Diverse patterns of stored water use among saplings in seasonally dry tropical forests

Abstract

Tree species in seasonally dry tropical forests likely vary in their drought-survival mechanisms. Drought-deciduousness, which reduces water loss, and low wood density, which may permit dependence on stored water, are considered key traits. For saplings of six species at two distinct sites, we studied these and two associated traits: the seasonal amount of water released per stem volume ("water released") and the hydraulic capacitance of the stem (C). Two deciduous species with low stem density, Cavanillesia platanifolia and Bursera simaruba, had high C and high dry-season stem water potential (Ψ(stem)), but differed in dry-season water released. C. platanifolia did not use stored water during the dry season whereas B. simaruba, in a drier forest, released stored water. In both, water released was highest while flushing leaves, suggesting that stored water supports leaf flushing. In contrast, two deciduous species with intermediate stem density, Annona hayesii and Genipa americana, had intermediate C, low dry-season Ψ(stem), and high seasonal change in water released. Meanwhile, two evergreen species with intermediate stem density, Cojoba rufescens and Astronium graveolens, had relatively low C, low dry-season Ψ(stem), and intermediate seasonal change in water released. Thus, at least three, distinct stored-water-use strategies were observed. Additionally, bark relative water content (RWC) decreased along with Ψ(stem) during the dry season while xylem RWC did not change, suggesting that bark-stored water buffers Ψ(stem) seasonally. Together these results suggest that seasonal use of stored water and change in Ψ(stem) are associated with functional groups that are characterized by combinations of deciduousness and stem density.

Does leaf shedding protect stems from cavitation during seasonal droughts? A test of the hydraulic fuse hypothesis

“…When plants were leafless, terminal twigs were collected to measure twig water potential (Ψ twig ) following the same protocol as for Ψ leaf , except that Ψ twig was measured only at predawn. Comparisons with psychrometric measurements of Ψ twig confirmed that the pressure chamber measurements accurately assessed Ψ twig (Wolfe & Kursar, ). As water potential is under a gradient within transpiring plants, we took the mean of Ψ leaf at predawn and midday to represent Ψ plant for each sapling in each census.…”

“…For Cavanillesia , direct measurements of stem water content revealed no difference between the wet and dry seasons, suggesting that leaf shedding contributed to the complete retention of stem water (Wolfe & Kursar, ). Meanwhile, Bursera had significantly lower stem water content in the dry season than in the wet season, suggesting that either leaf shedding did not stop water loss or that water loss occurred before leaf shedding (Wolfe & Kursar, ), and so it is unclear whether leaf shedding retained stem water. By contrast, Annona had Ψ plant that decreased at the onset of the dry season, before the saplings shed leaves, and then stabilized after leaf shedding (compare Figs c and S4c).…”

“…Comparisons with psychrometric measurements of Ψ twig confirmed that the pressure chamber measurements accurately assessed Ψ twig (Wolfe & Kursar, 2015). As water potential is under a gradient within transpiring plants, we took the mean of Ψ leaf at predawn and midday to represent Ψ plant for each sapling in each census.…”

“…S4). These species have high stem hydraulic capacitance that maintains high Ψ plant , even with significant water loss (Wolfe & Kursar, 2015). For Cavanillesia, direct measurements of stem water content revealed no difference between the wet and dry seasons, suggesting that leaf shedding contributed to the complete retention of stem water (Wolfe & Kursar, 2015).…”

Lignin composition is related to xylem embolism resistance and leaf life span in trees in a tropical semiarid climate

“…Increasing WD results in a decrease of QW sat and Ψ dry season (which became more negative) (Oliveira et al ., ). Low WD enables species in dry habitats to store water in their stem and to flush new leaves at the onset of rain (Borchert et al ., ; Wolfe & Kursar, ; Wolfe et al ., ). Indeed, Oliveira et al .…”