Article

Multi-cohort stand structure as a coarse filter of variation in mixedwood boreal bird communities

Authors:
  • Ontario Ministry of Natural Resources and Forestry, Peterborough, Canada
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Abstract

In targeting mature and over-mature forests for harvesting, management in the boreal forest has resulted in a net loss of older forests that often exhibit complex structural variation and multiple cohorts of trees. Multi-cohort forest management has been proposed as a management approach for these older forests that maintains structural wildlife habitat attributes. At the stand level, the approach relies on various partial harvest techniques to emulate the range of structural variation found in natural boreal landscapes. Here, we examine the extent to which boreal bird communities respond to multi-cohort-related structural variation in boreal mixedwood forests. In particular, we test the utility of parameters of Weibull distributions fitted to stand stem diameter distributions, which have figured prominently in methods to characterize multi-cohort structure, to explain variation in the entire bird community and in various species groupings defined by feeding guilds and forest-type associations. We also compare the explanatory power of the two Weibull parameters against 21 forest structure variables and stand age. In general, Weibull parameters outperformed stand age as a correlate of bird community variation and they were significant explanatory variables for the matrix of all species and for four species groupings, whereas age was significant for only one species grouping. When one or the other Weibull parameter was significant, it also tended to be significant even when variation due to the other was partialled out, supporting the importance not only of forest stature, but also of forest heterogeneity in understanding bird community composition. Thus, we found that multi-cohort-associated structural variation was important in explaining variation among boreal bird communities, supporting the idea of silvicultural approaches that aim at diversifying stand structural characteristics.

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... More recent studies have expanded upon the MCM approach to classify stand structure (Kuttner et al. 2013;Latrémouille et al. 2013;Malcolm and Harvey 2013), however, little is known about the potential of MCM to improve upon existing forest management strategies in terms of biodiversity conservation (Etheridge and Kayahara 2013;Malcolm and Harvey 2013). In a recent study, Burrell et al. (2013) showed that four classes of structural variation associated with MCM were relevant in explaining variation among boreal bird communities, but to date, the responses of other faunal communities remains unexplored. ...
... Mixedwood forest sites were all of type MS2 (as classified by the Ontario Ministry of Natural Resources) and were composed of poplar (Populus spp.), white birch, white spruce, black spruce, and balsam fir, such that 21-72 % of stems C10 cm diameter at breast height (DBH) were deciduous, with all sites having at least some poplar (1-65 %) and spruce (1-61 %; Table 1). Sites ranged from 35 to 130 years since last disturbance and, following Burrell et al. (2013), were classified based on their stem-diameter distribution as either, Cohort Class 1 (n = 4), Cohort Class 2 (n = 3), Cohort Class 3 (n = 5) or Cohort Class 4 (n = 6). Stands that were dated prior to when logging began in the region were considered unlogged. ...
... Weibull parameters from the diameter distribution of all stems were better predictors of ground beetle abundance and community attributes than any other habitat variables measured in this study. Similar results were found when analyzing boreal bird community responses to multi-cohort related structural variation in boreal mixedwood forests (Burrell et al. 2013). This predictive power appears to be related primarily to the scale parameter, which in turn was correlated with the grain of the understory vegetation, canopy height, and shrub openness. ...
