Bird richness and composition along an agricultural gradient in New Guinea: The influence of land use,habitat heterogeneity and proximity to intact forest

Abstract We examined variation in bird species richness, abundance and guild composition along an agricultural gradient in New Guinea, and looked for any additive influence of habitat heterogeneity on these variables. The study was based on a grid of survey plots, six plots wide and 24 plots long with the long axis running from a settlement 2.4 km through active and abandoned agricultural plots towards a large area of forest. Each circular survey plot (25 m radius) was assigned to a broad habitat type, ten habitat measures taken, and birds counted for 1 h in each plot. Principal component analysis (PCA) habitat axis 1 described an axis of decreasing forest alteration (larger trees, greater tree densities, fuller canopy) that was positively correlated with distance from the settlement. Bird richness and abundance were highest at intermediate disturbance levels (plots with mid‐range axis 1 scores). Proportions of insectivores and frugivores increased with decreasing forest alteration, while proportions of nectarivores decreased. We calculated three measures of habitat heterogeneity by comparing each plot's PCA score to those of eight neighbouring plots (50–110 m away). These measures reflected how different the plot was to its neighbours, how variable the habitat was around the plot, and the degree to which the plot bordered less disturbed forest. We related these measures to plot bird variable scores independently, and to residuals following regressions of bird scores against PCA scores. Heterogeneity measures had no significant influence on abundance or richness measures, but there were greater proportions of frugivores in plots showing a given degree of habitat alteration if they bordered more pristine habitat. While we readily identified differences in bird communities along the agricultural gradient, the influences of habitat heterogeneity were not striking for birds at this fine scale.     

Long‐term effects of prairie restoration on plant community structure and native population dynamics

The key to restoring degraded grassland habitats is identifying feasible and effective techniques to reduce the negative impacts of exotic species and promote self‐sustaining native populations. It is often difficult to extend monitoring of restoration efforts to evaluate long‐term success, but doing so is essential to understanding how initial outcomes change over time. To assess how initial treatment effects persist, we revisited degraded patches of Pacific Northwest prairie habitat 6 years after experimental restoration efforts ceased. We evaluated plant community composition to determine the lasting effects of supplemental native seeding and disturbance treatments (burning, mowing, and herbicide to reduce exotic species). We tracked the persistence of seeded species and measured spread of their populations to evaluate suitability of species for restoration and the ability of the habitat to support native plant populations. We found that plots that received supplemental seeding continued to exhibit higher richness of native species than those left unseeded, and that both seeding and disturbance treatments could positively influence native species abundance over the long term. The initially observed effects of disturbance treatments on reducing exotic grass abundance had diminished, highlighting the importance of long‐term monitoring and ongoing control of exotic species. Nevertheless, these treatments significantly influenced the population trajectories of 4 out of 8 seeded native species. There was evidence of spatial advance of most seeded species. Results from extended monitoring confirm that dispersal limitation of native species and difficulties maintaining the reduction of exotic grasses continue to be major barriers to success in restoration of invaded grasslands.     

Experimental reduction of pollinator visitation modifies plant–plant interactions for pollination

The strength of interactions between plants for pollination depends on the abundance of plants and pollinators in the community. The abundance of pollinators may influence plant associations and densities at which individual fitness is maximized. Reduced pollinator visitation may therefore affect the way plant species interact for pollination. We experimentally reduced pollinator visitation to six pollinator‐dependent species (three from an alpine and three from a lowland community in Norway) to study how interactions for pollination were modified by reduced pollinator availability. We related flower visitation, pollen limitation and seed set to density of conspecifics and pollinator‐sharing heterospecifics inside 30 dome‐shaped cages partially covered with fishnet (experimental plots) and in 30 control plots. We expected to find stronger interactions between plants in experimental compared to controls plots. The experiment modified plant–plant interactions for pollination in all the six species; although for two of them neighbourhood interactions did not affect seed set. The pollen limitation and seed set data showed that reduction of pollinator visits most frequently resulted in novel and/or stronger interactions between plants in the experimental plots that did not occur in the controls. Although the responses were species‐specific, there was a tendency for increasing facilitative interactions with conspecific neighbours in experimental plots where pollinator availability was reduced. Heterospecifics only influenced pollination and fecundity in species from the alpine community and in the experimental plots, where they competed with the focal species for pollination. The patterns observed for visitation rates differed from those for fecundity, with more significant interactions between plants in the controls in both communities. This study warns against the exclusive use of visitation data to interpret plant–plant interactions for pollination, and helps to understand how plant aggregations may buffer or intensify the effects of a pollinator loss on plant fitness.     

