Changes In Understory Vegetation Research Articles

Drought-induced mortality of a foundation species (Juniperus monosperma) promotes positive afterlife effects in understory vegetation

Climate change-induced droughts have contributed to large-scale die-offs of dominant tree species throughout much of the southwestern United States. These mortality events provide ecologists with the opportunity to determine whether afterlife effects associated with the die-off occur and the potential implications for future ecosystem changes. We studied both the afterlife and interaction effects of condition (dead trees, living trees, and open areas) on understory vegetation in a Juniperus monosperma woodland of northern Arizona 7 years after a major mortality event. Five major findings resulted: (1) there was a positive afterlife effect on understory plants, in which vegetation under dead junipers contained almost double the amount of cover; (2) the competitive effect on understory plants was exemplified by a 1.3 times greater cover and 1.6 additional species in open areas compared to under living junipers; (3) plant community composition significantly differed by aspect and condition; (4) the highly invasive cheatgrass (Bromus tectorum) was 1.5 times greater under dead junipers compared to live junipers; and (5) litter depth and light availability were negatively and positively correlated with plant cover, respectively, but weakly correlated with afterlife effects. Our results indicate that mortality events can promote changes in understory vegetation through afterlife effects. In ecosystems where foundation species suffer high rates of mortality, changes in plant population dynamics and ecosystem function may promote an altered trajectory in community composition with the potential to increase the presence of invasive species. Continued species die-offs associated with climate change-induced drought may contribute to an increased occurrence and legacy of afterlife effects.

Understory vegetation response to thinning disturbance of varying complexity in coniferous stands

AbstractQuestion: Can augmented forest stand complexity increase understory vegetation richness and cover and accelerate the development of late‐successional features? Does within‐stand understory vegetation variability increase after imposing treatments that increase stand structural complexity of the overstory? What is the relative contribution of individual stand structural components (i.e. forest matrix, gaps, and leave island reserves) to changes in understory vegetation richness?Location: Seven study sites in the Coastal Range and Cascades regions of Oregon, USA.Methods: We examined the effects of thinning six years after harvest on understory plant vascular richness and cover in 40‐ to 60‐year‐old forest stands dominated by Douglas‐fir (Pseudotsuga menziesii). At each site, one unthinned control was preserved and three thinning treatments were implemented: low complexity (LC, 300 trees ha−1), moderate complexity (MC, 200 trees ha−1), and high complexity (HC, variable densities from 100 to 300 trees ha−1). Gaps openings and leave island reserves were established in MC and HC.Results: Richness of all herbs, forest herbs, early seral herbs and shrubs, and introduced species increased in all thinning treatments, although early seral herbs and introduced species remained a small component. Only cover of early seral herbs and shrubs increased in all thinning treatments whereas forest shrub cover increased in MC and HC. In the understory, we found 284 vascular plant species. After accounting for site‐level differences, the richness of understory communities in thinned stands differed from those in control stands. Within‐treatment variability of herb and shrub richness was reduced by thinning. Matrix areas and gap openings in thinned treatments appeared to contribute to the recruitment of early seral herbs and shrubs.Conclusions: Understory vegetation richness increased 6 years after imposing treatments, with increasing stand complexity mainly because of the recruitment of early seral and forest herbs, and both low and tall shrubs. Changes in stand density did not likely lead to competitive species exclusion. The abundance of potentially invasive introduced species was much lower compared to other plant groups. Post‐thinning reductions in within‐treatment variability was caused by greater abundance of early seral herbs and shrubs in thinned stands compared with the control. Gaps and low‐density forest matrix areas created as part of spatially variably thinning had greater overall species richness. Increased overstory variability encouraged development of multiple layers of understory vegetation.

Changes in Understory Vegetation and Soil Characteristics Following Silvicultural Activities in a Southeastern Mixed Pine Forest1

