Statistical analyses
Descriptive statistics were expressed as arithmetic means and standard
errors (SE). When feasible, errors were propagated to maintain an
estimate of variance. We tested correlations between our predictor
variables and herbivory levels as well as the annual proportion of
elemental fluxes using linear mixed effect modelling (‘lmer’ function in
‘lme4’ package, Bates et al. 2015). Confidence intervals (95%) were
constrained with 999 bootstrap simulations. The relative position above
the valley bottom was included in the models as Z-scores based on the
elevation grouped by transect, i.e. yielding values close to -1 for the
lowest site, close to 0 for the middle site, and close to +1 for the
highest site. For herbivory levels, we tested volumetric soil water
content, mean growing season soil temperature, annual solar radiation,
relative position above the valley bottom and absolute site elevation as
abiotic predictors, while leaf C:N and CT:C ratios were included as
biotic predictors. When testing these as fixed effects, the other
variables were classified (4 approximately normally distributed groups
per variable) and included as random effects. Further, the transect
number was included as a random effect in all models (9 groups). For the
annual proportion of elemental fluxes, we tested herbivory level, leaf
production, leaf nutrient (N or P) content, and nutrient (N or P)
resorption as fixed effects. Nutrient resorption was included, as strong
nutrient resorption would decrease the nutrient flux through senesced
litter, and result in a larger proportion of the annual nutrient fluxes
through insect deposits for a given leaf area loss. Variables were
transformed when needed to comply with model assumptions and assure
similar variance for all variables. All statistical analyses were
conducted in R version 3.6.2 (R Core Team, Vienna, Austria).
To assess the consistency in the relationship between local and regional
elevation and herbivory, we linearly regressed BIH against the local
elevation (elevation centred per transect) and the regional elevation
(absolute elevation centred). We included green leaf N-content (% of
mass) plotted against elevation to check the design, as we expected a
more consistent positive relationship between leaf N and elevation
(Körner 1989, Read et al. 2014), while the expectation to foliar loss to
herbivores was less straight forward (see introduction).