Non-performance responses from stem-meta-analysis

We searched for experimental and observational studies that investigated effects of insect stem herbivores on plant performance, including studies that compared plant performance at sites where the stem herbivore was present to plant performance at sites where the stem herbivore was absent. Only woody plants were considered. All types of attack to living stems were considered including sap-suckers, bark beetles, gallers, stem borers, bark chewers. We included ovipositional damage and species that attacked other parts of the plant in addition to the stem (often in different life-history phases). We also included studies where herbivory was simulated to mimic attack by a particular species of insect herbivore. Studies that simulated attack without specifying a herbivore model or where the model was a mammal were excluded.

The studies that we used compared woody plant survival, growth, reproduction or photosynthesis on plants (or parts of plants) with and without stem herbivore attack. We also recorded changes in allometry, phenology, defence and carbon and nitrogen chemistry. Google Scholar was used to locate studies. Only studies in English were included and our final search for studies was in late April 2014. We searched for the terms "herbivor-" and "damage" with a range of possible terms for different parts of the plant stem. In addition, we searched for each of the major insect families that attack plant stems together with the terms "herbivor-" and "damage". A full list of search terms is provided in the Supplementary Material. Reference sections of papers were also scanned for additional studies.

Data were obtained from text or tables in the paper or, where necessary, by extraction from graphs using Plot Digitizer (Huwaldt 2010). We recorded the mean and total sample size for each variable measured. Where multiple levels of herbivory were assessed, we used only the most severe treatment. This might be the treatment that involved the most insects, removed most biomass, or exposed plants to herbivores for the longest period of time. If impacts of additional variables (e.g. drought, nutrients) were assessed in conjunction with herbivory, the control of this treatment was taken. If different sites were presented, we calculated the mean across sites for each response variable and if the same response was measured multiple times, we used either the final measurement (e.g. seed production in the final year) or the peak measurement (e.g. photosynthesis rate at solar noon). We removed all response variables where the number of replicates was less than three for either the treatment or the control.

In addition to performance metrics, we also scored measures of other allometric changes (i.e. those not covered by the lateral vs. leader comparison, see below), chemistry, defence and phenology. Unlike the performance variables, a change in these measurements does not necessarily imply a change in plant fitness (although of course these changes may ultimately lead to changes in plant fitness). Defence measurements and phenology measurements were each assessed as a group. We only had sufficient data to compare specific leaf area (SLA) and root:shoot for allometric measurements and nitrogen (including protein) and carbon (including carbon-based compounds such as sugars, carbohydrates, starch) for chemical analyses. These responses were analysed individually.