Non‐native plants have greater impacts because of differing per‐capita effects and nonlinear abundance–impact curves

Invasive, non‐native species can have tremendous impacts on biotic communities, where they reduce the abundance and diversity of local species. However, it remains unclear whether impacts of non‐native species arise from their high abundance or whether each non‐native individual has a disproportionate impact – that is, a higher per‐capita effect – on co‐occurring species compared to impacts by native species. Using a long‐term study of wetlands, we asked how temporal variation in dominant native and non‐native plants impacted the abundance and richness of other plants in the recipient community. Non‐native plants reached higher abundances than natives and had greater per‐capita effects. The abundance–impact relationship between plant abundance and richness was nonlinear. Compared with increasing native abundance, increasing non‐native abundance was associated with steeper declines in richness because of greater per‐capita effects and nonlinearities in the abundance–impact relationship. Our study supports eco‐evolutionary novelty of non‐natives as a driver of their outsized impacts on communities.     

Canopy thinning,not agricultural history,determines early responses of wild bees to longleaf pine savanna restoration

Longleaf pine savannas are highly threatened, fire‐maintained ecosystems unique to the southeastern United States. Fire suppression and conversion to agriculture have strongly affected this ecosystem, altering overstory canopies, understory plant communities, and animal populations. Tree thinning to reinstate open canopies can benefit understory plant diversity, but effects on animal communities are less well understood. Moreover, agricultural land‐use legacies can have long‐lasting impacts on plant communities, but their effects on animal communities either alone or through interactions with restoration are unclear. Resolving these impacts is important due to the conservation potential of fire‐suppressed and post‐agricultural longleaf savannas. We evaluated how historical agricultural land use and canopy thinning affect the diversity and abundance of wild bees in longleaf pine savannas. We employed a replicated, large‐scale factorial block experiment in South Carolina, where canopy thinning was applied to longleaf pine savannas that were either post‐agricultural or remnant (no agricultural history). Bees were sampled using elevated bee bowls. In the second growing season after restoration, thinned plots supported a greater bee abundance and bee community richness. Additionally, restored plots had altered wild bee community composition when compared to unthinned plots, indicating that reduction of canopy cover by the thinning treatment best predicted wild bee diversity and composition. Conversely, we found little evidence for differences between sites with or without historical agricultural land use. Some abundant Lasioglossum species were the most sensitive to habitat changes. Our results highlight how restoration practices that reduce canopy cover in fire‐suppressed savannas can have rapid benefits for wild bee communities.     

Spatial variability in a plant–pollinator community across a continuous habitat: high heterogeneity in the face of apparent uniformity

Large‐scale spatial variability in plant–pollinator communities (e.g. along geographic gradients, across different landscapes) is relatively well understood. However, we know much less about how these communities vary at small scales within a uniform landscape. Plants are sessile and highly sensitive to microhabitat conditions, whereas pollinators are highly mobile and, for the most part, display generalist feeding habits. Therefore, we expect plants to show greater spatial variability than pollinators. We analysed the spatial heterogeneity of a community of flowering plants and their pollinators in 40 plots across a 40‐km² area within an uninterrupted Mediterranean scrubland. We recorded 3577 pollinator visits to 49 plant species. The pollinator community (170 species) was strongly dominated by honey bees (71.8% of the visits recorded). Flower and pollinator communities showed similar beta‐diversity, indicating that spatial variability was similar in the two groups. We used path analysis to establish the direct and indirect effects of flower community distribution and honey bee visitation rate (a measure of the use of floral resources by this species) on the spatial distribution of the pollinator community. Wild pollinator abundance was positively related to flower abundance. Wild pollinator visitation rate was negatively related to flower abundance, suggesting that floral resources were not limiting. Pollinator and flower richness were positively related. Pollinator species composition was weakly related to flower species composition, reflecting the generalist nature of flower–pollinator interactions and the opportunistic nature of pollinator flower choices. Honey bee visitation rate did not affect the distribution of the wild pollinator community. Overall, we show that, in spite of the apparent physiognomic uniformity, both flowers and pollinators display high levels of heterogeneity, resulting in a mosaic of idiosyncratic local communities. Our results provide a measure of the background of intrinsic heterogeneity within a uniform habitat, with potential consequences on low‐scale ecosystem function and microevolutionary patterns.     

