Pure Conifer Stands Research Articles

Conversion of pure spruce to mixed spruce beech stands: Effects on alpha and beta diversity of multiple taxonomic groups

The conversion from pure coniferous stands to mixed forests is a major goal of forest management strategies in the light of climate change. However, the effects of forest conversion on the associated species diversity and the factors driving diversity changes remain poorly understood. To investigate these effects, we established 54 plots in a managed forest in southern Germany, where large areas have been converted from pure spruce to mixed spruce-beech stands since the middle of the 20th century. Our sampling plots consisted of three different age classes and increasing proportions of beech, measured by the number of stems. We sampled fungi, plants, invertebrates, and birds and combined classical methods (identification by specialists) and metabarcoding for species identification. Furthermore, important forest structure and site parameters were recorded to test their influence on species alpha and beta diversity. Alpha diversity of embryophytes, and therein tracheophytes, increased with increasing availability of light, whereas fungi decreased. With increasing humus thickness alpha diversity of insects, especially dipterans and hymenopterans increased, whereas embryophytes, tracheophytes and fungi decreased. Increasing numbers of habitat trees increased the alpha diversity overall, especially in tracheophytes and hymenopterans. The proportion of beech determined the alpha diversity of one out of 12 taxa and influenced the beta diversity of 10 from 12 taxa. However, factors influencing the diversity are taxa dependent. Our study indicates that the proportion of beech had less impact on alpha diversity but impacts beta diversity. Changes in forest structures and environmental conditions occurring during forest conversion are as important as the introduction of beech for alpha and beta diversity.

Tree mixtures mediate negative effects of introduced tree species on bird taxonomic and functional diversity

Abstract Recent biodiversity loss has emphasized the necessity to critically evaluate the consequences of human alterations of forest ecosystems. Stand diversification via tree species mixtures and the use of non‐native trees are two such alterations currently gaining importance as climate change adaptations. However, the effects of local versus regional tree mixing on associated biodiversity and notably the modifying role of tree species growing outside their natural range remain poorly understood. We assessed how monocultures and mixtures of native and introduced tree species influence the taxonomic and functional diversity of bird communities at stand and landscape scales in north‐west Germany. We focused on the dominant natural tree species (Fagus sylvatica) and economically important conifer species planted outside their natural range (the native Picea abies and non‐native Pseudotsuga menziesii). We found that bird species richness and functional diversity were generally higher in pure and mixed stands of native F. sylvatica than in pure conifer stands, especially in comparison to non‐native P. menziesii. These differences were particularly strong at the landscape scale. Pure conifer stands harboured only a reduced set of functionally similar bird species. Structural diversity based on tree microhabitat availability emerged as a key predictor of bird diversity. Synthesis and applications. Our study suggests that tree species mixtures do not necessarily increase bird diversity compared to pure stands of native trees, but can promote bird diversity relative to pure stands of species planted outside their natural range. Moreover, local mixtures, rather than a mosaic of pure stands, may promote bird diversity also at the landscape scale. By contrast, pure stands of tree species planted outside their natural range can increase biotic homogenization of forest birds. Promoting structural diversity of microhabitats via tree retention and ensuring that non‐native trees are planted in mixtures with native trees may alleviate potential limitations of climate change‐oriented management for biodiversity.

Influence of woody vegetation on physico-chemical properties of sod-podzolic soils at Lesnaya Opytnaya Dacha of RSAU-MTAA named after K.A. Timiryazev

A research of the dependence of Lesnaya Opytnaya Dacha (LOD) soil properties on the varieties of the woody vegetation grown on these soils has been conducted. Measurements of the humus content, hydrolytic soil acidity, saline soil acidity and the sum of exchange base cations were used as main soil properties. Different types of vegetation were studied, including pure deciduous and pure coniferous stands and their mixtures in different proportions. Some controversial data was acquired, according to which, it appears that deciduous stands and coniferous stands do not have a statistically significant impact on the studied soil properties.

