Long-term Patterns of Climate, Tree Growth, and Tree Mortality in Permanent Forest Plots of Hawaii Island
Friday, 19 December 2014
Long-term permanent vegetation plots provide opportunities for in-depth examinations of forest dynamics and climate. We used Center for Tropical Forest Science (CTFS) methodology to establish 4-ha forest dynamics plots in two contrasting climates on Hawaii Island. We established a montane wet forest dynamics plot in a site with 1150 m elevation, mean annual temperature (MAT) of 16.0 C, and mean annual precipitation (MAP) of 3440 mm. A second plot was established in a lowland dry forest site at 240 m elevation, 20.0 C MAT, and 835 mm MAP. The lowland wet forest site averaged only one month per year with < 100 mm rainfall (considered a dry season month), while the lowland dry forest had 12 dry season months. All trees greater or equal to 1 cm diameter were tagged, mapped, and followed from 2008/2009 to 2013/2014 as part of a 5-year census, and a subset of trees were measured annually. Climate variables measured were shortwave and longwave radiation, air temperature, photosynthetically active radiation, relative humidity, windspeed, soil moisture, and rainfall. At both sites, rainfall was the best predictor of annual growth rates. Rainfall and soil moisture were the two variables that demonstrated the greatest interannual variation; coefficients of variation were 36.7% and 61.6% for rainfall at the montane wet forest and lowland dry forest sites, respectively, and 13.4% and 66.1% for soil moisture at the two sites. Preliminary results from the five-year resurvey demonstrate that Hawaiian trees grow slowly, averaging 0.05 cm/y among 19 species in the montane wet forest, at much slower rates for the 15 species in the lowland dry forest plot. Preliminary mortality rates are 11.8% in the montane wet forest and 14.5% in the lowland dry forest. Forest dynamics appear highly related to water availability, even in wet forests, and are likely to be sensitive to climate change, under which reduced rainfall is predicted for much of the Hawaiian Islands.