Spatial and seasonal variation in soil respiration along a slope in a rubber plantation and a natural forest in Xishuangbanna, Southwest China

Soil respiration is a key component of the global carbon cycle, and even small changes in soil respiration rates could result in significant changes in atmospheric CO_2 levels. The conversion of tropical forests to rubber plantations in SE Asia is increasingly common, and there is a need to understand the impacts of this land-use change on soil respiration in order to revise CO_2 budget calculations. This study focused on the spatial variability of soil respiration along a slope in a natural tropical rainforest and a terraced rubber plantation in Xishuangbanna, Southwest(SW) China. In each land-use type, we inserted 105 collars for soil respiration measurements.Research was conducted over one year in Xishuangbanna during May, June, July and October 2015(wet season) and January and March 2016(dry season). The mean annual soil respiration rate was 30% higher in natural forest than in rubber plantation and mean fluxes in the wet and dry season were 15.1 and 9.5 Mg C ha^(-1) yr^(-1) in natural forest and 11.7 and 5.7 Mg C ha^(-1) yr^(-1) in rubber plantation. Using a linear mixedeffects model to assess the effect of changes in soil temperature and moisture on soil respiration, we found that soil temperature was the main driver of variation in soil respiration, explaining 48% of its seasonal variation in rubber plantation and 30% in natural forest. After including soil moisture, the model explained 70% of the variation in soil respiration in natural forest and 76% in rubber plantation. In the natural forest slope position had a significant effect on soil respiration, and soil temperature and soil moisture gradients only partly explained this correlation. In contrast, soil respiration in rubber plantation was not affected by slope position, which may be due to the terrace structure that resulted in more homogeneous environmental conditions along the slope. Further research is needed to determine whether or not these findings hold true at a landscape level.

Seedling survival after simulating grazing and drought for two species from the Pamirs,northwestern China

For plant populations to persist,seedling recruitment is essential,requiring seed germination,seedling survival and growth.Drought and grazing potentially reduce seedling recruitment via increased mortality and reduced growth.We studied these seederelated processes for two species indigenous to the Pamir Mountains of Xinjiang in northwestern China:Saussurea glacialis and Plantago lessingii.Seeds collected from Taxkorgan,Xinjiang,had a viability rate of 15.8%for S.glacialis but 100%for P.lessingii.Of the viable seeds,the highest germination rates were 62.9%for S.glacialis and 45.6%for P.lessingii.In a greenhouse experiment,we imposed a series of stressful conditions,involving a combination of simulated grazing and drought events.These had the most severe impact on younger seedlings.Modelling showed that 89%of S.glacialis mortality was due to early simulated grazing,whereas 80%of P.lessingii mortality was due to early simulated drought.Physiological differences could contribute to their differing resilience.S.glacialis may rely on water storage in leaves to survive drought events,but showed no shifts in biomass allocation that would improve grazing tolerance.P.lessingii appears more reliant on its root system to survive grazing,but the root reserves of younger plants could be insufficient to grow deeper in response to drought.After applying all mortality factors,17.7 seedlings/parent of P.lessingii survived,while only<0.1 seedlings/parent of S.glacialis survived,raising concerns for its capacity to persist in the Pamirs.Inherent genetic differences may underlie the two species’contrasting grazing and drought responses.Thus,differing conservation strategies are required for their utilization and protection.