Pathogen's level and parasitism rate in Ips typographus at high population densities: importance of time

We compared the levels of pathogen infection in parental beetles, parasitism of the offspring, abundance of predators and breeding performance success of univoltine populations of Ips typographus in plots characterized by short‐term (2–3 years) outbreaks vs. those with long‐term (>10 years) outbreaks on two localities at ca. 1100 m altitude in the ?umava Mts. The numbers of I. typographus were high in all plots, whether the plots were characterized by long‐term outbreaks or short‐term outbreaks. The numbers of maternal galleries in the sample areas ranged from 300 to 400 per m². The density of parental beetle galleries, abundance of surviving specimens of I. typographus, and length of maternal galleries did not differ between plots. The short‐term outbreaks had only fewer ectoparasitoids of I. typographus and a lower percentage of parasitism and infection level of Mattesia schwenkei than the long‐term outbreaks even though the maternal gallery densities and beetle production were the same. The most mortality appeared to be caused by intraspecific larval competition, and the identical reproductive success in plots with short‐term and long‐term outbreaks indicates that the negative feedback resulting from parasitoids and entomopathogens does not substantially reduce beetle numbers. Although entomopathogenic fungi as Beauveria bassiana occur naturally in the galleries of spruce bark beetles, there was no evidence of its presence in the studied population. The low levels of predation and/or parasitism in both kinds of plots indicate that natural enemies did not play a significant role in reducing outbreak numbers of I. typographus.     

Intra- and Interannual Vegetation Change: Implications for Long-Term Research

To draw reliable conclusions from forest restoration experiments, it is important that long‐term measurements be repeatable or year‐to‐year variability may interfere with the correct interpretation of treatment effects. We used permanent plots in a long‐term restoration study in southwestern Colorado to measure herbaceous and shrub vegetation at three dates within a single year (June, July, and August), and between years (2003 and 2005), on untreated control plots in a warm, dry mixed conifer forest. Growing season precipitation patterns were similar between 2003 and 2005, so differences in vegetation should be related primarily to differences in the sampling month. Significant indicator species for each sampling month were present within a single year (2005), primarily reflecting early‐season annuals. We found no significant differences for total species abundance (2005). Species richness, abundance, and indicator species were significantly different between years for different sampling months indicating that sampling should be conducted within a similar time frame to avoid detecting differences that are not due to treatment effects or variations in year‐to‐year climate. These findings have implications for long‐term research studies where the objectives are to detect changes over time in response to treatments, climate variation, and natural processes. Long‐term sampling should occur within a similar phenological time frame each year over a short amount of time and should be based on the following criteria: (1) the sampling period is congruent with research objectives such as detecting rare species or peak understory abundance and (2) the sampling period is feasible in regard to personnel and financial constraints.     

