Evaluation of the riparian forest habitat status on the basis of the analysis of the trees and shrubs flora of Rakowiecki Forest in Wrocław

Przemysław Bąbelewski, Katarzyna Blitek, Sebastian Cegiełka, Magdalena Pancerz

Abstract

The aim of the study was to evaluate the forest habitat on the basis of the analysis of the trees and shrubs flora of a part of Rakowiecki Forest in Wrocław. Changes in the composition and structure of the tree plants were examined. The geographical and historical structure of dendroflora was determined, and selected indices of anthropogenic changes in the flora were calculated. The syntaxonomic affiliation of this community and the compliance of the dendroflora with the potential flora were investigated. Moreover, the indices of natural habitat conservation, recommended in Natura 2000 – monitoring of natural habitats, were determined.

The total number of dendroflora species in the tested area was 57, including 39 trees and 18 shrubs taxa. Among them, 23 species belong to antropophytes; according to the current classification, 19 are characterized as kenophytes. There were also 4 diaphytes, species with no permanent settlement in the flora of our country; no archeophytes were observed. 34 taxa are native and account for 59,65% of species on the tested area, while their share in the tree stand is higher and reaches 79,86%. Individuals of native species ecologically extraneous for riparian forest account for 1,4% of the tree stand. The north-western part is dominated by taxa typical for willow-poplar gallery forest, the alliance Salicion albae: Salix alba, Populus alba and others. With the increase in the distance from the Oława river, there is an increasing share of species from the alliance Alno-Ulmion, belonging to Ass. Ficario-Ulmetum. The occurrence of broadleaf species, characteristic for the alliance Carpinion betuli, is the highest in the north-eastern part of the tested area, bordering with the watercourse Krzywa Grobla and along the northern forest boundary.

Keywords: analysis of tree flora; anthropogenic index; habitat conservation

Article type: original research paper

Running head: Evaluation of the riparian forest habitat

Addresses:

Przemysław Bąbelewski – email: [e-mail]

Katarzyna Blitek – email: [e-mail]

Sebastian Cegiełka – email: [e-mail]

Magdalena Pancerz – email: [e-mail]

Zakład Roślin Ozdobnych i Dendrologii, Uniwersytet Przyrodniczy we Wrocławiu

pl. Grunwaldzki 24 A, 50-363 Wrocław

Introduction

Habitats of riparian forests are among the most endangered ones in Poland. During the last twenty years, their surface area has been reduced by as much as 95%, and only 1% remained in its unchanged form. Natural elm-ash forests managed to remain in the country only fragmentarily. In the Lower Silesia voivodeship, significant areas of riparian wetlands are located in the Odra and Warta valleys (mesoregions of Central Odra Valley, Śrem Cirque, Grodków Plain and Wrocław Urstromtal) [1]. Forests in Odra Valley are considered to be one of the best preserved riparian forests in Poland and Europe [2, 3]. Among the forests located within the boundaries of Wrocław, occupying 2286.21 ha, which is approx.. 7.5% of the city’s area, most of the woodlands are the remnants of past elm-ash and willow-poplar forests, located in the wetlands along the rivers. However, only 10% of the tree stand is consistent with its habitat [4]. Due to the natural river course of the Odra, Bystrzyca, Oława and Widawa rivers, their valleys are the crucial wildlife corridors in Wrocław.

Each plant or plant community is an excellent reflection of its living conditions, representing through biological reactions the current state of the ecological environment as well as occurring transformations, both natural and anthropogenic [5, 6, 7, 8]. There are many test methods regarding the transformation of plant vegetation on the phytocenosis level. The evaluation of the state of ecosystems is conducted i.a. based on the differences between potential and real vegetation, by distinguishing substitute communities for specific communities [9], and taking into account the flora compatibility with the characteristic species combination. On a higher syntaxonomic rank, disappearance or a decrease in the number of taxa is the evidence of more advanced degeneration of communities [10]. Anthropopression often leads to the emergence of distorted units with specific structure features, floristic composition and plant vitality [9]. Forms of forest degradation include simplification of age structure, rejuvenation of the tree stand, reduction in the number of species and composition. One of the unfavourable phenomena occurring in riparian forests is the hornbeam and oak succession, which is often connected to a change in hydrological conditions [11, 12]. What can be used in the analysis of the anthropogenic transformations of plant vegetation is a geological-historical classification, which divides species into indigenous ones and xenophytes. Two phenomena resulting from the phytocenosis imbalance indicate a change in the structure or in the natural processes of a given ecosystem: apophitization – diffusion of species that are indigenous but foreign to a particular habitat; and neophitization – an increase in foreign species prevalence [13]. Riparian habitats, with their periodic floods, are especially vulnerable to foreign species diffusion. This vulnerability is also further increased by anthropogenic changes in the water system [14].

