Overstory Canopy Cover Research Articles

Woody Plant Community in the Cross Timbers Over Two Decades of Brush Treatments

The closed overstory of fire-adapted forests throughout the world precludes surface fires of sufficient intensity to open the overstory. The cross timbers, an upland oak forest that spans an area from Texas to Missouri, developed with frequent fire, but removal of fire since European settlement has increased canopy cover of dominant overstory trees. To assess response of woody plant species cover and plant community compositional trajectories to brush treatments that included herbicides and fire in the cross timbers, we analyzed a 20-year data set (1982‐2001) from untreated pastures, pastures treated with tebuthiuron or triclopyr in 1983, and pastures treated with tebuthiuron or triclopyr in 1983 and burned periodically thereafter (tebuthiuron þ fire and triclopyr þ fire). We used analysis of variance to determine change in overstory canopy cover and understory canopy cover in response to experimental treatment, and we used detrended correspondence analysis to assess trajectories of species composition over time. Overstory canopy cover was reduced from more than 100% to less than 20% following initial herbicide application, but canopy cover increased subsequently to over 60% as herbicide-resistant woody species increased in all but the tebuthiuron þ fire treatment. The most striking change in overstory occurred in the tebuthiuron þ fire pastures, which contained almost 20% cover of sumac (Rhus spp.) by 1994, and in the tebuthiuron pastures, which contained . 40% cover of eastern redcedar (Juniperus virginiana L.) by 1994. Tebuthiuron, both with and without fire as a follow-up, influenced composition of the overstory more than did triclopyr. Application of tebuthiuron appears to be a long-term unidirectional disturbance in the cross timbers. Composition also remained distinct from pretreatment condition in triclopyr-treated pastures after almost 20 years of postherbicide succession. In contrast, the understory woody plant community was quite resilient to both herbicides.

Woody Plant Community in the Cross Timbers Over Two Decades of Brush Treatments

The closed overstory of fire-adapted forests throughout the world precludes surface fires of sufficient intensity to open the overstory. The cross timbers, an upland oak forest that spans an area from Texas to Missouri, developed with frequent fire, but removal of fire since European settlement has increased canopy cover of dominant overstory trees. To assess response of woody plant species cover and plant community compositional trajectories to brush treatments that included herbicides and fire in the cross timbers, we analyzed a 20-year data set (1982-2001) from untreated pastures, pastures treated with tebuthiuron or triclopyr in 1983, and pastures treated with tebuthiuron or triclopyr in 1983 and burned periodically thereafter (tebuthiuron + fire and triclopyr + fire). We used analysis of variance to determine change in overstory canopy cover and understory canopy cover in response to experimental treatment, and we used detrended correspondence analysis to assess trajectories of species composition over time. Overstory canopy cover was reduced from more than 100% to less than 20% following initial herbicide application, but canopy cover increased subsequently to over 60% as herbicide-resistant woody species increased in all but the tebuthiuron + fire treatment. The most striking change in overstory occurred in the tebuthiuron + fire pastures, which contained almost 20% cover of sumac (Rhus spp.) by 1994, and in the tebuthiuron pastures, which contained > 40% cover of eastern redcedar (Juniperus virginiana L.) by 1994. Tebuthiuron, both with and without fire as a follow-up, influenced composition of the overstory more than did triclopyr. Application of tebuthiuron appears to be a long-term unidirectional disturbance in the cross timbers. Composition also remained distinct from pretreatment condition in triclopyr-treated pastures after almost 20 years of postherbicide succession. In contrast, the understory woody plant community was quite resilient to both herbicides.

Responses of ground flora to a gradient of harvest intensity in the Missouri Ozarks

The relationship between the intensity of timber harvest and the composition and diversity of ground vegetation was investigated in upland mixed oak forests in the Missouri Ozarks. Pre- and post-harvest data were collected from 420 permanent vegetation plots to determine if changes in the composition and diversity of ground flora depended upon the intensity of the harvest treatment and, if so, how this varied among life forms of ground flora. The five harvest treatments (no-harvest control, single-tree selection, group selection, thinning, and clearcut) imposed a gradient of overstory canopy cover with reductions ranging from 12.8% in the controls to 83.6% in clearcuts. Both total cover and species richness of the ground flora increased proportionally to harvest intensity. Harvest intensity also caused a proportional impact on the collective relative cover of seven identified life forms of ground flora: annuals/biennials, forbs, graminoids, legumes, woody vines, shrubs, and tree seedlings. The direction of this change was nearly identical for all harvest treatments, reflecting the consistent and proportional decrease in the relative cover of legumes, and increase in the relative cover of annuals/biennials, forbs, graminoids, and woody vines, following all harvest treatments. Closer inspection revealed that changes in relative cover, compared to levels in uncut stands, were observed at different levels (thresholds) of harvest intensity for different life forms, although almost all life forms showed significant differences between uncut and clearcut conditions. Managers may be able to use these differential thresholds of response to harvest intensity to develop silvicultural prescriptions specifically intended to increase or decrease the short-term abundance of certain life forms.

