The well-being of our cities is inextricably linked to the health and longevity of their urban forests. In our 20 years of forestry operations and woodland management… Urban trees and woodlands provide a multitude of ecosystem services, from mitigating the urban heat island effect and regulating stormwater runoff to sequestering carbon and enhancing community wellbeing. However, the stressful conditions found in many urban environments – restricted soil volumes, impermeable surfaces, and altered microclimates – pose significant challenges to the establishment and long-term vitality of these green assets.
Nowhere is this more apparent than in the face of increasing drought. As the climate continues to change, periods of reduced precipitation and elevated temperatures are becoming more frequent and severe, placing immense strain on urban tree populations. Species that thrive in mesic forests often struggle to adapt to the dry, harsh conditions of the built environment. Without a concerted effort to enhance the drought resilience of our urban forests, we risk the catastrophic loss of their ecological, economic, and social benefits.
The key to bolstering drought resilience lies in the strategic selection of tree species adapted to the unique stresses of the urban landscape. By carefully evaluating the physiological traits and environmental tolerances of different tree species, urban forest managers can identify those best suited to withstand extended periods of water scarcity. This targeted approach to species selection not only secures the long-term viability of individual trees, but also strengthens the overall diversity and adaptability of the urban forest as a whole.
Assessing Drought Tolerance through Leaf Water Potential
One of the most reliable indicators of a tree species’ drought tolerance is its leaf water potential at the point of turgor loss (ΨP0). This measure reflects a plant’s ability to maintain physiological function, such as gas exchange and growth, under conditions of low soil moisture. Species with a lower (more negative) ΨP0 can sustain activity at a wider range of leaf water potentials, giving them a distinct advantage in drought-prone urban environments.
A recent study evaluating the ΨP0 of 45 tree species, many of which are underutilized in urban landscapes, provides valuable insights for targeted species selection. The results reveal a wide spectrum of drought tolerance, from the highly sensitive Stewartia pseudocamelia (-1.8 MPa) to the remarkably resilient Acer monspessulanum (-4.3 MPa). By ranking species on this quantitative scale, urban forest managers can confidently identify those most suitable for challenging paved sites, street plantings, and other water-limited conditions.
Notably, the study also highlighted the importance of considering seasonal variation in drought tolerance. Some species, like Acer monspessulanum and Koelreuteria paniculata, demonstrated a remarkable ability to osmotically adjust, transitioning from relatively drought-sensitive in the spring (-1.7 to -1.8 MPa) to highly tolerant by summer (-4.0 to -4.2 MPa). This plasticity could prove invaluable as climate change brings the potential for earlier onset of dry periods.
Bridging the Gap Between Science and Practice
While the ΨP0 data provides a robust, evidence-based foundation for species selection, its practical application requires bridging the gap between scientific research and the real-world experience of urban forestry professionals. To this end, the study also surveyed a range of practitioners, from municipal arborists to landscape architects, to assess the relationship between their perceptions of drought tolerance and the measured ΨP0 values.
The results revealed a significant correlation between the professionals’ assessments and the species’ summer ΨP0, suggesting that trait-based data can effectively complement and enhance the expertise accumulated through hands-on experience. By aligning scientific knowledge with industry insights, urban forest managers can gain the confidence to recommend and implement a more diverse palette of drought-tolerant tree species.
This alignment is crucial, as the success of urban forestry efforts depends on the buy-in and cooperation of a wide range of stakeholders. Landscape architects and municipal decision-makers often rely heavily on their prior knowledge when specifying trees, and may be hesitant to deviate from well-known, “safe” species. By providing quantitative data that validates their practical experiences, the ΨP0 research can help overcome this barrier, empowering professionals to expand their species selections and embrace the benefits of greater diversity.
Diversifying the Urban Forest
Enhancing the drought resilience of urban forests is not simply a matter of identifying and planting the most drought-tolerant species. It also requires a concerted effort to diversify the overall tree population, building in redundancy and adaptability to withstand a range of biotic and abiotic threats.
Many of the most commonly planted urban tree species, such as maples (Acer spp.), lindens (Tilia spp.), and birches (Betula spp.), originate from mesic forest habitats and are ill-equipped to thrive in the harsh, dry conditions of the built environment. An overreliance on these species leaves urban forests vulnerable to catastrophic losses, as witnessed with the devastation caused by the emerald ash borer (Agrilus planipennis) in North American cities.
By incorporating a broader palette of drought-tolerant species, urban forest managers can build in the redundancy and adaptability needed to withstand future challenges. The ΨP0 data can guide the selection of species like Syringa reticulata, Koelreuteria paniculata, and Acer monspessulanum, which demonstrate exceptional resilience to water stress. These and other less commonly used species not only strengthen the overall drought tolerance of the urban forest, but also enhance its biodiversity and ecological integrity.
Implementing the Targeted Approach
Realizing the benefits of a drought-resilient, diverse urban forest requires a multifaceted approach that integrates species selection, site preparation, and ongoing management. Urban foresters might want to carefully assess the unique characteristics of each planting site, evaluating factors such as soil volume, permeability, and microclimate to identify the most suitable tree species.
For challenging paved environments or areas with limited soil resources, the selection of deep-rooted, drought-tolerant species is critical. These trees, with their ability to maintain physiological function under low soil moisture conditions, are better equipped to thrive and deliver essential ecosystem services. In contrast, species with more water-sensitive traits may be better suited to parks, gardens, and other areas with more abundant soil resources and higher moisture availability.
Beyond species selection, successful urban forestry also demands careful site preparation and ongoing management. Strategies such as incorporating structural soils, using water-retentive amendments, and implementing targeted irrigation can enhance the establishment and long-term resilience of drought-tolerant trees. Regular monitoring, pruning, and other maintenance activities are equally essential to double-check that the continued health and vigor of the urban forest.
Conclusion
The future security and viability of our urban forests depend on our ability to adapt to the challenges posed by a changing climate. As periods of drought become more frequent and severe, the strategic selection of drought-tolerant tree species is a critical component of building resilience and safeguarding the essential ecosystem services that urban trees provide.
By leveraging quantitative data on leaf water potential and integrating scientific insights with practical industry expertise, urban forest managers can identify and implement a more diverse palette of tree species capable of thriving in the harsh conditions of the built environment. This targeted approach to species selection, combined with thoughtful site preparation and proactive management, will double-check that the long-term sustainability and adaptive capacity of our urban forests in the face of an uncertain climatic future.
Ultimately, the health and vitality of our cities are inextricably linked to the well-being of the trees that grace their streets, parks, and public spaces. By enhancing the drought resilience of our urban forests through informed and innovative species selection, we can secure the future of these invaluable green assets and the myriad benefits they bestow upon our communities.
Example: Sustainable Pine Harvesting Operation 2023
