Research Article |
Corresponding author: Katharina Lapin ( katharina.lapin@bfw.gv.at ) Academic editor: Franz Essl
© 2019 Katharina Lapin, Janine Oettel, Herfried Steiner, Magdalena Langmaier, Dunja Sustic, Franz Starlinger, Georg Kindermann, Georg Frank.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Lapin K, Oettel J, Steiner H, Langmaier M, Sustic D, Starlinger F, Kindermann G, Frank G (2019) Invasive Alien Plant Species in Unmanaged Forest Reserves, Austria. NeoBiota 48: 71-96. https://doi.org/10.3897/neobiota.48.34741
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Invasive alien plant species (IAS) are one of the greatest threats to global biodiversity and the sustainable functioning of ecosystems and mitigating the threat posed by them is therefore of great importance. This study presents the results of a 15-year investigation into how IAS occur within natural forest reserves (NFR): unmanaged forest ecosystems within Austria, concluding that unmanaged forests are not resistant to plant invasions. The study comprised ground vegetation, regeneration, and stand structure surveys. The presence or absence of IAS in different forest types was assessed and the influencing variables for their presence or absence were determined. In addition, the study analysed whether the abundance of IAS has increased at the site level within the past decade. Significant differences in the probability of IAS presences between forest types (photosociological alliances) were found. The results of the study show that natural riparian and floodplain forests are among the forest types most vulnerable to biological invasions, which is reflected in elevation and soil type being determined as the main factors influencing the spread of IAS in unmanaged forests. The results of this study may be useful for persons responsible for sustainable forest management programmes or for managing forested areas within national parks. They provide a case study on non-intervention forest management policy in order to mitigate the impacts of IAS in protected areas. Forest areas, where IAS begin to spread can be identified, which in turn leads to measures in the early stages of invasion, and to optimise monitoring and control measures for relevant species in Central European forest types.
Austria, biological invasions, forest ecosystems, floodplains, natural forest reserves, nature conservation, neophytes
Invasive alien plant species (hereinafter “IAS”) are one of the greatest threats to global biodiversity and the sustainable functioning of ecosystems (
Databases and platforms such as the Global Register of Introduced and Invasive Species (GRIIS) have been established to collate information on the distribution of IAS for use by decision makers to plan and manage the spread of IAS (
In total, 42% (167 million ha) of the land surface of the EU is covered in forests, and preventing, controlling at early stages of invasion, and managing the spread of IAS in Europe’s protected forest areas is therefore of particular importance for insuring ecosystem services of European forests (
From previous studies we can conclude that alien species also occur in many unmanaged forests and that they are often invaded by similar sets of alien species (
Looking at Austria in particular, the proportion of land surface covered in forests is 47.9% (4.02 million ha), well above the EU-wide proportion. About 88% of this area (3.53 million ha) consists of managed productive forest (
The study area comprises the Austrian natural forest reserves (hereinafter “NFR”) as historically managed and now unmanaged forest sites, which provide novel areas for research, training, and education on forests. The aim of the NFR programme is to conserve, enhance, and monitor forest biodiversity by abstaining from forest utilisation, logging of dead wood, and artificial regeneration of forest trees (
Few studies have been conducted with the aim of understanding how human activity in forests causes changes in the presence of IAS. Validated knowledge on forest regeneration and tending, tree species selection, regeneration procedures, production and regeneration periods, competition control, and natural selection and differentiation in particular is needed to be able to compare the production, protection and recreation provided by forests depending on their ecological conditions. This type of information would help forest managers to identify forest regions or stands where IAS start to spread, which in turn gives rise for action in the early stages of invasion and optimise monitoring and control measures concerning the relevant species for Central European forest types.
This paper develops the current literature on IAS by providing an analysis of the alien flora in unmanaged NFR in Austria. The aims are (i) to identify invasive plant species in NFR and (ii) to analyse which variables are important for determining the presence or absence of IAS across multiple NFR, with particular reference to (iii) the composition of native plant species in those NFR. Additionally, the paper analyses at the site level (iv) whether the abundance of invasive alien plant species has increased within the past 15 years. The results of this paper are particularly useful for assessing “zero-IAS-management-scenarios” in forests, which is an ongoing challenge for persons responsible for sustainable forest management programmes or managing forested areas within national parks, by providing information on the spread of IAS in different unmanaged forest types.
The NFR analysed for this study are part of the Austrian Natural Forest Reserve Programme established in 1995. Sites in formerly managed forest areas were selected according to a set of criteria including naturalness of tree species composition. Today, the NFR network comprises 192 reserves in Austria with a total area of 8,355 ha. The aim of the programme is to represent the 118 forest communities (out of 159 known forest and shrub communities) found in 22 growth zones within Austria (
a) Vegetation sampling
The ground vegetation was recorded in 2,344 sample plots with sizes varying from 50 to 700 m² (average size: 240 m²) across all 192 NFR sites using the Braun-Blanquet cover abundance scale with 7 to 9 classes (
b) Tree sampling
The composition and development of tree species was systematically determined for the sampling plots within the NFR using Bitterlich’s angle count (AC) sampling with a basal area factor (BAF) of 4 (
The first data collection took place between 1997 and 1999 (period 1), and the survey was repeated 15 years later between 2013 and 2014 (period 2). The collected data included tree-related information like species, DBH, tree height, crown height, and location within the plot (distance and direction). For the analysis of tree species composition and development, the stem number (N) and basal area (G) per hectare were determined. The number of trees per hectare (N) was calculated by dividing the BAF (BAF = 4) by the circular area represented by each tree. The basal area per hectare (G) was calculated by summing up the trees in the AC and multiplying them with the BAF.