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Reconciling tree harvesting with the maintenance of forest bird populations is a major concern of integrated management. Because bird nest predation causes >50% of the nest losses in passerines and is known to vary according to habitat characteristics, we explored some aspects of avian nest predation in relation to forestry practices in a boreal coniferous landscape managed primarily for timber production in Quebec. Using artificial tree and ground nests with Common Quail (Coturnix coturnix) and plasticine eggs, we compared the risk of nest predation (1) in experimental riparian forest strips of different widths (20-m, 40-m, and 60-m unthinned strips; >300-m control strips; and 20-m thinned strips) and (2) in clearcuts experimentally subjected to different regeneration practices (plantations with chemical and mechanical weeding, and naturally regenerated clearcuts) between 1992 and 1995. The risk of ground nest predation was lower in naturally regenerated clearcuts (5% daily probability of predation) than in control forest strips (27%). We found no evidence that chemical and mechanical weeding affected the risk of nest predation in clearcuts. In forest strips, the predation risk was higher in forest strips 40-60 m wide than in 20-m and control strips. Birds accounted for 13% of predation signs, whereas red squirrels (Tamiasciurus hudsonicus) were the dominant mammalian predators, accounting for 36% of the total predation (n = 201 nests preyed upon). In our region, the low predation rates (30% for 24 real nests) and the absence of generalist foragers such as crows, raccoons, and skunks could be attributed to the near absence of human occupation in comparison to forest-dominated landscapes in Europe and northeastern United States.
Article
The relationship between mammal community structure and vertical variation in habitat physiognomy (complexity) and horizontal variation in habitat form (heterogeneity) was examined in the llanos of Venezuela. There was little association between habitat structure and the richness, diversity, abundance, and biomass of small mammals. Abiotic factors, such as the degree of wet-season flooding, probably play an important role in patterns of small mammal distribution and abundance. The total number of mammal species was positively correlated with habitat complexity but not correlated with habitat heterogeneity. Increasing species richness across the complexity gradient was probably accommodated by increasing potential food resources. New species were added to complex communities primarily through guild expansion rather than guild addition.-from Author
Article
Comparisons of the effects of logging and fire as disturbance agents on the composition of bird assemblages in boreal ecosystems are still lacking or are limited to the short-term impacts of clear-cutting. In Quebec, where the boreal forest is largely dominated by black spruce (Picea mariana (Mill.) B.S.P.) stands, we surveyed 140 point-count stations in 3 postlogging and 4 postfire development stages determined according to the height of the regenerating spruce trees. Species richness did not vary among forest development stages, but bird abundance was higher in recent clearcuts. Recently disturbed areas were characterized by open-land bird assemblages dominated by Neotropical migrants, which reached their highest abundance in clearcuts. Moreover, logged stands were distinguished from burned sites by the absence of cavity-nesting birds. Forest-bird assemblages reestablished themselves as soon as young spruces reached the sapling stage. However, the Black-backed Woodpecker (Picoides arcticus), Three-toed Woodpecker (Picoides tridactylus), and Brown Creeper (Certhia americana) were restricted to mature stands or recent burns and are thus likely to be negatively affected by modern forestry, which involves fire suppression and short logging rotations. We suggest that retention of larger areas of continuous mature forest might be essential to maintain these species in managed regions.
Article
We studied the impacts of low density, exurban housing developments on Wood Thrushes (Hylocichla mustelina) breeding in small forest fragments in two regions of rural southern Ontario. In both regions, Wood Thrushes breeding in woodlots with embedded houses (housing penetrating the forest border) experienced significantly higher rates of parasitism by Brown-headed Cowbirds (Molothrus ater) than Wood Thrushes breeding in woodlots with adjacent houses (houses within 100 m of the forest edge), or undeveloped woodlots (no houses within 100 m of the forest edge). Wood Thrushes breeding in Peterborough area woodlots with embedded or adjacent houses experienced significantly increased rates of nest predation compared to Wood Thrushes breeding in undeveloped woodlots. This increased nest predation resulted in significant reductions in seasonal productivity in developed woodlots. No increase in nest predation was experienced by Wood Thrushes nesting in developed woodlots in the Ottawa region. The effects of housing developments appear to be region-specific and may depend on other factors influencing the overall abundance of cowbirds.
