Effects of Nuisance/Invasive Species on Regeneration in New Hampshire

by William B. Leak, U.S. Forest Service, Northern Research Station, Durham, NH

A field note is a report from "the field," based on observation and experience. This is the second periodic offering of New England forestry field notes. If you have a field note you would like to share, contact, steven.roberge@unh.edu Please note, I can't guarantee I will publish your note.

During the fall of 2014, I was fortunate to visit with about a dozen forest managers in New Hampshire to view and discuss the problems they had encountered with nuisance/invasive plants during the regeneration process. I very much appreciate the time and effort spent by these folks. The contacts were facilitated through a very helpful announcement by UNH Cooperative Extension. I also very much appreciate the comments and references provided by Professor Tom Lee, UNH.

The properties were located throughout the State from far north to south, east to west. The purpose of the survey was to develop a beginning understanding of the scope of the problem: the nuisance plants involved, the forest types, the characteristics of the stands, and the possible silvicultural controls.

The primary competing plants were: glossy buckthorn (some common as well), hay-scented fern (coupled with additional fern species), beech, sweetfern, witchhazel, and mountain laurel coupled with a number of other species mostly encountered in this survey along forest-edges and roadsides (e.g. bittersweet and euonymus). In all areas of the State, the problems with competing plants were magnified by deer and moose browsing—generally heaviest on the desirable timber trees.

I’ll summarize my impressions and observations by species:

Glossy buckthorn: This is the most difficult species to deal with. It sprouts readily and seeds abundantly. It can survive in the shade but becomes vigorous and closely spaced in full sunlight; it will often outgrow/outcompete any desirable hardwood or softwood. The seeds are spread by birds, and may lie-over ungerminated in the soil for several years (3.5 or more, Godwin 1943). Appears to be most common south of the White Mountains on abandoned ag sites, especially moist soils, occupied by oak, pine and mixed hardwoods. Experience to date indicates that the species will not be deterred by heavy cutting or fire on the moist sites where it is most invasive; repeated cutting with brush saws appears ineffective in open areas.  Chemical applications would need to be repeated over several years due to the buried seed. We see areas where vigorous seed-producing buckthorn and other invasives are concentrated along road and stand edges. Possibly, repeated chemical applications along these borders would be useful to prevent invasion into the stand.

What to do? On drier sites, we see areas where group/patch openings or low-density shelterwoods will regenerate fast-growing species such as black birch that will outcompete the buckthorn (remember: black birch is not heavily browsed by deer). There may be moderate sites where buckthorn densities and patchy distributions may allow sufficient regeneration to succeed without additional treatment (see Koning and Singleton 2013) On moist sites where the buckthorn is most vigorous, individual mechanical release of minimum numbers of desired softwood/hardwood stems (50 plus per acre?) would provide some level of productivity to otherwise unproductive acres. The released stems would have to be well-developed—half the height of the buckthorn as a complete guess!!

The other option is to carefully look—prior to harvest—for areas of advanced regeneration (large seedlings to sapling/small-pole size), and release them completely (group/patch release) during the harvesting operation. I feel sure this will produce some level of success.

Hay-scented fern:This competitor is often present with other ferns such as New York fern (as well as others). Usually occurs under hardwood stands on fairly rich sites, often on old ag land, but we do see heavy fern invasion on cutover areas that always have been in forest. These ferns spread very rapidly through heavy spore production. A small area of fern in the spring can spread to acres by fall. These ferns are most aggressive in openings, but apparently (?) will persist for years under a closed canopy. The competition to the regeneration is primarily through intense root competition. However, some hardwood species, such as black and yellow birch, will gradually grow through a cover of fern—although sometimes we see fern cover so thick, it is doubtful that it can be penetrated by any desirable species. There have been trials of heavy cutting (groups/patches, low-density shelterwoods) coupled with HEAVY scarification that probably will successfully regenerate to yellow or black birch. On rich sites, we would see sugar maple gradually develop beneath the birch as the fern subsides. On poor sites ( e.g. granitic origin), beech would be the rule.