c) Regeneration sampling
Natural regeneration in the NFR was investigated from 2013 onward, and information is available for 36 NFR (784 samples). On each sample plot, 4 satellite samples of 1 m² in size (4 m² per plot) were collected to document the regeneration of tree species. Tree species, height (in 10 cm increments) and browsing damage were determined for each recorded plant. For the analysis of tree species regeneration, the number of regeneration trees per hectare was calculated by multiplying the number of trees with 2,500.
Each of the alien plant species found in the 192 NFR was evaluated to determine its invasive potential in Austrian bioregions as well as at the European level and to estimate the likelihood of its future spread and negative ecological impact in Austrian forests (
List of invasive alien plant species (IAS) and non-invasive alien plant species (AS) in NFR. The 16 invasive alien plant species with their family, life form (Raunkiær system), native range, number of NFR with occurrence.
# | Family | Species | Life form (Raunkiær system) | Native range | number of NFR |
---|---|---|---|---|---|
Invasive alien species | |||||
1 | Balsaminaceae | Impatiens parviflora DC. | Therophyte | Asia | 42 |
2 | Fabaceae | Robinia pseudoacacia L. | Phanaerophyte | Northern America | 16 |
3 | Compositae | Solidago gigantea Aiton | Hemikryptophyte | Northern America | 11 |
4 | Balsaminaceae | Impatiens glandulifera Royle | Therophyte | Asia | 6 |
5 | Sapindaceae | Acer negundo L. | Phanaerophyte | Northern America | 4 |
6 | Compositae | Bidens frondosa L. | Therophyte | Northern America | 4 |
7 | Oleaceae | Fraxinus pennsylvanica Marshall | Phanaerophyte | Northern America | 4 |
8 | Compositae | Symphyotrichum lanceolatum (Willd.) G.L.Nesom | Geophyte | Northern America | 2 |
9 | Simaroubaceae | Ailanthus altissima (Mill.) Swingle | Phanaerophyte | Asia | 1 |
10 | Elaeagnaceae | Elaeagnus angustifolia L. | Phanaerophyte | Asia | 1 |
11 | Compositae | Erigeron annuus (L.) Pers. | Hemikryptophyte-Therophyte | Northern America | 1 |
12 | Compositae | Erigeron canadensis L. | Therophyte | Northern America | 1 |
13 | Phytolaccaceae | Phytolacca americana L. | Geophyte-Hemikryptophyte | Northern America | 1 |
14 | Polygonaceae | Reynoutria japonica Houtt. | Geophyte | Asia | 1 |
15 | Compositae | Solidago canadensis L. | Hemikryptophyte | Northern America | 1 |
16 | Compositae | Symphyotrichum novi-belgii (L.) G.L.Nesom | Geophyte | Northern America | 1 |
Non-invasive alien species | |||||
17 | Fagaceae | Quercus rubra L. | Phanaerophyte | Northern America | 3 |
18 | Sapindaceae | Aesculus hippocastanum L. | Phanaerophyte | Southeastern Europe | 2 |
19 | Amaranthaceae | Atriplex sagittata Borkh. | Therophyte | Southwestern Asia | 2 |
20 | Compositae | Erechtites hieraciifolia (L.) Raf. ex DC. | Therophyte | America | 2 |
21 | Amaranthaceae | Atriplex prostrata Boucher ex DC. | Therophyte | Western Europe | 1 |
22 | Cucurbitaceae | Echinocystis lobata (Michx.) Torr. & A.Gray | Therophyte | Northern America | 1 |
23 | Onagraceae | Epilobium ciliatum Raf. | Hemikryptophyte | Northern America | 1 |
24 | Compositae | Galinsoga quadriradiata Ruiz & Pav. | Therophyte | Northern America | 1 |
25 | Leguminosae | Lupinus polyphyllus Lindl. | Hemikryptophyte | Northern America | 1 |
26 | Moraceae | Morus alba L. | Phanaerophyte | Asia | 1 |
27 | Oxalidaceae | Oxalis stricta L. | Geophyte-Therophyte | Northern America | 1 |
28 | Pinaceae | Pinus strobus L. | Phanaerophyte | Northern America | 1 |
29 | Salicaceae | Populus balsamifera L. | Phanaerophyte | Northern America | 1 |
30 | Salicaceae | Populus × canadensis Moench | Phanaerophyte | Northern America | 1 |
31 | Rosaceae | Potentilla indica (Jacks.) Th.Wolf | Hemikryptophyte | Asia | 1 |
32 | Rosaceae | Rhodotypos scandens (Thunb.) Makino | Phanaerophyte | Asia | 1 |
33 | Compositae | Telekia speciosa (Schreb.) Baumg. | Geophyte-Hemikryptophyte | Southeastern Europe | 1 |
34 | Ericaceae | Vaccinium macrocarpon Aiton | Chamaephyte | Northern America | 1 |
Explanatory variables (alliance, soil type, elevation, exposition, inclination, bedrock class, soil layer depth, and relief) used for the quasi-binomial logistic regression and the Random Forest model of the presence and absence of IAS in 2344 sample plots in 192 NFR.
