Adam Berland

The role of trees in urban stormwater management

Urban impervious surfaces convert precipitation to stormwater runoff, which causes water quality and quantity problems. While traditional stormwater management has relied on gray infrastructure such as piped conveyances to collect and convey stormwater to wastewater treatment facilities or into surface waters, cities are exploring green infrastructure to manage stormwater at its source. Decentralized green infrastructure leverages the capabilities of soil and vegetation to infiltrate, redistribute, and otherwise store stormwater volume, with the potential to realize ancillary environmental, social, and economic benefits. To date, green infrastructure science and practice have largely focused on infiltration-based technologies that include rain gardens, bioswales, and permeable pavements. However, a narrow focus on infiltration overlooks other losses from the hydrologic cycle, and we propose that arboriculture - the cultivation of trees and other woody plants - deserves additional consideration as a stormwater control measure. Trees interact with the urban hydrologic cycle by intercepting incoming precipitation, removing water from the soil via transpiration, enhancing infiltration, and bolstering the performance of other green infrastructure technologies. However, many of these interactions are inadequately understood, particularly at spatial and temporal scales relevant to stormwater management. As such, the reliable use of trees for stormwater control depends on improved understanding of how and to what extent trees interact with stormwater, and the context-specific consideration of optimal arboricultural practices and institutional frameworks to maximize the stormwater benefits trees can provide.

Adaptive governance to promote ecosystem services in urban green spaces

Managing urban green space as part of an ongoing social-ecological transformation poses novel governance issues, particularly in post-industrial settings. Urban green spaces operate as small-scale nodes in larger networks of ecological reserves that provide and maintain key ecosystem services such as pollination, water retention and infiltration, and sustainable food production. In an urban mosaic, a myriad of social and ecological components factor into aggregating and managing land to maintain or increase the flow of ecosystem services associated with green spaces. Vacant lots (a form of urban green space) are being repurposed for multiple functions, such as habitat for biodiversity, including arthropods that provide pollination services to other green areas; to capture urban runoff that eases the burden on ageing wastewater systems and other civic infrastructure; and to reduce urban heat island effects. Because of the uncertainty and complexities of managing for ecosystem services in urban settings, we advocate for a governance approach that is adaptive and iterative in nature—adaptive governance—to address the ever changing social order underlying post-industrial cities and offer the rise of land banks as an example of governance innovation.

Unexpected connections between residential urban forest diversity and vulnerability to two invasive beetles.

Invasive pests pose a threat to the key environmental and social benefits provided by urban forests, and diverse tree planting is a primary management strategy for reducing pest vulnerability. For example, past urban forest losses to Dutch elm disease (DED) prompted municipal foresters to emphasize diversification, but it is unclear whether residential properties developed after the peak DED outbreak are actually more diverse than older properties. To address this issue, we inventoried all public and private trees on 150 residential properties in the Twin Cities Metropolitan Area, Minnesota, USA, and compared genus diversity on pre- and post-Dutch elm properties. We then quantified vulnerability to two current invasive pest threats, emerald ash borer (EAB) (Agrilus planipennis) and Asian longhorned beetle (ALB) (Anoplophora glabripennis), to evaluate whether higher diversity corresponds with lower pest vulnerability. We assessed vulnerability based on two fundamental urban forest metrics–frequency and size of vulnerable trees. Surprisingly, properties developed after the peak DED outbreak were less diverse than older properties. At the same time, less diverse post-Dutch elm properties exhibited low ALB vulnerability and modest EAB vulnerability, while more diverse older sites were highly susceptible to ALB. The importance of pest host specificity in characterizing urban forest vulnerability was underscored by low EAB vulnerability and high ALB vulnerability on our oldest study sites. This research highlights an apparent disconnect between the theoretical notion that higher diversity should reduce invasive pest vulnerability, and our empirical data indicating that genus diversity does not necessarily correspond with pest vulnerability.

Influence of catchment land cover on stoichiometry and stable isotope compositions of basal resources and macroinvertebrate consumers in headwater streams

Anthropogenic land use affects aquatic landscapes. For example, landscape-level conversion to urban or agricultural land can heavily influence nutrient cycles in headwater streams via increased nutrient loading and altered hydrologic patterns. Recent studies in headwater streams have found that the stoichiometry and stable isotope compositions of basal resources and consumers can vary as a result of landscape-level change. To this end, we examined the stoichiometry and stable isotope compositions (δ13C and δ15N) of headwater stream flora and fauna in 16 streams located within forested, agricultural, urban, and mixed (urban, forested, and agricultural) catchments. Our results suggest basal resource stoichiometry varied across streams, with leaf litter being the most variable basal resource. Macroinvertebrate consumers maintained stoichiometric homeostasis across stream groups, but consumer stoichiometry differed across families. Values of δ13C did not vary across stream groups for basal resources; however, consumer δ13C did. Although δ15N did not differ among basal resources across stream groups, macroinvertebrate consumer δ15N differed because of the interaction between stream group and family. Our results show catchment land cover did not predictably alter the stoichiometry or stable isotope compositions of basal resources or consumers in headwater streams. The quality of basal resources in headwater streams could differ across catchments with varying land cover, but it is evident that differences in stoichiometry of basal resources did not lead to differences in stoichiometry of consumers in our study. Given the variability of stable isotope compositions, additional effort should be made to improve our understanding of the landscape factors that might influence isotopic data.

Asian longhorned beetle complicates the relationship between taxonomic diversity and pest vulnerability in street tree assemblages

Urban foresters routinely emphasise the importance of taxonomic diversity to reduce the vulnerability of tree assemblages to invasive pests, but it is unclear to what extent diversity reduces vulnerability to polyphagous (i.e. generalist) pests. Drawing on field data from seven communities in metropolitan Cincinnati, Ohio, USA, we tested the hypothesis that communities with higher diversity would exhibit lower vulnerability to the polyphagous Asian longhorned beetle, which currently threatens the region. Based on street tree compositions and the beetle’s host preferences, Asian longhorned beetle threatened up to 35.6% of individual street trees and 47.5% of the total basal area across the study area, but we did not see clear connections between taxonomic diversity and beetle vulnerability among study communities. For example, the city of Fairfield was among the least diverse communities but had the lowest proportion of trees vulnerable to Asian longhorned beetle, whereas the city of Wyoming exhibited high diversity and high vulnerability. On the other hand, Forest Park aligned with our original hypothesis, as it was characterised by low diversity and high vulnerability. Our results demonstrate that relatively high taxonomic diversity in street tree assemblages does not necessarily lead to reduced vulnerability to a polyphagous pest. Considering the threats posed by polyphagous pests, selecting a set of relatively pest resistant trees known to perform well in urban areas may promote long-term stability better than following simple heuristics for maximising taxonomic diversity, but further study is warranted.