Insect herbivory and elemental fluxes
The mean leaf area lost to BIH across all sites was 1.6±0.33%, while
10.0±1.6% of the leaves sampled across all sites were subjected to some
degree of insect herbivory (Table 1). The proportional leaf area lost to
herbivory was positively correlated with the relative position above the
valley bottom (p<0.001), annual solar radiation (p=0.028), and
leaf N-content (p=0.028), yet showed no link to temperature, absolute
elevation, or leaf content of condensed tannins (Table 2).
The overall mean elemental canopy loss to herbivores was estimated to
0.25±0.048 g C m-2 yr-1,
0.017±0.0033 g N m-2 yr-1, and
0.0017±0.00032 g P m-2 yr-1 (Table
1). The mean leaf production across sites was 47±5 g dry matter
m-2 yr-1, corresponding to 21±2 g C
m-2 yr-1, 0.6±0.08 g N
m-2 yr-1, and 0.09±0.011 g P
m-2 yr-1 (Table 1). The estimated
proportion of nutrients resorbed from the birch leaves towards the end
of the season was 56±2.8% for N and 30±4.1% for P (Table 1). The
resulting proportion of the annual flux from the canopy through insect
deposits make up 1.5±0.3% of the total C flux (litter+herbivore input),
while the proportion of N and P were slightly higher (3.6±0.7% and
2.4±0.5%, respectively) due to foliar nutrient resorption during
senescence. The annual proportion of canopy-to-soil N and P fluxes
caused by herbivory were positively correlated with herbivory level (N
and P: p<0.001) and resorption (N: p=0.002, P: p=0.024).
Moreover, the proportional insect mediated flux of N was negatively
correlated with leaf production (N: p=0.005). There was no significant
link to leaf N or P concentrations (Table S2).