Article
The consequences of forest harvest to birds nesting in spatially and temporally dynamic landscapes are poorly understood. The boreal forests of Newfoundland have a low density of nesting birds, are naturally heterogeneous, and experience extreme annual variation in weather. Against this backdrop, we tested whether breeding success of passerines was affected by forest harvest or whether natural heterogeneity masked potential consequences of harvesting. During 2004 and 2005, we monitored 98 nests and banded 439 hatch-year (HY) birds of three migratory passerines in landscapes that varied in the extent of natural or human-induced heterogeneity: the White-throated Sparrow (Zonotrichia albicollis), Yellow-rumped Warbler (Dendroica coronata), and Blackpoll Warbler (Dendroica striata), We monitored nest-initiation date, clutch size, hatching success, nests' daily survival rates, fledging success, and productivity (ratio of HY to adults captured) relative to proportion of forest harvested within radii of 20 m (local scale), 115 m (neighborhood scale), and 1250 m (landscape scale). Local habitat and year significantly predicted nest-initiation date. Hatching success, fledging success, and productivity were significantly influenced by year and proportion of harvest at all scales, although associations varied by species. Nests' daily survival rates were significantly related to proportion of harvest at all three scales for the White-throated Sparrow; for all species they were consistently higher in 2005, and the direction of the harvest's effect differed by species. Our results reveal that larger scales are important later in breeding and that spatial and temporal heterogeneity may not only conceal consequences of forest harvest, they may also have complex interactions that make predictions difficult in these dynamic landscapes.
Article
 The notion that species might exhibit thresholds in their response to habitat alteration is appealing from a conservation perspective. Such thresholds could be used as targets for conservation in managed landscapes. In New Brunswick, Canada, forest management produces mosaics of varying stand age, species composition, and structure. We sampled this gradient in habitat suitability to examine the shape of species response functions and to look for evidence of statistically significant thresholds. We focused our attention on bird species breeding in late-seral forest and surveyed them at 390 point-count stations sampling broad-leaved deciduous to pure coniferous stands and a variety of silvicultural treatments (patch cutting, single-tree selection, spruce plantation [35–45 years old], and no recent treatment). A principal components analysis (PCA) on local vegetation separated stations along two axes reflecting gradients in stand composition and habitat alteration (increasingly open canopy and decreasing density of large trees/snags), respectively. We combined logistic regression and receiver-operating characteristic (ROC) analysis to detect thresholds in species occurrence along these gradients. Of the 42 species frequent enough to be included in the analyses, 13 (31%) showed a significant (p < 0.01) negative response to habitat alteration. Eight of the 13 species sensitive to habitat alteration exhibited thresholds in their occurrence after controlling for the suitability of local habitat. According to curves of the expected number of sensitive species (based on their ROC-derived thresholds), canopy closure and the density of large trees (>30 cm dbh) should be at least 70% and 80 stems/ha, respectively, to expect to find the complete assemblage of bird species. However, these values should be viewed as liberal, given the nature of our response variable. More refined (e.g., fitness) parameters should be used to be conservative. Nonetheless, the approach allowed us to establish preliminary quantitative targets for conservation planning based on time-efficient sampling methods, and to explicitly account for the continuous variability existing within and among silvicultural treatments rather than to assume homogeneity within treatments.Resumen: La noción de que las especies pueden presentar umbrales en su respuesta a las alteraciones del hábitat es atrayente desde una perspectiva de conservación. Tales umbrales podrían ser utilizados como objetivos de conservación en paisajes bajo gestión. En Nuevo Brunswick, Canadá, la gestión de bosques produce mosaicos de bosques de edades, composición y estructura de especies diferentes. Muestreamos este la aptitud del hábitat en este gradiente para examinar la forma de las funciones de respuesta de las especies y para buscar evidencia de umbrales estadísticamente significativos. Centramos nuestra