Establishment of mountain birch (Betula pubescens ssp. tortuosa) on a glacial outwash plain: spatial patterns and decadal processes

Most of the Earth’s surface has now been modified by humans. In many
countries, natural and semi-natural ecosystems mostly occur as islands,
isolated by land converted to agriculture and a variety of other
land-uses. In this fragmented state, long-distance dispersal may be the
only option for species to adapt their ranges in response to changing
climate. The order of arrival of species may leave a lasting imprint on
community assembly. Although mostly studied at and above the species
level, such priority effects also apply at the intraspecific level. We
suggest that this may be particularly important in subarctic and arctic
ecosystems. Mountain birch (Betula pubescens ssp. tortuosa) is
characterised by great intraspecific variation. We explored
spatio-temporal patterns of the first two mountain birch generations on a
homogeneous, early successional glacial outwash plain in SE Iceland that
was the recipient of spatially extensive long-distance dispersal ca. 30
years ago. We evaluated the decadal progress of the young population by
remeasuring in 2018, tree density and growth form, plant size, and
reproductive effort on 30 transects (150 m2) established in 2008 at four
sites on the plain and two adjacent sites ca. 10 km away. All
measured variables showed positive increases, but contrary to our
predictions of converging dynamics among sites, they had significantly
diverged. Thus, two of the sites (only 500 m apart) could not be
distinguished in 2008 but by 2018, one of them had much faster growth
rates than the other, a higher growth form index reflecting more upright
tree stature, greater reproductive effort, and much greater
second-generation seedling recruitment. We discuss two hypotheses that may
explain the diverging dynamics, site-scale environmental heterogeneity and
legacies of intraspecific priority effects.