Plant Water Relations in a Cloudy World: Plant Hydraulics, Water Sources, and Niche Relations

Tropical montane cloud forests make up only 1.4% of the world’s tropical forests, yet they make a tremendous contribution to global biodiversity with large numbers of endemic species. Additionally, these critically threatened forests perform ecosystem services important to many people in developing countries, including cloud-water interception which reduces downstream erosion damage, river flooding, and aids in the supply of drinking water year-round. Climate change forecasts an increase in global temperature and disappearance of the cloud-base that supplies water to these forests. This saturated cloud-base and the nutrient-rich water subsidies it provides are principally what define a cloud forest, and are essential to the growth and survival of cloud forest species. As such, it is critical to study the vulnerability of cloud forest trees to shifting water availability in order to predict how these forests might respond to a changing climate. Plant sensitivity to water deficit can be directly measured by uncovering their hydraulic strategy, or their approach for maintaining a balance between water conservation and growth. My primary research questions are: Which hydraulic strategies make trees more vulnerable or resilient to drought? How much of a “safety margin” do tropical forests have in surviving droughts? This research illuminates vulnerability of tropical trees in response to drought, and contributes to the conservation of forests in the face of climate change./nMy dissertation forms three chapters, which develop novel methods and elucidate the importance of plant water-use strategies for forests traditionally assumed to be non-water limited. Chapter 1) Cloud forest trees possess wider hydraulic safety margins than rainforest trees below the clouds./nI investigated hydraulic strategies of congeneric pairs of tree species that occupied forests habitually immersed in the clouds vs. pre-montane wet rainforest in Monteverde, Costa Rica, by quantifying vulnerability to embolism and stomatal regulation in response to water loss. These measurements allowed me to quantify a hydraulic “safety margin” to evaluate species susceptibility to drought and identify whether cloud forest endemics are vulnerable to cloud loss because they’ve adapted their water-use strategy to a disappearing water source. My results show greater safety margins for cloud forest tree species compared to their rainforest species counterparts, raising new questions about the determinants of the distribution of cloud forests and their expected dynamics in a changing climate. Chapter 2) Seasonal variation in plant-water sourcing reveals cloud-water uptake and its importance for hydrologic niche strategies in a tropical montane cloud forest. I examine the degree to which plant species might change their water sourcing under reduced cloud conditions by focusing on seasonal plant water sourcing via analyses of δ18O and δ2H stable isotopes in rainwater precipitation, cloud water, soil profiles, and tree stems in a Peruvian tropical montane cloud forest. Our results show cloud forest trees source a greater percentage of cloud water in the wet season as compared to the dry or “mixed” season, and show a larger range of isotopic values and thus water sources between species in the mixed season than in the wet season. This suggests that when water becomes limiting, hydrologic niche segregation likely plays an important role in the survival and co-existence of the cloud forest tree community. Under future climate scenarios with reduced occult precipitation, certain cloud forest taxa may, therefore, perform better than others based on their ability to diversify their water sourcing via hydrological niche segregation. Chapter 3) Avoiding the threshold: the case study of the native endemic latex-bearing species, California Buckeye, in a hotter, drier environment. In the tropics, latex-bearing plants are abundant yet poorly understood because they defy many of our most basic ecophysiological techniques. I developed a novel method to measure the hydraulic strategies of latex plants. Using a previously enigmatic California endemic latex plant, I demonstrated that we can now measure the drought vulnerability of latex-produces and I shed light on controversial plant damage thresholds. Taken together, these projects aid our understanding of the mechanisms that determine the fate of these tropical cloud forests in the face of climate-change-type droughts.

Advisor: Dawson, Todd E