LECTURE MANAGEMENT OF STREET TREES: PLANTING ( Chapter 11 )

Transcription

1 LECTURE MANAGEMENT OF STREET TREES: PLANTING ( Chapter 11 ) STREET TREE ESTABLISHMENT Street trees are planted by either a public agency, or by the property owner. Some communities are completely public, others completely private, but now, most communities are a combination of both. Species selection Usually, a list of approved species is developed for a city, or for zones of a city. This should take into account the values and limitations of each species on the list. You have a copy of the City of Davis Master Tree List and I have a copy of the Sacramento Tree List no longer in print. Certainly no species would be desirable in all sites in a city. One species selection model presented in Miller is: *********** SLIDES ON We have seen various site factors in our field trips and in your project sites. These 1

2 include Environmental, Cultural, Social and Economic Constraints, or Factors. Environmental factors are those that would be important even without human influences. Hardiness, i.e. tolerance of Winter cold temperatures, should be considered, although urban heat islands may be cultural in origin. Thus, microclimate becomes a factor, as well as Meso or Macro Climatic factors Soil Chemistry and structure are very important, and are illustrated by comparisons between Davis, with high salt levels in its water, and Sacramento, with very high quality American River water. An important Biological Constraint is Mistletoe, which is common in both Davis and Sacramento. Another biological pest is the Eastern Tree Squirrel. It used to be a problem in Sacramento, but did not occur in Davis. But, it has since moved to the Davis area, and they do damage new growth of trees. Susceptibility to insects and disease, as well, will influence which tree species will succeed. Cultural Factors: Sidewalks and streets should also have a large influence on tree selection, often limiting the choices available. Buildings and utilities are Cultural Constraints that often limit tree selection to shorter or narrower trees. Basic tree shapes can be important. Tree shapes vary in terms of human preference, as we will see later, but here we are considering how tree shape determines whether a particular species will be suitable for a particular site. Social factors include safety, aesthetics and values associated with areas, ethnic preferences. In Milwaukee only linden trees are planted in one neighborhood 2

3 where it is an ethnic custom to make wine from the linden flowers. Joe McBride explained how religion can influence what trees should be planted or not planted in certain situations. Economic factors basically include establishment, maintenance and removal costs. Survival rate is particularly important because if a tree fails it must be removed and the costs start over again. Pruning can also a major expense, as it is important to select tree species that do not have high pruning needs which is generally the case for fast growing species. This may conflict with the social preference for larger trees in a short time, i.e. instant gratification. SLIDES OFF **************************** Sources of Trees Trees are either purchased from commercial nurseries, grown in municipal nurseries, or transplanted from the wild. When transplanted from the wild they were grown for awhile in the municipal nursery to add some growth. This was a cheap method in the old days, especially in the East and Midwest. This limited the species selection and accounted for the excessive use of American Elms and Sugar Maple. Today most cities purchase plants from commercial nurseries. Planting Generally the goal of a planting program is to achieve full stocking as soon as 3

4 is possible. This may seem like a simple thing, but with replacements, some of which may be unexpected, in addition to new developments, it is not so easy. However, you need to consider that more planting leads to increased maintenance costs in the future. Yet, this also leads to increased value of the trees and the forest. Computer simulation can be used to project various impacts of the planting strategy is being considered. Full stocking Although funds may vary from year to year, as well as the numbers of trees that need to be removed, and even priorities may change, cities need an objective method to plan and to rationalize to the public what is going on. We have avoided the math throughout the Quarter, but this model is so simple you will have the privilege here to look the math right in the eye. We want to know: (N) how many trees need to be planted per year, with (S) their survival probability, into (V) vacant spaces, and (R) trees are removed each year due to death or risk, and (G) the number of years it will take to reach full stocking. ************* SLIDE ON shows the formula with an example. 4

5 This shows us that if we want full stocking in 8 years we must plant 411 new trees each year. Another use of this formula is to analyze your budget, choose trees according to their mortality rates, and estimate how long it will take to realize full stocking. The formula can be attacked with the tools of high school algebra to obtain a formula for G = f(v,n,r,s). It would be a good idea to do this. For example: SLIDE OFF ********************** Suppose there are 2000 vacancies, you have the resources to plant 300 trees per year, their survival is 0.7 and replacements are 150 per year. How long will it take to reach full stocking? Would you be willing to wait that long or would you choose to increase the planting rate to reach full stocking sooner?? Is full stocking the right goal??? TREE MOVING Sometimes at tree like our Mexican Fan Palm needs to be eliminated from its site, but can be moved. This tree was in the way of the Fire Lane access to the new hotel being build 5

6 behind EH. It was moved about 8 feet, and cost $1,800. The patient is doing well. ******************* SLIDES ON SLIDES OFF ******************* PRUNING CYCLES, Miller, Chapter 12 It is important to recognize that as trees grow and develop from saplings to mature trees, and finally old, declining trees, their pruning needs will change. It is particularly important to carefully prune young trees in order to establish the basic primary branching that hopefully will establish a solid structure for many years to come. Miller discusses general needs, but species and climatic conditions in different areas vary so much that this section is very problematic. I think that trees should be examined every year for about the first 5 years, especially for rapidly growing species like Bradford Pears. However, Ginkgo could probably go 2 to 3 years for the first 7 to 8 years. But,it does not take much time to examine a young tree and not much time to remove what must be only a few primary branches that are not well placed. Better that than dealing with major pruning later, that will be costly and may result in a less that desirable structure. ****************** 2 SLIDES ON/OFF *************** 6

7 We have also seen in Paula Peper s lecture how over pruning in Santa Monica and Claremont reduced the vigor and life expectancy of some trees. Since pruning is covered in some detail in ENH 133, I will not get into more details on pruning techniques. However, I would like to suggest the value of having a pruning cycle for trees after ages of about 5 years. The cycles should vary from as frequently as every 5 years, although in practice that is unusual, to 10 to 12 years for slow growing, hard wooded trees. We have already discussed crisis management, and a current problem is that crisis management takes all of the labor force available and leaves little time for systematic cycle pruning. While eliminating some of the cycle pruning will reduce maintenance costs, and hence appear to improve the B/C ratio, it is only deferring the maintenance to a later date when costs may be much greater. For example, 3 large Eucalyptus (camaldulensis, I think) behind the Art Building were not pruned for many years, as far as I can remember, so when they were in dyer need the cost to the Grounds Department was about $1440, and a contracting with a private contractor would have been $2,000 to $2,500. Therefore, the per tree cost can be at least $500 to $1,000 for major surgery. TREE FAILURE AND REMOVAL Actually, 3 other large Eucalyptus trees were removed by Grounds for less. Each cost about $270 to remove, plus about $15 each for cleanup. It was estimated that a private 7

8 contractor would have charged more than $400 each. ************ SLIDES ON The European Hackberry between the Alumni Center and EH failed this year, and its removal cost Grounds $1,300. The removal cost was probably higher due to the sensitive site. The Eucalyptus trees were simply dropped onto the parking lot. The City had a private contractor remove the landmark Foothill Pine, Pinus sabiniana on E street by Mansion Square, in the Holiday Project Site, because it required a crane to safely remove the large branches. This tree was about 100 ft tall, but started leaning toward the building slightly soon after construction. Eventually, after maybe 15 years a big north wind pushed it against the building and it had to go. These cranes cost a lot, and Rob Cain, City Forester, estimated the cost at about $15,000 for the 3 days it took to remove it all. We were very lucky that so little damage was done to Mansion Square. And Nicole LaPatka took some photos after a rain storm in Davis SLIDES OFF *********** 8