UC Berkeley

Amazonian droughts are becoming more frequent and intense, having a profound effect on water availability for plants. However, we still have limited abilities to predict the effect of climate change on plant species survival and distribution in mega-diverse systems like the Amazon. Plant hydraulic traits coupled with the assessment of environmental characteristics emerge as an important tool to assess species physiological performance and resilience, especially during extreme climatic events. Here we investigate plant hydraulic strategies and physiological performance of tropical tree species (i) across contrasting environments, (ii) during an extreme drought event and (iii) during the course of a day to understand how water availability may shape species distribution and plant vulnerability to drought in the Amazon Basin.

In the first chapter, I investigate if the current patterns of species’ distribution in the main Amazonian habitats can be explained by species hydraulic strategies and how variation in the hydraulic properties of Amazonian forests influences species-specific vulnerability to drought across different habitat types. I found strong segregation between species from wet vs. dry environments in relation to their functional traits, suggesting that water availability could be a strong predictor of species functional composition in the different Amazonian environments. Also, the xylem of dry-habitat species are more embolism resistant, but it may not be correct to assume that these species will be the ones performing better under a warmer and drier climate.

The second chapter assesses plant physiological performance during a strong El Niño. This is the first study in the Amazon to measure in situ tree physiological stress before, during and after a natural drought event. I show that the warmer and drier conditions imposed by the El Niño greatly amplified trees’ physiological stress and could affect growth, phenology and potentially lead to tree mortality. Given the extreme nature of the this El Niño and that temperatures are predicted to increase in the tropics, this work can serve as a case study of the possible impact climate warming can have on tropical trees.

In the third chapter, I analyzed the diurnal patterns of stomatal conductance, water potential, and sap velocity during the dry season and investigate how they change with canopy temperature and vapor pressure deficit. I found that unlike preview reported, plants have a re-hydration period during the hottest time of the day probably due to an imbalance between stem sap-velocity and leaf transpiration rates. This study helps to elucidate the array of processes influencing diurnal patterns of plant-water balance in tropical trees.