Wyoming Big Sagebrush Recovery and Understory Response With Tebuthiuron Control

Field data collected over a 20-year period at 8 sites in northwestern New Mexico was used to determine Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle and Young) recovery following control with tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-N′-dimethylurea) and to relate understory perennial grass yield to overstory canopy cover. Tebuthiuron killed between 80% and 95% of mature Wyoming big sagebrush plants within 18 months of chemical treatment, but through recruitment plant numbers equaled or exceeded pretreatment density (plants/m2) at 3 of the 8 sites and were increasing at other locations near the study's end. Wyoming big sagebrush canopy cover averaged 15%) at 2 sites, to between 5% and 10% at 4 sites, and to less than 3% at the remaining 2 sites. Treatment life was projected to exceed 35 years for 6 of the 8 study sites. Higher rates of tebuthiuron generally extended treatment life. Annual average perennial grass yield increased on treated areas relative to untreated rangeland at all study sites over the 20-year study period. Grass yield was highly variable between years, with pronounced increases when weather and environmental conditions were favorable for grass growth. A nonlinear S-shaped curve best described overstory–understory relationships and also defined the time path of Wyoming big sagebrush recovery, which differed by study site.

Wyoming Big Sagebrush Recovery and Understory Response With Tebuthiuron Control

Field data collected over a 20-year period at 8 sites in northwestern New Mexico was used to determine Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle and Young) recovery following control with tebuthiuron (N-[5-(1,1- dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-N9-dimethylurea) and to relate understory perennial grass yield to overstory canopy cover. Tebuthiuron killed between 80% and 95% of mature Wyoming big sagebrush plants within 18 months of chemical treatment, but through recruitment plant numbers equaled or exceeded pretreatment density (plants/m2) at 3 of the 8 sites and were increasing at other locations near the study’s end. Wyoming big sagebrush canopy cover averaged <2% the first 10 years after herbicide treatment but had returned to near pretreatment levels (>15%) at 2 sites, to between 5% and 10% at 4 sites, and to less than 3% at the remaining 2 sites. Treatment life was projected to exceed 35 years for 6 of the 8 study sites. Higher rates of tebuthiuron generally extended treatment life. Annual average perennial grass yield increased on treated areas relative to untreated rangeland at all study sites over the 20-year study period. Grass yield was highly variable between years, with pronounced increases when weather and environmental conditions were favorable for grass growth. A nonlinear S-shaped curve best described overstory-understory relationships and also defined the time path of Wyoming big sagebrush recovery, which differed by study site.

Are Spotted Owl Populations Sustainable in Fire-Prone Forests?

We examined territory selection, occupancy and reproduction among northern spotted owls (Strix occidentalis caurina) relative to environmental factors influencing wildfire regimes in the eastern Cascade Mountains, Washington, USA. Territory selection was influenced positively by the amount of (a) forest with 30-70% overstory canopy cover; (b) grand fir (Abies grandis) and (c) riparian habitat, and negatively by (a) late-successional forest (LSF); (b) trees 13-19 cm diameter; and (c) elevation. Owl pairs in late-successional reserves (LSRs) were not more productive than those in intervening forests. Owl reproductive rates were lower in territories with more pole-sized trees and with greater annual precipitation at higher, wetter elevations and glaciated landtypes where LSF was abundant. As a result, LSF was not correlated to owl reproduction. The most productive owl pairs occurred in dry forests that were at highest risk to uncharacteristic wildfires. Short-term successional advances toward shade-tolerant, pole-sized trees may have led to abandonment of 45 owl territories in mesic forests. Sustainable conservation in this area should account for physical environmental factors that influence owl population performance and reduce continuity and density of fuels.

