Influence of Timber Harvesting Alternatives on Forest Soil Respiration and Its Biophysical Regulatory Factors over a 5-year Period in the Missouri Ozarks

We investigated the variability of soil respiration and several potential regulatory factors and modeled their interrelationships from May to August over a 5-year period in oak forests subjected to alternative harvesting treatments as part of the Missouri Ozark Forest Ecosystem Project (MOFEP). Treatments included even-aged management (EAM), uneven-aged management (UAM), and no-harvest management (NHM) and were implemented 7-8 years prior to this study. Summer mean soil respiration did not differ among the treatments, possibly because of changes in treatment differences in the separate months and years that tended to cancel each other out when averaged. Summer mean soil respiration and soil moisture tended to be higher in wet years (2004, 2006, and 2008) and lower in dry years (2005 and 2007) in EAM and UAM than in NHM. Summer precipitation was assumed to be the primary driver of variability in summer mean soil respiration through its control on soil moisture and the normalized difference vegetation index (NDVI) in the harvested forests. Nonlinear models using soil temperature, soil moisture and day-of-the-year (DOY) were used to predict within-summer soil respiration for all the treatments. A sensitivity analysis of the model using 30 min interval data suggested that soil respiration was more sensitive to soil moisture in the EAM and UAM treatments than in NHM. We also found a change in the soil respiration-soil temperature relationship in the summer for all the treatments. Simulated data sets that removed the covariance structure between soil temperature and moisture suggested that the change in the respiration-temperature relationship resulted from the combined effect of moisture stress and low temperature sensitivity at high temperatures during July and August. Simulations also showed the effect of moisture stress to be more limiting to soil respiration in the harvested forests than in the control at high temperatures, even resulting in a negative relationship at high temperatures