There is a noticeable difference between the studied shelterwood and seed tree harvest. Birch regeneration in the shelterwood harvest is roughly 10 times the amount found in the seed-tree, especially in the regenerant category. Many of the regenerants were found underneath competing vegetation, most notably red raspberry and bindweed. On the shelterwood site the competition of bindweed was sampled as a medium low presence with an average height of 1.1 ft. Raspberry was measured to have a medium low presence with an average height of 2.1. But when you put the two together as they were sampled on the site you have a medium to medium-high amount of competition for those paper birch seedlings with an average height of 1.4 feet to grow through. On the seed-tree site the competition averaged a medium-low presence for bracken fern with an average height of 2.6 feet and beaked hazel had a low presence and average height of 3.3 feet. Together competition was at medium level for bracken fern and beaked hazel with paper birch at an average height of 2.3 feet. Although some of the regenerants may not be able to recruit through the competition to become established trees, this still shows that the shelterwood harvest had a significant positive effect on germination. This leads us to believe that in this specific situation, the shelterwood harvest provided a better environment for the germination of paper birch seed.
For the shelterwood, Wisconsin DNR guidelines recommend a minimum stems/acre of 2000 for paper birch stands within 1-2 years of regeneration. The guidelines qualify a stem as an established seedling when it reaches 1-2 feet in height for paper birch. Our data show 1278 seedlings per acre, which doesn’t meet these requirements. However with over 8000 regenerants under 1 foot tall it is likely at least 10% of these may grow to become 1-2 feet tall in the growing season or after the canopy is removed, depending on what occurs first. Also if you consider the presence of 4800 maple seedlings in addition to the paper birch and use the regeneration stocking guidelines for northern hardwoods from the WI DNR, then this stand meets or will likely meet the criteria for sufficient regeneration by that standard of 2000-5000 seedlings 2-4’ 3-8 years post-harvest. The main goal of this harvest was to retain paper birch as the dominant species with a component of maple. The stand appears to be ready to plan the final overstory removal harvest within the next two years.
Our sampling on the seed tree harvest was biased. We avoided significant areas of Aspen regeneration. These pockets of aspen would have likely choked out any paper birch regeneration that may have been present in the same area. Even with this bias, we still discovered significantly less paper birch regeneration with only 720 seedlings sampled and 870 regenerants. We aren’t certain exactly why there was less birch regeneration but we can speculate. Some possible causes could be the lack of adequate scarification which is necessary to prepare the appropriate seedbed for birch to germinate. It could also be possible that the lack of shade affected birch germination negatively and led to the dessication of germinates. Competition with other plant species could have made it more difficult for paper birch seedlings to persist on the site. Most of the reserve trees in the seed tree harvest were paper birch, and the amount of trees that were left should have been sufficient for a seed tree harvest, so lack of seed or quality of seed trees likely wasn’t an influential factor.
On a positive note an important difference in regeneration in this seed tree stand is the greater percentage of birch and other species originating from stump sprouts. Paper birch, red maple, sugar maple, and basswood were all observed to be stump sprouting, and the sprouts were significantly more vigorous and taller than the observed seed origin seedlings. This is something worth monitoring throughout the future of the stand. The sprouts are above the competition, but because there are so many growing so close together from the same stump, it could lead to a decrease in timber quality if many of the stems remain vigorous and crowd each other for space and nutrients. Stems from sprout origin have less chance of become bolt quality as the cluster of stem usually remain smaller in diameter and grow more pulpwood than bolts. In contrast, the shelterwood birch regeneration method should grow a future forest with a greater percentage of seed origin birch which should yield higher quality future wood products.
Paper birch regenerates well when bare mineral soil is exposed, and no additional scarification occurred on this site, other than what was disturbed by equipment during the timber harvest. Birch regeneration on this site was substantial in areas that had been scarified, but unscarified areas showed little to no paper birch regeneration. This leads us to believe that if paper birch is a desired future species, scarification pre or post-harvest should be done in most situations when possible to maximize the potential for birch seed germination and set back competition. Mechanical scarification may still occur on this shelterwood site before the final overstory removal, if the site conditions allow. We will focus this scarification near the reserve trees that did not receive adequate scarification during the initial timber harvest. This additional post-harvest scarification could provide another significant increase in the amount of paper birch regeneration and result in a uniformly stocked site.
Paper birch is an important part of forest diversity, and there are concerns about its decline as a speciesacross the forested landscape of Minnesota.Climate change may exacerbate this problem, since the state is on the southern end of the paper birch’s range. This case study has provided some insight to combat regeneration issues that may appear as the effects of climate change become more visible. The shelterwood harvesting method could potentially become a necessity for paper birch regeneration as average temperatures continue to rise.
Overall, this stand will be a diverse mix of species as it matures, especially if it stays consistent with the collected regeneration data. The paper birch reserve trees did well seeding in and providing shade for seeds to germinate. Additional scarification after the harvest would likely increase the amount of paper birch regeneration, and should be an important part of management plans for future birch shelterwood harvests if a greater birch density is desired in the stand.