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

Abstract

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 (Rs). Materials and Methods: We measured Rs 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 Rs were controlled by soil temperature (Ts) and soil water content (SWC). The increase in Rs with Ts depended on SWC. The temperature-normalized Rs exhibited a parabolic relationship with SWC, suggesting a reduced root and microbial respiration associated to drought and waterlogging. The response of Rs to SWC did not vary among tree-decline classes. However, the sensitivity of Rs to Ts was higher around vigorous trees than around those with early symptoms of decline. Spatial variations in Rs were governed by soil carbon to nitrogen ratio, which had a negative effect on Rs, and SWC during summer, when drier plots had lower Rs 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 Rs. However, the mean Rs did not change among tree-decline classes. These results indicate that Rs 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 Rs homeostasis along forest succession.

Responses of Soil Respiration to the Interactive Effects of Warming and Drought in Alfalfa Grassland on the Loess Plateau

“…In our experiment, drought relatively reduced Rs by 3.4%. This was consistent with previous studies on the semi-arid grassland ecosystem in Inner Mongolia and the Mediterranean ecosystem [14,49,[51][52][53]. In arid and semi-arid ecosystems, water availability is the primary factor influencing the productivity and carbon source for Rs.…”

Linking functional composition moments of the sub-Mediterranean ecotone with environmental drivers

“…Additionally, slope, orientation, altitude and soil depth were also measured in the same plots (Supplementary Material, Appendix S3, Table S3). For the description of the methods employed to measure total soil N see Rodrıǵuez-Calcerrada et al (2019); for the methods employed to measure the rest of abiotic variables see .…”

Linking Soil CO2 Efflux to Individual Trees: Size-Dependent Variation and the Importance of the Birch Effect

“…The locations of background sampling locations were adjusted slightly upon collar installation to exclude local low points with a tendency to accumulate water and locations with shallow soils or exposed bedrock that prevented insertion of flux collars. Patches of understory vegetation may also contribute to soil F CO2 [35], and so were also avoided. After study trees were selected, six soil efflux collars were inserted at a distance of approximately one meter from the base of each tree.…”

“…However, counterexamples exist [10,31], and several chrono sequence studies have found reduced F CO2 with stand age in even-aged plantations [32,33], or no consistent relationship [34]. Rodríguez-Calcerrada et al, [35] tested for associations between visible crown health indicators and F CO2 in an open woodland in Spain but found that increased plant recruitment near declining trees offset potential reductions in F CO2 related to tree decline. The character of C supply could theoretically shift as trees age from active extrusion of labile root exudates by more productive trees to less bioavailable necrotic tissues, but the connection between local tree effects on F CO2 and tree senescence or health status has received little attention.…”