Differing effects of shade-induced facilitation on growth and survival during the establishment of a chenopod shrub

1 Although plants growing below tree or shrub canopies in xeric ecosystems often exhibit higher growth or survival than conspecifics growing in the open, few studies have sought to identify the processes that facilitate plant performance. 2 We tested the relationship between the importance of facilitation and environmental severity in a field experiment in which seedlings of the perennial shrub Enchylaena tomentosa were shaded at different times of year. We measured plant growth and survival on six occasions between June 1999 (winter) and February 2000 (summer). 3 Although seedling relative growth rates (RGR) changed with growing conditions, shade always increased mean RGRs. The intensity of facilitation (the difference in mean RGRs due to shade) did not differ between measurement periods. 4 Shade reduced survival during winter and spring, but had a strong positive effect during summer. Larger seedlings had lower mortality rates during summer, suggesting that prior facilitation of growth indirectly aided survival. 5 Our results provide only partial support for the hypothesis that the importance of facilitation increases with environmental severity. Whereas the importance of facilitation of seedling survival was high under severe conditions, the importance of facilitation of seedling growth did not increase under more severe conditions. 6 We conclude that the relationship between facilitation and environmental severity is more complex than previously recognized, and that the effects of plant interactions can vary between different aspects of plant performance.