Ontogenetic differences in leaf physiology in tropical tree species: Comparison between seedlings and adult trees

Background/Question/Methods

Vertical variation of light level from forest floor to canopy provides a light gradient which is similar to variation from forest understory to large gaps (or clearings). In this study, we tested whether three non-pioneer tropical tree species would regulate their leaf traits over ontogeny. Alternatively, we hypothesized that canopy leaves differed from juveniles by light induced physiological adjustment as shown by seedlings grown in gaps and the understory. The dicots Dacryodes excelsa and Sloanea berteriana and the palm Prestoea acuminata var. montana are common canopy species in subtropical wet forest in Puerto Rico. Potted seedlings were provided with two nutrient levels: a low nutrient level, representing an average forest soil and a high nutrient level, representing a well fertilized soil. They were placed in a large gap (58.3% of full sun), a gap edge (15.7%) and an understory site (3.6%). The objective of this design was to evaluate the response of leaf traits to a combination of light and nutrient levels and to compare the seedlings growing in preferred light and nutrient conditions with the sun leaves of adult trees.

Results/Conclusions

The canopy leaves of adult trees differed in light-saturated photosynthetic rate (A_max) with a rank of D. excelsa (7.95 μmol m^-2 s^-1) > S. berteriana (7.35 μmol m^-2 s^-1) > P. acuminata var. montana (6.78 μmol m^-2 s^-1). The A_max of adult trees was significantly (p < 0.05) greater than that of seedlings grown in the large gap for all the three species. The difference between seedlings and adult trees was more pronounced for the two dicots than for P. acuminata var. montana. The adult stage also had greater stomatal conductance (G_s) than the seedling stage for the two dicots but not for P. acuminata var. montana due to its relatively high G_s at seedling stage.

We used the method of Cavender-Bares and Bazzaz (2000) to calculate the ontogenetic components of the total difference in A_max between seedlings and adult trees. Oontogenetic components were 67%, 56%, and 44% for D. excelsa, S. berteriana, and P. acuminata var. montana, respectively. The smaller ontogenetic components for P. acuminata var. montana may reflect the narrower vertical gradient of light during its ontogeny. Adult trees of P. acuminata var. montana occupy a sub-canopy stratum where light levels are lower than those in the upper canopy.