Among the most general features of forest dynamics worldwide are the occurrence of canopy gaps creating heterogeneity and the differentiation of forest tree species in their response to light availability. The hypothesis that canopy gaps contribute to maintain species diversity is a longstanding paradigm in forest ecology. Surprisingly, considering the intense efforts devoted to test this hypothesis, there are no formal theoretical studies that identify the mechanisms allowing species coexistence by small-scale disturbances. Our objective in this study is to elucidate the mechanisms of coexistence of forest trees species based on the low light survival/high light growth trade-off and the spatial and temporal heterogeneity in light availability created by canopy gaps. We develop a theory of forest dynamics in a variable environment from principles based on a general theory of coexistence and assess it with a simple simulation model of forest dynamics. Here, we report only the results from the simulations on temporal fluctuations in light availability. Such fluctuations allow stable coexistence because of a coexistence mechanism termed “relative non-linearity”. The parameter space of the low light survival/high light growth trade-off allowing coexistence is affected by the disturbance regime, with the widest space occurring at an intermediate disturbance regime. The most important result is that this mechanism allows stable coexistence of only two species, contrary to what is commonly assumed in the literature. The rate of extinction is, however, extremely slow and there is transient coexistence of a larger number of species for a long period of time.