Paying the Extinction Debt in Southern Wisconsin Forest Understories

Abstract: The lack of long‐term baseline data restricts the ability to measure changes in biological diversity directly and to determine its cause. This hampers conservation efforts and limits testing of basic tenets of ecology and conservation biology. We used a historical baseline survey to track shifts in the abundance and distribution of 296 native understory species across 82 sites over 55 years in the fragmented forests of southern Wisconsin. We resurveyed stands first surveyed in the early 1950s to evaluate the influence of patch size and surrounding land cover on shifts in native plant richness and heterogeneity and to evaluate changes in the relative importance of local site conditions versus the surrounding landscape context as drivers of community composition and structure. Larger forests and those with more surrounding forest cover lost fewer species, were more likely to recruit new species, and had lower rates of homogenization than smaller forests in more fragmented landscapes. Nearby urbanization further reduced both alpha and beta understory diversity. Similarly, understory composition depended strongly on local site conditions in the original survey but only weakly reflected the surrounding landscape composition. By 2005, however, the relative importance of these factors had reversed such that the surrounding landscape structure is now a much better predictor of understory composition than are local site conditions. Collectively, these results strongly support the idea that larger intact habitat patches and landscapes better sustain native species diversity and demonstrate that humans play an increasingly important role in driving patterns of native species diversity and community composition.     

Effects of an Exotic Plant Invasion on Native Understory Plants in a Tropical Dry Forest

Abstract: The dry forests of southern India, which are endangered tropical ecosystems and among the world's most important tiger (Panthera tigris) habitats, are extensively invaded by exotic plants. Yet, experimental studies exploring the impacts of these invasions on native plants in these forests are scarce. Consequently, little is known about associated implications for the long‐term conservation of tigers and other biodiversity in these habitats. I studied the impacts of the exotic plant Lantana camara on understory vegetation in a dry‐forest tiger habitat in southern India. I compared the richness, composition, and abundance of tree seedlings, herbs, and shrubs and the abundance of grass among plots in which Lantana was cleared or left standing. These plots were distributed across two blocks—livestock free and livestock grazed. Removal of Lantana had an immediate positive effect on herb–shrub richness in the livestock‐free block, but had no effect on that of tree seedlings in either livestock block. Tree‐seedling and herb–shrub composition differed significantly between Lantana treatment and livestock block, and Lantana removal significantly decreased survival of tree seedlings. Nevertheless, the absence of trees, in any stage between seedling and adult, indicates that Lantana may stall tree regeneration. Lantana removal decreased the abundance of all understory strata, probably because forage plants beneath Lantana are less accessible to herbivores, and plants in Lantana‐free open plots experienced greater herbivory. Reduced access to forage in invaded habitats could negatively affect ungulate populations and ultimately compromise the ability of these forests to sustain prey‐dependent large carnivores. Additional research focused on understanding and mitigating threats posed by exotic plants may be crucial to the long‐term protection of these forests as viable tiger habitats.     

Changes in Avian and Plant Communities of Aspen Woodlands over 12 Years after Livestock Removal in the Northwestern Great Basin

Abstract: Riparian and quaking aspen (Populus tremuloides) woodlands are centers of avian abundance and diversity in the western United States, but they have been affected adversely by land use practices, particularly livestock grazing. In 1990, cattle were removed from a 112,500‐ha national wildlife refuge in southeastern Oregon. Thereafter, we monitored changes in vegetation and bird abundance in years 1–3 (phase 1) and 10–12 (phase 2) in 17 riparian and 9 snow‐pocket aspen plots. On each 1.5‐ha plot, we sampled vegetation in 6 transects. Three times during each breeding season, observers recorded all birds 50 m to each side of the plot's 150‐m centerline for 25 minutes. We analyzed data with multivariate analysis of variance and paired t tests with p values adjusted for multiple comparisons. In both periods, riparian and snow‐pocket aspen produced extensive regeneration of new shoots ( stems/ha and 7079 stems/ha, respectively). By phase 2, a 64% increase in medium‐diameter trees in riparian stands indicated successful recruitment into the overstory, but this pattern was not seen in snow‐pocket stands, where the density of trees was over 2 times greater. By phase 2 in riparian and snow‐pocket stands, native forb cover had increased by 68% and 57%, respectively, mesic shrub cover had increased by 29% and 58%, and sagebrush cover had decreased by 24% and 31%. Total avian abundance increased by 33% and 39% in riparian and snow‐pocket aspen, respectively, ground or understory nesters increased by 133% and 67% and overstory nesters increased by 34% and 33%. Similarly, ground or understory foragers increased by 25% and 32%, aerial foragers by 55% and 57%, and overstory foragers by 66% and 43%. We interpreted the substantial regeneration of aspen shoots, increased densities of riparian forbs and shrubs, and increased avian abundances as a multitrophic‐level response to the total removal of livestock and as substantial movement toward recovery of biological integrity.     