Tree diversity reduces the risk of bark beetle infestation for preferred conifer species, but increases the risk for less preferred hosts

Abstract In recent decades, European temperate forests have repeatedly suffered from severe droughts. Drought‐weakened forests have often become more susceptible to pest outbreaks such as bark beetle infestations. Tree species diversity is expected to increase resistance to drought and pests, but evidence for a positive tree diversity effect on insect pest reduction is largely circumstantial. Here we tested the effects of tree diversity and biogeographic origin of tree species on bark beetle infestation in a large, young tree diversity experiment, with six broadleaved and six conifer species from Europe and North America. Lower infestation risk was expected for the exotic tree species in each congeneric pair (spruce, larch and pine) and for mixtures with higher species richness and higher broadleaf proportion. Following a severe drought in summer 2018, the conifer trees were attacked by the six‐toothed spruce bark beetle Pityogenes chalcographus. Bark beetle boreholes were recorded in winter 2018/2019 on all conifer species. Norway spruce Picea abies and European larch Larix decidua were the most infested species and thus considered main hosts of the bark beetle. For these two species, probability of infestation decreased with increasing tree diversity (although this was only significant for Larix). In contrast, Pinus, which were less infested overall, were more likely to be infested in plots with high tree diversity. Exotic trees tended to be less infested, with clearest support for enemy release found at the level of infestation intensity when considering pure conifer stands. Overall, the effects of tree diversity and tree species origin were not as strong as the effect of position within the experimental site, where higher rates of infestation were observed at the edge than in the centre. Synthesis. Increasing tree diversity may reduce the risk of bark beetle infestation for genera prone to high infestation rates (Picea and Larix), but risk for less preferred genera (Pinus, and to some extent the exotic tree species) may increase with tree diversity due to spillover from preferred hosts. In mixed forests, the risk of infestation, even by relatively specialized insect pests, may be redistributed among tree species rather than reduced for all.

Modelling potential density of natural regeneration of European oak species (Quercus robur L., Quercus petraea (Matt.) Liebl.) depending on the distance to the potential seed source: Methodological approach for modelling dispersal from inventory data at forest enterprise level

The use of natural oak regeneration from admixed oaks and neighbouring oak stands provides an interesting alternative to cost-intensive artificial oak regeneration when aiming for forest conversion of pure coniferous stands. In this study analysis of forest inventory data is done on how far and in what density natural regeneration of both Pedunculate and Sessile oak occurs in coniferous stands.In order to investigate as exclusively as possible the effect of distance to the seed source on the regeneration density of both oaks, the regeneration potential was determined by using quantile regression. By applying a .995th quantile, reducing factors on seedling density, e.g. desiccation, browsing, pathogens or limited resource availability, were excluded as much as possible. Thus, the effect of zoochorus vectors on effective dispersal could be quantified.The regeneration potential was determined based on data from the forest inventory of the Saxony state forest enterprise, Germany, including 2357 sample plots. Remote sensing data, including the location of oaks in the overstorey, were used to determine the distance to the nearest potential seed source.The results of the present study demonstrate that the highest regeneration densities are not found in the immediate vicinity of the nearest seed source, but at distances between 60 and 140 m to it, i.e. the maximum of seedling density per area unit is in some distance to the trees trunk. In the present study, dispersal distances of established regeneration up to 1565 m were detected. From a distance of 1570–2300 m on, there was no oak regeneration.The results prove that acorns are taken from the seed source and that, in addition to barochorus dispersal, the zoochorus dispersal is of great importance for the succession of coniferous stands. The position of potential seed sources is therefore an important information for silvicultural planning, in order to estimate potential oak regeneration densities.

