Differential effects of precipitation on production and decomposition along a rainfall gradient in Hawaii

I assessed the differential effects of water availability on aboveground net primary production (ANPP) and decomposition along a natural rainfall gradient ranging from 500 to 5500 mm mean annual precipitation (MAP). All sites were native forest vegetation on the island of Hawai'i, at similar elevation and of similar substrate age. Measurements of production were conducted for 12 months, with monthly collections of litterfall, biannual measurement of tree diameter increase, and nondestructive measures of grass production. Aboveground net primary production increased linearly with increasing site rainfall in the first four sites (up to 2000 mm MAP) and then declined at the wettest site. Grass production was a small component of total ANPP at the two driest Sites and was absent elsewhere. When compared with decomposition of surface litter along the same gradient, patterns of ANPP were distinct from those of rates of mass loss and nutrient release. The balance between production and decomposition was illustrated with a simple model that predicted changes in detrital mass and nutrient content and concentration of detritus along the precipitation gradient. The change in the relative effect on these two processes along the gradient of precipitation suggests a complexity to the regional control of rainfall on ecosystem functioning that could not be seen when examining a single process alone.