Variable | Classification | Range/Categories | Description |
---|---|---|---|
Alliance | Categorical | N = 21 categories | Root category of phytosociological associations ( |
Soil type | Categorical | N = 32 categories | Expert aggregation of soil types determined by Anleitung zur Forstlichen Standortskartierung in Österreich (Englisch and Kilian 1998; Appendix |
Elevation (m) | Continuous | 120–2080 | The elevation was measured with an accuracy of ±10 m. |
Exposition | Categorical | Plain | The aspects of each site was measured in grade and assigned in cardinal directions. |
N-NE | |||
E-SE-S | |||
SW-S-NW | |||
Inclination | Continuous | 0–170% | The slope inclination of each site in percent (%) was estimated. |
Bedrock class | Categorical | Carbonate | Classification into bedrock classes was performed based on the Geological Map of Austria (1: 50,000) |
Flood plain sediments | |||
Intermediate | |||
Loess soil | |||
Silicate | |||
Soil depth (cm) | Categorical | 0 | The soil depth describes the thickness of the soil horizons over solid rock. This was determined by way of 3 to 5 samples per site. Classification was performed according to the sample mean values. |
0–15 | |||
15–30 | |||
30–60 | |||
60–120 | |||
>120 | |||
Relief | Categorical | Deposition | Sedimentation stages were classified according to the description of the macro- and mesoreliefs. |
Erosion | |||
Solid |
To prevent possible autocorrelations between the sometimes heavily spatially clustered vegetation recordings, neighbouring vegetation recordings were conflated into a single unit, with a threshold distance of 2 km defined for this purpose. This distance is based on the close proximity of six NFR located in the lower March floodplains in a 4.5 km radius. Although the distances between some individual areas is nearly 2 km, their specific species composition, especially that of alien species, is owed to their spatial proximity according to expert opinion. They were therefore aggregated into a single NFR site. This aggregation of neighbouring NFR was pragmatically extended to all other NFR as well, which provides the additional advantage of increasing the number of observations per NFR unit of area. In all, 21 groups of two NFR, five groups of three NFR, three groups of five NFR, two groups of six NFR, and one group of seven NFR were aggregated, while 101 NFR remain as individual sites. The 192 NFR were thus reduced to 133 NFR sites. Furthermore, the presence/absence data for each observation were weighted by the number of observations in each NFR site (weights = 1/n).
The relationship between the explanatory variables and the presence of IAS in the NFR was analysed for all 16 IAS together, as well as separately for Impatiens parviflora and Robinia pseudoacacia, using generalized linear models (GLM) with a logit link function. This analysis was performed using the freeware R (R version 3.4.2 (2017–09–28)) for personal computers (
In addition, the Random Forest (RF) method (
In total, 16 IAS and 18 non-invasive alien species (AS) were recorded (Table
Proportion of sites with occurrence of IAS (dark green), IAS and AS (green), AS (light green) and without IAS (grey) by forest alliances [%] (
The final quasi-binomial model showed that soil type, alliance and elevation were the strongest explanatory variables (p < 0.05) for the 189 sample plots with recorded IAS presence and 2,151 sample plots with recorded IAS absence. The probability of IAS presence was highest for the following soil types: pseudogley on unconsolidated sediments (130), gley (210), gray-alluvial soils (240), mature brown alluvial soils (250), and half bog (anmoor) (260) (Table
Parameter estimates of generalized linear models (error structure = quasi-binomial; link function = logit) explaining the probability of the presence of IAS in NFR sites. Only significant explanatory variables occurring in the minimal adequate GLM were included in the model. Values are on the logit scale. * = factors significant at the p<0.05 level. Data included in the logistic regression model were not transformed.
