Late-seral Forests Research Articles

Old-growth forests in the Dinaric Alps of Bosnia-Herzegovina and Montenegro: a continental hot-spot for research and biodiversity

IntroductionAccording to various censuses, Europe has less than 1.5 million ha of old-growth forests (OGF). Most of them are in the boreal zone, while their presence in the temperate zone is residual and fragmented.In the framework of the EU biodiversity strategy, it has been adopted a broad definition of OGF which includes late-seral forests and forests with some management legacies. However, research purposes need to identify strictly defined OGFs characterized by structure, disturbance history, and processes typical and exclusive of the last stage of the forest dynamic.MethodsThe present paper wants to contribute to this debate by presenting a research network of four mixed (Fagus-Abies-Picea) montane OGFs in the Dinaric Alps (Lom, BiH; Janj, BiH; Perućica, BiH; Biogradska Gora, MNE), summarizing 20 years of multidisciplinary research by focusing on the structural characteristics and the disturbance history of the whole network and their coherency with strict OGF indicators. These sites were selected in relatively structurally uniform study areas, where 142 permanent plots have been established since 2002.Results and discussionThe study sites have a high living (747–1,201 m3 ha−1) and coarse woody debris (CWD) biomass (304–410 m3 ha−1), resulting in the highest forest carbon sink at the continental level (398–484 Mg C ha−1). The presence of large and old trees is one of the critical characteristics of the old-growth stage: in Lom and Perućica, there are 19 trees and 14 ha−1 larger than 1 m at breast height, respectively, and 14 trees and 15 trees ha−1 older than 400 years. In the last three centuries, continuous small-scale disturbances have driven forest dynamics, developing stands characterized by gap-phase dynamics and quasi-equilibrium structure. The Dinaric OGF network presents robust indicators of old-growthness, similar structural characteristics, and dynamic processes across all four sites. Identifying this sub-set of OGF using strict criteria is critical for recognizing conservation priorities and for quantifying, along an old-growthness chronosequence, the current structural differences of managed or recently abandoned forests. Besides, only OGF selected with rigorous criteria can act as a reliable reference for ecological restoration and sustainable forest management as a benchmark for carbon sink and for quantifying the impact of climate change on forests.

Direct and Indirect Effects of Overstory Canopy and Sex-Biased Density Dependence on Reproduction in the Dioecious Shrub Shepherdia canadensis (Elaeagnaceae)

This study assessed the reproductive success of a temperate dioecious shrub, Canada buffaloberry, Shepherdia canadensis (L.) Nutt., in central Alberta, Canada, by examining the effects of spatial patterns and overstory canopy on flower and fruit production. S. canadensis is more abundant and productive (more fruit) at forest edges and in forest gaps, suggesting a dependence on higher light conditions than is typical of late-seral forests. We used path analysis to demonstrate that flower and fruit production exhibited density-dependent effects at a scale of 50 m2 around focal female plants. Fruit production was positively affected by male intraspecific density (pollen supply) and negatively affected by female intraspecific density (pollen competition), but not correlated with overall intraspecific density. The effects of sex-differentiated density are partly due to pollinator responses to male plant density. Flower production was positively affected by overall intraspecific density. A pollen supplementation trial doubled fruit production relative to a control, demonstrating that local male density (pollen availability) and pollinator activity can limit fruit production in S. canadensis. Canopy cover was negatively related to both flower and total fruit production, with approximately one-third (34%) of the total effect of canopy on fruit production due to the effect of canopy on flower production. The commonly observed negative association between canopy cover and fruit production in buffaloberry, therefore, is partly a result of the reduction first in flower number and second in fruit set. This study clarifies the mechanisms associated with the often-noted observation, but not previously assessed at the level of individuals, that reproductive output in S. canadensis is density dependent, limited by canopy cover through reductions in both flowering and fruit set, and pollinator limited. These findings hold implications for managing animal species that depend on the fruit of S. canadensis and suggest future directions for research on dioecious and actinorhizal species.

