One‐sided competition for light promotes coexistence of forest trees that share the same adult height

Abstract

Summary Coexistence of tree species is usually explained in relation to horizontal heterogeneity of forests caused by disturbance. In contrast, it remains unknown whether one‐sided competition for light in vertically structured forests contributes to stable coexistence of tree species that share the same adult height, even without horizontal heterogeneity. And if so, it is needed to specify the conditions of stable coexistence in terms of demographic and allometric traits between species. To answer these questions, we employ a theoretical model that describes dynamics of tree populations in forests of which vertical structure is expediently divided by height classes. We carry out analytical examination for a forest with two height classes where tree populations are adult in canopy and juvenile in understorey. Demographic processes in the canopy are regulated by foliage density in the canopy, whereas juvenile survival is suppressed by overall foliage density in both classes. A between‐species difference in juvenile/adult ratio of foliage density is required for coexistence equilibrium of two species to exist. The difference in juvenile/adult ratio in equilibrium is determined by demographic and allometric parameters of species. Trade‐offs between demographic parameters are necessary for a coexistence equilibrium which is either stable or unstable. A species with high fecundity and/or shade‐free juvenile survival can stably coexist with a species with high shade‐free growth rate and/or adult survival, whereas it cannot stably coexist with a species with high shade tolerance in growth rate and/or in adult survival. Difference in shade tolerance in juvenile survival does not contribute to stable coexistence. Height‐foliage allometry contributes only additionally to enhance coexistence equilibrium. When we divide forests into three height classes, up to three species can coexist, by means of demographic trade‐offs that bring about interspecific differentiation in vertical foliage distribution. We show all possible cases of coexistence among canopy (C), subcanopy (S) and understorey (U) species in terms of maximum adult height as follows: CCC, CCS, CCU, CSS and CSU. Synthesis. Our theoretical model shows that demographic trade‐offs among tree species with identical demographic responses to shade promotes stable coexistence through vertical foliage partitioning. This mechanism works independently of the coexistence mechanism through horizontal partitioning of patch mosaic, which is associated with interspecific difference in shade tolerance.