How Increasing Temperatures Affect Tropical Forests

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Tropical forests take in and store more carbon than any other biome in the world, but increasing temperatures may pose a threat to this invaluable service. This research aims to explore how temperature affects key tropical forest functions, such as plant photosynthesis and soil respiration. To do this, the Tropical Responses to Altered Climate Experiment (TRACE) project will use infrared heating to warm the soil and plants of the understory, as well as warming individual leaves and branches in the forest canopy. Within TRACE, scientists are working together to measure the potential impacts of increasing temperatures on tropical forest productivity, carbon cycling, plant physiology, and soil fertility. The ultimate goal of the work is to improve our understanding of how tropical forests will respond to altered temperatures, and to determine how their response could be important at local, regional, and global scales. This integrated experiment is the first of its kind in any tropical forest, and we expect to learn important truths about the fundamental relationships between temperature and how tropical forests work.

Background & Importance

Six metal poles support heating lamps several feet above the tropical floor. Tropical forest vegetation surrounds the structure
Experimental warming plot in Tropical Rainforest in Puerto Rico’s Luquillo Experimental Forest inside El Yunque National Forest. Infrared warming lamps are held by posts embedded in concrete (spring 2016). (Credit: Sasha Reed, USGS. Public domain.)

Tropical forests contain ~25% of Earth’s terrestrial biomass and exchange more carbon and energy with the atmosphere than any other type of ecosystem. Accordingly, our poor understanding of how tropical forests will respond to changing temperatures severely hold back our understanding of how tropical forests will function into the future. To meet the increasing need for improved understanding of tropical forest responses to changing temperature, we have constructed a field warming experiment in a wet tropical forest in Puerto Rico. This research will allow us to assess temperature responses of the most influential and active rain forest tissues and organisms: leaves, fine roots, and soil microbes. Our research focus on both above- and belowground parts of the forest will provide an integrated understanding of carbon storage and flux, which is critical to considerations of how temperature and precipitation effects on tropical forests will create feedbacks to future carbon cycling and climate at the global-scale. Our specific objectives are two-fold: 

1) Assess the mechanisms behind and effects of warming on carbon and nutrient cycling and storage in tropical forest soils

2) Investigate threshold temperature responses of both canopy and understory tropical tree foliage

We expect this work to make significant advances in our understanding of coupled biogeochemical processes in a globally important and poorly understood ecosystem that has strong potential to create feedbacks to future climate. Also, this field warming experiment is the first of its kind in any tropical forest, and the experimental research would be the first to investigate warming response to tropical processes from coordinated plant-soil perspectives.  Our focus on the mechanisms regulating temperature responses will allow us to consider the results beyond that of a single tropical forest site, and the work will provide critical information regarding the vulnerability and adaptation potential of the only tropical forest in the U.S. National Forest System (The Luquillo Experimental Forest inside El Yunque National Forest). 

General Methods

We warm the understory vegetation and upper layers of soil with an array of infra-red heaters. We use a complimentary warming of individual canopy leaves and branches to assess the relationship between temperature and canopy photosynthesis and respiration. We assess many fluxes of CO2 (including leaf photosynthesis and respiration and soil respiration), as well as monitoring the stocks and fluxes of numerous carbon and nutrient pools. At the same time, we are assessing the effect of altered temperature on plant and soil microbial community composition and soil moisture.

This is a highly collaborate research endeavor and involves scientists from USGS, the U.S. Forest Service, U.S. National Laboratories, and Michigan Tech University. Additionally, we together with a USGS Powell Center (https://powellcenter.usgs.gov/) data synthesis activity, where we are using published data and modeling approaches to helping assess how tropical forests will respond to increasing temperatures.

TRACE project’s three principal investigators in Puerto Rico with each person wearing a poncho.
TRACE project’s three principal investigators in Puerto Rico: Sasha Reed (left-USGS), Tana Wood (middle-US Forest Service), and Molly Cavaleri (right-Michigan Tech) (2010). (Credit: Sasha Reed, USGS. Public domain.)
USGS Scientist, Sasha Reed, with a sloth in a Brazilian rain forest.
USGS Scientist, Sasha Reed, with a sloth in a Brazilian rain forest. (Credit: Sasha Reed, USGS. Public domain.)