Exotic buckthorn comprised the majority of the woody vegetation at half of the sites that we examined closely in Schmeeckle Reserve, but was especially noticeable in certain habitat settings. We therefore concentrate our remarks on those habitat associations that seem to coincide with high presence of buckthorn, low presence of buckthorn, and possible future distribution trends.
Buckthorn was especially abundant (a majority of all woody plants) in mesic soils at transitions (edges, ecotones) between early and later seres, where sunlight was abundant, groundstory and leaf litter were relatively thin, and few native competitors existed. Although buckthorn occurred at close proximity to such human habitat disturbances as trails, it also was thriving (and often dominating) at open gaps well away from human clearings due to natural disturbances such as windfall, so long as these gaps also had unsaturated soil and were relatively free of competitors.
The two sites that had recently experienced corrective mechanical clearing exhibited rapid re-infestations. Site F (Figure 9) had heavy re-infestation (80% of all woody plants, and over ten stems per square meter) despite removal a mere two years earlier (Figure 2). Site H, which managers cleared in 2000, already had 50% average buckthorn dominance of all woody plants, although it still had substantially low density of under three stems per square meter. The implication is that not only buckthorn may very quickly re-establish itself into dominance following mechanical clearing, but in fact it may even “prefer” such disturbance locations over those not receiving such control efforts. This warrants further research.
A combination of favorable buckthorn conditions apparently can occur in a variety of forest communities. Buckthorn dominated the understory (see gray cells in Figure 3 and Figure 5) in stands of native jack pine (Pinus banksiana) at Sites F and S; white pine (Pinus strobus) at Sites A, B, I, T, and V; red maple (Acer rubrum) at Sites L, Q, and R; red oak (Quercus rubra) at Site J, and aspen (Poplus tremuloides) at Sites C and U. Seemingly, buckthorn has no particular affinity for any single tree species native to central Wisconsin, but rather thrives regardless of overstory when there are suitable combinations of sub-canopy conditions, including one or more of:
Moderately moist (mesic) soils
High sunlight levels at ground surface
Relatively low soil surface cover
Few groundstory competitors
Availability of disturbances, edges, and gaps.
The largest buckthorn stems occurred at sites having mostly broadleaf deciduous overstory, yet other sites with these same canopy species had lesser buckthorn size. Overstory species may therefore be happenstance rather than a consistent association with buckthorn size.
The sites having largest stem diameters may be locations and conditions where buckthorn originally established itself. However, we lack corroborative data enabling to us to confirm any assumption that “larger is older”, and larger size could be the result of particularly favorable local growth conditions. Lacking verifiable buckthorn age data from tree ring analyses, we therefore make no suggestion as to the location of buckthorn entry into the Reserve, or by what mechanism it arrived.
We can, however, suggest that concentrations of large mature buckthorn are potential sources of fruits. If the seeds are viable, then these locations may serve as nodes for further dispersal, and should perhaps become priority sites for control programs.
Buckthorn was notable for its
relative scarceness, and sometimes its virtual absence, at sites having certain
habitat conditions. We
consistently—but subjectively—observed less prevalent and smaller buckthorn to
correspond with the following circumstances:
Wet and/or very low-lying ground surfaces
Deep (>100mm) leaf litter layers
Deep shade without gaps in mature overstory
Dense presence of blackberry or grass/sedges.
Buckthorn, despite its reputation as a hydrophilic (water-loving) plant, seemed intolerant of permanent or excessive seasonal soil saturation by our observations and location data. It appears also to fare poorly (or at least less well) on soil surfaces receiving less direct sunlight, either by shading from continuous overstory or due to thick leaf litter. The litter barrier that we observed could be either broadleaf or needleleaf, so long as it was deep.
Wetland and decomposing litter associations with buckthorn scarcity suggest yet another possibility; perhaps buckthorn does not germinate well in excessively acidic soil. Again, further experimental research appears necessary.
Finally, buckthorn occurred less abundantly, and perhaps grows more slowly, at locations where aggressive ground-spreading competitors occur in quantity.
Some of our Schmeeckle findings corroborate those of earlier researchers working elsewhere, especially that exotic buckthorn is an aggressive competitor that is difficult to eradicate. We, however, identified some specific habitat factors (e.g., gaps allowing high sunlight) that had association with dense concentrations of this plant, while others (e.g., overstory species) seemed to make little difference with buckthorn abundance. Perhaps most importantly, we have indications that several habitat conditions—particularly soil saturation, pre-existing and intact groundstory competitors, and relatively thick litter layers—appear to coincide with relatively sparse buckthorn presence.
