Increasing the broad-leaved tree fraction in European forests mitigates hot temperature extremes

Abstract

Forests influence climate through a myriad of chemical, physical and biological processes and are an essential lever in the efforts to counter climate change. The majority of studies investigating potential climate benefits from forests have focused on forest area changes, while changes to forest management, in particular those affecting species composition, have received much less attention. Using a statistical model based on remote sensing observations over europe, we show that broadleaved tree species locally reduce land surface temperatures in summer compared to needle-leaved species. The summer mean cooling effect related to an increase in broad-leaved tree fraction of 80% is relatively modest (~ 0.3-0.75 K), but is amplified during exceptionally warm periods. The reduction of daily maximum temperatures during the hottest days reaches up to 1.8 K in the Atlantic region and up to 1.5 K in Continental and Mediterranean regions. Hot temperature extremes adversely affect humans and ecosystems and are expected to become more frequent in a future climate. thus, forest management strategies aiming to increase the fraction of broad-leaved species could help to reduce some of the adverse local impacts caused by hot temperature extremes. However, the overall benefits and trade-offs related to an increase in the broad-leaved tree fraction in European forests needs to be further investigated and assessed carefully when adapting forest management strategies. Forests are expected to play an essential role in climate change mitigation as they can generally sequester more carbon than non-forested ecosystems 1-3. In addition, forests affect water and energy fluxes at the earth surface through biogeophysical processes including changes in evapotranspiration, albedo, and surface roughness 4,5. Various observation-based studies have shown that forests, through these biogeophysical processes, either reduce or increase local temperatures depending on location and time of observation 6-9. In contrast to a comparison of forested and non-forested ecosystems, the potential impacts on temperatures of forest management or more generally changes in forest characteristics are less well documented 10-12. Facilitating an increase of the broad-leaved tree fraction (BTF) in forests is a promising management strategy to enhance the provision of ecosystem services and to adapt to climate change 13-16. For example, increasing the BTF can lead to reduced risk of fires, wind throw and bark beetle outbreaks 15,17. However, the potential benefits of broad-leaved trees through their biogeophysical influence on temperature, in particular on extreme temperatures, have not yet been investigated beyond the site-level scale 18 even though changes on extreme temperatures are highly relevant in terms of impacts on humans and ecosystems 19,20. To investigate how an increase in the BTF in Europe would influence local land surface temperature (LST) we linked observed patterns of LST with patterns of the BTF. In contrast to previous studies, we use remote sens...

The role of urban trees in reducing land surface temperatures in European cities

“…Our analysis of remote-sensing based LST profits from high spatial resolution and geographic coverage but is limited by temporal resolution. A low temporal resolution and early observation time (around 10:15 a.m.) leads to increased uncertainties particularly when predicting LULC temperature differences during hot extremes for which ideally highly resolved temporal data should be used 51 . In addition, remote sensing LST data is mainly derived during cloud-free conditions 52 , and hence it is rather impossible to infer LST differences between vegetated land and urban fabric during cloudy conditions.…”

“…The models were fitted using the package mgcv 80 embedded in the R computing environment 81 . GAMs can be used to estimate temperatures based on a variety of predictor variables and hence can account for potential confounding factors, which has shown to be very relevant in the analysis of LULC temperature impacts 6 , 51 . All GAMs are calibrated including LST observations as response variable and several predictor variables (Fig.…”

Forest resilience to global warming is strongly modulated by local‐scale topographic, microclimatic and biotic conditions

“…We observed that forest resilience indices differed in areas affected by contrasting levels of thermal exposure during extreme heatwave events. Thermal buffering and amplification effects during extreme heatwave events remain still poorly described in Mediterranean ecosystems and elsewhere (Carnicer, Stefanescu, et al., 2019; Frey et al., 2016; Schwaab et al., 2020). In this paper we report a first detailed description of the modulation of the thermal exposure in tree saplings during an unprecedented extreme summer heatwave event, based on detailed field measurements (van Oldenborgh et al., 2019; Zhao et al., 2020).…”

Empirical estimate of forestation-induced precipitation changes in Europe

“…7) with a Generalized Additive Model (GAM) to infer the contribution of LC on the spatial distribution of precipitation (Extended Data Table 1). Statistical models were successfully used as a tool to assess both local 32 and remote 33 temperature changes induced by LCC. For the GAM presented here, we consider topographic effects, which strongly modulate the spatial distribution of precipitation 34 , by including a number of topographic metrics as predictors (Supplementary Fig.…”