Above ground forest biomass and carbon status in the Rio Grande de Arecibo watershed

Abstract

Tropical forest ecosystems, especially young secondary-rapid growing forests, naturally or artificially developed, have great potential for long term carbon sequestration and storage. This ecosystem function helps to reduce the continuous CO2 emissions to the atmosphere. The objective of this investigation is to generate a base line that will serve as basis to estimate above-ground biomass and carbon fluxes of forest lands in the Rio Grande de Arecibo watershed. Biomass and carbon contents in above-ground vegetation were estimated from forest inventory data taken in the summer, 2003 in the Rio Grande de Arecibo watershed (RGA). The data collected also allowed for structural and species composition analyses of the vegetation. A Geographic Information System using land use, geology and ecological life zone maps of the study area, was developed and used as the basis for stratified random sampling of forests lands in the study area. Above-ground biomass was calculated from prediction equations developed for tropical forests as a function of life zones and rock formation types. Mean above-ground biomass for the RGA was estimated to be 72.8 Ton/ha, and it varies from 69.0 to 88.3 Ton/ha, depending on life zone. These estimates correspond mostly to early secondary forests, with crown closure between 50 and 80%. Statistical analyses showed no significant differences in above-ground biomass content when taking into account rock formation types within a life zone, or among volcanic, plutonic or limestone rock formations. However, a slight, but important difference was detected at the 5% level of significance in above-ground biomass content between life zones (wet, moist and wet lower montane forests). A total of 81 trees species were found in the RGA watershed with Importance Value Indices (I.V.I.) ranging from 0.5 to 16.23. Dominant species are Guarea guidonia, Cecropia schereberiana, Inga vera, Prestoea montana, Dendropanax arboreus, Didymopanax morototoni, and Syzygium jambos, accounting for 50.1% of the I.V.I value and almost 54.6% of the basal area. Dead standing trees accounted for almost 8% of the total I.V.I. value and third ranking position, indicating a large proportion of snags in the forest. The I.V.I. curves of each life zone are fairly steep with long tails, reflecting high dominance of few species, especially in wet lower montane forests where Prestoea montana has an I.V.I. value of 31.54.