Carpathian tree-ring network for European beech and Norway spruce

Basic ecological theory suggests that a tradeoff between competitiveness
and stress tolerance dictates species range limits at regional extents.
However, empirical support for this key theory remains deficient because
the necessary spatial and temporal coverage and scalability of field
observations have rarely been achieved. We harnessed an extensive
dendroecological network (>22,000 tree-ring samples from 816 forest
inventory plots) to disentangle competition-limited from climate-limited
growth in both overstory and understory trees. Growth synchrony among
trees thereby served as an integral metric of climate sensitivity, an
approach that we justify in supplementary analyses of growth responses to
temperature, precipitation, and the standardized
precipitation-evapotranspiration index. Sampling plots were arranged along
elevational climate and vegetation gradients throughout the Carpathian
Mountains, ranging from mixed-species lowland forests to coniferous
forests at high elevations. With mixed-effect modelling, we also
identified non-climatic factors (stand characteristics, species diversity,
and disturbance history) that modulate spatial patterns in the growth rate
and synchrony of European beech (Fagus sylvatica L.) and Norway spruce
(Picea abies (L.) Karst.). Beech exhibited reduced growth and increased
climate sensitivity towards higher elevations but performed better when
species diversity was higher. The growth of spruce increased towards its
lower range boundary, but understory cohorts grew poorly under
interspecific competition. Overall, climate sensitivity was lower in more
productive stands with benign climatic conditions and in recently
disturbed sites with reduced stand density. These contrasting performances
at mid-elevations where the two species overlap (900 – 1300 m a.s.l.)
reflect their evolutionary history, which enables them to be competitive
(beech) or cold-stress tolerant (spruce). This history will affect
interactions between the two species under climate warming and shape
macroecological patterns in the Carpathian ecoregion and likely other
parts of Europe. Our findings point to a growing advantage of
competitively stronger species in montane and subalpine vegetation zones.