Floodplain Forest Soil Nematode Communities as Influenced by Non-Native Acer negundo L. Invasion

Abstract

Invasive plants can significantly alter the composition and functioning of soil ecosystems, which in turn affects soil fauna such as microorganisms; mesofauna including mites, springtails, nematodes, and insects; and other invertebrates. We used clusters of three different tree species to investigate how they affect the composition of belowground soil nematode communities. The clusters included Acer negundo (L.) (an invasive, non-native species), Fraxinus excelsior (L.), and Alnus glutinosa (Gaertn.) (both as native representatives) in floodplain forest habitats of the Morava River. We investigated the families, genera, trophic groups, and functional guilds of soil nematodes in each tree cluster to assess the usefulness of nematodes as indicators of the impact of alien tree species on native communities. The study was complemented by measuring basic soil physico-chemical properties. The data show that nematode communities were not sensitive to A. negundo invasion, as clusters of invasive trees had similar nematode abundance, genus richness, diversity, family and genus composition, and trophic structure compared to species-specific clusters of two native tree species. A cumulative total of 96 nematode genera, belonging to 52 families, were recorded in the investigated floodplain forest sites. The most abundant families across all clusters were Alaimidae, Cephalobidae, Hoplolaimidae, and Rhabditidae for all tree clusters. Among the genera, Helicotylenchus, Pratylenchus, Paratylenchus (as obligate plant parasites), Filenchus, and Malenchus (as facultative plant parasites), as well as Acrobeloides, Eucephalobus, Plectus, and Rhabditis (as bacterivores), were the most dominant taxa. The measured soil properties did not differ significantly among tree species (p < 0.05). Nevertheless, redundancy analysis identified a significant correlation between soil moisture content and abundance of several nematode genera, nematode trophic groups, and functional guilds. The results indicate that the presence of invasive ash-leaved maple trees in the studied floodplain forests had no adverse effect on the diversity and functional structure of soil nematode communities. This study offers initial insights into nematode communities in Acer negundo invaded habitats, but further studies are needed to verify these findings.

1. Introduction

Forests are much more than just a group of trees. Forests are complex functional systems of interacting and often interdependent biological, physical, and chemical components, whose biological component has evolved to sustain itself via the production of new organic matter by successive generations of plants and animals [1]. This complexity creates combinations of climate, soil, animal, and plant species that are unique to each location, resulting in hundreds of different forest types worldwide. A number of factors within the forest ecosystem affect forest biodiversity; primary factors enhancing wildlife abundance and biodiversity are the presence of diverse tree species within the forest and the absence of timber management [2]. On the other hand, several factors may deteriorate specific aspects of forest biodiversity.

Many of the introduced plant species, both intentionally and unintentionally, provide considerable benefits to society and cause no problems in new habitats around the world [3]. Sometimes, however, non-native plant species can thrive in the specific conditions of a new area and may expand rapidly. Many of them become agricultural weeds or invade native ecosystems, displacing native plants and reducing their diversity. In recent decades, invasive plants have mostly occupied riverside forest habitats. Floodplain forests serve as buffers between the terrestrial and aquatic systems performing various hydrological and microclimatic functions due to their specific vegetation. Several tree species contribute to these functions.

The ash-leaved maple (Acer negundo L.) is an invasive tree species of North American origin and one of the most aggressive alien plants inhabiting various forests throughout Europe, especially in riparian habitats. In such habitats dominated by open-canopy forests, the chances of successful invasion by introduced tree species such as A. negundo are greater [4]. This is also confirmed by the fact that its spread in riparian communities has been described for several European rivers [5,6,7,8,9]. This North American floodplain tree species was introduced into Europe as a park tree at the end of the 17th century [10]. Nowadays, the species belongs to a group of invasive species with high environmental impact and very high dispersal potential, as well as being very difficult to control [11]. There is evidence that the diversity of native plants decreases in stands dominated by A. negundo [12,13,14]. However, knowledge about the effect of ash-leaved maple on native soil fauna is lacking.

Our study focused primarily on small soil metazoans (Nematoda), which are considered to be the most abundant, species-rich, and diverse organisms in the soils globally, with densities of up to millions of individuals per square metre [15]. It is estimated that approximately one in five animals is a nematode [16]. As a species-rich and abundant component of the belowground system, soil nematodes occupy a wide range of trophic positions and contribute significantly to soil food web interactions [17]. Soil nematode communities are thus useful bioindicators, with their functional shifts providing valuable information on the state of an ecosystem, thus allowing inferences regarding other biotic groups and soil health [17]. The representation of nematode species/genera within a community or the abundance of trophic groups, as well as coloniser–persister values of taxa, allow the calculation of various ecological indices and ratios; these parameters facilitate functional interpretation concerning disturbance. Most soil nematodes are free-living, and some are plant parasites. Plant composition, identity, and diversity [18,19,20], as well as various soil physical, chemical, and microbial properties [21,22,23], influence, shape, and alter soil nematode communities. Database literature sources describe the various consequences (negative/positive/none) of invasive plant herbs [24,25], shrubs [26], or trees [27] on soil nematode abundance, genera and species diversity, and functional and structural composition compared to related native vegetation included in the studies. However, the effects of ash-leaved maple on the structure of soil nematode communities in the invaded area have not yet been reported, although nematodes are considered excellent bioindicators of soil health [28,29] as well as soil degradation processes [30,31].

Protected areas (PAs) in Europe are among the most threatened by invasive alien species worldwide, and this threat is predicted to persist throughout the 21st century [32]. The Záhorie PA (Slovak Republic) was established in 1988 and is characterised by the largest area of interior aeolian sands created by the Morava River, which forms a border 71 km in length between Slovak Republic and Austria. This regularly flooded alluvium provides good conditions for the growth of floodplain forests and alluvial meadows. A high diversity of plant species and their communities in the Záhorie PA along the river in flooded meadows, considered to be the largest complex of flooded meadows in Central Europe, has been recorded on the Slovak side of the Morava River [33]. In forests, different deciduous species are present, mainly F. excelsior, Caprinus betulus (L.), A. glutinosa, Populus alba (L.), and others [34]. However recent investigation by [35] revealed a significant increase and cover of invasive tree neophytes in this area in the recent years, mainly A. negundo, Ailanthus altissima (Mill.), and Robinia pseudoacacia (L.). Such vegetation development and composition therefore provide an opportunity to investigate the effect of tree neophyte A. negundo on a specific group of soil mesofauna, the Nematoda.

In the present study, we explored how ash-leaved maple expansion in Záhorie PA floodplain forests influence the composition and structure of nematode communities. Considering that invasive species are generally considered to be a negative factor for biodiversity, we hypothesised that ash-leaved maple would have an unfavourable effect on nematode abundance, genera richness and diversity, functional guilds ratio, feeding groups ratio, and feeding structure of nematodes compared to communities present in the rhizosphere of two native tree species included in the study. The study was complemented by measuring soil physico-chemical properties. We hoped to find possible correlations between nematode community indicators and values of abiotic environmental factors, which may also be affected by the presence of the studied tree species.

2. Materials and Methods

2.1. Site Description

The Záhorie PA is located in the south-west part of the Slovak Republic (48°36′–48°20′ N and 16°56′–16°52′ E). Two native tree species, F. excelsior (FE, ash) and A. glutinosa (AG, alder), and one expanding non-native invasive species, A. negundo (AN, ash-leaved maple), were included in our study. To differentiate between the effects of tree species identity on soil Nematoda, 45 tree clusters (15 clusters for each tree species) were randomly established in an area with loamy Fluvisols, which is the dominant soil type in the proximity of the Morava in the floodplain zone. All the clusters were located in the floodplain forests downstream along the Morava River on the Slovak side, along a line approx. 45 km long (Figure 1).

