Variação intra-anual da dinâmica etária foliar e características ecofisilógicas de árvores de dossel na Amazônia Central

Abstract

Knowledge of the variation in the age structure of leaves on tree species during seasonal periods of rainfall can help a better understanding of carbon dynamics in forest ecosystems of the Amazon. This is because the leaf phenology influences the photosynthetic capacity of the species. However, the ecophysiological mechanisms and tolerance of species to extreme drought events (e.g. El Niño) are still poorly understood. The aim of this study was to investigate the ecophysiological characteristics in leaves of different age classes subject to seasonal rainfall in 19 canopy tree species in one year of occurrence of the El Niño phenomenon (2015/2016). The leaf analyzes were performed in groups of four leaves age classes as follows: class I = <99 days; class II = 100-267 days; class III = 268-436 days and class IV => 437 days. Field campaigns were performed during four seasonal periods of rainfall: rainy (May), rainy-dry transition (July), dry (September) and dry-rainy transition (November). Leaf demography was monitored monthly, while the various classes of leaf age were analyzed variables of gas exchange, chlorophyll fluorescence a, leaf nutrient content and chloroplastid pigments. The rainy periods (52%), wet-dry transition (55%) and dry (34%) had greater abundance of leaves in class III (268-436 days). In the dry-wet transition, the greater amount of leaves (45%) was observed in class II (100-267 days). Seasonal periods influenced gas exchange, photosynthetic capacity and efficient use of water and nutrients. In the dry season, there was a strong drop of photosynthesis (A), respiration (Rd), stomatal conductance (gs) and transpiration (E). Nevertheless, with the onset of the first rains (dry-wet transition), it was the recovery of all the variables of gas exchange. The greatest amount of leaves produced, especially in periods rainy-dry and dry transition, resulted in higher nutrient content in the canopy (nitrogen, phosphorus and potassium) and reduced specific leaf mass. Seasonal periods did not affect the photochemical performance (chloroplastid pigments and chlorophyll fluorescence a) species. The control of leaf age class in photosynthetic capacity (Vcmax and Jmax) was influenced by the precipitation period. The highest values of Vcmax and Jmax were observed in classes I and II during the dry and dry-rainy transition. Seasonal periods of precipitation in El Niño year, affect the photosynthetic capacity (Asat, Vcmax and Jmax), but the photochemical performance (chloroplastid pigments and chlorophyll fluorescence a) on trees is not changed. The effect of leaf age class on the photosynthetic capacity of trees depends on the seasonal period of precipitation, with more pronounced during the dry season due largely to the leaf age structure in the forest canopy.