Variable | Categories | Estimate | Std. error | t value | Pr (>|t|) | |
---|---|---|---|---|---|---|
Soil type | 130 | Pseudogley on unconsolidated sed. | 5.77E+03 | 1.93E+03 | 2.988 | 0.00 |
250 | Mature, brown alluvial soil | 4.14E+03 | 1.50E+03 | 2.763 | 0.01 | |
260 | Half-bog | 5.70E+03 | 2.47E+03 | 2.306 | 0.02 | |
210 | Gley | 3.87E+03 | 1.82E+03 | 2.124 | 0.03 | |
240 | Gray-alluvial soil | 3.28E+03 | 1.64E+03 | 1.996 | 0.05 | |
80 | Minor Brown earth | 3.20E+03 | 1.67E+03 | 1.916 | 0.06 | |
150 | Gley/Pseudogley on slopes | 3.39E+03 | 1.98E+03 | 1.712 | 0.09 | |
21 | Colluvial deposits | 4.57E+03 | 3.94E+03 | 1.158 | 0.25 | |
20 | Poor brown earth | 1.43E+03 | 1.31E+03 | 1.087 | 0.28 | |
10 | Rankers | 1.34E+03 | 1.32E+03 | 1.016 | 0.31 | |
31 | Minor calcaric cambisol | 1.30E+03 | 1.42E+03 | 0.91 | 0.36 | |
30 | Eutrophic brown earth | 7.43E+02 | 1.26E+03 | 0.59 | 0.56 | |
180 | Rendzinas | –8.79E+02 | 1.50E+03 | –0.59 | 0.56 | |
100 | Brown earth on loess | 1.40E+03 | 2.66E+03 | 0.53 | 0.60 | |
120 | Pseudogley on solid bedrocks | 7.21E+02 | 1.88E+03 | 0.38 | 0.70 | |
200 | Terra fusca | –5.87E+02 | 1.61E+03 | –0.37 | 0.71 | |
90 | Cohesive brown earth | –1.87E+02 | 1.82E+03 | –0.10 | 0.92 | |
190 | Mixed soil | –1.71E+04 | 2.73E+06 | –0.01 | 0.99 | |
40 | Semi-Podzols | –1.47E+04 | 3.86E+06 | 0.00 | 1.00 | |
22 | Podzolic brown soil | –1.79E+04 | 5.35E+06 | 0.00 | 1.00 | |
280 | Bog | –1.49E+04 | 4.74E+06 | 0.00 | 1.00 | |
160 | Loamy soil | –1.83E+04 | 7.09E+06 | 0.00 | 1.00 | |
181 | Pararendzina | –1.78E+04 | 7.46E+06 | 0.00 | 1.00 | |
270 | Low peat bog, bog general | –1.43E+04 | 6.81E+06 | 0.00 | 1.00 | |
220 | Alluvial soil, streamside marshes | –1.78E+04 | 1.09E+07 | 0.00 | 1.00 | |
60 | Substrate-induced Podzol | –1.49E+04 | 9.74E+06 | 0.00 | 1.00 | |
110 | Chromic luvisols | –1.77E+04 | 1.16E+07 | 0.00 | 1.00 | |
131 | Pseudogley on loess | –1.76E+04 | 1.18E+07 | 0.00 | 1.00 | |
50 | Climate-induced Podzol | –9.51E+03 | 7.87E+06 | 0.00 | 1.00 | |
92 | Slightly gleyed brown earth | –1.54E+04 | 1.64E+07 | 0.00 | 1.00 | |
202 | Cohesive calcaric cambisol | –1.61E+04 | 2.27E+07 | 0.00 | 1.00 | |
140 | Stagnogley | –1.82E+04 | 2.61E+07 | 0.00 | 1.00 | |
132 | Pseudogley on clay | –1.31E+04 | 2.09E+07 | 0.00 | 1.00 | |
81 | Brown podzolic soil | –1.42E+04 | 2.70E+07 | 0.00 | 1.00 | |
Alliance | Fagion sylvaticae | –2.31E+03 | 1.07E+03 | –2.159 | 0.03 | |
Quercion roboris | –2.56E+03 | 1.24E+03 | –2.062 | 0.04 | ||
Dicrano-Pinion | –2.84E+03 | 1.58E+03 | –1.793 | 0.07 | ||
Quercion pubescentis-petraeae | –1.97E+03 | 1.24E+03 | –1589.00 | 0.11 | ||
Alnion incanae | –1.58E+03 | 1.26E+03 | –1.253 | 0.21 | ||
Alnion glutinosae | –2.62E+03 | 2.32E+03 | –1.129 | 0.26 | ||
Carpinion betuli | –7.88E+02 | 1.01E+03 | –0.78 | 0.44 | ||
Salicion cinereae | 7.99E+02 | 1.84E+03 | 0.43 | 0.66 | ||
Tilio-Acerion | –2.62E+02 | 9.54E+02 | –0.27 | 0.78 | ||
Salicion albae | –4.22E+02 | 1.78E+03 | –0.24 | 0.81 | ||
Pinion mugo | 5.85E+02 | 2.84E+03 | 0.21 | 0.84 | ||
Vaccinio-Piceion | –1.92E+04 | 2.54E+06 | -0.01 | 0.99 | ||
Erico-Pinion sylvestris | –1.75E+04 | 2.71E+06 | -0.01 | 0.99 | ||
Vaccinio uliginosi-Pinion | –1.95E+04 | 6.15E+06 | 0.00 | 1.00 | ||
Fraxino orni-Ostryion | –1.79E+04 | 6.62E+06 | 0.00 | 1.00 | ||
Alnion viridis | –1.36E+04 | 7.98E+06 | 0.00 | 1.00 | ||
Sambuco-Salicion capreae | –1.80E+04 | 1.07E+07 | 0.00 | 1.00 | ||
Salicion triandrae | –2.47E+04 | 1.78E+07 | 0.00 | 1.00 | ||
Berberidion | –2.14E+04 | 1.78E+07 | 0.00 | 1.00 | ||
Populo tremulae-Corylion | –1.85E+04 | 1.78E+07 | 0.00 | 1.00 | ||
Elevation | –6.46E+00 | 1.33E+00 | -4.847 | 0.00 | ||
Atan (inclination/100) | 2.60E+03 | 1.13E+03 | 2.301 | 0.02 |
The presence of Impatiens parviflora seems to be driving the model, however. We also applied the quasi-binomial model to I. parviflora and Robinia pseudoacacia, the two species with the highest incidence in the vegetation sample set. The strongest explanatory variables for I. parviflora are soil type (df = 32) and alliance (df = 21) (Table
Parameter estimates of generalized linear models (error structure = quasi-binomial; link function = logit) explaining the probability of the presence of Impatiens parviflora in NFR sites. Only significant explanatory variables occurring in the minimal adequate GLM were included in the model. Values are on the logit scale. * = factors significant at the p<0.05 level. Data included in the logistic regression model were not transformed.
