Abstract
This study addressed some questions about how a suitable leaf carbon balance can be attained for different functional groups of tropical tree species under contrasting forest light environments. The study was carried out in a fragment of semi-deciduous seasonal forest in Narandiba county, São Paulo Estate, Brazil. 10-month-old seedlings of four tropical tree species, Bauhinia forficata Link (Caesalpinioideae) and Guazuma ulmifolia Lam. (Sterculiaceae) as light-demanding pioneer species, and Hymenaea courbaril L. (Caesalpinioideae) and Esenbeckia leiocarpa Engl. (Rutaceae) as late successional species, were grown under gap and understorey conditions. Diurnal courses of net photosynthesis (Pn) and transpiration were recorded with an open system portable infrared gas analyzer in two different seasons. Dark respiration and photorespiration were also evaluated in the same leaves used for Pn measurements after dark adaptation. Our results showed that diurnal-integrated dark respiration (Rdi) of late successional species were similar to pioneer species. On the other hand, photorespiration rates were often higher in pioneer than in late successional species in the gap. However, the relative contribution of these parameters to leaf carbon balance was similar in all species in both environmental conditions. Considering diurnal-integrated values, gross photosynthesis (Pgi) was dramatically higher in gap than in understorey, regardless of species. In both evaluated months, there were no differences among species of different functional groups under shade conditions. The same was observed in May (dry season) under gap conditions. In such light environment, pioneers were distinguished from late successional species in November (wet season), showing that ecophysiological performance can have a straightforward relation to seasonality.
Highlights
Tropical forest environments often show a wide range of light availability in both spatial and temporal scales
In order to grow in low light environments such as the forest understorey, where plants have low carbon gain (Chazdon et al, 1996; Strauss-Debenedetti and Bazzaz, 1996), plants must minimize carbon loss reducing both respiration and tissues construction cost (Givnish, 1988), attaining a positive leaf carbon balance
We address the following hypotheses: 1) saplings in understorey conditions show reduced diurnal respiration rates regardless of successional status; 2) diurnal leaf carbon balance responds to seasonal variation; 3) saplings growing in high-light environments show higher net CO2 assimilation, respiration and photorespiration rates than those growing in the shade
Summary
Tropical forest environments often show a wide range of light availability in both spatial and temporal scales. In order to grow in low light environments such as the forest understorey, where plants have low carbon gain (Chazdon et al, 1996; Strauss-Debenedetti and Bazzaz, 1996), plants must minimize carbon loss reducing both respiration and tissues construction cost (Givnish, 1988), attaining a positive leaf carbon balance. This hypothesis suggests that species acclimate to low light down-regulating respiration which would reduce carbon losses (Reich et al, 1998). Late successional species respond rapidly to light flecks maintaining respiratory losses at a minimum (Strauss-Debenedetti and Bazzaz, 1996)
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