Nature + Green  

 

City Trees

The Status of Berlin’s Street Trees (Vitality)


Urban street trees are exposed to various types of damage that can have a negative effect on their vitality and health. To monitor the status and the long-term development of Berlin’s street trees, an evaluation using aerial color-infrared (CIR; also known as false-color) photography has been carried out in Berlin’s city center every five years for more than 30 years now. A Street-Tree Status Report (“Straßenbaum-Zustandsbericht Berliner Innenstadt”) documents the results of each evaluation.

The current report on the condition of Berlin’s urban street trees in 2010 shows that roughly 60% of the trees in the city center are healthy and around 40% show at least some damage. In other words, just under two-thirds of our city-center street trees more than 15 years old had no crown damage (or at least none visible in aerial photographs) that year.

A comparison with the report for 2005 reveals an overall trend toward decline in the crown vitality of urban street trees planted before 1990, due to a general rise in detrimental factors. Planes and chestnuts in particular have experienced a clear increase in damage caused by pests since the late 1980s.

Aerial color-infrared photography does not, however, say anything about the source of existing damage, a tree’s stability, or the likelihood of broken branches.


The CIR method

By revealing differences in reflected light, which in turn indicate the relative health of foliage, aerial color-infrared photographs can help to record and evaluate the condition of street trees. The differences in reflected light are caused by damage to foliage, loss of foliage, and poor branching. Foliage color and density, crown shape, and branching should be compared with reference trees when evaluating tree health with aerial photography. At the same time as the photography flight, these reference trees are examined from the ground to determine their crown condition and any symptoms of damage or peculiarities, and an “interpretation key” is used to contrast the appearance of the trees on-site with their aerial photographs. Only a comparison of this kind makes it possible to evaluate tree vitality with aerial CIR photography.

After evaluating a random sample of around 6,000 trees of the major genera (linden, maple, horse chestnut, and plane) in 180 specific areas, the results are statistically extrapolated to the entire population of trees planted before 1990 in inner-city areas of Berlin, and the trees are assigned to different crown condition categories. The major tree genera being examined make up more than three-fourths of the inner-city tree population. Because stocks of the other genera are so small, they are not included in these assessments. The city center is defined here as the area inside the S-Bahn circle, as well as the densely built residential areas of the former Steglitz, Weissensee, and Pankow boroughs and all of the old Wedding borough.

2010 Street-Tree Status Report

The table below shows the results of the evaluation, classified according to the four genera examined and as a whole. Berlin’s linden (Tilia) trees – around 65% of which were undamaged – showed the best results in terms of crown vitality, followed by maples (Acer) (60% undamaged). At 50%, the horse chestnut (Aesculus) had roughly equal numbers of damaged and undamaged trees, while the majority of plane (Platanus) trees (around 53%) showed damage.

Damaged trees as a percentage of Berlin's four major tree genera:
linden, maple, plane, and horse chestnut (data as of 2010)
  Percentage
(of all randomly
sampled trees)
Category 1
(undamaged)
Categories 2-4
(damaged)
Mean
standard
deviation *)
Linden 59.9 % 64.6 % 35.4 % ± 1.9 %
Maple 22.1 % 59.7 % 40.3 % ± 2.6 %
Chestnut 5.5 % 49.5 % 50.5 % ± 7.4 %
Plane 12.5 % 46.6 % 53.4 % ± 5.2 %
all 4
genera **)
100.0 % 60.4 % 39.6 % ± 1.6 %

Results of the 2010 street-tree evaluation by genera and as a whole (percentages of the various crown-vitality categories)

*) Margin of uncertainty in the statistical extrapolation to the total population of all street trees planted before 1990 in Berlin’s city center (for category 1)

**) The percentages for all 4 species (category 1 and categories 2-4) are not the average of the single values of the 4 species. This is due to the variation in the number of trees per species.

Explanation of the crown-vitality categories
1: no (noticeable) damage
2: slight or medium damage
3: heavy damage
4: extremely damaged or moribund
5: dead or with no foliage

Age and crown condition are related here: the older the street trees, the worse the crown condition. Lindens show the greatest difference in this regard between younger and older trees.

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Comparison of the 2005 and 2010 Street-Tree Status Reports

A comparison with the report for 2005 reveals the following changes in street-tree health in Berlin’s city center:

  1. Statistical extrapolation of the crown vitality of inner-city street trees planted before 1990 to the total population shows an overall trend towards decline from 2005 to 2010.

  2. Extrapolated change in street-tree damage varied according to borough. In only one of the 15 former boroughs examined did the condition of the street-tree population show definite improvement (Neukölln*); in another borough (Treptow*), a slight improvement was not statistically significant. In 7 of the other 13 former boroughs (or parts of them), there was a statistically reliable change for the worse in tree condition (Prenzlauer Berg, Kreuzberg, Charlottenburg*, Wilmersdorf*, Tempelhof*, Weißensee*, Pankow*). No statistically reliable changes could be identified in the remaining 6 boroughs.

    *) Former boroughs for which data was only partially collected

  3. The 4 species examined differed in their development.
    • Improvement was significant in lindens of age-class I, while the condition of middle-aged and older trees worsened.
    • In the case of maples, the condition of young trees remained the same, while middle-aged and older trees deteriorated somewhat.
    • Horse chestnuts of all age classes showed no significant change in condition.
    • Plane trees deteriorated to a similar degree in all age classes.
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Causes of street-tree damage

As a rule, the interaction of very different factors on any given urban street makes it impossible to determine the etiology of street-tree damage. The 2010 Street-Tree Status Report confirms the striking differences observed earlier with regard to tree health. High- and low-stress environments can be found in close proximity, and detrimental factors themselves are subject to change, depending on the time and place (e.g., gas leaks, excavation, traffic accidents involving trees). While the results presented here give no clear indication (with the exception of biotic influences in the case of horse chestnuts and planes) as to the origins of specific damage, we can nevertheless draw a general conclusion:

The decline in crown health identified in Berlin’s inner-city street trees from 2005 to 2010 demonstrates that at least the sum of all detrimental factors has increased.

You can download the entire 2010 Street-Tree Status Report (in German) here:

You can download the entire 2005 Street-Tree Status Report (in German) here:




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Oak (Quercus) with
crown damage

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Maple (Acer) with
medium crown damage

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Linden (Tilia) without crown damage

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Aerial color-infrared photograph (CIR) Berlin 2005