EXAMINING ECOHYDROLOGICAL IMPLICATIONS OF PLANT INVASION IN THE LEEWARD KOʻOLAU MOUNTAINS THROUGH TRANSPIRATION

Abstract

Invasive alien species are regarded as a global threat to biodiversity and human well-being. Specifically, plant invasion can cause landcover change with negative results at multiple scales, from species impact (i.e., biodiversity loss) to community changes (i.e., species homogenization) and even disruptions to ecosystem functions. In particular, invasive alien plants can alter hydrologic flows by introducing aggressive water-use strategies and transforming the landcover's physical structure. Water resources of island communities are already vulnerable to the impacts of both climate change and overuse––and increasingly so, given the high invasibility associated with island ecosystems. On Oʻahu, the fourth-largest yet most populous of the Hawaiian Islands, groundwater resources contribute more than 90% of domestic water use. Furthermore, areas of high recharge in the Koʻolau Mountains overlap with critical native ecosystems threatened by plant invasion, particularly strawberry guava. To investigate how stand transpiration changes in response to strawberry guava invasion, a study site was identified where native ʻōhiʻa-uluhe forest and strawberry guava invaded forest occurred as adjacent stands. Estimates of stand transpiration were derived from sapflow measurements of ʻōhiʻa and strawberry guava, as well as vegetation surveys quantifying the physical structure in each plot. Additionally, sapflow responses to environmental conditions were investigated through concurrent measures of atmospheric and soil variables (i.e., solar radiation, humidity, and soil moisture). These data made it possible to examine how transpiration differs between these contrasting forest types and determine what characteristics explain them. Estimates of monthly transpiration were up to 5 times greater from strawberry guava than from ʻōhiʻa; largely driven by greater stem density and the resultant higher sapwood area per ground area generated by the monotypic strawberry guava stand. However, the ʻōhiʻa canopy in this forest type is open, with uluhe–an indigenous, sprawling fern–effectively acting as a canopy layer as well. Therefore, while native stand transpiration was estimated under two scenarios, which were still less than the total strawberry guava stand transpiration, sapflow measurements from uluhe are essential to quantifying native stand transpiration at this site. There was variation in sapflow velocity responses to environmental conditions, with ʻōhiʻa experiencing saturation of sapflow velocities with no evidence of strawberry guava sapflow velocities saturating under the same environmental conditions. As such, future work examining the physiological controls of transpiration for key native and invasive species and quantifying the changes to forest structure due to invasion is critical for future assessments of the ecohydrological impacts of plant invasion. This work further emphasizes how understanding the ecohydrological impacts of landcover changes due to plant invasion is critical for water management on islands.