Spatial distribution of four sympatric owl species in Carpathian montane forests

Abstract Knowledge about spatial distribution of owl species is important for inferring species coexistence mechanisms. In the present study, we explore spatial patterns of distribution and habitat selection of four owl species u Eurasian pygmy owl (Glaucidium passerinum), boreal owl (Aegolius funereus), tawny owl (Strix aluco) and Ural owl (Strix uralensis) u ranging in body mass from 50 g to 1300 g, with sympatric occurrence in temperate continuous montane forests in the Veľká Fatra Mts., Western Carpathians, central Slovakia. Locations of hooting owl males were surveyed between 2009–2015 in an area of 317 km2. Spatial point pattern analysis was used for analysis of owl distribution. Random patterns of owls’ spatial arrangement dominate at both intra- and interspecific levels within the studied area. Only intraspecific distribution of pygmy owls and interspecific distribution of Ural owls toward tawny owls exhibited positive associations. This discrepancy with other studies can be explained in terms of pygmy owlsy preference for high-quality nest sites and/or spatial clustering in their prey distribution, and due to aggressive behaviour of dominant Ural owls toward subdominant tawny owls, respectively. Moreover, we found considerable overlap in habitat preferences between owl species, considering stand age, stand height, tree species richness, distance to open area, elevation, slope, percentage of coniferous tree species and position on hillslope, although pygmy owls were not registered in pure broadleaved stands, Ural owls were not registered in pure coniferous stands, and boreal and Ural owls were more common on slope summits and shoulders than tawny and pygmy owls. The observed patterns of spatial arrangement might suggest developed coexistence mechanisms in these owl species; differences between studies may indicate complex interactions between intra- and interspecific associations and habitat quality and quantity, food availability and owl species involved in those interactions on a landscape scale.

Accounting for Carbon Flux to Mycorrhizal Fungi May Resolve Discrepancies in Forest Carbon Budgets

Carbon (C) fluxes among different components of plant growth are important to forest ecosystem C cycling and are strongly influenced by species composition and resource availability. Although mycorrhizal fungi are crucial for nutrient acquisition and can receive a large fraction of annual net primary production, most studies do not explicitly include carbon flux to mycorrhizal fungi in ecosystem C budgets. We measured annual production of plant components (foliage, wood, fine roots) and mycorrhizal fungi across temperate forest stands varying in species composition. Production of mycorrhizal fungi was estimated using both mass balance and isotopic techniques. Total plant production varied from about 600 g C m−2 y−1 in nearly pure deciduous broadleaf stands down to about 300 g C m−2 y−1 in conifer-dominated stands. In contrast, the production of mycorrhizal fungi was highest in conifer-dominated stands, varying from less than 25 g C m−2 y−1 in deciduous broadleaf stands to more than 175 g C m−2 y−1 in nearly pure conifer stands. Isotopic data indicated that both tree species composition and ecosystem nitrogen (N) availability influenced rates of fungal production. The large investment in mycorrhizal fungi in low-N, conifer-dominated stands demonstrated that a full accounting of ecosystem carbon fluxes to plant and fungal components may help resolve current discrepancies observed in broadscale forest carbon budgets, especially across forest types.

Tree Diversity Drives Forest Stand Resistance to Natural Disturbances

Forests are frequently exposed to natural disturbances, which are likely to increase with global change, and may jeopardize the delivery of ecosystem services. Mixed-species forests have often been shown to be more productive than monocultures, but it is unclear whether this results from mixed stands being in part more resistant to various biotic and abiotic disturbance factors. This review investigates the relationships between tree diversity and stand resistance to natural disturbances and explores the ecological mechanisms behind the observed relationships. Mixed forests appear to be more resistant than monocultures to small mammalian herbivores, soil-borne fungal diseases and specialized insect herbivores. Admixing broadleaves to conifers also increases the resistance to fire and windstorms when compared to pure conifer stands. However, mixed forests may be more affected by drought depending on the species in the mixture. Overall, our findings suggest that mixed forests are more resistant to natural disturbances that are relatively small-scale and selective in their effect. However, benefits provided by mixtures are less evident for larger-scale disturbances. Higher tree diversity translates into increased resistance to disturbances as a result of ecological trait complementarity among species, reduction of fuel and food resources for herbivores, enhancement of diversion or disruption processes, and multi-trophic interactions such as predation or symbiosis. To promote resistance, the selection of tree species with different functional characteristics appears more important than increasing only the number of species in the stand. Trees with different levels of susceptibility to different hazards should be intermixed in order to reduce the amount of exposed resources and to generate barriers against contagion. However, more research is needed to further improve associational resistance in mixed forests, through a better understanding of the most relevant spatial and temporal scales of species interactions and to optimize the overall provision of ecosystem services.