Consequences of Buffelgrass Pasture Development for Primary Productivity,Perennial Plant Richness,and Vegetation Structure in the Drylands of Sonora,Mexico

Abstract: In large parts of northern Mexico native plant communities are being converted to non‐native buffelgrass (Pennisetum ciliare) pastures, and this conversion could fundamentally alter primary productivity and species richness. In Sonora, Mexico land conversion is occurring at a regional scale along a rainfall‐driven gradient of primary productivity, across which native plant communities transition from desert scrub to thorn scrub. We used a paired sampling design to compare a satellite‐derived index of primary productivity, richness of perennial plant species, and canopy‐height profiles of native plant communities with buffelgrass pastures. We sampled species richness across a gradient of primary productivity in desert scrub and thorn scrub vegetation to examine the influence of site productivity on the outcomes of land conversion. We also examined the influence of pasture age on species richness of perennial plants. Index values of primary productivity were lower in buffelgrass pastures than in native vegetation, which suggests a reduction in primary productivity. Land conversion reduced species richness by approximately 50% at local and regional scales, reduced tree and shrub cover by 78%, and reduced canopy height. Land conversion disproportionately reduced shrub species richness, which reflects the common practice among Sonoran ranchers of conserving certain tree and cactus species. Site productivity did not affect the outcomes of land conversion. The age of a buffelgrass pasture was unrelated to species richness within the pasture, which suggests that passive recovery of species richness to preconversion levels is unlikely. Our findings demonstrate that land conversion can result in large losses of plant species richness at local and regional scales and in substantial changes to primary productivity and vegetation structure, which casts doubt on the feasibility of restoring native plant communities without active intervention on the part of land managers.     

Recovery Patterns of Understory Herbs and Their Use as Indicators of Deciduous Forest Regeneration

Abstract: Habitat fragmentation has reduced the richness of native species of forests in northeastern North America. Despite recent large-scale increases in forest cover, studies indicate that understory herbaceous plant communities may take many decades to recover. We studied recovery patterns of vegetation following up to 35 years of forest regeneration in restored former cottage and road sites at Point Pelée National Park, Ontario, Canada, to assess the vulnerability of the understory herbaceous species. Overall, there were no significant differences in the diversity of native species between restored and relatively undisturbed reference sites. There was, however, significant among-site variation in the composition of the native species component of these plant communities. When only restored sites were examined, variation in native species composition was associated with time since site restoration, soil moisture, canopy cover, and distance to continuous forest. Native species were assigned vulnerability rankings according to their relative occurrence in reference and restored sites. Spring-flowering herbs, with ant- or gravity-dispersed seeds, were absent from restored sites and were defined as highly vulnerable. In contrast, summer- and fall-flowering herbs, with vertebrate- and wind-dispersed seeds, dominated restored sites and were less vulnerable. Species of low and intermediate vulnerability had colonized restored sites successfully, and the latter should function as indicators of recovery. In contrast, species with high vulnerability rankings had not recovered at all and, because of their limited dispersal ranges, may recolonize restored sites only if they are actively reintroduced.     