Multi-scale habitat selection by Northern Goshawks (Accipiter gentilis) in a fire-prone forest

Increasing frequency and severity of wildfire may jeopardize persistence of large tracts of late seral forest, raising concerns over population viability of forest-dependent species like the Northern Goshawk (Accipiter gentilis). We tracked 20 adult Northern Goshawks with GPS loggers over 4 years to investigate roosting (nocturnal) and foraging (diurnal) habitat selection in a heterogeneously burned forest landscape of the Sierra Nevada mountains of California, United States. Goshawks selected late seral forest attributes for both roosting and foraging at multiple spatio-temporal scales, although at the finest (daily) scale, goshawks selected more diverse forest structure that included small trees and medium canopy cover. Less than 6% of roosts were in areas burned in the last 50 years and goshawks avoided areas burned at high severity when roosting and when foraging across spatial scales. Four goshawks (3 males, 1 female) undertook forays >5 km from their nest location, two of which forayed into burned areas during at least one season. High severity fire is likely to make forests unsuitable foraging or roosting habitat for Northern Goshawks, although lower severity fire may provide foraging opportunities for this generalist predator. Eighty percent of foraging space use and 87% of roost locations were considered high fire hazard potential, suggesting that goshawk habitat in western North America is likely to be reduced by predicted increases in fire frequency and severity in the region.

Corralling a black swan: natural range of variation in a forest landscape driven by rare, extreme events.

The natural range of variation (NRV) is an important reference for ecosystem management, but has been scarcely quantified for forest landscapes driven by infrequent, severe disturbances. Extreme events such as large, stand-replacing wildfires at multi-century intervals are typical for these regimes; however, data on their characteristics are inherently scarce, and, for land management, these events are commonly considered too large and unpredictable to integrate into planning efforts (the proverbial "Black Swan"). Here, we estimate the NRV of late-seral (mature/old-growth) and early-seral (post-disturbance, pre-canopy-closure) conditions in a forest landscape driven by episodic, large, stand-replacing wildfires: the Western Cascade Range of Washington, USA (2.7million ha). These two seral stages are focal points for conservation and restoration objectives in many regions. Using a state-and-transition simulation approach incorporating uncertainty, we assess the degree to which NRV estimates differ under a broad range of literature-derived inputs regarding (1) overall fire rotations and (2) how fire area is distributed through time, as relatively frequent smaller events (less episodic), or fewer but larger events (more episodic). All combinations of literature-derived fire rotations and temporal distributions (i.e., "scenarios") indicate that the largest wildfire events (or episodes) burned up to 105 -106 ha. Under most scenarios, wildfire dynamics produced 5th-95th percentile ranges for late-seral forests of ~47-90% of the region (median 70%), with structurally complex early-seral conditions composing ~1-30% (median 6%). Fire rotation was the main determinant of NRV, but temporal distribution was also important, with more episodic (temporally clustered) fire yielding wider NRV. In smaller landscapes (20,000ha; typical of conservation reserves and management districts), ranges were 0-100% because fires commonly exceeded the landscape size. Current conditions are outside the estimated NRV, with the majority of the region instead covered by dense mid-seral forests (i.e., a regional landscape with no historical analog). Broad consistency in NRV estimates among widely varied fire regime parameters suggests these ranges are likely relevant even under changing climatic conditions, both historical and future. These results indicate management-relevant NRV estimates can be derived for seral stages of interest in extreme-event landscapes, even when incorporating inherent uncertainties in disturbance regimes.

Characterizing Habitat Elements and Their Distribution over Several Spatial Scales: The Case of the Fisher

In past studies of the fisher (Pekania pennanti) most researchers have concluded that fisher habitat must consist of mostly mature to late-seral forest with few, if any, openings. Without doubt, certain elements found in mature to late-seral forests are required by females to successfully rear their young, but some recent work casts doubt on the extent that a continuous canopy of tree coverage and a preponderance of older stands are necessary as long as certain components exist. This paper explores this issue with an attempt to better characterize essential elements of habitat for the female fisher. This characterization is based upon fine-scale inventory plot data that is analyzed across several spatial scales that represent a small neighborhood about den sites, the forest of the 75% kernel density estimate for female home ranges, and the forested region as a whole. We present results of a test of significance in comparing habitat elements across these three scales. Our findings suggest that certain habitat elements typically found in mature to late seral forests must be present at a certain fraction of the landscape for the fisher. The approach described here may be of considerable value in developing guidelines for conservation agreements.

