CLIMATE CHANGE THREATENS CARBON STORAGE IN EUROPE’S URBAN TREES

This github site contains all relevant code for the above research paper

KEYWORDS

Ecosystem Services, Rainfall, Thermal Tolerance, Urban Heat Islands, Urban Forests, Urban Planning

HIGHLIGHTS

We calculate climate niches of ~ 200 tree species recorded in 22 European cities We predict thermal and hydraulic safety gaps/margins in 2050 & 2070 for each species/city By 2070 many species will experience climates outside their current niches Carbon storage, and other services, are threatened across European cities Urgent mitigation is needed to increase climate change resilience of urban forests

ABSTRACT

Urban trees contribute substantially to numerous ecosystem services. Here we quantify the threat to carbon stored by urban trees from increased heat and drought arising from climate change. We use data from tree inventories in 22 European cities, spread across five Köppen-Geiger climatic zones, that record ~1.2 million trees from 188 species. We calculate species’ climatic niches using global tree distribution data and estimate species-specific thermal and hydraulic safety gaps and margins for each city in 2050 and 2070 using the RCP 8.5 emissions scenario. This scenario provides the best match for emissions to at least 2050 under current and stated policy plans, and highly plausible emission levels to 2100. We then assess the proportion of current carbon storage at risk from changes in temperature (associated with thermal stress) and precipitation changes (associated with hydraulic stress). By 2070 a substantial amount of the current carbon storage in urban trees is projected to be threatened by climatic stress. Average values (depending on the precise methods used for calculating climatic niches) are: 99.96% - 99.98% in the cold semi-arid climate zone; 82.97% - 92.61% in the humid subtropical zone, 69.72% - 72.00% in the warm Mediterranean zone, 44.18% - 55.06% in the humid continental zone and 29.60% - 43.22% in the temperate oceanic zone – although within each climatic zone risks are lower in some cities. In each climatic zone the vast majority of this threat is associated with thermal stress, with precipitation changes projected to be a comparatively minor threat. Our analyses highlight individual species which are particularly vulnerable to future climatic conditions, and more resilient species that if rapidly planted on mass could improve resilience of urban tree stocks to climate change. Our findings inform the development of climate-ready urban forestry and planning strategies that will facilitate long term carbon storage capacity of Europe’s urban forests, and emphasise the urgency of doing so.

FIGURES

Some key figures are included here but please see supplementary material and the main manuscript for all figures.

Figure 1. Location of the 22 study cities coloured by Köppen-Geiger climate zone (as defined by Beck et al., 2018). Cities located at the edges of climatic zones were assigned to the zone that covered the largest proportion of the focal urban area.

Figure 2. Schematic explaining the calculation of a) thermal safety margin, b) thermal safety gap, c) hydraulic safety margin, and d) hydraulic safety gap using the Common Oak Quercus robur and climatic niches defined using both the 5th and 95th percentiles and 2nd and 98th percentiles. Yellow and blue boxes respectively illustrate the species’ thermal and hydraulic niche. Downward arrows show climatic conditions of the focal city in 2070 and horizontal arrows indicate safety margins (when inside the niche) and gaps (when outside the niche). In 2070 Quercus robur is thus predicted, when using the 5th and 95th percentiles, to have thermal and hydraulic safety margins respectively of 1.3°C and 7.4 mm in Bristol (UK, located within the temperate oceanic climate zone), but have significant thermal (11.7°C) and hydraulic (20.8 mm) safety gaps in Madrid (Spain, located within the cold semi-arid climatic zone).

Figure 3. Mean hydraulic and thermal safety margins/gaps of all species within each climate zone in 2070. Each point represents a species, with its size representing the percentage of the climate zone’s current carbon stored in that species. Species in the lower left green quadrant are safest with species having both thermal and hydraulic safety margins. Species in orange areas have only thermal safety gaps (bottom right quadrant) or hydraulic safety gaps (top left quadrant). Species in the red quadrant (top right) have both hydraulic and thermal safety gaps. Percentages in each quadrant show the amount of carbon stored by all species in each quadrant. Note the variation in the Y axis scales across climate zones. Examples of species in each climate zone’s quadrant are provided in table 2 (5th-95th percentiles) and table S2 (2nd-98th percentiles).

Table 1 Results from binomial logistic models of the percentage of carbon stored in urban trees at baseline that will be threatened in the future due to thermal or hydraulic safety gaps arising from climate change across 22 European cities. Parameter estimates (± one standard error) are reported for time point (reference set to baseline) and climatic zone (reference set to the temperate oceanic climate). Analyses are conducted using alternative climatic niche definitions (percentiles of occupied climatic niches).