Differentiation in the water-use strategies among oak species from central Mexico

Abstract

Oak species (Fagaceae: Quercus) differ in their distribution at the landscape scale, specializing to a certain portion of environmental gradients. This suggests that functional differentiation favors habitat partitioning among closely related species. To elucidate the mechanisms of species coexistence in oak forests, we explored patterns of interspecific variation in functional traits involved in water-use strategies. We tested the hypothesis that oak species segregate along key trade-offs between xylem hydraulic efficiency and safety, and between hydraulic safety and drought avoidance capacity, leading to species niche partitioning across a gradient of aridity. To do so, we quantified biophysical and physiological traits in four red and five white oak species (sections Lobatae and Quercus, respectively) across an aridity gradient in central Mexico. We also explored the trade-offs guiding species differentiation, particularly between the drought tolerance versus water acquisition capacity, and determined whether the water-use strategy was associated with the portion of the environmental gradient that the species occupy. In a trait-by-trait analysis, we detected differences between white and red oak species. However, a larger part of the variation was explained at the species rather than at the section level. We detected two primary axes of trait covariation. The first exhibited differences between species with dense tissues and species with soft tissues (the tissue construction cost axis); however, the oak sections did not constitute separate groups, while the second suggested a trade-off between xylem resistance to cavitation and tree deciduousness. As expected, the water-use strategies of the species were related to the environment; oak species from arid areas had more deciduousness and a higher instantaneous water-use efficiency. In contrast, their humid counterparts had less deciduousness and had a xylem that was more resistant to embolisms. Altogether, these results suggest that aridity filters closely related species, resulting in habitat partitioning and niche divergence.

Diversification, adaptation, and community assembly of the American oaks (Quercus), a model clade for integrating ecology and evolution

“…More broadly, repeated variation in form and function in the oaks (Tucker, ) is evident in multiple lineages related to their habitats across precipitation and moisture gradients (Bahari et al ., ; Kaproth & Cavender‐Bares, ; Aguilar‐Romero et al ., ; Figs ), across fire gradients (Myers, ; Jackson et al ., ; Cavender‐Bares et al ., ; Cavender‐Bares & Reich, ; Schwilk et al ., ), elevation (Poulos et al ., ; Schwilk et al ., ; Fallon & Cavender‐Bares, ), temperature gradients (Koehler et al ., ), and successional gradients (Monk, ; Petit et al ., ; Lagache et al ., ). Ecological diversification across climatic gradients is well documented (Cavender‐Bares et al ., ; Hipp et al ., ).…”

“…Across rainfall and aridity gradients in California (Skelton et al, 2018) and in Michoac an, Mexico (Aguilar-Romero et al, 2017), trade-offs between traits linked to xylem vulnerability, xylem hydraulic transport capacity, and drought avoidance have allowed species to partition the abiotic environment. Oak species from more arid regions have greater resistance to embolism (Skelton et al, 2018) and/or tend to abscise a greater proportion of leaves during the dry season, allowing them to avoid dry-season water stress, whereas those in more mesic regions with less severe dry seasons show less deciduousness and xylem that is more resistant to embolism (Aguilar-Romero et al, 2017). In Sierra del Carmen in Coahuila, Mexico, two arid species partition altitudinal gradients through physiological and growth mechanisms that are associated with tradeoffs in growth rate and drought tolerance (Poulos et al, 2007).…”

Leaf‐level trade‐offs between drought avoidance and desiccation recovery drive elevation stratification in arid oaks

“…These relationships show differences in resource acquisition strategies that were not reflective of deep phylogenetic divergences, but rather are correlated evolutionary traits in species of both clades. Q. rugosa (white oak) and Q. hypoleucoides (red oaks), the true evergreens, showed the highest wet season transpiration and the higher seasonality and precipitation of their ranges may influence a strategy of conservative behavior in dry seasons, such as higher C leaf , to avoid leaf loss during less severe droughts until the following, voluminous rainy season, when they transpire more and can continue growth (Aguilar-Romero et al 2017). The lowest elevation species, also both red and white oaks, experienced overall less precipitation during the hottest season and did not show as much change in transpiration between wet and dry seasons (Appendix S1: Table S6).…”

“…In oaks, deciduousness has been found to be important in structuring populations and species distributions with respect to aridity: Xeric populations in a tropical oak were more likely to be deciduous than their mesic counterparts (Ramírez‐Valiente and Cavender‐Bares ) and species occupying more arid regions or with lower seasonal water access were more likely to have shorter‐lived leaves (Griffin , Aguilar‐Romero et al. ), although elevationally stratified species may also exhibit drought deciduousness despite habitat differences (Poulos et al. ).…”

“…Among oak species (Quercus L., Fagaceae), a highly diverse genus globally and within the Americas (Nixon 2002(Nixon , 2006, both drought and freezing resistances have been found to structure some species distributions at local and regional scales, making it difficult to parse how species coexisting within any system may be limited at elevation extremes. In oaks, deciduousness has been found to be important in structuring populations and species distributions with respect to aridity: Xeric populations in a tropical oak were more likely to be deciduous than their mesic counterparts (Ram ırez-Valiente and Cavender-Bares 2017) and species occupying more arid regions or with lower seasonal water access were more likely to have shorter-lived leaves (Griffin 1973, Aguilar-Romero et al 2017, although elevationally stratified species may also exhibit drought deciduousness despite habitat differences (Poulos et al 2007a). Oaks are generally more drought-tolerant than other co-occurring angiosperms (Abrams 1990), but differences among species are not always clearly delineated by tolerances like turgor loss points.…”