Following the Duona landslides that occurred in 2009, secondary forests, with many juveniles growing in the understory, have since developed in some sites. This study compares the predawn leaf water potential (Ψpd) and net photosynthetic rate (Pn) of saplings of shade-tolerant tree species of the secondary forests during both rainy and dry seasons, and explores the effects of water and light availability on the photosynthetic rates of these saplings. The gap fractions of 100 small plots within the secondary forests were measured in both seasons, and the total diameter at breast height (T-dbh) of trees growing in each plot was calculated. Results indicate that during the late-dry season, the shedding of upper canopy leaves resulted in a 74% increase of photosynthetic photon flux density available for saplings compared to the rainy season. We also found significant negative correlations between gap fractions and T-dbh values in both dry and rainy seasons. The Ψpd values of all ten tested species during the early-and late-dry seasons were significantly lower than those during the rainy season, although the extent of reduction varied among species. By the late-dry season, the Ψpd value for Glycosmis parviflora var. parviflora saplings was -2.70 MPa, while that of Acer albopurpurascens saplings was -0.74 MPa, indicating higher water availability for the latter species. The Pn values of saplings of Neolitsea parvigemma, Litsea hypophaea, Machilus philippinensis, and Acer albopurpurascens did not show significant variation across the three seasons. However, when data from these four species were pooled, the Pn measured in the two dry seasons were significantly lower than those in rainy season. The Pn measured in the late-dry season was 78% of that observed in the rainy season. During the late-dry season, the Ψpd values of the saplings were significantly negatively correlated with the T-dbh values of trees in the small plots, suggesting competition for water between the saplings and nearby trees. The results suggest that T-dbh of trees can serve as an indicator of both water and light availability in small plots during the late-dry season. In conclusion, the study found that light availability increased, while soil water availability significantly decreased in dry seasons resulting in reduction of the photosynthetic rate of the understory saplings in the secondary forests of Duona landslide sites.