Beech: This is perhaps the most common competitor on many of the low/medium northern hardwood sites in New Hampshire. On soils derived from rich (calcareous) bedrock, beech gives way to a component of sugar maple and ash. The best method for dealing with beech is through group/patch/clearcut harvests where the understory is eliminated (e.g. by whole-tree harvesting) and soil scarification is produced. A seed source of birch and sugar maple should be nearby. These species will outgrow the beech. We also run into beech competition on sites with a desirable oak or pine component on abandoned ag land. If advanced competitive regeneration of desirable species is present, a complete release will allow the desirable regen to outgrow the beech. There is some feeling that minimal soil/root disturbance, such as a winter or frozen-soil harvest, will minimize beech root sucker production. Also, that suckering is less if cutting is done in the growing season when carbohydrate reserves are down. I don’t believe these precautions will have earth-shattering effects—especially over the long run. Cutting beech sapling/pole stems 2-3 feet above ground will significantly reduce stump sprouting and there is some feeling that suckering also is reduced; I’m not sure about the last item. Some additional research and observation is needed.

Mountain laurel: Found in certain southern New Hampshire locations as a thick understory that is heavily competitive on wet sites with very little advanced regeneration of other species. Oak/pine/hemlock is the usual forest type. The best control is group/patch/clearcut with as much ground disturbance as possible. On these wet sites, winter harvests with a minimal/moderate snow cover and frozen skid trails should be adequate. Resprouting of laurel does not appear to be a heavy competitive problem. Following the harvest, there should be a scattering of oak on the skid trails, some pine in the areas between, as well as a significant proportion of black birch (which is not browsed by deer, and perhaps by moose). Mountain laurel also occurs on dry, somewhat rocky sites where it does not appear as vigorous as on the wet sites.

Witch hazel: Found as a heavy understory on historic ag land in the Connecticut river valley, and other locations in mid/southern New Hampshire. Group/patch harvests to regenerate/release pine and oak appear to control this competitor or keep it out of a dominant position.

Sweetfern: Found as a dense cover in young patch cuts near Rumney. Also, as a scattered competitor in other areas following patch harvests. The presence of sweetfern may not be evident prior to harvest since the seed apparently lies dormant in the soil until a harvest provides more warmth and light (Del Tredici 1977). Dense sweetfern appears to provide a deterrent to desirable regeneration such as pine. More time is needed to see what happens.

Grass/sedges: In some areas, we saw large harvest areas almost completely occupied by grass/sedge cover. Soils appeared wet. I suspect that browsing was a significant factor. Where this occurrence seems common, it may be useful to employ smaller harvest openings, attempt to develop and then release advanced regeneration, and attempt browsing control (see below).

Rubus: occurred as a component on many of the areas described above. It does not appear to be a serious or long-lasting regeneration problem.

Prevention: Often the threat of invasive/nuisance species that will disrupt regeneration efforts can be anticipated by the presence of these species, alone or in combination, along forest edges and roads or in prior skid trails. Prior to harvest, a survey of the stand to detect the presence of these species is well worthwhile. If present in these fringe areas, this might be the optimum time to deal with the problem by chemical means, or a combination of chemical/mechanical approaches. If this is not possible, a strategy to deal with the potential problem should be carefully developed using the suggestions above.

Browsing: All of the nuisance/invasive problems mentioned above are accentuated by deer and/or moose browsing. Browsing is seldom severe or even noticeable on the unwanted species. However, we saw one area near moose winter habitat where browsing was quite severe on beech after all the palatable species had been nearly eliminated. Browsing control is difficult but some control is possible through facilitating hunting, brush barriers or tops on or surrounding the harvested area, large clearcuts or numerous smaller cuts to overwhelm or spread out the browsing pressure. Black birch is perhaps the only desirable hardwood that is lightly browsed. Red spruce, in contrast to balsam fir, is also lightly browsed.