Quantifying the roles of climate, herbivory, topography, and vegetation on tree establishment in the treeline ecotone

The transition zone between the upper forest boundary and the tree-less alpine vegetation (i.e., the treeline ecotone) is expected to shift upwards in elevation with global warming. Multiple factors in addition to climate are influencing tree establishment, but it is uncertain to what degree. In this study, we took advantage of a unique dataset with field measurements of mountain birch trees acquired from 32 boreal-alpine treeline ecotone sites situated along an 1,100 km latitudinal transect in Norway to investigate the role of climate, herbivory, topography, and vegetation on the establishment success of trees. To represent tree establishment, we recorded the occurrence, survival, and growth of short juvenile trees (stem height ≤1 m) at each site in 2008, 2012, and 2018. In addition to using already available datasets on climate and herbivory at each site, we used airborne laser scanning (ALS) and unmanned aerial vehicle (UAV) surveyed imagery together with field measurements to compute topography and vegetation predictor variables. We used GLMM-LASSO regression to quantify the role of the predictor variables on the three responses. We found that short juvenile trees do not necessarily occur, survive, and grow within the same conditions. In general, short juvenile tree establishment decreased with an increase in herbivore livestock density. Winter precipitation and the two vegetation types tree-covered area and lee side were also important for tree establishment, but if the relationships were positive or negative differed between tree occurrence, survival, and growth. Our results suggest that treeline ecotones will shift towards higher elevations in areas with little or no livestock herbivory and close to already established trees and in lee sides. In areas with less winter precipitation, the treeline ecotones may be more stable.