Bengt Gunnar Jonsson

Biodiversity in Dead Wood

Preface 1. Introduction 2. Wood decomposition 3. The saproxylic food web 4. Other associations with dead woody material 5. Host tree associations 6. Mortality factors and decay succession 7. Microhabitats 8. Tree size 9. The surrounding environment 10. Evolution of saproxylic organisms 11. Species diversity of saproxylic organisms 12. Natural forest dynamics 13. Dead wood and sustainable forest management 14. Population dynamics and evolutionary strategies 15. Threatened saproxylic species 16. Dead wood in agricultural and urban habitats 17. The value and future of saproxylic diversity References Index.

Exploring potential biodiversity indicators in boreal forests

The present study evaluates indicators in Swedish spruce forests. We ask whether different species groups co-vary in their occurrence and to what extent species richness and composition is predictable from habitat structures. We studied 10 boreal spruce forest stands constituting a gradient in degree of selective logging. Occurrences of vascular plants, bryophytes, epiphytic lichens and wood-inhabiting fungi as well as habitat structures was inventoried. In addition, in five of the stands, beetles were sampled with windows traps. Total species richness was correlated with several habitat factors, mainly particular substrates and degree of forestry impact. However, the richness of a set of species regularly used as indicators did not correlate with habitat factors. Correlation in species richness among different organism groups were few and scale dependent. Only lichens and vascular plants formed nested subset patterns (i.e. species composition at poorer sites is subsets of the species present at richer sites) among the study sites. The study shows that in this forest type one cannot a priori assume that richness in one group of species correlated with richness in other, and measures of single habitat features may be relevant only to particular groups of species. Instead, monitoring and inventories should be based on a set of factors reflecting important aspects for different groups of organisms and if indicator species are to be used these should be chosen from several species groups.

Treefall disturbance maintains high bryophyte diversity in a boreal spruce forest

(1) The effect of disturbance on diversity patterns of cryptogams has received little attention. We compared bryophyte diversity in thirty-seven patches formed by uprooting with that of the undisturbed forest floor in a north Swedish Picea abies forest. (2) Bryophyte diversity and species richness were significantly higher in patches with soil disturbance compared with undisturbed forest floor. Species richness and diversity were highest at intermediate patch ages, but disturbed patches had an altered vegetation composition for at least 100 years. (3) Four causes of high bryophyte diversity in disturbed patches are proposed: (i) uprooting creates space for bryophyte colonization that is free from potential competitors; (ii) disturbed patches have high habitat heterogeneity; (iii) within-patch disturbance continues long after patch formation through erosion from the tip-up mound; and (iv) the small patch size implies a short distance to potential sources of bryophyte diaspores which should increase the chance of establishment. (4) Both early and late successional bryophytes establish shortly after an uprooting has occurred. This suggests that no facilitation by early species is necessary before late successional species invade. (5) Results show that treefall disturbances are important for both the persistence of colonists and the maintenance of high bryophyte diversity in boreal-forest ecosystems. Consequently, the studied system has clear parallels with the gap-phase systems widely recognized in temperate and tropical forests.

Availability of coarse woody debris in a boreal old-growth Picea abies forest

. This study reports temporal (based on cross-dated dead trees) and spatial patterns of availability of coarse woody debris (CWD) from Picea abies in a Swedish boreal landscape with discrete old-growth forest patches in a wetland matrix. Data were collected from 29 patches ranging in size from 0.3 to 28 ha. A total of 897 dead trees with a minimum diameter of > 15 cm occurred on the 7.2 ha area analysed. The year of death was established for 50% of these trees. CWD volume ranged from 17 to 65 m3/ha for downed logs and from 0.5 to 13 m3/ha for standing snags. CWD of all decay stages and diameter classes occurred abundantly and the probability of finding logs of all decay stages and sizes was very high at the scale of single hectares. Tree mortality differed among 5 yr periods. However, during the last 50 yr no 5 yr period produced less than 3 logs/ha. Decay rates were highly variable among different logs. Logs with soft wood and some wood pieces lost (decay stage 5) died ca. 34 years ago. This suggests a fairly rapid decay in this northern forest. The data indicate a high and continuous availability of CWD of all types. It is likely, therefore, that selection pressures for efficient dispersal among CWD dependent species may not be very high. Consequently, species with narrow habitat demands and/or low dispersal ability may have evolved and this may contribute to the decrease of certain species in the managed landscape.

