Dynamic effects of insect herbivory and climate on tundra shrub growth: roles of browsing and ramet age

1. To predict shrub responses under climate change in tundra, we need to
understand how thermal conditions and herbivory contribute to growth. We
hypothesise that shrub growth increases with thermal conditions and
precipitation, but that this increase is counteracted by insect herbivory,
and that these climate-insect herbivory relationships are modified by both
browsing and plant age. 2. We use empirical dynamic modelling (EDM) to
analyse a 20-year time series on willow (Salix phylicifolia) shoot growth,
growing degree days, summer precipitation and herbivory from an experiment
at forest-tundra ecotone. The experiment includes manipulations of avian
and mammal browsing (fences) and ramet age (pruning to rejuvenate
willows). 3. Negative effects of insect herbivory on willow shoot growth
were intensified during warmer years, whereas increasing precipitation led
to reduced effects. Moreover, the effect of insect herbivores on shoot
growth varied with ramet age and vertebrate browsing: Younger ramets
generally experienced less negative insect herbivore effects, whereas
Ptarmigan browsing was associated with more positive temperature effect on
shoot growth, and reindeer browsing with more negative effects of insect
herbivory and precipitation. 4. Synthesis. Our findings show that the
negative effects of insect herbivory on shoot growth likely intensify
under warmer thermal conditions, but that increasing precipitation can
counteract these effects. Moreover, changing thermal conditions,
precipitation and vertebrate browsers all have predictable, albeit complex
and nonlinear, effects on shrub growth, highlighting the importance of
long-term experimental data and flexible analytical methods such as EDM
for characterizing climate and community interactions in artic systems.