Overstory Stem Research Articles

Community Attributes Predict the Relationship between Habitat Invasibility and Land Use Types in an Agricultural and Forest Landscape

Finding ecosystem or community level indicators for habitat invasibility may provide natural resource managers with environmentally friendly measures to control alien plant invasion; yet, ecosystem invasibility remains understudied. Here, we investigated alien plant invasion into various ecosystems representing different land use types in a subtropical peri-urban area of south China. Four invasive alien species were found from five out of the six ecosystems. Lower plant diversity in both the overstory and understory was consistently associated with more severe alien plant invasion to the ecosystems. The highest total abundance and plot occurrence of the invasive plants were found in the agroforestry ecosystem representing the highest disturbance. At plot scale, an increase in invasion severity was associated with a significant decrease in overstory stem density, species richness, and diversity, but with a significant increase in overstory plant dominance. The understory community attributes in response to the increase in invasion severity followed similar patterns, except that the stem density increased with invasion severity. Higher canopy openness and thus lower leaf area index and greater understory radiation were associated with higher invasion severity of invasive plants to the understory habitat. For predicting total abundance of the invasive species, the most important variable is land use type, while for the abundance of Lantana camara and Mikania micrantha, the most important predictor variable is overstory Berger–Parker index and canopy openness, respectively. Canopy structure and understory gap light regimes were among the most important factors determining the abundance of the worst invasive plant Mikania micrantha. Our results demonstrate that land use types with varying disturbance regimes determine the spatial heterogeneity in plant diversity and community structure, which predicts alien plant invasion and habitat invasibility; and that the severity of alien plant invasion in turn is a good indicator of habitat disturbance across the ecosystems.

Hardwood-Pine Mixedwoods Stand Dynamics following Thinning and Prescribed Burning

Restoration of hardwood-pine (Pinus L.) mixedwoods is an important management goal in many pine plantations in the southern Cumberland Plateau in north-central Alabama, USA. Pine plantations have been relatively unmanaged since initiation, and thus include a diversity of hardwoods developing in the understory. These unmanaged pine plantations have become increasingly vulnerable to insects, and management activities were initiated to facilitate transition towards hardwood-pine mixedwoods. We evaluated a combination of thinning and prescribed fire prescriptions in a randomized complete block design with a 3 × 3 factorial treatment arrangement and four replications of each treatment. Treatments were combinations of thinning to three residual basal areas (no thin, light thin to 17.2 m2 ha−1, heavy thin to 11.5 m2 ha−1) and three prescribed burn applications (no burn, one burn to be repeated every 9 years, three burns repeated every 3 years). Burning without thinning altered stand structure by reducing overstory stem density by 15%, whereas thinning without burning reduced density by 70% and burning coupled with thinning resulted in reduced overstory trees by 72%. Frequent fire had the greatest impact on midstory structure and regeneration. Midstory stem density was reduced by 90% following thinning and burning. Oaks (Quercus L.) and red maple (Acer rubrum L.) were the most common understory species after thinning only, burning only, and thinning combined with burning. There were more oak and red maple seedling sprouts following frequent burning. Currently, >75% of red maple sprouts dominate the regeneration, compared to only 40% of the oaks. Although the treatments have accelerated the transition toward hardwood-pine mixedwoods, the fate of oak and which hardwood species will be dominant in the future remains uncertain.

Impacts of Wildfire Recency and Frequency on an Appalachian Oak Forest

Cabwaylingo State Forest in southern West Virginia has experienced numerous anthropogenic wildfires over the past 36 years. In this case study, we assessed the relationship between fire frequency and recency and stand composition and structure, with emphasis on oak and its competitors. Frequent and recent fire was significantly correlated with reduced red maple overstory stem density and basal area. Overstory oak density did not significantly vary with either fire frequency or recency. Total overstory basal area was greatest in areas of either no fire or nonrecent fire. Oak sapling density was significantly greater with high frequency and recent fire. Red maple sapling densities were greatest when fires were infrequent and recent, and red maple seedlings were greatest in no fire and low-frequency nonrecent fire areas. Our results suggest that recurring fire can enhance the development of large oak advanced reproduction. However, frequent fires without a sufficient fire-free interval could prevent the recruitment of oaks into the overstory.

