Influence of species interactions on transpiration of Mediterranean tree species during a summer drought (original) (raw)
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Recent research has shown that interactions between species with different functional traits can promote forest ecosystem processes. In the context of climate change, understanding whether species interactions in mixed-species ecosystems can improve the adaptation of these ecosystems to extreme climatic events is crucial to developing new management strategies. In this study, we investigated the impact of species interactions on the sap flux density of three Mediterranean tree species (Quercus faginea, Pinus nigra and Pinus sylvestris) during a summer drought. Measurements of foliar carbon isotopic composition (d 13 C) were also conducted on the same trees. The decline in transpiration during drought was the greatest for P. sylvestris and the least pronounced for Q. faginea. For P. nigra and Q. faginea, the decrease in transpiration as the drought progressed was lower when these species where interacting with another tree species, particularly with P. sylvestris. In contrast, the decrease for P. sylvestris was higher when this species was interacting with another species. Differing drought effects were consistent with the d 13 C values. We showed that the identity of the species present in the direct neighbourhood of a given tree can differentially influence water availability and wateruse of these three co-existing Mediterranean tree species during a summer drought. Our findings suggest that species interactions play an important role in modulating the response of tree species to drought. Favouring tree species diversity in this region does not seem to be systematically beneficial in terms of soil water availability and water-use for all the interacting species.
Increasingly severe droughts are expected to negatively impact forest functioning in the future, especially in the Mediterranean region. Favoring mixed species stands has been advocated as a compromise between wood production and biodiversity conservation, but whether such management practices would allow forest ecosystems to acclimate to future climate conditions remains to be addressed. We tested whether the transpiration of Quercus cerris (Qc) and Quercus petraea (Qp) during droughts differ when they grow in pure or mixed forests. We measured sap flux density (F D ) and leaf carbon isotope composition (d 13 C), as a proxy for intrinsic water use efficiency (WUE int ), in pure and mixed Qc and/or Qp forest plots in Italy during the summer 2012. Qc and Qp showed a drop in F D of 41% and 52% respectively during the summer drought independently of the mixtures, but with different seasonal trends. Qp transpiration response to soil drought did not differ between pure and mixed plots. In contrast, Qc transpiration was reduced by 56% in mixed plots at the maximum of the drought and only by 31% in the pure plot. Furthermore, we observed higher WUE int for Qc in the mixed plot and no change for Qp, supporting the higher water stress intensity in the mixed plot for Qc. Our study illustrated the negative impact of Qp on Qc when these species compete for water resources. We suggest that managing for mixed stands in the Mediterranean region might, in some cases, increase the detrimental effect of drought on species functioning.
Oecologia, 2013
increased or maintained their growth at rising iWUE, suggesting a higher CO 2 uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C a even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions.
Tree Physiology, 2003
We studied the seasonal patterns of water use in three woody species co-occurring in a holm oak forest in northeastern Spain. The three species studied, Quercus ilex L., Phillyrea latifolia L. and Arbutus unedo L., constitute more than 99% of the total basal area of the forest. The study period included the dry seasons of 1999 and 2000. Water use was estimated with Granier-type sap flux sensors. Standard meteorological variables, soil water content and leaf water potentials were also monitored. All monitored individuals reduced leafrelated sap flow (Q l ) during the summer, concurrent with an increase in soil moisture deficit (SMD). Despite similar maximum Q l between species, the decline in Q l with increasing SMD was species-dependent. The average reduction in Q l between early summer and the peak of the drought was 74% for A. unedo (n = 3), 58% for P. latifolia (n = 3) and 87% for Q. ilex (n = 1). The relationship between canopy stomatal conductance (G s ) and vapor pressure deficit (D) changed during the course of the drought, with progressively lower G s for any given D. Summertime reductions of Q l and G s were associated with between-species differences in vulnerability to xylem embolism, and with the corresponding degree of native embolism (lowest in P. latifolia and highest in Q. ilex). Our results, combined with previous studies in the same area, outlined differences among the species studied in manner of responding to water shortage, with P. latifolia able to maintain water transport at much lower water potentials than the other two species. In an accompanying experiment, A. unedo responded to an experimental reduction in water availability by reducing Q l during the summer. This species also modified its water use between years according to the different seasonal patterns of precipitation. These results are discussed in relation to the possible impacts that climate change will have on Q. ilex-dominated forests.
