Consequências funcionais da variação intraespecífica: as respostas de uma palmeira ameaçada de extinção

Abstract

Understanding the mechanisms that cause plant mortality due to drought constitutes one of the biggest gaps in plant ecology. One of the associated mechanisms is described as the Hydraulic Limitation Hypothesis in which vulnerability to drought increases with plant height due to greater restrictions on the flow of water from the roots to the leaves. Such restrictions can limit carbon assimilation, biomass production and individual growth. Because they have an anatomical structure distinct from trees, palms may have different mechanisms to deal with water stress throughout their ontogeny. The forecast scenario for the Atlantic Forest of Southeastern Brazil includes a reduction in precipitation by up to 50% at the end of the century, which further threatens the vegetation of this biodiversity hotspot. We used the palm tree Euterpe edulis Martius, popularly known as juçara palm, to understand its vulnerability to drought at different stages of development by carrying out sequential monthly measurements of water potential and, simultaneously, measurements of stomatal conductance and chlorophyll a and b content. The study was carried out in the Tijuca National Park (PNT), located in the state of Rio de Janeiro. The functional characteristics of up to 16 individuals with heights ranging from 0.07 to 13.71 meters were collected - an important factor for developing environmental restoration plans based on the detection of the phases most susceptible to drought. We found a positive relationship between chlorophyll a and b concentrations, stomatal conductance and height, indicating that the increase in the height of individuals, in addition to implying greater photosynthetic pigments, does not impose stomatal restrictions and, therefore, there would be no limitation to carbon gain. These data corroborate what was recently found in the Atlantic Forest for this species, in which taller individuals develop a more competitive strategy in the context of Grime's CSR Triangle Theory. More specifically, taller individuals of the juçara palm appeared to be able to maintain their metabolic functions under greater degrees of dehydration (e.g. greater evaporative air demand), which could suggest greater resistance to embolism when accompanied by greater carbon gain. Small individuals, focused on the ruderality strategy, are probably unable to acquire as many resources as possible even if such a strategy is acquisitive, since these individuals located in the shade did not open their stomata during the study period, which could result in death due to deprivation. of carbon. This study shows how essential it is to assess the vulnerability of plants to drought to predict possible losses of species and changes in the structure and functioning of plant communities. In addition to the relevance of this study of intraspecific variation in the water and carbon use characteristics of a palm species threatened with extinction over time, it can improve the understanding of how unfavorable drought conditions can distance it from its metabolic optimum in an environment to predictions of an increase in the frequency, duration and intensity of drought events, adding to the understanding of the resilience and variability of growth response during drought.