Effects of Artificial Sand Fixing on Community Characteristics of a Rare Desert Shrub

Eremosparton songoricum (Fabaceae) is a rare, native, clonal small shrub of the deserts of central Asia. Although human activities have greatly fragmented the distribution of E. songoricum, it occurs in areas where artificial sand fixing (AS) has been implemented. We sought to explore whether AS promotes survival and growth of E. songoricum. In the Gurbantunggut Desert of northwestern China in June 2010, we established 10 plots in an area where sand fixing occurred (5–10 years previously) and 11 plots on original sand substrate on which some plants had settled without fixing sand. Sand fixing changed soil properties and biological characteristics in sand‐fixed plots. The soil surface where sand fixing occurred was covered by algal crusts and some lichen, but not bare sand (BS). Soil nutrients; water content of deep soil (30–150 cm); overall plant and herbaceous species richness, diversity, abundance, and cover; above‐ and belowground biomass; and cover, biomass, and height of E. songoricum in the sand‐fixed plots were significantly greater than in plots of BS. However, distribution of E. songoricum individuals in the 2 types of plots did not differ. Our results indicate AS may enhance survival of E. songoricum and increase the overall diversity and stability of the desert plant community. We suggest AS as a way to protect this rare desert plant in situ. Efectos de la Fijación Artificial de Arena sobre las Características de la Comunidad de un Arbusto Desértico Raro     

Thinning and burning result in low-level invasion by nonnative plants but neutral effects on natives.

Many historically fire-adapted forests are now highly susceptible to damage from insects, pathogens, and stand-replacing fires. As a result, managers are employing treatments to reduce fuel loadings and to restore the structure, species, and processes that characterized these forests prior to widespread fire suppression, logging, and grazing. However, the consequences of these activities for understory plant communities are not well understood. We examined the effects of thinning and prescribed fire on plant composition and diversity in Pinus ponderosa forests of eastern Washington (USA). Data on abundance and richness of native and nonnative plants were collected in 70 stands in the Colville, Okanogan, and Wenatchee National Forests. Stands represented one of four treatments: thinning, burning, thinning followed by burning, or control; treatments had been conducted 3-19 years before sampling. Multi-response permutation procedures revealed no significant effect of thinning or burning on understory plant composition. Similarly, there were no significant differences among treatments in cover or richness of native plants. In contrast, nonnative plants showed small, but highly significant, increases in cover and richness in response to both thinning and burning. In the combined treatment, cover of nonnative plants averaged 2% (5% of total plant cover) but did not exceed 7% (16% of total cover) at any site. Cover and richness of nonnative herbs showed small increases with intensity of disturbance and time since treatment. Nonnative plants were significantly less abundant in treated stands than on adjacent roadsides or skid trails, and cover within these potential source areas explained little of the variation in abundance within treated stands. Although thinning and burning may promote invasion of nonnative plants in these forests, our data suggest that their abundance is limited and relatively stable on most sites.     

Changes in Arthropod Assemblages along a Wide Gradient of Disturbance in Gabon

Abstract: Searching for indicator taxa representative of diverse assemblages, such as arthropods, is an important objective of many conservation studies. We evaluated the impacts of a wide gradient of disturbance in Gabon on a range of arthropod assemblages representing different feeding guilds. We examined 4 × 10⁵ arthropod individuals from which 21 focal taxa were separated into 1534 morphospecies. Replication included the understory of 3 sites in each of 4 different stages of forest succession and land use (i.e., habitats) after logging (old and young forests, savanna, and gardens). We used 3 complementary sampling methods to survey sites throughout the year. Overall differences in arthropod abundance and diversity were greatest between forest and open habitats, and cleared forest invaded by savanna had the lowest abundance and diversity. The magnitude of faunal differences was much smaller between old and young forests. When considered at this local scale, anthropogenic modification of habitats did not result in a monotonous decline of diversity because many herbivore pests and their associated predators and parasitoids were abundant and diverse in gardens, where plant productivity was kept artificially high year‐round through watering and crop rotation. We used a variety of response variables to measure the strength of correlations across survey locations among focal taxa. These could be ranked as follows in terms of decreasing number of significant correlations: species turnover > abundance > observed species richness > estimated species richness > percentage of site‐specific species. The number of significant correlations was generally low and apparently unrelated to taxonomy or guild structure. Our results emphasize the value of reporting species turnover in conservation studies, as opposed to simply measuring species richness, and that the search for indicator taxa is elusive in the tropics. One promising alternative might be to consider “predictor sets” of a small number of taxa representative of different functional groups, as identified in our study.     

