Canopy height changes of an old-growth evergreen broad-leaved forest analyzed with digital elevation models

Abstract

We analyzed the canopy height dynamics of an old-growth, evergreen, broad-leaved forest over 32 years by using digital elevation models (DEMs) derived from aerial photographs. The study site was situated in the Tatera Forest Reserve in southwestern Japan, an area through which typhoons frequently pass, where a 4 ha study plot was established in 1990. We estimated canopy heights using DEMs of the canopy and ground surfaces. The DEMs of the canopy surface were obtained from aerial photographs taken in four different years (1966, 1983, 1993, and 1998), whereas the ground surface DEM was obtained from ground measurements made in 1990. The resolution of the DEMs was 2.5 m, which resulted in 6400 2.5 m×2.5 m quadrats within the plot. The mean heights obtained from the field survey and from the DEM were not significantly different ( P>0.05). Therefore, DEMs can be used to analyze the average dynamics of canopy height in an entire plot. The maximum canopy height was 25.3–25.7 m, and it did not continuously increase over time. The increment and decrement of mean canopy height were 6.4–14.4 cm per year and −7.5 to −18.9 cm per year, respectively, in the three periods (1966–1983, 1983–1993, and 1993–1998). The decreases in the height of the canopy surface as a result of damage were significantly greater than increases with growth. The canopy state of each quadrat was classified into one of four categories, “gap–gap” (canopy gap maintained over time), “gap–closed” (canopy gap that changed to closed canopy), “closed–gap” (closed canopy that changed to a canopy gap), and “closed–closed” (closed canopy maintained over time), based on the temporal changes in the canopy state during each period. The canopy heights of all categories greatly decreased in 1983–1993 and 1993–1998, including the canopy heights of “closed–closed” quadrats, although they did not form gaps. These temporal changes in the canopy surface may have been correlated with typhoon disturbance, as strong typhoons passed near the reserve during these periods.