Respostas ecofisiológicas de plantas de sub-bosque ao aumento de CO₂ atmosférico e à seca intensificada pelo El Niño em uma floresta de terra firme na Amazônia Central

Abstract

Physiological and ecological processes, as well as the resilience and productivity of the Amazon Rainforest, can be profoundly affected by global environmental and climate changes. Among these factors, the increase in atmospheric carbon dioxide (CO₂) concentration and alterations in hydrological regimes stand out, especially the reduction in precipitation associated with the intensification of droughts caused by extreme events such as the El Niño–Southern Oscillation (ENSO), which are becoming increasingly frequent and severe in the Amazon basin. However, the responses of tropical forests to the combination of these factors are still not fully understood. In order to contribute to filling these gaps and improving the understanding of in situ plant responses to elevated CO₂ (eCO₂) and drought, this thesis aimed to investigate the effects of eCO₂ and drought intensified by El Niño on understory plants in a terra firme forest in Central Amazonia. To achieve this objective, we conducted a pioneering experiment using Open-Top Chambers (OTCs) to simulate future scenarios of elevated CO₂ concentration, with four chambers maintained under ambient conditions (control) and another four exposed to elevated CO₂ (treatment), with an increase of 250 ppmv relative to ambient levels. Measurements took place during the 2023 dry season, intensified by El Niño, and in the subsequent 2024 wet season. This climatic context provided a unique opportunity to investigate, under in situ conditions, the combined effects of eCO₂ and drought on the ecophysiological performance of understory plants. During the drought, there was a reduction in the photosynthetic efficiency of the plants, and traits related to growth appear to modulate the response to water stress (Chapter 1). Plants activate protective mechanisms to mitigate the effects of drought on photosystem II, promoting the reallocation of energy among utilization and dissipation pathways (Chapter 2). Additionally, eCO₂ may attenuate the negative effects of drought on understory plants. These results are relevant because they demonstrate how tropical species adjust physiological traits when faced with the combination of complex factors. Understanding these mechanisms provides support for predicting ecosystem responses and functionality in the Amazon as extreme drought events intensify and atmospheric eCO₂ levels continue to increase.