Plasticity to neighbour shade: fitness consequences and allometry

1. Phenotypic plasticity may confer a fitness benefit to organisms in a spatially heterogeneous environment. However, the evolution of such adaptive plasticity can be limited by the constraints imposed by interacting environments. Moreover, adaptive plasticity may be masked by reduced growth in a low-quality environment. The allometry between plasticity of morphological traits and overall growth is often overlooked. 2. We addressed these issues by examining the effect of shade produced by neighbouring plants (neighbour shade) on a small annual plant, Geranium carolinianum. Plants were grown in two field sites, a wood margin and an old field. A clipping treatment was used to manipulate neighbour shade at both planting sites. We measured petiole length to quantify shade avoidance response. Genotypic selection analysis was used to assess the fitness consequences of petiole length. 3. Petioles were longer in the wood margin than in the old field and were similar between neighbour environments. However, overall plant size differed among the environments. As a result, relative to plant size, petioles were longer when individuals were surrounded by neighbours than when neighbours had been removed, but were similar between sites. After taking the size into account, plants with longer petioles had higher fitness than plants with shorter petioles in all environments. 4. The functional response of petioles to neighbour shade was revealed only when the passive plasticity of plant size was considered. The lack of differential selection across neighbour environments together with active elongation of petioles in response to neighbours suggests that the current shade avoidance response is adaptive.