Toward a recovery time: forest herbs insight related to anthropogenic acidification

摘要

Atmospheric deposition is a global concern contributing to soil acidi?cation and biodiversity changes in forest ecosystems. Although acidifying deposition has decreased in the last decades in Europe, few evidence of ecosystem recovery from acidi?cation has been reported until now. The objective of this study was to reconstruct spatiotemporal changes in soil pH across the entire French forest territory over the last 100-year period through herb species assemblages. Data were collected from ?oristic databases resulting in a total of 120 216 plots covering French forests and spanning from 1910 to 2010. To de?ne acidity ?gures, pH values were inferred from herb assemblages for each plot of the prediction dataset based on a weighted averaging partial least squares (WA-PLS) model (R 2 = 0.80, SD = 0.59 for the validation dataset). Spatiotemporal trends of mean pH changes were obtained by comparing plots with respect to the period (mean year of the period = 1933, 1966, 1984, 1997, 2007) and substrate (acidic and nonacidic forest areas). Bioindicated pH highlighted a decrease in soil pH in both acidic and nonacidic forest areas. The sharpest and most signi?cant pH decrease occurred before 1984 in acidic areas, reaching 0.34 pH units. Subsequently, no signi?cant changes were observed, with a tendency toward stabilization. By contrast, the pH decrease reached 0.19 pH units in nonacidic areas, only reaching signi?cance between 1984 and 1997. Thereafter, we observed a slight and signi?cant pH increase. Spatially, pH trends revealed a regionalized character of acidi?cation regarding the substrate, which could not be related to the extent of deposition modeled by the European Monitoring and Evaluation Programme. Both temporal and spatial trends highlight the lagged responses of nonacidic areas compared with acidic areas. Hence, ?oristic reconstructed pH trends demonstrate a gradual cessation and recovery from acidi?cation of French forests after a period of intense atmospheric pollution.