A silvicultural chronosequence was studied in upland pine stands of Fort Benning, Georgia, to assess understory vegetation and soil characteristics following silvicultural disturbance activities. Hypotheses regarding patterns of understory vegetation distribution and abundance, and the impact of disturbance on soil properties were evaluated in 32 forest stands. The chronosequence encompassed various times following clear-cut regeneration: stand age (0–3 yrs), (8–10 yrs), (18–20 yrs), and (30–80 yrs). Soil pH, total carbon (C) and nitrogen (N) contents, soil texture, and bulk density were used to characterize soil conditions across the chronosequence. Foliar cover by species was used to characterize vegetation across this same chronosequence. Canonical correspondence analysis (CCA) was performed to determine the relationship between understory vegetation pattern and measured soil gradients and stand age. CCA identified stand age as the most important factor influencing distribution and abundance of understory vegetation. Herbaceous species composition and cover varied more with stand age than did understory woody species. Aside from a decrease in bulk density soil variables did not vary with recovery time. Indicator analysis identified Gaylussacia mosieri (Small) and Carya spp. as the only significant woody indicators of age class. Cyperus croceus (Vahl) and Bulbostylis barbata (Rottb.) C. B. Clarke were identified as herbaceous indicators of the 0–3 age class. Andropogon virginicus (L.), Dichanthelium sp. and Sporobolus junceus (Beauv.) Kunth were significant indicators of the 8–10 year age class. Significant indicators of the 15–20 year class were Pityopsis sp. and Tridens flavus (L.) A. S. Hitchc. Andropogon ternarius (Michx.), Schizachyrium scoparium (Michx.) Nash, Desmodium sp., Hieracium sp., and Rhynchosia tomentosa (L.) Hook. & Arn. were indicators of 30–80 year age class. Major changes in understory vegetation cover and composition continued for at least 15–20 years post clear-cut regeneration.

CONTROLS ON VEGETATION STRUCTURE IN SOUTHWESTERN PONDEROSA PINE FORESTS, 1941 AND 2004

Long-term studies can broaden our ecological understanding and are particularly important when examining contingent effects that involve changes to dominance by long-lived species. Such a change occurred during the last century in Southwestern (USA) ponderosa pine (Pinus ponderosa) forests. We used five livestock grazing exclosures established in 1912 to quantify vegetation structure in 1941 and 2004. Our objectives were to (1) assess the effects of historical livestock grazing on overstory structure and age distribution, (2) assess the effects of recent livestock grazing and overstory on understory vegetation, and (3) quantify and explain changes in understory vegetation between 1941 and 2004. In 1941, canopy cover of tree regeneration was significantly higher inside exclosures. In 2004, total tree canopy cover was twice as high, density was three times higher, trees were smaller, and total basal area was 40% higher inside exclosures. Understory species density, herbaceous plant density, and herbaceous cover were negatively correlated with overstory vegetation in both years. Most understory variables did not differ between grazing treatments in 1941 but were lower inside exclosures in 2004. Differences between grazing treatments disappeared once overstory effects were accounted for, indicating that they were due to the differential overstory response to historical livestock grazing practices. Between 1941 and 2004, species density declined by 34%, herbaceous plant density by 37%, shrub cover by 69%, total herbaceous cover by 59%, graminoid cover by 39%, and forb cover by 82%. However, these variables did not differ between grazing treatments or years once overstory effects were accounted for, indicating that the declines were driven by the increased dominance of the overstory during this period. Our results demonstrate that historical livestock grazing practices are an aspect of land-use history that can affect ecosystem development. Grazing history must be considered when extrapolating results from one site to another. In addition, the understory vegetation was more strongly controlled by the ponderosa pine overstory than by recent livestock grazing or by temporal dynamics, indicating that overstory effects must be accounted for when examining understory responses in this ecosystem.

Nesting Ecology of Boreal Forest Birds Following a Massive Outbreak of Spruce Beetles