The bryophyte diaspore bank and its role after small‐scale disturbance in a boreal forest

. The composition of the bryophyte diaspore bank in an old-growth Picea abies forest was studied before and after experimental disturbance of forest-floor patches. 40 species, both hepatics and mosses, emerged from spores, and probably also from gemmae and moss fragments, in greenhouse cultivations of soil samples. The three most abundant taxa were Pohlia nutans, Sphagnum spp. and Polytrichum commune/longisetum. Initially, the number of species from the diaspore bank in mineral soil, 9.9 species/sample, was higher than that in humus, 6.6. Four years after the disturbance took place, the reverse was found. Several species that are typical colonisers of disturbed soil were very common in the diaspore bank. By contrast, some of the most abundant forest floor species appeared to be absent. The diaspore bank of bryophytes appears to play a role similar to that of the seed bank in vascular plants: (1) it allows species to survive unfavourable periods (temporal dispersal); (2) it facilitates rapid colonisation after disturbance; (3) it influences the post-disturbance species composition and diversity. The results emphasise that the understanding of vegetation succession patterns is, to a large extent, dependent on quantitative data on the relative importance of spatial and temporal dispersal.

A null model for randomization tests of nestedness in species assemblages

Analysis of the degree of order in species assemblages in terms of nested subsets has received increased interest during the last decade. However, recently a series of papers have questioned the validity of methods employed for testing whether observed patterns deviate from random expectations. The current view seems to be that the randomization procedure should control for both number of species per site and species frequencies. The randomization procedures used also choose to keep the total number of observations constant in each resample. In this paper I question some of these assumptions when analyzing species-by-site matrices for detecting whether the biota is significantly nested or not. My basic assumption is that the observed species frequency is only an estimate of the probability of occurrence for the particular species. For a test of degree of nestedness all sites should be regarded as being equal. To what extent size, isolation or habitat quality may influence species distribution is a secondary question if nestedness can be statistically proven. This implies that generation of random matrices should only consider the frequency of the species (as an estimate of their probability of occurring in any patch). Such matrices are computationally simple and besides providing a test of nestedness also open the possibility of testing whether the range in species richness is smaller or larger than expected under random expectations. The choice of null model for the test should always be viewed in relation to the question asked. If nestedness is concerned the methods proposed here should be used. However, if other questions are at hand the restrictions of previous approaches may be valid. This is for instance the case if pairwise species co-occurrences are analyzed. In this case, the richness of each site should obviously be incorporated in the randomization to control for the higher probability of co-occurrence at species-rich sites.

Predictability of plant and fungal species richness of old‐growth boreal forest islands

The aims of this thesis are to (1) analyze the predictability (indicators) of plant and fungal species diversity in old-growth forests, and (2) assess the history and biodiversity of woodland key habitats (WKHs) and their potential to maintain species diversity in fragmented boreal forest landscapes. Predictability was explored in Granlandet nature reserve, an unexploited landscape composed of discrete old-growth Picea forest patches of varying size isolated by wetland, reflecting conditions of insular biota at stochastic equilibrium. Data from 46 patches (0.2-12 ha) showed that most species were rare. However, species richness and composition patterns exhibited a high degree of predictability, which strengthen the possibility to apply biodiversity indicators in old-growth forest stands. Area was a key factor. The increase in species richness starts to level out at 2-3 ha. Large patches host more Red-list species in their interiors than do small ones, i.e. stand size is an important qualitative aspect of old-growth habitat. Nestedness emerged in relation to area but also in equal-sized plots. Structural complexity and habitat quality were important for species richness and compositional patterns, and small habitats of high quality could harbor many rare species. Monitoring of wood-fungi on downed logs showed that species diversity on downed logs changed over periods of 5-10 years and that the occurrences of annual species were unpredictable. It is suggested that monitoring of species with durable fruit bodies (mainly polypores) is likely to be a feasible approach to obtain comparable data over time.Assessments of biodiversity of WKHs were performed in two areas with contrasting histories of forest exploitation, namely in south boreal and north boreal Sweden. Analyses of the history of 15 south boreal WKHs showed that fire-suppression, selective logging until mid-20th century and abandonment by modern forestry has shaped their forest structure. These WKHs are not untouched forests, they lack key structural components and harbor few Red-list species. Artificial interventions to restore natural processes and patterns are needed to further increase their suitability for threatned species. Modeling analyses of species richness in 32 WKHs in north boreal Sweden, some of which have not been isolated by modern forestry until recently, indicated an excess of crustose lichen species, i.e. WKHs may face delayed species extinctions. By contrast, the results indicate that wood-fungi have tracked the environmental changes. Differences in substrate dynamics between epiphytes on living trees and species growing on decaying logs may explain the diffeence between species groups. The results indicate that population densities of Red-list species were low, which may result in further depletion of species diversity.Continuing species declines and extinctions are likely if not conservation of WKHs are combined with other considerations in th managed forest landscape. Both WKHs and their surroundings must be managed and designed to maintain biodiversity over time. For a successful future conservation of boreal forest biodiversity monitoring of WKHs must be combined with monitoring of refeence areas.