The legacy of disturbance on individual tree and stand-level aboveground biomass accumulation and stocks in primary mountain Picea abies forests

Disturbances, both natural and human induced, influence forest dynamics, ecosystem functioning, and ecosystem services. Here, we aim to evaluate the consequences of natural disturbances on the magnitude and dynamics of tree- and stand-level biomass accumulation from decadal to centennial scales. We use tree-ring data from 2301 trees and biometric data from 4909 trees sampled in 96 plots (each 1000m2) to quantify the influence of mixed severity disturbance regimes on annual aboveground biomass increment (AGBI) and total aboveground biomass accumulation (AGB) across a mountainous monotypic Norway spruce (Picea abies (L.) Karst.) primary forest. We hypothesise that the multiple internal and external factors constraining tree growth will cause differences in tree and stand-level biomass trajectories in these natural forests.Although we found that tree-level AGB growth increases with tree size, we also found that tree age and disturbance legacies plays a crucial role for AGB in the investigated Norway spruce forests. Importantly, while younger trees of the same diameter class have an average current AGBI rate that is ∼225% higher than older trees (300–400years), we find trees that have been suppressed for up to 120years can respond vigorously when competition is reduced. On average, post disturbance AGBI was ∼400% greater than pre-disturbance AGBI. Growth of suppressed trees, independent of their age, followed similar trajectories after canopy accession. While aboveground biomass generally increased through time, the time since disturbance and disturbance severity are important co-predictors for stand-level AGBI and AGB. These forests regained most of the above ground living biomass over short interval (∼50years) after low intensity disturbances. The highest stand-level living AGB was observed on plots that experienced >40% canopy removal 160–190years ago, whereas the highest AGBI occurred in plots disturbed recently within the past 40–50years.Our results emphasize the importance of including both individual tree age and disturbance legacies to accurately characterize biomass dynamics and trajectories in forest ecosystems. Importantly, the period of time that a tree is in the canopy, and not tree age, modulates the trajectory of tree level AGBI. Growth rates begin to decline after ∼30years (tree-rings width) and ∼100years (AGBI) in the canopy. We demonstrate that even late-seral forests can rapidly regain biomass lost to low intensity disturbance.

Landscape heterogeneity compensates for fuel reduction treatment effects on Northern flying squirrel populations

In the dry forests of the western United States frequent fires historically maintained a diversity of habitats in multiple seral stages. Over the past century, fire suppression and preferential harvest of large trees has led to a densification and homogenization of forests, making them more prone to larger and more severe wildfires. In response, fuel reduction treatments have become common practice in the management of dry western forests. However, the effect of fuel reduction treatments on late seral forest species, such as the Northern flying squirrels, remains a management concern.We captured and marked Northern flying squirrels within mixed conifer forest in the Stanislaus–Tuolumne Experimental Forest (California) on a continuous trapping grid (∼1400 traps) spanning a 120-ha study landscape in which 24 4-ha units were subject to different fuel reduction treatments (variable thin, even thin, and control, all with or without prescribed burning). The study spanned two pre-thinning and three post-thinning years. We divided the study landscape into three blocks (two with treatments, one control only). For each block we analyzed data with spatial capture–recapture models to estimate Northern flying squirrel density, and tested whether canopy closure before and after thinning and percent area burned were important predictors of density.Northern flying squirrel densities varied from 0.168 (SE 0.086) to 0.808 (SE 0.094) individuals/ha across blocks and years. Densities varied by year, independent of treatments. Percent area burned was not an important predictor of density. The effect of canopy closure was variable, but more consistently positive after thinning reduced overall canopy closure. When considered by treatment type, densities were highest in control and burn only units, and lowest in thinned units.Whereas thinning had negative effects on Northern flying squirrel density on the scale of a thinning treatment unit, our results suggest that these effects were largely absorbed by the heterogeneous landscape, as animals shifted their distribution into un-thinned areas without a decline in overall density. This highlights the need to incorporate the landscape context when evaluating the effects of forest management on wildlife.

Climate indices strongly influence old-growth forest carbon exchange

We present a decade and a half (1998–2013) of carbon dioxide fluxes from an old-growth stand in the American Pacific Northwest to identify ecosystem-level responses to Pacific teleconnection patterns, including the El Niño/Southern Oscillation (ENSO). This study provides the longest, continuous record of old-growth eddy flux data to date from one of the longest running Fluxnet stations in the world. From 1998 to 2013, average annual net ecosystem exchange (FNEE) at Wind River AmeriFlux was −32 ± 84 g C m−2 yr−1 indicating that the late seral forest is on average a small net sink of atmospheric carbon. However, interannual variability is high (>300 g C m−2 yr−1) and shows that the stand switches from net carbon sink to source in response to climate drivers associated with ENSO. The old-growth forest is a much stronger sink during La Niña years (mean FNEE = −90 g C m−2 yr−1) than during El Niño when the stand turns carbon neutral or into a small net carbon source (mean FNEE = +17 g C m−2 yr−1). Forest inventory data dating back to the 1930s show a similar correlation with the lower frequency Pacific North American (PNA) and Pacific Decadal Oscillation (PDO) whereby higher aboveground net primary productivity (FANPP) is associated with cool phases of both the PNA and PDO. These measurements add evidence that carbon exchange in old-growth stands may be more sensitive to climate variability across shorter time scales than once thought.