atención en la especies de aves que se reproducen en bosque seral tardío y las muestreamos en 390 estaciones conteo por puntos en bosques deciduos de hoja ancha hasta bosques puros de coníferas y una variedad de tratamientos silvícolas (corte por parches, selección de árbol único, plantación de abetos [35-45 años de edad] y sin tratamiento reciente). Un análisis de componentes principales (ACP) de la vegetación local separó estaciones a lo largo de dos ejes que reflejan los gradientes en composición del bosque y alteración del hábitat (dosel progresivamente abierto y reducción en la densidad de árboles/tocones grandes), respectivamente. Combinamos análisis de regresión logística y de característica receptor–operador (CRO) para detectar umbrales en la ocurrencia de especies a lo largo de estos gradientes. De las 42 especies suficientemente frecuentes para ser incluidas en el análisis, 13 (42%) mostraron una respuesta negativa significativa (p < 0.01) a la alteración del hábitat. Ocho de las 13 especies sensibles a la alteración del hábitat exhibieron umbrales en su ocurrencia después de controlar para la aptitud del hábitat local. De acuerdo con las curvas del número esperado de especies sensibles (con base en sus umbrales derivados del CRO), la cobertura del dosel y la densidad de árboles grandes (>30 cm dap) por lo menos deberían ser de 70% y 80 tallos/ha, respectivamente, para esperar encontrar al ensamble completo de especies de aves. Sin embargo, estos valores deben ser vistos como liberales, dada la naturaleza de nuestra variable de respuesta. Para ser conservadores se deben utilizar parámetros más refinados (e. g. adaptabilidad). Sin embargo, el método nos permitió establecer objetivos cuantitativos preliminares para la planificación de la conservación basada en métodos de muestreo rentables en tiempo y para dar cuenta explícita de la variabilidad continua que existe dentro y entre los tratamientos silvícolas en lugar de asumir que hay homogeneidad dentro de los tratamientos.
Article
Summary • In northern Scandinavia there are indications of a long-term decline in the abundance of the three dominant vole species, Clethrionomys glareolus, Clethrionomys rufocanus and Microtus agrestis, since the 1970s. One explanation proposes that intensified clear-cutting has created even-aged and homogeneous forest stands with poor overall conditions for survival and reproduction of the voles. • We investigated the relationship between forest age and structural habitat factors and its implications for the species richness and abundance of small mammals. In particular, we assessed the population dynamics of C. glareolus, a forest-dwelling species with rather general habitat requirements. • Extensive snap-trapping of small mammals was conducted during 1998–2000 on 24 study sites in boreal forests in northern Sweden. Trapping was carried out along transects running from immature forests of six age classes (0–50 years) into adjacent reference sites (> 100 years). At each trapping station we recorded 14 habitat variables that were reduced to three principal components (PCs). The PCs were related to late successional traits, such as forest age and cover of tree layers (PC1), cover of tall vegetation in the field layer (PC2) and structural heterogeneity in the forest floor (PC3). • The species richness of small mammals, as well as the total abundance of C. glareolus, was positively influenced by tall vegetation (PC2) and structural heterogeneity (PC3) but not by late successional traits (PC1). The youngest forests had higher scores for both PC2 and PC3 compared with older forests. • The youngest forests also had the highest species richness and total abundance of C. glareolus. This was associated with a generally higher rate of change in numbers of C. glareolus during summer in the youngest forests compared with adjacent reference sites. In contrast, survival during winter was lower in the youngest forests. We found this result to be consistent with a source–sink scenario where young individuals, primarily born in old forest stands in early summer, migrate into younger forests to breed, but where the probabilities for winter survival are poor. • Our study demonstrates that both the species richness of small mammals and the population dynamics of C. glareolus are influenced to a great extent by structural habitat factors that are altered by common forest management practices in northern Sweden. In order to conserve species richness of small mammals and to minimize population fluctuations of C. glareolus in northern Scandinavia, we outline forest management practices that will provide heterogeneous environments, such as leaving logging residues on site after forest harvesting.
