Filter-Feeding Forests: Epiphytes and Fog in the Self-Organization and Function of Ecosystems

Abstract

The interaction between organisms and their environment is often simplified to a passive view of organisms as primarily subject to their environments, but better understood as a series of feedbacks between organisms and environmental processes. These feedbacks can generate complex spatial patterns of organisms and processes. In this dissertation I explore the mechanisms and consequences of a particularly striking yet understudied feedback, the harvesting of fog-water by vegetation; and the role of epiphytes, a hallmark of fog-influenced environments, in ecosystem processes. The chapters of this dissertation address a range of scales, from the local effects of individual organisms and their organization into communities to the ecosystem and landscape effects that these local effects can have.

The harvesting of fog water requires the presence of a physical structure to intercept it. This simple observation leads to powerful predictions about the increased availability of water (and as a result, nutrients) in the near vicinity of plants in foggy places. The results of my field measurements and experiments are the first to experimentally show that plant structures (natural or artificial) in Perú and Chile can harvest fog water and generate ‘islands of fertility’ at their base. When there is sufficient overlap of plant canopies, this process can lead to the self-organization of entire forest-patches in arid but foggy locations in coastal Chile. These patches form predictable bands that appear to slowly move windwards across the landscape, leaving a soil chemistry ‘footprint’ in their wake. These bands were predicted by theorists for similar sites, but this dissertation provides novel insights into spatial and temporal dynamics of soil properties and organisms across such bands, supported by field data.

One of the plant growth-forms most commonly associated with fog is epiphytism (use of other plants as a substrate for growth). Epiphyte morphologies and communities can change across fog-availability gradients, and I show that morphologies theoretically better adapted to fog water harvesting are more abundant at higher fog availability. This chapter involves the novel application of theory from marine organisms and fractal geometry to epiphytes.

The abundance of epiphytes in foggy environments leads to the question of what effects of epiphyte presence may have on host plants. Epiphytes can harvest additional fog water, but also retain fog and rainwater in the canopy, preventing it from reaching the soil. I tested the effects of epiphytes by removing epiphytes from entire plants, as well as adding them to artificial plant mimics. In the South American ‘fog-oases’ wher this research was conducted, the retention outweighs the increased water input, but with positive consequences: the retention of water by epiphytes buffers the canopy microclimate and reduces transpiration rates for the host plant. Plants with epiphytes are more efficient in their water use and show greater growth and owering. Epiphytes can therefore have a significant effect on host plants to an extent likely to impact forest hydrology and vegetation-climate feedbacks.

Epiphytes may modify more than just hydrology. Fertilization experiments in a rainforest canopy in Panama show that epiphyte communities of nitrogen fixers are limited by phosphorus. This finding constitutes an important step towards understanding the dynamics of nitrogen availability in tropical forests, in which epiphytes respond to and affect nutrient availability in the forest canopy, with consequences for whole-forest nitrogen availability.