Research focus on the phenology of plants has accompanied current trends in climate warming, because the two are inextricably linked. Warmer temperatures have led to advanced plant phenology in a range of systems, although some responses are species specific. However, other stressors, such as herbivory, can delay or advance plant phenology, and few studies have addressed the combined impact of these drivers. We experimentally warmed plots with open top chambers (OTCs) and simulated herbivory resistance to insect outbreaks with Methyl jasmonate (MeJA), along an elevational climatic gradient. We then recorded vegetative and reproductive phenology in two functionally important dwarf shrubs, the deciduous Vaccinium myrtillus (bilberry) and the evergreen V. vitis-idaea (lingonberry), to test the responses to the combined treatments. Based on earlier findings, we expected larger phenological responses at higher elevations and for bilberry. We found that in the year after MeJA application, both bilberry and lingonberry delayed vegetative and reproductive phenology, although responses were stronger in bilberry. Warming with OTCs weakly advanced phenology in both species, but combined effects were not consistent. We further examined the consequences of this altered phenology on reproductive output with multigroup piecewise structural equation modelling and found that herbivory resistance driven changes to phenology had a strong mediating effect on berry numbers, particularly under warmed conditions and mainly in the year after MeJA application.

Synthesis: These results demonstrate the need to consider combined pressures in understanding the impacts of global change on plants and highlight that multiple drivers may have unidentified synergetic effects on species phenology and reproduction.