The effects of differing harvesting intensity on soil respiration rates and temperature sensitivity in a northern mixed-deciduous fores

Background/Question/Methods

Physical disturbances to forest soils and alterations in stand structure through harvesting can affect soil physiochemical and structural properties, altering soil respiration (R_s) rates and associated soil CO₂ efflux (FCO₂). Intensification of harvesting for commercial biomass threatens to amplify disturbance effects. However, the mechanisms behind harvesting effects on R_s are not well understood; existing studies of partial-harvest methods in temperate deciduous forests show mixed results regarding whether partial-harvesting increases or decreases FCO₂. To address this, we assessed harvesting intensity on R_s rates by measuring FCO₂ between June and October for three years following a 2010 winter harvest. Fifteen 1-ha plots were randomly distributed between an unharvested control, a tree-length (TL) partial-harvest (~30% basal area removed), or an intensified biomass (BIO) harvest (~40-50% basal area removed). An exponential regression model fitted between FCO₂ and temperature (r²=0.75-0.91) was used to determine annual Q₁₀ values. Q₁₀ was used to standardise R_s rates between treatments to the mean temperature at each measurement period (R_smean).

Results/Conclusions

Initial FCO₂ measurements were similar between treatments but by September 2010 harvested R_s was 38-43% higher than control (p=<0.01). Harvested R_s remained higher than controls throughout 2011-12, fluctuating between mid-summer lows (11-18%, p=>0.05) and autumn peaks every year (18-43%, p=<0.05). No significant difference was found between TL and BIO R_s. Q₁₀ was significantly lower for harvested plots than controls in 2010, but by 2012 control and TL values were similar and BIO was significantly higher. Temperatures were higher in harvested plots and accounted for ~42% of difference between harvested and control R_s in 2010 but declined to <14% by 2012. Overall harvested R_s declined 14-17% relative to control from 2011-12 while harvested R_smean increased 8-12%. Our results indicate partial-harvest initially suppressed Q₁₀ but increased FCO₂ which we attribute to increased decomposition rates from greater insolation, mixing of soils, and decomposing fine roots. FCO₂ attributed to elevated soil temperatures from harvesting declined as the understory and canopies regrew, decreasing harvested R_s, but the respiration model indicated these were largely offset by other harvesting-related effects and increasing Q₁₀ values. We suggest these effects are due to a combination of rising Rs rates from increasing root necromass decomposition and respiration from new roots.