Distribution and Apparent Decline of Aspen (Populus tremuloides) in the Broader Lake Tahoe Area: A Four-Decade Assessment

We used machine learning models to produce remotely sensed maps of aspen (Populus tremuloides) percent cover in the broader Lake Tahoe area (BLTA). Aspen is an important ecological and cultural resource both sensitive to and dependent on wildfire, and also vulnerable to climate change. Elsewhere in its range aspen declines have been well documented. More information on where aspen are and how aspen cover has changed over time is needed to inform management. Our ensemble model of aspen cover over the past decade (2014–2023) performed well in cross-validated metrics of predictive performance (e.g., R2 = 0.81). The maps provide a more accurate and detailed view of the distribution of aspen in our area compared with previous maps that delineated aspen presence but did not assess levels of cover. Model outputs indicate that aspen cover has declined in our study area over the past 40 years. This result is consistent across several distinct versions of our models, appears to be robust to potential sources of statistical bias, and is supported by multiple lines of outside evidence. Across the BLTA we provide an initial estimate that aspen cover has declined by about 26% (95% CI: 9–39%) over the 1984–2023 period. A greater focus on restoration treatments, such as prescribed fire, strategic management of naturally ignited fires, and targeted thinning of conifers overtopping aspen, could slow or reverse apparent aspen declines in our region. Detailed maps, such as our product, can serve to inform strategic and adaptive management. Maps are available at: https://stewartecology.org/TahoeAreaAspenMaps/