Differential drought resistance strategies of co-existing woodland species enduring the long rainless Eastern Mediterranean summer (original) (raw)
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bioRxiv (Cold Spring Harbor Laboratory), 2022
Woody species employ various strategies to cope with drought stress. We investigated similarities and differences in response to chronic drought to understand resistance strategies in co-occurring Mediterranean species. We studied five predominant Mediterranean species; Quercus calliprinos, Pistacia palaestina, Pistacia lentiscus, Rhamnus lycioides, and Phillyrea latifolia over two summers at three sites with different aridities. We measured key hydraulic and osmotic traits related to drought resistance, including resistance to embolism (Ψ50), carbon isotope signature (δ 13 C), pre-dawn (ΨPD) and midday (ΨMD) water potentials, and native (Ψs) and full turgor (П0) osmotic potentials. Significant differences among species appeared in resistance to embolism. The species also showed differences in the water potential plastic response over the dry season. This interspecific variation increased at the end of the dry season and resulted in very narrow hydraulic safety margins (HSM). Consequently, predicted loss of hydraulic conductivity revealed species with significant native embolism. Two of the species also had seasonal changes in osmotic adjustment. Our detailed analysis indicates that co-existing Mediterranean woody species combine various drought resistance strategies to minimize mortality risk. However, all of them risk mortality as they approach their hydraulic limit near the dry margin of their distribution.
Water use strategies of six co-existing Mediterranean woody species during a summer drought
Drought stress is known to limit plant performance in Mediterranean-type ecosystems. We have investigated the dynamics of the hydraulics, gas exchange and morphology of six co-existing Mediterranean woody species growing under natural field conditions during a drought that continued during the entire summer. Based on the observed minimum leaf water potentials, our results suggest that the six co-existing species cover a range of plant hydraulic strategies, from isohydric to anisohydric. These differences are remarkable since the selected individuals grow within several meters of each other, sharing the same environment. Surprisingly, whatever the leaf Communicated by Hermann Heilmeier.
Mediterranean trees coping with severe drought: Avoidance might not be safe
Environmental and Experimental Botany, 2018
Plants coexisting in Mediterranean regions differ widely in their sensitivity to drought and in their ability to maintain carbon assimilation and hydraulic function ranging from avoidance to tolerance of stressful periods. We studied the response of three coexisting Mediterranean tree species (Quercus ilex L. spp. Ballota (Desf.) Samp., Quercus faginea Lam. and Pinus nigra ssp. Salzmannii J.F. Arnold) under natural conditions for three years to test their functional strategies in dealing with extremely dry years. Despite P. nigra following a droughtavoiding strategy, it was unable to effectively avoid the hampering effects of a severe drought in the driest year, and experienced significant drops in the efficiency of the hydraulic system, in gas exchange at the leaf level and in tree water use estimated from daily sap flow. In contrast, the two Quercus species showed a more droughttolerant strategy by maintaining a more profligate water use at the leaf and tree level, with Q. ilex having a larger tolerance threshold to severe drought. The main decrease in carbon assimilation was due to stomatal closure, with a minor effect of photochemistry damage or down regulation in the three species. In the case of the two oaks, seasonal osmoregulation was inferred from changes in the osmotic potential at full turgor (π o), bringing about the lowest leaf osmotic potentials at turgor lost (π tlp) at late summer in both species, and maintenance of leaf turgor under the lowest midday leaf water potential (Ψ mid). Seasonal changes in osmotic potentials were observed in the three years, more during the driest year. Plasticity in modulating the tolerance to leaf dehydration as the season advances, especially in response to the intensity of inter-annual summer droughts was higher for the evergreen Q. ilex than in the semi-deciduous Q. faginea. In spite of the intensity of the drought and the associated stress experienced by the three forest tree species, especially by P. nigra, mortality or severe defoliation were not observed after the extreme dry year. This reveals a large operational range of coping with water stress by the three species regardless of the hydric strategy, which is crucial for survival under the increasing drought expected in the most likely climate change scenarios for the region.