Trees and shrubs are able to function only in a range of conditions which they tolerate. Their state is influenced by many factors: habitat conditions, damage caused by abiotic, biotic and anthropogenic factors. The impact of a given factor is further modified by the specificity of a tree species, including mechanical properties of wood, different endurance to unfavourable habitat conditions and ongoing changes [15]. Plants react to changes in the environment visually; one can observe for instance dried parts in the treetop, yellowing of the leaves or other signs of worsening condition. These symptoms may act as bioindicators of the state of the ecosystem. Yet, it is crucial to protect special specimens (or groups) of distinctive size or specific features, giving them high natural value [16,17].

The aim of this study was to assess the condition of the forest habitat based on the analysis of the trees and shrubs flora in an exemplary fragment of riparian forest in Wrocław. The state of preservation of the tree stand, changes in its composition and the species structure of woody plants were examined, and the most important protection recommendations were presented. The geo-historical structure of tree and shrub species was determined, and selected anthropogenic changes in the flora were calculated. The syntaxonomic association of the community and the compatibility of its dendroflora with the potential vegetation of the area were also investigated. In addition, habitat preservation rates were established, as recommended in the program Natura 2000 as to the monitoring of natural habitats.

Material and methods

The analyses were performed based on the data obtained during dendrological cataloguing. The survey was conducted in a triangular wooded area of 6 ha in the Rakowiecki Forest located in the eastern part of Wrocław. It is placed at the confluence of the Oława river, the south-western border of the forest, and the Krzywa Grobla watercourse, bordering it in the east. In the north of the forest there are Rakowiec housing development, family allotment gardens and the Oława backwater. This area is varied in height, with a slight slope toward the river. Potential natural vegetation are ash-elm trees (Ficario-Ulmetum typicum) [18]. Tree and shrub species were recorded and classified according to their syntaxonomic affiliation [1, 2, 18, 19, 20] and geo-historical division, including their origin, arrival time and affinity in the flora [21]. Metaphyte species were identified (only kenophytes were noted), diaphytes (mainly ergazophyphophytes) as well as taxa of unestablished origin. In terms of syntaxonomics, taxa typical for riparian forests and native taxa foreign to the habitat were included among native species (tab. 1-2). The nomenclature of trees and shrubs was adopted after Seneta and Dolatowski [22]. The percentage frequency of the above-mentioned species was calculated as well as its share in each layer.

The field maps used in the study had a scale of 1:500. A full list of dendroflora taxa recorded in the surveyed area, including detailed information regarding their size and phytosanitary status, is provided in Table 3. Each tree and shrub is presented on the 1:500 scale map.

General species richness of the dendroflora (i.e. the number of species – S) and the species richness of individual layers (Sa, Sk) were reported [13]. The attention was also drawn to which species dominate in the tree stand and how many of them are typical to the habitat [1]. These indicators allowed the study of the status of the natural habitat and its compatibility with the potential syntaxon. The synanthropization scale of the dendroflora and other processes ongoing in the studied phytocenosis were analyzed, including indicators such as antropofitization (An), archeofitization (Ar), kenofitization (Kn), modernization (M) and flora lability (L) (tab. 3-4). The analysis of the anthropogenic changes in the flora was needed to valorize the examined area with regard to flora naturalness and foreign species expansion as well as flora lability. Additionally, the analysis of species composition was to determine whether the specific structure and functions of the community were preserved [1, 13, 23, 24].

