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