The clusters were at least 150 m apart (the average distance between clusters was 250 m). Such a distance was considered accepted and reasonable because previous studies of fine tree root dynamics [36,37] showed a maximum horizontal spread of fine tree roots < 25 m, and therefore water and nutrient fluxes between adjacent clusters could be excluded. Individual clusters comprised four mature tree individuals of each species, with the canopy layer forming a square and the trees spaced 5–10 m around the cluster centre (Figure 2). Tree diameter at 150 cm height was on average 20–30 cm at AN, 20–40 cm at AG, and 40 to 60 cm at FE. No other trees or shrubs were present inside the clusters.

2.2. Soil Sampling, Nematodes Extraction, and Processing

In June 2022, soil samples were taken from each cluster to a depth of 0–25 cm using a garden trowel. Soil samples were collected using a systematic design because of the spatial heterogeneity of soil abiotic and biotic characteristics. In each of the tree clusters, five sub-samples were collected (200 g each), one from each quarter of the cluster and the fifth in its centre to obtain one composite soil sample (1 kg), which was used for all subsequent analyses (Figure 2). A total of 45 composite samples (15 samples from each tree species cluster) were thus collected. The soil samples were separately sealed in plastic bags, transferred to the laboratory, and kept at 5 °C until further processing, but no longer than two weeks after collection.

For nematode extraction, soil of each sample was gently homogenised by hand and mixed, and the stones were removed. Nematodes were extracted from 100 g of fresh soil from each sample soaked in 1 L of tap water for 60 min using a combination of sieving and a modified funnel technique [26]. The soaked sample was carefully passed through a 1 mm sieve (16 mesh) to remove plant parts and debris, and this suspension was passed through a 50 μm sieve (300 mesh) two minutes later to remove water and very fine soil particles. The nematodes were then extracted from the soil/water suspension by a set of two cotton-propylene filters in the funnels at room temperature (approx. 22 °C) for 48 h. This method requires the nematodes in water suspension to actively swim through the fine spaces in the filters into the water below, resulting in a nematode suspension without soil particles.

Extracted nematodes in water suspension were killed in a water bath (approx. 65 °C), fixed in a 4% formaldehyde and pure glycerol solution. The total number of individuals per sample was determined under low magnification (up to 60×) by using a LEICA S8APO stereomicroscope (Wetzlar, Germany). Of the counted nematodes, 10%, but not less than 100 individuals per sample, were microscopically (100, 200, 400, 600, and 1000× magnification) identified to the genus level from temporary slides using an Eclipse 90i light microscope (Nikon Instruments Europe BV, The Netherlands), based on the original species descriptions and accessible taxonomic keys of nematode genera and groups.

2.3. Nematode Community Analysis

Prior to analysis, total nematode abundance as well as abundance of nematode genera were expressed in terms of individuals per 100 g of dry soil. In order to assess nematode assemblages in relation to specific tree species, genera were sorted to feeding groups based on morphology of the stoma and oesophagus, namely, bacterivores (B), fungivores (F), plant parasites (PP), predators (P), and omnivores (O), as proposed [38,39], and revised and supplemented [40]. Furthermore, nematode genera were assigned to several functional guilds integrating nematode feeding strategies (trophic groups) and the nematode coloniser–persister (c-p) scale range from 1 to 5 [41]. C-p1-3 represents “r-strategists” (colonisers), with short life cycles, small eggs, high fecundity, high colonisation ability, and high tolerance to disturbance. Colonisers generally live in ephemeral habitats. At the other end of the scale, c-p4-5 nematodes represent “K-strategists” (persisters), with the longest generation times, largest bodies, lowest fecundities, and the highest sensitivity to disturbance. Persisters are never dominant in a soil and generally live in stable habitats [42].

Besides the total nematode abundance, trophic groups, and functional guild classification, the following parameters were determined from the soil nematode data: (1) the Wasilewska index (WI = B + F/PP), which refers to the ratio of bacteriovores and fungivores to plant parasites, which may reflect the risk that plant parasites pose to the health of soil plants [43]; (2) nematode channel ratio (NCR = B/B + F), which indicates the prevailing decomposition pathway of the soil food web [44]; (3) maturity index for free living nematodes (MI = (∑vi fi)/n, where vi is the c-p value of nematode family i, fi is the frequency of nematode family i, and n is the total number of individual nematodes in the sample [42], indicating the level of maturity and distubance of soil food web. For MI calculation, nematode genera were assigned to the “coloniser–persister” cp-scale according to their r and K life history characteristics [42]. Nematode diversity, richness, and evenness were measured by five indices: (4) genera richness (S); (5) the Shannon–Weaver diversity index (H’ = ∑pi × lnpi where pi is the frequency of taxon i in a sample), which estimates information about the composition and richness of nematode communities [45]; (6) the Pielou evenness index (J′ = H′/log(S), where H’ is the Shannon–Weaver diversity and “S” is the number of species in the community), which estimates evenness of species distribution [46]; (7) the Margalef index (SR = (S − 1)/Ln N, where “N” is total nematode abundance, which estimates genera richness [47]); (8) trophic diversity (TD = 1/∑Pi², where Pi is the relative proportion of each of the four trophic groups), which assesses the diversity of nematode communities at the trophic level [48].

2.4. Soil Properties Analysis

The basic physico-chemical characteristics of the soil samples were determined from a part of each homogenised sample used for nematode analysis. The gravimetric soil moisture was determined from 100 g of moist soil samples at 105 °C for 24 h. The soil pH was measured in water suspension (1:2.5; w:v) potentiometrically after 24 h. For the determination of total carbon and nitrogen, a MACRO Elemental Analyzer (CNS Version; Elementar, Germany) was used, employing the dry combustion method. For the estimation of organic carbon concentration, we determined the carbonate contents based on the volume of CO₂ released from the reaction of the carbonate minerals and excess HCl using the Jankov calcimeter (Kvant, Bratislava, Slovakia). The organic carbon was estimated as C_org = C_tot − (0.12 × wCaCO₃), where C_org is the organic carbon content, C_tot is the total carbon content, and wCaCO₃ is the content of CaCO₃ [49]. The concentrations of plant-available nutrients (Ca, Mg, K, Na, and P) were estimated using the Mehlich II soil extracts employing the ICP-AES (inductively coupled plasma atomic emission spectrometry); all analyses were performed in the laboratories of the National Forestry Centre in Zvolen, Slovakia.

2.5. Statistical Analysis

Statistical analyses were performed using STATISTICA version 14.0 (TIBCO, 2020, Santa Clara, CA, USA) and CANOCO 5 for Windows version 5 [50]. All response variables were subjected to a one-way ANOVA to determine the overall effect of tree species on the nematode communities, after checking homogeneity of variance using Levene’s test. When necessary, data were log(x + 1) transformed. Tukey’s honestly significant difference was applied to identify significant differences of the variables between tree species clusters at p < 0.05. Redundancy analysis (RDA) of major nematode genera associated with tree clusters was used to identify relationships between nematode genus abundance and soil properties, with soil properties as explanatory variables [51]. Forward selection of environmental variables and the Monte Carlo permutation test (999 permutations) were used to determine the proportion of variability in the genera or functional trait data matrix explained by each environmental variable and its significance. Only variables that were determined to be significant by the Monte Carlo test (p < 0.05) were included in future analyses. Data were log-transformed in all cases. For particular explanatory variables, a t-value biplot diagram with van Dobben circles was plotted to differentiate response variables, which answer positively or negatively to them. A significant response is represented by an approximation of the t-value of the regression coefficient being larger than 2 in the absolute value. The result of the analysis is the standard ordination biplot, which is provided with its axes eigenvalues (first two axes were used—most important in a case of variability) and the t-value biplot diagram with van Dobben circles.