Results

General richness of the dendroflora of the studied area (S) amounted to 57 (Table 3), including 39 tree and 18 shrub taxa. Among the trees, the most common species was Crataegus monogyna, present only in lower layers of the tree stand. In higher levels, Acer platanoides, Populus alba, Qercus robur, Tilia cordata, Salix alba and Robinia pseudoacacia showed the highest frequency of occurrence (more than 7% of the tree stand). Species such as Alnus glutinosa, Fraxinus pennsylvanica, Fraxinus excelsior, Ulmus minor, Populus x canadensis, Prunus cerasifera represented more than 2.5% of the tree stand. The share of other species did not exceed 1%. 23 species of anthropophytes were recorded in the studied area, and according to the currently accepted classification 19 of them belong to kenophytes – plants domesticated in Poland after the 15th century. Four diaphytes were noted – species not domesticated in Poland’s flora. Not a single archeophyte was identified [25, 21]. The antropofitization index (wAn), the share of geographically foreign taxa – metaphytes, was estimated at 40.4% of the overall number of species in the examined area (S). The ratio of lability, the share of diaphytes (D), was estimated at 7%, and the kenofitization ratio, the share of kenophytes (K) was estimated at 33.3%. Slightly smaller was the quantitative share of geographically foreign species in the tree stand, amounting to 20.8%, of which 20.7% were kenophytes, and the diaphytes were only 0.1%. Six taxa from among the antropophytes were classified as invasive: Acer negundo, Ailanthus altissima, Prunus cerasifera, Prunus serotina, Robinia pseudoacacia and Symphoricarpos albus. [13, 21, 23, 24, 26, 27, 28]. Thirty-four native taxa were noted, amounting to 59.65% of the overall number of species in the studied area. Their share in the tree stand was higher and equaled 79.86%. Native, ecologically foreign species accounted for 1.4% of the tree stand of the riparian habitat (Table 4).

Spatial distribution of species in the tree stand of the studied area was varied. Taxa typical for willow-poplar forests such as Salicion albae: Salix alba, Populus alba and other, were predominant in the north-western section. Unfortunately, the tree stand of this section was also heavily influenced by foreign taxa: Robinia pseudoacacia and Populus canadensis. With the increasing distance from the Oława river, species from Alno-Ulmion, Ficario-Ulmetum group were more common (Table 1). The frequency of hornbeam and oak species, characteristic for the Carpinion betuli group, was the highest in the north-eastern section bordering the Krzywa Grobla watercourse and along the northern border of the forest. In this area, the majority of species was noted, with the number of representatives not exceeding 10.

Discussion

Riparian forests are communities connected with the valleys of great rivers as well as smaller watercourses. They may also emerge on the flanks of valleys, in areas prone to episodic floods [20, 29]. The studied area ideally corresponds to this characteristic. Moreover, the structure of the tree stand with 2–3 layers is typical for this habitat. Potential vegetation of this area is Ficario-Ulmetum elm-ash forest [18], characterized by a lush layer of many species of shrubs, especially well developed in the parts subject to at least periodical floods [2, 3]. However, the floristic richness, the presence or absence of typical species (ash, elm, oak) are not good indicators of the habitat preservation. According to the Natura 2000 monitoring of habitats, the natural composition of the well-preserved phytocenoses of the 91F0 habitat is varied and heavily dependent on the micro-diversity of water conditions. Therefore, "the occurrence of phytosociologically characteristic species" may not be a good indicator. "Typicality of flora composition" should be evaluated comprehensively and in relation to the choice of "habitat-specific species that are key to biodiversity" [2]. The structure of the tree stand is varied, both in terms of species and age. Many self-sowing plants, manifestations of natural regeneration of the forest, can be spotted as well as aged specimens and dead trees, standing and lying. It means that there are no symptoms of monotonization or juvenalization. What is also valuable is the presence of dead wood, crucial to the biodiversity of the ecosystem. It can be concluded that forest management is conducted properly. Among the species typical for the habitat, oak is predominant, although it is not due to the economic origin of the forest. There is also a relatively small number of the individuals of this species, as they were intentionally placed to form an "oak alley" along the Krzywa Grobla watercourse. Numerous occurrences of Acer platanoides maple and small-leaved lime Tilia cordata can be a manifestation of the forest transformation and an example of the mosaic of habitats in a fragment of Rakowiecki Forest [11, 12]. The species richness of the tree stand, including a significant share of willows, poplars and field maples, is typical to riparian wetlands in the floodplains of large river valleys. In the studied area, the share of poplars and willows is inversely proportional to the distance from the river Oława; however, as the distance from the river increases, the share of ashes and alders does as well. This is probably due to the natural variation of the riparian habitat. With the increase in the distance from the river, the periodicity and the duration of floods change, and so do the habitat conditions, from the Salicion albae to the Alno Ulmion being favorable. In the areas furthest from the river there are more taxa typical for humid forests from the Qerco-Carpinetum group [2, 30].

There is a considerable concern about a significant contribution of foreign taxa, mainly kenophytes, both in the number of species and their quantitative share in the tree stand. Their placement is uneven. This is probably the result of the diversity of the habitat conditions [13, 14]. Among foreign species, Populus canadensis, which has dominated the native poplar (P. nigra), prevails in the periodically flooded western part and in the central area of research. A potential threat to these species of poplars may be the forming of hybrids from the cross pollination of black poplar with the pollen of Canadian poplar [31].

