Ecohydrologic impacts of plant invasion in a mesic-wet forest on Oʻahu, Hawaiʻi: Transpiration differences between invader Psidium cattleianum and dominant native species Metrosideros polymorpha and Dicranopteris linearis

Abstract:

Biological invasion is a global phenomenon threatening biodiversity and ecosystem processes. Land cover change incurred by plant invasion is recognized as a dominant mechanism of biodiversity loss in Hawaiʻi. Understanding hydrologic processes within native ecosystems, and being able to predict potential changes to these processes given transformations as a result of plant invasion is a crucial component of water resource management. We examine how Psidium cattleianum, a widespread invader across the state, alters hydrologic flows compared to a native forest dominated by an indigenous fern, Dicranopteris linearis, and an endemic tree, Metrosideros polymorpha by measuring transpiration in a wet-mesic climate. The study site, two adjacent stands of native and invaded forest were equipped with a meteorological station. The study species were instrumented with heat ratio method (HRM) sapflow sensors, and monitored continuously from 2019 to 2021. Leaf-level gas exchange measurements were conducted on the study species with the LI-6400XT Portable Photosynthesis System, and used to model canopy transpiration. Results will be used to examine emerging sapflux and transpiration patterns in the study species, highlighting differences in responses to varying environmental conditions. Survey data quantifying stem density, stand basal area, stand sapwood area and stand leaf area are used to quantify stand level water-use. Variable responses to environmental conditions, such as transpiring under lower levels of soil water content and/or performing nighttime transpiration could influence greater quantities of water being transpired from the invaded system. Vegetation surveys have shown that P. cattleianum transforms the stand structure by increasing the sapwood area per ground area compared to the native system, as well as increasing the leaf area index. Even without species level differences in the ecophysiological traits that influence transpiration, these structural differences can alter stand level transpiration. This study elucidates the hydrologic impacts of plant invasion on a remote island, ultimately addressing the potential impacts to groundwater recharge and illustrating the importance of concerted management efforts.