Woody competition during mid-rotation limits available nutrients to crop species. Mid-rotation competition control was evaluated to determine if applications would result in significant increase in growth. Two studies were initiated using herbicides and prescribed burning on mid-rotation loblolly pine plantations. The first study contained afforested stands in Mississippi. Combination of imazapyr and burn was applied. The second study contained reforested sites in Mississippi. Treatments consisted of imazapyr and burn, imazapyr only, burn only, and a control. Five-year post-treatment measurements for the first study showed no significant treatment differences in height, diameter, basal area, or volume growth. Nine-year post-treatment measurements for the second study showed no significant differences in growth using the same measurements. Although previous research has documented significant growth responses to mid-rotation competition control, results from this study demonstrate that increased growth does not always result from mid-rotation competition control, suggesting initial site conditions may dictate degree of response.

The effects of two forms of mid-rotation release on thinned, fertilized loblolly pine (Pinus taeda L.) plantations in Virginia were examined: aerial imazapyr and basal triclopyr application. Imazapyr measurement plots were installed in nine Piedmont and twelve Coastal Plain plantations operationally released with imazapyr, and triclopyr measurement plots were installed within a controlled fertilization/release study spanning both regions. No differences in volume were detected following triclopyr release. All release dates combined, Piedmont released areas averaged 0.06 m3/tree (18%) greater than the control and Coastal Plain released areas averaged 0.05 m3/tree (14%) greater than the control. Reductions in hardwood basal area, stem density, and shrub stratum cover were observed for both forms of release. Reductions in shrub stratum richness and diversity were also documented for imazapyr release; however, trends indicate that richness and diversity, as well as stem density and shrub stratum cover, may recover to pre-treatment levels. Herbaceous vegetation was increased on triclopyr sites, which was reflected in an elevated turkey (Meleagris gallopavo sylvestris L.) food/brood index. Following imazapyr release, habitat suitability index (HSI) values for pine warblers (Dendroica pinus L.) and black-capped chickadees (Parus atricapillus L.) increased due to reductions in canopy hardwoods and increases in snags. Reduced shrub stratum density resulted in a lower bobwhite quail (Colinus virginianus L.) cover index on imazapyr-released areas. / Master of Science

Appropriate management of mid-rotation pine plantations can improve habitat for early successional and pine-grassland adapted avian species. I tested effects of Imazapyr selective herbicide and prescribed fire on plant and avian communities in thinned, mid-rotation pine stands contracted under the Conservation Reserve Program. Within 12 replicate sites, 2 8.1-ha plots were assigned either herbicide and prescribed fire treatment or control. I described components of vegetation structure and composition in 2006. I tested effects of herbicide and prescribed fire treatment on avian relative abundance, species richness, total avian conservation value, and density of select species during 2003 - 2006. Hardwood midstory decreased and abundances of grasses and forbs increased following treatment. A shift occurred in the bird community from closed-canopy forest species to early successional and pine-grassland species. Treatment stands benefited many avian species exhibiting negative population trends.

Mid-rotation fertilization and vegetation control are some of the most common silvicultural treatments in loblolly pine (Pinus taeda L.) plantations in the southeastern United States. Competing vegetation is commonly thought to sequester fertilizer nitrogen (N) and reduce the potential growth response to a mid-rotation fertilization treatment. This experiment aims to identify what proportion of applied N fertilizer is retained in the crop tree pine foliage, and the degree to which understory vegetation is competing for this resource. Our mid-rotation loblolly pine plantation received an application of 15N fertilization (urea 365 kg/ha, at 46% N by weight, i.e. 168 kg/ha of N) and a portion of plots received an understory vegetation control (basal spray application of triclopyr; 13.6% active ingredient) treatment either before fertilization or not at all. One-year post-fertilization, 15N contents within pine foliage, leaf fall/leaf litter, forest floor, and soil were measured, as was competing vegetation presence. There was significant variation in applied nitrogen acquisition among the different ecosystem components measured, with 0-15 cm soils retaining a majority at 32-37% added 15N. Differences in fertilizer N acquisition in pine foliage between plots with and without understory vegetation control was marginally significant (p = 0.06) with pine foliage in plots without understory vegetation capturing greater 15N (4.3% greater). Red maple (Acer rubrum) and oak species (Quercus spp.) were the most common competitors but neither had a uniquely pronounced effect on pine nitrogen sequestration. My data indicate that increasing competition reduces fertilizer N foliar concentrations in crop pine trees but at a modest rate and equally across species groups. An unrefined threshold determining when fertilizer N capture in crop pine trees was affected was found at 3.1 m2/ha of competing vegetation basal area. This site will continue to be monitored over time to assess fertilizer N retention in loblolly pine each year after fertilization and evaluate the fertilizer N capture within competing vegetation. / Master of Science / Some of the most prevalent management practices for mid-rotation (age 15, i.e., roughly halfway through a crop cycle) loblolly pine (Pinus taeda L.) plantations in the southeastern United States are fertilization and vegetation control. Nitrogen (N) is consistently one of the most limiting factors to productivity. The addition of N via fertilization is therefore a common forestry practice. However, when a stand is fertilized, the added resource is partitioned and cycled throughout the ecosystem. It is presumed that the amount of fertilizer N obtained by crop trees in a plantation is dependent on the level of competing vegetation (i.e., weed-trees and shrubs) present on site. Controlling competing vegetation prior to fertilization may therefore be warranted under certain conditions. To date, the amount of competing vegetation where it begins to impact fertilizer uptake by the crop tree is unknown. This study aims to elucidate this competing vegetation threshold to better inform mid-rotation management of loblolly pine plantations. This study examined applied fertilizer N capture in ecosystem components with varying levels of understory vegetation, and found more fertilizer N in pine foliage when understory vegetation was completely removed prior to fertilization. No single understory hardwood weed species had a uniquely strong influence on crop tree productivity uptake. Plots that ranked in the upper third in competing vegetation presence did have significantly less foliar fertilizer N in the pine crop trees. Additional replication of this study would be necessary to determine a universal threshold of competing vegetation which would trigger the removal of competing vegetation prior to fertilization.