Shifts in priming partly explain impacts of long‐term nitrogen input in different chemical forms on soil organic carbon storage

Input of labile organic carbon can enhance decomposition of extant soil organic carbon (SOC) through priming. We hypothesized that long‐term nitrogen (N) input in different chemical forms alters SOC pools by altering priming effects associated with N‐mediated changes in plants and soil microbes. The hypothesis was tested by integrating field experimental data of plants, soil microbes and two incubation experiments with soils that had experienced 10 years of N enrichment with three chemical forms (ammonium, nitrate and both ammonium and nitrate) in an alpine meadow on the Tibetan Plateau. Incubations with glucose–¹³C addition at three rates were used to quantify effects of exogenous organic carbon input on the priming of SOC. Incubations with microbial inocula extracted from soils that had experienced different long‐term N treatments were conducted to detect effects of N‐mediated changes in soil microbes on priming effects. We found strong evidence and a mechanistic explanation for alteration of SOC pools following 10 years of N enrichment with different chemical forms. We detected significant negative priming effects both in soils collected from ammonium‐addition plots and in sterilized soils inoculated with soil microbes extracted from ammonium‐addition plots. In contrast, significant positive priming effects were found both in soils collected from nitrate‐addition plots and in sterilized soils inoculated with soil microbes extracted from nitrate‐addition plots. Meanwhile, the abundance and richness of graminoids were higher and the abundance of soil microbes was lower in ammonium‐addition than in nitrate‐addition plots. Our findings provide evidence that shifts toward higher graminoid abundance and changes in soil microbial abundance mediated by N chemical forms are key drivers for priming effects and SOC pool changes, thereby linking human interference with the N cycle to climate change.     

Environmental heterogeneity and biotic interactions mediate climate impacts on tropical forest regeneration

Predicting the fate of tropical forests under a changing climate requires understanding species responses to climatic variability and extremes. Seedlings may be particularly vulnerable to climatic stress given low stored resources and undeveloped roots; they also portend the potential effects of climate change on future forest composition. Here we use data for ca. 50,000 tropical seedlings representing 25 woody species to assess (i) the effects of interannual variation in rainfall and solar radiation between 2007 and 2016 on seedling survival over 9 years in a subtropical forest; and (ii) how spatial heterogeneity in three environmental factors—soil moisture, understory light, and conspecific neighborhood density—modulate these responses. Community‐wide seedling survival was not sensitive to interannual rainfall variability but interspecific variation in these responses was large, overwhelming the average community response. In contrast, community‐wide responses to solar radiation were predominantly positive. Spatial heterogeneity in soil moisture and conspecific density were the predominant and most consistent drivers of seedling survival, with the majority of species exhibiting greater survival at low conspecific densities and positive or nonlinear responses to soil moisture. This environmental heterogeneity modulated impacts of rainfall and solar radiation. Negative conspecific effects were amplified during rainy years and at dry sites, whereas the positive effects of radiation on survival were more pronounced for seedlings existing at high understory light levels. These results demonstrate that environmental heterogeneity is not only the main driver of seedling survival in this forest but also plays a central role in buffering or exacerbating impacts of climate fluctuations on forest regeneration. Since seedlings represent a key bottleneck in the demographic cycle of trees, efforts to predict the long‐term effects of a changing climate on tropical forests must take into account this environmental heterogeneity and how its effects on regeneration dynamics play out in long‐term stand dynamics.     

Long‐term mammal and nocturnal bird trends are influenced by vegetation type,weather and climate in temperate woodlands

Many studies have documented the individual effects of variables such as vegetation, long‐term climate and short‐term weather on biodiversity. Few, however, have explicitly explored how interactions among these major drivers can influence species abundance. We used data from a 15‐year study (2002–2017) in the endangered temperate woodlands of south‐eastern Australia to test hypotheses associated with the effects of vegetation type, long‐term climate and short‐term weather on population trajectories of seven species of (largely) nocturnal mammals and birds. Despite prolonged drought conditions, there was a significant increase in the abundance of some species over time (e.g. the Eastern Grey Kangaroo). It is possible that destocking of domestic livestock may have reduced competition with Kangaroos, thereby facilitating increases in abundance. The Common Brushtail Possum and Common Ringtail Possum were significantly less likely to occur in replanted woodlands, possibly because of the paucity of nesting sites. We found no evidence that replanted woodlands are refuges for exotic pest species like the European Rabbit and Red Fox. Short‐ and long‐term rainfall and vegetation type had important independent and combined effects on animal abundance. That is, responses to periods of high short‐term rainfall were dependent on vegetation type and whether sites occurred in long‐term climatically wet versus climatically dry locations. For example, the Red Fox responded positively to high levels of short‐term rainfall, but only at climatically dry sites. Our results highlight the complementary value of different vegetation types across the landscape and the context‐specific responses of animals to short‐term fluctuations in moisture availability. They also underscore the value of long‐term monitoring at a landscape scale for examining how multiple interacting factors influence trends in animal abundance.     