A foundation tree at the precipice:Tsuga canadensishealth after the arrival ofAdelges tsugaein central New England

Hemlock (Tsuga canadensis) plays a unique role in Eastern forests, producing distinctive biogeochemical, habitat, and microclimatic conditions and yet has begun a potentially irreversible decline due to the invasive hemlock woolly adelgid (Adelges tsugae; HWA) that causes foliar damage, crown loss, and mortality of host trees. Understanding the regional, landscape, site, and stand factors influencing HWA spread and impact is critical for predicting future landscape dynamics and directing effective management. Using aerial photographs, we documented hemlock distribution throughout central Massachusetts and subsampled 123 stands to examine the spatial pattern of HWA and its impact on tree vigor and mortality since its arrival in 1989. In the study region, over 86,000 ha of hemlock forest were mapped in 5,127 stands. White pine (Pinus strobus), red oak (Quercus rubra), red maple (Acer rubrum), and black birch (Betula lenta) were common overstory associates. Hemlock abundance increased from south to north, commonly on western and northwestern slopes. Average stand size was 55 ha, overstory basal area ranged from 23 to 55 m2ha−1and overstory stem densities averaged 993 ha−1.By 2004, 40% of sampled stands were infested, but most stands remained in good health overall; only 8 stands contained high HWA densities and only two had lost >50% overstory hemlock. Out of fifteen stand and landscape predictor variables examined, only latitude and winter climate variables were related to HWA density. Cold temperatures appear to be slowing the spread and impact of HWA at its northern extent as HWA infestation intensity and hemlock mortality and vigor were significantly correlated with average minimum winter temperature. Contrary to predictions, there was no regional increase in hemlock harvesting. The results suggest that regional HWA‐hemlock dynamics are currently being shaped more by climate than by a combination of landscape and social factors. The persistence and migration of HWA continues to pose a significant threat regionally, especially in the northern portion of the study area, where hemlock dominates many forests.

Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range

Abstract Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km2 of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height > 2 m and diameter at breast height 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P = 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P = 0.0001), conifer establishment (P = 0.0001), and cattle (Bos spp.) grazi...

Overstory and understory development in thinned and underplanted Oregon Coast Range Douglas-fir stands

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests managed for timber in western Oregon frequently lack structure and diversity associated with old-growth forests. We examined thinning effects on overstory and understory development for 8 years after treatment. Three 30- to 33-year-old Oregon Coast Range plantations were partitioned into four overstory treatments: unthinned (~550 trees/ha) and lightly (~250 trees/ha), moderately (~150 trees/ha), and heavily (~75 trees/ha) thinned. Within each overstory treatment, two understory treatments were established: underplanted with Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.) or not underplanted. Thinning increased overstory stem growth, crown expansion, and retained crown length. Thinned overstory canopies began to close rapidly the third year after thinning, decreasing % skylight by approximately 2%/year, whereas % skylight in unthinned stands increased slightly. All seedlings planted in unthinned stands died, whereas eighth year survival in thinned stands averaged 88%. Natural regeneration densities and distributions were highly variable. Understory shrub cover was reduced by harvesting disturbance but recovered by the fifth year. Thinning increased understory plant species diversity, and no shrub species were lost. Thinning to low densities and underplanting has the potential to accelerate development of multilayered stands characteristic of old-growth Douglas-fir forests.

Stand structure over 9 years in burned and fire-excluded oak stands on the Cumberland Plateau, Kentucky

