Trees and shrubs in urban green areas constitute a significant potential for carbon sequestration, and therefore, for climate change mitigation in our planet. Knowledge, however, of the responses of these organisms to the changes in temperature and precipitation predicted for the coming decades is necessary to understand their future, as well as the future of management planning of forested urban green areas.
Climate change projections foresee global warming between 2ºC and 4ºC by the end of the century, which will lead to an increase in the frequency and intensity of severe droughts and heat waves. This scenario could jeopardize the functioning of terrestrial ecosystems worldwide and compromise the provision of multiple ecosystem services essential to human societies. So far, the combination of high temperatures and droughts has been associated with a significant reduction in plant growth, as well as an increase in mass die-off events and vigour decline. There is evidence to predict that many of the ecosystems we know today will change over the next few decades. However, it is many of these ecosystems that paradoxically help us most significantly to mitigate climate change through the absorption of atmospheric CO2.
Carbon sink capacity is currently studied in natural and managed forests around the world. The most recent studies suggests that forests contribute very positively to removing carbon from the atmosphere through photosynthesis. Any tree or woody plant in general thus contributes to removing CO2 from the atmosphere and store it in the form of wood. In this sense, a multitude of trees and garden areas in urban environments could play an important role, despite having received less attention from scientists because they are artificial environments. Gardens in general represent highly diverse, ad hoc created ecosystems, where the constituent elements, in this case trees and bushes, are distributed in space following aesthetic and/or cultural criteria. Many of the species present in these gardens are native to the place where they are planted, however, others have remote origins sometimes in completely different climatic environments, and therefore can have little capacity to adapt to rising aridity and warming under climate change. This makes a large part of the tree species present in the gardens especially vulnerable to climate change, which limits their mitigating capacity and, in parallel, also their aesthetic, botanical and even historical interest. A project leaded by one of the members of the EiFAB within the Association for Nature protection “El Espadañal” and funded by the Obra Social La Caixa (2021-2023), aimed to evaluate the climate change mitigation capacity of garden areas and avenues in the village of Cuéllar (Segovia, Spain). Evaluating this mitigating capacity and its contingency in the vulnerabilities to drought and heat waves will allow us to anticipate the level of risk faced by the different tree species in the gardens and with it, the potential changes of urban green areas in the long run.
The project proposed the use of two complementary databases: 1) dendrochronology in the most representative species, and 2) general inventory of trees in the most important parks and avenues. So far, almost 1500 trees of 67 species have been inventoried. Growth samples in turn, were taken from 196 trees distributed in 9 different species. The average diameter at breast height (DBH) is 41.8 cm [±13.7], being the individuals of the species Styphnolobium japonicum, those with larger diameters (78.8 cm [±0.6] DBH), and the species Cupressus lusitanica the one with smaller DBH (30.3 cm [±8.2]). Regarding height, the overall average of the sampled trees was 11. 49 m [±3.1], with the species Cedrus atlantica presenting the highest average height values (14.32 m [±3.7]), and the species Styphnolobium japonicum the lowest values (7.9 m [±2.12]). Finally, the largest tree sampled, with 89 cm DBH and 20 m in height, is an Atlas cedar (Cedrus atlantica) located in the courtyard of the CEIP Santa Clara School.
Early analyses of growth and carbon sequestration suggest that tree species such as cypresses (non-irrigated) have fixed ≈25,472 g of C per year in wood over the past 50 years. Most interestingly, while the radial growth of trees in non-irrigated gardens remarkably shows the impact of dry years such as 2005 or 2012, it is those in irrigated areas that recover the worst after drought events. This result highlights the potential vulnerability of many urban tree species to dry years, especially in irrigated areas where trees may develop characteristics or traits that make them poorly adapted to the lack of water in particularly dry and warm years.
Results of dendrochronology applied to Cedrus atlantica trees. a) Tree ring width values averaged for watered (purple) and unwatered (orange) trees of the species Cedrus atlantica. Highlighted in dotted red arrows are the dry years 2005, 2012, 2017. Annual precipitation in blue. b) Resilience for the dry years 2005 (gold), 2012 (light blue) and 2017 (grey) in watered and unwatered trees.
The climatic challenge over the coming decades imposes drastic changes in the definition and configuration of wooded urban areas in the face of the dual role of adaptation-mitigation of these formations. Urban planning policies will have to change the current priorities based on the aesthetic aspect for priorities focused on adaptation and resilience. Thus, it will be necessary to change the traditional aesthetic paradigms for more ecological models aimed at promoting native species, well adapted to drought and heat waves, and maintained with the least input of resources (water and nutrients).