Variable | Categories | Estimate | Std. error | t value | Pr (>|t|) | |
---|---|---|---|---|---|---|
Alliance | Abieti-Piceion | –1.39E+00 | 1.10E+00 | –1.26 | 0.21 | |
Alnion glutinosae | –1.92E+01 | 6.83E+03 | 0.00 | 1.00 | ||
Alnion incanae | –2.04E+00 | 9.36E–01 | –2.18 | 0,03 | ||
Alnion viridis | –1.52E+01 | 5.41E+03 | 0.00 | 1.00 | ||
Berberidion | –2.13E+01 | 1.19E+04 | 0.00 | 1.00 | ||
Carpinion betuli | –1.68E+00 | 6.85E–01 | –2.46 | 0.01 | ||
Dicrano-Pinion | –2.66E+00 | 1.14E+00 | –2.34 | 0.02 | ||
Erico-Pinion sylvestris | –1.82E+01 | 1.89E+03 | –0.01 | 0.99 | ||
Fagion sylvaticae | –2.29E+00 | 7.87E–01 | –2.91 | 0.00 | ||
Fraxino orni-Ostryion | –1.89E+01 | 4.35E+03 | 0.00 | 1.00 | ||
Pinion mugo | –4.49E–01 | 2.06E+00 | –0.22 | 0.83 | ||
Populo tremulae-Corylion | –1.92E+01 | 1.19E+04 | 0.00 | 1.00 | ||
Quercion pubescentis-petraeae | –2.52E+00 | 9.16E–01 | –2.75 | 0.01 | ||
Quercion roboris | –2.58E+00 | 8.88E–01 | –2.90 | 0.00 | ||
Salicion albae | –3.37E+00 | 1.07E+00 | –3.15 | 0.00 | ||
Salicion cinereae | –2.33E+01 | 5.73E+03 | 0.00 | 1.00 | ||
Salicion triandrae | –2.45E+01 | 1.19E+04 | 0.00 | 1.00 | ||
Sambuco-Salicion capreae | –1.76E+01 | 7.38E+03 | 0.00 | 1.00 | ||
Tilio-Acerion | –4.21E–01 | 6.69E–01 | –0.63 | 0.53 | ||
Vaccinio-Piceion | –1.83E+01 | 1.78E+03 | –0.01 | 0.99 | ||
Vaccinio uliginosi-Pinion | –3.12E+00 | 5.86E+03 | 0.00 | 1.00 | ||
Soil type | 80 | Minor Brown earth | 2.51E+00 | 7.54E–01 | 3.34 | 0.00 |
130 | Pseudogley on unconsolidated sediments | 2.84E+00 | 7.55E–01 | 3.77 | 0.00 | |
250 | Mature. brown alluvial soil | 2.94E+00 | 9.33E–01 | 3.16 | 0.00 | |
21 | Colluvial deposits | 4.36E+00 | 1.76E+00 | 2.48 | 0.01 | |
180 | Rendzinas | –1.60E+00 | 6.78E–01 | –2.36 | 0.02 | |
150 | Hanggley. Hangpseudogley | 2.81E+00 | 1.21E+00 | 2.32 | 0.02 | |
30 | Eutrophic brown earth | –1.29E+00 | 5.82E–01 | –2.22 | 0.03 | |
200 | Terra fusca | –1.95E+00 | 1.18E+00 | –1.66 | 0.10 | |
22 | Podzolic brown soil | –1.83E+01 | 3.73E+03 | –0.01 | 0.10 | |
240 | Gray-alluvial soil | 1.37E+00 | 1.10E+00 | 1.25 | 0.21 | |
31 | Minor calcaric cambisol | 6.83E–01 | 6.93E–01 | 0.99 | 0.32 | |
120 | Pseudogley on solid bedrocks | –2.15E+00 | 2.90E+00 | –0.74 | 0.46 | |
20 | Poor brown earth | 2.88E–01 | 4.42E–01 | 0.65 | 0.52 | |
210 | Gley | 6.09E–01 | 1.50E+00 | 0.41 | 0.69 | |
90 | Cohesive brown earth | –4.18E–01 | 1.11E+00 | –0.38 | 0.71 | |
190 | Mixed soil | –1.79E+01 | 1.89E+03 | –0.01 | 0.99 | |
40 | Semi-Podzols | –1.60E+01 | 2.76E+03 | –0.01 | 1.00 | |
280 | Bog | –1.62E+01 | 3.70E+03 | 0.00 | 1.00 | |
160 | Loamy soil | –1.87E+01 | 5.00E+03 | 0.00 | 1.00 | |
181 | Pararendzina | –1.86E+01 | 5.26E+03 | 0.00 | 1.00 | |
270 | Low peat bog, bog general | –1.52E+01 | 5.02E+03 | 0.00 | 1.00 | |
260 | Half-bog | –1.43E+01 | 5.52E+03 | 0.00 | 1.00 | |
220 | Alluvial soil, streamside marshes | –1.82E+01 | 7.28E+03 | 0.00 | 1.00 | |
60 | Substrate-induced Podzol | –1.65E+01 | 6.66E+03 | 0.00 | 1.00 | |
131 | Pseudogley on Loess | –1.83E+01 | 7.90E+03 | 0.00 | 1.00 | |
110 | Chromic luvisols | –1.82E+01 | 7.95E+03 | 0.00 | 1.00 | |
50 | Climate-induced Podzol | –1.23E+01 | 5.59E+03 | 0.00 | 1.00 | |
140 | Stagnogley | –1.88E+01 | 1.74E+04 | 0.00 | 1.00 | |
100 | Brown earth on loess | –2.04E+01 | 1.98E+04 | 0.00 | 1.00 | |
132 | Pseudogley on clay | –1.51E+01 | 1.49E+04 | 0.00 | 1.00 | |
202 | Cohesive calcaric cambisol | –1.72E+01 | 1.69E+04 | 0.00 | 1.00 | |