Can partial‐cut harvesting be used to manage terrestrial lichen habitat? A review of recent evidence

Recent research suggests that partial-cut harvesting techniques can be used to alter successional trajectories in pine- and spruce-lichen woodlands, allowing forest managers to extend the period of reindeer lichen growth in mid- to late seral boreal forest stands. In Quebec, a fully replicated partial-cutting trial found that terrestrial lichen abundance remained at least as high in the partial cut as in the clearcuts or unlogged stands, and that the partial cut appeared to be on a trajectory to have even more terrestrial lichen due to sustained higher growth rates. In Alberta, a retrospective study found higher terrestrial lichen abundance in an early horse-logged partial cut than in undisturbed adjacent old forests or in clearcuts. Follow-up studies of partial-cut harvesting trials in British Columbia found that group selection plots 10 years after harvesting had lichen cover equivalent to that of undisturbed forest. In contrast, studies on lichen woodlands that have been defoliated by mountain pine beetle showed a major decline in reindeer lichen cover and a corresponding increase in vascular plant cover, similar to the results of previous studies on clear-cut logging impacts. Taken together these studies provide qualified support for the hypothesis that partial-cut harvesting can be used to enhance, or at least maintain, terrestrial lichen mats used as forage by caribou.

Barred owls and landscape attributes influence territory occupancy of northern spotted owls.

We used multi-season occupancy analyses to model 2 fates of northern spotted owl territories in relation to habitat amount, habitat fragmentation, and the presence of barred owls in Washington State, USA, 1989–2005. Local colonization is the probability a territory unoccupied by a spotted owl in year i would be occupied in year i + 1, and local extinction is the probability a territory that was occupied by a spotted owl in year i would be unoccupied in year i + 1. We found a negative relationship between local extinction probability and amount of late-seral forest edge. We found a negative relationship between colonization probability and the number of late-seral forest patches (higher fragmentation), and a negative relationship between colonization probability and the amount of non-habitat within 600 m of a spotted owl territory center (Akaike weight = 0.59). The presence of barred owls was positively related to extinction probability and negatively related to detection probability of spotted owls. The negative relationship between presence of barred owls and detectability of spotted owls indicated that spotted owls could be modifying their calling behavior in the presence of barred owls. The positive relationship between barred owl detections and local extinction probability suggests that because of competition with barred owls, spotted owls are being displaced. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Biological associates of early-seral pre-forest in the Pacific Northwest

Traditionally overlooked by foresters as unproductive and ecologists as disorganized, naturally regenerating forests in the Pacific Northwest (PNW) are perhaps the least understood forest condition in the region. More recently, concerns over the rarity of this forest condition have sparked interest in identifying ecological characteristics unique to forested sites after a canopy-opening disturbance and before the re-establishment of a closed conifer canopy. Here we review the literature to identify the plant and animal associates of early-seral pre-forests in the PNW in order to provide baseline information pertaining to the recognition and conservation role of early-seral pre-forest ecosystems. We describe a number of bird, mammal, insect, amphibian and reptile species associated with PNW early-seral pre-forests either by empirical observation or inferred through life-history characteristics in an attempt to formally identify unique species indicators of naturally regenerating pre-forest communities. For Washington, Oregon, and northern California, we also review the state lists of endangered, threatened, monitored or otherwise conservation-listed species (664 unique species or subspecies for the combined region) to assess the proportion of protected species that rely on the structural attributes of early-seral pre-forests. Here, we found that these proportions are comparable to the proportions reliant on mature or late-seral forest. In addition, greater than 50% of all listed species for each of the three regions were partial or facultative users of early-seral pre-forest ecosystems. This assessment suggests that naturally structured early-seral pre-forests in the PNW provide key habitat for many species, including obligates and near-obligates, and that future research should seek to refine our understanding of the specific structural and compositional attributes that form the basis of these associations.

