Groundwater availability mediates the ecosystem effects of an invasion of Prosopis pallida

Abstract

Groundwater levels in arid environments are dropping worldwide due to human extraction, and precipitation events are predicted to become rarer and more intense in many arid areas with global climate change. These changes will likely alter both primary productivity and plant–soil nutrient cycles. To better understand the nature of such alterations, we examined effects of groundwater availability on plant–soil nitrogen (N) cycling in areas invaded by the N-fixing phreatophyte, Prosopis pallida, on the dry leeward coast of Hawai‘i Island. Our aims were to quantify effects of groundwater availability to P. pallida on rates of litterfall N inputs and accretion in soils and to quantify effects of groundwater availability on N mineralization and leaching rates of inorganic N under natural rainfall conditions and simulated rain events. Stem water δ18O values indicate that P. pallida trees in lowland plots accessed shallow groundwater, while in upland plots they relied solely on rainfall. During drought periods, P. pallida at upland plots experienced water stress, evidenced by lower stem water potentials, higher water-use efficiency, and lower predawn photosynthetic performance than at lowland plots. Prosopis pallida basal area was 5.3 times greater at lowland plots, and these plots exhibited 17 times higher carbon (C), 24 times higher N, and 35 times higher phosphorus (P) additions via litterfall, indicating that productivity of this phreatophyte was decoupled from rainfall where groundwater was present. Total N mass in soils was 4.7 times greater where groundwater was accessible, supporting the case that groundwater access increased N2 fixation at a stand level. In contrast, N mineralization and leaching losses from soils, though substantially greater in lowland relative to upland areas, were strongly controlled by rainfall. Results provide clear examples of how invasive species with particular functional attributes (i.e., N-fixing phreatophytes) exploit otherwise inaccessible resources to dramatically alter the functioning of the systems they invade and how anthropogenic changes to hydrological processes can also alter ecosystem-level impacts of biological invasions. Results also illustrate a mechanism by which regional groundwater drawdown may reduce soil nutrient accretion and availability in arid regions.

Invasive alien plant species: Their impact on environment, ecosystem services and human health

“…IAPS are also known to affect quantity of surface and ground water (Shackleton et al, 2019). Prosopis pallida, a N-fixing IAPS in arid regions of Hawaii Island exploits groundwater resources to a level that alters the soil's environment (Dudley et al, 2014). Some IAPS exploit an enormous amount of water, which can compound the impact of water scarcity and bring a paradigm shift in socio-ecological regimes (Gaertner et al, 2014;Shackleton et al, 2019).…”

“…Notably, in this respect Global Assessment Report on Biodiversity and Ecosystem Services of UN, recently declared the IAPS/alien invaders as major driver of biodiversity loss (IPBES, 2019) Human mediated transport, migration, and commerce are continuing to disperse an ever-increasing array of IAPS across previously insurmountable environmental barriers such as fresh and marine aquatic ecosystems, mountain ranges and even inhospitable climate zones (Rai, 2015;Kueffer, 2017). Modern intensive agriculture managed for food security, however, has remarkably increased the spread of IAPS (Mack et al, 2000;Gilbert and Levine, 2013;Pimentel et al, 2005;Simberloff et al, 2013;Dudley et al, 2014;Rai et al, 2018).…”

Non‐native tree in a dry coastal area in Hawai'i has high transpiration but restricts water use despite phreatophytic trait

“…However, low leaf water potential, high iWUE and high canopy leaf stable carbon isotope ratio (Dudley et al, 2014) suggest constraints to groundwater absorption by this species and partly explain the modest transpiration rates and stomatal conductance at their daily peaks. However, low leaf water potential, high iWUE and high canopy leaf stable carbon isotope ratio (Dudley et al, 2014) suggest constraints to groundwater absorption by this species and partly explain the modest transpiration rates and stomatal conductance at their daily peaks.…”

“…Roots extend into the ground through cracks in the rock. Our study site was established in a closed P. pallida stand in May 2010 at the same location as Dudley et al (2014). Dominant vegetation consists of P. pallida stands and ephemeral herbaceous species, primarily Sida fallax ('Ilima) and Waltheria indica ('Uhaloa), which partially cover the area following heavy rain events.…”

“…This groundwater, derived primarily from high-altitude recharge, is a critical resource to coastal human communities as well as terrestrial flora and fauna. Dudley et al (2014) documented P. pallida's access to groundwater along leeward coasts and found that this access resulted in considerable increases in the size and density of P. pallida individuals. Unfettered groundwater use by P. pallida, a species capable of high rates of transpiration (Scott et al, 2004), has the potential to reduce freshwater inputs and alter nutrient transport to near-coastal marine environments.…”