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 vegetation on tree seedling survival and growth during secondary succession in tropical old fields, and few studies have elucidated the physiological responses of tree seedlings to different vegetational communities. We compared growth 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-successional abandoned pasture in Costa Rica; in addition, we made measurements for two species in forest gaps. We tested the general hypothesis that early-colonizing shrubs facilitate growth of forest tree seedlings. Specifically, we measured microclimate, growth, CO₂ assimilation, stomatal conductance, photosystem II quantum yield (KPSII), and xanthophyll pigment pools for all seedlings. Photosynthetic flux density (PFD) was higher under grass than shrubs or forest gaps, but was highly variable in each growth environment. For three of the four species, height growth was greatest in the grass compared to the shrubs and forest gaps; growth was similar below grass and shrubs for O. whitei. Photosynthetic capacity, apparent quantum yield, and stomatal conductance did not vary across habitats, but light compensation point and PFD at light saturation tended to be higher in the grass compared to forest and shrub growth environments. Water use efficiency differed across growth environments for three of the species. For plants in ambient PFD and dark-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 KPSII 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 de-epoxidation in the different growth environments. Xanthophyll pigment pool size on an area basis was highest in the grass compared to shrubs and forest gaps for all four species. The results suggest that shrubs do not provide a facilitative effect for growth or photosynthesis for ~1.5-year-old seedlings of these four species. We conclude that site differences in success 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.