The response of pinon and juniper leaf respiration to drought and warming

We established a rainfall and temperature manipulation experiment in June 2012 in a pinon-juniper woodland near Los Alamos, NM. In situ, mature trees were exposed to one of the five following treatments: drought (-40% precipitation), warming (+4° C), drought and warming, control, and chamber control. Respiration of leaf tissue (CO2 efflux) was measured once per month during the growing season (April - October) from 2012 to 2013 using polycarbonate branch chambers and a closed-path infra-red gas analyzer. Growth was recorded concurrent with respiration during the 2013 and 2014 growing seasons. In October 2013, we measured respiration temperature response curves over a 10° to 20° C range overnight. Our results show that heated treatments have higher cumulative rates of leaf respiration than drought or control groups, generally applied to both species, during most of the growing season. During periods of high growth and high heat, instantaneous respiration rates were highest. Little growth was observed in heat or drought treatments compared to control treatments each year measured and in October 2013, growth had ceased in all treatments for both species. The October 2013 measurement cycle revealed no differences between treatments in the Q10 of respiration, with a mean of 2.6 for each species. Data from previous months suggested a possible decoupling of respiration from temperature for the non-drought juniper groups, but the October measurement period showed a well-coupled response. These results suggest that neither the heat nor drought treatment affected leaf maintenance respiration in October 2013. Although measurements during the two seasons of protracted precipitation and temperature stress of the treatment groups indicated diverging patterns in the respiration signal, by the end of the second season, any acclimation or temperature decoupling response was not observed, possibly due in whole or part by a large input of precipitation in September 2013.