#urban_vegetation
#urban_trees
#species_distribution_models

2019 study by Hugh Burley from Macquarie University, Australia and co-authors

Background information

(1) Although global governments are placing great emphasis on using urban trees to improve the adaptability and sustainability of cities under cilmate change, there are little consideration on whether the selected urban tree species will be resilient to climate changes that may happen during their lifespan. Climate change has been demonstrated to shift species taxa for many natural plants, crops, and invasive species, but few such attention has been paid to urban trees.

(2) Several previous studies reported shifts in the growth of urban trees, which were likely due to shifting climate [Lanza & Stone 2016; Nitschke et al. 2017; Pretzsch et al. 2017]. Analysis across climate gradients showed that trees that were adapted to cool environments in the natural setting tended to be planted in warmer cities, and vice versa [Kendal et al. 2018].

(3) The urban heat island may exacerbate heat stress on trees, but irrigation may be available to urban trees [Jenerette et al. 2016; Vogt et al. 2017].

(4) Climate suitability models, i.e. species distribution models, can describe the environmental tolerances of the species, and can be used to identify un-occupied suitable areas, or project climate impacts on species.

Study objective

(1) Assess the climatically suitable habitat for 176 native Australia urban tree species during the baseline (1960-1990), short term (2030) and long term (2070) periods

- which tree species are more likely to experience increases and decreases in the habitat sizes

- which urban areas are likely to see more/fewer tree species that can use it as a suitable habitat

- hypothesis: urban areas in cooler regions will gain in species, and warmer regions lose species; previous findings in the natural setting revealed this to be true [O'Donnell et al. 2012]

Data and methods

(1) The native tree species kept by Australia nurseries were obtained from online resources. The existence of these tree species were further verified by contacting local government authorities, resulting in data for 44 local councils spanning 49 "significant urban areas" (SUAs). The final results were 248 species.

(2) The spatial occurrences of the species were obtained from GBIF and the Atlas of Living Australia (www.ala.org.au, rgbif package, ALA4R package). Due to the relative lack of data for the arid and tropical regions of Australia, the analysis was limited to the temperate Köppen zones in Australia.

(3) The climatic variables for the species distribution modeling were obtained from WorldClim and included 8 bioclimatic variables: 1) annual mean temperature, 2) temperature seasonality, 3) maximum temperature of the warmest month, 4) minimum temperature of the coldest month, 5) annual precipitation, 6) precipitation seasonality, 7) precipitation of the wettest month, 8) precipitation of the driest month.

(4) The species distribution model was Maxent. The performance was assessed using the Area Under the Receiver Operating Characteristic (AUC) and the Truee Skill Statistic (TSS) through five-fold cross-validation. The Multivariate Environmental Similarity Surfaces (MESS) was further used to indicate whether the models were applied on novel states of individual variables, which can cause the model to over-project suitability. Note MESS cannot indicate whether novel combinations of variables occurred. The MESS results further excluded 72 species, due to the insufficient ability of the above 8 variables to characterize their niche. Therefore, 176 species were analyzed in the end.

Results

(1) The climatically suitable habitat will decline from 16,152 km2 in the baseline to 13,043 km2 in 2030, and 12,300 in 2070. Among the 176 species, 18% will lose >50% of their habitat by 2030, 34% will lose >50% by 2070, and 11 species will increase their habitat by >50%. Generally, suitable habitat will shift polewards, so most of the species gained habitat in the south, and lost in the north.

(2) The hotter places (higher mean annual temperature & mean annual maximum temperature) tended to gain fewer species, and lose more species.

(3) At present, each SUA contained suitable climate for 10-139 of the 176 species, and on average 74 species, but by 2070, the average suitable species declined to 63. 21 of the 82 SUAs, mainly in the cooler regions, increased in the number of suitable species between the baseline and 2070.

Discussion

(1) The poleward shift in climatically suitable habitat in urban areas (SUAs) parallels the change in natural areas.

(2) Species and urban areas in cooler regions would fare better than those in warmer regions, which also supports the findings of previous studies [Jenerette et al. 2016; Kendal et al. 2018].

(3) Some caveats are that: the study area is limited to the temperate regions of Australia, and the tropical & subtropical & arid area trees may respond differently; only macroclimatic predictors were considered, microclimate, extreme weather, and edaphic conditions need to be considered by future studies; management factors like irrigation may increase the habitat range of some species despite warming.

(4) Heat waves and drought can affect the growth of urban tree species [Nitschke et al. 2017]. Climate change also affects the growth rate [Pretzsch et al. 2017; Jia et al. 2018].

Referenced studies

Jenerette, G.D., Clarke, L.W., Avolio, M.L., Pataki, D.E., Gillespie, T.W., Pincetl, S., Nowak, D.J., Hutyra, L.R., McHale, M., McFadden, J.P., Alonzo, M., 2016. Climate tolerances and trait choices shape continental patterns of urban tree biodiversity. Glob. Ecol. Biogeogr. 25, 1367–1376.

Jia, W., Zhao, S., Liu, S., 2018. Vegetation growth enhancement in urban environments of the conterminous United States. Glob. Chang. Biol. 24, 4084–4094.

Kendal, D., Dobbs, C., Gallagher, R.V., Beaumont, L.J., Baumann, J., Williams, N.S.G., Livesley, S.J., 2018. A global comparison of the climatic niches of urban and native tree populations. Glob. Ecol. Biogeogr. 27, 629–637.

Nitschke, C.R., Nichols, S., Allen, K., Dobbs, C., Livesley, S.J., Baker, P.J., Lynch, Y., 2017. The influence of climate and drought on urban tree growth in Southeast Australia and the implications for future growth under climate change. Landsc. Urban Plan. 167, 275–287.

Pretzsch, H., Biber, P., Uhl, E., Dahlhausen, J., Schütze, G., Perkins, D., Rötzer, T., Caldentey, J., Koike, T., Con, T.V., Chavanne, A., Toit, B.D., Foster, K., Lefer, B., 2017. Climate change accelerates growth of urban trees in metropolises worldwide/631/158/858/704/158/ 2165 article. Sci. Rep. 7, 15403.

https://www.sciencedirect.com/science/article/pii/S0048969719323289?via%3Dihub