The role of desiccation tolerance in determining tree species distributions along the Malay–Thai Peninsula

Abstract

Summary 1.Patterns of water availability are frequently implicated in local and regional tree species distributions. A major floristic and climatic transition from aseasonal to seasonal evergreen tropical forest is the Kangar-Pattani Line (KPL) in the Indo-Sundaic region of Southeast Asia. We hypothesize that differences in species' drought tolerance will correspond with their distribution with respect to the KPL. Using a common garden study, we assess the role of differences in physiological drought tolerance traits to geographic distributions for 24 tropical tree species in relation to rainfall seasonality. 2. Inherent differences in desiccation tolerance of seedlings were quantified as water potentials and relative water contents (RWCs) below which the plant could no longer support living tissue, and plant water relation parameters were measured using pressure-volume analysis. The relationships among these traits were examined using bivariate trait relationships and a principal components analysis (PCA). The physiological traits contributing most to lethal water potential and RWC were assessed using multiple regression analysis. 3. Distribution-related differences in all desiccation tolerance and pressure-volume traits were detected both with and without phylogenetic correction. Widespread species that occurred in seasonally dry forests were able to maintain living tissue at more negative water potentials and lower RWCs than were species restricted to aseasonal forests. Likewise, widespread species demonstrated more negative water potentials at turgor loss, more negative saturated osmotic potential, lower symplastic water fraction and higher bulk modulus of elasticity values. Turgor loss point (TLP) and bulk modulus of elasticity were the best predictors of lethal water potential while symplastic water fraction and bulk modulus of elasticity were included as predictors of lethal RWC. 4. Inherent differences in physiological traits contributing to drought tolerance are associated with differences in tropical tree species distributions in relation to rainfall seasonality. These results, combined with lack of support for hypothesized historical factors, strongly implicate climate as a determinant of tree species distributions around the KPL.

The correlations and sequence of plant stomatal, hydraulic, and wilting responses to drought

“…These values correlate with π tlp (11), as shown here, and with leaf and stem hydraulic traits across small species sets (n ≤ 5) (19,37,41), and it is increasingly critical for further studies to determine how these traits interact to influence plant mortality during drought. (11). The sequence is determined from pairwise comparisons between all of the traits (SI Appendix, Table S7), but, for clarity, B shows the mean of each trait from its pairwise comparison with the trait immediately after (i.e., more negative than) it in the sequence.…”

“…Indeed, measurements of plant Ψ lethal are sparse in the literature, and most studies use different definitions for plant death (11,41). These values correlate with π tlp (11), as shown here, and with leaf and stem hydraulic traits across small species sets (n ≤ 5) (19,37,41), and it is increasingly critical for further studies to determine how these traits interact to influence plant mortality during drought. (11).…”

“…Capacitance, embolism recovery, and metabolic synthesis of abscisic acid and nonstructural carbon reserves have all been predicted to influence drought survival, but the roles of these traits and their interactions with the classical drought tolerance traits, or their influence on plant Ψ lethal , are not well understood (15,23). Indeed, measurements of plant Ψ lethal are sparse in the literature, and most studies use different definitions for plant death (11,41). These values correlate with π tlp (11), as shown here, and with leaf and stem hydraulic traits across small species sets (n ≤ 5) (19,37,41), and it is increasingly critical for further studies to determine how these traits interact to influence plant mortality during drought.…”

“…Previous studies have shown that these water potential thresholds are intercorrelated for small species sets (8)(9)(10)(11)(12). We tested these correlations for a large dataset to produce a framework for extrapolating plant responses to a wide range of water stress from one or two traits.…”

Biogeography and conservation in Southeast Asia: how 2.7 million years of repeated environmental fluctuations affect today’s patterns and the future of the remaining refugial-phase biodiversity

“…Reports of such adaptive variation and of shifts in species ranges and phenology illustrate the ability of some species to respond individualistically to significant climate change (Parmesan 2006). The following recent regional examples are informative: (1) Baltzer et al (2007Baltzer et al ( , 2008 describe current determinants of tree species distributions and the evolution of drought tolerance in trees north and south of the Kangar-Pattani Line; (2) Sheridan (2009) found three frog species that occur in both ever-wet Singapore and seasonal Thailand have adapted to the different environments with changes in clutch size, body size, and the timing of oviposition; (3) Round and Gale (2008) found that the lowland Siamese fireback pheasant Lophura diardi, has increased in abundance at higher elevations over 25 years in central Thailand; (4) Peh (2007) found evidence that other bird species have also extended their upper limits along elevation gradients; (5) Chen et al (2009) found that the average altitudes of individuals of 102 montane geometrid moth species on Mount Kinabalu in Borneo increased by 67 m between 1965 and 2007; (6) Corlett (2009b) discussed the innate dispersal abilities of trees and other plants and concluded that although altitudinal shifts are feasible as they involve short distances (a 3°C increase in mean annual temperature is equivalent to an elevational shift of *500 m), the required latitudinal range shifts, which may require dispersal of [500 km, and are unlikely to occur naturally in the time available; and (7) Bickford et al (2010) also discuss herpetological examples but argue that many regional amphibians and some reptiles will soon reach the physiological limits of their adaptability. Wright et al (2009) make the same point but more generally: tropical species are likely to be particularly sensitive to global warming because they are adapted to limited geographic and seasonal variation in temperature, already live at or near the highest temperatures on Earth before global warming began, and are often isolated from potential cool refugia.…”