## 6 Discussion  
Soil respiration rates and the contribution of this flux to \(R_{s}\) varies considerably among forest types, though the fundamental variables regulating carbon cycling process rates are generally conserved across ecosystems [53]. In the mixed deciduous forests of Michigan, USA, respiration from soils is the largest component of ecosystem respiration, comprising approximately 70% of the total flux of CO\({}_{2}\)[30]. Ecological transitions and regional climatic changes can affect Rs. Carbon sequestration by an ecosystem is determined as a small difference between two large fluxes: respiration, which emits CO2 to the atmosphere and gross primary productivity (\(GPP\)) that takes up CO\({}_{2}\) from the atmosphere through the process of photosynthesis. The small difference between these two opposing fluxes makes the net CO\({}_{2}\) sequestration rate highly sensitive to any change in either respiration of \(GPP\). These climatic changes have motivated the scientific community to quantify \(R_{s}\) and to better understand the factors controlling it, particularly during and after disturbance [54]. Primary climatic controls on \(R_{s}\) are temperature, solar radiation, and precipitation [55]. Soil respiration increases as soil temperatures rise, because microbial community abundance and activity both respond positively to warming [27,56]. Drought and extremely wet conditions limit microbial activity and decrease respiration [57]. Over periods of hours to days, photosynthetic activity also controls carbohydrate supply from leaves to roots resulting in increased C availability to mycorrhizal fungi and microorganisms in the root environment (rhizosphere), thereby increasing \(R_{s}\)[58, 59, 60, 61]. Forest disturbance can change the rates of photosynthetic activity through a reduction in live leaf area and thus \(GPP\), and resulting in changes the soil's energy budget as temperature and humidity shift in response to reduced shading and water uptake by trees. Disturbance also changes canopy structure and the sub-canopy environment in ways that may affect \(R_{s}\), modifying light and nutrient use efficiency [62], roughness length, canopy-atmosphere coupling, and increasing turbulence mixing above the soil [63,64]. In both 2008 and 2009 respiration was lower in the treatment site than in the control. Due to the lack of respiration measurements prior to the girdling treatment we cannot rule out that this difference is a result of the treatment. However, the difference in respiration between the sites was rather constant over the period from 3 months to a year and a half past the treatment, despite stem girdling and eventual and slow mortality (trees where fully alive in summer 2008, and mostly dead in 2009) of canopy dominant trees that occupied roughly 30% of the canopy in 2008. Additionally, the dynamics and causality structure of the drivers of respiration in both sites was not different. Given these two indications, it seems unlikely that that difference is a result of the treatment. This was not because of an ineffective treatment. In summer 2009 the aspen and birch trees were visibly defoliated. However our study indicates resilience of ecosystem functioning to moderate disturbance, and particularly the soil ecosystem which maintained high respiration rates despite a reduction of photosynthates from girdled trees. In part, this resilience is due to compensatory growth and increased productivity of the remaining un-girdled trees from increased light and nitrogen availability [65]. Alternatively, soil respiration may have declined in the treatment very abruptly following the 2008 girdling, shortly before our chamber measurement started. Several studies document an almost immediate decline in \(R_{s}\) that reaches its lowest point within days following stem girdling [54,66], suggesting that the exhaustion of stored labile carbohydrates progressively limits root and rhizosphere respiration.