Blinded by the Light: The Functional Ecology of Plant-Light Interactions

Abstract

The capture of sunlight by plants and other primary producers is the greatest driver of the world’s carbon cycle. The photosynthetic machinery that plants use to fix carbon dioxide and light energy into storable carbohydrates must be able to handle intense fluxes of energy, and both lack and excess of light put plants at a disadvantage—either from starvation or from damage. Plant leaves evolve in how they absorb, reflect, or avoid light in ways that can be explained as functional adaptations to their environment. Here, I present four studies on the interactions between plant tissue and the light environment—two of which concern the functional role of light capture or avoidance in ecological strategies, and two of which are methodological studies that explain how we can use plants’ interactions with light to understand their strategies more broadly. Chapter 1 reports on a study in the Big Biodiversity (BioDIV) experiment that seeks to characterize the range of strategies that plants have to cope with excess light under stressful conditions. In a survey of prairie plants, we find that species may either primarily use biochemical or structural strategies to protect themselves from excess light. The position along this continuum is phylogenetically conserved. Communities with more species relying on biochemical mechanisms are more resilient aboveground during water-limited periods. Chapter 2 uses growth surveys and physiological measurements in the Forests and Biodiversity (FAB) experiment to show how broadleaf trees respond to shade from faster-growing conifer neighbors. While most species were harmed by shade, growing slower and assimilating less carbon, two species showed the opposite trend. These two species were the most shade-tolerant in the experiment and were exceptionally susceptible to photoinhibition, such that shade from their neighbors facilitated their growth. All species relied on photoprotection more in sunnier environments. Chapters 3 and 4 use reflectance spectroscopy to estimate traits in different kinds of leaf tissue. Chapter 3 focuses on leaf litter, whose chemical traits are often measured to gain insight into components of nutrient cycle such as nutrient resorption and decomposition. We show that we can estimate a fiber content and elemental composition using pressed-leaf spectra and, with somewhat higher accuracy, ground-leaf spectra. Chapter 4 is about pressed leaves, such as herbarium specimens, whose functional traits ecologists increasingly seek to measure in order to fill in trait databases or understand the impacts of global anthropogenic changes. We show that reflectance spectroscopy can provide non-destructive estimates of several leaf functional traits from pressed leaves, which may extend the possibility of using a wider variety of herbarium specimens in functional ecology.