Morgan et al. (2025) Co-regulation of water use and canopy temperature in desert trees

Identification

• Journal: Agricultural and Forest Meteorology
• Year: 2025
• Date: 2025-11-20
• Authors: Bryn E. Morgan, Anna T. Trugman, Kelly K. Caylor
• DOI: 10.1016/j.agrformet.2025.110929

Research Groups

• Department of Geography, University of California, Santa Barbara, Santa Barbara, CA, USA
• Earth Research Institute, University of California, Santa Barbara, Santa Barbara, CA, USA
• Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, CA, USA

Short Summary

This study assessed how desert trees co-regulate water use and canopy temperature under varying hydroclimatic stress using UAV-based remote sensing, finding species-specific water-use strategies but similar thermal responses, with a decoupling of water and temperature regulation under hot, wet conditions.

Objective

• To assess how desert trees co-regulate their water status and temperature under fluctuating water supply and atmospheric demand conditions.
• To evaluate the relationships between water and temperature regulation in individual plants, particularly the tradeoffs between hydraulic function and avoiding thermal stress.

Study Configuration

• Spatial Scale: Three riparian forest sites along the ephemeral Kuiseb River in the central Namib Desert, Namibia (one wetter site at 15.2942° E, -23.6666° S, 140 km upstream; two drier sites at 15.0315° E, -23.5569° S, 104 km upstream, and 14.8702° E, -23.3601° S, 74 km upstream). Data were collected at the scale of individual tree canopies (approximately 40-50 cm spatial resolution) for 306 individual trees (267 included in final analysis).
• Temporal Scale: Two seasonal campaigns in 2022: hot, wet season (10–14 May) and cool, dry season (22–26 September). Five diurnal flights were conducted at each site in approximately 90-minute intervals between 09:00 and 15:30 local time over 1–2 days per season. Data from 20 wet season and 17 dry season flights were analyzed.

Methodology and Data

• Models used:
  • Novel surface energy balance algorithm (Morgan and Caylor, 2023) for retrieving plant-scale transpiration.
  • Flux-gradient equation for latent heat (Eq. 1) to derive canopy resistance.
  • Logarithmic relationship (Oren et al., 1999) to describe canopy conductance ($gc$) sensitivity to leaf-to-air vapor pressure deficit ($VPDl$) (Eq. 2).
  • Second-order polynomials to fit transpiration rates.
  • Linear regression to derive canopy warming rates ($dTc/dTa$).
• Data sources:
  • Near-surface remotely sensed retrievals of canopy conductance, transpiration, and temperature using Unmanned Aerial Vehicles (UAVs).
  • UAV platform: DJI Matrice 600 Pro hexacopter equipped with a MicaSense Altum multispectral and thermal infrared sensor, Emlid Reach M2 GNSS module, two LI-200R pyranometers, and a TriSonica Mini Wind and Weather Sensor.
  • Meteorological data (air temperature, relative humidity, air pressure, wind speed) collected at 5 Hz at 1.5 m height.
  • Field surveys of tree locations, species (Acacia erioloba and Faidherbia albida), diameter-at-breast height (DBH), trunk locations, and crown perimeters using a high-precision GNSS unit (Reach RS2).
  • Normalized Difference Vegetation Index (NDVI) from UAV images to exclude non-vegetated pixels.
  • Historical meteorological data from Gobabeb meteorological station (2015–2022).
  • Flood magnitude and frequency data from Grodek et al. (2020) and the Ministry of Agriculture, Water and Forestry of Namibia (1960–2006).