Red Alder-Conifer Stands in Alaska: An Example of Mixed Species Management to Enhance Structural and Biological Complexity

There is worldwide interest in managing forests to improve biodiversity, enhance ecosystem services and assure long-term sustainability of forest resources. An increasingly important goal of forest management is to increase stand diversity and improve wildlife and aquatic habitat. Well-planned silvicultural systems containing a mixture of broadleaf-conifer species have potential to enhance stand diversity and provide other ecosystem services earlier than typical even-aged conifer plantations. Here, we use the example of mixed Sitka spruce/western hemlock and red alder in young, managed stands in southeast Alaska to achieve these goals. We briefly describe the silvics of Sitka spruce, western hemlock and red alder plantations as pure conifer stands or pure broadleaf stands. Then, we synthesize studies of mixed red alder-Sitka spruce/western hemlock stands in southeast Alaska and present their potential for improving stand structural complexity, biodiversity and other ecosystem services over pure conifer forests. Finally, we discuss some of the opportunities and potential tradeoffs for managing mixed broadleaf-conifer stands for providing a number of natural resources and the influence of these broadleaf-conifer forests on ecosystem linkages and processes.

Soil acidity and exchangeable base cation stocks under pure and mixed stands of European beech, Douglas fir and Norway spruce

The establishment of mixed forest stands is nowadays seen as an opportunity to maintain forest services in the course of global climate change. Thus, we determined forest floor and mineral soil pH, base saturation (BS) as well as exchangeable base cation stocks in adjacent groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) as well as single-tree mixtures of beech with either Douglas fir or spruce at two forest sites in Southern Germany that differ in site and soil properties. Spruce forest floors had lowest pH and BS, while beech favoured less acidic forest floors with higher BS. The impact of Douglas fir on soils varied depending on the site. Under beech–Douglas fir and beech–spruce mixtures, forest floor and mineral soil pH and BS were higher than under the respective pure conifer stands. While beech depletes soil exchangeable Ca and Mg stocks more than Douglas fir and spruce, respectively, total soil exchangeable K stocks under beech were among the highest. Again, beech–conifer mixtures were intermediate. Mixed species stands might maintain forest soil fertility by mitigating soil acidification, nutrient leaching and concomitant soil base cation depletion compared to pure conifer stands.

Soil organic carbon and nitrogen stocks under pure and mixed stands of European beech, Douglas fir and Norway spruce

Numerous studies have addressed tree species effects on forest soil carbon (C) and nitrogen (N); however, knowledge of how and to what extent specific tree species and species mixtures impact forest soil C and N stocks is scarce and inconsistent across soil types. Therefore, we studied three forest sites in Southern Germany differing in parent material, soil properties as well as nutrient and water supply. Each site comprises adjacent groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) as well as single-tree mixtures of beech with Douglas fir or Norway spruce. To account for tree-species-specific spatial heterogeneity, we sampled the forest floor and mineral soil to a depth of 60cm at different distances from the trees.Significant tree species and species mixing effects on soil organic carbon (OC) and N concentrations, C/N ratios and soil OC and N stocks were mainly found in the forest floor and in the uppermost (0–15cm) mineral soil. Forest floor OC and N stocks and total soil OC stocks were higher under Douglas fir and Norway spruce compared with beech. While tree species effects on soil OC and N were present across sites, the influence of soil type induced variations in their magnitude. The forest floor C/N ratio under Douglas fir was low and comparable with beech in soils developed from nutrient-rich parent material, whereas it was higher and similar to spruce in the soil formed from sandstone. Tree species-specific differences in foliar nutrient concentrations between beech and conifer stands might influence litter decomposition rates among the species and thus modify soil OC and N stocks.Forest floor OC stocks were significantly higher in mixed beech–conifer stands compared with pure beech, and most often smaller than or similar to pure conifer stands. Forest floor N stocks showed the same tendency, but differences were inconsistent and not always significant across sites. Admixture of beech with Douglas fir or Norway spruce reduced the share of OC and N stored in the forest floor compared with the pure conifer stands and significantly increased mineral topsoil (0–15cm) OC stocks compared with pure beech stands. Hence, the vertical distribution of OC and N in the soil profile varied depending on the tree species composition. Total soil (forest floor+mineral soil) OC and N stocks of mixtures were similar to pure beech, pure conifers or intermediate depending on site and soil type.