Associations of Forest Cover,Fragment Area,and Connectivity with Neotropical Understory Bird Species Richness and Abundance

Theoretical and empirical studies demonstrate that the total amount of forest and the size and connectivity of fragments have nonlinear effects on species survival. We tested how habitat amount and configuration affect understory bird species richness and abundance. We used mist nets (almost 34,000 net hours) to sample birds in 53 Atlantic Forest fragments in southeastern Brazil. Fragments were distributed among 3 10,800‐ha landscapes. The remaining forest in these landscapes was below (10% forest cover), similar to (30%), and above (50%) the theoretical fragmentation threshold (approximately 30%) below which the effects of fragmentation should be intensified. Species‐richness estimates were significantly higher (F= 3715, p = 0.00) where 50% of the forest remained, which suggests a species occurrence threshold of 30–50% forest, which is higher than usually occurs (<30%). Relations between forest cover and species richness differed depending on species sensitivity to forest conversion and fragmentation. For less sensitive species, species richness decreased as forest cover increased, whereas for highly sensitive species the opposite occurred. For sensitive species, species richness and the amount of forest cover were positively related, particularly when forest cover was 30–50%. Fragment size and connectivity were related to species richness and abundance in all landscapes, not just below the 30% threshold. Where 10% of the forest remained, fragment size was more related to species richness and abundance than connectivity. However, the relation between connectivity and species richness and abundance was stronger where 30% of the landscape was forested. Where 50% of the landscape was forested, fragment size and connectivity were both related to species richness and abundance. Our results demonstrated a rapid loss of species at relatively high levels of forest cover (30–50%). Highly sensitive species were 3‐4 times more common above the 30–50% threshold than below it; however, our results do not support a unique fragmentation threshold. Asociaciones de la Cobertura Forestal, Superficie del Fragmento y Conectividad con la Riqueza y Abundancia de Aves Neotropicales de Sotobosque     

Effects of Earthworm Invasion on Plant Species Richness in Northern Hardwood Forests

Abstract:  The invasion of non-native earthworms ( Lumbricus spp.) into a small number of intensively studied stands of northern hardwood forest has been linked to declines in plant diversity and the local extirpation of one threatened species. It is unknown, however, whether these changes have occurred across larger regions of hardwood forests, which plant species are most vulnerable, or with which earthworm species such changes are associated most closely. To address these issues we conducted a regional survey in the Chippewa and Chequamegon national forests in Minnesota and Wisconsin (U.S.A.), respectively. We sampled earthworms, soils, and vegetation, examined deer browse in 20 mature, sugar-maple-dominated forest stands in each national forest, and analyzed the relationship between invasive earthworms and vascular plant species richness and composition. Invasion by Lumbricus was a strong indicator of reduced plant richness in both national forests. The mass of Lumbricus juveniles was significantly and negatively related to plant-species richness in both forests. In addition, Lumbricus was a significant factor affecting plant richness in a full model that included multiple variables. In the Chequamegon National Forest earthworm mass was associated with higher sedge cover and lower cover of sugar maple seedlings and several forb species. The trends were similar but not as pronounced in Chippewa, perhaps due to lower deer densities and different earthworm species composition. Our results provide regional evidence that invasion by Lumbricus species may be an important mechanism in reduced plant-species richness and changes in plant communities in mature forests dominated by sugar maples.     

Providing Habitat for Native Mammals through Understory Enhancement in Forestry Plantations

The Convention on Biological Diversity (CBD) expects forestry plantations to contribute to biodiversity conservation. A well‐developed understory in forestry plantations might serve as a surrogate habitat for native species and mitigate the negative effect of plantations on species richness. We experimentally tested this hypothesis by removing the understory in Monterey pine (Pinus radiata) plantations in central Chile and assessing changes in species richness and abundance of medium‐sized mammals. Frequency of occurrence of mammals, including kodkods (Leopardus guigna), culpeo foxes (Pseudalopex culpaeus), lesser grisons (Conepatus chinga), and Southern pudu deer (Pudu puda), was low in forest stands with little to no understory relative to stands with well‐developed undergrowth vegetation. After removing the understory, their frequency of occurrence decreased significantly, whereas in control stands, where understory was not removed, their frequency did not change. This result strongly supports the idea that facilitating the development of undergrowth vegetation may turn forestry stands into secondary habitats as opposed to their containing no habitat for native mammals. This forestry practice could contribute to conservation of biological diversity as it pertains to CBD targets. Proporcionando Hábitat para Mamíferos Nativos Mediante el Mejoramiento del Sotobosque en Plantaciones Forestales     

