A historical ecology of two Quebec watersheds: Long-term changes in land use and management

Background/Question/Methods

Historical land use and management can have long-term implications for ecological function in a watershed, particularly for aquatic ecosystems. Past land use, such as agriculture, can amend soil nutrient levels, and may take centuries to rectify. Knowledge of the spatial distribution of land use historically thus provides valuable insight, but tells us little about actual management practices over time. We investigate the historical ecology of two large watersheds in southern Quebec, the Yamaska and Richelieu, by quantifying indicators of both land use and management over a 65-year period. Georeferenced aerial photographs for each watershed were used to map and interpret land use in the late 1930s and late 1990s. These maps were then compared to existing land use maps for the mid-1960s, developed as part of the Canada Land Inventory project. Using agricultural census data for crops, fertilizer, and livestock, we also computed soil-surface phosphorus (P) balances every ten years from 1931-1971 as a proxy for changes in agricultural management in each watershed; surplus P balances indicate the potential magnitude of P accumulation in watershed soils that could accompany spatial changes in agriculture.

Results/Conclusions

From the 1930s to 1960s, estimates show that agricultural land area decreased substantially in each of the watersheds, by approximately 16% in the Yamaska and 13% in the Richelieu, while the P balances increased by 130% and 280%, respectively. Results for the Richelieu watershed indicate that between the 1960s and 1990s, both agricultural and woodland area decreased by about 3% each, while the urban land area doubled. Although overall changes were small, the median forest patch size decreased by more than 80% from the 1960s to 1990s, and agricultural lands became more densely concentrated. Changes in agricultural management in the Richelieu during this period resulted in an increase in the P balance from 12.9 kg ha^-1 in the 1960s to 16.5 kg ha^-1 by the 1990s. These results indicate a trend toward agricultural intensification in the Richelieu in conjunction with increasingly mixed land uses. Agricultural land area in the Yamaska watershed actually increased over this 30-year period, with a concurrent 75% increase in the P balance (from 13.1 kg ha^-1 to 22.9 kg ha^-1), resulting particularly from more intensive livestock production. Legacies of past land use and management can outlast transitions to new land uses, so knowledge of how these have changed in conjunction provides insight on the long-term ecological implications of land use planning and management.