81 | Brown podzolic soil | –1.56E+01 | 1.85E+04 | 0.00 | 1.00 | |
Relief | Erosion | 5,69E–01 | 4.71E–01 | 1.21 | 0.23 | |
Solid | 7,17E–01 | 3.54E–01 | 2.03 | 0.04 | ||
Elevation | -4,77E-03 | 8,92E–04 | –5.35 | 0.00 | ||
Atan (inclination/100) | 2,14E+00 | 8,83E–01 | 2.42 | 0.02 |
Parameter estimates of generalized linear models (error structure = quasi-binomial; link function = logit) explaining the probability of the presence of Robinia pseudoacacia in NFR sites. Only significant explanatory variables occurring in the minimal adequate GLM were included in the model. Values are on the logit scale. * = factors significant at the p < 0.05 level. Data included in the logistic regression model were not transformed.
Variable | Categories | Estimate | Std. error | t value | Pr (>|t|) |
---|---|---|---|---|---|
Bedrock class | Intermediate | 3.68E+00 | 1.82E+00 | 2.02 | 0.04 |
Carbonate | 3.44E+00 | 1.75E+00 | 1.97 | 0.05 | |
Silicate | 1.86E+00 | 1.80E+00 | 1.03 | 0.30 | |
Flood plain sediments | 9.13E-01 | 1.32E+00 | 0.69 | 0.49 | |
Loess soil | –1.45E+01 | 6.73E+03 | 0.00 | 1.00 | |
Relief | Erosion | –1.68E+01 | 1.13E+03 | –0.02 | 0,99 |
Solid | –1.28E+00 | 6.85E–01 | –1.87 | 0,06 | |
Elevation | -2,03E-02 | 5.54E–03 | –3.66 | 0.00 |
Variable significance plot for IAS presence by mean decrease in Gini values (MeanDecreaseGini) using the Random Forest model, ranked by significance. The points represent the mean decrease in Gini value, indicative of the importance of each variable. A higher value indicates the significance of that variable for predicting IAS occurrence in NFR sites.
In total, 11 native tree species and two invasive alien tree species were present in the 64 AC plots of the tree sampling. The native trees species with the highest occurrence by stem number were Fraxinus angustifolia, Acer campestre, and Ulmus spp. (Ulmus spp. includes U. glabra, U. minor, and U. laevis). In comparison, the tree species with the highest basal areas were F. angustifolia, Populus spp. (Populus spp. includes P. alba, P. canescens, P. tremula, P. nigra, and P. × canadensis) and Quercus spp. (Quercus spp. includes Q. petraea and Q. robur), owing to a high proportion of large tree dimensions in DBH. During the 15-year monitoring period, the average total stem number increased from 591 to 718 trees per ha and the averaged total basal area increased from 31.1 to 39.3 m² per ha. The invasive tree species recorded were Fraxinus pennsylvanica and Acer negundo. Invasive tree species occurred in 8% (5 plots) of the total sampled area (64 plots). Figures
Changes in tree species composition in unmanaged floodplain forests (period 1: 1997 to 1999 – period 2: 2013 to 2014) using the angle count sampling method (Bitterlich, 1984). Average stem number per hectare in floodplain forests alongside the river March on 64 sample plots in 6 NFR in period 1 (light green) and period 2 (dark green); error bars denote standard errors. The large error bars indicate a low number of plots with a high increase in stem number for A. negundo. Ulmus spp. includes U. glabra, U. minor und U. laevis; Tilia spp. includes T. cordata, T. platyphyllos and T. × vulgaris; Quercus spp. includes Q. petraea, and Q. robur; Populus spp. includes P. alba, P. canescens, P. tremula, P. nigra, and P. × canadensis; Salix spp. includes S. alba, S. fragilis and S. × rubens.
