Fluctuations of climatic conditions, elemental cycling and forest growth at the watershed scale

The relationships between fluctuations in climatic conditions, forest productivity and elemental cycling were studied from 1994 to 1997 in a headwater catchment of the southern Laurentians dominated by sugar maple (Acer saccharum Marsh.) growing on podzolic soils. Annual budgets show that H+, K, and NO3 were retained in the watershed while Ca, Mg and Na were lost. The magnitude of the net annual budget for Ca, Mg and Na was correlated to annual variations in precipitation with the absolute budget value decreasing during dry years. Stemwood (r2 = 0.85) and total tree biomass production (r2 = 0.99) were correlated with mean annual temperature but fine roots and leaf litter were not. During the growing season, the pH of the organic horizons (FH) decreased as the volumetric water content of soil decreased. A positive association was also found between air temperature and H2O-soluble (r2 = 0.88) and PO4-extractable (r2 = 0.99) SO4 in the upper B horizon. On a multiyear scale, we suspect that the decrease in the storage of inorganic SO4 in the soil results from the cumulative effects of annual variations in climatic conditions superimposed on the long-term decrease in SO4 deposition from the atmosphere. These soil changes were reflected by a decline in SO4, Ca and Mg concentrations in the stream. The generalisation of the observed short-term patterns to longer time scales must be approached with caution. Yet, our results indicate that the associations between climatic variations and the biogeochemistry of the ecosystem occur at different spatial and temporal scales and integrate a broad range of chemical components and ecosystem compartments. This reflects the inherent complexity of natural systems and offers a vast palette of indicators of the response of terrestrial ecosystems to variations in the intensity of environmental factors such as climatic conditions.