Data from: Herbivory and climate as drivers of woody plant growth: Do deer decrease the impacts of warming?

Vegetation at ecotone transitions between open and forested areas is often
heavily affected by two key processes: climate change and management of
large herbivore densities. These both drive woody plant state-shifts,
determining the location and the nature of the limit between open and tree
or shrub-dominated landscapes. In order to adapt management to prevailing
and future climate, we need to understand how browsing and climatic
factors together affect the growth of plants at biome borders. To
disentangle herbivory and climate effects, we combined long-term tree
growth monitoring and dendroecology to investigate woody plant growth
under different temperatures and red deer (Cervus elaphus) herbivory
pressures at forest-moorland ecotones in the Scottish highlands.
Reforestation and deer densities are core and conflicting management
concerns in the area, and there is an urgent need for additional
knowledge. We found that deer herbivory and climate had significant and
interactive effects on tree growth: in the presence of red deer, pine
(Pinus sylvestris) growth responded more strongly to annual temperature
than in the absence of deer, possibly reflecting differing plant-plant
competition and facilitation conditions. As expected, pine growth was
negatively related to deer density and positively to temperature. However,
at the tree population level, warming decreased growth when more than 60%
of shoots were browsed. Heather (Calluna vulgaris) growth was negatively
related to temperature and the direction of the response to deer switched
from negative to positive when mean annual temperatures fell below 6.0°C.
In addition, our models allow estimates to be made of how woody plant
growth responds under specific combinations of temperature and herbivory,
and show how deer management can be adapted to predicted climatic changes
in order to more effectively achieve reforestation goals. Our results
support the hypothesis that temperature and herbivory have interactive
effects on woody plant growth, and thus accounting for just one of these
two factors is insufficient for understanding plant growth mechanics at
biome transitions. Furthermore, we show that climate-driven woody plant
growth increases can be negated by herbivory.