Efeito da sazonalidade da precipitação no crescimento e trocas gasosas em espécies arbóreas numa floresta de terra-firme da Amazônia Central

Abstract

The objectives of this study were to determine the effect of the rainfall pattern on growth and leaf gas exchange in saplings of ten tree species in a terra-firma rainforest of Central Amazonia. I also studied the effect of diurnal variation on photosynthetic rates (A), and the relationship between A and growth rates of saplings. The study was conducted at the Experimental Station of Tropical Forestry (ZF2 reserve) of the National Institute for Research in the Amazon (02º 36 21 S; 60º 08 11 W). I utilized tree species in the juvenile stage, 1-3 m tall. Data of monthly increase in diameter (at 50 cm above the stem base) and height were collected in 2007 and 2008. Whereas, other data were collected in the rainy and dry seasons of 2008. The fraction of visible sky (FCV) in microsites was measured with a canopy analyzer (LAI-2000, Li-Cor, USA). Gas exchange rates were measured with a portable photosynthesis system (Li-6400, Li-Cor, USA) using one to three leaves per plant and three plants per species. Light-saturated photosynthesis (Amax) and potential photosynthesis (Apot) were measured at light saturation and CO2 concentrations of 380 and 2000 μmol mol-1, respectively. The effect of time of day on stomatal functioning and photosynthetic rates (Amax) was assessed by collecting data between 06:00 and 18:00 h. Chlorophyll and nitrogen content were obtained by the Arnon and Kjeldahl´s methods, respectively. Average annual increase in diameter (IAD) was 0.90 mm, while mean annual increase in height (IAA) did not exceed 0.07 m. Stomatal conductance (gs) and Amax had high diurnal variation, but there was no effect time of day on Apot. Because Apot remained quite constant for most of the day, the effect of studied variables (growth rates, specific leaf area, AFE, etc) on photosynthetic rates were examined with respect to Apot. I found no effect of the rainy pattern (dry and rainy season) on IAD or IAA. However, I found effect of the rainfall regime on Amax, leaf area index (IAF), FCV, and chlorophyll content. Also, there was no effect of the rainfall regime and species on Apot. However, FCV had significant effects on IAD, IAA, Apot, and leaf traits (AFE, leaf thickness) and chlorophyll content. I conclude that because Apot is less sensitive to gs changes, Apot best describes photosynthetic characteristic of saplings growing at the forest understorey. I also found that little variation in FCV has a significant effect on IAA, IAD, leaf physiology (A) and leaf anatomy traits (AFE, leaf thickness).