Changes in tree species composition in unmanaged floodplain forests (period 1: 1997–1999; period 2: 2013–2014) using the angle count sampling method (Bitterlich, 1984). Average basal area per hectare in floodplain forests alongside the river March on 64 sample plots in six NFR in period 1 (light green) and period 2 (dark green); error bars denote standard errors. Ulmus spp. includes U. glabra, U. minor, and U. laevis; Tilia spp. includes T. cordata, T. platyphyllos, and T. × vulgaris; Quercus spp. includes Q. petraea, and Q. robur; Populus spp. includes P. alba, P. canescens, P. tremula, P. nigra, and P. × canadensis; Salix spp. includes S. alba, S. fragilis, and S. × rubens.
The species development data shows an increase in stem number for the native tree species F. angustifolia (from 126 to 177 trees per ha) and Ulmus spp. (Ulmus spp. includes U. glabra, U. minor, and U. laevis) (from 57 to 176 trees per ha), whereas a decrease was recorded for A. campestre (from 202 to 174 trees per ha), Populus spp. (Populus spp. includes P. alba, P. canescens, P. tremula, P. nigra, and P. × canadensis) (from 61 to 30 trees per ha) and Quercus spp. (Quercus spp. includes Q. petraea and Q. robur) (from 37 to 22 trees per ha). In terms of basal area, an increase in the proportion of F. angustifolia (from 11.7 to 17.0 m² per ha) and Ulmus spp. (from 1.3 to 2.8 m² per ha) was determined during the observation period, while the proportion of Salix spp. (Salix spp. includes S. alba, S. fragilis, and S. × rubens) was the only one to decrease slightly (from 1.1 to 0.9 m² per ha).
Focusing on the IAS, the proportion of F. pennsylvanica in terms of both stem number and basal area was very low and remained stable during the observation period. The stem number for A. negundo increased from 3 to 42 trees per hectare, but its proportion in basal area only increased from 0.2 to 0.7 m² per ha, indicating that the current tree population consists predominantly of small trees ranging from DBH 5 to 30 cm.
In total, IAS occurred in 0.4% (n = 3) of the investigated regeneration site plots (n = 784). The invasive species found were Robinia pseudoacacia, Ailanthus altissima, and Acer negundo, each on one plot. Individuals of R. pseudoacacia and A. altissima were found in the forest alliance Galio sylvatici-Carpinetum (Oberdorfer 1957) at elevations between 250 and 300 m above sea level. Acer negundo was found in the regeneration of the Fraxino pannonicae-Ulmetum floodplain forest alongside the river March at an elevation of 150 m above sea level. As this data is insufficient for further detailed statistical evaluation, vegetation surveys were used to analyse the spread of alien species in the forest communities of the NFR.
The IAS and AS identified in both the herbaceous layer and the tree layer are not new to Europe; all of them are commonly known alien species in European temperate forests (
As was likewise to be expected, herbaceous IAS were found more frequently than tree species. Especially common were Impatiens parviflora (106 plots in 42 NFR), Solidago gigantea (38 plots in 11 NFR), and Bidens frondosa (31 plots in 4 NFR). The most common tree species were Fraxinus pennsylvanica (17 plots in 4 NFR), Robinia pseudoacacia (15 plots in 16 NFR), and Acer negundo (9 plots in 4 NFR). Interestingly, one of the most widespread IAS in Europe, Ailanthus altissima (the tree of heaven) (
We conclude from the calculated statistical models that the best predictors for the number of IAS in unmanaged forests are alliance, elevation, and soil type (Table
Besides elevation and alliance, soil type was found to be a highly significant variable. Our observations are similar across European woodlands, where I. parviflora has successfully established itself in a wide range of habitat niches with soils of intermediate to high nutrient content (
The abundance of IAS and AS (in total 34 alien plant species), especially of invasive alien tree species, in the unmanaged forest habitats investigated for this report is lower than in other European forest habitats (
In many studies, human disturbances, which increase propagule pressure are mentioned as important predictors of the range and abundance of IAS in forest ecosystems (
Riparian areas, defined by
The occurrence of IAS was highest in the natural floodplain forest communities, with IAS were found in the herbaceous layer and the tree layer. Over the past 15 years, Fraxinus pennsylvanica and Acer negundo (a tree species of North American origin) increased in stem number and DBH in these communities. This increase may signal the beginning of species composition changes in the Fraxino pannonicae-Ulmetum community. Over the 15-year period examined in this study (1998/99 to 2013/14), the proportion of invasive tree species increased in the floodplain forest community.
