Positive effect of shade on plant growth: amelioration of stress or active regulation of growth rate?

1. While plant competition for light has received considerable attention, the facilitative effect that shading can have on plants and the role of species-specific adaptations in mediating this phenomenon are still poorly understood. Evidence is accumulating, however, that positive interactions can play as important a role in community structuring as has been shown for competition. 2. We examined interspecific variation in growth responses to shade using 46 temperate grassland species grown for 10 weeks in a common garden under identical soil conditions but at different levels of shading. The importance of morphological plasticity and habitat preference as species traits determining the net effect of shading on plant growth was tested. 3. Moderate shade (50% daylight) had, on average, a net facilitative effect on plant mass. Plant growth in the 25% daylight treatment was not significantly different from that in full daylight, and it was only when 90% of natural light was made unavailable that shaded plants attained a significantly lower dry mass than plants in full daylight. 4. Species that exhibited the most pronounced reduction in specific leaf area and increased allocation to roots in full compared with 50% daylight were least facilitated by shading. This finding supports the hypothesis that morphological plasticity is an important trait determining the strength of facilitative interactions. 5. The growth response to shade was also dependent on species' ecological optima, with species characteristic of nutrient-poor or dry habitats most facilitated by shade. Greater growth enhancement by shade in grassland stress-tolerant plants is surprising because such species frequently occupy unproductive vegetation with little shade and should therefore be adapted to high irradiance. This result calls into question the notion that a positive effect of shade on growth always reflects stress amelioration. 6. Synthesis. Our study demonstrates a widespread ability of plant species to decouple growth from resource availability and challenges our understanding of the processes determining plant productivity.