Regeneration in the Understory of Declining Overstory Trees Contributes to Soil Respiration Homeostasis along Succession in a Sub-Mediterranean Beech Forest

Research Highlights: Tree decline can alter soil carbon cycling, given the close relationship between primary production and the activity of roots and soil microbes. Background and Objectives: We studied how tree decline associated to old age and accelerated by land-use change and increased drought in the last decades, affects soil properties and soil respiration (R<inf>s</inf>). Materials and Methods: We measured R<inf>s</inf> over two years around centennial European beech (Fagus sylvatica L.) trees representing a gradient of decline in a sub-Mediterranean forest stand, where the number of centennial beech trees has decreased by 54% in the last century. Four replicate plots were established around trees (i) with no apparent crown dieback, (ii) less than 40% crown dieback, (iii) more than 50% crown dieback, and (iv) dead. Results: Temporal variations in R<inf>s</inf> were controlled by soil temperature (T<inf>s</inf>) and soil water content (SWC). The increase in R<inf>s</inf> with T<inf>s</inf> depended on SWC. The temperature-normalized R<inf>s</inf> exhibited a parabolic relationship with SWC, suggesting a reduced root and microbial respiration associated to drought and waterlogging. The response of R<inf>s</inf> to SWC did not vary among tree-decline classes. However, the sensitivity of R<inf>s</inf> to T<inf>s</inf> was higher around vigorous trees than around those with early symptoms of decline. Spatial variations in R<inf>s</inf> were governed by soil carbon to nitrogen ratio, which had a negative effect on R<inf>s</inf>, and SWC during summer, when drier plots had lower R<inf>s</inf> than wetter plots. These variations were independent of the tree vigor. The basal area of recruits, which was three times (although non-significantly) higher under declining and dead trees than under vigorous trees, had a positive effect on R<inf>s</inf>. However, the mean R<inf>s</inf> did not change among tree-decline classes. These results indicate that R<inf>s</inf> and related soil physico-chemical variables are resilient to the decline and death of dominant centennial trees. Conclusions: The development of advanced regeneration as overstory beech trees decline and die contribute to the R<inf>s</inf> homeostasis along forest succession.