Hidden effects of forest management practices: responses of a soil stored seed bank to logging and repeated prescribed fire

Forest management practices have the potential to impact upon native vegetation. Most studies focus on the effects of management on the above‐ground vegetation communities, with little attention given to the soil stored seed bank. Here we examine the soil stored seed bank of a long‐term experimental site in south‐eastern Australia, which has experienced timber harvesting and repeated prescribed burning over a 20‐year period. At each of 213 long‐term vegetation measurement plots, 3.5 kg of soil was collected and germinated in a glasshouse over a period of 2 years. Comparisons were made between the experimental treatments considering differences in species richness, abundance and community composition of the understorey seed bank. Logged sites had a higher diversity and abundance of seedlings compared with unlogged sites, which is consistent with observed changes in standing vegetation within 10 years following logging. Prescribed burning resulted in a lower diversity and abundance of seedlings, which contrasts with the increase in species diversity observed in response to frequent fire in standing vegetation. Individual taxa that declined in the seed bank in response to frequent fire were all taxa for which germination is enhanced by exposure to smoke. Contrary to expectations, these did not exhibit a corresponding decline as standing plants. While management actions above ground are having minor impacts, greater effects were seen in the soil stored seed bank.     

The effects of livestock grazing on biodiversity are multi‐trophic: a meta‐analysis

Anthropogenic disturbance has generated a significant loss of biodiversity worldwide and grazing by domestic herbivores is a contributing disturbance. Although the effects of grazing on plants are commonly explored, here we address the potential multi‐trophic effects on animal biodiversity (e.g. herbivores, pollinators and predators). We conducted a meta‐analysis on 109 independent studies that tested the response of animals or plants to livestock grazing relative to livestock excluded. Across all animals, livestock exclusion increased abundance and diversity, but these effects were greatest for trophic levels directly dependent on plants, such as herbivores and pollinators. Detritivores were the only trophic level whose abundance decreased with livestock exclusion. We also found that the number of years since livestock was excluded influenced the community and that the effects of grazer exclusion on animal diversity were strongest in temperate climates. These findings synthesise the effects of livestock grazing beyond plants and demonstrate the indirect impacts of livestock grazing on multiple trophic levels in the animal community. We identified the potentially long‐term impacts that livestock grazing can have on lower trophic levels and consequences for biological conservation. We also highlight the potentially inevitable cost to global biodiversity from livestock grazing that must be balanced against socio‐economic benefits.     

Do local conspecific density and floral display size influence fruit set via pollinator visitation in Orchis militaris?

Plant density varies naturally, from isolated plants to clumped individuals, and this can influence pollinator foraging behaviour and plant reproductive success. In addition, the effect of conspecific density on reproduction may depend on the pollination system, and deceptive species differ from rewarding ones in this regard, a high density being often associated with low fruit set in deceptive plants. In our study, we aimed to determine how local conspecific density and floral display size (i.e. number of flowers per plant) affect fruit set in a deceptive orchid (Orchis militaris) through changes in pollinator visitation. We measured fruit set in a natural population and recorded pollinator abundance and foraging behaviour within plots of different O. militaris densities. Detailed data were recorded for the most abundant potential pollinators of O. militaris, i.e. solitary bees. Floral display size was negatively correlated to fruit set in medium‐density plots, but uncorrelated in low‐ and high‐density plots. Plot density had no effect on solitary bee abundance and visitation, which may be due to low pollinator abundance within the study site. The proportion of visited flowers per inflorescence was negatively influenced by floral display size, which is in line with previous studies. In addition, solitary bees spent decreasing time in successive flowers within an inflorescence, and the time spent per flower was negatively affected by ambient temperature. Our results suggest that pollinator behaviour during visitation is poorly linked to pollen deposition and reproductive success in O. militaris.     

