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

Abstract

Citation: Fallon, B., and J. Cavender-Bares. 2018. Leaf-level trade-offs between drought avoidance and desiccation recovery drive elevation stratification in arid oaks. Ecosphere 9(3):e02149. 10. 1002/ecs2.2149 Abstract. Understanding the extent to which climate limitations drive elevation stratification among species is integral to predicting the impacts of climate change. Zonation patterns of species within mountains have been well documented, and shifts in these patterns have been correlated with recent warming. However, the physiological mechanisms that explain these zonation patterns are not well understood. We used a system of broadly sympatric oak species within semi-arid mountains to (1) investigate the extent to which species elevation ranges correlate with climate, (2) test for associations of cold and drought resistances with upper and lower elevation limits, and for trade-offs between resistance mechanisms with elevation, and (3) examine the extent to which species-wide climatic ranges predict traits that drive local community assembly along an elevation gradient. We found that aridity gradients but not winter minimum temperatures predict oak stratification. Species differed in drought resistance, demonstrating a tradeoff between drought avoidance and drought recovery. At lower elevations, species avoided drought stress during the dry season through leaf abscission; at upper elevations, they maintained transpiration but recovered from daily desiccation via higher leaf water storage capacity, rather than tolerating desiccation via lower turgor loss points. Freezing resistance, measured as stem electrolyte leakage, was not correlated with elevation differences. Taken together, these results indicate that elevation stratification is linked to drought resistance rather than freezing resistance. We also found evidence of niche partitioning among closely related oaks linked to contrasting leaf phenology. The functional, phenological, and physiological traits important for elevation stratification were correlated with species' range-wide mean annual precipitation and precipitation seasonality, but not aridity. Our findings indicate that drought resistance along a leaf avoidance-recovery trade-off is integral to species stratification within this semi-arid montane system. Additionally, the mechanism of stratification acts upon traits and strategies conserved at the species level. Species within this system are likely vulnerable to range retraction under increased drought as a consequence of this phenological avoidance-physiological tolerance trade-off.

The role of diversification in community assembly of the oaks (QuercusL.) across the continental U.S.

“…A) (Schwilk et al., ; Fallon and Cavender‐Bares, ), two oaks from the predominantly Mexican red oak clade. Evolutionary divergence and convergence in leaf function likely drives species current distributions along hydrologic, edaphic, and fire gradients (Cavender‐Bares and Holbrook, ; Schwilk et al., ; Fallon and Cavender‐Bares, ). For example, leaf dissection, or the perimeter × leaf length per area (PLA) in oaks varies with mean rainfall during the growing season across a range of species (Kaproth and Cavender‐Bares, ).…”

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

“…As a consequence, species in different sections (red and white) have overlapping niche distributions, whereas species within a section do not. Zonation patterns of oaks in this system are well explained by species differences in phenology and leaf capacitance, with species showing a trade‐off between drought avoidance, on the one hand, and drought tolerance through rapid desiccation recovery, on the other hand (Fallon & Cavender‐Bares, ).…”

“…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 ., ).…”

“…Continuous leaf traits associated with leaf lifespan and phenology, including the timing of leaf abscission and SLA, appear to be relatively flexible both in terms of their plasticity (Cavender‐Bares, ; Firmat et al ., ; Ramírez‐Valiente et al ., ) and in their ability to evolve (Fig. d–f), in a manner that appears critical for survival in and adaptation to novel or changing climates (Cavender‐Bares & Holbrook, ; Cavender‐Bares et al ., ; Cavender‐Bares, ; Center, ; Ramírez‐Valiente & Cavender‐Bares, ; Fallon & Cavender‐Bares, ; Hipp et al ., ; Ramírez‐Valiente et al ., ). Flexibility in these leaf traits, even within a limited range, may thus be important in the long‐term persistence of the oaks across varying environments.…”

“…Compared with plants globally (Wright et al, 2004), leaf lifespan in the oaks shows a relatively limited range of variation (Cavender-Bares et al, 2004b, 2005. Nevertheless, even small evolutionary changes within this limited range (Fallon & Cavender-Bares, 2018) may be critical to adaptive differentiation and persistence in novel and changing environments.…”