Abstract:We studied breeding dark‐eyed juncos (Junco hyemalis), yellow‐rumped warblers (Dendroica coronata), and spruce‐nesting birds from 1997 to 1998 among forests with different levels of spruce (Piceaspp.) mortality following an outbreak of spruce beetles (Dendroctonus rufipennis) in Alaska, USA. We identified species using live and beetle‐killed spruce for nest sites and monitored nests to determine how the outbreak influenced avian habitat selection and reproduction. We tested predictions that 1) nesting success of ground‐nesting juncos would increase with spruce mortality due to proliferation of understory vegetation available to conceal nests from predators, 2) nesting success of canopy‐nesting warblers would decrease with spruce mortality due to fewer live spruce in which to conceal nests, and 3) both species would alter nest‐site selection in response to disturbance. Juncos did not benefit from changes in understory vegetation; nesting success in highly disturbed stands (46%) was comparable to that in undisturbed habitats throughout their range. In stands with low spruce mortality, nesting success of juncos was low (5%) and corresponded with high densities of red squirrels (Tamiasciurus hudsonicus). Yellow‐rumped warblers nested exclusively in spruce, but success did not vary with spruce mortality. As disturbance increased, nesting warblers switched from selecting forest patches with high densities of live white spruce (Picea glauca) to patches with beetle‐killed spruce. Warblers also placed nests in large‐diameter live or beetle‐killed spruce, depending on which was more abundant in the stand, with no differences in nesting success. Five of the 12 other species of spruce‐nesting birds also used beetle‐killed spruce as nest sites. Because beetle‐killed spruce can remain standing for >50 years, even highly disturbed stands provide an important breeding resource for boreal forest birds. We recommend that boreal forest managers preserve uncut blocks of infested forest within managed forest landscapes and practice partial harvest of beetle‐killed spruce rather than commercial clear‐cutting of infested stands in order to sustain breeding bird populations until natural reforestation occurs. Because breeding densities do not always reflect fitness, assessing impacts of a massive natural disturbance should include measuring impacts of changes in vegetation on both reproductive success and predator‐prey dynamics.

Changes in understory vegetation over 10 years after the exclusion of sika deer browsing pressure in cool temperate deciduous forests, Central Japan

Changes in understory vegetation over 10 years after the exclusion of sika deer browsing pressure in cool temperate deciduous forests, Central Japan

Changes in understory vegetation of a ponderosa pine forest in northern Arizona 30 years after a wildfire

Wildland fires can cause shifts in understory species composition and production. Many studies have examined short-term changes in understory vegetation following a wildfire; however, very few long-term studies are available. The objective of this study was to examine changes in understory (herbaceous and shrub) species composition and production since the 1972 Rattle Burn wildfire on the Coconino National Forest near Flagstaff, Arizona. Understory species composition and production were originally sampled in 1972, 1974, and 1980 and were re-sampled during July and August of 2002 and 2003 on 30 plots in each of four sites: high severity burn, low severity burn, unburned site prescribed burned in 1977, and an unburned site. Repeated measures analysis was used to test for the effects of fire and time on understory species production. The effects of fire on understory species composition were tested using multi-response permutation procedures (MRPP). A lingering effect of the Rattle Burn wildfire on the understory plant production and composition was revealed. Severely burned sites, which had the least pine basal area, may have greater understory production as compared to unburned sites up to 30 years after a wildfire. However, species composition on all sites was altered. A significant, negative relationship was found between tree basal area and understory species composition and production for 1972 and 2003.

Changes in Understory Vegetation of a Thinned Japanese Larch (Larix leptolepis) Plantation in Yangpyeong, Korea

Photosynthetically active radiation (PAR) beneath the forest canopy, understory species richness and diversity, and biomass were measured in a Larix leptolepis plantation seven years after thinning in Yangpyeong. Four different thinning intensities (control, <TEX>$10\%,\;20\%\;and\;40\%$</TEX> stocking reduction) were applied in 1997. The current PAR values were lower than those measured four years after thinning, and PAR at the heavy thinning plots was significantly higher than that of other thinning intensities. A total of 23 species including 9 tall-trees and 14 shrubs were found for the high layer while a total of 82 species including 10 tall-trees, 29 shrubs, and 43 herbs for the low layer. Species richness and diversity generally increased with thinning intensities, and the trends were more evident for the low layer. Aboveground biomass significantly differed among thinning intensities for both shrubs and herbs. Also there was a negative correlation between biomass and the current number of stems per hectare. The current study suggested that the effects of thinning on light conditions at the forest floor, species richness and diversity and production of understory vegetation continued seven years after the treatment.

Characterizing woodland caribou habitat in sub-boreal and boreal forests

Woodland caribou ( Rangifer tarandus caribou) are sensitive to changes in understory vegetation resulting from forest harvesting and are, therefore, of special concern for foresters and habitat biologists. Effective management of this species requires reliable habitat inventories which, because of the large heterogeneous areas over which caribou range, can be costly. We used Landsat Thematic Mapper (TM) imagery and digital elevation data to identify 23 vegetative cover types across the 5100 km 2 range of the Wolverine caribou herd of northcentral British Columbia, Canada. The classification was augmented with available geographical information system (GIS) data for a total of 27 cover types. We achieved an overall accuracy of 76.7% based on known ground samples; however, accuracy varied according to cover type. Considering the size of the study area, the procedure we employed was relatively cost effective and efficient. We discuss the advantages of such an approach for wildlife-habitat studies reliant on large-scale vegetation maps.

