Abstract
The response of small understory trees to long-term drought is vital in determining the future composition, carbon stocks and dynamics of tropical forests. Long-term drought is, however, also likely to expose understory trees to increased light availability driven by drought-induced mortality. Relatively little is known about the potential for understory trees to adjust their physiology to both decreasing water and increasing light availability. We analysed data on maximum photosynthetic capacity (Jmax , Vcmax ), leaf respiration (Rleaf ), leaf mass per area (LMA), leaf thickness and leaf nitrogen and phosphorus concentrations from 66 small trees across 12 common genera at the world's longest running tropical rainfall exclusion experiment and compared responses to those from 61 surviving canopy trees. Small trees increased Jmax , Vcmax , Rleaf and LMA (71, 29, 32, 15% respectively) in response to the drought treatment, but leaf thickness and leaf nutrient concentrations did not change. Small trees were significantly more responsive than large canopy trees to the drought treatment, suggesting greater phenotypic plasticity and resilience to prolonged drought, although differences among taxa were observed. Our results highlight that small tropical trees have greater capacity to respond to ecosystem level changes and have the potential to regenerate resilient forests following future droughts.
Highlights
Climate change can simultaneously affect multiple environmental variables across ecosystems globally (IPCC, 2019)
Some studies have investigated the impacts of short-term drought events on tropical understory trees (e.g., Newbery, Lingenfelder, Poltz, Ong, & Ridsdale, 2011; Phillips et al, 2010), but, to our knowledge, no studies to date have investigated the effects of prolonged drought in tropical forests on understorey trees exposed to elevated light conditions
Whichever mechanism small trees use, our results suggest that these trees are likely to be able to minimize the impact of the drought, relative to larger trees, in order to facilitate the upregulation of photosynthetic capacity and a release from extreme light limitation
Summary
Climate change can simultaneously affect multiple environmental variables across ecosystems globally (IPCC, 2019). Trees modify their leaf physiology to optimize the balance between carbon gain and carbon and water loss, in order to maximize growth, reproduction and competitiveness Both large and small understory trees in tropical forests have been shown to be highly responsive and plastic to changes in their light environment (Kitajima et al, 2013; Quevedo-Rojas et al, 2018), suggesting light is likely to be the most limiting factor for photosynthesis in intact tropical forests. Increases in light availability under drought conditions may result in excessive photon flux density, elevated leaf temperatures and elevated VPD, inducing photoinhibition (Kamaluddin & Grace, 1992; Krause, Virgo, & Winter, 1995; Mulkey & Pearcy, 1992), prolonged stomatal closure (Reynolds-Henne et al, 2010) and xylem embolism To avoid these negative consequences of concurrent high light and drought stress, small understory trees may need to modify their physiology in different ways compared to canopy trees. Leaf physiological traits (Jmax, Vcmax, Rleaf and LMA) are more responsive to reduced soil moisture availability and canopy openings following prolonged soil moisture deficit in small understory trees than large trees
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