RESPONSES TO LIGHT CHANGES IN TROPICAL DECIDUOUS WOODY SEEDLINGS WITHCONTRASTING GROWTH-RATES

We evaluated the responses in growth, biomass allocation, photosynthes is and stomatal conductance, to changes in light in woody seedlings fr om the tropical deciduous forest in Mexico, which shows a highly seaso nal rain pattern. We studied ten species, which differed by 30-fold in relative growth rate (RGR). We analyzed plant growth in two contrasti ng light levels during 52 days and two transfers: from high to low (HL ) and from low to high (LH) light intensity, and the respective contro ls in high (HH) and low (LL) light for another 52 days. The photosynth etic capacity (A(max)) and stomatal conductance were measured at the d ay of the transfer between light conditions and at the end of the expe riment. Species with high RGR showed the largest changes in RGR in res ponse to contrasting light conditions (HH/LL ratio), and species with low RGR showed low responses. The fast-growing species were the most p lastic, followed by species with intermediate growth rates, with the s low-growing species being the least plastic. Fast-growing species achi eved higher maximum photosynthetic capacities (A(max)) and stomatal co nductance and higher response to light than slow-growing species. Spec ies with high RGR showed a low RGR HH/LH ratio, suggesting a large res ponse of L plants when transfered to H. The RGR of the species were as sociated with species specific leaf area and with the response in the leaf area, net assimilation rate and leaf weight ratio, suggesting the importance of the leaf area produced and the leaf characteristics rat her than root:shoot ratio in determining RGR. Considering that seed ge rmination is expected at the beginning of the rainy period, seedlings of most of the species will experience high-light conditions during it s early growth. There are large annual variations in the time required for canopy closure (35-75 days). The influence of these variations ma y have different effect on the species studied. Species with intermedi ate growth rate and intermediate response to light changes were less a ffected by light reduction than fast-growing species. The intermediate -RGR species Caesalpinia eriostachys is the most abundant and widely d istributed species, perhaps this could be in part due to its ability t o acclimate to both light increases and decreases. The fast-growing sp ecies studied here can be found in open sites in the forest and in are as cleared for pasture growth. These fast-growing species eventually r each the canopy, although this may require several canopy openings dur ing their lives, which implies juvenile shade tolerance. In the tropic al deciduous forest juvenile pioneer trees also benefit from the tempo rary high light available caused by the dry period during the rainy se ason. The slow-growing species Celaenodendron mexicanum forms small pa tches of monospecific forest; the adult trees are not completely decid uous, and they retain their old leaves for a long time period before s hedding. Thus seedlings of this species may receive lower levels of li ght, in agreement with its shade tolerance and its lower response to l ight increases.