LONG-TOED SALAMANDERS IN HARVESTED AND INTACT DOUGLAS-FIR FORESTS OF WESTERN MONTANA

There is little known about how timber harvest practices have affected ter- restrial amphibians in the northern Rocky Mountains. Especially lacking is information on the effects of revised harvest methods that fall within the framework of environmental or New Forestry. We estimated the relative abundance of a common forest amphibian, the long-toed salamander (Ambystoma macrodactylum) captured in pitfall arrays on intact, environmentally harvested, and overstory-removal harvested sites in mixed-conifer forests of western Montana. Pitfall data from 1994 through 1996 showed that previously logged sites contained significantly fewer long-toed salamanders regardless of harvest method used. The number of salamanders captured on intact sites (3.1 salamanders.(array)-1.(100 d)-') was nearly three times the number captured on logged sites (1.2 salamanders4(array)-14(100 d)-1). Habitat conditions measured in conjunction with trapping efforts indicated that lower amphibian abundance was associated with decreased numbers of large live trees. Declines in amphibian abundance occurred in the absence of changes in understory vegetation that typically occur when forest canopy is reduced. Our findings suggest that long-toed sala- manders responded to changes in the physical environment, probably increased temperatures and decreased moisture. That salamanders should respond so dramatically indicates that immediate changes in physical conditions may profoundly alter habitat quality even when other components of the habitat are unaffected.

Managing the Abundance and Diversity of Breeding Bird Populations through Manipulation of Deer Populations

Abstract: Deer densities in forests of eastern North America are thought to have significant effects on the abundance and diversity of forest birds through the role deer play in structuring forest understories. We tested the ability of deer to affect forest bird populations by monitoring the density and diversity of vegetation and birds for 9 years at eight 4‐ha sites in northern Virginia, four of which were fenced to exclude deer. Both the density and diversity of understory woody plants increased following deer exclosure. The numerical response of the shrubs to deer exclosure was significantly predicted by the soil quality (ratio of organic carbon to nitrogen) at the sites. Bird populations as a whole increased following exclosure of deer, particularly for ground and intermediate canopy species. The diversity of birds did not increase significantly following exclosure of deer, however, primarily because of replacement of species as understory vegetation proceeded through successional processes. Changes in understory vegetation accounted for most of the variability seen in the abundance and diversity of bird populations. Populations of deer in protected areas are capable of causing significant shifts in the composition and abundance of bird communities. These shifts can be reversed by increasing the density and diversity of understory vegetation, which can be brought about by reducing deer density.

A DECADE OF RECOVERY OF UNDERSTORY VEGETATION BURIED BY VOLCANIC TEPHRA FROM MOUNT ST. HELENS

We examined changes in understory vegetation under an intact forest canopy during the first decade following the deposition of tephra (aerially transported volcanic ejecta) during the 1980 eruption of Mount St. Helens, Washington State, USA. Objectives were (1) to document vegetation response to a major disturbance that has received little attention but is widespread and relatively frequent in the northwestern United States, and (2) to analyze vegetation responses in terms of characteristics of the disturbance, responses of growth forms as well as those of species, components of vegetation change, and species autecology. We used permanent plots at four study sites, representing two tephra depths (≈4.5 and 15 cm), to examine understory vegetation change in old-growth, subalpine conifer forests. The two sites at each tephra depth differed in understory vegetation and amount of snowpack at the time of disturbance. At each site, plant cover and density were measured in 100 1-m2 plots with undisturbed tephra c...

Changes in understory vegetation following prescribed fire in maritime pine forests

Secondary succession of understory communities in Pinus pinaster forests after prescribed fire is evaluated in terms of changes in cover and frequency. To have some information on density changes, relationships were established between percentage of absence and density for the major species. Frequency composition is not significantly affected by fire. However, cover composition is affected but rapidly recovers to preburn levels. Diversity is almost unaffected by fire. From the analysis of the individual species it is concluded that in general, forb and grass cover increase initially after fire. Shrub cover decreases after fire but changes in density and rate of recovery are dependent upon species' sprouting ability. Pine seedlings increase greatly after fire. It is concluded that postfire communities are easily predicted from preburn composition because of the adaptations of the major species to fire.

