Differences in the Light Environment at Different Heights between Monolayered Sowing Slope and Multilayered Mixed-Species Plantation Slope

Objective:
To maintain a sustainable forest, the existence of saplings plays an important role. The light environment is one of the most important parameters influencing the forest floor vegetation. To restore a natural forest, two techniques were introduced 25 years ago on a slope in the Akandana parking lot,
Japan: a mixed-species plantation method and a sowing method. The mixed-species plantation slope is a multilayered forest where the number of saplings was abundant, whereas the sowing slope is a monolayered forest where the dominance of herbaceous species and less number of saplings were confirmed. The purpose of this study was to determine the differences in the light environment at different heights as well as the daily fluctuation in the light conditions between the two slopes, and to decipher the influence of the light environment on the forest floor vegetation, especially on the saplings.
Methods:
We established 36 plots (5 m × 5 m) and measured relative photosynthetic photon flux density (rPPFD) at 1.0-m height intervals (using the midpoints) from 0.0 to 10.0 m. To continuously measure the fluctuation in the daily light environment for three days, one sensor was placed inside a typical point of the sowing slope and two sensors were placed inside the mixed-species plantation slope, one under the canopy gap point and one under the multilayered canopy.
Results:
The rPPFD at 0.0 m height on the sowing slope was significantly lower than that on the other slope because of the dominance of herbaceous species which hindered sapling establishment (P < 0.01). Moreover, the multilayered mixed-species plantation slope showed a wide variation in rPPFD in vertical and horizontal directions, whereas the monolayered sowing slope showed a uniform value.
Conclusion:
These findings suggest that the forest created by mixed-species plantation is more appropriate to be called a sustainable forest.