Crown development in tropical rain forest trees in gaps and understorey

Crown development was monitored for juvenile trees (4-18 m) of the canopy t ree species Dicorynia guianensis and Vouacapoua americana in a tropical rai n forest in French Guiana. A comparison was made between crown development in closed understorey forest sites (low light) and in canopy gaps (high lig ht); development was expressed by plant traits at multiple levels of organi sation. Dicorynia and Vouacapoua responded to gap creation at all organisational le vels, but not for all traits. Both species increased their light intercepti on efficiency in the understorey. Firstly, understorey trees produced relat ively wide crowns because they did not favour the growth of shoots in the s ummit of the crown, as did gap trees. Secondly, they reduced self-shading ( expressed by leaf area index) by lower sympodial unit production rates, low er leaf production rates per growth unit (only Vouacapoua), and smaller lea f size (only Dicorynia). The reduction in self-shading was weakened by othe r traits that had the opposite effect on self shading (e.g. longer leaf lif e span in understorey), and cannot be considered an adaptive response in it self. (3) Understorey trees of Vouacapoua reduced leaf display costs by pro ducing a higher specific leaf area, a shorter space among leaves, a smaller leaf spacing to leaf size ratio, and a longer leaf life span. Thus, all tr aits contributed to the more economical use of carbon in the understorey. D icorynia showed the same trends, but not significantly. These results suggest that light availability plays a major role in the dev elopment and morphology of trees through its influences at multiple levels of organisation within the crown hierarchy. The two species studied were ra ther similar in their response at crown level, while they sometimes differe d in their responses at the underlying lower organisational levels. The res ponse directions of some individual plant traits were similar for the tall trees studied here and small saplings studied elsewhere. Some of traits inv estigated here may not be important in the crown level responses of smaller seedlings and saplings, while they are important mechanisms for crown leve l responses in taller trees.