Our work in Puerto Rico started with the tropical erosion project, but has since transformed into a series of projects focusing on understanding the relative importance of parent rock, topographic position, and forest community on determining ecosystem properties. In collaboration with the late Fred Scatena and Art Johnson and the Luquillo Critical Zone Observatory, we sampled soils from across the landscape of El Yunque National Forest in Puerto Rico. The sampling was designed to capture the relavant heterogeneity of the place – we collected soils derived from the two main parent materials, overlain by the three dominant forest communities, and in three topographic positions (ridge, slope, valley). In total, the team dug 216 soil pits to a depth of 80cm in little over a month!
From this Herculean effort, we have learned an enormous amount about the relative importance of these different state factors. The availability of phosphorus is more a function of parent rock than anything else, though topography also plays a role. Nitrogen is more complicated, and seems to be most influenced by forest community composition. Maya Almaraz is investigating this link in her dissertation. Our exploration of cation availability illuminated something we hadn’t thought about. Like the Osa Peninsula, the El Yunque forest is not in geomorphic equilibrium. The mountains were uplifted about 5 million years ago. The lower portion of the landscape has adjusted to this uplift, erosion rates are really fast everywhere, ridges are narrow and eroding almost as fast as slopes. In contrast, the upper portion of the landscape is more hetergeneous. There are steep slopes and narrow ridges (similar to lower down) in some portions of the landscape. But there are also remnant surfaces, broad, flat ridges that are isolated, for now, from the river incision propagating through the landscape in response to the recent uplift.
Cooper Tamayo and Bekah Stein are working on these different geomorphic surfaces, while tightly controlling for parent rock, climate, and forest community. Stay tuned as their projects continue. Ultimately, the lessons we learn in El Yunque about the link between geomorphic disequilibrium and biogeochemistry will be compared to what we learn in Osa. That’s a few years out, but it’s where we’re going.
Former post doc Steve Goldsmith uses an 8ft tall slingshot to sample canopy leaves for nitrogen isotopes.