Downed woody debris carbon emissions in a European temperate virgin forest as driven by species, decay classes, diameter and microclimate.

Downed woody debris (DWD) plays an important role as regulator of nutrient and carbon (C) cycling in forests, accounting for up to the 20 % of the total C stocks in primary forests. DWD persistence is highly influenced by microbial decomposition, which is determined by various environmental factors, including fluctuations in temperature and moisture, as well as in intrinsic DWD properties determined by species, diameter, or decay classes (DCs). The relative importance of these different drivers, as well as their interactions, remains largely unknown. Moreover, the importance of DWD for C cycling in virgin forests remains poorly understood, due to their scarcity and poor accessibility. To address this research gap, we conducted a study on DWD respiration (RDWD), in a temperate virgin forest dominated by European beech and silver fir. Our investigation analysed the correlation between RDWD of these two dominant tree species and the seasonal changes in climate (temperature and moisture), considering other intrinsic DWD traits such as DCs (1, 2 and 4) and diameters (1, 10 and 25 cm). As anticipated, RDWD (normalized per gram of dry DWD) increased with air temperature. Surprisingly, DWD diameter also had a strong positive correlation with RDWD. Nonetheless, the sensitivity to both variables and other intrinsic traits (DC and density) was greatly modulated by the species. On the contrary, water content, which exhibited a considerable spatial variation, had an overall negative effect on RDWD. Virgin forests are generally seen as ineffective C sinks due to their lack of net productivity and high respiration and nutrient turnover. However, the rates of RDWD in this virgin forest were significantly lower than those previously estimated for managed forests. This suggests that DWD in virgin forests may be buffering forest CO2 emissions to the atmosphere more than previously thought.

Contrasting net primary productivity and carbon distribution between neighboring stands of Quercus robur and Pinus sylvestris.

Standing biomass, net primary production (NPP) and soil carbon (C) pools were studied in a 67-year-old pedunculate oak (Quercus robur L.) stand and a neighboring 74-year- old Scots pine (Pinus sylvestris L.) stand in the Belgian Campine region. Despite a 14% lower tree density and a lower tree height in the oak stand, standing biomass was slightly higher than in the pine stand (177 and 169 Mg ha(-1) in oaks and pines, respectively), indicating that individual oak trees contained more biomass than pine trees of similar diameter. Moreover, NPP in the oak stand was more than double that in the pine stand (17.7 and 8.1 Mg ha(-1) year(-1), respectively). Several observations indicated that soil organic matter accumulated at higher rates under pines than under oaks. We therefore hypothesized that the pines were exhibiting an age-related decline in productivity due to nutrient limitation. The poor decomposability of pine litter resulted in the observed accumulation of organic matter. The subsequent immobilization of nutrients in the organic matter, combined with the already nutrient-poor soil conditions, resulted in a decrease in total NPP over time, as well as in a substantial shift in the allocation of NPP toward fine roots. In the oak stand, litter is less recalcitrant to decay and soil acidity is less severe; hence, organic matter does not accumulate and nutrients are recycled. This probably explains why NPP was much higher in the oaks than in the pines and why only a small proportion of NPP was allocated to oak fine roots.

Soil respiration in a mixed temperate forest and its contribution to total ecosystem respiration.

Soil respiration (SR) was measured with an infrared gas analyzer in nine plots representative of the heterogeneous vegetation in a mixed coniferous-deciduous forest in the Belgian Campine region. Selected plots included the two most representative overstory species (Pinus sylvestris L. and Quercus robur L.) in combination with the most representative understory species of the forest. A model that includes temperature and water as the main controlling variables was fitted to the data. We found large spatial variability in SR among plots, with typically lower fluxes under the coniferous overstory than under the deciduous overstory (means of 4.8 +/- 0.4 and 8.8 +/- 0.5 Mg C ha(-1) year(-1), respectively). Total annual soil carbon (C) emissions were estimated by weighting fluxes from different types of vegetation according to their relative contribution to the footprint area of the eddy covariance flux measurement. The relative contribution of the two main tree species to the footprint-weighted total SR varied among seasons with the more abundant coniferous overstory contributing the most to total SR during most of the year. Nonetheless, during summer, the contribution of deciduous plots to total SR was disproportionally high because of the more pronounced seasonality of belowground metabolic activity. Net ecosystem carbon dioxide exchange was measured by eddy covariance, and we estimated total ecosystem respiration (TER) with footprint-constrained nighttime fluxes. Mean total annual SR and TER were 6.1 +/- 0.11 and 9.1 +/- 1.15 Mg C ha(-1) year(-1), respectively. The 95% confidence interval of the ratio of annual SR:TER ranged from 0.58 to 0.76, with a mean of 0.67. The contribution of SR to TER tended to vary seasonally, with minimum contributions during summer (less than 50% of TER) and maximum contributions during winter (about 94% of TER).

Interactive effects of temperature and precipitation on soil respiration in a temperate maritime pine forest.

Soil respiration (SR) was monitored periodically throughout 2001 in a Scots pine (Pinus sylvestris L.) stand located in the Belgian Campine region. As expected for a temperate maritime forest, temperature was the dominant control over SR during most of the year. However, during late spring and summer, when soil water content (SWC) was limiting, SR was insensitive to temperature (Q(10) = 1.24). We observed that during prolonged rain-free periods, when SWC was less than 15% (v/v), SR decreased dramatically (up to 50%) and SWC took over control of SR. During such drought periods, however, rain events sometimes stimulated SR and restored temperature control over SR, even though SWC in the mineral soil was low. We hypothesize that restoration of temperature control occurred only when rain events adequately rewetted the uppermost soil layers, where most of the respiratory activity occurred. To quantify the rewetting capacity of rain events, an index (I(w)) was designed that incorporated rainfall intensity, time elapsed since the last rain event, and atmospheric vapor pressure deficit (a proxy for evaporative water losses). To simulate SR fluxes, a model was developed that included the effects of soil temperature and, under drought and non-rewetting conditions (I(w) and SWC < threshold), an SWC response function. The model explained 95% of the temporal variability in SR observed during summer, whereas the temperature function alone explained only 73% of this variability. Our results revealed that, in addition to temperature and SWC, rain plays a role in determining the total amount of carbon released from soils, even in a maritime climate.