Valuing Toronto's urban forest: seeing the forest for the trees

Many news outlets in Toronto reported on a study released by the chief economist at TD bank about the value of urban trees in Toronto. Toronto has been called ‘the city in a park’ because of the heavily forested urban landscape we have here (though when you Google ‘city in a park’ a plethora of cities have the same view of themselves). The value of Toronto’s urban forest estimated by the economists was 7 billion dollars. This seems like an astronomical amount, and that a large bank is the one forwarding this view of the value of an urban forest is in itself an amazing development (note: I do have an obvious conflict of interest as my professorship is endowed by TD).

TD's valuation of the urban forest relied on per-species estimates of net benefits, including carbon sequestration, air quality improvement, storm water flow, and energy savings though shading of buildings. These economic returns more than justify municipal expenses for maintaining parks and urban trees. This approach to quantifying the value of trees has been forwarded by new initiatives such as iTree that provide information on the benefits of tree species. The TD report does go on to say that there are other unquantified benefits of the urban forest such as aesthetic values and importance to communities. But the question is, is cumulative economic benefit a sum of individual trees or is there something more to a forest?

While individual trees have clear economic benefits, captured nicely in the report, and which often increase with the age or size of the tree, there may be direct economic benefit from forested lands that is greater than the sum of the individual trees. In essence, we need to see the additional value of the forest for the trees. Individual trees do not make a forest, and there is something special about a forest.

The simplest way in which a forest supplies additional value is through diversity effects. Different tree species may utilize differing resources or niches and by occurring together are able to turn more of the total local resources into growth, thus sequestering more carbon dioxide then if they were grown alone or only with other trees of the same species. As an example, if you grow a tall canopy tree and a medium shade tolerant species underneath it, the cumulative energy savings through shading are much greater than growing two tall canopy trees or two medium shade tolerant trees. This is often referred to as ‘complementarity’

Photo I took while on a hike in Toronto's Rouge National Urban Park

More than species complementing one another, in forests we often see species facilitating one another, meaning that individual trees perform better with other tree species around it, then when grown alone. Again, using carbon sequestration as the example, facilitation means that more carbon is taken up then when trees are isolated from one another.

Forests also provide habit for other plants and animals that individual trees do not provide. A forest can also better support pollinators by including different tree species that flower at different times of the season. Further, forests provide recreational activities (biking, hiking, camping, etc.) that are economically measurable (gas costs to travel there, user fees, tax support, etc.).

Taken all together, intact forests supply even greater economic, health, and environmental benefits than individual trees. If the trees of Toronto are valued at 7 billion dollars, then the forests of Toronto must be worth much more.

To intervene or not to intervene: this is a real question

Should land managers actively manipulate the structure and function of ecosystems within protected areas? Is intervention appropriate to protect or maintain native biodiversity and natural processes in areas such as national parks and wilderness areas? These are the questions that stem from a new paper by Richard Hobbs and others in Frontiers in Ecology and the Environment. US national parks and wilderness areas have legislative mandates to maintain ‘naturalness’, but what does this mean in the context of dynamic ecosystems with current and future changes including invasions by nonnative organisms and climate change?

Hobbs and his colleagues challenge concepts of naturalness and propose several ‘guiding principles’ for stewards of national parks and wilderness. They suggest that more useful concepts for managing protected areas relate to ecological integrity and resilience. Concepts of ecological integrity have been adopted by Parks Canada and relate to maintaining ecosystem components. Resilience concepts focus on the ability of a system to “absorb change and persist” without undergoing a “fundamental loss of character”. While maintaining ecological integrity in the face of global changes may - by definition - require protection of species, maintaining ecological resilience tends to focus more attention on ecosystem function “over preserving specific species in situ”.

Rather than protecting an area to maintain naturalness, focusing on ecological integrity and resilience acknowledges that a diversity of approaches - from non-intervention to actively managing systems - may be required. The flexibility in this view, demands that conservation planning span gradients of land uses across landscapes. Management objectives and success need to be re-evaluated in an adaptive and experimental framework, which requires careful and robust monitoring.

At The Wilderness Society and specifically here in Montana, these very questions are being wrestled with in terms of forest restoration, fire management, and climate change. Current forest conditions have been shaped by historic logging practices and fire suppression leading to altered structure and function – including increasing the severity of fires. Through active management, including removing small diameter trees and lighting prescribed fires, managers hope to restore forests and fire intensities to conditions more closely resembling those that historically occurred. Much of the research on restoration was conducted in dry forests in the American Southwest where low-severity fires occurred across large areas. However, in the Northern Rockies, many forests were shaped by a ‘mixed severity’ fire regime, where fires crept along the forest floor in some areas and torched trees in others. In many cases, these forests have not been fundamentally altered and need only the return of fire to restore their resilience. In other cases, forests are recovering from past logging practices and may benefit from thinning to restore a fire-resilient structure.