Article
Regional variation in spore deposition and viability was studied for two fungi, Fomitopsis rosea (Alb. & Schwein.: Fr.) P. Karst. and Phlebia centrifuga P. Karst., both confined to old-growth spruce forests in the boreal zone. Seven regions in Sweden were studied along a north-south transect in which the historical impact from forestry increases and the amount old forests decreases towards the south. The two southernmost regions were located outside the distribution border of the species. Spore deposition was measured species specifically as heterokaryotisation of homokaryotic mycelia growing on wood discs. There was a significant decline in spore deposition towards the south for both species. F. rosea deposited an average amount of 111 spores m−2 24 h−1 in the northernmost region compared to less than 1 spore in the four southernmost regions. The corresponding values for P. centrifuga were 27 spores m−2 24 h−1 in the north compared to less than 2 spores in the 4 southernmost regions. No deposition was found south of the distribution borders. The viability of spores from local populations within each region was measured as germination success on nutrient media. Individual fruiting bodies from large populations in the north generally produced spores with higher germinability than fruiting bodies from geographically isolated populations in the central and southern regions. However, there was a high variation among the southern populations. Our data suggest that some populations in mid- and south Sweden may suffer from negative genetic effects, possibly associated with fragmentation and loss of habitat. Thus, the combination of low spore deposition and low germinability of spores may be a threat to the long-term persistence of F. rosea and P. centrifuga in southern Sweden. Several other species may experience the same situation, especially when considering the severe decline of dead wood in Swedish forests.
Article
We studied edge effects on the American three-toed woodpecker (Picoides dorsalis) foraging behaviour in 18 remnant forest edges of black spruce and feather moss forests managed with a dispersed checkerboard pattern of clear-cuts. Our objectives were to assess (1) the characteristics of foraging substrates used by woodpeckers, (2) whether birds foraged according to the availability of high-quality foraging substrates found at varying distances from edges and (3) to characterize the movement patterns of foraging individuals near clear-cut boundaries. Behavioural observations of individuals allowed us to characterize all trees used for foraging according to their DBH, decay state, bark cover, tree species and top condition. We also georeferenced those trees, which allowed us to determine their distance from the edge and the orientation of the bird movements with regards to the edge. We sampled snags and downed woody debris along 80 m line transects that were oriented perpendicularly to the edge. Our results show that woodpeckers foraged in a relatively high proportion of live trees (35%). For live trees, woodpeckers used trees of larger diameter and black spruce was underused with regards to its availability. Among snags, woodpeckers preferred snags with a larger diameter, a lower decay class and a higher bark cover than nearest available snags. The density of high-quality foraging substrates (large recently dead trees) increased near the edge and decreased as we got farther into the forest interior. When comparing the distribution of used foraging snags with the one of available high-quality foraging substrates, our results show that these two distributions are significantly different. High-quality substrates located at 40 m or less from an edge were used less frequently than their availability. Hence, we can conclude that foraging woodpeckers can use snags near edges but are less prone to use these foraging trees even though they become more available than in the interior of remnant stands of managed forests. Nevertheless, bird movements were oriented parallel to the edge as far as 80 m away from the clear-cut boundary. Considering the under-utilisation of high-quality substrates near edges, we suggest that foraging substrate availability cannot explain the results obtained; the hypothesis that edges are acting as movement conduits likely explain woodpecker movement patterns we observed. Finally, the retention of larger tracks of mature and overmature forests would reduce the amount of edge habitat and provide better foraging conditions for American three-toed woodpecker in extensively managed landscapes.
Article
Fire history was reconstructed for an area of 15 000 km 2 located in the transition zone between the mixed and coniferous forests in Quebec's southern boreal forest. We used aerial photographs, archives, and dendroecological data (315 sites) to reconstruct a stand initiation map for the area. The cumulative distribution of burnt area in relation to time since fire suggests that the fire frequency has decreased drastically since the end of the Little Ice Age (about 1850) in the entire region. However, a large part of the area was burned between 1910 and 1920 during intensive colonization and when the climate was very conducive to fire. For the period 1920–1945, large fires have mainly been concentrated in the more populated southern area, while few fires have been observed in the virgin coniferous forest in the north. Despite slight differences between the south and the north, fire cycles or the average number of years since fire are not significantly different. Since 1945, there have been far more fires in the south, but the mean fire size was smaller than in the north. These results suggest that the transition between the mixed and coniferous forests observed in the southern boreal forest cannot be explained by a difference in fire frequency, at least during the last 300 years. As climatic factors and species potential distribution did not vary significantly from south to north, we suggest that the transition from mixedwood to coniferous forests is mainly controlled by fire size and severity. Smaller and less severe fires would favor species associated with the mixedwood forests as many need survivors to reinvade burnt areas. The abundance of deciduous species in mixedwood forests, together with the presence of more lakes that can act as firebreaks, may contribute to decreases in fire size and severity. The transition between the two vegetation zones could be related to the initial setting following the vegetation invasion of the area during the Holocene. In this context, the limit of vegetation zones in systems controlled by disturbance regimes such as fires may not have reached a balance with current climatic conditions. Historical legacies and strong positive feedback between disturbance regimes and com-position may filter and delay the responses to changes in climate.
