Abstract
•Replacing western white pine (WWP) by shade-tolerant conifers greatly reduced transpiration.•Model simulations revealed changes in water budget as forests changed.•Harvesting, WWP mortality, and regrowth created > 30% more annual streamflow.
Vegetation changes can strongly influence the hydrological cycle, including streamflow, but the effects of plant diseases have seldom been described. In the mid-20th century, the invasion of an exotic disease, white pine blister rust, precipitated widespread mortality of western white pine (WWP; Pinus monticola Dougl.) in the northern Rocky Mountains, USA. These events converted a forest dominated by white pine into one dominated by more shade-tolerant tree species. The long-term hydrological implications of this historical shift in forest composition have not been adequately explored, in part because collocated long-term vegetation, meteorology, and hydrology data are rare. We assembled long-term streamflow, climate, and vegetation records for a small (4.0 km2) forested watershed in the U.S. northern Rocky Mountains. We used a minimally-calibrated, physically-based model to simulate historical changes in the water budget based on observed vegetation changes along with the historical climate. The observed hydrological anomalies were attributed to a combination of white pine blister rust induced tree mortality, harvest, and eventual species replacement. Small increases in streamflow began in the basin in the 1940s, when the basal area of WWP began to decrease. More dramatic increases in streamflow started after harvesting began in 1966. Based on both observations and modeling, streamflow increased by ∼131–179 mm between the 1940s and the 2000s (30–40% of the 2000s’ streamflow) in contrast to documented regional streamflow declines. Approximately 1/3 of the flow changes were attributed to blister rust and 2/3 to harvest, as shade-tolerant species instead of WWP regenerated in response to both. This study highlights the importance of long-term ecohydrological data, which made it possible to detect the influence of the invasion of an exotic pathogen against a backdrop of periodic timber harvest. It also showed that the species replacement after these disturbances caused long-term changes in the flow regime, providing a clear and relevant example of how land cover changes can affect interannual hydrological dynamics.