Avian community responses to restoration efforts in a complex volcanic landscape

Monitoring of restoration efforts has focused primarily on the effects of restoration activities for the vegetation component of ecosystems, but restoration efforts should consider other wildlife groups to fully assess ecosystems recovery. We assessed differences in avian community responses among managed (restoration and reforestation sites), disturbed (abandoned cropfields and volcanic ash deposits), and natural (undisturbed and late-seral forests) habitats within a volcanic landscape with a complex disturbance history. Our results indicate that bird communities vary among managed habitats, as a consequence of both their disturbance history and current management strategy. Data indicate that restoration activities promote more diverse, even, and heterogeneous (both taxonomic and functional) bird communities that resemble those of natural habitats. Although we found that reforestation activities could enhance diversity and evenness of bird communities, planting only tree species as a management strategy might not be sufficient to achieve bird communities similar to those of natural habitats. Thus, we encourage restoration efforts, even in productive ecosystems, to enhance the effectiveness of biodiversity recovery. Based on our results, we highlight the importance of defining particular restoration strategies for each site, as local disturbance histories and site-specific characteristics could produce particular responses in bird communities, despite the presence of rich species pools in adjacent forests. We conclude that understory complexity is an important factor to consider in restoration programs that include wildlife components in their goals.

Level and pattern of overstory retention interact to shape long‐term responses of understories to timber harvest

In many regions of the world, variable retention has replaced clear-cutlogging as the principal method of regeneration harvest. Partial retention of the overstory is thought to ensure greater continuity of the species and ecological processes that characterize older forests. Level (amount) and spatial pattern of overstory retention are two basic elements of forest structure that can be manipulated to achieve specific ecological or silvicultural objectives. However, experiments that elucidate the relative importance of retention level and pattern (or their interaction) are rare. Here we assess long-term (> 10 yr) responses of forest understories to experimental harvests of mature coniferous forests replicated at five sites in the Pacific Northwest (PNW). Treatments contrast both the level of retention (40% vs. 15% of original basal area) and its spatial distribution (dispersed vs. aggregated in 1-ha patches). For most vascular plant groups (early seral, forest generalist, and late seral), postharvest changes in cover and richness were reduced at higher levels of retention and in dispersed relative to aggregated treatments. Although retained forest patches were stable, changes in adjacent harvested (cleared) areas were significantly greater than in dispersed treatments. Late-seral herbs were highly sensitive to level and pattern of retention, with extirpations most frequent in the cleared areas of aggregated treatments and at low levels of dispersed retention. In contrast, early-seral species were most abundant in these environments. Forest-floor bryophytes exhibited large and persistent declines regardless of treatment, suggesting that threshold levels of disturbance or stress were exceeded. Our results indicate that 15% retention (the minimum standard on federal forestlands in the PNW) is insufficient to retain the abundance or diversity of species characteristic of late-seral forests. Although 1-ha aggregates provide refugia, they are susceptible to edge effects or stochastic processes; thus, smaller aggregates are unlikely to serve this function. The ability to achieve multiple ecological or silvicultural objectives with variable retention will require the spatial partitioning of habitats to include dispersed retention and larger undisturbed aggregates along with cleared areas.

Response of an ecological indicator to landscape composition and structure: Implications for functional units of temperate rainforest ecosystems

We evaluated the northern flying squirrel (Glaucomys sabrinus griseifrons), a known associate of key habitat features and processes of old-growth forest, for its capacity as a broad-scale ecological indicator of temperate rainforest ecosystem condition in southeastern Alaska, USA. We utilized a spatially explicit, resource-selection function to evaluate its distribution relative to landscape composition and structure at local (within home-range) and broad (home-range selection) spatial scales, followed by a moving-window analysis to model patch occupancy across this landscape. We found strong support for the influence of type, size, and compositional elements: large, old-growth patches were selected at both spatial scales, and regenerating forest patches ≤40yrs old were selected against at the broader scale. More importantly, we found that occupancy was related to critical thresholds in composition: patches required ≥73% old-growth forest cover or a minimum total area of 73ha of old-growth forest to be occupied by flying squirrels. A non-uniform pattern of selection for patches with higher structural connectivity at the local scale, but not at the broader scale, was likely a result of the “empty forest” phenomenon, in which remnant patches were inaccessible in this fragmented landscape. These results are consistent with recent studies of this and related species and suggest that occurrence of northern flying squirrels in southeastern Alaska is influenced by a number of landscape structure and compositional variables that relate critically to late-seral forest conditions. These findings shed new light on the utility of this species as an ecological indicator and its functional relevance to the resilience of fragmented forest ecosystems.