RED-BACKED VOLE (CLETHRIONOMYS GAPPERI) RESPONSE TO DISTURBANCE IN SUBALPINE FORESTS: USE OF REGENERATING PATCHES

We investigated scale-dependent habitat selection by the southern red-backed vole (Clethrionomys gapperi) in subalpine forests and 20- to 30-year-old regenerating timber harvests of the central Rocky Mountains. At the macrohabitat level, C. gapperi preferred forest stands, which had more overstory canopy cover, more uniformly distributed coarse woody debris (CWD), and more dwarf huckleberry (Vaccinium scoparium) than regenerating harvested stands. Further, C. gapperi was captured in forest stands up to harvest boundaries but crossed them significantly less than expected by chance. Analysis of microhabitat selection indicated that, in forest stands, C. gapperi showed selection at a fine spatial scale, strongly favoring CWD microhabitats and avoiding those dominated by V. scoparium. Our results suggest that older, regenerating clear-cuts in the Rocky Mountains can have clearly defined impacts on C. gapperi similar to those immediately after timber harvests, but that regenerating clear-cuts do support some use by C. gapperi. The results support the association of C. gapperi with CWD and forest overstory in western montane habitats and indicate a noticeable response to forest boundaries but no direct edge effect.

Northern Bobwhite Population and Habitat Response to Pine-Grassland Restoration

We compared northern bobwhite (Colinus virginianus) abundance and habitat characteristics in unmanaged mixed shortleaf pine (Pinus echinata)-hardwood stands and restored pine-grassland stands managed for the red-cockaded woodpecker (Picoides borealis) on the Ouachita National Forest, Arkansas, USA. To determine northern bobwhite (hereafter, bobwhite) population response in untreated control, thinned, and thinned and burned stands either 1, 2, or 3 growing seasons (Mar to mid-Oct) post-burn, we used whistling-male counts and covey-call counts as indices of population abundance. We estimated woody stem density, understory and overstory canopy cover, conifer and hardwood basal area, and the disc of vulnerability to characterize habitat response. Relative abundance of whistling males in the spring was greatest in thinned stands 3 growing seasons post-burn and in thinned but unburned stands. These stands had the smallest disc of vulnerability and the greatest understory shrub cover <2 m in height compared with other treatments. A threshold-like increase in bobwhite abundance was observed as a function of woody structure <2 m. Pine-grassland restoration provided suitable structure for bobwhites in spring, summer, and fall, but may not be adequate in winter. Further, data suggested that bobwhite density within a stand also was related to the amount of suitable habitat surrounding the stand. Bobwhite management efforts in similar shortleaf pine forests should include thinning to reduce midstory and overstory cover and frequent fire to maintain open woodland conditions-i.e., low basal area stands with limited midstory.

An approach to deriving roughness length and zero-plane displacement height from satellite data, prototyped with BOREAS data

Global climate models typically use relatively crude estimates of roughness length, often based on constant values for land cover category. However, large variations may exist within any of the vegetation categories. Global large-scale estimates of roughness length, such as required by climate models, cannot rely on field experiments for practical reasons and must, therefore, be derived from satellite observations. This paper develops a satellite retrieval method for the roughness parameters, momentum roughness length, z o, and displacement height, d. The formulas, based on the observational data of Raupach [Raupach, M.R., 1992. Bound.-Layer Meteor. 60, 375–395; Raupach, M.R., 1994. Bound.-Layer Meteor. 71, 211–216] and Lindroth [Lindroth, A., 1993. Bound.-Layer Meteor. 66, 265–272], are applicable to most types of vegetation. Raupach relates the momentum roughness length and zero-plane displacement height to the frontal area index, λ, and to the height of the vegetation, h. The frontal area index is related to the shape of an average tree or plant crown and to the average density of the canopy elements. The crown shape factor includes the aspect (height-to-width) ratio of the crown and the geometric shape of the crown. Lindroth relates the momentum roughness length to the leaf area index (LAI) and to the height of the trees. The functional form of z o/ h is peaked in both frontal area index, with the maximum occurring at λ=0.152, and average plant LAI ( L p), with the maximum occurring at L p=0.878. As λ and L p increase, d/ h also increases and approaches an upper limit asymptotically at high values of λ and L p. As the vegetation height is difficult to obtain from present satellite data, retrievals are restricted to fractional values (i.e. z o/ h and d/ h) for global application. This retrieval then requires data for the crown aspect ratio, the canopy density (fractional overstory canopy cover) and the LAI. Data from BOREAS ( BOReal Ecosystem Atmosphere Study) are used to illustrate this satellite retrieval method.