The results of this study do not show any competitive interactions between alien and native floodplain plant species. Rather, they highlight that there has been an increase in total tree diversity. These observations are also reflected when considering the overall species composition of trees in floodplain NFR sites (Figs
The average stem number and basal area increased during the 15-year monitoring period, reflecting the observed high diversity and rapid development of tree species in riparian NFR. The numbers of individual tree species differ considerably between NFR sites, however. High stem numbers combined with low basal areas indicate a large proportion of small trees, whereas high basal areas indicate a large proportion of bigger trees. According to Figures
The most severe changes caused by invasive tree species were detected in NFR Herrschaftsspitz (n = 9 AC plots), where the number of A. negundo individuals increased from 20 per hectare in period 1 to 117 per hectare in period 2 (results not shown here). Acer negundo has spread widely across Europe and Central Asia (
Given the presence of IAS with well-documented negative ecological impacts such as A. negundo, Robinia pseudoacacia, and Ailanthus altissima in comparably local spreads within the NFR sites on the one side and a high frequency of natural disturbances in the NFR sites on the other, the results of this study can be considered under a “no IAS management scenario” in the temperate climate of Central Europe. At high elevations (>800m above sea level) almost no alien plant species were recorded. Nevertheless, the proportion of alien tree species in the sampled plots of the NFR is similar to that in the National Forest Inventory overall (<2% of the total forest cover). The non-intervention management policy in the NFR sites examined in this study offered an opportunity to observe changes in species composition, provide reference data for nature-based silviculture and contribute to management options in unmanaged forests.
In total, 16 IAS were identified in the study; this is in line with many other investigations into the spread of IAS in Europe. Similarities include a common set of IAS led by Impatiens parviflora, the small balsam, invading the ground vegetation of temperate forests. The findings of this study show that unmanaged forests at low elevations are not resistant to plant invasions. Instead, the monitoring of invasive plant species in NFR sites shows that biological plant invasions do occur in unmanaged temperate forest ecosystems, albeit at a slower pace than in many other habitat types. The results of the study also show that an absence of human disturbance may lower but not entirely mitigate the propagule pressure in forest ecosystems. Further studies are necessary to investigate the effects on propagule pressure. Nevertheless, it must to be taken into consideration that AC sampling may not be an appropriate methodology for evaluating spontaneous regeneration of IAS in NFR and that the observation period was comparatively short in the context of tree species development. Long-term studies with specific IAS monitoring are, therefore, necessary to achieve a better understanding of IAS development in unmanaged forest reserves. The aim of this study was to determine drivers for plant invasions in unmanaged European temperate forests. The explanatory variables alliance, elevation, bedrock class, soil type, and relief were found to be significant predictors for the presence of IAS. Ultimately, the findings of this study show that climatic limitation (elevation) is the main driver for the spread of IAS into unmanaged temperate European forests.
The BFW is acknowledged for providing the authors with access to its resources. We thank Christian Neureiter and many others who supported the data collection throughout the study period. Finally, we would like to thank Stephan Stockinger, Max Fancourt, and Elaine Paterson, whose methodological recommendations as well as comments on English language and style helped to improve this paper. We sincerely thank the two reviewers, Thomas Wohlgemuth and Giuseppe Brundu, whose comments/suggestions helped improve and clarify this manuscript.
Legend of soil type classification (
Order | Classification | Soil type | Description |
---|---|---|---|
Hydromorphic | Riparian soils | 220 | Alluvial soil, streamside marshes |
240 | Gray-alluvial soil | ||
250 | Mature, braun alluvial soil | ||
Gley | 150 | Gley/Pseudogley on slopes | |
210 | Gley | ||
Bog and half-bog | 260 | Half-bog | |
270 | Low peat bog, bog general | ||
280 | Bog | ||
Pseudogley | 120 | Pseudogley on solid bedrocks | |
130 | Pseudogley on unconsolidated sediments | ||
131 | Pseudogley on Loess | ||
132 | Pseudogley on clay | ||
140 | Stagnogley | ||
Terrestrial | Loamy soil & red clay | 160 | Loamy soil |
190 | “Mixed soil” (Rendzina/ Terra fusca transition) | ||
200 | Terra fusca | ||
203 | Gleyed Terra fusca | ||
Brown elevation earth (Braunerde) | 20 | Poor brown earth | |
22 | Podzolic brown soil | ||
30 | Eutrophic brown earth | ||
31 | Minor calcaric cambisol | ||
80 | Minor Brown earth on unconsolidated sediments | ||
81 | Brown podzolic soil on unconsolidated sediments | ||
90 | Cohesive Brown earth on unconsolidated sediments | ||
92 | Slightly gleyed brown earth on unconsolidated sediments | ||
100 | Brown earth on loess | ||
110 | Chromic luvisols (Para-brown earth) | ||
202 | cohesive calcaric cambisol | ||
Humid black soil | 171 | Humid black soil | |
Podzols | 40 | Semi-Podzols | |
50 | Climate-induced Podzol | ||
60 | Substrate-induced Podzol | ||
Rendzinas, Ranker | 10 | Rankers | |
180 | Rendzinas | ||
181 | Pararendzina | ||
Relocated soils | 21 | Colluvial deposits |