3. Results

3.1. Soil Properties

Clusters of three tree species could not be distinguished by their soil property values (

Table 1. Soil physical and chemical properties associated with studied tree clusters in the floodplain forest along the Morava River (n = 15).

Parameter	Acer negundo (AN)			Alnus glutinosa (AG)			Fraxinus excelsior (FE)			p-Value
	Mean ± S.D.	min	max	Mean ± S.D.	min	max	Mean ± S.D.	min	max
pH (H2O)	7.37 ± 0.42	6.41	7.81	7.11 ± 0.46	6.00	7.56	7.09 ± 0.43	5.97	7.53	0.658 *
C (g/kg)	29.72 ± 9.99	9.07	43.5	35.27 ± 14.84	10.40	65.35	30.36 ± 13.57	14.51	45.29	0.268
N (g/kg)	2.307 ± 0.96	0.68	3.85	2.822 ± 1.19	0.73	4.03	2.353 ± 0.90	1.24	3.86	0.324
CO32− (g/kg)	25.952 ± 38.83	1.92	37.51	9.515 ± 8.78	0.50	22.62	16.293 ± 41.43	1.50	20.47	0.097
Corg (g/kg)	26.608 ± 10.42	8.64	46.66	34.209 ± 15.10	10.21	65.24	28.527 ± 11.15	14.59	45.01	0.287
Mg (mg/kg)	421.53 ± 155.2	128.8	681.2	430.67 ± 177.5	147.0	817.5	441.80 ± 193.7	192.1	742.5	0.168
Ca (mg/kg)	3497.93 ± 1074.9	1696.1	5449.2	3093.53 ± 1180.9	1330.1	4984.2	3339.73 ± 1240.9	1902.1	5401.2	0.133
K (mg/kg)	187.93 ± 44.8	130.8	273.3	199.59 ± 90.8	111.1	341.8	228.01 ± 171.2	71.7	742.6	0.207
P (mg/kg)	41.283 ± 19.6	6.1	84.6	39.544 ± 25.5	11.2	99.6	40.442 ± 27.6	5.83	93.7	0.387
SM (%)	26.738 ± 4.5	15.7	32.4	23.192 ± 3.4	21.3	30.9	22.585 ± 3.8	16.9	26.3	0.294

pH—soil acidity; C—carbon; N—nitrogen; CO₃—carbonates; Corg—organic carbon; Mg—magnesium; Ca—calcium; K—potassium; P—phosphorous; SM—soil moisture % of dry weight, * not significant by the Tukey HSD test (p < 0.05).

). Soil pH ranged from 7.09 in FE to 7.37 in AN tree clusters, indicating neutral floodplain forest soils. The clusters of non-native Acer negundo were characterised by the highest content of carbonates, calcium, phosphorous, and soil moisture, although these values did not differ significantly from those of the native species. Average soil carbon and nitrogen contents ranged from 29.72 to 35.27 mg/kg and 2.307 to 2.822 mg/kg soil, respectively, but did not differ significantly between tree species. The highest content of magnesium and potassium was found in soil samples collected in FE tree clusters.

The average total abundance of nematodes ranged from 1354 to 1646 individuals per 100 g of dry soil. No significant effect of tree species identity on total nematode abundance was recorded in this study, but abundance was lower in AG compared to FE and AN clusters (

Table 2. Nematode abundance, diversity, and community indices associated with Acer negundo (AN), Alnus glutinosa (AG), and Fraxinus excelsior (FE) tree clusters in the floodplain forests along the Morava River (n = 15).

Indices	AN	AG	FE	p-Value
Nematode abundance	1646.1 ± 1027.9 a	1354.9 ± 1127.6 a	1608.1 ± 997.2 a	0.127
Genera richness (S)	33.5 ± 7.7 a	36.7 ± 8.1 a	36.6 ± 7.3 a	0.369
Maturity index (MI)	2.68 ± 0.34 a	2.72 ± 0.23 a	2.82 ± 0.28 a	0.087
Shannon–Weaver diversity index (H′)	3.03 ± 0.29 a	3.31 ± 0.25 a	3.19 ± 0.15 a	0.455
Pielou evenness index (J′)	0.82 ± 0.02 a	0.92 ± 0.05 a	0.89 ± 0.03 a	0.241
Margalef index (SR)	4.51 ± 0.73 a	5.22 ± 0.82 a	5.05 ± 0.84 a	0.493
Trophic diversity index (TD)	17.69 ± 5.99 a	19.53 ± 5.15 a	18.92 ± 4.41 a	0.697
Nematode channel ratio (NCR)	0.80 ± 0.13 a	0.79 ± 0.12 a	0.81 ± 0.07 a	0.875
Wasilewska index (WI)	1.25 ± 0.52 a	3.52 ± 0.27 b	1.12 ± 0.61 a	0.038

The numerical is average ± standard error (n = 15). Different small letters in the same row indicate significant difference at (p < 0.05) by Tukey’s HSD test.

).

Nematodes from a total of 52 families were found in the soils of three tree species clusters (

Table 3. Mean abundance ± S.E. of nematode families per 100 g dry soil associated with Acer negundo (AN), Alnus glutinosa (AG), and Fraxinus excelsior (FE) tree clusters in the floodplain forests along the Morava River.

Family	Genera	AN (n = 15)	AG (n = 15)	FA (n = 15)
Alaimidae	Alaimus, Apmhidelus	56.7 ± 46.40	43.9 ± 45.3	62.9 ± 52.5
Anatonchidae	Anatonchus, Miconchus	6.0 ± 11.3	4.6 ± 8.4	8.4 ± 11.6
Anguinidae	Ditylenchus, Nothotylenchus	27.6 ± 42.1	21.8 ± 30.1	27.4 ± 57.7
Aphanolaimidae	Aphanolaimus	1.1 ± 3.5	1.3 ± 3.9	-
Aphelenchidae	Aphelenchus	70.1 ± 26.4	19.6 ± 24.4	35.5 ± 42.7
Aphelenchoididae	Aphelenchoides	25.8 ± 42.2	17.7 ± 41.2	23.3 ± 22.6
Aporcelaimidae	Aporcelaimellus, Aporcelaimus, Paraxonchium	43.1 ± 30.8	61.2 ± 64.1	53.5 ± 24.8
Aulolaimidae	Aulolaimus	11.5 ± 8.6	12.8 ± 2.3	22.7 ± 6.9
Bastianiidae	Bastiania	13.0 ± 7.5	2.6 ± 5.5	15.8 ± 3.7
Belondiridae	Axonchium, Oxydirus	54.1 ± 107.2	19.3 ± 26.4	21.9 ± 44.3
Boleodoridae	Basiria, Boleodorus	16.9 ± 25.5	8.8 ± 19.6	8.8 ± 12.8
Cephalobidae	Acrobeles, Acrobeloides, Cephalobus, Cervidellus, Eucephalobus, Heterocephalobus, Chiloplacus	262.1 ± 169.8	304.8 ± 187.5	215.9 ± 95.8
Criconematidae	Criconema, Mesocriconema, Ogma	16.1 ± 5.7	13.4 ± 22.3	26.3 ± 8.2
Cylindrolaimidae	Cylindrolaimus	5.1 ± 1.2	9.2 ± 7.7	6.7 ± 3.9
Diphtherophoridae	Diphtherophora	27.7 ± 34.6	5.3 ± 2.8	14.1 ± 10.8
Diplogasteridae	Allodiplogaster	-	1.4 ± 0.7	1.3 ± 1.0
Dorylaimidae	Labronema, Mesodorylaimus, Prodorylaimus	33.9 ± 45.9	13.6 ± 18.8	20.2 ± 11.7
Ecphyadophoridae	Ecphyadophora, Lelenchus	2.0 ± 4.5	1.4 ± 3.0	2.0 ± 2.5
Hemicycliophoridae	Hemicycliophora	-	4.9 ± 10.2	9.5 ± 4.3
Heteroderidae	Heterodera	3.3 ± 5.6	4.5 ± 5.8	19.9 ± 6.3
Hoplolaimidae	Helicotylenchus, Pratylenchoides, Rotylenchus	126.1 ± 110.1	113.1 ± 58.8	110.1 ± 69.4
Leptonchidae	Tylencholaimellus	1.3 ± 0.5	3.2 ± 5.2	7.7 ± 10.2
Longidoridae	Longidorus, Paralongidorus	25.5 ± 40.1	31.2 ± 25.4	32.5 ± 15.6
Meloidogynidae	Meloidogyne	0.5 ± 1.8	1.7 ± 2.3	4.3 ± 9.8
Mesorhabditidae	Mesorhabditis	13.2 ± 10.2	16.8 ± 24.1	12.7 ± 20.9
Metateratocephalidae	Euteratocephalus	-	-	1.0 ± 1.5
Microlaimidae	Prodesmodora	2.5 ± 7.6	-	2.8 ± 2.0
Monhysteridae	Geomonhystera, Monhystrella	5.9 ± 17.7	7.7 ± 12.4	9.9 ± 1.3
Mononchidae	Clarkus, Mononchus	17.7 ± 10.8	13.7 ± 24.4	16.3 ± 15.1
Mydonomidae	Dorylaimoides	17.6 ± 21.5	3.7 ± 6.2	13.3 ± 19.9
Mylonchulidae	Mylonchulus	26.6 ± 11.2	32.2 ± 21.4	30.1 ± 30.5
Neotylenchidae	Deladenus, Hexatylus	2.3 ± 8.9	2.1 ± 4.2	1.6 ± 5.7
Nordiidae	Longidorella	12.8 ± 9.2	18.0 ± 23.4	9.6 ± 15.4
Nygolaimidae	Nygolaimus	3.3 ± 6.7	4.5 ± 9.3	7.5 ± 14.7
Odontolaimidae	Odontolaimus	-	2.5 ± 5.3	0.6 ± 2.1
Panagrolaimidae	Panagrolaimus	4.0 ± 8.9	17.9 ± 10.5	2.3 ± 6.2
Paraphelenchidae	Paraphelenchus	3.3 ± 2.5	3.1 ± 5.8	2.0 ± 6.7
Paratylenchidae	Gracilacus, Paratylenchus	44.7 ± 72.5	54.4 ± 35.5	63.5 ± 77.7
Plectidae	Anaplectus, Ceratoplectus, Ereptonema, Plectus, Wilsonema	21.7 ± 45.6	12.9 ± 25.3	14.2 ± 23.4
Pratylenchidae	Pratylenchus	58.8 ± 61.2	21.9 ± 15.2	20.6 ± 30.7
Prismatolaimidae	Prismatolaimus	16.0 ± 15.2	5.6 ± 10.6	18.7 ± 10.9
Qudsianematidae	Crassolabium, Discolaimus, Ecumenicus, Epidorylaimus, Eudorylaimus, Microdorylaimus	10.4 ± 22.1	41.4 ± 26.1	62.6 ± 23.5
Rhabiditdae	Rhabditis	45.5 ± 26.9	57.9 ± 50.1	71.2 ± 56.3
Rhabdolaimidae	Rhabdolaimus	-	-	3.3 ± 2.1
Seinuridae	Aprutides, Seinura	2.1 ± 3.5	2.5 ± 5.4	3.4 ± 11.4
Steinernematidae	Steinernema	10.9 ± 5.3	9.8 ± 18.6	12.7 ± 19.1
Telotylenchidae	Amplimerlinius, Bitylenchus, Geocenamus, Trophurus	10.8 ± 21.5	5.3 ± 15.2	8.3 ± 9.9
Trichodoridae	Paratrichodorus, Trichodorus	7.8 ± 21.5	4.9 ± 14.5	9.1 ± 11.9
Tripylidae	Tripyla	2.2 ± 5.9	1.2 ± 3.2	5.5 ± 9.3
Tylenchidae	Coslenchus, Filenchus, Malenchus, Psilenchus, Tylenchus	45.5 ± 24.8	34.2 ± 59.9	51.7 ± 79.1
Tylencholaimidae	Tylencholaimus	8.8 ± 15.9	16.1 ± 25.1	2.4 ± 6.1
Xiphinematidae	Xiphinema	7.6 ± 18.1	6.7 ± 10.2	20.2 ± 39.9

). The most abundant families were Alaimidae, Cephalobidae, Hoplolaimidae, and Rhabditidae across all tree clusters. The least abundant families were Metateratocephalidae, Aphanolaimidae, Diplogasteridae, and Rhabdolaimidae. The abundance of Aporcelaimidae, Mylonchulidae, Paratylenchidae, and Quidsianematidae was higher in clusters of native tree AG and FE than in clusters of invasive ash-leaved maple. In contrast, the abundance of Aphelenchidae, Aphelenchoidide, Belondiridae, Diphtherophoridae, Hoplolaimidae, and Pratylenchidae was greater in clusters of non-native AN than in clusters of native AG and FE species (Table 3).

A cumulative total of 96 genera were recorded in the investigated floodplain forest sites along the Morava River, of which were 83 genera in AN, 87 genera in AG, and 77 genera in FE tree clusters (

Table 4. Nematode genera composition: their total (mean) abundance (A) of individuals 100 g/dry soil and dominance (D%) associated with studied tree clusters in the floodplain forest along the Morava River (n = 15).

Trophic Group/Genus		Acer negundo (AN)		Alnus glutinosa (AG)		Fraxinus excelsior (FE)
Bacterivores	c-p	A	D%	A	D%	A	D%
Acrobeles	2	920.3 (61.2)	3.42	617.4 (41.2)	2.34	744.8 (49.7)	3.21
Acrobeloides	2	1009.2 (67.4)	4.19	722.9 (48.2)	3.71	978.3 (65.2)	3.99
Alaimus	4	754.0 (50.3)	3.28	597.9 (39.9)	2.53	833.9 (55.6)	3.77
Allodiplogaster	3	-	-	22.2 (1.5)	0.04	19.1 (1.3)	0.05
Amphidelus	4	96.9 (6.5)	0.53	61.6 (4.1)	0.36	109.5 (7.3)	0.49
Anaplectus	2	248.0 (16.5)	0.85	150.5 (10.1)	1.01	223.3 (14.9)	0.78
Aphanolaimus	3	17.0 (1.1)	0.11	19.6 (1.3)	0.12	-	-
Aulolaimus	3	173.1 (11.5)	0.55	193.0 (12.9)	0.50	340.0 (22.7)	1.33
Bastiania	3	194.5 (12.8)	0.61	39.8 (2.7)	0.16	236.8 (15.8)	0.87
Cephalobus	2	643.9 (42.9)	1.93	527.9 (35.2)	2.62	356.2 (23.8)	1.99
Ceratoplectus	2	53.9 (3.4)	0.12	93.9 (6.3)	0.38	100.6 (6.7)	0.40
Cervidellus	2	305.8 (20.9)	1.14	538.4 (35.9)	1.65	154.1 (10.3)	0.70
Cylindrolaimus	3	75.9 (5.0)	0.18	137.6 (9.2)	0.36	77.5 (5.2)	0.29
Ereptonema	3	34.4 (2.3)	0.07	39.7 (2.6)	0.11	25.1 (1.7)	0.04
Eucephalobus	2	809.9 (53.9)	4.70	1279.9 (85.1)	6.28	906.1 (60.4)	3.88
Euteratocephalus	2	-	-	-	-	15.1 (1.0)	0.11
Geomonhystera	2	77.8 (5.2)	0.17	113.8 (7.7)	0.72	149.2 (10.0)	0.53
Heterocephalobus	2	177.4 (11.8)	0.88	79.3 (5.3)	0.63	54.3 (3.6)	0.41
Chiloplacus	2	65.1 (4.4)	0.28	200.9 (13.4)	1.12	34.2 (2.3)	0.15
Mesorhabditis	1	198.2 (13.2)	0.84	252.2 (16.8)	1.09	185.4 (12.4)	0.67
Monhystrella	2	11.4 (0.8)	0.04	-	-	-	-
Odontolaimus	3	-	-	35.5 (2.4)	0.16	8.6 (0.6)	0.05
Panagrolaimus	1	56.0 (3.7)	0.16	251.2 (16.8)	1.19	32.8 (2.2)	0.10
Plectus	2	1212.5 (80.8)	5.89	561.5 (34.7)	3.28	553.2 (36.9)	2.52
Prismatolaimus	3	223.6 (14.9)	0.71	78.5 (78.5)	0.55	282.6 (18.8)	1.05
Prodesmodora	3	37.9 (2.5)	0.14	-	-	42.2 (2.8)	0.09
Rhabditis	1	744.0 (49.9)	5.18	887.2 (59.2)	4.89	1021.8 (68.2)	4.66
Rhabdolaimus	3	-	-	2.4 (0.2)	0.04	46.8 (3.1)	0.13
Wilsonema	2	78.0 (5.2)	0.29	120.5 (8.1)	0.64	163.6 (10.9)	0.68
Fungivores
Aphelenchoides	2	587.0 (25.8)	1.13	265.3 (17.7)	1.19	349.1 (23.4)	1.36
Aphelenchus	2	1053.5 (70.2)	4.16	290.4 (19.4)	2.23	532.5 (35.5)	2.38
Aprutides	2	-	-	57.6 (3.8)	0.33	101.7 (6.8)	0.48
Deladenus	2	34.4 (2.3)	0.07	9.2 (0.6)	0.06	23.6 (1.6)	0.09
Diphtherophora	3	415.0 (27.7)	1.54	375.5 (25.1)	1.97	211.1 (14.1)	1.21
Ditylenchus	2	414.1 (27.6)	1.38	258.1 (17.2)	2.03	399.4 (26.6)	1.35
Ecphyadophora	2	-	-	-	-	10.2 (0.7)	0.05
Hexatylus	2	-	-	20.9 (1.4)	0.10	-	-
Nothotylenchus	2	-	-	69.0 (4.6)	0.22	11.8 (0.8)	0.05
Paraphelenchus	2	47.2 (3.1)	0.14	44.3 (3.0)	0.09	27.3 (1.8)	0.09
Tylencholaimellus	4	19.5 (1.3)	0.05	45.6 (3.1)	0.27	115.3 (7.6)	0.61
Tylencholaimus	4	123.4 (8.2)	0.34	256.2 (17.1)	1.22	34.0 (2.7)	0.32
Plant parasites
Amplimerlinius	3	37.2 (2.5)	0.10	-	-	-	-
Basiria	2	13.0 (0.9)	0.05	44.6 (3.0)	0.33	74.4 (4.5)	0.42
Bitylenchus	3	21.5 (1.5)	0.11	22.2 (1.5)	0.04	-	-
Boleodorus (f)	2	240.6 (16.1)	1.26	87.9 (5.9)	0.24	58.4 (3.9)	0.26
Coslenchus (f)	2	317.5 (21.2)	1.20	362.1 (24.1)	1.41	256.1 (17.1)	1.32
Criconema	3	14.5 (1.0)	0.05	7.8 (0.5)	0.05	41.3 (2.8)	0.21
Filenchus (f)	2	1515.4 (101.1)	5.62	997.8 (65.5)	4.56	1394.3 (92.8)	5.46
Geocenamus	3	405.1 (27.1)	3.08	283.6 (18.9)	1.78	402.0 (26.8)	1.48
Gracilacus	2	239.9 (16.2)	0.60	444.9 (29.7)	2.62	1240.8 (82.7)	4.30
Helicotylenchus	3	1546.1 (103.8)	6.56	1280.5 (85.4)	6.89	1200.4 (80.1)	4.83
Hemicycliophora	3	-	-	72.8 (4.8)	0.31	136.2 (9.1)	0.38
Heterodera	3	49.4 (3.3)	0.36	67.2 (4.7)	0.26	298.6 (19.9)	1.02
Lelenchus (f)	2	29.8 (2.0)	0.30	21.1 (1.4)	0.14	19.5 (1.3)	0.10
Longidorella	4	190.3 (12.9)	0.59	343.4 (22.9)	1.26	153.8 (10.3)	0.94
Longidorus	5	378.7 (25.5)	1.19	470.7 (31.4)	2.20	473.7 (31.6)	1.42
Malenchus (f)	2	1287.8 (85.9)	6.23	1067.3 (71.5)	6.35	1778.6 (118.5)	8.32
Meloidogyne	3	7.1 (0.4)	0.05	28.1 (1.9)	0.28	64.2 (4.3)	0.69
Mesocriconema	3	228.0 (15.2)	0.79	200.5 (13.4)	0.82	340.6 (22.7)	1.13
Ogma	3	-	-	-	-	13.1 (0.9)	0.05
Paralongidorus	5	-	-	-	-	18.7 (1.3)	0.10
Paratrichodorus	4	-	-	22.2 (1.5)	0.04	88.7 (5.9)	0.39
Paratylenchus	2	1100.1 (73.3)	4.58	288.2 (19.2)	0.96	655.7 (43.7)	2.86
Pratylenchoides	3	118.3 (7.9)	0.34	85.3 (5.7)	0.42	266.0 (17.7)	0.79
Pratylenchus	3	824.4 (54.9)	2.17	399.3 (26.6)	1.66	289.0 (19.3)	0.95
Psilenchus (f)	2	130.1 (8.8)	0.41	56.7 (3.7)	0.44	185.4 (12.4)	0.83
Rotylenchus	3	226.0 (15.7)	0.67	338.9 (22.6)	1.35	85.4 (24.5)	0.50
Trichodorus	4	235.2 (11.8)	1.18	124.9 (8.3)	0.37	104.6 (15.6)	0.55
Trophurus	3	186.4 (12.4)	0.41	2.2 (0.2)	0.13	95.7 (6.4)	0.44
Tylenchus (f)	2	126.1 (8.4)	0.39	75.0 (5.0)	0.49	242.6 (16.2)	0.67
Xiphinema	5	106.6 (7.1)	0.35	124.5 (8.3)	0.60	292.7 (19.1)	1.18
Omnivores
Aporcelaimellus	5	492.9 (32.9)	1.94	616.1 (41.7)	2.74	611.2 (25.5)	2.14
Aporcelaimus	5	146.0 (9.7)	0.56	199.9 (13.3)	0.88	187.9 (10.2)	0.87
Axonchium	5	355.2 (23.6)	0.87	74.1 (4.5)	0.44	67.4 (9.9)	0.25
Crassolabium	4	365.6 (24.7)	1.39	379.7 (25.3)	1.63	354.9 (20.4)	1.70
Dorylaimoides	4	264.0 (17.6)	1.16	49.1 (3.3)	0.32	196.5 (21.7)	0.79
Ecumenicus	5	67.3 (4.5)	0.30	44.8 (3.0)	0.50	50.9 (5.5)	0.38
Epidorylaimus	4	15.3 (1.1)	0.04	29.9 (2.0)	0.09	25.1 (6.9)	0.04
Eudorylaimus	4	301.5 (20.1)	1.01	210.8 (14.1)	1.07	247.6 (15.1)	1.42
Labronema	4	17.5 (1.2)	0.18	65.7 (4.4)	0.26	30.4 (2.5)	0.15
Mesodorylaimus	4	455.9 (30.4)	1.23	206.4 (13.7)	0.99	255.9 (26.3)	1.06
Microdorylaimus	4	352.1 (23.5)	1.56	429.1 (128.5)	1.68	543.5 (15.8)	2.18
Oxydirus	5	455.6 (15.4)	1.29	216.0 (14.4)	1.40	261.5 (30.1)	0.92
Paraxonchium	5	6.9 (0.5)	0.08	101.9 (6.7)	0.51	3.3 (0.4)	0.07
Prodorylaimus	4	47.7 (3.2)	0.16	-	-
Predators
Anatonchus	4	62.8 (4.2)	0.16	13.2 (0.9)	0.05	-	-
Clarkus	4	213.5 (14.5)	0.82	151.5 (10.1)	0.73	116.9 (10.5)	0.49
Discolaimus	5	7.1 (0.5)	0.05	59.4 (4.0)	0.25	89.8 (11.1)	0.37
Miconchus	4	27.6 (1.8)	0.29	49.0 (3.3)	0.28	27.0 (6.9)	0.26
Mononchus	4	56.2 (3.8)	0.19	54.3 (3.6)	0.28	77.1 (5.6)	0.43
Mylonchulus	4	398.4 (26.5)	1.48	485.2 (32.3)	2.48	462.1 (25.3)	2.03
Nygolaimus	5	46.5 (3.1)	0.14	78.5 (5.2)	0.37	105.5 (2.3)	0.32
Seinura	2	62.7 (4.2)	0.13	18.5 (1.2)	0.11	-	-
Tripyla	3	31.3 (2.1)	0.37	17.6 (1.9)	0.09	76.4 (7.8)	0.29
Insect parasite
Steinernema		152.7 (10.2)	0.89	136.6 (15.5)	0.60	171.8 (6.3)	0.6

c-p: coloniser–persister value (Bonger, 1990) [42]; (f) facultative plant parasites, root-fungal feeders.

). The mean richness of genera ranged from 35.5 in AN to 36.5 in AG, with no statistical differences between tree species (Table 2). Most nematode genera occurred in soils of all tree species, but Monhystrella and Prodorylaimus were observed exclusively in the AN cluster. Several genera were found only in FE clusters, including Euteratocephalus, Ogma, and Paralongidorus, while the genus Hexatylus was recorded only in AG clusters. Furthermore, the genus Aprutides (Seinuridae) identified in multiple clusters of AG and FE, represents the first record of this fungivorous nematode in the Slovak Republic (Table 4).

Among the clusters of three tree species, the composition of the community represented by all nematode trophic groups as well as functional guilds (with the exception of P₃) did not differ significantly (

Table 5. Abundance of nematode trophic groups and functional guilds associated with Acer negundo (AN), Alnus glutinosa (AG), and Fraxinus excelsior (FE) tree clusters in the floodplain forests along the Morava River.

	AN	AG	FE	p-Value
Bacterivores	547.1 ± 324.1 a	508.5 ± 425.1 a	513.1 ± 328.5 a	0.236
Ba1	22.2 ± 28.9 a	30.9 ± 42.2 a	27.6 ± 44.8 a	0.125
Ba2	26.7 ± 46.1 a	23.8 ± 30.4 a	21.1 ± 43.6 a	0.340
Ba3	5.0 ± 13.2 a	3.8 ± 10.6 a	7.2 ± 19.4 a	0.154
Ba4	28.4 ± 38.5 a	22.0 ± 35.7 a	31.5 ± 42.7 a	0.369
Predators	60.40 ± 49.9 a	61.5 ± 62.7 a	73.6 ± 65.2 a	0.247
P2	4.2 ± 6.7 a	1.2 ± 11.8 a	-	0.239
P3	2.1 ± 5.5 a	1.7 ± 3.1 a	8.1 ± 1.2 b	0.041
P4	10.1 ± 4.0 a	12.0 ± 5.4 a	11.1 ± 2.6 a	0.687
P5	1.8 ± 4.7 a	3.3 ± 4.2 a	7.0 ± 12.9 a	0.183
Fungivores	166.2 ± 145.6 a	112.8 ± 98.7 a	121.1 ± 127.4 a	0.237
Fu2	14.3 ± 37.8 a	7.5 ± 19.9 a	10.8 ± 28.9 a	0.269
Fu3	27.7 ± 34.6 a	25.0 ± 32.8 a	14.0 ± 22.8 a	0.188
Fu4	5.1 ± 11.7 a	9.7 ± 19.3 a	5.3 ± 11.1 a	0.214
Omnivores	222.8 ± 112.1 a	174.2 ± 157.1 a	192.3 ± 129.1 a	0.255
Om4	15.2 ± 27.1 a	11.4 ± 25.9 a	14.2 ± 23.8 a	0.547
Om5	16.9 ± 38.3 a	13.2 ± 26.4 a	13.1 ± 27.6 a	0.473
Plant parasites	638.3 ± 423.1 a	487.4 ± 258.3 a	696.7 ± 464.9 a	0.164
Pp2	33.3 ± 59.6 a	23.0 ± 45.5 a	39.4 ± 69.9 a	0.234
Pp3	17.5 ± 45.5 a	13.3 ± 41.2 a	15.9 ± 42.2 a	0.277
Pp4	9.5 ± 23.5 a	10.9 ± 25.7 a	9.5 ± 23.2 a	0.754
Pp5	10.8 ± 26.9 a	13.2 ± 32.9 a	17.5 ± 44.4 a	0.269
Insect parasites	10.2 ± 5.2 a	9.1 ± 10.8 a	5.5 ± 6.6 a

The numerical is average ± standard error (n = 15). Different small letters in the rows indicate significant difference at (p < 0.05) by Tukey’s HSD test. Ba_1.2.3.4: bacterivores; Fu_2.3.4: fungivores; P_2.3.4.5: predators; Om_4.5: omnivores; Pp_2.3.4.5: plant parasites.

