Carbon storage landscapes of lowland Hawaii: the role of native and invasive species through space and time

Abstract

Tropical forests are important storehouses of carbon and biodiversity. In isolated island ecosystems such as the Hawaiian Islands, relative dominance of native and nonnative tree species may influence patterns of forest carbon stocks and biodiversity. We determined aboveground carbon density (ACD) across a matrix of lava flows differing in age, texture, and vegetation composition (i.e., native or nonnative dominated) in wet lowland forests of Hawaii Island. To do this at the large scales necessary to accurately capture the inherent heterogeneity of these forests, we collected LiDAR data across areas of interest and developed relationships between LiDAR metrics and field-based estimates of forest ACD. This approach enabled us to inventory, rather than merely sample, the entire populations (i.e., forests) of interest. Native Hawaiian wet lowland forests exhibited ACD values similar to those of intact tropical forests elsewhere. In general, ACD of these forests increased with increasing lava flow age, but patterns differed between native and nonnative forest stands. On the youngest lavas, native-dominated forest ACD averaged < 60 Mg/ha, compared to -100 Mg C/ha for nonnative-dominated forests. This difference was due to the presence of the nonnative, N2-fixing trees F. moluccana and C. equisetifolia in the nonnative-dominated forest stands, as well as the corresponding absence of N2-fixing trees in native-dominated forest stands. Following -500 years of primary succession and thereafter, however, both forest types exhibited ACD values averaging -130 Mg C/ha, although it took nonnative forests only 75 80 years of post-establishment succession to reach those values. Given the large areas of early-successional M. polymorpha-dominated forest on young lava flows, further spread of F. moluccana and C. equisetifolia populations would likely increase ACD stocks but would constitute a significant erosion of the invaluable contribution of Hawaii's native ecosystems to global biodiversity.

Landscape‐scale GPP and carbon density inform patterns and impacts of an invasive tree across wet forests of Hawaii

“…, Hughes et al. ), our results demonstrate in particular how subtle increases in invasion severity change forest carbon storage. Together, these studies suggest species‐specific and density‐specific effects of invasion on losses or gains of forest carbon stock.…”

“…Landscape‐scale forest carbon stocks increase with substrate age, but this relationship is controlled by native vs. nonnative tree dominance (Hughes et al. ). Increases in elevation often lead to decreases in nutrient cycling and availability, which consequently reduce primary productivity (Raich et al.…”

“…Researchers have analyzed impacts of invasive species on ecosystem function by comparing invaded habitats with adjacent native counterparts (e.g., Ehrenfeld , Hughes et al. ). However, native forests often show diverse levels of invader dominance (invasion severity), which lead to varying degrees of change in ecosystem functioning as landscapes are gradually invaded (Stohlgren and Rejmanek ).…”

Invasion of Hawaiian rainforests by an introduced amphibian predator and N2‐fixing tree increases soil N2O emissions

“…Alternatively, higher total N‐oxide emissions may be related to likely higher rates of litterfall from lowland F. moluccana compared to montane M. faya (Hughes and Denslow , Hughes et al. ). In one example, extensive invasion across the southern United States by the N 2 ‐fixing vine kudzu ( Pueraria montana ) increased the number of regional high ozone days relative to native vegetation in part by doubling soil NO fluxes, which were in the same range of emissions from F. moluccana (Hickman et al.…”

Determining Subcanopy Psidium cattleianum Invasion in Hawaiian Forests Using Imaging Spectroscopy

“…Forest ecosystems invaded by exotic plant species can experience changes on native species abundance and richness [1,2], altered ecosystem function [3,4], or economic losses [5]. These and other negative impacts of invasion may also vary as landscapes are gradually invaded [6].…”