Incorporating differences between genetic diversity of trees and herbaceous plants in conservation strategies

Reviews that summarize the genetic diversity of plant species in relation to their life history and ecological traits show that forest trees have more genetic diversity at population and species levels than annuals or herbaceous perennials. In addition, among-population genetic differentiation is significantly lower in trees than in most herbaceous perennials and annuals. Possible reasons for these differences between trees and herbaceous perennials and annuals have not been discussed critically. Several traits, such as high rates of outcrossing, long-distance pollen and seed dispersal, large effective population sizes (N_e), arborescent stature, low population density, longevity, overlapping generations, and occurrence in late successional communities, may make trees less sensitive to genetic bottlenecks and more resistant to habitat fragmentation or climate change. We recommend that guidelines for genetic conservation strategies be designed differently for tree species versus other types of plant species. Because most tree species fit an LH scenario (low [L] genetic differentiation and high [H] genetic diversity), tree seeds could be sourced from a few populations distributed across the species’ range. For the in situ conservation of trees, translocation is a viable option to increase N_e. In contrast, rare herbaceous understory species are frequently HL (high differentiation and low diversity) species. Under the HL scenario, seeds should be taken from many populations with high genetic diversity. In situ conservation efforts for herbaceous plants should focus on protecting habitats because the typically small populations of these species are vulnerable to the loss of genetic diversity. The robust allozyme genetic diversity databases could be used to develop conservation strategies for species lacking genetic information. As a case study of reforestation with several tree species in denuded areas on the Korean Peninsula, we recommend the selection of local genotypes as suitable sources to prevent adverse effects and to insure the successful restoration in the long term.     

Effects of landscape design of forest reserves on Saproxylic beetle diversity

Increasing the density of natural reserves in the forest landscape may provide conservation benefits for biodiversity within and beyond reserve borders. We used 2 French data sets on saproxylic beetles and landscape cover of forest reserves (LCFR) to test this hypothesis: national standardized data derived from 252 assessment plots in managed and reserve stands in 9 lowland and 5 highland forests and data from the lowland Rambouillet forest, a forested landscape where a pioneer conservation policy led to creation of a dense network of reserves. Abundance of rare and common saproxylic species and total saproxylic species richness were higher in forest reserves than in adjacent managed stands only in highland forests. In the lowland regional case study, as LCFR increased total species richness and common species abundance in reserves increased. In this case study, when there were two or more reserve patches, rare species abundance inside reserves was higher and common species richness in managed stands was higher than when there was a single large reserve. Spillover and habitat amount affected ecological processes underlying these landscape reserve effects. When LCFR positively affected species richness and abundance in reserves or managed stands, >12‐20% reserve cover led to the highest species diversity and abundance. This result is consistent with the target of 17% forested land area in reserves set at the Nagoya biodiversity summit in 2010. Therefore, to preserve biodiversity we recommend at least doubling the current proportion of forest reserves in European forested landscapes.     

Effects of productivity on biodiversity in forest ecosystems across the United States and China

In the global campaign against biodiversity loss in forest ecosystems, land managers need to know the status of forest biodiversity, but practical guidelines for conserving biodiversity in forest management are lacking. A major obstacle is the incomplete understanding of the relationship between site primary productivity and plant diversity, due to insufficient ecosystem‐wide data, especially for taxonomically and structurally diverse forest ecosystems. We investigated the effects of site productivity (the site's inherent capacity to grow timber) on tree species richness across 19 types of forest ecosystems in North America and China through 3 ground‐sourced forest inventory data sets (U.S. Forest Inventory and Analysis, Cooperative Alaska Forest Inventory, and Chinese Forest Management Planning Inventory). All forest types conformed to a consistent and highly significant (P < 0.001) hump‐shaped unimodal relationship, of which the generalized coefficients of determination averaged 20.5% over all the forest types. That is, tree species richness first increased as productivity increased at a progressively slower rate, and, after reaching a maximum, richness started to decline. Our consistent findings suggest that forests of high productivity would sustain few species because they consist mostly of flat homogeneous areas lacking an environmental gradient along which a diversity of species with different habitats can coexist. The consistency of the productivity–biodiversity relationship among the 3 data sets we examined makes it possible to quantify the expected tree species richness that a forest stand is capable of sustaining, and a comparison between the actual species richness and the sustainable values can be useful in prioritizing conservation efforts.     

