Timothy B. Harrington

Response of understory vegetation important to the northern bobwhite following imazapyr and mechanical treatments

Abstract In southern pine (Pinus spp.) stands, managers have used a variety of treatments to control hardwood encroachment and improve habitat conditions for northern bobwhites (Colinus virginianus). We compared use of the herbicide Arsenal® (BASF Corporation, Research Triangle Park, N.C.) (imazapyr) and traditional mechanical treatments, with and without fire, to control hardwood encroachment on study sites on Tall Timbers Research Station in the Red Hills Region near Tallahassee, Florida. Hardwood stem density decreased on herbicide and herbicide+burn plots but increased on all mechanically treated plots at 1 year after treatment. Herbicide and herbicide+burn treatments resulted in a >3-fold increase in forb coverage 1 year after treatment, whereas forb coverage did not increase in mechanically treated plots. A single application of imazapyr, with or without prescribed fire, can control hardwood encroachment and stimulate herbaceous species growth. Following treatment, vegetative communities likely can be maintained for prolonged periods by using traditional methods such as prescribed fire.

Effects of logging debris treatments on five-year development of competing vegetation and planted Douglas-fir

Although considerable research has focused on the influences of logging debris treatments on soil and forest regeneration responses, few studies have identified whether debris effects are mediated ...

Dissolved carbon and nitrogen leaching following variable logging-debris retention and competing-vegetation control in Douglas-fir plantations of western Oregon and Washington

We examined the effect of logging-debris retention and competing-vegetation control (CVC, initial or annual applications) on dissolved organic carbon (DOC), dissolved organic nitrogen, and nitrate-N leaching to determine the rela- tive potential of these practices to contribute to soil C and N loss at two contrasting sites. Annual CVC resulted in higher soil water nitrate-N concentration and flux, with the magnitude and duration of the effect greatest at the high-N site. Most of the increase in nitrate-N at the low-N site occurred in treatments where logging debris was retained. Dissolved organic nitrogen increased at the high-N site in March of each year following annual CVC, but the contribution of this increase to total N concentration was small (2%-4% of total N flux). There was no effect of logging-debris retention or CVC treat- ment on soil water DOC concentrations, indicating that DOC inputs from logging debris and competing vegetation were either retained or consumed in the mineral soil. The estimated increase in leaching flux of dissolved C and N associated with the treatments was low relative to total soil pools, making it unlikely that loss of these elements via leaching will negatively affect future soil productivity at these sites.

Soil and Douglas-fir (Pseudotsuga menziesii) foliar nitrogen responses to variable logging-debris retention and competing vegetation control in the Pacific Northwest.

Experimental treatments of logging-debris retention (0%, 40%, or 80% surface coverage) and competing vegetation control (initial or annual applications) were installed at two sites in the Pacific Northwest following clearcutting Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) stands to assess short-term effects on tree N acquisition, soil N supply, and total soil N. Vegetation control treatments began in the first year after harvest, and logging-debris manipulations were installed 2 years after harvest. Annual vegetation control increased foliar N concentration and content in most years at both sites, which was associated with higher available soil N and increased soil water content. Logging-debris retention treatments had no detectable effect on any of the foliar variables or soil available N at either site. There were no treatment effects on total soil N at the site with relatively high soil N, but total soil N increased with logging-debris retention when annual vegetation control was ...

Belowground competition from overstory trees influences Douglas-fir sapling morphology in thinned stands

We evaluated effects of belowground competition on morphology of naturally established coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) saplings in 60- to 80-year-old thinned Douglas-fir stands in southwestern Washington. We separately quantified belowground competition from overstory and understory sources using trenching and understory removal. In this light-limited environment of 26xa0±xa016% (std. dev.) full sunlight, 2-year exclusion of tree root competition by trenching increased sapling stem biomass by 18%, total aboveground biomass by 21%, number of interwhorl buds by 68%, total foliar biomass by 33%, and foliar biomass on branch components over 4xa0years old by 143%. Belowground competition did not influence shoot:root ratio or foliar efficiency (i.e., stem growth per unit foliage biomass). Sapling needle size, specific leaf area, and internodal distance also were not affected by belowground competition; these variables were apparently a function of the low-light environment. The principal source of belowground competition was roots of overstory trees; effects of belowground competition from understory vegetation were minor. Thus, under a partial overstory, morphology of Douglas-fir regeneration was influenced by both belowground and aboveground competition from overstory trees. In this environment, understory vegetation control would not likely influence belowground competition to an extent that would affect sapling morphology.

Invasive scotch broom alters soil chemical properties in Douglas-fir forests of the Pacific Northwest, USA

Backgrounds and aimsScotch broom is an N-fixing invasive species that has high potential to alter soil properties. We compared soil from areas of Scotch broom invasion with nearby areas that had no evidence of invasion to assess the influence of broom on soil P fractions and other chemical properties.MethodsThe study was conducted at two contrasting Douglas-fir sites in Oregon (OR) and Washington (WA), USA with broom invasion for 10xa0years. We used the Hedley sequential fractionation procedure to assess effects of Scotch broom invasion on P pools of varying bioavailability, and also measured total C, N and extractable nutrient cations.ResultsTotal soil C and N were significantly higher with broom present at the fine-textured OR site, but there was no effect at the coarse-textured WA site. There was no difference in labile-P measures between the presence and absence of Scotch broom at either site, but there were notable reductions (25–30xa0%) in the intermediately-available P fraction when broom was present. Extractable nutrient cations (notably K) were lower in the presence of broom at both sites, with the effects most pronounced at the fine-textured OR site.ConclusionsLasting effects of Scotch broom invasion are likely to be associated with variable changes in soil C, N, and decreases in extractable nutrients and available P. These changes, and other documented effects of Scotch broom on soil, are likely to have lasting effects on Douglas-fir growth after Scotch broom removal that will vary depending soil nutrient status at a given site.

Aboveground growth interactions of paired conifer seedlings in close proximity

Where belowground resources are relatively abundant, naturally established trees sometimes occur in very close proximity to one another. We conducted a two-year study to assess the aboveground interactions between Douglas-fir (Pseudotsuga menziesii), grand fir (Abies grandis) and noble fir (Abies procera) seedlings planted in closely spaced (stems 10xa0cm apart) conspecific and heterospecific pairs. Paired seedling growth also was compared to seedlings planted with no neighbor. Stem height growth was not affected by the presence of a neighbor seedling, although diameter growth was slightly reduced. Branch diameter growth and weight were reduced where seedling crowns overlapped; branch morphological data suggested that this was caused by shading rather than mechanical interactions. Light measurements showed the potential for significant shading, particularly by the relatively large, dense crowns of Douglas-fir. Heterospecific pairs including Douglas-fir demonstrated the competitive production principle in that their mean growth was greater than the average of conspecific pairs of both species. Neighbor seedling height significantly affected subject seedling growth; neighbor effects were similar whether the neighbor seedling was growing on the north or south side of the subject seedling. Light reflected from Douglas-fir crowns had a lower red: far-red ratio than that of noble fir, although there was no evidence of a phytochrome-mediated growth response to the neighbor seedling. While heterospecific seedling pairs showed reduced competition, we found no evidence of facilitation for seedlings growing in very close proximity.