Upland oak-dominated forests in the central Appalachian region are being managed increasingly with prescribed fire, ostensibly to reduce midstory stem density of fire-sensitive species and to promote oak regeneration, creating the need to document the effectiveness of fire alone for accomplishing these objectives. On three ridges in the Daniel Boone National Forest, Kentucky, changes in stand structure and tree regeneration were examined from 1995 to 2003 in oak-dominated stands where fire-sensitive red maple ( Acer rubrum L.) and eastern white pine ( Pinus strobus L.) were gaining dominance. Fire-excluded and burn treatments were applied to the three ridges, such that each ridge contained three units receiving one of the following treatments: fire-excluded, burned 2×, or burned 3×. Burning reduced midstory (2–10 cm DBH) stem density by 91% and midstory basal area by 86%, whereas on fire-excluded units, midstory stem density decreased 24% and basal area 28%. Burning reduced overstory (≥10 cm DBH) stem density 30% but did not significantly affect overstory basal area. Overstory stem density on fire-excluded units did not change but overstory basal area decreased 10%. Repeated burning reduced midstory red maple stem density 94%, compared to fire-excluded units, which experienced a 39% decrease in midstory red maple stem density and an 85% increase in overstory red maple stem density and basal area. Burning reduced midstory oak ( Quercus spp.) stem density 82%, while on fire-excluded units, a decline in midstory oak stem density was not significant. Overstory oak stem density decreased 25% on both fire-excluded and burned units. Damaged and dead stems on burned units responded by sprouting vigorously, with red maple being the most prolific. The number of basal sprouts/stem increased with each additional fire and the basal sprout density (sprouts/ha) was highest after the third fire. Burning also affected tree regeneration by eliminating eastern white pine stems in the sapling layer (≥50 cm height and <2 cm DBH). Red maple stem density in the sapling-layer was highest on the 3×-burned units primarily due to basal sprouting. Seedling-layer (<50 cm height) oak and flowering dogwood stems were also highest on units burned 3×. Multiple prescribed fires successfully reduced midstory densities of fire-sensitive species such as red maple, but also negatively affected oaks. Additional silvicultural methods to augment prescribed fire, or changing the timing and/or intensity of burning may be necessary for restoring oak-dominated stands.

Effects of Salmon-Borne Nutrients on Riparian Soils and Vegetation in Southwest Alaska

Spawning Pacific salmon (Oncorhynchus spp.) contribute marine-derived nutrients to riparian ecosystems, potentially affecting characteristics of the associated soils and vegetation. We quantified these effects by comparing soil and vegetative characteristics upstream and downstream of natural migratory barriers on ten spawning streams in southwest Alaska. Mean δ15N values—indicative of salmon-borne nutrients—were significantly higher in the O horizon and surface mineral soils downstream of barriers (near spawning reaches) than in soils upstream of barriers (near non-spawning reaches). However, the mean total N concentration in surface mineral soil was lower downstream than upstream. Mean foliar δ15N values were higher downstream for three plant species (Picea glauca, Salix alaxensis and Arctagrostis latifolia) with contrasting physiognomies. Mean overstory stem density was 100% higher downstream, primarily due to a fivefold difference in the density of large-diameter willows (Salix spp.). Mean understory stem density was 47% lower downstream, also driven by a difference in willow density. Mean ground layer non-vascular and dwarf shrub species covers were 28% and 73% lower downstream, respectively. Of the ten soil and vegetative characteristics that differed upstream to downstream, two (O horizon and Picea glauca δ15N) were correlated with the density of spawning salmon. Collectively, the data suggest that salmon-borne nutrients alter riparian soils and vegetation, while factors unrelated to salmon are responsible for the ultimate expression of many community characteristics.

Shadow allometry: Estimating tree structural parameters using hyperspatial image analysis

We present a novel approach to generating regional scale aboveground biomass estimates for tree species of the Lake Tahoe Basin using hyperspatial (< 1 m 2 ground resolution) remote sensing imagery. Tree crown shadows were identified and delineated as individual polygons. The area of shadowed vegetation for each tree was related to two tree structural parameters, diameter-at-breast height (DBH) and crown area. We found we could detect DBH and crown area with reasonable accuracy (field measured to image derived cross correlation results were 0.67 and 0.77 for DBH and crown area, respectively). Furthermore, the counts of the delineated polygons in a region generated overstory stem densities validated to manually photointerpreted stem densities (photointerpreted vs. image-derived stem densities correlation was 0.87). We demonstrate with accurate classification maps and allometric equations relating DBH or crown area to biomass, that these crown-level parameters can be used to generate regional scale biomass estimates without the signal saturation common to coarse-scale optical and RADAR sensors.

