UFR questions

You note that the committee is “…directed at the prevention of clear-cutting…”. Clear-cutting is an overworked term and refers to a silvicultural system that, when applied correctly, is used to regenerate forests with an even-aged structure. I won’t get into a discussion of what happens when it is applied incorrectly. I assume that when you say “clear-cutting” you mean the removal of all the trees in a particular stand, woodland or forest and replacing it with another land-use. I consider this to be deforestation, the consequences of which are far more serious than the worst clear-cutting you will see anywhere. The conversion of woodlands to other land use implies a permanent (or virtually permanent - at least as long as people are around) loss of all functions and services provided by the woodland to the general environment and the people who benefit from it.

Your list of questions is very comprehensive and I feel you raise some extremely good points. Unfortunately, providing answers won’t be simple. Most of the difficulty arises with the use of the term “canopy cover”. While this is a widely-used measure of the extent of our urban forests, it has its pros and cons. On the affirmative side, it is a relatively easy parameter to estimate either from above using remote sensing or simply aerial photographs or from the ground. As I am sure you are aware, either approach usually involves an estimate of the proportion of the ground area that is covered by tree (and shrub) crowns resulting in a value expressed as a percentage canopy cover. Often this is presented as a single value, which of course, may make good reading in a newspaper or brochure but is of little value in real planning. While as value of 20% canopy cover (or whatever is being considered) tells us something, it is really the distribution of the canopy cover that is most informative. A number of years ago we applied the UFORE model to the city of Toronto and found the AVERAGE canopy cover to be about 21%. However, it ranged from 0 to 100%. On another note, I feel that the use of canopy cover targets while an easy concept to visualize (perhaps too easy) and communicate is also fraught with difficulties. If we set a canopy cover target of, let’s say 30%, then we have to be sure that this target is, in fact, achievable. Such a target must also be based on specific strata or land use types within the city because, as you know, not all land use types are created equal (this is partly behind the variation I referred to earlier). Imagine for example an area dominated by commercial use with a very extensive and intensive grey infrastructure with virtually no growing space for trees. Let’s say for the sake of argument the current canopy cover is 10%. This may represent a completely stocked portion of the urban forest with a tree in every available spot. Imagine now a relatively new subdivision with large lots and few trees planted. In this case, the canopy cover may also be 10% but the POTENTIAL is substantially greater. I won’t hazard a guess here because the whole point is the potential canopy cover (something seldom measured) is completely dependent upon the extent AND CONFIGURATION of the grey infrastructure.

Another issue with canopy cover and the way it is usually measured is that it is an aggregate of all trees and shrubs and, as such, it is difficult to refer to many of the individual tree parameters you mention. If we are dealing with forest or woodlands which may be managed in an extensive manner (e.g. the stand or woodland is unit of management and not the individual tree) then this isn’t really a problem. If, on the other hand, we are in the part of the urban forest where we are managing individual trees on an arboricultural basis (what I consider to be intensive management) then canopy cover creates some problems. While we could measure the crown radius and then convert this to a crown projection area (the circle delineated by the outer perimeter of the tree crown as seen from directly above) and convert this to a proportion of all the similarly calculated crown projection areas for all trees to give us a measure of the part of the canopy cover attributed to that tree, this is not usually done.

Another issue with canopy cover is that it is two dimensional and, as such doesn’t consider the crown depth of trees. It is also difficult to represent the proportion of canopy represented by trees of a certain species or size-class. Finally, canopy cover also fails to represent any measure of the health of the crowns.

The UFORE model that estimates among other things, the proportion of some air pollutant that are sequestered by the urban forest is driven primarily by the leaf area of the urban forest (there are many other inputs relating to the trees and the atmosphere but leaf area is the main gross feature if I can put it that way). Leaf area is estimated from sample plots across the various land use types and is based on actual tree measurements taking into account tree size (crown width and length), species and condition. Put very simply, it is these measurements that are used to describe the Town’s urban forest and it is these measurements that contribute to the estimation of the benefits derived from the forest. While we can imagine that there should be a relationship between canopy cover and leaf area, it doesn’t represent a simple conversion.

