Variabilidade do Déficit de Pressão de Vapor em uma Área de Floresta Densa na Amazônia Central

Abstract

The Vapor Pressure Deficit (VPD) in the atmosphere is a difference between the saturation vapor pressure of water and the actual vapor pressure of the air. In recent years, several studies have shown an increase in VPD values, which has generated several consequences for vegetation. This increase can reduce the photosynthetic processes of plants, thus decreasing their capacity to absorb carbon dioxide (CO₂) from the atmosphere. The main objective of this work was to study the variability of VPD, within and above a forest canopy, in Central Amazonia, comparing and characterizing VPD in the different atmospheric layers and also its temporal variability at different scales. For this purpose, data measured in the micrometeorological towers of the Observatório da Torre Alta da Amazônia (ATTO) site were used, using the temperature and relative humidity of the air, obtained by thermohygrometers installed at different heights, from 1.5 m to 316 m above ground in two towers (ATTO with 316 m and INSTANT with 81 m), which helped in the calculation of the DPV through the discovery of Tetens. In addition, observation data, obtained by a rain gauge located at an altitude of 81 m, as well as information on soil temperature and humidity, were used, which are developed for a more in-depth interpretation of the DPV analyses. The results show that the DPV undergoes greater variation at heights closer to the forest canopy, that is, the difference between the maximum and minimum values ​​is greater in this region, while at higher heights the daily DPV cycle undergoes little oscillation. Such behaviors were expected, since the DPV has direct radiation with the air temperature, which in turn increases/decreases as the surface heats/cools. Note the maximum DPV values ​​around 2:00 p.m. (local time) with values ​​that exceed 20 hPa, due to the peak of solar radiation, and minimum values ​​during the early morning hours, when the radiation from the sun ceases. The analysis of the monthly DPV time series shows severe seasonal and interannual variations, with a strong influence from the El Niño and La Niña climate characteristics. During El Niño events, specifically in 2015-16 and 2023-24, DPV values ​​reached higher monthly peaks, exceeding 16 hPa, especially in the dry period of those years and at heights from 50 m above the forest canopy. These peaks reflect the increase in temperature and the reduction in relative humidity associated with this specific event. On the other hand, in La Niña years, such as 2021-22, the DPV presented its minimum values, resulting from the increase in humidity and the specific characteristics of the features. Differences between the towers were also evident: the ATTO tower, surrounded by a small clearing, presented higher DPV values ​​compared to the INSTANT tower, surrounded by forest. This difference can be explained by the lower emission of humidity from the forest in the area impacted by the clearing. In addition, the analysis of the data revealed a gradual increase in DPV over the 13 years trained, bringing progressive changes in the temperature and humidity patterns of the region, which may be extremely limiting for plant physiology in the coming years. This increase may be related to ongoing climate change, which has direct impacts on the hydrological cycles and ecological processes of the Amazon rainforest. These changes reinforce the need for continuous monitoring to understand the future implications for the ecosystem and the environmental services provided by the Amazon.