenarios will remain either constant at the global scale [61], or a negative [62] and/or positive trend between VPD and RH at a regional scale [27] will be observed. In our research, the values of PPFD were generally maintained at high levels, as it was expected concerning the characteristics of the Mediterranean climate in the experimental site. The same values in air and leaf temperature imply that broadleaved shrubs possess such leaf tissues, where the transpiration flow is not sufficient to reduce leaf temperature in relation to the ambient temperature. The increase of T m and VPD m under drought conditions (after June) may be linked to the physiological performance and survival of deciduous and evergreen shrubs via either reducing g s and gas exchanges (feedforward mechanism), or increasing plant water loss (feedback mechanism) [63].
The seasonal patterns of Ψ p and Ψ m suggest that the co-occurring deciduous and evergreen shrubs on Olympus Mountain were able to regulate water loss and ensuring adequate hydration of leaf tissues overnight, even during the dry season, therefore Ψ p was retained to approximately −1.0 MPa; such scale of the Ψ p values is sufficient for the Mediterranean region (Quercetalia ilicis zone) where the studied species are growing [64]. Ψ p is an essential index to study the response of plants to drought, because its value can be considered equal to soil water potential, since plants and soil reach a hydration equilibrium during the night [65][66][67][68].
The higher (less negative) Ψ m in deciduous shrubs indicates higher values of relative water content, E and g s in comparison to the evergreen shrubs. The Ψ m seems to be the main factors for stomatal regulation because is directly linked to the turgor of guard cells [49,69,70]. After sunrise, the leaf water potential in Mediterranean plants decreases steadily, reaching its lower value at noon, while it begins to recover again during the afternoon [71][72][73]. This reflects a decline of stored water, and therefore water shortage [60]. The plants are the best indicators of soil water availability, which affect their water status [74,75]. Plants with both elevated (less negative) Ψ m and water transport capacity can better control their leaf water status during midday, and hence they experience a smaller decline of midday g s [49].
It is likely that the relatively higher E in deciduous species during midday is due to their elevated water status, i.e., higher Ψ m in leaves (Figures 4a and 5); a slight variation of E values was detected between deciduous and evergreen shrubs, while the lowest values of E occurred in the two groups of shrubs during the dry season (June-August), ( Figure 5).
Apparently, low E values during the dry season remained rather stable and above zero values, which indicates influx of carbon dioxide [49,76].
The deciduous shrubs exhibited higher Ψ m and lower midday g s than the evergreens; this may indicate that midday stomatal conductance is more related to stem rather than leaf water status, which is in accordance with earlier results [49]. In fact, g s was negatively correlated with VPD m ( Table 2), suggesting a response to environmental conditions, which is in agreement with the published results by Auge et al. [77] from other deciduous species. This trait may help deciduous species to regulate their stomatal apparatus in order to maintained Ψ m in a range of values that will support their water status and avoid xylem embolism under drought conditions [49,78]. Our results show that there was not any synchronization between g s and Ψ m in both shrub groups, which may also be due to the fact the evergreens are hypostomatic plants [79][80][81]. However, it is not clear which microclimate parameter, i.e., temperature and/or relative humidity, was directly linked to stomatal behaviour. It has been proposed that temperature had a greater impact on stomatal conductance and assimilation rate in the amphistomatic leaves of Eucalyptus tetrodonta independently of VPD [82,83]. Addington et al. [35] reported that the stomatal apparatus controls Ψ in a way that the tension on the water column created by decreasing Ψ did not cause extreme xylem cavitation. Several studies suggested that high VPD reduces g s , consequently affecting assimilation rate and growth [84,85]. On the contrary, it has been argued that the impact of high VPD on g s may not possess any impact on assimilation rate and growth [86]. Nevertheless, the impact of VPD on g s and/or assimilation rate varies amongst plant species [82]. In addition, the stomatal anatomy and structure affect water loss and carbon assimilation, demonstrating the evolution and adaptations of the plants to environmental conditions [84,87].
Plants in order to grow and survive under water limited conditions evolved mechanisms to control stomatal aperture and xylem water capacity [88]. The differences in wateruse strategies might be partially due to various hydraulic properties between the considered groups. The hydrodynamic status of leaf tissue expressed via leaf water potential [89] is determined by two mail functions taking place in the SPA continuum, i.e., transpiration rate and hydraulic conductance. Therefore, the favourable water status in deciduous shrubs could be attributed to the higher values of transpiration rate and/or at the highest values of the hydraulic conductance. The deciduous shrubs exhibited higher values of K Leaf when compared to evergreen shrubs, especially during the dry summer period. This advantage of deciduous shrubs could be attributed to their water status and anatomical features. The positive correlation between K Leaf and Ψ m , and E in deciduous shrubs (Table 2) and the negative correlation between K Leaf and VPD m suggest that the water transport from root to the leaf plays a role to the fluctuations of leaf water status. The seasonal K Leaf declined in both groups of plants when VPD increased during the summer dry period (Figure 7) especially at the end of July, when the highest value of VPD (3.99 KPa) was recorded. Probably, at a given transpiration rate in deciduous shrubs the leaf water status is maintained due to high hydraulic conductance [17,90,91]. Manzoni et al. [88] reported that, in some ecosystems, deciduous species have been found to be more hydraulically efficient than evergreen species. Choat et al. [92] suggested that deciduous species are more hydraulically efficient, but also more vulnerable to drought-induced embolism, than co-existing evergreens in a rainforest. It is well known that embolism occurs in plants under drought conditions when Ψ reaches very low values; however, the plants have the capacity to repair damage when the environmental conditions are favourable [89].
Our data are somehow in agreement with BIumler [93], who argued that although the Mediterranean climate is associated with evergreen species, some coexisting deciduous species exhibit some advantages in response to drought [92,94]. It is noteworthy that in our work deciduous and evergreen species are presented as two distinct life-history groups [91], although they probably form a continuum of variation in leaf life-history span [95].