So why don’t we just use leaf area instead of canopy cover. Well, I guess it is primarily because canopy cover is easy to measure and easy to visualize. As with most things that are convenient or easy, it doesn’t provide enough insight to answer many of the very good questions you raise.

Somewhere, someone had to make a decision of how small a tree should be to be measured. The UFORE model protocol has chosen this cut off point to be 1” or 2.5 cm dbh. What impact does this have on canopy cover? This would be difficult to say since canopy cover is measured as an aggregate of all canopies so we can’t simply extract trees from a certain dbh class from the data set. We could however extract certain dbh classes from the leaf area measurements and extrapolate this to the Town’s forest. Theoretically, would a change form say 2.5 cm to 15 cm as the threshold change the canopy cover? Perhaps, but that would depend on where the trees are located. If the small trees were all in the understory of areas with a high canopy cover, they may not be “seen” by the analysis so their elimination wouldn’t be “seen” either. Small trees in the open may not be detectable by some canopy cover methods. Trees of 15 cm should be visible if they are in the open and could be included in canopy cover estimates but that would depend on the method used.

Estimating the impact on the leaf area could be done from the UFORE data set but would require some relatively simple calculations. The UFORE analysis provides tables of leaf area by dbh class (among many others).

This could be calculated from the UFORE data using regression technique. I don’t have the data so I couldn’t do this calculation. I could dig up other data or request the information from Syracuse, but that would take some time. I’d rather not “hazard a guess’ here as these guesses sometimes end up being taken more seriously and I don’t want to have to come back a year from now explaining where such a figure came from. Numbers casually thrown about can be dangerous!

No, I can’t for the same reasons. However, I am comfortable in saying that, if the Town intends to eventually expand its canopy cover (or leaf area or whatever measure is used), then a logical first step would be to prevent the loss of existing trees! Yes, trees do die from many causes including the very difficult growing environment in our cities and town. However, the removal of medium to large healthy trees would have to be compensated for by the planting of MANY MORE (perhaps dozens) of smaller trees to retain the leaf area that is providing the benefits derived from the medium to large trees removed. In my opinion a reasonable objective for any municipality serious about their urban forest should be AT LEAST no net loss of leaf area.

The size distribution of trees in Oakville suggests a large percentage of Oakville trees are young and of an age class more susceptible to mortality from urban stress, improper planting, disease etc. In Appendix 10, it is stated that the UFORE Grow-out Model, "required an assumption" of a 2% tree mortality rate.

Dr. Nowak’s research group is meticulous in its work. Obviously, some estimate of mortality had to be included in the model. Since we have no real data for Oakville then this figure is the best available data and I am confident it is a reasonable reflection of the situation in Oakville. Yes, it might be higher in some areas and lower in others; this is the nature of an average. We simply don’t have any better estimate and for someone to suggest it is too high or too low would have to be supported by real data (as this estimate is) and not just a gut feeling that perhaps fits some desired outcome.

As noted above, making estimates for the impact on canopy cover for any particular dbh classes would be difficult at best. However, keeping in mind that leaf area increases exponentially with dbh then the loss of trees in the larger dbh classes will have a much greater impact on the leaf area (and hence most of the benefits) than would the loss of smaller trees.

While it would be safe to say that loss of 9% of the towns urban forest would be a significant impact (remember, we are talking about mortality here and not just a season of defoliation), translating this to canopy cover would be difficult. I am pretty sure the impact on leaf area is already outlined in the USDA’s UFORE report (to the best of my knowledge, the final report has not been released yet though it should be available soon.)

I think the development of the strategic plan will be a significant improvement. However, maintaining a diverse and healthy urban forest is critical. No single measure will prevent an infestation. In fact, with respect to EAB, it is almost a foregone conclusion that Oakville (and every other municipality in southern Ontario and beyond) will have to deal with it at some level or another. Researchers at the CFS also suggest that we can expect other invasive aliens (not already observed) to challenge us in the future. After the devastation caused by Dutch Elm Disease, many municipalities planted far too many of relatively few species (including ashes). Hopefully this time around we learned our lesson and will take a more ecological approach to urban forestry rather than a purely landscape architectural one that puts aesthetics ahead of sustainability.