Journal of Experimental Botany, 2011
Plants may exhibit some degree of acclimation after experiencing drought, but physiological adjustments to consecutive cycles of drought and re-watering (recovery) have scarcely been studied. The Mediterranean evergreen holm oak (Q. ilex) and the semi-deciduous rockrose (C. albidus) showed some degree of acclimation after the first of three drought cycles (S1, S2, and S3). For instance, during S2 and S3 both species retained higher relative leaf water contents than during S1, despite reaching similar leaf water potentials. However, both species showed remarkable differences in their photosynthetic acclimation to repeated drought cycles. Both species decreased photosynthesis to a similar extent during the three cycles (20-40% of control values). However, after S1 and S2, photosynthesis recovered only to 80% of control values in holm oak, due to persistently low stomatal (g s) and mesophyll (g m) conductances to CO 2. Moreover, leaf intrinsic water use efficiency (WUE) was kept almost constant in this species during the entire experiment. By contrast, photosynthesis of rockrose recovered almost completely after each drought cycle (90-100% of control values), while the WUE was largely and permanently increased (by 50-150%, depending on the day) after S1. This was due to a regulation which consisted in keeping g s low (recovering to 50-60% of control values after re-watering) while maintaining a high g m (even exceeding control values during rewatering). While the mechanisms to achieve such particular regulation of water and CO 2 diffusion in leaves are unknown, it clearly represents a unique acclimation feature of this species after a drought cycle, which allows it a much better performance during successive drought events. Thus, differences in the photosynthetic acclimation to repeated drought cycles can have important consequences on the relative fitness of different Mediterranean species or growth forms within the frame of climate change scenarios.
Functional traits predict drought performance and distribution of Mediterranean woody species
Water availability is one of the key environmental factors that affect plant establishment and distribution. In many regions water availability will decline with climate change, exposing small seedlings to a greater likelihood of drought. In this study, 17 leaves, stem, root, and whole-plant traits of ten woody Mediterranean species were measured under favourable growing conditions and seedling drought survival was evaluated during a simulated dry-down episode. The aims of this study were: i) to assess drought survival of different species, ii) to analyse which functional traits predict drought survival time, and iii) to explain species distribution in the field, based on species drought survival and drought strategies. Drought survival time varied ten-fold across species, from 19 to 192 days. Across species, drought survival was positively related to the rooting depth per leaf area, i.e., the ability to acquire water from deeper soil layers while reducing transpiring leaf area. Drought survival time was negatively related to species ability to grow quickly, as indicated by high relative growth and net assimilation rates. Drought survival also explained species distribution in the field. It was found that species were sorted along a continuum, ranging between two contrasting species functional extremes based on functional traits and drought performance. One extreme consisted of acquisitive fast-growing deciduous species, with thin, soft metabolically active leaves, with high resource use and vulnerability to drought. The opposite extreme consisted of conservative slow-growing evergreen species with sclerophyllous leaves, deep roots, a low transpiring area, and low water use, resulting in high drought survival and drought tolerance. The results show that these drought strategies shape species distribution in this Mediterranean area.
Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought
Tree Physiology, 2007
In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day -1 during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.
Phenological responses to extreme droughts in a Mediterranean forest
Global Change Biology, 2011
Mediterranean regions are projected to experience more frequent, prolonged and severe drought as a consequence of climate change. We used a retractable rainfall shelter, to investigate the impact of extreme droughts on the development of Quercus ilex leaves, flowers and fruit. In 2008, 97% of rainfall was excluded from a forest plot during the autumn, representing 50% of the 1127 mm of rain that fell during the year. In 2009, 87% of rainfall was excluded during the spring, representing 58% of the 749 mm that fell during the year. The rainfall shelter did not impact neither incident radiation nor air temperature. Autumn rainfall exclusion did not significantly affect leaf, flowers or fruit development. Spring rainfall exclusion resulted in larger and more sustained depression of leaf water potential during the key phases of foliar and floral development. Consequently, only half of the sampled trees (6) reached the shoot lengthening stage which leads to functionally mature leaves (phenophase 4), with one abandoning leaf development at budburst (phenophase 3) and the other two at the bud swelling stage (phenophase 2). All trees of the control plot passed phenophase 4, with most reaching complete leaf development. The impact of extreme droughts on flower development differed between the sexes. The spring exclusion had no effect on male flower, but only one of six trees completed female fruit maturation, compared with four in the control plot. The difference between the male and female drought impacts is likely attributable to the occurrence of male floral development before the period of lowest leaf water potential, and to the lower resource allocation requirements of male flowers. The information provided by our experimental approach may constitute a crucial step to evaluate the impact of increasing drought due to climate change on the most dominant Mediterranean tree species and to help drawing a full picture of the ecological consequences of the decline in water resource on forest dynamics under changing conditions.