Grassland responses to increased rainfall depend on the timescale of forcing

Forecasting impacts of future climate change is an important challenge to biologists, both for understanding the consequences of different emissions trajectories and for developing adaptation measures that will minimize biodiversity loss. Existing variation provides a window into the effects of climate on species and ecosystems, but in many places does not encompass the levels or timeframes of forcing expected under directional climatic change. Experiments help us to fill in these uncertainties, simulating directional shifts to examine outcomes of new levels and sustained changes in conditions. Here, we explore the translation between short‐term responses to climate variability and longer‐term trajectories that emerge under directional climatic change. In a decade‐long experiment, we compare effects of short‐term and long‐term forcings across three trophic levels in grassland plots subjected to natural and experimental variation in precipitation. For some biological responses (plant productivity), responses to long‐term extension of the rainy season were consistent with short‐term responses, while for others (plant species richness, abundance of invertebrate herbivores and predators), there was pronounced divergence of long‐term trajectories from short‐term responses. These differences between biological responses mean that sustained directional changes in climate can restructure ecological relationships characterizing a system. Importantly, a positive relationship between plant diversity and productivity turned negative under one scenario of climate change, with a similar change in the relationship between plant productivity and consumer biomass. Inferences from experiments such as this form an important part of wider efforts to understand the complexities of climate change responses.     

Long‐term persistence of wildlife populations in a pastoral area

Facilitating coexistence between people and wildlife is a major conservation challenge in East Africa. Some conservation models aim to balance the needs of people and wildlife, but the effectiveness of these models is rarely assessed. Using a case‐study approach, we assessed the ecological performance of a pastoral area in northern Tanzania (Manyara Ranch) and established a long‐term wildlife population monitoring program (carried out intermittently from 2003 to 2008 and regularly from 2011 to 2019) embedded in a distance sampling framework. By comparing density estimates of the road transect‐based long‐term monitoring to estimates derived from systematically distributed transects, we found that the bias associated with nonrandom placement of transects was nonsignificant. Overall, cattle and sheep and goat reached the greatest densities and several wildlife species occurred at densities similar (zebra, wildebeest, waterbuck, Kirk's dik‐dik) or possibly even greater (giraffe, eland, lesser kudu, Grant's gazelle, Thomson's gazelle) than in adjacent national parks in the same ecosystem. Generalized linear mixed models suggested that most wildlife species (8 out of 14) reached greatest densities during the dry season, that wildlife population densities either remained constant or increased over the 17‐year period, and that herbivorous livestock species remained constant, while domestic dog population decreased over time. Cross‐species correlations did not provide evidence for interference competition between grazing or mixed livestock species and wildlife species but indicate possible negative relationships between domestic dog and warthog populations. Overall, wildlife and livestock populations in Manyara Ranch appear to coexist over the 17‐year span. Most likely, this is facilitated by existing connectivity to adjacent protected areas, effective anti‐poaching efforts, spatio‐temporal grazing restrictions, favorable environmental conditions of the ranch, and spatial heterogeneity of surface water and habitats. This long‐term case study illustrates the potential of rangelands to simultaneously support wildlife conservation and human livelihood goals if livestock grazing is restricted in space, time, and numbers.     

