CREAF

Biochar is a high carbon material resulting from biomass pyrolysis that, when applied to croplands, can increase soil carbon and soil water retention. Both effects are of critical importance in semi-arid regions, where carbon decline and desertification are the main drivers of soil degradation. Since most environmental services provided by soil are mediated by belowground biota, effects of biochar on soil microbial and invertebrate communities must be evaluated under field conditions before its agricultural application can be recommended. We tested maize biochar for its mid-term effect on soil microbes and micro-arthropods of a Mediterranean vineyard. We applied biochar to three field plots with neutral sandy loam soils at a dose of 5 Mg ha−1. During two years, we monitored the abundance of functional groups of soil micro-arthropods and estimated the biomass of soil microbial groups. We also analyzed the δ13C value of microbial PLFA biomarkers to determine biochar-C utilization by each microbial group taking advantage of the δ13C natural abundance differences between the applied biochar and the soil. Biochar addition significantly reduced soil microbial biomass but did not alter the functional microbial diversity nor the abundance or biodiversity of soil micro-arthropods. The contribution of biochar-C to the diet of most microbial groups was very low through the monitoring period. However, two gram-negative bacterial groups increased their biochar-derived carbon uptake under extreme soil dryness, which suggests that biochar-C might help soil microbes to overcome the food shortage caused by drought. The decrease in microbial biomass observed in our experiment and the concomitant decrease of SOM mineralization could contribute to the carbon sequestration potential of Mediterranean soils after biochar addition. © 2019 Elsevier B.V.

Reforesting and managing ecosystems have been proposed as ways to mitigate global warming and offset anthropogenic carbon emissions. The intent of our opinion piece is to provide a perspective on how well plants and ecosystems sequester carbon. The ability of individual plants and ecosystems to mine carbon dioxide from the atmosphere, as defined by rates and cumulative amounts, is limited by laws of physics and ecological principles. Consequently, the rates and amount of net carbon uptake are slow and low compared to the rates and amounts of carbon dioxide we release by fossil fuels combustion. Managing ecosystems to sequester carbon can also cause unintended consequences to arise. In this paper, we articulate a series of key take-home points. First, the potential amount of carbon an ecosystem can assimilate on an annual basis scales with absorbed sunlight, which varies with latitude, leaf area index and available water. Second, efforts to improve photosynthesis will come with the cost of more respiration. Third, the rates and amount of net carbon uptake are relatively slow and low, compared to the rates and amounts and rates of carbon dioxide we release by fossil fuels combustion. Fourth, huge amounts of land area for ecosystems will be needed to be an effective carbon sink to mitigate anthropogenic carbon emissions. Fifth, the effectiveness of using this land as a carbon sink will depend on its ability to remain as a permanent carbon sink. Sixth, converting land to forests or wetlands may have unintended costs that warm the local climate, such as changing albedo, increasing surface roughness or releasing other greenhouse gases. We based our analysis on 1,163 site-years of direct eddy covariance measurements of gross and net carbon fluxes from 155 sites across the globe. © 2018 John Wiley & Sons Ltd

The performance and persistence of rear-edge tree populations are relevant issues for conserving biodiversity because these stands harbor high intraspecific biodiversity and play a key role during periods of climate change. The occurrence of these populations is associated with the influence of heterogeneous topography, creating suitable refugia with regionally rare environmental conditions. Climate is changing at a global-scale, but little is known about the long-term impact on local climatic singularities and the associated taxa. We analyzed tree-ring growth chronologies of the two species (Fagus sylvatica and Quercus ilex) forming the evergreen-deciduous forest ecotone, constitutive of the rear-edge of F. sylvatica distribution. The study area is a coastal range with frequent fog immersion, which has been hypothesized to favor the persistence of F. sylvatica in Mediterranean peninsulas. We analyzed the long-term effect of fog on tree growth along a topographical gradient and the sensitivity of growth to rainfall and temperature. The annual number of foggy days has decreased by 62% over the last four decades, concomitant with increasing temperatures. Fog frequency was a relevant factor determining tree growth; fog during summer had positive effects on F. sylvatica growth mainly through a temperature buffering effect. The positive effect of fog on the growth of Q. ilex, however, was likely caused by a collinearity with rainfall. Q. ilex growth was less sensitive to climate than F. sylvatica, but growth of both species was enhanced by a positive early-summer water balance. Our results indicate that a decrease in fog frequency and an increase in temperature may generally benefit Q. ilex in this forest ecotone. Although future changes in rainfall and temperature matter most for the fate of rear-edge tree populations, local climatic singularities such as fog should also be considered. Those can have complementary effects that can swing the balance in ecotones and rear-edge tree populations such as those studied here. © 2018 Elsevier B.V.

Understanding ecosystem responses to compound disturbance regimes and the influence of specific sequences of events in determining ecosystem shifts remains a challenge. We use a modelling framework for Mediterranean-type ecosystems to assess the effects of fire–drought interactions on long-term vegetation dynamics and to identify disturbance-driven changes in trait composition (tree seeder vs. tree resprouter prevalence) and ecosystem state (forest vs. non-forest). Changes in tree seeder and the tree resprouter dominance show nonlinear, threshold-type trends over gradients of increasing compound disturbance frequency. Vegetation composition thresholds mostly occur in a narrow range of the compound fire–drought disturbance space. Additionally, trait compositional switches and the likelihood of sudden changes in ecosystem state are promoted by fire-drought interactions. Distinct sequences of disturbance events cause vegetation transitions, disrupting ecosystem resilience, even under moderate recurrence of individual disturbances. An extreme drought year followed by one or two large fire events promotes shifts from resprouter- to seeder dominance. Contrastingly, a large crown fire followed by an extreme drought promotes changes from seeder to resprouter dominance. This disturbance sequence is also a mechanism strong enough to trigger sudden shifts in ecosystem state (from forest to non-forest). Synthesis. Thresholds of change in vegetation composition occur over a narrow range of the modelled gradients of compound fire-drought recurrence, and the loss of ecosystem resilience is contingent on particular sequences of disturbance events. Overall, our findings highlight that disturbance interactions define the relative location of tipping points in ecosystem state, and that effects and feedbacks of compound disturbance regimes increase the long-term likelihood of sudden ecosystem shifts and, therefore, uncertainty in predicting vegetation state. © 2018 The Authors. Journal of Ecology © 2018 British Ecological Society