Effects of experimental warming on oak defenses and herbivory across latitudes

Forest microclimatic variation can result in substantial temperature
differences at local scales with concomitant impacts on plant defences and
herbivory. Such microclimatic effects, however, may differ across
abiotically contrasting sites depending on background environmental
differences. To test these cross-scale effects shaping species'
ecological and evolutionary responses, we experimentally tested the
effects of aboveground microhabitat warming on insect leaf herbivory and
leaf defences (toughness, phenolic compounds) for saplings of sessile oak
(Quercus petraea) across two abiotically contrasting sites spanning 9°
latitude. We found higher levels of herbivory at the low-latitude site,
but leaf traits showed mixed patterns across sites. Toughness and
condensed tannins were higher at the high-latitude site, whereas
hydrolysable tannins and hydroxycinnamic acids were higher at the
low-latitude site. At the microhabitat scale, experimental warming
increased herbivory but did not affect any of the measured leaf traits.
Condensed tannins were negatively correlated with herbivory, suggesting
that they drive variation in leaf damage at both scales. Moreover, the
effects of microhabitat warming on herbivory and leaf traits were
consistent across sites, i.e., effects at the microhabitat scale play out
similarly despite variation in factors acting at broader scales. These
findings together suggest that herbivory responds to both microhabitat
(warming) and broad-scale environmental factors, whereas leaf traits
appear to respond more to environmental factors operating at broad scales
(e.g., macroclimatic factors) than to warming at the microhabitat scale.
In turn, leaf secondary chemistry (tannins) appears to drive both
broad-scale and microhabitat-scale variation in herbivory. Further studies
are needed using reciprocal transplants with more populations across a
greater number of sites to tease apart plant plasticity from genetic
differences contributing to leaf trait and associated herbivory responses
across scales and, in doing so, better understand the potential for
dynamics such as local adaptation and range expansion or contraction under
shifting climatic regimes.