Seasonal and inter-annual photosynthetic response of representative C₄ species to soil water content and leaf nitrogen concentration across a tropical seasonal floodplain

Abstract

We examined the seasonal and inter-annual variation of leaf-level photosynthetic characteristics of three C 4 perennial species, Cyperus articulatus, Panicum repens and Imperata cylindrica, and their response to environmental variables, to determine comparative physiological responses of plants representing particular microhabitats within a seasonal tropical floodplain in the Okavango River Delta, Botswana. Five measurement campaigns were carried out over a period of 2 y which covered two early rainy seasons, two late rainy seasons and one dry season. For all three species, light-saturated net photosynthetic rates (A sat ) and stomatal conductance (g sat ) decreased with decreasing soil water content with a seasonal range for A sat of approximately 5-45 μmol m −2 s −1 , and for g sat of 0.03-0.35 mol m −2 s −1 . The species representing the wettest microhabitat (Cyperus) had the highest g sat at low leaf-to-air vapour pressure deficits (D l ), the highest ratio of intercellular to ambient CO 2 concentration (C i /C a ), as well as the highest degree of variation in C i /C a from season to season. We interpret this as being indicative of its adaptation to a moist growth environment allowing for non-conservative water use strategies as soil moisture is usually abundant. For all three species there was significant variation in photosynthetic fluxes from one year to another that was related to variation in leaf nitrogen and phosphorus. This study shows that when assessing the role of savanna stands in large-scale carbon balance models, the remarkable inter-annual variation in leaf photosynthesis reported in this study should be taken into account.

Tropical savannas: Biomass, plant ecology, and the role of fire and soil on vegetation

“…The wet soils of riparian and lower slope sites and floodplains tend to develop analogous hydromorphic characteristics throughout the tropics, and are particularly striking in semi-arid savannas, as illustrated by the N and P accumulation in soils (Jacobs et al, 2007) and in related photosynthetic fluxes (Mantlana et al, 2008a). In Lamto (Ivory Coast), Boudsocq et al (2009) explain the nitrification-inhibiting nature of wet savannas that contribute to ecosystem fertility and primary production.…”

Ecophysiology of forest and savanna vegetation

“…Unfortunately, little attention seems to have been paid to the water relations and physiological functioning of cerrado grasses, but it is clear from studies on other continents that considerable reductions in stomatal conductance occur during the dry season as a consequence of dramatic reductions in soil water availability and much higher leaf-to-air vapor pressure differences [e.g., Mantlana et al, 2008b].…”