Vegetation Patterns in Lowland Wet Forests of Hawai'i

Abstract

The easternmost part of the Big Island contains some of Hawai'i's last remaining native lowland wet forests. My goals in this study were to examine how substrate age and elevation influence the degree of invasion present in this lower elevation (< 300 m) region by assessing native and non-native canopy cover of trees, to evaluate native-dominated communities for the influence of substrate age and elevation on diversity and species composition, to examine canopy conditions surrounding rare plant occurrences, and to identify priority areas for restoration. I focused on East Hawai'i's five forest reserves using Braun-Blanquet cover estimate methods to assess vegetation patterns. Plots were stratified to represent combinations of variables including three categories of substrate age (< 200 yrs, 200-750 yrs, >750 yrs), and three elevation zones (< 100 m, 101-200 m, 201-300 m). I analyzed 291 plots, of which 125 had a native-dominated canopy. On intermediate flows, absolute cover of native tree species was higher when compared to young flows; relative cover of native tree species was higher, and absolute cover of non-native species was lower, when compared to older substrates. On young flows, absolute cover of native trees equaled that of non-native trees. The oldest substrate proved to be most degraded, with the relative cover of native trees being lower than on other substrates. At native-dominated sites, there was higher native species richness on intermediate substrates relative to the young, and higher non-native species richness on the older substrate when compared to the young. Young and old substrates had higher numbers of non-native species than they did native species. There was a greater relative canopy cover of native trees where rare plants occur compared to plots where they do not occur. All 34 individuals from rare species were entirely on the 200-750 year old substrate. Together, these trends point to intermediate age substrates as holding the most intact native assemblages and highest potential for restoration.