In the south-western part of the studied area, elevated above the floodplains, near the Oława river bend, invasive Robinia pseudoacacia is most dominant. This species is also scattered throughout the whole studied area. In the north-western part, green ash (Fraxinus pennsylvanica) is seen, in quantities exceeding the share of native common ash (F. excelsior). This area is located in the river water-storage area, above the level of annual floods, but within the range of episodic ones. It is a typical area for the Ficario-Ulmetum habitat (Table 2), with favourable conditions for common ash. In the lower floor of the forest, the most common kenophyte (formerly classified as archeophyte) is domestic apple tree (Malus domestica) [21]. In natural conditions, this habitat is most often occupied by wild apple (Malus sylvestris), not found in the study area. Frequent occurrence of cherry plum (Prunus cerasifera) can also be observed in this forest level. The population of ash-leaved maple (Acer negundo) population, relatively small, is scattered across the studied area. However, since there is a lot of evidence of its invasiveness in other parts of the country, it can be deducted that the future population of this species will significantly increase in the studied area as well. The native taxa in their natural habitat are being replaced by their related or foreign species, which have similar habitat requirements [32, 33]. The remaining foreign species are rare (up to 10 plants), especially near the forest border, which may be due to the proximity of urbanized areas and gardens. The studied area is characterized by conditions conducive to the penetration of anthropophytes, which are either natural factors in the area of periodic floods, i.e. permanent "pioneering" of alluvial habitats (conditions favourable to Acer negundo L., Fraxinus pennsylvanica and Populus × canadensis) [2, 14] or already existing substitute communities, with a large share of foreign taxa, especially locusts.

The layer of shrubs in the studied area is rich and diverse, which is typical for riparian forests. There are 18 species of shrubs, and the tree stand is growing. Although there are only 7 native taxa among bushes, they definitely outnumber the foreign ones. Particularly clear is the dominance of black elder (Sambucus nigra). Whether this is a favourable phenomenon remains a matter of discussion, because although this taxon is typical for the understory of riparian forests [20, 29, 34], it also belongs to apophytes [27].

The mosaic distribution of the real vegetation patches corresponds well to the diversity of potential vegetation, although native species are replaced by their related species or kenophytes with similar habitat requirements. The condition of the habitat affects the condition and distribution of trees and shrubs. However, it is important to keep in mind that this data does not provide up-to-date information on the environment. Both qualitative and quantitative vegetation changes appear with a visible delay, particularly in the case of perennial species such as trees and shrubs. The results of the study do not reflect the state of the environment at any given moment, but instead they reflect the average state of the community, shaped for many years [12].

Tables

Table 1. Species composition of elm-ash riparian forest Ficario-Ulmetum according to the phytosociological classification

	Tree layer	Shrub layer
[20]	Ulmus laevis, U. minor, U. glabra, Quercus robur, Fraxinus excelsior, Prunus padus, Carpinus betulus, Tilia cordata , Acer campestre, A. platanoides, Malus sylvestris, Populus alba	Prunus padus, Crataegus monogyna, C. laevigata, C. rhipidophylla, Sambucus nigra, Viburnum opulus, Cornus sanguinea, Ribes spicatum, R. rubrum
[1, 2]	Qercus robur, Fraxinus excelsior, Ulmus laevis, U. minor, U. glabra, Malus sylvestris , Prunus padus, Tilia cordata, Carpinus betulus, Alnus glutinosa, Acer platanoides, A. campestre, A. pseudoplatanus , Populus alba, P. nigra, Salix alba, S. fragilis	Cornus sanguinea, Crataegus laevigata , Sambucus nigra, Euonymus europaea, Viburnum opulus, Ribes spicatum, Rhamnus catharticus, Corylus avellana

Table 2. Species composition of elm-ash riparian forest Ficario-Ulmetum minoris of the Silesia land [19]

Main species I fl.	Species dom. I fl.	Species II fl.	Species III fl.	Understory
Fraxinus excelsior, Ulmus sp., Quercus robur	Alnus glutinosa	Ulmus laevis, U. glabra, Populus alba	Prunus padus, Tilia cordata, Carpinus betulus, Acer platanoides, Malus sylvestris	Prunus padus, Sambucus nigra, Euonymus europaeus, Cornus sanguinea, Ribes spicatum

Table 3. List of trees and shrubs, including their syntaxonomic, geographical and historical affiliation, growing in the area of Rakowiecki Forest

