Shade tolerance, canopy gaps and mechanisms of coexistence of forest trees

Th e belief that canopy gaps are important for the maintenance of tree species diversity appears to be widespread, but there have been no formal theoretical models to assess under what conditions gap phase processes allow coexistence. Much of the empirical research on niche diff erentiation in response to gaps has focused on evidence for an interspecifi c tradeoff between low light survival and high light growth. Th e objectives of this study are fi rst to distinguish the possible mechanisms allowing coexistence based on this tradeoff , and second, to explore their limitations. We present a theory of forest dynamics driven by small-scale disturbances as a special case of the theory of coexistence in variable environments. We demonstrate that temporal and spatial heterogeneity in light conditions that results from canopy gaps can allow stable coexistence as a result of three previously documented general mechanisms: ‘relative non-linearity’, ‘the successional niche’ and the ‘storage eff ect’. We fi nd that temporal fl uctuations in light availability alone allow the stable coexistence of only two species. Spatial variation in disturbance synchronicity and intensity allows three species to coexist in a narrow parameter space. Th e rate of extinction is, however, extremely slow and there is transient coexistence of a larger number of species for a long period of time. We conclude that while the low light survival/high light growth tradeoff may be ubiquitous in forest tree species, it is unlikely to function as an important mechanism for the stable coexistence of several tree species.