Phytosociological changes in understory vegetation following timber harvest in northern Minnesota

Impacts on the understory of a balsam fir (Abiesbalsamea (L.) Mill.) – paper birch (Betuiapapyrifera Marsh.) stand, caused by winter and spring full-tree logging and by winter tree-length logging which was followed by summer burning, were monitored for two growing seasons after overstory harvest. New species invaded full-tree logging sites, while compositional changes on the burned area were due primarily to the disappearance of species. Prescribed burning on the tree-length logging area prevented the quick increase in density of woody species that occurred on full-tree harvested sites. However, understory production increased on all sites, and by the end of the second season aboveground biomass was 3847, 4516, and 2604 kg/ha on the winter full-tree, spring full-tree, and winter tree-length plus burning sites, respectively, compared with 942 kg/ha on the uncut control.The major factors causing differences were prescribed burning and snow cover during harvest operations. Since our results concur with previous findings and appear predictable, resource managers can choose the technique best suited to specific objectives. However, no matter which system is used, the same core species will persist afterward, and if succession is allowed to continue, an understory will develop similar to that existing before harvest.

Changes in understory vegetation with increasing stand age in New Brunswick forests: species composition, cover, biomass, and nutrients

Understory relationships were examined in 12 jack pine and 11 mixed hardwood stands representing a 7- to 57-year age sequence in New Brunswick. All stands were of fire origin. Percentage cover and frequency of vascular species and major mosses and lichens were presented for each stand. Accumulation of shrub layer and herb layer biomass and distribution of N, P, K, Ca, and Mg were also examined. The biomass showed maximum values in the 10- to 20-year period after fire: it then decreased and stabilized. Contribution of the understory biomass to the total ecosystem biomass ranged from 71–88% on 13-and 16-year-old jack pine stands to 1–6% on the older stands. Nutrient accumulation generally showed a similar pattern to biomass, but some differences occurred owing to variation in nutrient concentration. In particular, shrub layer K increased dramatically on the two oldest jack pine stands; this was postulated to be due to increased dominance of Pteridium aquilinum on these stands. Understory contribution to the total aboveground nutrient pool (tree + understory + forest floor) ranged from 25 to 65% on the younger stands and 5 to 31% on the older stands.

Browse and Herbage in Intensively Managed Pine Plantations

When 4 cultural treatments--cultivation alone and cultivation with 3 rates of fertilization-were applied to pine (Pinus spp.) plantations on the Mississippi Gulf Coast, pine basal area on the fertilized plots more than doubled that on the untreated control by age 12. All treatments reduced total browse density and crown cover; however, plots fertilized at medium or high rates contained more browse desirable for white-tailed deer (Odocoileus virginianus) than the control and had as much crown cover from desirable species as the control. On untreated plots the most abundant browse was gallberry (Ilex glabra), an undesirable species; on treated plots it was yaupon (I. vomitoria), a desirable species. Herbage yields on treated plots were less than half that on the control. Broomsedge bluestem (Andropogon virginicus) was the principal forage species on all plots. Only common carpetgrass (Axonopus affinis) increased in yield with intensive culture. J. WILDL. MANAGE. 39(3):557-562 Southern pine forest ranges commonly provide adequate forage for yearlong grazing by cattle and afford habitat suitable for white-tailed deer. However, the question of how intensive pine culture affects multiple land uses has not been answered. This study was initiated to determine changes in understory vegetation that accompany intensive site preparation for southern pine regeneration. Results apply directly to about 3.2 million hectares of longleaf (P. palustris) -slash pine (P. elliottii) range, where bluestems (Andropogon spp.) are the principal herbaceous species, and indirectly to about 20.2 million hectares of loblolly (P. taeda)-shortleaf pine (P. echinata)-hardwood range, where extensive areas currently are being prepared for regeneration. The study was on the Harrison Experimental Forest near Gulfport, Mississippi. Soils are relatively infertile upland fine sandy loams; slope on the gently rolling land is less than five percent. The site had been stocked with second-growth longleaf pines before it was clearcut in 1959. Little bluestem (A. scoparius) was the predominant grass, but other bluestems and panicums (Panicum spp.) were common. Grassleaf goldaster (Chrysopsis graminifolia) and other composites were the most abundant forbs. Important shrubs included southern waxmyrtle (Myrica cerifera), gallberry, and yaupon.