Since its introduction in the first half of the last century, the Black Cherry quickly became a problem species. Since the 1980s the Berliner Forsten has been making great efforts to reduce its numbers. Although there are a number of success stories in individual districts, the overall situation is still problematic. Drastic reductions in machinery and manpower have limited the opportunities for work in this area.
Standing on approximately one third of the total Berlin forest area of 29,000 ha, this neophyte is, next to damage by game, the biggest problem for the structure of the forest. The aim of changing the mono-culture pine forest is difficult in these particular affected areas.
The guidelines set out by the Berliner Forsten already hinted at the problem the Black Cherry may cause in relation to the goal of converting the forest (from an only pine forest) in 1992.
“For ecological as well as silvicultural reasons the Black Cherry needs to be removed, so that forests can develop with; horizontal and vertical structural variation, the presence of native tree and shrub species, and a well developed (in terms of location and age of species) ground vegetation layer.”
The existing procedures carried out in Berlin, include:
• Clearing of tree stumps.
• Extraction of seedlings/young plants.
• Chopping down of tree stumps.
These are however not satisfactory as they are all very resource-intensive. There has been no use of chemical herbicides in forests in Berlin since 1990. An alternative, in the form of a biological combative agent, is being worked on.
To create an effective combative method against the Black Cherry, it seems sensible to focus in detail on the following aspects.
I. Cause of the invasion
II. Development of a combative method/concept
III. Development of a combative biological agent.
I. Search for the cause of invasion:
The issue is raised as to what the particular factors which have favoured the spreading/proliferation of this North American species of tree over decades, where Pine and Oak forests are dwindling. Evidently Prunus serotina must occupy a niche in which its needs are almost entirely satisfied.
There are extensive publications noting the species’ special characteristics such as; shade tolerance (the ability to tolerate low light levels), preference for high precipitation levels, long-lasting seeds, animal seed-dispersal, allelopathy etc. As well as these factors, Prunus serotina also possesses the recognized and important quality of resistance to damage caused by game animals. Further study into this area would no doubt be beneficial.
The cells of Prunus serotina contain amygdalin (a cyanogenic glycoside). Damage from game animals creates contact with atmospheric oxygen which causes a reaction forming hydrogen cyanide, poisonous through absorption/consumption. This “hydrogen cyanide protection” gives the Black Cherry a fundamental advantage in the constant battle for space against the native species, especially in ecosystems where damage by game is prevalent. It often remains as the only species of tree, selected, as it were, by the game. Under the thinning cover of pine trees, there is a greater availability of light, water and nutrients, and most importantly little or no competition from native species. It is therefore realised that both damage by game, as well as resistance to this damage, increases the chance of successful establishment of Prunus serotina and as a result makes a considerable contribution to the further spreading/distribution of the species.
1. During the invasion phase, Prunus serotina often targets mono-culture pine and oak forests, preventing these species from forming a rejuvenation layer. This in turn means that the damage-to-game-resistant seedlings are able to germinate on extensive, largely competition-free ground.
2. During the population? phase, Prunus serotina is able to create an enclosed middle layer due to its good shade tolerance. Underneath this layer, the rejuvenation of native species is not possible due to the deficiency of natural light as well as the continued damage caused by game animals. The Rowan Berry can sometimes be an exception to this.
3. Following combative treatment, it is crucial to protect the new native seedlings from damage by game. Only once this has been achieved is it possible for a rejuvenation layer to develop, and for it to have the opportunity to out-compete Prunus serotina in the long term.
The relationship between game and the Black Cherry seems to be two-way. On the one hand damage by game is beneficial for the development of the Black Cherry population, but it has the opposite effect on the wildlife stock.
Game resistant neophytes, such as Robinia, Ash Leaf Maple, and the Common Snowberry, seem to utilise this “window” in a similar way. They colonise the available open areas, pathways and diminishing wooded areas. The Black Cherry, further to this, is able to settle under the expansive canopy of pine and oak populations due to its much higher shade tolerance. The invasion of the Black Cherry into native mixed-forests or self-contained Spruce and Douglas fir forests has not occurred yet however.
II. Black Cherry combat and security/protection concept:
As well as the affected forests, soon gardens, parks, embankments, graveyards and other green spaces in Berlin will become potential areas for the spread of Prunus serotina. Only the change from pure pine forest to a mixed native forest will help reduce the spread of the Black Cherry in the long term.
