Julie Tierney, doctoral student, Department of Ecology and Evolutionary Biology, Princeton University
After returning to Iquitos, Peru from a two-week field campaign in the rainforests along the upper Nanay River, I turned on my phone to receive a storm of worried messages from my family and friends: “Please tell us that you are safe from the fires!”; “Are you okay? Is there a lot of smoke?”; “I am reading about the fires – I am worried about you!”
While I was in the field, the world was gripped by the news that Jair Bolsonaro had swung open the doors for runaway deforestation in the Brazilian Amazon, a practice commonly accomplished by slashing burning old-growth forests. I was in no physical danger from this while working in the Peruvian Amazon, but I imagine the anxious messages I received from my friends and family stemmed in part from fear caused by the ominous implications of these fires.
The Amazon basin is a global asset in buffering against human-induced climate change – it accounts for one quarter of the world’s annual carbon sink. These fires release back into the atmosphere the carbon stored in the immense quantities of forest biomass, thus accelerating climate change and weakening the region’s carbon storage potential. Further, any potential efforts in the future to restore these forests will be far from simple. While the Amazon basin supports some of the tallest and most productive primary forests in the world, they curiously grow on some of the most nutrient-poor soil in the tropics. Because of this, understanding the cycling of chemical nutrients through these ecosystems is vital to conservation in this region. Indeed, the strategies that plants have evolved to recycle, conserve and access nutrients may hold the key to understanding how these forests function.
My doctoral research focuses the relationship between nutrient cycling and plant evolutionary strategies in Amazonian white-sand forests (locally called varillales in Peru), which are an extreme example of nutrient limitation in the neotropics. These curious, dwarf-like forests grow in small, island-like patches throughout the Amazonian and differ dramatically in structure and species composition from the towering tierra firme forests that surround them. A common explanation for this sudden transition in forest structure and function is that the underlying sandy soils are exceedingly poor in growth-limiting nutrients, like phosphorus or nitrogen. But this explanation is not entirely satisfactory, as it cannot resolve why white-sand forests are so very different when many Amazon forests in general have also evolved on extremely nutrient-poor soils. From this perspective, white-sand forests may hold answers to the fundamental question of how biodiversity, soils and ecosystem productivity are connected.
A white-sand forest (“varillal”) on the upper Nanay River. These forest are characterized by trees that generally do not grow taller than 7 meters and rarely exceed 10 centimeters in diameter.
This summer, I traveled to Iquitos, Peru to investigate the question: how do soil nutrients and plant nutrient-acquisition strategies control productivity in white-sand forests? My research team and I traveled by river to 20 previously-established plots of white-sand and terra firme forests scattered throughout the forests of Loreto. Along the way, we camped on beaches nearby the chattering of the pink Amazon river dolphins or stayed in the rural communities close to our study sites.
Our field work involved traveling by boat to remote forest locations, often for weeks at a time.
My first objective was to identify important differences in nutrient cycling between the two forest types. To accomplish this, we collected soil and leaf litter samples to determine the quantity and quality of the nitrogen and phosphorus in the ecosystem. We also performed in situ soil incubations to determine the rate at which mineral nutrients become available for plant uptake. Finally, in a subset of plots, we installed soil water collectors (called lysimeters) so we can measure the rate of loss nutrients from the ecosystem that are leached down the soil profile.
My next objective was to address an evolutionary question: which adaptive strategies of nutrient acquisition confer success in white-sand forests? Most critically, I wanted to understand the ability of the plant communities to overcome limitation by nitrogen or phosphorus, by fixing atmospheric nitrogen in symbiosis with soil bacteria, by exuding phosphatase enzymes (to gain phosphorus), or by symbiotic association with ecto- or endosymbiotic mycorrhizal fungi.
Excavating the roots of Caraipa densifolia, a tree endemic to the white-sand forests of Peru. This species associates with endosymbiotic mycorrhizal fungi to acquire soil nutrients.
To answer these questions, we excavated roots of white-sand specialist trees and their closely-related counterparts that grow in terra firme forests. In total, we collected roots from 240 individual trees throughout the 20 plots. I will be bringing these root samples back with me to Princeton where I will quantify their association with mycorrhizal symbionts and also quantify important morphological properties that can tell us if these plants employ a more “conservative” or “acquisitive” nutrient-foraging strategy. Finally, using previously-collected tree diversity and abundance data, I will be able to determine how community-level patterns of nutrient acquisition strategies change between these two forest types.
With the investigation of nutrient availability, nutrient losses and plant evolutionary strategies, my goal is to provide a holistic picture of ecosystem function in Amazonian forests. This knowledge is especially important in white-sands forests, which are under-studied, under threat, and are a reservoir of endemic species in the Amazon basin. More broadly, white-sand forests can lend insight into nutrient limitation of productivity in the tropics and offer a deeper understanding of the factors that constrain the land carbon sink.
A storm rolls in at San Juan de Hunguhuraul, a rural community along the upper Nanay River.
Tierney's summer research project was funded by the Program of Latin American Studies and a National Geographic Society Early Career Grant.