Protected forests worldwide face growing thermal stress as cities expand

Protected forests that shield biodiversity from development pressures are now facing a different, rapidly intensifying threat: heat. A new study drawing on global observations finds that forests inside protected areas are experiencing rising thermal stress as surrounding cities expand, amplifying temperature extremes that can disrupt ecosystem function.

Researchers synthesized evidence across multiple regions to quantify how urbanization alters the local climate experienced by forest canopies. Rather than relying solely on weather stations near cities, the team examined forest-level conditions to capture how heat is transported from built environments into adjacent natural landscapes.

The results indicate that the “urban heat island” effect extends beyond city boundaries and increasingly overlaps with protected forest habitats. As impervious surfaces grow and land cover shifts, heat retention increases at night, elevating minimum temperatures over time and raising baseline thermal stress for species that are not adapted to sustained warming.

Crucially, the study links thermal pressure to biological vulnerability. Higher heat loads can reduce photosynthetic efficiency, accelerate water loss, and increase the frequency of physiological strain during hot spells. Even when average temperatures do not appear dramatic, repeated exposure to heat extremes can drive cumulative damage.

To strengthen causal interpretation, the researchers controlled for major environmental gradients and compared trends across forested protection zones exposed to varying intensities of nearby urban growth. Where urbanization was stronger, thermal indicators were consistently elevated, suggesting that protection status does not fully insulate forests from regional climate forcing.

The paper also highlights spatial heterogeneity. Forests bordering expanding urban corridors face sharper increases, while more isolated protected areas show comparatively weaker thermal escalation. This pattern implies that planning decisions—such as maintaining green buffers and limiting high-density expansion near reserves—could meaningfully alter future exposure.

Because thermal pressure interacts with drought, wildfire risk, and habitat fragmentation, the authors emphasize that heat is not acting alone. Instead, urban-driven warming likely compounds multiple stressors that can push sensitive species past their tolerance thresholds.

The study appears in Nature Communications and provides a global framework for understanding how urban growth reshapes the climate inside protected areas. The authors call for conservation strategies that integrate urban planning with ecological resilience, treating heat exposure as a measurable threat requiring mitigation, not an unavoidable background condition.