1. Maintenance of free space:
Forested areas where the Black Cherry is not present should be kept free. The degradation of these areas needs to be avoided. The transformation to a natural forest with native tree species hinders the potential establishment of Black Cherry due to ‘natural’ competition.
a. Rejuvenation population
The continued control and securing of free spaces protects the particular areas by allowing rejuvenation and development of the forest structure.
b. Intermediate and younger populations
As well as controlling and holding free spaces, under-planting with native species should take place. This creates a competitor for the Black Cherry. The under-planting should continue to fulfil the role of a competitor once a fully established mixed forest has been formed.
c. Free spaces
Free spaces need continuous monitoring and control to ensure they remain free.
d. Deciduous tree trunks
The addition of a 10-30 metre long deciduous tree trunk can be useful in the structuring of an area. This greatly reduces the dispersion of the Black Cherry, as very few of its seeds are able to cross this barrier/zone.
2. Combat in affected areas:
a. Rejuvenation areas:
The use of the remaining rejuvenation period is the main aim here. The areas are cleared of Black Cherry, and deciduous tree species are introduced to “secure” the area. Older areas have priority. Individual Black Cherry trees present in the new rejuvenation layer are removed during follow up maintenance procedures.
b. Intermediate and younger areas
Due to the shortage of resources, it seems sensible/logical to… After this under-planting can compete with the Black Cherry.
c. Free spaces
d. Priority areas
Prompt action can be useful here.
- Sparsely occupied spaces can easily be freed from Black Cherry and following this can be further treated using the methods explained in 1. a, b, c.
- Affected ‘islands’ and solitary trees
III. Biological combat of the Black Cherry through the use of the Silver Leaf Fungus (Chondrostereum purpureum) as a “Bio-herbicide”.
On the search for an ecological and economically acceptable alternative to the current combative methods, the Silver Leaf Fungus came into focus. Research teams in the Netherlands, Switzerland and Canada have all been working intensively on this topic for many years. Since 2003, the Landesforstamt Berlin have taken on the further development of this research. The project is supported by the Landeskompetenzzentrum Forst Eberswalde (LFE) and the Pflanzenschutzamt Berlin.
A mycelial suspension of the native Silver Leaf Fungus (Chondrostereum purpureum) is applied to fresh cuts of Prunus serotina using a brush.
The results of a study into the risk of infection, recommend a distance of 500m from fruit farms during applications (de Jong et al., 1990, 1998). The fungus cannot penetrate intact bark. C. purpureum is very common and relatively short-lived, which means there is little or no risk of ecological side-effects on the native fungi species or other native species when used as a mycoherbicide (de Jong et al., 1998).
Outdoor trials in the Berliner Forsten
As of yet the outdoor trials do not give a definitive answer as to whether this procedure is suitable for further use in the field. Although, from the trials carried out so far it has become clear that C. purpureum has the ability to cause major damage to Prunus serotina, even under our specific climatic conditions.
2. Bekämpfung befallener Flächen:
Hier steht die Nutzung des verbleibenden Verjüngungszeitraumes im Vordergrund. Diese Bestände werden von STK befreit, mit Laubbaumarten verjüngt und somit „gesichert“. Ältere Bestände genießen Priorität. Vereinzelt in die Verjüngung eingewachsene Traubenkirschen werden im Zuge weiterer Pflegemaßnahmen entnommen.
b. Mittelalte und jüngere Bestände:
Es erscheint hinsichtlich der knappen Ressourcen sinnvoll, befallende Bestände mittleren und jungen Alters erst dann in die aufwendige STK Bekämpfung mit einzubeziehen, wenn sowohl die Sicherung der nicht befallenden Bestände 1 a - c, als auch die Bekämpfung in den befallenden Verjüngungsbeständen 2 a, sowie 2 d, erfolgreich gehandhabt wird. Dann kann eine STK Bekämpfung mit Voranbau, bzw. Unterbau erfolgen.
Freiflächen stehen nicht selten im Widerspruch zur natürlichen Sukzession der Bewaldung. Eine Sicherung durch waldbauliche Maßnahmen ist somit, „qua Definitione“ nicht erzielbar. Die notwendigen Ressourcen für notwendige Bekämpfungsmaßnahmen müssen daher dauerhaft eingeplant werden.
Hier kann schnelles Handeln sinnvoll sein.
- Gering befallende Flächen können mit geringem Mitteleinsatz von STK befreit und im Anschluss entsprechend 1. a, b, c weiter behandelt werden.
- Befallsinseln und Solitärbäume als Samenemittenten sind Ausgangpunkte von Besiedlungen und sollten vorrangig bekämpft werden.