Study Area and Climate
The study was conducted on Olympus Mountain (40 • 06 54 N, 22 • 28 42 E), which is a great, long-lived natural laboratory [96][97][98] in 2009, at an altitude of 554 m a.s.l., in an area located 5 Km from the town of Litochoro, 95 km south-east of Thessaloniki, in Greece. The climate of the study area is characterized as Cfa in the Köppen-Geiger system (http://www.en.climate-data.org, 7 January 2022) and as Mediterranean with cold and wet winters, dry and warm summer according to the bioclimatogram of Emberger. The mean annual rainfall in the study area ranges from 800 to 1000 mm, while substantially elevated precipitation is recorded during winter. The minimum air temperature ranges from 13 to 6 • C (during summer and winter, respectively). The maximum air temperature ranges from 4.9 to 6.7 • C during winter, and from 20 to 26 • C during summer. The warmest month is July and the coldest is December. The mean annual relative humidity (RH) ranges from 75 to 80%. Average RH values during the most humid month (December) is 85-90% and during the driest month (July) 30-50%. The monthly changes of temperature and precipitation (ombrothermic diagram) during the year of the measurements, from the nearest meteorological station of Dion (40 • 12 00 N, 22 • 30 00 E), are presented in Figure 9. The microclimatic conditions (temperature, humidity), in the study area during the experimental period was measured with the Hobo H8 Pro (Hobo H8 Pro Series 1997-2003, Onset Computer Coorporation, Bourne, MA, USA). Furthermore, on specific Julian dates and hours when plant physiological parameters were investigated, ambient air predawn and midday temperature and RH were measured using a Novasima MS1 microclimatic sensor (Novatron Scientific Ltd., Horsham, UK). Predawn (VPDp) and midday (VPDm) vapour pressure deficit were calculated according to Abtew and Melesse (2013). The Photosynthetic Photon Flux Density was measured with the steady-state diffusion porometer LI-1600 (LI-COR, Inc., Lincoln, NE, USA). The presented values of VPDp, VPDm, RH, PPFD, Tp and Tm are means of twenty-four measurements.

Plant Material
The study area is part of the Mediterranean zone of the evergreen broadleaved plants Quercus ilex L. and Arbutus andrachne L. Deciduous and evergreen shrubs and small trees such as Acer monspesulanum L., Carpinus betulus L., Cercis siliquastrum L., Cotinus coggygria Scop., Fraxinus ornus L., Pistacia terebinthus L., Coronilla emeroides Boiss. and Spruner, Quercus coccifera L., Arbutus unedo L., Phillyrea media L., and Juniperus oxycedrus L. are widespread in the study area.
Four evergreen shrubs Arbutus andrachne (commonly known as Grecian strawberry tree), Arbutus unedo (strawberry tree), Quercus ilex (holm oak) and Quercus coccifera (ker- The microclimatic conditions (temperature, humidity), in the study area during the experimental period was measured with the Hobo H8 Pro (Hobo H8 Pro Series 1997-2003, Onset Computer Coorporation, Bourne, MA, USA). Furthermore, on specific Julian dates and hours when plant physiological parameters were investigated, ambient air predawn and midday temperature and RH were measured using a Novasima MS1 microclimatic sensor (Novatron Scientific Ltd., Horsham, UK). Predawn (VPD p ) and midday (VPD m ) vapour pressure deficit were calculated according to Abtew and Melesse (2013). The Photosynthetic Photon Flux Density was measured with the steady-state diffusion porometer LI-1600 (LI-COR, Inc., Lincoln, NE, USA). The presented values of VPD p , VPD m , RH, PPFD, T p and T m are means of twenty-four measurements.

