Landslide Disturbance and Forest Regeneration in the Upper Luquillo Mountains of Puerto Rico

Publisher Website
Google Scholar
Abstract
The natural landslide disturbance regime and the major trends in post-landslide succession were studied within a 44-km2 belt between 530 and 850 m a.s.l. (subtropical lower montane wet forest, sensu Holdridge) in the Luquillo Mountains of Puerto Rico [West Indies]. Examination of aerial photographs from 1936 to 1988, and field surveys, revealed forty-six landslides. Forty per cent of the landslides had map areas (the horizontal projection of their actual areas) between 200 and 400 m2. Landslides > 1800 m2 comprised less than 10% of the sample, but accounted for about 40% of the total map area disturbed. Landslide occurrence is spatially and temporally heterogeneous. On average, landslides disturb a minimum of 0.3% and 0.08% of the forest map area per century on slopes underlain by intrusive and volcaniclastic rocks, respectively. On eight landslides less than one year old, the soil in the lower zone (which included deposits of organic soil, broken plant parts and uprooted trees) generally had a higher concentration of organic carbon and nutrients than that in the upper zone (usually consisting of exposed weathered bedrock and mineral soil). Germinable buried seeds of light-demanding species were found only in the lower zone (22 seeds m-2), about 5% of the average buried seed density found in adjacent mature forest. Vegetation analyses on a fifty-two-year chronosequence of twenty landslides showed that regrowth was consistently more vigorous in the lower zone. Species composition is dominated on landslides up to thirty-eight years old by light-demanding, fast-growing pioneer trees. Revegetation of the upper zone seemed to be retarded by extensive mats of light-demanding ferns. Basal area and floristic composition start to resemble pre-disturbance conditions on 300-600-m2, fifty-two-year-old landslides.