Ecofisiologia de dois ecotipos de Paubrasilia echinata em cenários de mudanças climáticas da floresta Atlântica, Brasil

Abstract

The Intergovernmental Panel on Climate Change (IPCC) has introduced some scenarios called Representative Concentration Pathways (RCP) due to increases in [CO2]. Among the forecasts for the Atlantic Forest, Brazil, two climate scenarios were selected: an optimistic one (RCP 4.5) composed of a moderate increase in [CO2], temperature and evaporative demand and another pessimistic (RCP 8.5) composed of a high increase in [CO2], temperature and evaporative demand, representing more extreme weather conditions. The ecophysiological responses of tropical tree species in extreme climate scenarios are poorly understood, and in this panorama, a question arises: how will shade-tolerant and sun-tolerant tropical trees respond to climate change? Paubrasilia echinata (brazilwood) is a species native to the Atlantic forest and has three ecotypes (small, medium and large leaf), which differ in terms of ecological habit in the initial phase of growth. The use of a species with intraspecific and ecophysiological variations such as P. echinata is quite interesting because it eliminates interspecific divergence. Seedlings of the small leaf (shade-tolerant) and medium leaf (sun-tolerant) ecotypes were cultivated for 90 days in OTC's inside climatized greenhouses, in three climate scenarios: one simulating the current climatic conditions of the Atlantic Forest (T= 26.1ºC, RH= 82% and [CO2]= 400 ppm) and two climate change scenarios-RCP 4.5 (T= 28.1ºC, RH= 80% and [CO2] = 580 ppm) and RCP 8.5 (T= 29.6ºC, RH= 74% and [CO2]= 936 ppm). For each scenario, two levels of substrate moisture were applied: 80% (well irrigated) and 40% (water restriction). At the end of the experiment, the following variables were evaluated: growth, gas exchange, water status, chlorophyll a fluorescence, non structural carbohydrates and cell wall polymers. The increase in [CO2] had a positive response in the growth of plants of the two ecotypes. Well irrigated plants showed higher stem height, total dry mass, net assimilation of CO2 and intracellular carbon. Furthermore, even with water restriction, plants grown in scenarios with increased [CO2] showed higher growth, CO2 assimilation, lower stomatal conductance and transpiration. These responses resulted in increased water use efficiency and carboxylation efficiency. Elevated [CO2] probably has a “buffering effect” on plants of the two ecotypes, increasing photosynthesis and early growth, reflecting greater mobilization and accumulation of carbohydrates in vegetative organs. Water availability had a greater influence on the contents of cell wall polymers, in relation to the scenarios. These mechanisms used by plants grown in high CO2 may have favored osmotic adjustment under these conditions, especially when grown with water restriction. The effect of the RCP 8.5 scenario was not limiting for the two ecotypes. In this scenario, plants of the two ecotypes with higher production of total dry mass were observed, indicating that the predicted climate changes will not promote so much limitation on the future of P. echinata.