Article
Extensive even-aged management of the boreal forest and its consequences on the loss of late-seral stages (>100 years) is raising concerns about the future of organisms associated with standing deadwood. The considerable reduction of deadwood not only at the stand but at the landscape level is considered to be one of the principal causes of biodiversity loss in managed forest ecosystems worldwide. Ecosystem-oriented management approaches propose a fundamental change in forestry practices whereby live and dead tree retention becomes an important consideration in forest harvesting. We use woodpecker assemblages and their association with standing deadwood for both nesting and foraging to emphasize the importance of the entire range of snag degradation stages for maintenance of key ecological processes in habitat remnants of managed landscapes. We argue that bridging foraging and nesting knowledge of woodpecker's snag requirements can refine conservation objectives for deadwood retention in the boreal forest.
Article
Clear-cut harvesting can alter ecosystem conditions and dynamics drastically compared to natural disturbance regimes, hence alternative harvesting systems are being developed in an attempt to better mimic natural forest structure. A recent approach is to harvest trees at variable intensities and spatial configurations in what is known as variable retention harvesting. Our study examines the responses of aerial insect assemblages to a gradient of forest retention at the landscape scale, and provides an assessment of the conservation benefits of alternative versus traditional harvesting systems in lowland boreal forest. The experimental design consisted of six treatments representing decreasing levels of structural retention at the landscape scale (with four replicates per treatment): (1) unharvested forest interior; (2) unharvested forest edge; (3) high-structural retention (strip retention harvesting areas at the edge of adjacent areas of unharvested forest); (4) medium-structural retention (strip retention harvesting areas in the interior of contiguous retention harvesting areas); (5) low-structural retention (strip retention harvesting areas adjacent to clear-cutted areas); (6) clear-cut harvesting. Response variables were the abundances of selected families and trophic assemblages of aerial insects, which were sampled with Malaise traps at each site. Univariate and multivariate analyses showed that the structural-retention harvesting influenced the abundance of most families and trophic assemblages. Most insect families and assemblages were most abundant in the strip retention harvested areas, especially in the medium retention treatment. These increases in abundance reflected strong edge effects, as evidenced by the fact that significant treatment effects were observed even within the two major habitat types of the study (cleared or forested habitat). Increasing structural retention favoured some assemblages such as Diapriidae, herbivores, and parasitoids whereas other groups such as predators decreased in abundance. Results support the potential use of high-level taxonomic and trophic assemblages of aerial insects in monitoring the ecological sustainability of forest harvesting practices.
Article
Efforts to preserve biological diversity must focus increasingly at the ecosystem level because of the immense number of species, the majority of which are currently unknown. An ecosystem approach is also the only way to conserve processes and habitats (such as forest canopies, belowground habitats, and hyporheic zones) that, with their constituent species, are poorly known. Continued concern with species is essential, however. Landscape-level issues also need much greater attention. Designing an appropriate system of habitat reserves is one landscape- level concern. Understanding and appropriately manipulating the landscape matrix is at least equal in importance to reserves issues, however, since the matrix itselfis important in maintaining diversity, influences the effectiveness of reserves, and controls landscape connectivity.
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