Effects of climate warming differ among plant functional groups in a temperate/subtropical forest

Abstract:

Climate warming is expected to shift some temperate/subtropical regions to a more tropical climate in the next 30 years. We conducted a 1.5-year-long passive warming experiment in a North Florida forest to determine how warming (mean daily air temperature +1.5 °C, maximum daily air temperature +5 °C) might impact leaf C and H₂O fluxes and productivity of four temperate/subtropical forest species representing different functional groups: Serenoa repens (shrub palm), Andropogon glomeratus (C4 grass), Pinus palustris (needled evergreen tree), and Quercus laevis (broadleaved deciduous tree). Over time we measured leaf gas-exchange (i.e. net photosynthesis (A), stomatal conductance to water vapor (g_s)) at prevailing leaf temperatures on seedlings of each species grown under ambient and warmed treatments. At the completion of the experiment we determined warming effects on photosynthetic biochemistry and total dry mass production. We hypothesized that 1) species with warmer distributions and adaptations to warmer temperatures (palm and C4 grass) would increase/maintain photosynthesis and productivity with warming compared to tree species with more temperate distributions, and 2) across species, periods of low soil moisture would diminish or reverse the positive effects of warming on photosynthesis. Species responded differently to experimental warming. Warming increased A in S. repens and A. glomeratus at most time points, and increased RuBP regeneration (J_max) in S. repens (+39%). Warming had little effect on A or photosynthetic capacity in Q. laevis. Total dry mass production increased slightly with warming in A. glomeratus (+22%), S. repens (+7%), and Q. laevis (+10%). In P. palustris, warming reduced A during summer (but increased A during autumn/winter), reduced J_max (-22%), and slightly reduced total dry mass production (-14%). In all species but A. glomeratus, A declined during dry spells and did so consistently across treatments. These results indicate that climate warming might have different effects on species at the temperate/subtropical-tropical ecotone. Warming may benefit grasses and palms at the expense of tree species, particularly P. palustris, foreshadowing a shift in forest structure, composition, and productivity.