Estimating habitat value using forest inventory data: The fisher (Martes pennanti) in northwestern California

Managing forests for multiple objectives requires balancing timber and vegetation management objectives with needs of sensitive species. Especially challenging is how to retain the habitat elements for species that are typically associated with late-seral forests. We develop a regionally specific, multivariate model describing habitat selection that can be used – when linked to an institutional forest inventory program – to assess, monitor and forecast habitat conditions for a key wildlife species. We use the fisher (Martes pennanti) in northwestern California as our example and develop a predictive model for resting habitat that is created using data from the specific region where it will be used. We explore how this resting habitat model differs from a similar model developed for the Sierra Nevada and consider the implications for forest management. We developed the model using MaxEnt by comparing vegetation data at 99 randomly selected fisher resting structures on public and tribal lands in northwestern California with 883 Forest Inventory and Analysis (FIA) plots within the same ecoregion. A total of 58 alternative vegetation models were specified and the top 10 were nearly identical in their performance (Gain>1.08; Area Under the Curve [AUC]>0.89). We chose a five-variable model (canopy closure, tree age, total basal area, volume of “large” wood and basal area of hardwoods) because it included the fewest variables and included only those that could be affected by management. This model was similar to the Sierra Nevada model, but did not include topographic features (e.g., slope) nor did it include a variable representing the density of small trees. The absence of variables related to topography may make it easier for managers to affect positive change in resting habitat suitability since all variables can be influenced by management actions. Moreover, the model indicates that small trees appear to be less important (compared to southern Sierra Nevada) and therefore the probability of producing high-value resting habitat without higher fire risk is greater. We also created a spreadsheet that simplifies the process of generating habitat predictions from new data. Since metrics of stand structure and wildlife habitat are sensitive to sample design, collecting new data with FIA protocols will provide the most accurate estimates of predicted habitat with this model. Together, the Sierra Nevada and northwest California models provide managers in California a quantitative means to assess and monitor resting habitat suitability using current and future data that are part of an institutionally supported program to inventory forest vegetation.

Patch dynamics and the development of structural and spatial heterogeneity in Pacific Northwest forests

Over time, chronic small-scale disturbances within forests should create distinct stand structures and spatial patterns. We tested this hypothesis by measuring the structure and spatial arrangement of gaps and canopy patches. We used airborne LiDAR data from 100 sites (cumulative 11.2 km2) in the Pacific Northwest, USA, across a 643 year chronosequence to measure canopy structure, patch and gap diversity, and scales of variance. We used airborne LiDAR’s ability to identify strata in canopy surface height to distinguish patch spatial structures as homogeneous canopy structure, matrix–patch structures, or patch mosaics. We identified six distinct stand structure classes that were associated with the canopy closure, competitive exclusion, maturation, and three patch mosaics stages of late seral forest development. Structural variance peaked in all classes at the tree-to-tree and tree-to-gap scales (10–15 m), but many sites maintained high variance at scales >30 m and up to 200 m, emphasizing the high patch-to-patch heterogeneity. The time required to develop complex patch and gap structures was highly variable and was likely linked to individual site circumstances. The high variance at larger scales appears to be an emergent property that is not a simple propagation of processes observed at smaller spatial scales.

Edge effect and matrix influence on the nest survival of an old forest specialist, the Brown Creeper (Certhia americana)

Conservation strategies should be based on a solid understanding of processes underlying species response to landscape change. In forests fragmented by agriculture, elevated nest predation rates have been reported in many forest bird species, especially near edges. In intensively-managed forest landscapes, timber harvesting might also be associated with negative edge effects or broader “context” effects on some species when the matrix provides additional resources to their major nest predators. In this study, we hypothesized that proximity to a forest edge and proportion of cone-producing plantations will increase nest predation risk in fragments of relatively undisturbed forest. We focused on the Brown Creeper (Certhia americana), an indicator species of late-seral forests. We compared habitat configuration and composition at four spatial scales (0.14, 0.5, 1 and 2 km) around 54 nests and related daily nest survival rate to the distance to the nearest forest edge, mean patch size of late-seral forest (r = 141 m), proportion of non-forested lands (r = 141 m), density of maintained roads (r = 1 km), proportion of cone-producing spruce plantations (r = 2 km), and year. The best model included distance to the nearest edge and proportion of cone-producing plantations. Distance of nests to the nearest edge was the best individual predictor of daily nest survival. A larger sample of nests showed a significant threshold in distance to the nearest forest edge; nests located at least 100 m away were more likely to fledge young. These results suggest that even in managed forest landscapes, matrix effects can be important and some bird species may exhibit negative edge effects.

