Response of pinion and juniper respiration to drought and warming

Background/Question/Methods

Drought and temperature-induced tree mortality is believed to be occurring globally, though the physiological mechanisms underlying documented mortality events are not well understood.  Understanding the controls on forest carbon cycling and their responses during drought and temperature stress is critical in informing vegetation models and thus predictions of forest response to climate change.  Pinon pine (Pinus edulis) and oneseed juniper (Juniperus monosperma) are widespread species in forests of the Southwestern United States and known to be susceptible to mortality due to altered precipitation and temperature regimes.  Respiration is a key component of the carbon budget and its response to abiotic stress is thought to play a role in mortality or survival.  The ability of these species to acclimate respiration to altered temperature and/or precipitation is a key model parameter, but is currently not known.  A careful examination of the response of pinon and juniper respiration to increased temperature and drought conditions is thus a necessary step in predicting their future distribution in a changing environment.

We established a rainfall and temperature manipulation experiment in a pinon-juniper woodland near Los Alamos, NM.  In-situ trees were exposed to one of five treatments:  warming alone, drought alone, warming plus drought, ambient control, and chamber control. 

Respiration measurements were conducted on the bole of each tree once per month between June and November 2012.  A polycarbonate gas-exchange chamber was temporarily sealed to the bole of each tree during the night of each measurement cycle.  Air was circulated from the chamber to a closed-path infra-red gas analyzer and CO₂ flux was measured hourly.

Results/Conclusions

Preliminary analysis of results shows marked differences between the two species.  Heated pinon showed elevated respiration and an unchanging Q₁₀ of respiration while all other pinon treatments were no different from ambient control in either parameter.  Juniper heat+drought and heat alone began the season with higher Q₁₀ values than other treatments but exhibited a diminishing Q₁₀ through the August measurement whereby the values matched control treatments.  Similarly, respiration was higher for both juniper heat+drought and heat alone at treatment initiation, but respiration declined to match control treatments by July 2012.

These results suggest that elevated temperature alone may increase pinon respiration over the course of one season, with no acclimation.  Juniper however showed marked acclimation, which may have served to reduce respiration to match trees unaffected by such imposed precipitation or temperature anomalies.