To return to the paper at hand: what is the appropriate level of intervention to maintain ecological integrity and resilience given past forest management and future climate change? If the current forest lacks integrity (novel stand structure) and resilience under a predicted climate of warmer, drier conditions, what is the appropriate level of management? While The Wilderness Society continues to work with diverse partners to answer these questions, one thing is clear: whatever actions take place, they need to be conducted with humility in an experimental framework that includes sufficient ecological monitoring. For the ‘experiment’ to be most helpful, we should maintain adequate hands-off “controls” along with the “treatments” to allow us to gauge the effects of intervention.

Richard J Hobbs, David N Cole, Laurie Yung, Erika S Zavaleta, Gregory H Aplet, F Stuart Chapin III, Peter B Landres, David J Parsons, Nathan L Stephenson, Peter S White, David M Graber, Eric S Higgs, Constance I Millar, John M Randall, Kathy A Tonnessen, Stephen Woodley (2009) Guiding concepts for park and wilderness stewardship in an era of global environmental change. Frontiers in Ecology and the Environment e-View.
doi: 10.1890/090089
http://www.esajournals.org/doi/abs/10.1890/090089

Hurricanes might contribute to global warming

In a large-scale study published this week in Proceedings of the National Academy of Sciences, Hongcheng Zeng and colleagues show that hurricane damage can diminish a forest’s ability to absorb carbon dioxide from the atmosphere. Their results suggest that an increase in hurricane frequency due to global warming may further amplify global warming.

The annual amount of carbon dioxide a forest absorbs from the atmosphere is determined by the ratio of tree growth to tree mortality each year. When hurricanes cause extensive tree mortality, not only are there fewer trees in the forest to absorb greenhouse gases, but these tree die-offs also emit carbon dioxide, thus potentially warming the climate.

Using field measurements, satellite image analyses, and empirical models to evaluate forest and carbon cycle impacts of hurricanes, the researchers established that an average of 97 million trees have been affected each year for the past 150 years over the continental United States, resulting in a 53-million ton annual biomass loss and an average carbon release of 25 million tons per year. Over the period of 1980–1990, released CO2 potentially offset carbon absorption by forest trees by 9–18% over the entire United States. Impacts on forests were primarily located in Gulf Coast areas such as southern Texas, Louisiana, and Florida, but significant impacts also occurred in eastern North Carolina.

These results have important implications for evaluating positive feedback loops between global warming and environmental change.


Zenga, H., J. Q. Chambers, R. I. Negrón-Juárez, G. C. Hurtt, D. B. Baker, and M. D. Powell. (2009). Impacts of tropical cyclones on U.S. forest tree mortality and carbon flux from 1851 to 2000. PNAS, 106 (19), 7888-7892. DOI:10.1073/pnas.0808914106

The evolutionary meaning of autumn colors

As a kid growing up in Ontario, Canada, I have vivid memories of vast expanses of forests set ablaze by their autumn colors. Whole landscapes look like the canvas of a painter whose love of red, orange, gold and yellow are readily apparent. But, like most biologists, I had been taught that these colors are simply the by-product of leaf senescence, nothing more than a biochemical accident. I was amazed to read Marco Archetti's recent work showing that there may actually be adaptive benefits to changing leaf color in autumn and for particular colors. Generally the adaptive benefits involve either protection against abiotic factors or as a response to plant-animal interactions. One of his interesting results is that autumn coloration has evolved repeatedly and cannot be explained by being related to an ancestor who changed colors, rather that there must be some other evolutionary or adaptive explanation. While he suggests a large number of candidate hypotheses, some more plausible than others, I'll list five for example:

1) Sunscreen: Pigments provide photoprotection against photooxidation during the recovery of nutrients.

2) Leaf warming: Colors absorb light and warm the leaves during cooling temperatures.

3) Coevolution: Tells overwintering insects that the tree is not suitable (poisonous or low nutrition) for hibernation.

4) Camouflage: Many insects lack red photoreceptor, making leaves difficult to see -thus protecting trees from overwintering pests.

5) Unpalatability: Pigments (e.g., red -anthocyanins) are unpalatable.

So, we may quibble about particular hypotheses, but the point for me is that there may be deeper explanations as to why certain species produce the vivid colors they do. At a minimum, Archetti provides ammunition to experimental botanists and evolutionary biologists for testing new hypotheses. I'll never look at an autumn forest the same again.

Archetti, M. (2009). Classification of hypotheses on the evolution of autumn colours Oikos, 118 (3), 328-333 DOI: 10.1111/j.1600-0706.2008.17164.x

Archetti, M. (2008). Phylogenetic analysis reveals a scattered distribution of autumn colours Annals of Botany, 103 (5), 703-713 DOI: 10.1093/aob/mcn259

Archetti, M., Döring, T., Hagen, S., Hughes, N., Leather, S., Lee, D., Lev-Yadun, S., Manetas, Y., Ougham, H., & Schaberg, P. (2009). Unravelling the evolution of autumn colours: an interdisciplinary approach Trends in Ecology & Evolution, 24 (3), 166-173 DOI: 10.1016/j.tree.2008.10.006