Above- and belowground strata isolation upon regenerating American chestnut and other hardwoods following midstory removal
Amidst heavy vegetative competition, regenerating shade-intermediate species is often problematic. Without silvicultural intervention, the trees often either fail to contend with faster growing pioneer species, or stagnate and deteriorate in heavily shaded environments while shade tolerant species establish. Since these shade intermediate species include many economically and ecologically desirable species, such as oak (Quercus spp.), management that favors these species is often applied. One such prescription is to remove the midstory strata prior to regenerative harvest, leading to a well-stocked understory of advanced regeneration of the desired species. ^ In Chapter 2, I evaluated combinations of manual, mechanical and herbicide midstory removal techniques by their inputs and for their treatment efficiency and efficacy. Across both study sites, midstory removal increased mean light transmission to between 14% and 17% of full sun, whereas non-thinned controls averaged 7% light transmission. Basal area was reduced by 16%, on average across both sites. Midstory removal costs ranged from $89 ha-1 to $1,003 ha-1. Without herbicide, 20% mortality was best achieved. Overall, I found that the manual removal with a cut stump herbicide treatment was the most cost effective on the two sites. ^ In Chapter 3, I also investigated underplanting performance of four hardwoods, most notably American chestnut (Castanea dentata (Marsh.) Borkh), after midstory removal. To further explore isolated competitive effects from the overstory and existing understory, trenching and weeding were used within stands and in adjacent afforested fields. Two years after planting, there was no overstory treatment effect upon survival at the wetter site, but midstory removal significantly increased both relative height and diameter. Trenching significantly increased relative diameter growth, and seedlings showed a positive response to weeding. Relative height increase was greatest for red maple (Acer rubrum L.), then chestnut, then northern red oak (106%, 76% and 33%, respectively). At the drier site, midstory removal increased survival of northern red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.) and chestnut compared to both the non-thinned control. Poor survival made growth comparisons difficult in the control, but there was a trend of decreased growth from field to midstory removal. Relative height and diameter of chestnut increased with trenching. Absolute height of maples remained below American chestnut and northern red oak. ^ Lastly, Chapter 4 focused on the physiology of chestnut and northern red oak within the midstory removal treatment. Volumetric soil water content was 44% higher within trenched plots at the wetter site, and 19% at the drier site. Within those trenched plots, photosynthetic rate was significantly higher for chestnut. Late season mean water potential within untrenched treatments was lower than those within trenched treatments. At the drier site, trenching effects upon carbon assimilation and transpiration were absent for both species, despite significantly increased relative height and diameter. Weeding had no discernable effect at the drier site. There was a difference in water potential measurements between the two species (P = 0.01), with chestnut having slightly lower readings. The favorable performance of chestnut across both sites is a good indicator of its suitability to midstory removal treatments. ^
Michael R. Saunders, Purdue University.
Agriculture, Forestry and Wildlife