Regeneration after stump harvesting in southern Finland

In southern Finland, large-scale stump harvesting from regeneration areas began in the early 2000s. At that time, stump harvesting and additional soil scarification for planting was performed simultaneously. The aim of this study was to investigate the regeneration result, particularly the outcome of the natural regeneration of birch after stump harvesting, compared to the result achieved on a conventionally soil prepared area without stump lifting.The study material consisted of 37 stump-harvested areas 4–9years old, and 10 conventionally soil prepared regeneration areas of a corresponding age. In the case of Norway spruce planting, the result did not differ between stump-harvested and conventionally soil prepared areas, but the share of pure coniferous stands after pre-commercial thinning was estimated to be 30% after stump harvesting and 50% in conventional regeneration areas.In stump-harvested areas, the share of disturbed soil surface seemed remarkably higher and the time for seedling emergence longer than in conventionally soil prepared areas without stump lifting. The number of Scots pine seedlings was somewhat higher after stump harvesting than after conventional soil preparation, but there was great variation between regeneration areas. As for Norway spruce, there were fewer natural seedlings on stump-harvested areas. The number of birch seedlings was somewhat higher after stump lifting, but there was huge variation between regeneration areas. Most birch-rich regeneration areas were discovered after the stump harvest; the maximum mean density was over 60,000 birch seedlings ha−1. Birch regeneration was most abundant on fine textured, moist mineral soils or peat layered spots. The mean temperature in May and June during the first summer, and rainfall in May and June during the second summer after the stump harvest, correlated positively with the abundance of birch seedlings.

Changes in soil faunal assemblages during conversion from pure to mixed forest stands

Replacement of native deciduous forests by coniferous stands was a common consequence of former European afforestation policies. However, these changes have proven to lead to serious ecological problems. Therefore, re-establishing mixed forests with native tree species became an increasingly popular management strategy to fulfil the demands of multi-functional forestry. We report about changes in collembolan assemblages and microbial performances during conversion of pure coniferous stands to mixed forests. The study was carried out in the Black forest area (SW of Germany), where a gradient of conversion from pure spruce stands (S1) to equally mixed stands (spruce, beech, and fir) at S4, through two intermediate stages (S2 and S3) was selected. Results clearly indicated strong modifications of the collembolan communities with an enrichment of the assemblages over the course of the conversion process. Mean species richness increased by 47% from S1 to S4 accompanied by diversity indices (Shannon and Simpson) higher at S4. Significantly different soil biota assemblages were found at each phase of the conversion process. Spatial turnover and nestedness contribute almost equally to the modification of assemblages from S1 to S2 while later on in the mixing process only spatial turnover was acting. Concomitantly, significant shifts in the functional structure of the collembolan assemblages were depicted, deep-dwelling collembolan (euedaphic) being, surprisingly, the most responsive group. In contrast, neither microbial nor coarse environmental parameters were influenced by the factor “conversion phase”. We suggest that stimulation of Collembolan communities after the mixing process was mainly due to the input of more suitable food sources and/or microhabitat increases. Our findings underline the crucial role of aboveground processes on the belowground system, with the intensity and scheme of the mixing-process as important factors to consider when aiming at soil biodiversity improvement in forest systems.

