Understorey light environment impacts on seedling establishment and growth in a typhoon-disturbed tropical forest

Some studies suggest that canopy gaps are not crucial for woody species recruitment in typhoon-disturbed forests because of frequently defoliated forest crowns and invariant high-light levels under the forest canopy. Here we examined this speculation focusing on seedling establishment stages and forest floor light in a forest with annual return frequencies of typhoons. We monitored light (photosynthetically active radiation; PAR) and seedling dynamics in 75 seedling quadrats (each 1 × 1 m²) for eight years in Nanjenshan tropical lowland forest, Taiwan (2043 newly recruited seedlings; 199 advance regeneration seedlings). Due to attenuation by the understorey vegetation, only 24% of the PAR that penetrated the forest canopy reached the forest floor. We found that PAR transmittance on the forest floor was heterogeneous temporally, ranging from 0.01 to 15.6%. Nonetheless, shade-intolerant species established at relatively high median values of light transmittance, compared to species of the moderate and tolerant groups, indicating that light is a crucial resource at the establishment stage. However, many seedlings died in relatively high-light conditions, suggesting that factors other than low light were more critical for survival. The generally low-light environment on the forest floor also resulted in similar composition and slow growth of seedlings across quadrats, except for a few in extraordinary high-light quadrats. In conclusion, the temporal light environment below the understorey layer was heterogeneous and was critical for differentiated establishment and growth patterns of newly recruited seedlings. Thus, besides canopy gaps, the light attenuation effect caused by understorey vegetation plays an essential role in the early establishment stage of seedlings in a forest frequently disturbed by typhoons.