HABITAT OF BIRDS IN PONDEROSA PINE AND ASPEN/BIRCH FOREST IN THE BLACK HILLS, SOUTH DAKOTA

Birds with both eastern and western distributions occur in the Black Hills of western South Dakota. This forest is mostly ponderosa pine (Pinus ponderosa) and is managed for timber. Logging alters forest characteristics and the bird community. We studied habitat relations of breeding songbirds at the stand- and site-level scales in ponderosa pine and quaking aspen (Populus tremuloides)/paper birch (Betula papyrifera) forest. Sixty bird species were observed ≤50 m from count points. Species richness was greater in aspen/birch than in ponderosa pine. Species richness was generally lower in ponderosa pine with >40% overstory canopy cover (OCC) than in ponderosa pine with ≤40% OCC and than aspen/birch of any structural stage. Seven bird species were associated with the ponderosa pine, while four species were associated with aspen/birch. Bird associations at the stand-level were further refined by OCC and diameter-at-breast-height (DBH) structural stage of each forest type. Habitats for most birds in the Black Hills can be managed using current forest inventory descriptions that include OCC and DBH. However, Red-naped Sapsuckers (Sphyrapicus nuchalis), Red-breasted Nuthatches (Sitta canadensis), White-breasted Nuthatches (S. carolinensis), Ovenbirds (Seiurus aurocapillus), and Western Tanagers (Piranga ludoviciana) were strongly associated with site-level vegetation characteristics. Snag density, snag condition, and deciduous trees beneath the ponderosa pine canopy should be included in forest inventories to better quantify habitats for these birds.

LANDSCAPE MANAGEMENT USING HISTORICAL FIRE REGIMES: BLUE RIVER, OREGON

Landscapes administered for timber production by the U.S. Forest Service in the Pacific Northwest in the 1950s-1980s were managed with dispersed patch clear- cutting, and then briefly in the late 1980s with aggregated patch clear-cutting. In the late 1990s, use of historical landscape patterns and disturbance regimes as a guide for landscape management has emerged as an alternative to the static reserves and standard matrix pre- scriptions in the Northwest Forest Plan. Use of historical information to guide management recognizes the dynamic and variable character of the landscape and may offer an improved ability to meet ecosystem management objectives. We describe a landscape management plan based in part on interpretations of historical disturbance regimes. The plan contains a reserve system and other landscape areas where three distinct types of timber harvest are prescribed. Timber harvest prescriptions approx- imate the frequency, severity, and spatial extent of past fires. Future harvest blocks are mapped and used to project forest patterns 200 yr forward and to map resulting landscape structure. This plan is compared with an alternative plan for the same area based on the extensive reserves and prescriptions for matrix lands in the Northwest Forest Plan. The management approach based on historical patterns produced more late-successional habitat (71% vs. 59%), more overstory structure in young stands (overstory canopy cover of 15-50% vs. 15%), larger patches (mean patch size of 48 vs. 26 ha), and less edge between young and old forest (edge density of 19 vs. 37 m/ha). While landscape structures resulting from both plans are historically unprecedented, we feel that landscape management plans incorporating key aspects of ecosystem history and variability may pose less risk to native species and ecological processes.

Comparison of linear and nonlinear overstory-understory models for ponderosa pine

Linear and nonlinear regression models of total understory biomass in relation to overstory canopy cover and several abiotic site factors were tested using data from 31 ponderosa pine ( Pinus ponderosa Dougl.) sites along the Front Range of Colorado and Wyoming. At the forest level, site factors were insignificant in explaining understory biomass. Canopy cover, however, was a significant predictor. The relationship between canopy cover ( X) and grass production ( Y) was best expressed by the nonlinear model, Y = A B X . Total understory production could be predicted with equal precision using simple linear and nonlinear regressions with canopy cover. In this case, the simple linear model captured the interaction of the nonlinear grass response and a suspected quadratic relationship between shrub production and overstory cover as well as any other model.

Overstory-Understory Relations in Pinyon-Juniper Woodlands in New Mexico

Herbage biomass for blue grama (Boutdoua grdlis [H.B.K. Lag& pinyon ricegrass (Pfph&tu&un jimbriatum [H.B.K.] Hit&c.), New Mexico muhly (Muhihkgfa pcsrciflorcl Buckl.), other grasseq and forbs was estimated on 25 pinyon-juniper stands of varying overstory cover on the Fort Stanton Experimental Ranch insouthcentral New Mexico. Negative 2”d degree polynomial curvea best expressed the relationshipa between total understory and blue grama biomass and overstory canopy cover. Positive polynomial relationships were shown for cool-season graazs, New Mexico muhly, and pinyon ricegrass. Reducing pinyon-juniper canopy cover would likely increase blue grama production and reduce production of New Mexico mubly and pinyon ricegraas.