Nematoda response to forest conversion

This study investigates the response of the Nematoda community to forest conversion from pure coniferous stands into semi-natural montane forests in the southern Black Forest (Germany). The investigation was carried out by comparing existing stands that represent the four major stages of the envisaged conversion process. Major results are: (i) the number of Nematoda genera was highest at the species enrichment stage pointing to a positive influence of a rich ground vegetation on the diversity of Nematoda, (ii) predaceous species reached a particularly high, and omnivorous species having a particularly low, density at the species enrichment stage, and (iii) root-hair feeding Nematoda profited from qualitative changes of fine roots during the conversion process and gradually increased in abundance from the even-aged pure spruce stands to the diverse continuous cover forests. Altogether the comprehensive investigation of the Nematoda of the four conversion stages indicates that forest conversion only weakly alters the structure of the Nematoda community. Neither an overall change nor a clear trend in Nematoda richness and abundance could be established. However, due to the specific responses of the different Nematoda feeding groups to the conditions of the individual conversion stages it seems to be ecological useful to maintain special qualities of each conversion stage (e.g. forest gaps in the species enrichment stage) during forest conversion protecting the structure and diversity of the Nematoda communities.

Soil macrofaunal response to forest conversion from pure coniferous stands into semi-natural montane forests

We studied the response of the macrofauna to forest conversion from pure coniferous stands into semi-natural montane forests in the southern Black Forest (Germany). The investigation was carried out by comparing existing stands that represent the four major stages of the envisaged conversion process. Major results are: (i) environmental parameters indicate a significant alteration of the soil environment, (ii) neither an overall change nor a clear trend in macrofaunal richness and abundance could be established, and (iii) the data for the different taxa suggest specific responses to the conditions of the individual conversion stages. In general, saprophagous taxa seem to be mainly driven by alterations of the resource base (litter quality, microbial parameters) while predatory taxa tend to respond to changes at the consumer level. Associated alterations in the functional structure occurred for macroengineers (earthworms), primary consumers (diplopods, isopods) as well as a wide range of predators. These changes could partly be explained by changes in environmental conditions that did not follow a gradual adjustment during the conversion process. One essential step is the shift from an organic layer composed of low-quality coniferous litter that is dominated by fungi to a litter layer with higher quality food sources. Within this framework, the specific response of the macrofauna is modulated by factors such as differences in structural features of the ground vegetation and availability of woody litter in a stage-specific way. Since the response of the macrofauna depended far less on site conditions than any of the other environmental factors, this invertebrate group may be used as a valuable indicator of the changes associated with different stages of the conversion process. As long as no dramatic change of soil conditions – in particular pH – occurs, however, no fundamental restructuring of the soil community is to be expected.

Does conversion of even-aged, secondary coniferous forests affect carbon sequestration? A simulation study under changing environmental conditions

To circumvent problems associated with even-aged, pure coniferous stands propagated outside their natural range alternative management strategies and conversion programs are currently discussed in Central Europe. However, a mainstreaming of such adapted silvicultural systems with climate change mitigation objectives is missing to date. In this study the objective was to assess in situ C storage under conditions of climate change in a secondary Norway spruce (Picea abies (L.) Karst.) forest management unit in Austria. Four management strategies (Norway spruce age class forestry, transition to continuous cover forestry with Norway spruce, conversion to mixed conifer/broadleaved stands, no management) were investigated under current climate and two transient climate change scenarios in a simulation study. By comparing the results of two independent forest ecosystem models (PICUS v1.41, 4C) applied under identical forcings and boundary conditions we aimed at addressing uncertainties in model-based projections. A transition to continuous cover forestry increased C storage in all climate scenarios (+45.4 tC·ha to +74.0 tC·ha over the 100 year analysis period) compared to the approximately balanced C budget under the age class system. For the mixed conifer/broadleaved management variant predictions of the two models diverged significantly (+29.4 tC·ha and –10.6 tC·ha in PICUS and 4C respectively, current climate). With regard to climate change impacts both models agreed on distinct effects on productivity but lower sensitivity of C stocks due to compensation from respiration and adaptive harvest levels. In conclusion, considering the potential effects of silvicultural decisions on C stocks climate change mitigation should be addressed explicitly in programs advocating targeted change in management paradigms.–1–1–1–1