Main Results

• The two desert tree species (Acacia erioloba and Faidherbia albida) exhibited different water-use strategies in response to supply- and demand-driven water stress but showed similar responses to thermal stress.
• Under unstressed (cool, wet) conditions, F. albida had higher reference conductance ($g{c,ref}$ = 0.166 mol m⁻² s⁻¹) and peak transpiration rates ($E{max}$ = 0.117 mm h⁻¹) than A. erioloba ($g{c,ref}$ = 0.078 mol m⁻² s⁻¹, $E{max}$ = 0.083 mm h⁻¹).
• A. erioloba displayed profligate (anisohydric) behavior, sustaining $gc$ and transpiration near well-watered rates under hot, dry conditions ($g{c,ref}$ = 0.051 mol m⁻² s⁻¹, $E_{max}$ = 0.071 mm h⁻¹), despite a decline under cool, dry conditions.
• F. albida showed conservative (isohydric) behavior, strongly downregulating $gc$ under hot, dry conditions ($g{c,ref}$ = 0.103 mol m⁻² s⁻¹) but surprisingly upregulating $gc$ under cool, dry conditions ($g{c,ref}$ = 0.131 mol m⁻² s⁻¹).
• In the cool season, canopies warmed faster than the air ($dTc/dTa$ > 1) under both wet (1.16 °C °C⁻¹) and dry (1.07 °C °C⁻¹) conditions.
• In the hot season, canopies warmed slower than the air ($dTc/dTa$ < 1) under both wet (0.82 °C °C⁻¹) and dry (0.87 °C °C⁻¹) conditions, preventing average daily maximum canopy temperatures from exceeding 40 °C.
• Under cool conditions (wet: slope 1.63, $r^2$ = 0.09, $p$ = 0.05; dry: slope 1.11, $r^2$ = 0.05, $p$ = 0.001) and hot, dry conditions (slope 3.93, $r^2$ = 0.14, $p$ < 0.001), a positive relationship between $m$ (sensitivity of $gc$ to $VPDl$) and $dTc/dTa$ indicated a tradeoff between water and temperature regulation.
• Under hot, wet conditions, the relationship between $m$ and $dTc/dTa$ was negative (slope -0.88, $r^2$ = 0.05, $p$ = 0.08), indicating a decoupling of water and temperature regulation, where increased sensitivity to $VPD_l$ enhanced temperature regulation.
• Observed maximum canopy temperatures exceeded the suggested critical temperature for photosynthetic function (∼46.7 °C) in 47.6% of observations, more frequently in A. erioloba and under hot, dry conditions.

Contributions

• Introduced a novel co-regulation framework to holistically evaluate the linkages and tradeoffs between plant water and temperature regulation, accounting for responses to water supply, atmospheric demand, and thermal stress.
• Provided fine-scale, individual-plant level insights into water and temperature co-regulation in desert trees using UAV-based remote sensing, resolving behaviors often obscured at coarser scales.
• Revealed two unexpected plant behaviors critical for understanding climate change vulnerability:
  1. Plasticity in F. albida's water-use strategy, with upregulation of canopy conductance ($g_c$) under cool, dry conditions, suggesting a key productivity window vulnerable to increasing winter temperatures.
  2. Decoupling of water and temperature regulation under hot, wet conditions, highlighting the essential role of evaporative cooling for mitigating thermal stress when water availability is sufficient.
• Identified critical gaps in current process-based vegetation models, emphasizing the need to incorporate environmentally dependent hydraulic parameters and the use of water for thermal regulation beyond carbon assimilation.

Funding

• Fulbright U.S. Student Grant (B.E.M.)
• American Philosophical Society Lewis and Clark Fund for Exploration and Field Research (B.E.M.)
• University of California, Santa Barbara Department of Geography Leal Anne Kerry Mertes Award (B.E.M.)
• Zegar Family Foundation (Grant No. SB200109) (B.E.M. and K.K.C.)
• National Science Foundation (Grant No. 2003205) (A.T.T.)
• Gordon and Betty Moore Foundation (Grant No. GBMF11974) (A.T.T.)
• USDI National Park Service (Award Nos. P24AC00910 and P24AC01425) (A.T.T.)
• CALFIRE Forest Health Research Program (Grant No. 60164685) (A.T.T.)

Citation

@article{Morgan2025Coregulation,
  author = {Morgan, Bryn E. and Trugman, Anna T. and Caylor, Kelly K.},
  title = {Co-regulation of water use and canopy temperature in desert trees},
  journal = {Agricultural and Forest Meteorology},
  year = {2025},
  doi = {10.1016/j.agrformet.2025.110929},
  url = {https://doi.org/10.1016/j.agrformet.2025.110929}
}