Cambiamenti nel regime pluviometrico in ecosistemi mediterranei: il progetto MIND

Changes in rainfall patterns in Mediterranean ecosystems: the MIND project. Will Mediterranean terrestrial ecosystems be affected by the expected changes in precipitation regimes? If so, by how much and in which direction? These questions are at the basis of the research performed in context of the EU MIND project, whose key objectives were: i) to investigate the potential effects of increasing drought on Mediterranean terrestrial ecosystems at the process, ecosystem and regional scales and ii) to assess ecosystem vulnerability to changes in rainfall patterns. A network of experimental study sites has been created in Portugal, Spain, France and Italy, where field manipulations alter the amount of water available to the ecosystem. The most up-to-date methods of ecophysiology, micrometeorology, soil ecology and remote sensing have been used to elucidate the mechanisms that regulate the response of vegetation and soil to changes in water availability. This information is providing the basis for the implementation and validation of simulation models capable of predicting the drought response of Mediterranean terrestrial ecosystems, and their vulnerability to future climate change, on a larger scale. The out-coming results are elucidating how water availability affects plant ecophysiological processes, the dynamics of soil carbon and the overall exchange of mass and energy between the land and the atmosphere. This paper focuses on some of the important, yet preliminary, results on C and energy fluxes that have been obtained at the large scale troughfall manipulation experiment (Tolfa, Italy), in a forest dominated by Arbutus unedo L

Litter decomposition and soil CO2 efflux on the Mediterranean island of Pianosa

Mediterranean ecosystems are particularly vulnerable to changes in climate and land use forecasted for the near future, with likely perturba- tions of the carbon cycle. The aim of our study was to quantify particular aspects of the carbon cycle in typical Mediterranean ecosystems, in particular (1) the decay rates of litter from common tree and shrub species, (2) the efflux of CO2 from the soil and its relation to soil and litter moisture, and (3) the dynamics of the stable isotope 13 C during litter decomposition. Field work was conducted on the island Pianosa, which comprises a range of common Mediterranean ecosystem types. Litter decay rates of three selected species (Cistus monspenliensis, Pistacia lentiscus and Juniperus phoenicia) were found to be low with an average of 70 % of initial mass remaining after 2 years of field incubation. Over the same period, all litter types showed only a slight (<10 %) net loss of N. Despite relatively high initial N contents, litter decay rates were comparable to those reported in the literature, suggesting that C and N dynamics are decoupled during litter decomposition. Over the two years of incubation, 13 C dynamics were not unanimous between the three litter types, with only a slight enrichment in one species. Continuation of this ongoing experiment is likely to resolve the long term effects of decomposition on 13 C enrichment on litter. Soil CO2 efflux was found to be unusually high (peak rates of over 9 \ub5mol m-2 s-1 ), owing to both high soil water content and soil temperature during an intensive measuring campaign in October 2003. Mean daily fluxes in woodland ecosystems were significantly higher than in either macchia or ex agricultural ecosystems, exceeding the latter about twofold. However, when scaled to the relative surface representation on Pianosa, the highest contribution of daily soil CO2 efflux stems from Macchia type vegetation, followed by abandoned agricultural sites and woodland ecosystems (around 20, 22, and 8.5 t C d-1 , respectively). With the exception of one site, soil CO2 efflux correlated positi- vely with litter content at different sites across the island. Rather than causing the higher fluxes directly, higher litter contents are likely to indicate higher site productivity rates, resulting in higher CO2 turnover dynamics and hence higher overall soil CO2 efflux rates. Owing to the only small range of soil moisture conditions during the measuring campaign, no dependence of soil CO2 efflux on soil moisture could be detected. However, a range of moisture conditions between sites was noted, indicating the significance of site specific conditions also within the same ecosystem types

Decreased summer drought affects plant productivity and soil carbon dynamics in a Mediterranean woodland

Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20 % reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10 % v/v. Treatments were compared to control plots which received ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130 %, 26 %, 58 % and 220 %, respectively, as compared to the control. In contrast, the 20 % reduction in throughfall (equivalent to 10 % reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10 % reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodlands. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving modelling of long-term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long-term soil C stocks. \ua9 Author(s) 2011

Decreased summer drought affects plant productivity and soil carbon dynamics in a Mediterranean woodland

Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20 % reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10 % v/v. Treatments were compared to control plots which received ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130 %, 26 %, 58 % and 220 %, respectively, as compared to the control. In contrast, the 20 % reduction in throughfall (equivalent to 10 % reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10 % reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodlands. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving modelling of long-term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long-term soil C stocks

Precipitation pulses enhance respiration of Mediterranean ecosystems: the balance between organic and inorganic components of increased soil CO2 efflux

In regions characterized by arid seasons, such as the Mediterranean basin, soil moisture is a major driver of ecosystem CO(2) efflux during periods of drought stress. Here, a rain event can induce a disproportional respiratory pulse, releasing an amount of CO(2) to the atmosphere that may significantly contribute to the annual ecosystem carbon balance. The mechanisms behind this pulse are unclear, and it is still unknown whether it is due to the stimulation of autotrophic, heterotrophic and/or inorganic CO(2) fluxes. On the Mediterranean island of Pianosa, eddy flux measurements showed respiratory pulses after rain events following prolonged drought periods, which occurred in the summer of 2003 and 2006. To investigate the mechanisms of this observed enhanced respiration fluxes and partition of the soil CO(2) sources, two water manipulation experiments were performed. The first was designed to estimate the effect of soil rewetting on soil CO(2) efflux, in the different ecosystem types existing on the island (i.e. woodland, ex-agricultural and Mediterranean shrubland). The second was a soil CO(2) partitioning experiment to investigate the relative contribution of inorganic and organic CO(2) sources to soil respiration, under dry and wet soil conditions. Our results suggest that the pulse in the CO(2) efflux is primarily due to the enhancement of heterotrophic respiration, likely caused by the degradation of easily decomposable substrates, accumulated in soils during the dry period. In fact, the vegetation at the site was senescent and did not play any significant role in CO(2) exchange, as suggested by the absence of diurnal CO(2) uptake in eddy covariance measurements. In addition, soil rewetting did not significantly enhance inorganic CO(2) efflux