Diversity and quantitative analysis of dominant tree species in district Chamba of Himachal Pradesh

A study on diversity, composition and quantitative analysis of dominant tree species was carried out in twelve different sites varying from 1700 m to 2800 m above msl of District Chamba, Himachal Pradesh. Selected sites favoured most of the forests on North-western aspect and were different in their ecoclimatic and phytogeographical features. Overall diversity of tree species in their number, varied from 4 to 12. Forests having high basal area, canopy cover and average tree height were mostly dominated by the conifers, comparatively with less disturbance. The sites with low basal area supported mixture of broadleaved with coniferous species, with high level of degradation. Ban oak (Quercus leucotrichophora) is heavily lopped as winter fodder, fuel wood, to prepare agricultural implements and for the preparation of coal. It is observed that species diversity and density decrease with increasing basal area. Species richness, Hill diversity indices (N1 and N2), Simpson’s (l) and Shannon’s diversity indices (H’) are reported maximum for the sites, dominated by mixed coniferous broadleaved forests. The minimum diversity value is observed in the localities that are occupied by pure conifer stands. The present study reflects that the sites possessing maximum diversity are under high level of disturbance and needs priority for conservation measures.

Relations between red alder composition and understory vegetation in young mixed forests of southeast Alaska

Interest in mixed red alder (Alnus rubra Bong.) – conifer young-growth stands has grown in southeast Alaska, USA, because they appear to provide much more productive understory vegetation and wildlife habitat than do similar-aged pure conifer stands. We studied understory vegetation in nine even-aged young-growth stands (38–42 years old) comprising a gradient of red alder – conifer overstory composition, with red alder ranging from 0% to 86% of stand basal area. Conifers were Sitka spruce (Picea sitchensis (Bong.) Carr.), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and western redcedar (Thuja plicata Donn ex D. Don). We measured understory biomass and net production (current annual growth) in each stand by species and plant part and estimated carrying capacity for black-tailed deer (Odocoileus hemionus sitkensis Cowan) with a food-based habitat model. Highly significant positive relations (P < 0.002) were found between red alder basal area and all of the following: total understory biomass (r2 = 0.743), net production of shrubs (r2 = 0.758) and herbs (r2 = 0.855), and summer carrying capacity for deer (r2 = 0.846). The high correlation between red alder and herbaceous production is especially important, because herbs are least abundant and most difficult to maintain in young-growth conifer forests of this region. Red alder offers prospects for increasing understory vegetation biomass and its food value for deer and other wildlife when included as a hardwood overstory species in mixed hardwood–conifer young-growth forests.

Thresholds in Forest Bird Response to Habitat Alteration as Quantitative Targets for Conservation

Abstract: 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.

Red alder ( Alnus rubra) alters community-level soil microbial function in conifer forests of the Pacific Northwest, USA

Nitrogen-fixing tree species have been shown to improve site fertility and increase N transformation rates, but the influence of N-fixing plants on the soil microbial community as a whole is largely unknown. We used patterns of individual carbon-source utilization and enzyme activities to assess the relative effects of N-fixing red alder on the soil microbial community in three adjacent stands (pure conifer, mixed alder-conifer, and pure alder) of a highly productive coastal Oregon forest where the density of red alder has been experimentally manipulated for over 65 years. Two major patterns were revealed: (1) bacterial and fungal carbon-source utilization patterns in soil from pure conifer stands were significantly different from both pure alder soils and mixed conifer-alder soils, while there was no difference in substrate utilization patterns between soils from the mixed alder-conifer and pure alder stands; and (2) the activities of nine extracellular enzymes involved in ligno-cellulose degradation and the mineralization of organic nitrogen, phosphate, and sulfate compounds were all significantly greater in pure alder soils compared to either pure conifer or mixed conifer-alder soils. Our results show that, in addition to an overall increase in soil fertility, microbial biomass, and microbial activity, the presence of N-fixing red alder significantly alters the physiological profile of the microbial community-even in an ecosystem already of high N status.