In Germany, about one third of the population lives in cities. This results in a high degree of sealing and heating of settlement and traffic areas, as well as a high level of emissions. In addition to climate protection, adaptation measures are also required to sustainably improve the small-scale urban climate, air quality and ultimately the overall living conditions of the population. Urban greenery and especially urban trees can reduce heat stress and high irradiation enormously and thus contribute to mitigating the negative effects.
Which tree species are particularly sensitive to the effects of air pollution and climate change, such as heat and drought? And which tree species are particularly resistant? To answer these and other questions, cores were taken from selected trees in Karlsruhe in the "Dendrochonology and Dendrochemistry" subproject. Based on these, a vulnerability assessment for urban trees and forests along an urban-rural and air pollution gradient was subsequently performed.
Divided into two subinvestigations, the dendrochronological subinvestigation served to characterize the resistance, recovery capacity, and resilience (stability indicators) of selected Karlsruhe tree species. The isotope analysis served to determine the influence of heat stress and drought on tree physiology. Among other things, this method was intended to detect evidence of air pollution and to document the spatial variability of air pollution in Karlsruhe.
The cores were first taken manually using a special drill. The cores are about the thickness of a pencil and provide a cross-section of the tree up to the center of the tree. This allows the annual rings to be seen, but the tree itself continues to live. In addition to the age of the tree (number of annual rings), the width of the rings provides information about annual tree growth. The wider the ring, the more favorable the environmental conditions were, and vice versa. Extreme weather events (such as drought or heat) or environmental stresses (such as exhaust gases or fine dust) can cause disturbances in growth and can be additionally analyzed with the help of weather and climate records. For example, it is possible to find out how well an individual tree has recovered from a drought year (2003, 2008, 2015), i.e., how resilient it is, and whether or how this ability differs among different tree species. Like tree growth, the ratio of stable isotopes (chemical property of trees) is dependent on environmental conditions and is stored anew in each tree ring. Together with the dendrochronological studies described above, past stress from climatic and environmental stresses can be traced and the ability of trees to resist and recover can be illuminated.
Of the five most common tree species (Norway maple, littleleaf linden, English oak, hornbeam, and maple-leaved sycamore), ~30 trees each were studied. The first map shows the selection of tree individuals and their spatial distribution, as well as the level of ambient air pollution (NO2) per tree. The investigations clearly showed that the five investigated tree species, which together make up more than one third of the tree population of the study area, differed partly strongly in their tolerance to drought with respect to stem thickness growth. Across all tree species (except sycamore), it is clear that water availability in spring plays a decisive role for stem thickness growth in general as well as the reaction to drought in particular.
The quantified responses (top resistance, bottom resilience) of stem thickness growth of the dry years are shown for all five tree species investigated (from left to right: Norway maple, hornbeam, maple-leaved plane, English oak, and littleleaf linden) in the figure on the left. In 2003, only Norway maple showed a decline in growth. In 2011, all tree species except maple-leaved sycamore showed low resilience, and in 2015, all tree species except English oak and maple-leaved sycamore. Resilience was low after 2003 and 2015 for all tree species except English oak while after 2011 only maple-leaved sycamore and English oak showed lower increment than before the drought. Modeling of potential influencing factors revealed few significant effect of site characteristics on resistance and resilience, but mainly high intraspecific variability.
Analyses of resilience, as well as analyses of stable carbon and oxygen isotopes, can provide important quantitative assessments regarding drought tolerance and should therefore be considered as criteria for tree species selection in urban areas. Based on our results, especially sycamore and pedunculate oak proved to be suitable for urban areas with regularly occurring dry periods.
The influence of weather on tree ring width, δ13C and δ18.
In addition to tree ring width, isotopic signatures of carbon and oxygen in wood were analyzed. These relate to the ratio of stable isotopes to each other and are influenced by weathering. In this context, high values of δ13C and δ18O in wood indicate drought.
Our results clearly show that precipitation in May - especially for maple, hornbeam, oak and lime - is the most important influencing factor. Higher precipitation is associated with higher stem thickness growth and lower δ13C values. The dependence of stem thickness growth on sufficient water availability in spring is also reflected in the low resistance of all tree species (except sycamore) in the dry year 2011 compared to the dry year 2003, because in 2011 the most severe drought occurred in spring and in 2003 only later in summer.
C and O isotopes in/after drought years
In order to analyze the influence of drought on tree physiology - first and foremost the isotopic composition of carbon - in more detail, the individual series of measured values were standardized. Subsequently, it was investigated whether the annual mean values (per tree species) in the dry years and the respective following year (for oxygen) deviated significantly from the mean value of the series or (for carbon) were higher than the mean value.
For carbon, English oak and littleleaf linden showed elevated values after the 2003 dry year, and for littleleaf linden and Norway maple the values in the 2011 dry year were also elevated. Only sycamore and English oak did not show elevated values, so comparatively low drought stress can be assumed here.
Isotopic composition of nitrogen in the xylem
Tree species was thus the most important predictor explaining the variability in isotopic composition of nitrogen. Distance to the nearest road and (general, estimated) intensity of traffic emissions (NOx) also had a significant influence. Thus, the closer the tree was to a road and the higher the NOx emissions at that location, the higher the estimated δ15N values were. This result confirms that exposure to traffic emissions is reflected in the isotopic composition of nitrogen in wood.
The investigations clearly showed that the five tree species studied, which together make up more than one third of the tree population in the study area, differ in terms of stem thickness growth, in some cases strongly in their tolerance to drought. Based on our investigations, sycamore and pedunculate oak prove to be particularly suitable for urban areas with regularly occurring dry periods. Across all tree species (except sycamore), it becomes clear that especially water availability in spring plays a crucial role for stem thickness growth in general as well as the response to drought in particular. These findings are important for translating the results into specific irrigation strategies. For sycamore and pedunculate oak, where resistance and resilience are generally higher, irrigation does not seem to be absolutely necessary, or for oak only in the case of drought in spring, to which it reacts particularly sensitively. This is also true for the tree species European lime, hornbeam and Norway maple, which then react above all with a substantial drop in growth and also a low resilience.