If buckthorn continues to expand in Schmeeckle Reserve, it seems probable that it may displace less competitive native understory plants, and may prevent regeneration of the aging overstory native trees. The even-aged jack pine (Pinus banksiana) stand at Site F may be in particular peril, because the existing trees are nearing the end of their life expectancy, few seedlings exist, buckthorn now has high density and comprises 80% of all woody plant stems, and the area has experienced recent disturbance stresses during buckthorn removal operations.
Should such radical composition transformations occur, the effects seem likely to extend beyond displacement of the many native wildlife species dependent upon native stand structures, and unable to utilize buckthorn as habitat. What are these? Lower stature and less continuous canopy, for example, might result in increases in both runoff and consequent erosion, or in earlier and accelerated snowmelt and escalation of flooding. Further, such changes probably would not confine themselves to the reserve, but likely would affect the human community and campus surrounding the study area. Additionally, this biotic island that formerly served as a haven to native plants and animals might well become the locus of buckthorn dispersal, especially if rapid transport of viable seed by birds is in fact a regular phenomenon.
As we thought before preliminary reconnaissance, it seems likely that much of the general public is either insufficiently, or perhaps completely, unaware of the magnitude and rapidity of Schmeeckle Reserve’s buckthorn invasion. Aside from possible mis-identification or a mistaken belief that buckthorn is native, this plant’s lasting dense green foliage may create a false impression of ecosystem health. Beyond direct control efforts, we now believe that an education campaign relating environmental information and potential costs to the public is essential as a consistent buckthorn management practice.
Lacking prior buckthorn data or detailed original vegetation maps to compare with our findings, we instead used our presence/absence correspondence factors to pose hypotheses of future buckthorn dispersion at our twenty-two observation sites (Figure 10). Four sites (A, C, I, and T) we believe clearly have favorable conditions for buckthorn expansion, and the space available for expansion. Eleven further sites (B, D, H, J, N, O, P, R, S, U, and V) afford favorable growth conditions, but lack room for expansion. Three sites (E, F, and L) are now so crowded with buckthorn that intraspecific competition should restrict further density increases. Only four sites (G, K, M, and Q) seem unlikely to acquire any greater buckthorn presence.
We then also portrayed these site hypotheses as a map (Figure 11).
On the basis of our correspondence observations, we speculate that the non-wetland areas southwest of Michigan Avenue and two areas along the north shore of Lake Joanis face the highest risk for further infestation. If our projections are valid, future monitoring efforts should be especially watchful around Sites A, C, T, I, and L. We hold it premature to base any management action upon our initial findings until further examination of the study area affords opportunity for verification.
Our preliminary investigation suggests numerous specific areas where additional research might be beneficial. Of necessity, many of these topics require cooperative application of skills from multiple environmental disciplines, as well as from economics, public administration, and communications.
Longitudinal studies to document changes over time are desirable. These might include, beyond buckthorn itself, the mapping of native plant and leaf litter distributions, the durability of diversity alterations, and the conduct of environmental gradient transect studies. Projects more specific to buckthorn might include the verification of its absence in wetlands, the reconstruction of its historical distribution using tree ring analyses, and experimentally determining its dispersal potential, monitoring its rates of re-establishment in cleared sites, and identification of the influence from mechanical clearing on invasion/re-infestation.
The ecological sciences could provide valuable insights into dispersal mechanisms through studies of buckthorn phenology—especially pertaining to its reproduction by means of roots, shoots, or fruits, and by establishing age-to-size correlations through tree-ring analyses of stumps in mechanically cleared areas. Laboratory experiments to determinine buckthorn tolerances for shade, nutrients, and soil acidity, and the influence of litter decomposition could provide applicable knowledge as well. Ultimately, a multivariate risk factor scoring scale, weighting the relative influence of these and other habitat controls over buckthorn germination and growth, might become a useful prediction and management tool.
Various impact studies are necessary to provide information capable of convincing the public of the gravity of this—and other—problem exotics; the general public might thus acquire both awareness and motivation to demand corrective or containment actions. Impact studies might include assessing the environmental and human costs of native plant and animal displacements, of hydrologic regime modifications, and of control management practices.
Under extremely careful oversight, we propose that confined-area field experiments should test the management viability of multi-phase buckthorn control, by artificially creating at least three conditions that we particularly suspect have association with lesser buckthorn abundance. These are soil saturation, litter accumulation, and understory competition. It may be possible to reduce or eliminate buckthorn by a sequential program of prolonged inundation, seeding and nurturing native competitors (such as blackberry or grasses), and deep mulching. Artificial shading might enhance the effect. However, in addition to assessing the efficacy of such a program for reducing buckthorn, it is also important to gather and evaluate information as to how much such control practices might impose further stresses on the native species, so as to avoid replacement of buckthorn with other exotic invaders or hazards (e.g., wildfire).