The Viability of Reserve Trees and Stump Sprouting on Dry, Nutrient-Poor Oak Sites

Abstract

Low quality oak growing on dry, nutrient poor sites, colloquially called scrub oak, constitutes a common forest type in Central Wisconsin. Northern pin oak (Quercus ellipsoidalis) is one of the dominant tree species on these sites. It is shade intolerant and an effective stump sprouter; however, it is also short-lived. The combination of effective stump sprouting and shade intolerance suggests coppice as one of the soundest silvicultural practices for use with this species. However, many of the associated tree species that grow on these sites, such as white pine (Pinus strobus), white oak (Q. alba) and bur oak (Q. macrocarpa), tend to be longer-lived and more shade tolerant. The potential for retaining a percentage of these longer-lived trees as reserve trees has not been well researched. Because these longer-lived species also tend to have slower growth rates, retention of them for a longer period of time than the standard rotation length of 45-70 years will allow them to reach sawlog size and also provide omproved habitat for wildlife and better aesthetics. Because of the desirability of maintaining some reserve trees on these sites, one of the objectives of this research endeavor is to assess the persistence (as demonstrated by health and growth rate) of reserve trees for a 15 year time period after the surrounding stand was harvested. Two species of reserve trees were selected for use in this study: white oak and northern pin oak. Reserve trees that result from harvests which occurred over a 15 year timeframe were assessed for volume growth, crown condition, vigor and epicormic branching. This assessment indicated that white oak makes an excellent reserve tree with improvement in periodic annual increment (PAI; an index of volume growth) and vigor. Northern pin oak improved in PAI, indicating a positive growth response to the release; however, tree vigor declined through time after release. This corresponded well with our expectations based on the autecology of both species. Northern pin oak is a relatively fast-growing species with early maturity and quick senescence. White oak, by contrast, is a relatively slow-growing species, with late maturity and delayed senescence. Both species appear to be viable species for use as reserve trees; however, where white oak is present, it will make a far superior reserve tree because it has the ability to improve in both vigor and growth rate after release. Because stump sprouting is the dominant form of regeneration in a coppice harvest system for oaks, determining the rate of stump sprouting for the dominant species is essential. This information is lacking for northern pin oak in Wisconsin. The objective of the second portion of this study is to determine the expected sprouting rate of northern pin oak on scrub oak sites. Northern pin oak stumps were assessed for sprouting on four recent harvests sites (winter of 2006 and 2007). Overall, northern pin oak was found to sprout at a high frequency (overall average of 84.7%). As stump diameter increased, frequency of sprouting decreased; however, even for large stumps, the frequency of sprouting was quite high (over 70%). With sprouting frequencies that are this high on sites where oaks are very competitive, only moderate amounts of advanced regeneration would be necessary for a site to restock after harvest. The results from first studies suggest that retention of reserve trees can be quite viable in scrub oak sites. Additionally, the frequency of stump sprouting of the dominant vegetation in these sites, northern pin oak, was found to be quite high. The combination of high frequency of stump sprouting and good viability of reserve trees suggests the need for future research with the use of “coppice with standard” harvest system on scrub oak sites.