Fotossíntese e crescimento em diâmetro de árvores em função da temperatura e da precipitação numa floresta primária de terra-firme na Amazônia central

Abstract

Climate changes predicted over the next decades (changes in air temperature and precipitation) may affect leaf carbon assimilation and, therefore tree biomass accumulation in the Amazon region. The objective of this work was to determine the influence of temperature and precipitation on photosynthesis and diameter increment of trees in a terra-firme rain forest in central Amazonia. Climatic parameters were measured with a micrometeorological station (Li 1400, Li-Cor, USA). Diameter increment of 400 trees was measured monthly during 2006 and 2007 and an allometric equation used to estimate aboveground biomass. Light-saturated photosynthesis (Amax) and potential photosynthesis (Apot) were assessed with an infra-red gas analyzer (IRGA 6400, Li-Cor), whereas fluorescence parameters (Fo, Fm and the Fv/Fm) were measured in intact leaves using a portable fluorometer (PEA, Hansatech, UK) in three diurnal periods (6-10 h, 10-14 h, 14-18 h) during the dry and wet season. Mean annual diameter increment was 1.99 mm year-1. Annual diameter increment was greater in Byrsonia duckena W. R. Anderson (Malpighiaceae), Tachigalia venusta Dwyer (Fabaceae: Leguminosae Caesalpinioideae), Inga laurina (Swartz) Willd. (Fabaceae: Mimosoideae) e Sclerorema micranthum Ducke (Bombacaceae) (4.3 to 5.8 mm year-1) than in Gustavia augusta L. (Lecythidaceae) and Mezilaurus itauba (Meiss.) Taubert ex Mez (Lauraceae) (0.6 mm year-1). Above-ground biomass accumulation and carbon storage of all 400 trees were estimated to 4.5 Mg yr-1 and 2.2 Mg yr-1, respectively. There were no significant effects of the mean monthly precipitation, irradiance (mol m-2 day-1) and air temperature (mean, maximum and minimum) on the means of the monthly diameter increment. However, the number of hours the air temperature was below 22.6 ºC (the mean minimum temperature of two years of study) and diameter increment showed a negative relationship, suggesting a decline in respiration with a decrease in night temperature, which leads to increase in tree diameter growth. Wood water content was higher in the wet than in the dry season, as long as bark water content there was no difference between the seasons. Water content of wood ranged from 26 to 50% and bark water content from 31 to 66% and, in all species, bark moisture was higher than that of wood. Wood water content and wood density showed a negative relationship, suggesting that in species with dense wood less space remained for water to accumulated than in species with soft wood. Wood density varied among species (mean of 0.74 g cm-3), with 53.4% of species with wood density greater than 0.75 g cm-3. Wood density showed negative correlation with the diameter increment, because species with high density grow more slowly than species with low density. There was no relation between diameter increment and the leaf nitrogen content, suggesting that within a range of moderate nitrogen content, trees with the same leaf nitrogen content may present variation in diameter growth. Light-saturated photosynthesis (Amax) varied between species, where Coussapoa orthoneura Standl (Cecropiaceae) reached 14 μmol m-2 s-1 and Mabea angulares G. Dem Holl. (Tiliaceae) 4 μmol m-2 s-1. For all species, Amax was higher in the dry season (06 -10 h). Diurnal and seasonal change in Amax may be attributed to stomatal conductance. Potential photosynthesis (Apot) varied between species, but Apot was similar in diurnal and seasons periods, with the exception of M. angulares (at 14 - 18 h), which may be attributed to reduction of mesophyll conductance to CO2 diffusion. Fluorescence parameters were affected by irradiance, though there was no difference between the seasons. There was no photoinhibition in Goupia glabra Aubl. (Celastraceae) and Inga paraensis Ducke (Fabaceae: Mimosoidae), however the recovery of Fv/Fm ratio showed that there was dynamic photoinhibition in M. angulares and C. orthoneura. In conclusion, night temperatures above the mean minimum temperature lead to a lower tree diameter increment, which could provide important effect on the global carbon balance, considering the increase in global mean temperature predicted for the next decades. Finally, diameter increment was constant throughout the year, although in general the light-saturated rates of photosynthesis (Amax) were higher during the dry season than during the wet season. This indicates that diameter increment is the result of complex interactions among a number of factors and not only affected by changes in photosynthetic carbon gain.