One troublesome item presented in the report is the fact that European buckthorn is one of the top ten species in terms of leaf area. As you are well aware, Common European buckthorn (Rhamnus cathartica) and European alder buckthorn (Rhamnus frangula) are particularly invasive shrubs in this region of Ontario and represent a significant threat to native habitats. While Canada struggles with its invasive alien species (IAS) legislation, we will undoubtedly need to address mitigation and removal of these species. While replacement with such species as American elder (Sambucus canadensis), Black chokeberry (Aronia melanocarpa), and Juneberry (Amelanchier alnifolia) would be ideal, replacement would of course result in the removal of a mature shrubs with only young specimens as replacements.

IAS are, and will continue to be a big challenge. However, I do not subscribe to the idea that the solution is to revert solely to planting native species. One of the reasons many of the exotics are over-planted is because they have shown themselves capable of withstanding the incredibly poor growing environments we put them in. Yes, aesthetics has played a big role (crimson king for example). Control of IAS will likely be an issue for decades to come and should continue to be a part of our management regime. However, at the same time we should be more imaginative in terms of what we plant and where. IAS should not be planted in areas where they can threaten “natural” areas. This is a big issue.

I’m sorry I couldn’t be more specific in most of my responses but the answers aren’t always black and white. Unfortunately, there will be people who will prefer to suggest they are and to suggest overly simplistic solutions. I guess that’s the sort of approach that has got us to where we are today with respect to the environment.

….to Dr. Kenney,

I very much appreciate you taking the time to respond to my long query. Your responses were most helpful. I now better understand the difficulties around canopy coverage determination and I fully concur that leaf area density is a much better measure of the health of our urban forest.

Thank you as well for pointing out how we are using the term "clear cutting" improperly. While I am not sure that the term deforestation would make sense to the general public when applied to our sparse urban forest, it is essentially what is happening. We are indeed observing deforestation through the removal of healthy mature trees from private properties to make room for home expansions and new construction. As you may know, we were unable to pass legislation in the fall to prohibit tree removal in general, thus we are now trying to prohibit "large scale" tree removal. Hence we are forced to define what "large scale" means.

We can define it based on either stem count and size, by percent canopy cover or by leaf area density? The first is the easiest to administer, but may still allow for the loss of a significant number of trees. Percent canopy cover, may protect smaller species, but would be more difficult for the property owner to determine - although it is being used in some US jurisdictions. While leaf area density would provide the most significant measure, this would not be simple for the average person to determine. Although, I suppose one could develop some kind of databank that could be used - I have not seen any such examples. Anyway, this is where we are struggling.

On a positive note, I believe that we will be able to pass legislation to protect "heritage" trees (and I have suggested that trees over 76cm, of which there are only 8,500 estimated in Oakville, be granted automatic Heritage status) and endangered species.

I would be continually grateful for any suggestions you may have in helping us to protect our urban forest. Thank you again for all your time.

Catherine

Your comments about deforestation are good and precisely why I prefer this term. It seems the general public are comfortable with the term used in Brazil or the boreal but I never see it used with respect to urbanization. So, let’s call a spade a spade. MY reference to clear-cutting wasn’t meant to be critical but just to help point out that terms can be misused. I once read an article in the Ottawa Citizen that said a tree (that’s a single tree) had been clear-cut! Enough said.

I share your concerns about using leaf area and the fact that it is more complex. However, I think we should continue to try to ramp up the understanding. I urge you NOT to base any protection measures on stem counts however. The idea of protecting big trees is important and we have covered it in our discussion. However, we can’t lose sight of the fact that big trees start out as small trees.

Some day we should chat about protecting growing space, I would suggest that this is just as important, perhaps more important, than protecting existing trees. But that’s for another time perhaps.