Use of Multispecies Occupancy Models to Evaluate the Response of Bird Communities to Forest Degradation Associated with Logging

Forest degradation is arguably the greatest threat to biodiversity, ecosystem services, and rural livelihoods. Therefore, increasing understanding of how organisms respond to degradation is essential for management and conservation planning. We were motivated by the need for rapid and practical analytical tools to assess the influence of management and degradation on biodiversity and system state in areas subject to rapid environmental change. We compared bird community composition and size in managed (ejido, i.e., communally owned lands) and unmanaged (national park) forests in the Sierra Tarahumara region, Mexico, using multispecies occupancy models and data from a 2‐year breeding bird survey. Unmanaged sites had on average higher species occupancy and richness than managed sites. Most species were present in low numbers as indicated by lower values of detection and occupancy associated with logging‐induced degradation. Less than 10% of species had occupancy probabilities >0.5, and degradation had no positive effects on occupancy. The estimated metacommunity size of 125 exceeded previous estimates for the region, and sites with mature trees and uneven‐aged forest stand characteristics contained the highest species richness. Higher estimation uncertainty and decreases in richness and occupancy for all species, including habitat generalists, were associated with degraded young, even‐aged stands. Our findings show that multispecies occupancy methods provide tractable measures of biodiversity and system state and valuable decision support for landholders and managers. These techniques can be used to rapidly address gaps in biodiversity information, threats to biodiversity, and vulnerabilities of species of interest on a landscape level, even in degraded or fast‐changing environments. Moreover, such tools may be particularly relevant in the assessment of species richness and distribution in a wide array of habitats. Uso de Modelos de Ocupación para Múltiples Especies para Evaluar la Respuesta de las Comunidades de Aves a la Degradación de Bosques Asociada con la Tala     

Earthworm Invasion as the Driving Force Behind Plant Invasion and Community Change in Northeastern North American Forests

Abstract:  Identification of factors that drive changes in plant community structure and contribute to decline and endangerment of native plant species is essential to the development of appropriate management strategies. Introduced species are assumed to be driving causes of shifts in native plant communities, but unequivocal evidence supporting this view is frequently lacking. We measured native vegetation, non-native earthworm biomass, and leaf-litter volume in 15 forests in the presence and absence of 3 non-native plant species ( Microstegium vimineum, Alliaria petiolata, Berberis thunbergii ) to assess the general impact of non-native plant and earthworm invasions on native plant communities in northeastern United States. Non-native plant cover was positively correlated with total native plant cover and non-native earthworm biomass. Earthworm biomass was negatively associated with cover of native woody and most herbaceous plants and with litter volume. Graminoid cover was positively associated with non-native earthworm biomass and non-native plant cover. These earthworm-associated responses were detected at all sites despite differences in earthworm species and abundance, composition of the native plant community, identity of invasive plant species, and geographic region. These patterns suggest earthworm invasion, rather than non-native plant invasion, is the driving force behind changes in forest plant communities in northeastern North America, including declines in native plant species, and earthworm invasions appear to facilitate plant invasions in these forests. Thus, a focus on management of invasive plant species may be insufficient to protect northeastern forest understory species.     

元江干热河谷植物群落特征及土壤肥力研究

对云南省元江县境内元江河谷山坡上的5种主要植被类型的群落特征、植物多样性现状、植物区系组成以及不同群落土壤理化性质进行了比较研究.结果表明:干热河谷区内,植物群落生活型组成主要以小乔木、灌木和草本植物为主,基本没有高大的乔木树种和大型藤本植物.草本植物生长旺盛,物种多样性和丰富度均较乔灌层高,但均匀度较其低.5种群落之间的相似性程度较低,且各群落均处于相对稳定的状态.各群落中共记录有61个科,144个属,194个物种,其植物区系的泛热带特征比较明显,其中泛热带分布科有31个,占除去广布科以外所有科的77.5%,泛热带分布属有50个,占除去广布属以外所有属的38.2%.蝶形花科Papilionaceae、禾本科Gramineae、大戟科Euphorbiaceae、菊科Compositae、锦葵科Malvaceae等科在元江干热河谷植被中占有极为重要的地位.群落土壤理化性质的研究表明,元江干热河谷各类植被的土壤元素中,除了磷元素极其缺乏以外,大多数处于中等以上水平.土壤肥力状况与植被的生长状况之间关系紧密,其中,土壤肥力最好的是群落结构比较复杂、残存的稀树旱生林.     

