Ecophysiological controls on water use of tropical cloud forest trees in response to experimental drought
Description
Tropical montane cloud forests (TMCFs) are expected to experience more frequent and prolonged droughts over the coming century, yet understanding of TCMF tree responses to moisture stress remains weak compared to the lowland tropics. We simulated a severe drought in a throughfall reduction experiment (TFR) for two years in a Peruvian TCMF and evaluated the physiological responses several dominant species (Clusia flaviflora, Weinmannia bangii, Weinmannia crassiflora, Prunus integrifolia). Measurements were taken of: i) sap flow ii) diurnal cycles of stem shrinkage, stem moisture variation, and water use; iii) intrinsic water use efficiency (iWUE) estimated from foliar δ13C. In Weinmannia bangii, we used dendrometers and volumetric water content (VWC) sensors to quantify daily cycles of stem water storage. In two years of sap flow (Js) data, we found a threshold response of water use to VPD > 1.07 kPa independent of treatment, though control trees used more soil water than the treatment trees. The daily decline in water use in the TFR trees was associated with a strong reduction in both morning and afternoon Js rates at a given VPD. Soil moisture also affected the hysteresis strength between Js and VPD. Reduced hysteresis under moisture stress implies that TMCFs are strongly dependent on shallow soil water. Additionally, we suggest that hysteresis can serve as a sensitive indicator of environmental constraints on plant function. Finally, six months into the experiment, the TFR treatment significantly increased iWUE in all study species. Our results highlight the conservative behavior of TMCF tree water use under severe soil drought and elucidates physiological thresholds related to VPD and its interaction with soil moisture. The observed strongly isohydric response likely incurs a cost to the carbon balance of the tree, and reduces overall ecosystem carbon uptake.
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This study was conducted in a tropical montane cloud forest located near the Wayqecha Biological Station in the Kosñipata catchment, Peru. The study site was situated at an elevation of 3000 meters on the eastern slope of the Andes. The soils in the area were classified as Umbrisols, characterized by a shallow depth of 30 cm and consisting of an acidic organic-rich A layer with a thin stony B/C horizon. The local climate was highly humid due to prevailing winds and topography, resulting in frequent moist cloud immersion from the Amazonian lowlands. The rainy season spanned from October to May, with an annual precipitation of approximately 1800 mm. Additionally, fog deposition contributed around 10% of the total hydrological input. The forest at the study site exhibited a closed, multi-level canopy with a sparse understory and a diverse array of epiphytes covering the tree stems and branches. To investigate the effects of throughfall exclusion on tree physiological responses, a transparent plastic sheet was attached to a wooden frame in a 30 × 30 m throughfall reduction (TFR) plot. The TFR plot effectively excluded throughfall at an estimated rate of 95%. Control trees of the same species and size were selected outside the TFR plot for comparison. Soil moisture sensors were installed at different depths in both the TFR plot and control treatment, revealing a drastic reduction in soil moisture within the TFR plot. The volumetric water content (VWC) in the TFR plot was approximately 85% lower than in the control plot during the rainy season. Hourly measurements of air temperature and humidity were recorded, and vapor pressure deficit (VPD) was calculated as a measure of water demand. Sap flow sensors were used to measure sap flux density (Js), and daily sap flow values were integrated to estimate stand transpiration. Stem diameter variation (SDF) and stem volumetric water content (VWCstem) were monitored using point dendrometers and stem moisture sensors, respectively. Leaf samples were collected for carbon isotope ratio analysis to estimate intrinsic water use efficiency (iWUE) based on δ13C values.