Rainfall during parental care reduces reproductive and survival components of fitness in a passerine bird

Adverse weather conditions during parental care may have direct consequences for offspring production, but longer‐term effects on juvenile and parental survival are less well known. We used long‐term data on reproductive output, recruitment, and parental survival in northern wheatears (Oenanthe oenanthe) to investigate the effects of rainfall during parental care on fledging success, recruitment success (juvenile survival), and parental survival, and how these effects related to nestling age, breeding time, habitat quality, and parental nest visitation rates. While accounting for effects of temperature, fledging success was negatively related to rainfall (days > 10 mm) in the second half of the nestling period, with the magnitude of this effect being greater for breeding attempts early in the season. Recruitment success was, however, more sensitive to the number of rain days in the first half of the nestling period. Rainfall effects on parental survival differed between the sexes; males were more sensitive to rain during the nestling period than females. We demonstrate a probable mechanism driving the rainfall effects on reproductive output: Parental nest visitation rates decline with increasing amounts of daily rainfall, with this effect becoming stronger after consecutive rain days. Our study shows that rain during the nestling stage not only relates to fledging success but also has longer‐term effects on recruitment and subsequent parental survival. Thus, if we want to understand or predict population responses to future climate change, we need to consider the potential impacts of changing rainfall patterns in addition to temperature, and how these will affect target species' vital rates.     

Increasing Liana Abundance and Basal Area in a Tropical Forest: The Contribution of Long‐distance Clonal Colonization

Recent evidence suggests that liana abundance and biomass are increasing in Neotropical forests, representing a major structural change to tropical ecosystems. Explanations for these increases, however, remain largely untested. Over an 8‐yr period (1999–2007), we censused lianas in nine, 24 × 36 m permanent plots in old‐growth and selectively logged forest at La Selva Biological Station, Costa Rica to test whether: (1) liana abundance and basal area are increasing in this forest; (2) the increase is being driven by increased recruitment, decreased mortality, or both; and (3) long‐distance clonal colonization explains the increase in liana abundance and basal area. We defined long‐distance clonal colonization as lianas that entered and rooted in the plots as vegetative propagules of stems that originated from outside or above the plot, and were present in 2007, but not in 1999 or 2002. Our hypotheses were supported in the old‐growth forest: mean liana abundance and BA (≥1 cm diameter) increased 15 and 20 percent, respectively, and clonal colonization from outside of the plots contributed 19 and 60 percent (respectively) to these increases. Lianas colonized clonally by falling vertically from the forest canopy above or growing horizontally along the forest floor and re‐rooting—common forms of colonization for many liana species. In the selectively logged forest, liana abundance and BA did not change, and thus the pattern of increasing lianas may be restricted to old‐growth forests. In summary, our data support the hypothesis that lianas are increasing in old‐growth forests, and that long‐distance clonal colonization is a major contributor.     

Interactive effects of local and landscape factors on farmland carabids

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Quantifying variation in forest disturbance,and its effects on aboveground biomass dynamics,across the eastern United States

The role of tree mortality in the global carbon balance is complicated by strong spatial and temporal heterogeneity that arises from the stochastic nature of carbon loss through disturbance. Characterizing spatio‐temporal variation in mortality (including disturbance) and its effects on forest and carbon dynamics is thus essential to understanding the current global forest carbon sink, and to predicting how it will change in future. We analyzed forest inventory data from the eastern United States to estimate plot‐level variation in mortality (relative to a long‐term background rate for individual trees) for nine distinct forest regions. Disturbances that produced at least a fourfold increase in tree mortality over an approximately 5 year interval were observed in 1–5% of plots in each forest region. The frequency of disturbance was lowest in the northeast, and increased southwards along the Atlantic and Gulf coasts as fire and hurricane disturbances became progressively more common. Across the central and northern parts of the region, natural disturbances appeared to reflect a diffuse combination of wind, insects, disease, and ice storms. By linking estimated covariation in tree growth and mortality over time with a data‐constrained forest dynamics model, we simulated the implications of stochastic variation in mortality for long‐term aboveground biomass changes across the eastern United States. A geographic gradient in disturbance frequency induced notable differences in biomass dynamics between the least‐ and most‐disturbed regions, with variation in mortality causing the latter to undergo considerably stronger fluctuations in aboveground stand biomass over time. Moreover, regional simulations showed that a given long‐term increase in mean mortality rates would support greater aboveground biomass when expressed through disturbance effects compared with background mortality, particularly for early‐successional species. The effects of increased tree mortality on carbon stocks and forest composition may thus depend partly on whether future mortality increases are chronic or episodic in nature.