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.
Highlights
Soil respiration (Rs ) has been measured in most terrestrial biomes, from deserts, grasslands and shrublands, to forests in temperate, boreal, tropical and Mediterranean regions [1]
The impact of forest decline on Rs is likely to vary among ecosystems depending on how sensitive the roots of dominant species are to defoliation, and how fast microbes are in the decomposing litter and other plant species in colonizing the canopy gaps opened by defoliation and tree death
We explored the influence of soil temperature and water availability on Rs seasonality in two years of contrasted climate conditions, and whether soil temperature, soil water content (SWC) or their influence on Rs changed across the tree decline stages
Summary
Soil respiration (Rs ) has been measured in most terrestrial biomes, from deserts, grasslands and shrublands, to forests in temperate, boreal, tropical and Mediterranean regions [1]. From forest studies where carbon (C) fluxes have been partitioned among their components, it has been shown that Rs typically accounts for more than half of the total annual ecosystem respiration [2,3,4]. Tree ageing and forest succession involve changes in the overstory and, in turn, in water, nutrient and C fluxes between the atmosphere and the rhizosphere that can translate into different rates of Rs [5,6]. The impact of forest decline on Rs is likely to vary among ecosystems depending on how sensitive the roots of dominant species are to defoliation, and how fast microbes are in the decomposing litter and other plant species in colonizing the canopy gaps opened by defoliation and tree death. Ávila et al [11] found that the effect of tree decline on soil variables differed in woodlands and closed forests, varied between seasons, and was higher as declining trees were larger
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