RESISTÊNCIA DO XILEMA À SECA OU RETARDO DA DESSECAÇÃO: DETERMINANTES DA MORTALIDADE VEGETAL ASSOCIADA AO EMBOLISMO EM ESPÉCIES REPRESENTATIVAS DA AMAZÔNIA

Abstract

The Amazon Rainforest is the largest tropical forest in the world, with a wide diversity of endemic species and home to around 7.000 species of trees. However, this Brazilian domain has suffered several environmental pressures that put its entire biodiversity at risk, such as deforestation, fires, and climate change. Increases in tree mortality have been observed in the Amazon in recent years due to the occurrence of droughts. Considering that the species of this domain evolved in an environment with wide water availability throughout the year, it is possible that they may present high sensitivity to drought, mainly as a consequence of the occurrence of embolism. The present study aimed to evaluate the main determinants of plant mortality in representative species of the Amazon Rainforest, evaluating how the balance between xylem tolerance to embolism, and water conservation and accumulation determine the time for mortality associated with hydraulic failure. For this, species from the Amazon domain were cultivated in a greenhouse for 4 months, and then used to determine gas exchange, xylem vulnerability to embolism, water conservation traits (water potential inducing stomatal closure, transpiration until stomatal closure, residual transpiration in leaves and bark, cuticular and stomatal characterization) and water storage traits (stem, root and leaf water content, total water content, water balance, wood characteristics, and leaf succulence). It was observed that the analyzed plants have low xylem tolerance to embolism and that the balance between xylem tolerance, water accumulation, and water conservation was essential to delay the catastrophic hydraulic failure associated with drought-induced mortality. The relative importance of each factor, however, varied according to the species, with cocoa, the plant that died last, distinguished from the others by accumulating more water and presenting a greater hydraulic safety margin. This dependence on water accumulation to delay mortality, at the expense of xylem tolerance, may reflect the environment in which these species evolved. However, it should be noted that water accumulation can only delay mortality for a short period of time. In this way, the data suggest that Amazon trees species will face a risk of increased mortality if drought events become more frequent and intense. These results have major implications for reforestation programs and also for understanding the natural regeneration dynamics of forests.