Tree Physiology, 2019
In anticipation of a drier climate and to project future changes in forest dynamics, it is imperative to understand species-specific differences in drought resistance. The objectives of this study were to form a comprehensive understanding of the drought resistance strategies adopted by Eastern Mediterranean woodland species, and to elaborate specific ecophysiological traits that can explain the observed variation in survival among these species. We examined leaf water potential (𝛹), gas exchange and stem hydraulics during 2–3 years in mature individuals of the key woody species Phillyrea latifolia L., Pistacia lentiscus L. and Quercus calliprinos Webb that co-exist in a dry woodland experiencing ~ 6 rainless summer months. As compared with the other two similarly functioning species, Phillyrea displayed considerably lower 𝛹 (minimum 𝛹 of −8.7 MPa in Phillyrea vs −4.2 MPa in Pistacia and Quercus), lower 𝛹 at stomatal closure and lower leaf turgor loss point (𝛹TLP ), but reduced hydr...
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.
2012
"We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (d13CLW and d18OLW, respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, d13CLW and d18OLW were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher d18OLW and similar d13CLW values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands."
Establishment of co-existing Mediterranean tree species under a varying soil moisture regime
Journal of Vegetation Science, 2004
We investigated the responses of two co-existing Mediterranean trees with different regeneration strategies (Phillyrea latifolia seedlings and Quercus ilex sprouts) to experimental drought below the forest canopy. We considered different recruitment stages and used leaf isotopic discrimination to estimate water use efficiency (WUE) and nitrogen availability and use. Drought decreased the emergence and survival of seedlings and sprouts. Survival and growth of older saplings were not influenced by drought. Seedling emergence of P. latifolia was higher than Q. ilex sprout production, but Q. ilex sprouts had higher survival and growth rates. These differences disappeared in the sapling stage. Carbon isotopic discrimination suggested that Q. ilex sprouts had higher WUE than P. latifolia seedlings. Drought increased WUE of recruits, particularly in Q. ilex. Water use regulation increased with ageing, particularly in P. latifolia. Q. ilex had higher d 15 N values than P. latifolia; these were also higher under drier soil conditions. Current year seedlings had higher d 15 N than saplings, particularly in P. latifolia, suggesting they exploit superficial soil layers. These results suggest that sprouts obtain benefit from resources stored in parent plants. At earlier stages, they perform better than seedlings. This response is not coupled to adult vulnerability to drought for these species, revealing the difficulty of predicting species dynamics during climate change.
As episodes of drought-induced forest mortality are being increasingly reported worldwide and may become more frequent in the future as a result of climate change, it is essential to characterize their functional implications in terms of ecosystem carbon and water fluxes. We investigated the spatial variability of soil respiration in a mixed Mediterranean forest located on rugged terrain, where Scots pine (Pinus sylvestris) is affected by drought-induced dieback and appears to have been replaced by Holm oak (Quercus ilex) as the dominant tree species. Soil respiration was measured in spring 2010 on two plots (16.2 Â 16.2 m) using a static closed chamber method (soda lime technique) and a systematic sampling (1.8-m grid) including 100 points per plot. Biotic and abiotic variables, such as soil moisture, soil temperature, soil organic matter content, stoniness, pH, fine root C:N ratio and biomass, tree basal area and tree species and health condition of nearest neighbouring tree were also recorded. Our results showed that the spatial variability of soil respiration under optimal environmental conditions (spring) was high and showed no spatial autocorrelation on the scale studied (1-18 m). A mixed-effects model applied to explain the spatial variability of soil respiration indicated that only the variables related to forest structure (i.e., health condition and basal area) explained any of the observed variability of soil respiration (R 2 = 0.45). Our model revealed that soil respiration was highest in soils close to dead pines and under Holm oak trees, suggesting that tree mortality and species replacement of pine trees by Holm oak may lead to higher soil respiration fluxes. The direct effect of tree mortality on soil respiration may be a transitory response caused by fine root mortality. Furthermore, the fact that tree species replacement as a result of drought-induced die-off is accompanied by concomitant changes in soil respiration has important implications for soil and ecosystem carbon balance.
Oecologia, 2009
The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO 2 have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; d 13 C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO 2 concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in d 13 C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high-(H) and a lowelevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive microenvironment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in lowelevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming.