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Alien Plant Invasion in Mixed-grass Prairie: Effects
of Vegetation Type and Anthropogenic Disturbance

Results


Of 847 transects in the south unit, only 110 (13%) were free of alien plants; in the north unit, 121 (26%) of the 467 transects were free of alien plants. We found 38 alien plant species on our transects (Appendix), of which 27 occurred in both units and 11 species occurred only in the south unit. Seventeen alien species occurred on at least 10 transects and thus were analyzed individually. Eleven of the 23 vegetation types occurred in both the north and south units and had both disturbed and undisturbed transects (Table 1). Of the 17 species analyzed individually, eight occurred at least once in each of the 11 vegetation types in our analysis; 15 occurred in seven or more vegetation types. Thus, propagules generally were not isolated from any of the vegetation types in the analysis. Seventy-six and 77% of transects in the north and south units, respectively, were located in the 11 vegetation types. In the south unit, 23% of all transects and 20% of those in the 11 selected vegetation types were classified as disturbed; in the north unit, 21% of all transects and 19% of those in the selected vegetation types were classified as disturbed. The number and frequency of alien species were higher in vegetation types excluded from the analysis (2.7 ± 0.13 vs. 2.1 0.06 alien species/transect and 34.6 1.4 vs. 27.8 0.66% of plots occupied/transect in excluded and included vegetation types, respectively).

Table 1.  Properties of vegetation in the north and south units of Theodore Roosevelt National Park, North Dakota, USA.
Vegetation Type North Unit South Unit Transects
Undisturbed Disturbed Undisturbed Disturbed Total Alien (%)
Used in model selection
     Chrysothamnus nauseosus/Agropyron smithii 29 7 55 18 109 53
     Artemisia tridentata/Atriplex confertiflora 26 9 51 9 95 59
     Populus deltoides/Juniperus scopulorum 18 3 11 1 33 91
     Artemisia cana 22 7 45 15 89 94
     Symphoricarpos occidentalis/Prunus virginiana 32 4 50 16 102 98
     Andropogon scoparius 31 5 56 16 108 68
     Andropogon scoparius/Juniperus horizontalis 23 10 57 3 3 63
     Stipa comata/Bouteloua gracilis 24 6 32 10 72 75
     Juniperus scoparius/Oryzopsis micrantha 26 10 52 11 99 88
     Agropyron smithii/Stipa viridula 31 4 58 15 108 95
     Agropyron smithii/Stipa comata 32 3 45 15 95 96
Not used in model selection
     Achenbach Hills complex 5 1 0 0 6 83
     Populus tremuloides 0 0 3 0 3 100
     Symphoricarpos occidentalis 10 5 3 0 18 94
     Artemisia tridentata/Bouteloua gracilis 0 0 23 1 24 83
     Grassed sand floodplain 19 2 0 0 21 71
     Marsh 6 3 0 0 9 67
     Petrified Forest complex 0 0 1 2 3 100
     Populus tremuloides/Betula occidentalis 4 5 0 0 9 89
     River bottom 9 0 17 3 29 100
     Rolling scoria complex 0 0 33 12 45 95
     Steep scoria complex 0 0 29 10 39 95
     Salix spp. 15 3 0 0 18 100
Notes: Alien transects are those with at least one alien plant. "Disturbed" refers to those transects that were within 100 m of a road, trail, seeded field, campground, or picnic area. The vegetation types that were not used in the analysis were excluded because they lacked representation in one of the park units and/or in one disturbance category.

Number and frequency of alien species. — The number and frequency of alien plant species depended on the interaction among all three variables: vegetation type, disturbance, and park unit (Table 2). In each case, vegetation type as a main effect had the least difference in AIC score from the best model (Table 2), which indicated that this variable had the largest effect on the number and frequency of alien plants.

Table 2.  Variables included in the three best models of vegetation structure, as determined by Akaike's Information Criterion (AIC).
Response variable Sample size (no. transects) Best model
(difference in AIC score from third)
Second-best model
(difference in AIC score from third)
Third-best model
(difference in AIC score from fourth)
Difference in AIC score from best for each main effect
V U D
Total number of alien species 1003 V × U × D (32) V × U (18) V, U, D (4) 69 258 265
Mean frequency of alien species 796 V × U × D (74) V × D (1) V × U (30) 108 217 219
Salsola iberica 27 V (3) V, D (1) V, U (3) 0 44 43
Tragopogon dubius 242 V (2) V, U (1) V, D (1) 0 83 83
Bromus japonicus 204 V, U (1) V × U (1)  V, U, D (10) 12 108 121
Melilotus officinalis 434 V, U or V, U, D (5) V × U (1) V or V, D or V × U × D (1) 6 22 27
Taraxacum officinale 225 V, U, or V × U (3) V or V, U, D (2) V, D (5) 3 74 78
Camelina microcarpa 68 V × U (14) V × U × D (5) V, U or V, U, D (14) 33 85 98
Cirsium arvense 39 V × U (15) V × U × D (15) V, U (2) 32 72 80
Euphorbia esula 112 V × U (18) V × U × D (37) V, U (2) 98 112 148
Poa pratensis 377 V × U (7) V (1)  V, U (1) 7 172 172
Descurainia sophia 37 D, U or D × U (1)  D (1) U (8) 13 2 1
Convolvulus arvensis 34 D (2)  D × U (1) U (1) 4 3 0
Poa compressa 25 U (1) D (3) U, D (1) 10 0 1
Lactuca serriola 17 U (2) D (1) U, D (3) 26 0 2
Chenopodium album 59 D (1) U (3) U, D (1) 14 1 0
Alyssum desertorum 13 D or U, D (1) U or D × U (21) V, U, D (2) 24 1 0
Agropyron cristatum 25 U or D (2)  U, D (2) U × D (7) 11 0 0
Bromus inermis 144 V, D or V × D (1) V, U, D (7) V (1) 8 46 38
Notes: Numbers in parentheses report the difference between the AIC score for that model and for the next-best model. The final three columns indicate the difference in AIC score between models containing each main effect alone and the best model. The magnitude of the AIC score is dependent on sample size, so scores can only be compared among models for a given response variable. The difference in AIC scores between models reflects uncertainty in model selection, with relatively small difference indicating greater uncertainty. V = native vegetation type, U = park unit, D = anthropogenic disturbance. Variables separated by "×" or commas indicate interaction between main effects of main effect without interaction, respectively.

The number of species per transect varied more among than within vegetation types (Fig. 1a). For the most part, the interactions reflected the magnitude rather than direction of differences in numbers of alien species. The number of alien species on disturbed transects either did not differ from, or was greater than, the number on undisturbed transects, regardless of vegetation type or park unit. However, in Agropyron smithii/Stipa viridula and Populus deltoides/Juniperus scopulorum vegetation types, disturbed transects in the north unit had far more alien species than did undisturbed transects. This was true for A. smithii/S. viridula but not for P. deltoides/J. scopulorum vegetation in the south unit, where both disturbed and undisturbed transects had very high numbers of alien species. With a single exception (in A. smithii/S. viridula vegetation), the number of alien species per transect in the south unit was the same as or greater than the number in the north unit in each vegetation type.

Two bar graphs showing (a) number and (b) frequency of alien plant species located in 11 different vegetation types.  Each vegetation type is separated into South unit, disturbed; South unit, undisturbed; North unit, disturbed; and North unit, undisturbed.
Fig. 1  Akaike's Information Criterion identified the fully parameterized model, including the vegetation type × park unit × disturbance interaction, as the best model for both (a) mean number of alien species on transects and (b) mean frequency of alien plants on occupied transects. Shown are least-squares means with 1 SE. The vertical line on each graph represents the mean of the least-squares means, as a point of reference.

There was less variation in frequency of alien species on occupied transects than there was in number of alien species (Fig. 1b). Frequency was, in fact, only very weakly related to number of species (r = 0.07, P = 0.046). Differences between frequencies of alien plants on disturbed and undisturbed transects in general were small, as were differences between the north and south units. Interaction among the three variables was primarily evident in Juniperus scopulorum/Oryzopsis micrantha, where disturbed transects had higher frequencies of aliens in both north and south units, and in Symphoricarpos occidentalis/Prunus virginiana vegetation, where disturbed transects had higher frequencies only in the north unit. Again, it was the magnitude rather than the direction of differences in frequency that resulted in the interaction: alien species were never more frequent on undisturbed compared with disturbed transects and in only one vegetation type were they more frequent in the north than in the south unit.

Presence of individual alien species on transects. — AIC identified, either as a main effect or as an interaction, vegetation type as a variable in the best model for 10 species, park unit for 12 species, and disturbance for seven species (Table 2). For the seven species best described by either park unit or disturbance, AIC scores differed little between models containing either variable. This suggests considerable uncertainty in model selection between these two variables. In general, vegetation type was unlikely to be included in models for those species that occurred infrequently. Vegetation type more closely approximated the best model (and therefore had the largest effect in the model) for seven species, park unit and disturbance for six; the remainder were either well defined by interactions present in the best model (e.g., Cirsium arvense, Camelina microcarpa, and Euphorbia esula), or showed little difference among any of the models (e.g., Convolvulus arvensis) (Table 2).

For two species, Tragopogon dubius and Salsola iberica, the best model included only vegetation type (Table 2). Tragopogon dubius was most likely to occur in grasslands dominated by cool-season grasses with little or no shrub or tree canopy (Fig. 2a). Salsola iberica occurred most often on transects in the Chrysothamnus nauseosus/Agropyron smithii vegetation type, which is primarily composed of bare ground, and was never recorded in vegetation types dominated by sod-forming grasses (Fig. 2a).

Two bar graphs showing transect proportions for (a) Salsola iberica and (b) Tragopogon dubius.
Fig. 2  Proportion of transects on which Salsola iberica and Tragopogon dubius were found in 11 vegetation types at TRNP. Shown are least-squares means with 1 SE.

The best models for Bromus japonicus and Melilotus officinalis included vegetation type and park unit as main effects (Table 2). In each case, vegetation type had a larger effect than park unit as indicated by the smaller difference in AIC score between the main effect of vegetation type and the best model. Both species tended to be more commonly encountered in the south unit, but relative occurrence among vegetation types was similar in both units (Fig. 3a, b). The best models for Camelina microcarpa, Cirsium arvense, Euphorbia esula, and Poa pratensis included the interaction between vegetation type and park unit (Table 2). The AIC score was the same with and without this interaction term for Taraxacum officinale, and vegetation type alone had nearly as high a score as the two-term model (Table 2). Taraxacum officinale occurred on either the same or greater proportion of transects in the north than in the south unit in each vegetation type (Fig. 3c). Camelina microcarpa, Cirsium arvense, and Euphorbia esula frequencies were strongly defined by the interaction; main effects alone had relatively large differences in AIC scores from the best model. Camelina microcarpa was very uncommon in the north unit, but occurred there in the two vegetation types in which it was most common in the south unit (Fig. 3d). Cirsium arvense was unusual in that it was more commonly encountered in the north than the south unit, and occurred in the south unit in vegetation types different from where it occurred in the north (Fig. 3e). Euphorbia esula varied substantially among vegetation types in the south unit, and although present on two transects in the north unit, was absent from the 11 vegetation types in this analysis (Fig. 3f). Poa pratensis was more frequently encountered in the south unit than the north in Populus deltoides/Juniperus scopulorum vegetation, but was more common in the north unit in Agropyron smithii/Stipa viridula and Symphoricarpos occidentalis/Prunus virginiana vegetation (Fig. 3g).

Seven bar graphs showing transect proportions for (a) Bromus japonicus, (b) Melilotus officinalis, (c) Taraxacum officinale, (d) Camelina microcarpa, (e) Cirsium arvense, (f) Euphorbia esula, (g) Poa pratensis.  Each vegetation type is separated into South unit, and North unit.
Fig. 3  Proportions of transects on which alien species were found in 11 vegetation types, by park unit. Vegetation type and park unit were main effects in models for Bromus japonicus, Melilotus officinalis, and Taraxacum officinale; for Camelina microcarpa, Cirsium arvense, Euphorbia esula, and Poa pratensis, models also included the interaction between vegetation type and park unit. Shown are least-squares means with 1SE.

The best models for Lactuca serriola, Poa compressa, Agropyron cristatum, Alyssum desertorum, Chenopodium album, and Convolvulus arvensis included either park unit or disturbance as main effects, although AIC did not distinguish well between the two variables (Table 2). All of these species occurred relatively infrequently in the vegetation types under consideration. Agropyron cristatum and L. serriola tended to be more common in the south unit, P. compressa in the north unit (Fig. 4). Where significantly different, each species was more commonly encountered on disturbed than on undisturbed transects (Fig. 4). Descurainia sophia was the only species for which the interaction between disturbance and park unit defined the best model (Table 2). This species was far more common on disturbed than undisturbed transects in the south unit, but did not differ by disturbance in the north unit, where it was substantially less abundant (Fig. 4). The best model for Bromus inermis included the interaction between vegetation type and disturbance, the only species for which this was the case (Table 2). However, the model containing all three main effects (but no interaction) differed from the best model by only one unit (Table 2). Bromus inermis was always more commonly encountered on disturbed transects, especially in the Agropyron/Stipa communities (Fig. 5).

Bar graph showing transect proportions for seven species.  Each species is separated into South unit, disturbed; South unit, undisturbed; North unit, disturbed; and North unit, undisturbed.
Fig. 4  Proportion of transects occupied by species for which the best model contained park unit and/or disturbance. AIC did not differentiate these two variables well, so both effects are depicted. Shown are least-squares means with 1 SE.

Bar graph showing transect proportions for Bromus inermis.  Each vegetation type is separated into Disturbed and Undisturbed. Fig. 5  Proportion of transects occupied by Bromus inermis, for which the best model contained the interaction between vegetation type and disturbance. Shown are least-squares means with 1 SE.


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