Influence of Inundation Potential and Forest Overstory on the Ground-layer Vegetation of Allegheny Plateau Riparian Forests

In the nonglaciated Allegheny Plateau of northwestern Pennsylvania ground-layer plant communities of headwater riparian forests are among the most species-rich of the region. We examined the influence of inundation potential (geomorphic surfaces with high, moderate or low probability of seasonal inundation) and forest overstory (stems ≥2.5 cm dbh) characteristics (percent overstory cover, overstory basal area, overstory stem density and basal area of eastern hemlock, Tsuga canadensis) on species richness, biomass and cover of the summer ground-layer (all vascular plants ≤1 m tall) at six riparian sites. The study design used inundation as a class variable with sites or portions of sites within each class as replicates (n = 4 for each inundation class). Mean total species richness (7.3 ± 1.3 se species/0.25 m2) and biomass (51.0 ± 12.4 se gm dry weight/0.25 m2), forb species richness (4.3 ± 0.8 se species/0.25 m2) and biomass (9.3 ± 4.5 se gm dry weight/0.25 m2) and graminoid species richness (2.4 ± 0.3 se species/0.25 m2) and biomass (40.3 ± 8.7 se gm dry weight/0.25 m2) were significantly greater for geomorphic surfaces of high inundation potential than for those of moderate and low inundation potential. Overstory cover, overstory stem density and basal area of T. canadensis were not correlated with ground-layer species richness, biomass and cover within inundation classes. Wetland species (obligate facultative wetland species), chiefly forbs and graminoids, occurred most often on geomorphic surfaces of high inundation potential (66.7% of total species for which wetland indicator status has been determined); upland species (facultative facultative upland upland species), mainly tree seedlings, occurred principally on geomorphic surfaces of moderate and low inundation potential (81.5% and 74.3% respectively, of total species for which wetland indicator status has been determined). Differences in species composition, richness, biomass and cover suggest that the ground-layer of Allegheny Plateau riparian forests is composed of a series of wetland and upland species guilds that replace each other along the stream to mesic forest gradient.

Effects of Long-Term Military Training Traffic on Forest Vegetation in Central Minnesota

Abstract We studied vegetation on traveled and untraveled loamy sands on military training land in central Minnesota to identify traffic-induced differences in species composition, cover, diversity, growth rates, and tree condition. Long-term military traffic resulted in a 23% reduction in overstory basal area, a 14% reduction in canopy cover, a 43% reduction in overstory stem density, and a 20% reduction in overstory species diversity compared with adjacent untraveled areas. Most of the overstory basal area reduction was accounted for by a 78% reduction in quaking aspen basal area. Crown dieback was not significantly affected by traffic. Midstory tree and shrub density was 70% lower on traveled areas. Herbaceous vegetation was denser and more diverse in traveled areas. Traffic caused a six-fold increase in bare soil area on sloping soils, but had no effect on bare soil area on level sites. Our data suggest that moderate, long-term traffic on loamy sand soils results in a relatively stable system which can be thought of as in equilibrium between traffic, natural regenerative forces, and management activities. We caution managers that quaking aspen may decline under traffic, and remind them of the risk of erosion on traveled slopes. North. J. Appl. For. 13(4): 157-163.

Community changes in a deciduous forest exposed to air pollution

Vegetation was measured in seven stands on similar sites in a 50-km portion of the upper Ohio River Valley. These stands lay along gradients of chronic exposure to airborne chloride (Cl−), sulfur dioxide (SO2), fluorides (F−), and possibly other pollutants. Species richness, evenness, and Shannon diversity index were generally depressed within the overstory, subcanopy, and herb strata near industrial sources of air pollutants. A significant relationship between coefficient of community and combined air pollutant index (relative exposure of stands to Cl−, F−, and SO2) showed that similarity in species composition decreased along a gradient of increasing air pollutant exposure. Increasing air pollutant exposure reduced overstory stem density, but abundance of vegetation in other strata tended to increase along the same gradient. The relative importance of Acersaccharum Marsh. was greatly reduced in all strata with increasing pollutant exposure, whereas Aesculusoctandra Marsh. appeared tolerant of air pollutant stress. In understory strata, the importance of Linderabenzoin (L.) Blume increased with increasing pollutant exposure.