Willaredt et al. (2026) The effects of extreme climatic events on urban trees in Mediterranean regions: A review

Identification

• Journal: Current Plant Biology
• Year: 2026
• Date: 2026-03-24
• Authors: Moreen Willaredt, Luisa Velasquez-Camacho, Elizeth Cinto Mejía, Pooja Singh, Alessandro Ossola
• DOI: 10.1016/j.cpb.2026.100605

Research Groups

• Department of Plant Sciences, University of California Davis, Davis, CA 95616, USA
• Department of Entomology and Nematology, University of California Davis, Davis, CA 95616, USA

Short Summary

This review synthesizes existing literature on the effects of climatic and weather-related extremes on urban trees in Mediterranean cities, revealing consistent impairment of urban tree functioning across multiple scales and methods, and highlighting significant research gaps.

Objective

• To systematically synthesize scientific literature to evaluate and compare the impacts of climatic and weather extremes on urban trees in Mediterranean cities.
• To identify the methods used to study the effects of climatic extremes on urban Mediterranean trees.
• To determine which extreme climatic events have been studied across Mediterranean urban contexts.
• To identify the impacts on tree growth, physiology, and canopy response.
• To highlight key knowledge gaps to inform climate-resilient urban forest management and stimulate new research efforts for practical applications like efficient water management and risk assessment.

Study Configuration

• Spatial Scale: Mediterranean cities worldwide, classified by Köppen-Geiger climate system (Csa, Csb). The review analyzed 13 studies focusing on locations including Los Angeles and Santa Barbara (USA), Santiago (Chile), Cuéllar (Spain), Cape Town (South Africa), Lisbon (Portugal), and Naples (Italy). Studies covered individual tree, population, and community ecological scales.
• Temporal Scale: The systematic literature review had no date restrictions, but all 13 included studies were published within the last decade (earliest in 2018). Drought periods were characterized using various indices, with some studies analyzing multidecadal time series. The review summarized 59 drought years across all study locations, with 24 in Los Angeles.

Methodology and Data

• Models used:
  • Standardized Precipitation–Evapotranspiration Index (SPEI), specifically 6-month estimates (SPEI06), was calculated for comparison across studies.
  • Other indices used in reviewed studies include Martonne Index (MI), Aridity Index, and Palmer Drought Severity Index (PDSI).
  • Simulation approaches were used in some reviewed studies to evaluate effects of irrigation regimes.
• Data sources:
  • Systematic literature review following PRISMA guidelines, using Web of Science, Scopus, and Google Scholar databases.
  • Remote sensing data (53% of studies): Landsat imagery (Landsat 8, 5, 4–7, 8–9) and Airborne Visible/Infrared Imaging Spectrometer (AVIRIS, AVIRIS-NG, AVIRIS-C). Derived products included Land Surface Temperature (LST), Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI), Modified Chlorophyll Absorption Ratio Index (MCARI), Photochemical Reflectance Index (PRI512), fractional green vegetation cover, canopy cover, and equivalent water thickness (EWT).
  • Experimental studies (23% of studies): Physiological measurements (e.g., leaf-level thermal tolerance, water relations, gas exchange, photosystem II (PSII) efficiency, photosynthetic pigments, antioxidant activity) and functional traits (e.g., leaf water potential, specific leaf area) collected under controlled laboratory/greenhouse conditions or in situ.
  • Field-based studies (23% of studies): Dendrochronological analyses (tree-ring records) and observational field studies.

Main Results

• Only 13 studies met the review's inclusion criteria from over 3000 screened, highlighting a significant research gap.
• Most studies focused on drought and heat waves, with only one addressing extreme wind effects on tree mortality.
• Climatic extremes consistently impaired urban tree functioning across multiple scales and methods.
• Remote sensing analyses detected declines in canopy greenness, canopy cover (e.g., an average 14% decline in urban tree canopy reported in one study), and reduced cooling effects. An average decline in NDVI by 0.023 during drought was found in Los Angeles.
• Field-based studies documented reductions in leaf area and photosynthesis. Drought consistently reduced tree growth across species, though the magnitude and recovery rates varied. Irrigation can accelerate growth but may compromise drought resilience in mature trees, while non-irrigated trees showed greater growth sensitivity to interannual precipitation variability.
• Simulation approaches highlighted the loss of evaporative cooling in irrigated trees under water restriction.
• Physiological responses included reduced photosynthesis (more pronounced in deciduous species), increased stomatal closure and water use efficiency (particularly in deciduous trees), reduced specific leaf area, and increased leaf dry matter content.
• No drought-focused study reported tree mortality; the only study on extreme wind documented wind-induced tree fall and loss.
• Studied drought intensities (based on SPEI06) ranged from -2.4 (Cuéllar, Spain, 2019) to -0.6 (Los Angeles, CA, 2016).

Contributions

• Provides a systematic synthesis of the fragmented and geographically uneven literature on the impacts of climatic extremes on urban trees in Mediterranean cities.
• Identifies a critical research gap in understanding tree physiology, growth, and adaptive capacity at species and individual tree levels in these vulnerable urban environments.
• Highlights the prevailing anthropocentric focus in urban forestry research (ecosystem services) and calls for more studies on tree vulnerability and resilience.
• Underscores the need for interdisciplinary research integrating soil science, hydrology, and social sciences with urban forestry.
• Emphasizes the limitations of remote sensing for fine-scale processes and the need for more experimental and detailed field-based studies.
• Questions the applicability of regional climatic drought indices in urban environments due to urban heat island (UHI) effects and impervious surfaces.
• Offers crucial insights for developing climate-adaptive urban forest management practices, informing sustainable tree species selection, and predicting tree damage and failure to sustain ecosystem services.

Funding

• Deutsche Forschungsgemeinschaft (DFG, German Research Foundation - 529031514)
• Challenge Cost Share Agreement, Pacific Southwest Research Station, USDA Forestry Service (23-CS-11272123–031)
• National Institute of Food and Agriculture (Award number 202-67019-37136)
• Foundation for Food & Agriculture Research (New Innovator Award, Grant ID: 22–000107)
• Hatch project CA-D-PLS-2735-H

Citation

@article{Willaredt2026effects,
  author = {Willaredt, Moreen and Velasquez-Camacho, Luisa and Mejía, Elizeth Cinto and Singh, Pooja and Ossola, Alessandro},
  title = {The effects of extreme climatic events on urban trees in Mediterranean regions: A review},
  journal = {Current Plant Biology},
  year = {2026},
  doi = {10.1016/j.cpb.2026.100605},
  url = {https://doi.org/10.1016/j.cpb.2026.100605}
}