No.	Latin name	Number	Form	% share in the tree stand	Syntaxonomic affiliation
1	Acer campestre	14	d/t	0.61	Ł
2	Acer negundo	25	d/t	1.28	K
3	Acer platanoides	209	d/t	10.70	Ł
4	Acer pseudoplatanus	116	d/t	5.94	Ł
5	Acer saccharinum	5	d/t	0.26	K
6	Aesculus hippocastanum	10	d/t	0.51	K
7	Ailanthus altissima	3	d/t	0.15	K
8	Alnus glutinosa	77	d/t	3.94	Ł
9	Betula pendula	2	d/t	0.10	O
10	Carpinus betulus	24	d/t	1.23	Ł
11	Cornus alba	4	k/s		D
12	Cornus mas	1	k/s		D
13	Cornus sanguinea	3	k/s		Ł
14	Corylus avellana	5	k/s		Ł
15	Crataegus laevigata	3	d/t	0.15	Ł
16	Crataegus monogyna	254	d/t	13.00	Ł
17	Eonymus europaeus	14	k/s		Ł
18	Fagus sylvatica	2	d/t	0.10	O
19	Fraxinus excelsior	70	d/t	3.58	Ł
20	Fraxinus pennsylvanica	75	d/t	3.84	K
21	Juglans regia	10	d/t	0.51	K
22	Juniperus chinensis	1	k/s		D
23	Ligustrum vulgare	1	k/s		O
24	Lonicera xylosteum	9	k/s		Ł
25	Malus domestica	33	d/t	1.69	K
26	Morus alba	2	d/t	0.10	K
27	Philadelphus coronarius	1	k/s		K
28	Picea abies	8	d/t	0.41	O
29	Platanus x hispanica	1	d/t	0.05	K
30	Populus alba	166	d/t	8.49	Ł
31	Populus x canadensis	55	d/t	2.81	K
32	Populus canescens	12	d/t	0.61	Ł
33	Populus nigra	37	d/t	1.89	Ł
34	Populus tremula	4	d/t	0.20	O
35	Prunus avium	9	d/t	0.46	O
36	Prunus cerasifera	46	d/t	2.35	K
37	Prunus cerasifera 'Pissardi'	1	d/t	0.05	K
38	Prunus domestica	1	d/t	0.05	K
39	Prunus mahaleb	6	d/t	0.31	K
40	Prunus padus	18	d/t	0.92	Ł
41	Prunus persica	2	d/t	0.10	D
42	Prunus serotina	4	d/t	0.20	K
43	Pyrus pyraster	3	d/t	0.15	O
44	Quercus robur	164	d/t	8.39	Ł
45	Ribesnigrum	1	k/s		Ł
46	Robinia pseudoacacia	134	d/t	6.86	K
47	Rosa canina	19	k/s		O
48	Salix alba	156	d/t	7.98	Ł
49	Salix caprea	1	d/t	0.05	Ł
50	Salix fragilis	2	d/t	0.10	Ł
51	Salix viminalis	1	k/s		Ł
52	Sambucus nigra	223	k/s		Ł, Ap
53	Symphoricarpos albus	2	k/s		K
54	Syringa vulgaris	1	k/s		K
55	Tilia cordata	162	d/t	8,29	Ł
56	Ulmus minor	56	d/t	2,87	Ł
57	Viburnum opulus	1	k/s		Ł
	Overall	2269

Native species: Ł – typical for ass. Ficario-Ulmetum; O – extraneous in the habitat; Ap – apophytes; species extraneous geographically (antropophytes): K – kenophytes; D –diaphytes (ergasiophygophytes); life forms: t – trees; s – shrubs, t – mainly tree shape.

Table 4. Summary of trees and shrubs growing in the areas of Rakowiecki Forest according to the geographical and historical classification

Geographical and historical groups	Species number	Number of plants	% of species	% of the tree stand
Native, proper to riparian forest (N1)	26	1798	45.61	78.43
Native, extraneous in the habitat (Ne)	8	48	14.04	2.46
Apophytes (Ap)	1	223	1.75
Native total (N)	34	1846	59.65	79.86
Archeophytes (Ar)	0	0		0
Kenophytes (K)	19	415	33.33	20.69
Diaphytes (D)	4	8	7.02	0.10
Extraneous total	23	423	40.35
Tree layer	39	1954
Extraneous species in the tree stand	15	414	26.32	20.79
Trees and shrubs total	57	2269
Native in the tree stand	27	1539		79.21
Native extraneous in the tree stand	4	16		1.43

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Authors' contributions

Przemysław Bąbelewski: research designing, factual supervision, methodological consultation,

Blitek Katarzyna: conducting experiments, writing the manuscript

Cegiełka Sebastian: conducting experiments, assistance in analysing

Pancerz Magdalena: writing the English version of manuscript, research designing, factual supervision