The effects of slope orientation on plant growth, developmental instability and susceptibility to herbivores

South-facing slopes (SFS) may receive six times higher solar radiation than north-facing slopes (NFS). Although located only a few hundred meters apart, the microclimatic conditions on the slopes vary dramatically, affecting the biology of organisms at all levels. This study was conducted on both slopes of three canyons, where we examined leaf traits of Pistacia lentiscus L. and characterized the populations of its specific herbivore—the gall-forming aphid Aploneura lentisci Pass. Both a larger leaf size and lower gall density were found on NFS. However, gall size and leaf symmetry were not affected by slope. The unique biology of the aphids may explain their higher density on the SFS. Gall formers usually prefer and perform better on large plant organs (leaves), in accordance with the plant vigour hypothesis. Our results demonstrate that larger shaded leaves are an exception.