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

In arid systems, phreatophytes are often among the most effective invaders because of their capacity to access and exploit groundwater resources otherwise unavailable to native vegetation. On Hawai'i Island, a non-native phreatophyte, Prosopis pallida (kiawe), has invaded extensive dry lowland areas following its introduction in the 19th century. To better understand the influence of this invader on the host ecosystem, and vice versa, we determined transpiration characteristics of P. pallida by monitoring sap flux of mature individuals over an 18-month period on the arid leeward coast of Hawai'i Island. Daily sap flux increased with increasing atmospheric vapour pressure deficit (D) and exhibited no clear response related to rainfall events or prolonged drought. Annual transpiration (i.e. 308 mm) was 80% higher than rainfall. Stomatal conductance decreased with increasing vapour pressure deficits more slowly than the theoretical stomatal conductance–D relationship that assumes stomatal regulation of leaf water potential above a critical level. While daily peak stomatal conductance was relatively high, it nonetheless appeared limited by high intrinsic water use efficiency (net photosynthetic rate per stomatal conductance), suggesting a constraint to unlimited groundwater absorption. In this young volcanic environment where rainfall is low and quite episodic, the success of this species likely stems from its anisohydric strategy even at the risk of cavitation, and it is altering the hydrological cycling of the areas where it is dominant.