). Plant parasites (including both obligate and facultative root-fungal feeders) and bacterivorous nematodes were the most abundant trophic groups in each tree species cluster. Among them, the genera Helicotylenchus, Pratylenchus, Paratylenchus (as obligate plant parasites), Filenchus, and Malenchus (as facultative plant parasites), as well as the genera Acrobeloides, Eucephalobus, Plectus, and Rhabditis (as bacterivores), were the most abundant taxa (Table 4). Furthermore, invasive AN had no significant effect on the relative abundance of omnivorous (14 taxa), fungivorous (12 taxa), and predaceous nematodes (9 taxa) (Table 4). The abundance of P₃ nematodes was significantly higher in FE than in AN and AG clusters (p < 0.05). Tripylidae (Tripyla) were the most abundant in P₃ and were responsible for the positive influence of ash on this nematode guild (Table 4).

In addition, the partial RDA analysis also confirmed that tree species had no significant influence on the composition of nematode genera, nematode trophic structure, and functional guilds, which explained between 2.8 and 1.8%, 1.2 and 0.7%, and 1.5 and 0.9% of the species variability, respectively (

Table 6. Effect of tree species and soil properties on nematode communities determined by RDA ordination. Simple effects.

	Explained Variability	pseudoF	p
Nematode genera composition
Acer negundo	2.8	1.2	0.184	+
Alnus glutinosa	2.2	1.0	0.345	+
Fraxinus excelsior	1.8	1.5	0.445	+
Phosphorus	1.9	0.9	0.646	+
Soil moisture	3.6	1.6	0.034	+
Potassium	1.5	0.7	0.886	+
Calcium	3.6	1.7	0.022	+
Magnesium	2.6	1.2	0.569	+
Corg	2.2	1.0	0.382	+
pH	4.5	2.0	0.006	+
Nitrogen	1.9	0.9	0.648	+
Carbon	0.7	0.3	0.338	+
Nematode trophic structure
Acer negundo	1.2	0.7	0.957	+
Alnus glutinosa	1.0	0.5	0.746	+
Fraxinus excelsior	0.7	0.3	0.908	+
Phosphorus	3.92	1.9	0.132	+
Soil moisture	9.15	4.3	0.020	+
Potassium	2.30	1.1	0.785	+
Calcium	1.41	0.7	0.506	+
Magnesium	0.84	0.4	0.758	+
Corg	0.28	0.4	0.966	+
pH	1.03	0.5	0.684	+
Nitrogen	0.98	0.4	0.715	+
Carbon	0.51	0.2	0.866	+
Nematode functional guilds
Acer negundo	0.9	0.4	0.962	+
Alnus glutinosa	1.8	0.8	0.546	+
Fraxinus excelsior	1.5	0.8	0.642	+
Phosphorus	6.9	3.2	0.006	+
Soil moisture	4.9	2.4	0.039	+
Potassium	2.8	1.4	0.182	+
Calcium	3.1	1.5	0.144	+
Magnesium	2.0	1.0	0.396	+
Corg	1.1	0.5	0.864	+
pH	1.2	0.6	0.792	+
Nitrogen	1.1	0.5	0.890	+
Carbon	0.8	0.4	0.964	+

+ included as factor by forward selection.

).

Overall, the direct effect of several soil properties on the composition of nematode genera, trophic groups, and functional guilds was significant (Table 2). RDA identified a significant negative correlation between soil moisture and abundance of several nematode genera (Figure 3), nematode trophic groups (Fu, O, Pp, and IN), and functional guilds (Fu2, Pp5, and Om4), which explained 3.6, 9.15, and 4.9% of the variability (Figure 4 and Figure 5a). Moreover, nematode genera composition negatively correlated with soil pH and Ca content, explaining 3.9 and 4.5% of variability (Figure 4), while nematode functional guilds (Fu3, Fu4, and Ba3) negatively correlated with P content, explaining 6.9% of the variability (Figure 5b).

3.2. Nematode Community Indices and Faunal Profile Analysis

With the exception of the Wasilewska index (WI), which was the highest in the AG clusters (p < 0.05), no significant differences were observed in the Shannon–Weaver index (H′), Margalef index (SR); Pielou evenness index (J′), trophic diversity index (TD), nematode channel ratio (NCR), and maturity index (MI) (Table 2). Nevertheless, in the clusters of invasive ash-leaved maple, the values of H′, SR, J′, and TD indices were slightly lower than in the native tree species (FE and AG).

Similar food web and environmental conditions for nematodes under AN, FE, and AG tree species demonstrated also “weighted faunal analysis” in the faunal profile scatter plot (Figure 6). The graphical representation for the enrichment and structure ecosystem indicators (values of EI and SI) situated all soil samples from the three tree species clusters to quadrant B and C, which are characterised as a mature soil food web, weakly to moderately disturbed, with the decomposition of organic matter controlled by both bacteria and fungi (Figure 6).

4. Discussion

Despite the high diversity of habitats grouped under a single term, forest ecosystems are characterised by the dominance of trees, which influence not only the aboveground environment but also belowground life. Forests support complex aboveground vegetation and diverse soil faunal communities, and even in temperate climates, they represent one of the largest storehouses of animal biodiversity, primarily belonging to the phyla Arthropoda and Nematoda [52]. Although forests may be considered relatively stable ecosystems in which the tree component mediates the flow of organic matter through long-term cycles, changes in forest ecosystem biodiversity have become a global issue in the second half of the 20th century due to intensive human activities [53], particularly those resulting from biological invasions of herbaceous and woody plants [54].

The impacts of non-native ash-leaved maple on the taxonomic composition of ground vegetation in invaded areas are generally harmful. A study by [55,56] revealed a 40% and 55% decrease in the taxonomic diversity of vascular ground plant species, respectively, in areas dominated by A. negundo compared to areas dominated by native tree and shrub species. They also found a negative relationship between the canopy cover of A. negundo and the abundance of understory herbaceous plants. However, despite the recognised links between above-ground and below-ground biotic communities, no study has investigated the influence of A. negundo on any group of soil organisms.

Contrary to our hypothesis, the results showed that the invasive ash-leaved maple A. negundo did not alter the overall structure of soil nematodes. However, further studies involving nematodes and/or various soil organisms in other invaded areas, repeated monitoring, or experimental testing are needed to confirm any causal relationships. As there is currently no available literature specifically addressing the impact of A. negundo on soil nematodes, our findings offer a novel contribution. Therefore, we can only speculate why nematode communities had similar composition as those recorded under native tree species. The observed lack of clear impact might be explained by functional redundancy among nematode taxa, where multiple genera fulfil similar ecological roles in soil, which helps the community stay stable [57]. Nematode communities in long-term ecosystems as forests may also show resilience, meaning they can resist or recover from changes in environmental conditions [58], including those caused by the invasion of A. negundo. The similarity of nematode communities beneath both native and non-native tree species suggests that key soil functions, such as nutrient cycling and decomposition, can remain stable despite changes in tree species composition and leaf litter quality. Research indicated that the quality of leaf litter varies strongly between tree species, and the quality of leaf litter as a substrate likely affects soil microbiota directly [59] or indirectly via tree species effects on soil chemistry [60]. But regardless of species origin (non-native, native), our findings did not confirm the conclusions by [61], who stated that structure of nematode soil food webs varies markedly with tree species and point to the importance of basal resources, i.e., leaf litter quality and rhizodeposits. This suggests bottom-up forces mediated by individual tree species to control major decomposition pathways [61]. However, our findings underline the resilience of soil nematode communities in floodplain forests and may carry practical significance for these ecosystem management, protection, and conservation in general. In our study, total nematode abundance, genus richness, and the structure of trophic groups and functional guilds were nearly identical under ash, alder, and ash-leaved maple.

