Water relations and microclimate around the upper limit of a cloud forest in Maui, Hawai'i

Abstract

The goal of this study was to determine the effects of atmospheric demand on both plant water relations and daily whole-tree water balance across the upper limit of a cloud forest at the mean base height of the trade wind inversion in the tropical trade wind belt. We measured the microclimate and water relations (sap flow, water potential, stomatal conductance, pressure-volume relations) of Metrosideros polymorpha Gaudich. var. polymorpha in three habitats bracketing the cloud forest's upper limit in Hawai'i to understand the role of water relations in determining ecotone position. The subalpine shrubland site, located 100 m above the cloud forest boundary, had the highest vapor pressure deficit, the least amount of rainfall and the highest levels of nighttime transpiration (EN) of all three sites. In the shrubland site, on average, 29% of daily whole-tree transpiration occurred at night, while on the driest day of the study 50% of total daily transpiration occurred at night. While EN occurred in the cloud forest habitat, the proportion of total daily transpiration that occurred at night was much lower (4%). The average leaf water potential (Ψleaf) was above the water potential at the turgor loss point (ΨTLP) on both sides of the ecotone due to strong stomatal regulation. While stomatal closure maintained a high Ψleaf, the minimum leaf water potential (Ψleafmin) was close to ΨTLP, indicating that drier conditions may cause drought stress in these habitats and may be an important driver of current landscape patterns in stand density.

Vapor pressure deficit predicts epiphyte abundance across an elevational gradient in a tropical montane region

“…While elevation at the large scale has clear effects on microclimate, our results indicate that canopy VPD was the best predictor of epiphyte abundance. The VPD has been shown to be an important driver of evapotranspiration at the stand level in a number of ecosystems (Bucci et al, 2004; Dawson et al, 2007; Eller et al, 2015; Gotsch et al, 2014a, b; Motzer et al, 2005). Estimates of epiphyte abundance can be used as a proxy for canopy VPD, which can be used to compare the microclimate of TMCF sites and facilitate comparative research.…”

“…Our sites span an elevation of 1100–1635 m above sea level (m a.s.l.) and represent a range in microclimate that is representative of this and other TMCF ecosystems (Gotsch et al, 2014a, b; Jarvis and Mulligan, 2011). Sites were selected in 2014 as part of a long‐term study on the ecophysiology of epiphytes.…”

Strategies of leaf water uptake based on anatomical traits

“…Leaf water uptake may play a particularly important role in periods of soil water deficit if leaf-wetting events, such as fog, occur. Absorption of fog water through leaves is capable of increasing leaf water potential and stomatal opening, positively affecting the carbon balance Eller et al 2013;Gotsch et al 2014Gotsch et al , 2016Darby et al 2016). In tropical montane ecosystems, the formation of orographic fog is favoured by winter climate conditions, which in some regions coincide with the dry season.…”

Evaluating canopy transpiration and water use of two typical planted tree species in the dryland Loess Plateau of China

“…Meanwhile, the response of plant water use to precipitation and soil water conditions is associated with stomatal control de Dios et al, 2014). Stomatal closure is the main mechanism that regulates transpiration, avoiding irreversible damage to plant hydraulic systems (Gotsch et al, 2014). Hysteresis is one of the most obvious phenomena related to plant transpiration that has been examined as a function of micrometeorological variables (Rodriguez-Dominguez et al, 2012;Zheng & Wang, 2014).…”