Tree Physiology, 2003
We studied stomatal responses to decreasing predawn water potential (Ψ pd) and increasing leaf-to-air water vapor pressure difference (VPD) of co-occurring woody Mediterranean species with contrasting leaf habits and growth form. The species included two evergreen oaks (Quercus ilex subsp. ballota (Desf.) Samp. and Q. suber L.), two deciduous oaks (Q. faginea Lam. and Q. pyrenaica Willd.) and two deciduous shrubs (Pyrus bourgaeana Decne. and Crataegus monogyna Jacq.). Our main objective was to determine if stomatal sensitivity is related to differences in leaf life span and leaf habit. The deciduous shrubs had the least conservative water-use characteristics, with relatively high stomatal conductance and low stomatal sensitivity to soil and atmospheric drought. As a result, Ψ pd decreased greatly in both species during the growing season, resulting in early leaf abscission in the summer. The deciduous oaks showed intermediate water-use characteristics, having maximum stomatal conductances and CO 2 assimilation rates similar to or even higher than those of the deciduous shrubs. However, they had greater stomatal sensitivity to soil drying and showed less negative Ψ pd values than the deciduous shrubs. The evergreen oaks, and especially the species with the greatest leaf longevity, Q. ilex, exhibited the most conservative water-use behavior, having lower maximum stomatal conductances and greater sensitivity to VPD than the deciduous species. As a result, Ψ pd decreased less during the growing season in the evergreens than in the deciduous species, which may contribute to greater leaf longevity by avoiding irreversible damage during the summer drought. However, the combination of low maximum CO 2 assimilation rates and high stomatal sensitivity to drought must have a negative impact on the final carbon budget of leaves with a long life span.
Global change biology, 2013
Like many midlatitude ecosystems, Mediterranean forests will suffer longer and more intense droughts with the ongoing climate change. The responses to drought in long-lived trees differ depending on the time scale considered, and short-term responses are currently better understood than longer term acclimation. We assessed the temporal changes in trees facing a chronic reduction in water availability by comparing leaf-scale physiological traits, branchscale hydraulic traits, and stand-scale biomass partitioning in the evergreen Quercus ilex across a regional precipitation gradient (long-term changes) and in a partial throughfall exclusion experiment (TEE, medium term changes). At the leaf scale, gas exchange, mass per unit area and nitrogen concentration showed homeostatic responses to drought as they did not change among the sites of the precipitation gradient or in the experimental treatments of the TEE. A similar homeostatic response was observed for the xylem vulnerability to cavitation at the branch scale. In contrast, the ratio of leaf area over sapwood area (LA/SA) in young branches exhibited a transient response to drought because it decreased in response to the TEE the first 4 years of treatment, but did not change among the sites of the gradient. At the stand scale, leaf area index (LAI) decreased, and the ratios of stem SA to LAI and of fine root area to LAI both increased in trees subjected to throughfall exclusion and from the wettest to the driest site of the gradient. Taken together, these results suggest that acclimation to chronic drought in long-lived Q. ilex is mediated by changes in hydraulic allometry that shift progressively from low (branch) to high (stand) organizational levels, and act to maintain the leaf water potential within the range of xylem hydraulic function and leaf photosynthetic assimilation.
Tree Physiology, 2020
Nowadays, evergreen sclerophyllous and winter-deciduous malacophyllous oaks with different paleogeographical origins coexist under Mediterranean-type climates, such as the mixed forests of the evergreen Quercus ilex subsp. rotundifolia Lam. and the winter-deciduous Quercus faginea Lam. Both Mediterranean oaks constitute two examples of contrasting leaf habit, so it could be expected that they would have different functional strategies to cope with summer drought. In this study, we analysed photosynthetic, photochemical and hydraulic traits of different organs for Q. faginea and Q. ilex subsp. rotundifolia under well-watered conditions and subjected to very severe drought. The coordinated response between photosynthetic and hydraulic traits explained the higher photosynthetic capacity of Q. faginea under well-watered conditions, which compensated its shorter leaf life span at the expense of higher water consumption. The progressive imposition of water stress evidenced that both types...