Where Are Forests Going in the North-American Mountains Under Recent Climate Change?

Abstract:

In global mountain ecosystems, the transition zone from closed-canopy montane forests to treeless alpine tundra areas is often referred to as “alpine treeline ecotone” (ATE). It is an essential habitat for numerous species, such as diverse trees, understory plants, mammals, and breeding birds. Under the human-mediated climate change, global ATEs are expected to be driven upslope, which could lead to a variety of cascading ecological consequences, including changes in carbon sequestration, alpine biodiversity, nutrient and water cycling, snow retention, albedo, surface roughness, etc. Consequently, the dynamics of ATEs has the potential to serve as a powerful indicator of the changing climate. However, as field data collection is often labor-intensive and time-consuming in mountainous regions, it is very difficult to monitor ATE changes through time consistently. Therefore, most existing studies on the ATE dynamics were limited to a single study site or a set of individual locations with relatively small geographic ranges.

Accordingly, the objectives of this study are: 1) to define an automated ATE detection metric using easily accessible remote sensing datasets, and 2) to apply the developed metric to monitor the spatio-temporal dynamics of ATEs during the past decades in North America. We first determined “climatic ATEs” in the North-American mountains as our study domain. Then, we defined three characteristics of ATEs, which included: a) sharp spatial gradient in normalized difference vegetation index (NDVI), b) intermediary values of NDVI, and c) spatial co-variation of elevation and NDVI. According to these key features, we developed an ATE-detection Index (ATEI) ranging from 0 to 1. The annual ATEIs were then calculated from 1984 to 2019. Additionally, based on the ALOS Landform dataset, we generated 1,153,928 elevational transects across the study area and estimated the ATE elevation trend over the 36 years within each transect.

This study generates a continental consistent ATE detection metric. It allows us to assess the ATE change over the full time-series of available remote sensing data, which improves our understanding of the mechanisms of geographic range dynamics of plant species in mountain ecosystems under the recent changing climate.