Age and thinning effects on the temperature sensitivities of respiration in loblolly pine plantations in eastern North Carolina

Age and silvicultural interventions are two important factors to characterize the development of commercial plantations. Effects of the two factors are of importance in quantifying the carbon dynamics in these artificial systems, but remain unclear. From 9 years (2005-2013) of carbon exchange observations in a mature (13 years old in 2005) and a young (2 years old in 2005) loblolly pine plantations located on the lower coastal plain in eastern North Carolina, we investigated the difference in temperature sensitivities (Q10) of ecosystem respiration (ER) of the two plantations of different ages. The mature plantation was thinned in August 2009, therefore, we compared the ER before and after thinning to estimate the potential response of Q10 to thinning in the mature plantation. Before thinning (years of 2005 - early half year of 2009), Q10 of ER in the mature plantation was similar across years. Even during a severe drought (e.g. 2007), Q10 did not differ from other years (p = 0.2 - 0.8 between years). The mean Q10 before harvesting was 2.33±0.05 (mean±SE). After thinning (years of 2010-2013), there was significant inter-annual variation in Q10 with an overall estimate at 2.23±0.05. The young plantation also exhibited significant inter-annual variations in Q10, and the overall Q10 was 2.99±0.11, higher than that of the mature plantation. The increased variability of temperature sensitivity after thinning in the mature plantation might be associated with the altered physical environment by thinning such as soil bulk density and soil water content, whereas the similar pattern in the young plantation might result from the primary contribution of heterotrophic respiration and also be confounded with the fast growth of young seedlings.