Local richness along gradients in the Siskiyou herb flora: R. H. Whittaker revisited

In his classic study in the Siskiyou Mountains (Oregon, USA), one of the most botanically rich forested regions in North America, R. H. Whittaker (1960) foreshadowed many modern ideas on the multivariate control of local species richness along environmental gradients related to productivity. Using a structural equation model to analyze his data, which were never previously statistically analyzed, we demonstrate that Whittaker was remarkably accurate in concluding that local herb richness in these late-seral forests is explained to a large extent by three major abiotic gradients (soils, topography, and elevation), and in turn, by the effects of these gradients on tree densities and the numbers of individual herbs. However, while Whittaker also clearly appreciated the significance of large-scale evolutionary and biogeographic influences on community composition, he did not fully articulate the more recent concept that variation in the species richness of local communities could be explained in part by variation in the sizes of regional species pools. Our model of his data is among the first to use estimates of regional species pool size to explain variation in local community richness along productivity-related gradients. We find that regional pool size, combined with a modest number of other interacting abiotic and biotic factors, explains most of the variation in local herb richness in the Siskiyou biodiversity hotspot.

Conserving biodiversity in managed forest landscapes: The use of critical thresholds for habitat

In Canada, as in other large forested countries of the world, managers and scientists alike question what can happen to forest biodiversity under long-term industrial forest management. Recent studies may help us understand how species react when habitat is lost past a certain threshold in the landscape. In the case of population, a “critical threshold for habitat” does exist in forested habitat, which is defined by the minimal proportion of habitat needed to be preserved to avoid drastic population declines or massive species loss. In this paper, two types of thresholds are described, the first refers to population, and the second refers to the community of species. Many ecologists agree with the assumption that the specialist, sensitive species are the first to disappear (local extirpation for specialist species). For most species with large home range (such as birds), the threshold may generally be located between 30% and 40% of the habitat still remaining, compared to the proportion observed under a natural disturbance regime. We suggest, in order to protect the most sensitive species and to deal with uncertainty associated with thresholds, to maintain at least 40% of residual habitats. Although there is still much to understand concerning these thresholds, we nevertheless recommend their use for the diagnostic analysis that must be performed in the context of forest management planning and biodiversity conservation, as these thresholds could represent the minimal proportion of habitat to preserve integrity of the forest ecosystem. However, to be effective, the application of thresholds should be based on detailed knowledge of ecosystem characteristics and dynamics. Key words: ecological threshold, forest management, forest ecosystem, habitat loss, older or late-seral forests, population, community, biodiversity, conservation

Canopy gaps decrease microbial densities and disease risk for a shade-intolerant tree species

Canopy disturbances such as windthrow events have obvious impacts on forest structure and composition aboveground, but changes in soil microbial communities and the consequences of these changes are less understood. We characterized the densities of a soil-borne pathogenic oomycete ( Pythium) and a common saprotrophic zygomycete ( Mortierella) in nine pairs of forest gaps created by windthrows and adjacent forest understories. We determined the levels of Pythium necessary to cause disease by performing pathogenicity experiments using two Pythium species, a range of Pythium densities, and two common tree species ( Acer rubrum and Prunus serotina) from the study sites. Three years post-disturbance, densities of Mortierella remained suppressed in soil from forest gaps compared to levels in intact forest understories while varying across sites and sampling dates. Pythium were infrequently detected likely because of soil handling effects. Expression of disease symptoms increased with increasing inoculum density for seedlings of P. serotina with each Pythium spp. having a similar effect on this species. Conversely, A. rubrum appeared resistant to the two species of Pythium. These results suggest that Pythium densities at sites where they were detected are sufficient to cause disease and possibly affect establishment of susceptible species like P. serotina. Because early seral environments have lower loads of the saprotrophic Mortierella, pathogen loads may follow a similar pattern, causing susceptible species to establish more frequently in those habitats than in late-seral forests. Forest disturbances that alter the disease landscape may provide an additional mechanism for explaining succession of temperate forests in addition to the shade-tolerance paradigm.