The variability of soil respiration (R_S) and its regulatory factors, e.g., soil temperature (T_S) and soil moisture (M_S) were investigated over five years in similar Ozark managed forests. The instruments used in for the measurements includes: EGM-4(RGA, EGM-4 Environmental Gas Monitor, PPsystems, UK) for R_S and TDR-100 for M_S ( Campbell Scientific Inc., Logan, UT). The Missouri Ozarks Forest Ecosystem Project (MOFEP) has three forest treatments including: control no-harvest (NHM), even-aged clear-cut harvest (EAM), and uneven-aged single-tree harvest (UAM).

Results/Conclusions

Our results showed that: seasonally, R_S shared a similar trend with T_S; interannually, total summer precipitation (defined from May-August) (PPT_S) was highly related to summer mean R_S in the EAM and UAM. The five-year measurements of R_S showed more fluctuations at EAM and UAM than at NHM. Overall, R_S was higher in wet years and lower in dry years in harvested treatments, although these differences were statistically significant only in 2007 when the area experienced a severe summer drought. The lower interannual variability in R_S at NHM suggested that NHM was more resistant to natural disturbance than the harvest treatments. The NDVI of the measured plots supports this conclusion too. There existed thresholds in the relationship between T_S and R_S (~23°C) over the five-year period for all the treatments. Based on minor changes in M_S, the T_S threshold changed monthly from May to August, with the lowest threshold in May (~19°C) and the highest in July (~25°C). The threshold also ranges from the lowest (~20°C) occurring in 2007 and the highest (~25°C) in 2008 annually. The low M_S and depleted substrate supply at high T_S is reasoned for the T_S thresholds.