Plant Material
The study area is part of the Mediterranean zone of the evergreen broadleaved plants Quercus ilex L. and Arbutus andrachne L. Deciduous and evergreen shrubs and small trees such as Acer monspesulanum L., Carpinus betulus L., Cercis siliquastrum L., Cotinus coggygria Scop., Fraxinus ornus L., Pistacia terebinthus L., Coronilla emeroides Boiss. and Spruner, Quercus coccifera L., Arbutus unedo L., Phillyrea media L., and Juniperus oxycedrus L. are widespread in the study area.
Four evergreen shrubs Arbutus andrachne (commonly known as Grecian strawberry tree), Arbutus unedo (strawberry tree), Quercus ilex (holm oak) and Quercus coccifera (kermes oak) and four deciduous shrubs Carpinus betulus (common hornbeam), Cercis siliquastrum (Juda's tree), Coronilla emeroides (scorpion senna), and Pistacia terebinthus (terebinth) were selected for this study. The shrubs were randomly selected for sampling. More specifically, the area of interest was equal to 20 ha. We divided this area into 20 tiles, 1 ha each, and then we randomly selected 6 tiles where each one of the eight species was randomly chosen. Concerning dendrometric parameters, the considered species possessed the same height (3-4 m) and diameter (1.5-2.0 m).

Physiological Parameters
Leaf water potential (Ψ), transpiration rate (E), stomatal conductance (g s ) and leaf temperature (T l ) were measured on the newest fully developed mature leaves of six different individuals from sun-exposed terminal branches. Using the pressure-bomb technique (PMS, Albany, OR, USA), leaf water potential was measured twice during the day, i.e., at predawn (Ψ p ) and midday (Ψ m ). Transpiration rate, g s and T l were measured using steady state porometer (Li1600, LI-COR Lincoln, NE, USA). Seasonal measurements of Ψ p were obtained before sunrise, while the measurements of Ψ m , E, and g s were obtained on clear sunny days at around solar noon (12:00-14:30 h), approximately 10-15 days intervals. The presented values, for each of the parameters, are means of six replications per studied shrub.
Leaf hydraulic conductance (K Leaf ) was calculating according to Ohm's law following the formula: It has been assumed that Ψ soil is in equilibrium with Ψ p and the lowest diurnal Ψ leaf is equal to Ψ m [66,67,99]. However, sometimes the first assumption may lead to overestimation of K plant [100]. Additionally, values of K Leaf reflect the capacity of evergreen and deciduous plants, grown under ambient conditions, for water exploitation during a period of soil drying.

Statistical Analysis
The Kolmogorov-Smirnov test was employed to evaluate data normality. To explore whether the ecophysiological response of deciduous and evergreen shrubs vary during the dry season, a two-way analysis of variance (ANOVA) was performed on the studied parameters (T L , Ψ p , Ψ m , E, g s , K leaf ) [81]. Student's t-test for independent samples was used to examine differences in T L , E, g s , Ψ m , Ψ p and K Leaf between deciduous and evergreen shrubs. The climatic parameters VPD p , VPD m, T p , and T m at each sampling date were compared using also the Student's t-test for independent samples. Polynomial regression analysis was used to determine the relationship between K Leaf and Ψ m , between deciduous and evergreen shrubs. In addition, a Principal Component Analysis (PCA) with varimax rotation was used to assess the relationships among the measurements of interest between the eight shrubs to see whether they could be classified according to their ecophysiological response in the life form in which belong (deciduous, evergreen). Pearson correlation was used to explore links among VPD m , E, g s , Ψ p, Ψ m and K Leaf . P-values lower than 0.05 were considered statistically significant. All statistical analyses were performed using the SPSS statistical package v. 27.0 (IBM Corp. in Armonk, NY, USA).

Conclusions
The results of this research work demonstrate different hydraulic strategies between cooccurring deciduous and evergreen shrubs grown on Olympus Mountain. The deciduous life-form presented a strategy of higher hydraulic and lower stomatal conductance, while the evergreen exhibited lower hydraulic and higher stomatal conductance. Although,