On the other hand, Lazzaro [27] investigated the impact of the invasive deciduous tree black locust (R. pseudoacacia) on soil nematode communities. They found that black locust significantly lowered nematode taxon richness, especially plant parasitic nematodes, while the relative proportions of feeding groups was similar for bacterivores (the major group), fungivores, omnivores, and predators in black locust and native oak stands. Similarly, the invasive herb Heracleum sosnowskyi (Manden.) or shrub Fallopia japonica (Houtt.) negatively affected total abundance, abundance of plant parasitic nematodes along with omnivores or predators [25,26]. In contrast, the invasive herbs Asclepias syriaca (L.) and Impatiens parviflora (DC.) did not affect the structure of soil nematode communities in the habitats where they were dominant [24,62]. In addition, total abundance increased following Solidago gigantea invasion of grassland, especially plant-parasitic species, in the study by [59]. Thus, the response of soil nematode communities to the dominance of certain plants (including invasive ones) in their living habitat seems to be species-specific.

We found that A. negundo, F. excelsior, and A. glutinosa fostered similar nematode assemblages, regardless of tree species origin. Nematode weighted faunal analysis indicated a mature, weakly to moderately disturbed soil food web, with the decomposition of organic matter controlled by both bacteria and fungi under all tree species investigated. Bacterivores together with plant parasitic nematodes were the most abundant and taxonomically rich trophic groups (in terms of families and genera) in clusters of all investigated tree species. Among the plant parasitic genera, Helicotylenchus, Pratylenchus, Paratylenchus, Filenchus, and Malenchus prevailed, while among the bacterivores, Acrobeloides, Eucephalobus, Plectus, and Rhabditis dominated. This is consistent with previous findings from European deciduous forests [62,63,64,65,66,67], where the above-mentioned genera prevailed. Therefore, it is reasonable to consider these genera as indicators of deciduous forests. All genera benefited from the presence of ash, alder, and ash-leaved maple in our study, as well as other tree species from the above-mentioned studies.

Omnivores and predators are considered ‘extreme persisters’ that are more sensitive to changes in their habitat than colonisers due to their low reproduction rate (they produce few, large eggs) and long generation times, and therefore they tend to reach higher abundances in unchanged and stable ecosystems. In line with the statement of [68] that changes in plant communities do not alter the abundance of nematodes of higher trophic groups, in our study, omnivores and predators showed similar overall abundance across ash-leaved maple, ash, and alder clusters. Although some genera such as Prodorylaimus (Om), Anatonchus, or Seinura (Pr) were not recorded under all tree species investigated, the overall diversity and abundance of individual genera of omnivorous and predaceous nematodes indicated an even spatial distribution across the floodplain forest sites.

Acer negundo invasion resulted in no significant changes in nematode genera diversity and evenness represented by H′, SR, and J′; in soil food-web condition represented by MI and TD; and in decomposition and nutrient mineralisation pathway represented by NCR. This indicates that there is no meaningful shift in soil ecological processes and functions provided by soil nematode communities in invasive ash-leaved maple compared to clusters of native ash and alder tree species.

We found that invasive A. negundo did not affect nematode community structure, nor did it alter soil physical and chemical properties or nutrient pools compared to native trees. This is in line with the conclusions by Wohlgemuth [69]. They reviewed and analysed the impacts of seven non-native tree species widely distributed in Europe on soil properties, namely, Acacia dealbata (Link), A. altissima, Eucalyptus globulus (Labill.), Prunus serotina (Ehrh.), Pseudotsuga menziesii (Mirb.), Quercus rubra (L.), and R. pseudoacacia. The data show that despite their ecological relevance, information on ecological impacts is still limited for most species. However, overall impacts on soil properties are small, and in some cases, non-native tree species may even increase soil fertility.

RDA identified a significant correlation between soil moisture and the abundance of several nematode genera, nematode trophic groups, and specific functional guilds. For example, increasing soil moisture had a negative effect on the abundance of omnivorous (Om4), fungivorous (Fu2), and plant-parasitic nematodes (Pp5) of genera Mesodorylaimus, Ditylenchus, Aphelenchoides, and Longidorus. Soil moisture is one of the key factors influencing the distribution, abundance, and activity of nematodes, which rely on a thin film of water surrounding soil particles for movement and feeding. Local climate, soil type, and plant community all influence how moisture affects nematode trophic groups, taxa, and diversity [70,71]. As our study showed, some nematode taxa within a community may be more sensitive to higher soil moisture content than others. On the other hand, [23] recorded the highest nematode diversity and nematode trophic level at 30% soil moisture. This is in line with our findings, where nematode-taxa-rich communities represented by 96 genera from 52 families were recorded in the investigated floodplain forests along the Morava River. Similarly rich nematode communities (111 species, 67 genera) were found by [72] on the banks of five Slovak rivers and by [30] in alluvial meadows (78 genera) along the Litavka River with varying levels of heavy metal contamination. This suggests that wet riverside biotopes support higher diversity and heterogeneity of nematode communities than drier ecosystems, e.g., deserts [73,74].

It seems that invasive A. negundo may not disrupt key belowground processes in floodplain forests, which could inform forest management by identifying systems where native soil biodiversity and function remain stable despite ash-leaved maple invasion [75,76]. Future studies should explore long-term trends and experimental setups to confirm the mechanisms behind this resilience.

5. Conclusions

Invasive species have the potential to threaten local biodiversity. We are familiar with examples of invasive plant and animal species. However, it is not always clear whether a species introduced from another continent should be classified as invasive and whether it poses a real threat to native flora and fauna. In the case of the invasive tree A. negundo, currently restricted or banned in several European countries including the Slovak Republic, only a few studies have been conducted worldwide, especially on its impacts on native vegetation. Therefore, we assumed that a study focusing on soil Nematoda as bioindicators could highlight the harmfulness of ash-leaved maple to edaphic fauna and local floodplain forests.

The invasive ash-leaved maple A. negundo did not modify overall structure of soil nematodes when compared to the nematode fauna under two native tree species at the locality. The total nematode abundance, genus richness, composition of nematode trophic groups, and functional guilds were nearly identical in the cluster of ash, alder, and ash-leaved maple. A cumulative total of 96 nematode genera (83 in AN; 87 in AG; and 77 in FE), belonging to 52 nematode families, were recorded in the investigated floodplain forest sites, including the genus Aprutides not previously reported in the Slovak Republic. Most nematode genera occurred in the rhizosphere of all tree species, with few exceptions, e.g., the genus Monhystrella and Prodorylaimus were only found in clusters of ash-leaved maple, while the genus Hexatylus was only observed in alder clumps. A relatively high total nematode abundance and genera diversity across all tree species with no significant differences suggests that nematode communities are supported by all three species.

Although our data represent the first study on the effect of ash-leaved maple on soil nematodes, the results indicate that ash-leaved maple may not pose a threat to the soil food web and soil nutrient pools, as shown by soil physico-chemical analyses and contrary to general assumptions about invasive plants.