Photosynthetic responses of tree seedlings in grass and under shrubs in early-successional tropical old fields, Costa Rica

Only recently have studies addressed the effect of early-colonizing vegetat ion on tree seedling survival and growth during secondary succession in tro pical old fields, and few studies have elucidated the physiological respons es of tree seedlings to different vegetational communities. We compared gro wth and various photosynthetic parameters for seedlings of four rain-forest tree species, Cedrela tonduzii, Inga punctata, Ocotea whitei, and Tapirira mexicana, growing in areas of pasture grass and shrubs in early-succession al abandoned pasture in Costa Pica, in addition, we made measurements for t wo species in forest gaps. We tested the general hypothesis that early-colo nizing shrubs facilitate growth of forest toe seedlings. Specifically, we m easured microclimate, growth, CO2 assimilation, stomatal conductance, photo system II quantum yield (Phi PSII), and xanthophyll pigment pools for all s eedlings. Photosynthetic flux density (PFD) was higher under grass than shr ubs or forest gaps, but was highly variable in each growth environment. For three of the four species, height growth was greatest in the grass compare d to the shrubs and forest gaps; growth was similar below grass and shrubs for O. whitei. Photosynthetic capacity, apparent quantum yield, and stomata l conductance did not vary across habitats, but light compensation point an d PFD at light saturation tended to be higher in the grass compared to fore st and shrub growth environments. Water use efficiency differed across grow th environments for three of the species. For plants in ambient PFD and dar k-adapted plants, the efficiency of excitation energy transfer through PSII was lowest for plants in the grass compared to shrubs and forest gaps and also differed across species. Measurement of steady-state responses of Phi PSII to increasing PFD indicated a significant effect of growth environment at low PFD for all species and significant effects at high PFD only for I. punctata. All species exhibited a high degree of midday xanthophyll deepox idation in the different growth environments. Xanthophyll pigment pool size on an area basis was highest in the grass compared to shrubs and forest ga ps for all four species. The results suggest that shrubs do not provide a f acilitative effect for growth or photosynthesis for similar to1.5-year-old seedlings of these four species. We conclude that site differences in succe ss of tree seedlings during succession are a result of complex interactions of facilitation and competition and are not simply based on physiological responses to PFD.