Competition and climate dictate range limits of Picea and Fagus,
respectively
The processes regulating tree performance at their respective upper and
lower range limits differed for F . sylvatica and P .abies . In the Carpathians, the growth rate of F .sylvatica declines as growth synchrony increases towards its
upper elevation limit, which is highly indicative of a species growing
at the limit of its climatic tolerance (Jump et al. 2006). Aspects of
xylem anatomy related to hydraulic conductivity are closely associated
with the upper altitudinal limits of most European tree species,
including F . sylvatica , with large vessels amplifying the
risks of embolism formation under freezing and thawing cycles (Charrier
et al. 2013). The capacity of F . sylvatica to repair
cavitation is temperature sensitive (Cochard et al. 2001), similarly
reflected in the heightened sensitivity of F . sylvatica to
winter temperatures in the Carpathians (Supporting Info 1) and towardsF . sylvatica ’s northern distribution (Weigel et al. 2018),
which almost certainly prevents this species from reaching
dominant/mature size classes at high altitudes in the Carpathians.
Unlike F . sylvatica , the maximum growth rates of P .abies increased as its abundance declined towards its lower
altitudinal limit; this misalignment between abundance and growth
optimum strongly suggests that P . abies is outcompeted at
sites that offer more favorable growing conditions (Canham et al. 2004).
Species composition had fundamentally different impacts on the growth
patterns of F . sylvatica and P . abies within
the overlapping segments of their altitudinal distributions, especially
for understory trees. Understory F . sylvatica grew faster
and in greater synchrony with climate under interspecific competition,
implying an alleviation of competitive suppression. The opposite was
true for P . abies , which exhibited lower growth rates and
growth synchrony under mixed-species canopies. These patterns reinforce
the notion of shade tolerant angiosperms as species that rise in
abundance through monopolization of resources, namely light (Bazzaz
1979), and the competitive advantages afforded to angiosperms by their
improved conductivity, higher leaf area, and faster growth (Bond et al.
1987).