Shifts in southern Wisconsin forest canopy and understory richness, composition, and heterogeneity

We resurveyed the under- and overstory species composition of 94 upland forest stands in southern Wisconsin in 2002-2004 to assess shifts in canopy and understory richness, composition, and heterogeneity relative to the original surveys in 1949-1950. The canopy has shifted from mostly oaks (Quercus spp.) toward more mesic and shade-tolerant trees (primarily Acer spp.). Oak-dominated early-successional stands and those on coarse, nutrient-poor soils changed the most in canopy composition. Understories at most sites (80%) lost native species, with mean species density declining 25% at the 1-m2 scale and 23.1% at the 20-m2 scale. Woody species have increased 15% relative to herbaceous species in the understory despite declining in absolute abundance. Initial canopy composition, particularly the abundance of red oaks (Quercus rubra and Q. velutina), predicted understory changes better than the changes observed in the overstory. Overall rates of native species loss were greater in later-successional stands, a pattern driven by differential immigration rather than differential extirpation. However, understory species initially found in early-successional habitats declined the most, particularly remnant savanna taxa with narrow or thick leaves. These losses have yet to be offset by compensating increases in native shade-adapted species. Exotic species have proliferated in prevalence (from 13 to 76 stands) and relative abundance (from 1.2% to 8.4%), but these increases appear unrelated to the declines in native species richness and heterogeneity observed. Although canopy succession has clearly influenced shifts in understory composition and diversity, the magnitude of native species declines and failure to recruit more shade-adapted species suggest that other factors now act to limit the richness, heterogeneity, and composition of these communities.     

A century of increasing pine density and associated shifts in understory plant strategies

We analyzed one of the longest-term ecological data sets to evaluate how forest overstory structure is related to herbaceous understory plant strategies in a ponderosa pine forest. Eighty-two permanent 1-m2 chart quadrats that were established as early as 1912 were remeasured in 2007. We reconstructed historical forest structure using dendrochronological techniques. Ponderosa pine basal area increased from an average of 4 m2/ha in the early 1900s to 29 m2/ha in 2007. Understory plant foliar cover declined by 21%, species richness declined by two species per square meter, and functional diversity also declined. The relative cover of C4 graminoids decreased by 18% and C3 graminoids increased by 19%. Herbaceous plant species with low leaf and fine root nitrogen concentrations, low specific leaf area, high leaf dry matter content, large seed mass, low specific root length, short maximum height, and early flowering date increased in relative abundance in sites where pine basal area increased the most. Overall, we observed a long-term shift in composition toward more conservative shade- and stress-tolerant herbaceous species. Our analysis of temporal changes in plant strategies provides a general framework for evaluating compositional and functional changes in terrestrial plant communities.     

Reduced Density and Nest Survival of Ground‐Nesting Songbirds Relative to Earthworm Invasions in Northern Hardwood Forests

Abstract: European earthworms (Lumbricus spp.) are spreading into previously earthworm‐free forests in the United States and Canada and causing substantial changes, including homogenization of soil structure, removal of the litter layer, and reduction in arthropod abundance and species richness of understory plants. Whether these changes affect songbirds that nest and forage on the forest floor is unknown. In stands with and without earthworms in the Chequamegon‐Nicolet National Forest, Wisconsin (U.S.A.), we surveyed for, monitored nests of, and measured attributes of habitat of Ovenbirds (Seiurus aurocapillus) and Hermit Thrushes (Catharus guttatus), both ground‐dwelling songbirds, and we sampled earthworms at survey points and nests. Bird surveys indicated significantly lower densities of Ovenbirds and Hermit Thrushes in relation to Lumbricus invasions at survey point and stand extents (3.1 and 15–20 ha, respectively). Modeling of Ovenbird nest survival (i.e., the probability that nestlings successfully fledge) indicated that lower survival probabilities were associated with increased sedge cover and decreased litter depth, factors that are related to Lumbricus invasions, possibly due to reduced nest concealment or arthropod abundance. Our findings provide compelling evidence that earthworm invasions may be associated with local declines of forest songbird populations.