Figure 3. Relationship between climate effects on willow shoot growth, and effects of insect herbivores on shoot growth. Tests are the same as those in Fig. 2a, except that analyses are run separately for each of the experimental treatments. Abbreviations indicate: no vertebrate herbivores (N ); only herbivory by ptarmigans (P ), herbivory by ptarmigans and reindeer (RP ), and the interaction of each of these treatments with “rejuvenation” (i.e. pruning of shoots in year 1 of the experiment) (J , PJ , andRPJ , respectively) (Table 1).
When separated by browsing and rejuvenation treatments, we found that the general pattern of responses of willow shoot growth to climate and insect herbivores was similar to that in our pooled analysis (Fig. 3, compare to Fig. 2a). However, the test revealed that the treatments differ and that relationships within treatments were not uniform. Recall that there are six different experimental treatments: neither vertebrate herbivores (N ); only herbivory by ptarmigans (P ), herbivory by ptarmigans and reindeer (RP ), rejuvenation (i.e. pruning of shoots at the initiation of the experiment) (J ), rejuvenation plus browsing by ptarmigans (PJ ), and rejuvenation plus browsing by ptarmigans and reindeer (RPJ ). For most treatments, there was still a negative relationship between the effects of climate and the effects of insect herbivory, except in the case of rejuvenation treatments, for which the relationship reversed (J, PJ treatments in Fig. 3). Additionally, the treatments that included vertebrate herbivores tended to experience more positive effects of climate on shoot growth, while for treatments that included rejuvenation, herbivory effects were generally less negative (compare treatments P vs. PJ, RP vs. RPJ in Fig. 3).