Determination of urban trees resilience, resistance and recovery to drought under variable nutrient availability in the city of Zurich

Climate change is becoming increasingly evident, and as it progresses, cities are facing environmental challenges that directly affect the growth of urban trees, essential elements from which people derive numerous benefits. Urban trees are exposed to high temperatures, reduced precipitation, and spatial limitations, all of which impact their growth and health, thereby compromising the environmental services they can provide. The analysis of urban tree growth serves as an important tool for understanding their response to drought events. Tree response to drought is determined by their growth during a drought event, the speed at which they resume pre-drought growth, and their ability to maintain stable growth once favorable conditions return. Factors such as available soil volume and nutrient levels in the soil can significantly influence how well trees cope with water scarcity. This study assessed the drought response of three tree species in the urban context of Zurich, Acer platanoides, Betula pendula, and Robinia pseudoacacia. Specifically, it aimed to understand how differences in nutrient availability and location conditions affect tree recovery following drought events. To evaluate tree response, annual ring widths were measured. The values were detrended to eliminate age-related biases and were cross-dated to minimize error. These values were then compared with two drought indices representing seasonal and annual drought severity, SPEI-3 and SPEI-12. In addition, chemical and isotopic analyses were carried out on wood samples, as well as on soil and leaf samples from the same trees. For each species, individuals located near roads, subject to the presence of dogs and therefore likely to receive additional nutrient input, were compared to trees growing in open areas, such as schools or playgrounds, where such external inputs were absent. The results highlight that the species exhibit different responses, both in how drought events are reflected in their ring growth and in their nutrient uptake under varying urban conditions. Acer platanoides showed greater sensitivity to drought in its growth patterns, yet demonstrated high recovery capacity in environments with greater nutrient availability. Betula pendula exhibited a less pronounced response to drought in its ring width and showed greater resistance in areas with increased soil volume. Robinia pseudoacacia, on the other hand, displayed a generally more stable response across conditions. Among the nutrients examined, potassium was most strongly associated with drought tolerance indices, particularly in A. platanoides and B. pendula. These findings suggest that drought response in urban trees is highly species-specific and influenced by local urban factors. This work introduces new elements for understanding how urban trees adapt to climate-related stressors and draws attention to the importance of urban management strategies that aim to create better integration between urban development and the environment. In the context of a changing and increasingly challenging climate, such insights may serve as valuable guidance for planning more resilient urban spaces.