Comparative Analysis of Pathology of Introduced and Indigenous Tree Species in Urban Plantings of Voronezh

One of the problems of urban planting is the necessity for choosing between indigenous and introduced tree species. Under conditions of Voronezh, Russia, the ratio is 1:4 in favor of the former, though the many years’ experience shows that it is not substantiated from the perspective of vital capacity of trees in urban environment. The conducted research showed that the most popular introducents in the city (common horse chestnut, Lombardy poplar) are in better condition than the indigenous species (tillet, drooping birch, Norway maple). This difference is especially seen in linear, street, and riverside plantings.


Introduction
Green plantations are an essential element of improvement of city. To a greater extent, this applies to major cities. Green plantations affect temperature and humidity of the air of the city, have a bactericidal effect, reduce dust, and restore normal composition of the air 6 .
Green areas have a number of distinctive features, such as broken or artificially created soils or artificial flora; duration of their existence is limited by lifespan of the constituent trees, since the complex is not capable of self-renewal 7 .
It has long been known that inside city, tree species tend to have worse health status and live less than in the natural environment 1 . Besides, it was noticed that trees of indigenous nature in urban plantings often feel worse than introduced species. Considering that while creating of urban planting, gardeners always face a difficult choice, which species to use and in which proportions, knowledge of the comparative characteristics of the status of these species in adulthood can substantially facilitate this choice 5 .

Materials and Methods
Based on this, we set ourselves to make a comparative analysis of forest pathologic state of indigenous and introduced trees in the urban plantings, on the example of the city of Voronezh.
The objects of research were chosen in such a way that, on the one hand, all types of urban planting and plant conditions were taken into account, and on the other, the most common in urban greening tree species were examined. Since 92% of urban planting in Voronezh are represented by deciduous vegetation, we restricted ourselves to examination of this particular species group.
The state of the plantings was determined by standard methods of forest pathology examination by conducting surveys on routes and temporary sample plots 4,8,9 . Field investigations were conducted during 2011-2014 in various types of plantations in 6 districts (Table 1).

Results
Trees of more than 100 species 2 can be found in street and park plantings of Voronezh; however, 89.6% of the urban trees are represented by 8 species, 6 of which are indigenous (white poplar, small-leaved lime, Norway maple, European ash, elm, and silver birch) and 2 species -introducents (horse-chestnut, cottonwoods) ( Figure 1). These very species became the subject of our research in the first turn.
According to our research, the leading position of 8 major tree species used in greening of Voronezh is taken by Norway maple (22.9%) which is related to the typical natives of the forest-steppe zone 3 . The total share of natives in landscaping is 79.1%. Horse-chestnut is leading among introduced species (11.3%).
The pathological characteristics, registered during the process of research, were divided into 12 groups. Their quantitative and percentage values in group park plantings are presented in ( Table 2).
Analysis of the data of the table shows that the most common type of pathology of all trees under research is pathology of trunk form. It is found in 96% of the analyzed trees. This pathology manifests itself at a high level (42.8-73.3%) in almost all indigenous species. This pathology is found in introducents many times rarer. Thus, in cottonwoods it constitutes 9.6%, and in horse-chestnut -12.8%. Most often it is bifurcation (51.6%), slopes (28.4%) and thick main branches (13.9%).
Status of separate species by districts is assessed as weakened and poor ( Table 2). Weakening of plants was assessed visually and was fixed by primary and secondary external pathological signs of trees, indicating the category of its weakening ( Table 3).
The listed abnormal forms of trunk do not directly affect the state and vitality of tree, but significantly increase the risk of windthrow, breaking of trunk, or large branches breakup. Accordingly, they can justifiably be considered as pathological, i.e. reducing the prospects of survival and prospects of tree.
Indigenous species, as compared with native species, in the park plantings have significantly less pathologies, such as damage by stem pests and mechanical damage (up to 10 times).
If we compare two systematically close woody species by presence of pathologies, one of which is an unconditional aborigine (Norway maple), and the second is an explicit introducent (Ash-leaved maple), it is evident that in a number of pathologies (abnormal shape of trunk, water sprouts, number of overgrown knots, etc.), the two species are almost identical (Figure 2). At the same time, it is evident that introduced tree suffers much stronger from low temperatures and is damaged by leaf-mining insects.
Indigenous species have more mechanical damages, as well as more burns and eating of leaves by insects.
The analogous tendency in occurrence and diversity of pathological symptoms is noticed when comparing pairs of poplar (white and cottonwoods).
Thereby, it is possible to state that in group park plantings, pathological condition of introduced tree species is not much, but significantly better than the state of indigenous trees. Occurrence of pathological signs in linear street plantings significantly differs from park landings (Table 4).
This situation is diametrically opposed to that observed in park plantings. In our view, it is logical to assume that a higher percentage of damaged crowns of introducents by phytophages in park plantings is due to their heterogeneity and thickening, which encourages formation of natural biocenosis, which also implies presence of a parasite entomocomplex. In linear one-species plantings, this factor is excluded.
While comparing the most common species in street plantings -Small-leaved lime and horse-chestnut ( Figure  3), it is seen that both the set of pathological symptoms and level of their importance for each of the species are very specific. Trees of lime do not have any pathology associated with damage of leaves and influence of low temperatures, but have much more mechanical damage, not overgrown

Discussion
As compared to indigenous species, the introducents in park plantings feature less damage from trunk pests and mechanical damages (up to 10 times less).
More leaf burns, damages by leaf miners, and spongecreators were noted in introduced species. Tumors, fruit bodies of wood-destroying fungi, and wood-defoliation are isolated cases.
All other pathologies have comparable percentage ratio between the analyzed groups of species.
Almost on all of the trees under research external symptoms of pathology are marked in some way. Most of the pathologies are not fatal (leaf burns, partial defoliation of crowns, insignificant mechanical damages of bark, etc.), but there is also a significant amount of pathologies which are irreversible (fruit bodies of wood-destroying fungus, large hollows, cancerous tumors, drying of a part of crown, etc.).
In street plantings, the introduced species have considerably less damaged leaves (burns, defoliation, leaf miners, etc.). The exception is the horse-chestnut, up to 89% of the trees of which have leaf burns in the middle of summer.
Trees of such introducent as brittle willow, which is not in park plantings, are also noted for extremely unsatisfactory state. The majority of trees of this species has pathological forms of trunk (85.2%) and dried out main branches (94.1%).
According to the other indicators, the introducents practically do not differ from indigenous species in street plantings.

Conclusion
According to the given data and analysis, it is possible to conclude that in park and in street plantings of Voronezh introduced species of trees, with all things being equal, feel at least no worse than the trees of indigenous species.
Considering that introduced species constitute only 20% of the green spaces of the city, their share may be significantly increased, which will undoubtedly improve the aesthetic qualities of urban landings without reducing their sanitary state.