Thesis (MScFor (Forest and Wood Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Mid rotation fertiliser application is generally practised in forestry to enhance nutrient availability in areas were soils are impoverished and do not provide sufficient nutrients for high productivity. Generally speaking there is great potential for mid rotation fertiliser in pine plantations, but stand and site characteristics such as water availability, soil depth, stand density and available nutrients need to be considered before fertiliser treatments are implemented. Foliar nutrient analyses were used to estimate nutrient availability. These levels were measured throughout the study and were used to implement specific fertiliser treatments that would boost nutrient availability. Treatments consisted of an unfertilised control, a single fertiliser application (1F) and two fertiliser applications over two consecutive years (2F). Leaf area index (LAI) and stem volume increment were measured in order to evaluate its influence on growth efficiency. LAI was estimated using the gap fraction method with the use of a ceptometer. Volume increment was calculated with diameter and height measurements. Basal area was calculated by means of diameter measurements. These growth responses were used to determine the effect of increased nutrient availability and although increases were found in LAI, volume increment, basal area increment and growth efficiency, none were significant. The lack of significance may be due to relatively large variations in other factors such as stand density and initial volume of the experimental plots. The 18 month monitoring period apparently did not allow complete reaction time to increased nutrient availability and limited our understanding of the responses somewhat. Despite this, the magnitude of some growth responses was large as nutrient ratios in the foliage increased to levels within the norms range. Increases in current annual volume increment (CAI) of 3.48 m3 ha-1 a-1 and 3.62 m3 ha-1 a-1 in 1F plots at Grabouw and La Motte plantations indicated that it may be economically feasible to fertilise at mid rotation age as the NPV and IRR increased over a projected 25 year rotation. The Grabouw site had the most significant response with regards to CAI in 2F treatment with a mean volume increment of 5.43 m3 ha-1 a-1. The mechanism of the response was examined further by taking water availability and soil characteristics into account. The seasonal climatic effect (length of the moisture growing season) had a significant influence on the response to fertilisation. / AFRIKAANSE OPSOMMING: Die toediening van mid-rotasie bemesting word algemeen in bosbou toegepas om voedingstofbeskikbaarheid te verhoog in areas waar voedingstowwe onvoldoende is vir hoë produktiwiteit. Daar is oor die algemeen ruim potensiaal vir mid-rotasie bemesting in denne plantasies, maar eienskappe soos waterbeskikbaarheid, gronddiepte, opstanddigtheid en beskikbaarheid van voedingstowwe moet in ag geneem word voor optimum bemestingtoedienings bepaal kan word. Blaaranalise is gebruik om voedingstofbeskikbaarheid in plantasies te skat. Hierdie voedingstofvlakke is deurgans gemeet en is gebruik om spesifieke bemestingsbehandelings te implementeer wat voedingstofbeskikbaarheid kon opstoot. Behandelings het bestaan uit ‘n onbemeste kontrole, ‘n eenmalige kunsmistoediening (1F) en twee kunsmistoedienings in opeenvolgende jare (2F). Blaar oppervlak indeks en toename in stamvolume is gemeet om die invloed daarvan op die effektiwiteit van groei te bepaal. Blaaroppervlakindeks is bepaal deur middel van die gapingfraksie metode met behulp van ‘n stralingsmeter. Toename in volume is bereken met stamdeursnee en hoogte meetings. Basale oppervlakte is bereken deur middel van deursnee metings. Hierdie groeireaksies is gebruik om die effek van verbeterde voedingstofbeskikbaarheid te bepaal. Al die groeireaksies het toegeneem maar was nie statisties beduidend nie. Die gebrek aan beduidende toename kan toegeskryf word aan variasies in opstandsdigtheid en oorspronklike volume van die bome in die navorsingspersele. Die toetstydperk van 18 maande het moontlik nie genoeg tyd gegee vir die bome om op die toename in voedingstofbeskikbaarheid te reageer nie. ‘n Goeie groeirespons is wel waargeneem waar die voedingstofverhoudings in die naalde aanvaarbare norme bereik het. Die toename in volume aanwas van tussen 3.48m3 ha-1 a-1 en 3.62 m3 ha-1 a-1 in 1F persele by Grabouw en La Motte plantasies het aangedui dat dit ekonomies lewensvatbaar is om op mid-rotasie ouderdom bemesting toe te dien aangesien die netto teenswoordige waarde en die interne opbrengs koers toegeneem het op ‘n geprojekteerde 25 jaar rotasie. Die persele op Grabouw plantasie het die mees beduidende respons getoon met betrekking tot huidige jaarlikse aanwas (5.43 m3 ha-1 a-1 in die 2F perseel). Die meganisme van die respons is verder nagevors met inagneming van waterbeskikbaarheid en grondeienskappe. Die seisoenale klimaatseffek (lengte van die vog-groeiseisoen) het ‘n beduidende impak op die respons tot bemesting.