Ecosystem responses to soil warming in New England forests

Seeta Sistla, with Jerry Melillo

In an even-aged mixed hardwood stand in the Harvard Forest, I work with Dr. Melillo's research group to characterize an array of plant and soil community responses to chronic soil warming.  Currently, Northern Hemisphere temperate and boreal forests are critical sinks for CO₂, accounting for a large fraction of the global terrestrial C sink.  Whether these forests will continue to act as a net C sink depends largely upon plant and soil responses to climate change.  Despite numerous studies on the influence of these drivers on aboveground vegetative and belowground biogeochemical, microbial, and rhizosphere responses, the causal links between them remain poorly understood.  However, because plant and soil microbial communities act together, a combined aboveground-belowground research paradigm is critical to projecting whether Northern forests will remain a C sink or will transition to a source as global climate change occurs over the next century.  

To better describe the potential net C balance as northern forests warm, I am working on a project concurrently study belowground rhizosphere and aboveground vegetative dynamics as part of an integrated characterization of the biotic and abiotic responses to chronic soil warming of a temperate forest experimental area. 

The soil in a 30 x 30 m experimental area in the Harvard Forest has been heated to 5C above ambient area soil temperature since 2003.  We observed two primary responses in this experiment:  (1) an increased rate of CO₂ loss from the heated soils due to increased microbial decomposition and potentially, increased root respiration; (2) an increased rate of C storage in woody tissues of certain species due to an increased soil N availability and lengthened growing season. After 4 years, this ecosystem acts as a net C source. Based on our previous small-scale warming experiment, we hypothesize that over time the elevated soil respiration rate will decline to control levels, while N mineralization remain elevated.  The decoupling of C and N cycling observed in the previous study may result in elevated N availability and drive increased vegetative C storage while soil respiration declines.

Currently, I am working on measurements of photosynthetic and related leaf physiological characteristics in response to soil warming in order to understand species-specific responses to the soil warming driver.  I am also following belowground responses to warming, with the goal of trying to understand how the driver influences roots and associated rhizosphere dynamics.  These studies include an adaptation of a novel method for reversibly and non-invasively separating root respiration from total soil respiration and several strategies to quantify changes in fine-root biomass in response to warming. This work will contribute towards increasing our understanding of the relative importance of changing aboveground and belowground dynamics in regulating the net C balance of warming northern forests.

 

 

Updated 05/22/2008
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