Urban Forests For the Birds – Part 2. Management
Composition of the urban forest
Landscape architects and urban foresters recommend which species of trees to plant on public lands and private land. These decisions have a long term effect on the habitat value of the urban forest. But, what tree species provide the best habitat for birds in the city?
Local and migratory native birds have evolved to depend on native trees and plants for food and shelter. Insects, particularly the larvae of butterflies and moths, are critical to the reproductive success of our local terrestrial birds. So, does it really matter whether we use native or non-native tree species … absolutely (Woods and Esaian 2020).
Native insects, such as butterflies and moths, and native tree species have co-evolved. Butterfly and moth caterpillars have developed specific mouth structures and digestive systems to successfully use a very narrow suite of native plants. 96% of all terrestrial birds in North America rely on insects (and other arthropods – spiders that eat insects) to feed their young. Non-native vegetation reduces the amount of caterpillar food available, reducing nesting success, a critical element of bird conservation (Narango 2018).
Tallamy’s 2009 study concluded that woody plants supported more species of butterflies and moths than herbaceous plants; native plants supported more species than introduced plant; and native woody plants with ornamental value supported more butterflies and moth species than introduced woody ornamentals. Native trees produced 4x more biomass than non-native woody plants; supported 3x as many herbivorous insects associated with native than non-native woody plants; and native woody plants produced 35x more caterpillar biomass than non-native woody plants.
Structure of the urban forest
Urban forest habitat structure has two attributes that are directly affected by landscape design and arboricultural operations. Vertical structure, that is the vegetation from the ground to top of tree canopy. A complex, or connected, vertical structure is required to meet the needs of a large number of perching songbirds. Other attributes of vertical structure include tree health – stages of growth and decay; tree canopy cover; shrub canopy cover; and tree density. It is at this level of habitat that the arborist can play an important role. She/he can consider effects of timing and intensity of pruning on songbirds. For instance: will operations occur during the bird breeding season. Will pruning remove branches or limbs needed for nesting and interfere with reproductive success. Arborists can make recommendations to clients concerning how pruning and other arboricultural operations may affect bird habitat.
Horizontal structure, or the distribution of the urban forest across the urban landscape, concerns the connectivity of individual trees, small clumps of trees with larger patches and woodlands often found in city parks and increasingly along rivers and coastal zones. This attribute of habitat is often addressed in landscape planning, the purview of urban planners and landscape architects. (see fig. 2)
Parkland and natural areas should be managed to develop and maintain large continuous patches of forest with a representative complement of native trees and shrubs. Composition and density for trees and shrubs should mimic the native forest. Examples can be drawn from state parks and forests within the metropolitan region.
These two parts of structure often determine the suitability of a site for nesting, resting and protection from predators. All critically important to survival and reproduction. The larger the patch size the more birds can use it and the greater diversity of bird species will be present. The size of the habitat patch has also been correlated with habitat quality. The larger the patch the higher the quality. Interestingly the recommendations we have for developing wind resistant urban forests include the planting and management of trees in patches.
All this, of course, is not to say that street trees have no value as habitat. In fact, they become the bridges or connectors from one habitat patch to another for birds. They allow birds to traverse otherwise dangerous open areas used by predators; and avoid hitting windows of our tall buildings (windows – 1 billion die each year in N.A.). Investigations highlight the positive influence of street trees on urban avifauna (Woods and Esaian 2020). They found that eighty-three percent of the street-tree species were used in a lower proportion than their availability by feeding birds, and that nearly all were nonnative in origin. Improved street-tree management in lower-income communities would likely positively benefit birds as well as these neighborhoods that have historically had less tree canopy cover. Affluent communities often harbor a broader composition of street trees, including denser and larger trees than lower-income communities, which in turn, has been shown to attract higher the densities of feeding birds (Woods and Esaian 2020).
Many Florida cities and towns are conducting inventories and analysis of their urban forest using iTree methodologies and software. The data collected during an iTree inventory provides a wealth of important habitat information. When coupled with a GIS generated map of the urban forest’s distribution, it provides a comprehensive description of all the key attributes of bird habitat. The US Forest Service has begun development of bird habitat models using these data (Lerman et al. 2014). A better understanding of how to access, understand and use these valuable sets of data by the urban forester or municipal arborist is the initial step we must take to understand and manage urban forest habitat.
The goal of urban forest management is sustainability – the sustainability of the ecological processes that produce resources, beauty and all the rest. In a world where people live increasingly urbanized lifestyles, the nature around where they live and work forms a critical component of their daily nature interaction. A major challenge in harnessing people’s interest in local and broader conservation issues is that many people simply do not notice the nature that is around them. The urban forest – the whole forest that includes the insects, birds, amphibians, mammals and fishes – reenforces this connection.
Lerman, S. B., Nislow, K. H., Nowak, D. J., DeStefano, S., King, D. I., & Jones-Farrand, D. T. (2014). Using urban forest assessment tools to model bird habitat potential. Landscape and Urban Planning, 122, 29–40.
Narango, D. L., Tallamy, D. W., & Marra, P. P. (2018). Nonnative plants reduce population growth of an insectivorous bird. Proceedings of the National Academy of Sciences, 115(45), 11549–11554. Open access- https://www.pnas.org/content/pnas/115/45/11549.full.pdf
Tallamy, D.W. and K.J. Shropshire. 2009. Ranking lepidopteran use of native versus introduced plants. Conservation Biology v.23 (4) Open access – https://pubmed.ncbi.nlm.nih.gov/19627321/
Wood, E.M. and S. Esaian. 2020. The importance of street trees to urban avifauna. Ecological Applications v. 30 (7): Open access – https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/eap.2149