UC Berkeley

AbstractPlant Functional Traits and Water Relations Across Succession

in the Atlantic Rainforest of Brazil By Ilana Rose Stein Doctor of Philosophy in Integrative Biology University of California, Berkeley Professor Todd E. Dawson, Chair

Tropical forests host the majority of the world’s terrestrial biodiversity and provide many ecosystem services. Throughout the tropics, forests are threatened by human activities such as logging, hunting, urbanization, and the expansion of agriculture, and these disturbances have pushed many tropical forests towards earlier successional states. In the context of anthropogenic climate change, the severe loss of habitat and biodiversity currently underway poses many novel and unknown outcomes for tropical forests, as well as the people and planetary cycles that depend on them.

One of the starkest examples of tropical biodiversity at risk is the Brazilian Atlantic forest. It is one of the most biodiverse forest biomes in the world – by some estimates, more so than Amazonia – with extremely high rates of endemism and species richness, despite the fact that it has been so heavily fragmented that less than 10% of the original forest cover remains. The importance of both protecting remaining tracts of the Atlantic Forest and restoring biological corridors between them is well-recognized, but there remains a paucity of ecological studies to inform effective management decisions, especially relative to the geographic scope and ecological complexity of the biome. Within the Atlantic Forest, the coastal forests of southern Bahia (a state in the economically-depressed Northeastern region of Brazil) are especially biodiverse as well as under-studied.

Of critical importance for our ability to effectively conserve and restore forests in the Anthropocene is understanding how ecosystems and communities recover from disturbance, and the relative vulnerabilities of secondary and old-growth forests to climate change. The relationships between plant physiology and community dynamics following disturbance are a key aspect of the intersection between climate and land-use change that has been largely neglected in Atlantic Forest research, especially in the southern Bahia region. My dissertation seeks to fill some of the knowledge gaps regarding the ecophysiology of tropical forest succession in this under-studied and critically threatened ecosystem, with the hope of improving future restoration and conservation outcomes.

In chapter 1, I investigate the leaf-level water relations traits and functional tradeoffs of ten ecologically important species in the Atlantic Forest that are specialized to different successional habitats. I measured several drought tolerance and avoidance mechanisms that allow leaves to perform photosynthesis and maintain other physiological functions under water limitation, high temperature, and high evaporative demand. I used this analysis to compare the drought resistance, functional trade-offs, and differences in trait-coordination between early- and late-successional tree species in the field. I found a similarly broad range of drought tolerance in both groups, but that they differed in the drought-avoidance mechanisms that they rely on for low drought tolerance. I concluded that the water-relations physiology of early- and late-successional species are shaped by different environmental factors, and that the leaf water relations and energy balance traits in young forest specialists are more coordinated than in mature forest species. In chapter 2, I explore how leaf functional traits vary between forest stands representing 8, 19, 32, and 48 years since agricultural abandonment. I measured leaf mass per area, leaf nitrogen content, and the stable isotope ratios of both carbon and nitrogen in leaves to understand how resource acquisition and allocation strategies change with forest recovery from disturbance. I found that leaf economics traits (leaf mass per area and nitrogen content) in my study system did not follow the patterns that are typical of other tropical rainforests, raising questions about what other processes drive trait variation in the Atlantic Forest. I also found that symbiotic nitrogen fixation in legumes is a key part of the recovery of N cycling following disturbance, and saw strong evidence for niche differentiation across succession in terms of the sources of nitrogen used by plants.

In chapter 3, I test the drought responses at the seedling stage in ten tree species commonly used in restoration plantings, representing different successional guilds, in the context of their overall physiological and life-history strategies. I conducted a greenhouse drought experiment and measured a suite of physiological parameters to characterize their water-relations strategies and test how effectively each strategy confers drought resistance in seedlings. I found three main life-history strategies that were associated with water relations and drought response, and that the strongest predictor of seedling survivorship under drought is biomass, even for species with ostensibly drought-tolerant traits at maturity. Additionally, I observed that the Atlantic Forest species included in the study were more drought resilient than anticipated, offering hope for restoration efforts in the Atlantic Forest under climate change.