A stage-based matrix model for decay-class dynamics of woody debris

Dead trees contribute to the structural diversity in forested ecosystems, providing habitat, shelter, and substrate for a wide range of organisms. Forecasting the dynamics of dead trees is a key to incorporating woody debris in managed forests to conserve those species dependent on dead trees. We present a new model for predicting woody-debris dynamics over time using a stage-based matrix, with transition rates between decay classes. It is constructed for use with the type of classification systems commonly used for assessments of dead trees as a plant and animal habitat. Dendrochronological measurements of time since death are the basis for transition rates between decay classes. Calculating mean residence time in decay classes from a single time point sample, rather than using longitudinal long-term data, tends to overestimate residence time due to a higher probability of inclusion of slow-decaying trees. A new method for correcting this bias is presented, and incorporated into the model. The stage-based decay-class model approach is suggested as a general model, applicable to many tree species and forest regions. We illustrate how such a model can be used for management planning for woody debris by forecasting decay-class distributions of woody debris over time at five-year intervals. In combination with functions for growth and mortality, different ways to reach woody-debris goals can be explored. The model is parameterized for Picea abies (L.) Karst. (Norway spruce) in mid-northern Sweden. Because there is a lack of data from long-term studies in similar conditions, model validation is difficult. However, comparisons with available independent data support the conceptual basis for the model. Future studies should concentrate on the relative importance of factors that influence residence time in decay classes, e.g., tree size, site productivity, and decomposer communities.

Nested plant and fungal communities; the importance of area and habitat quality in maximizing species capture in boreal old-growth forests

Knowledge of the distribution of rare species is crucial for species conservation in fragmented habitats. Species communities often exhibit nestedness, i.e. species in species-poor sites comprise a subset of richer ones. Thus, rare species are confined to species-rich sites. We evaluate whether plant and fungal communities in 46 old-growth spruce forest patches (0.17–12 ha) exhibit nestedness. The question whether a single large patch or several small patches capture most species (i.e. the SLOSS-issue) is evaluated in combination with species saturation analyses. All species groups exhibited significant nestedness. Area was generally related to nestedness, i.e. rare species were over-represented in the largest patches. Species saturation analysis indicated that large patches accumulated more Red-list species in patch interiors than small patches. Thus, rare and Red-list species were best captured in large patches. However, nestedness also emerged in equal sized sample plots, i.e. rare species were over-represented in high quality habitats. Thus, small habitats of high quality should not be neglected in a conservation perspective.

Refining volume estimates of down woody debris

Down woody debris (DWD) plays a vital role in forest ecosystem structure and function. Although volume is likely the most common metric used to characterize DWD, an evaluation of the formulae used for volume estimation on individual DWD pieces has received little attention. We determined actual volume of 155 diverse DWD pieces (types, species, lengths, and diameters) by detailed field measurements. By comparing the actual and calculated volumes from six commonly used formulae, we assessed their bias, precision, and accuracy. Based on observed DWD forms, we developed a new formula, namely the “conic−paraboloid”, which was included in the assessment. Among the formulae that require length and two end diameter measurements, the conic−paraboloid had the lowest bias, highest precision, and hence greatest accuracy. Newton’s and the centroid formulae had higher accuracy yet require more field measurements. Smalian’s, conical frustum, and average-of-ends formulae had poor performance relative to the others. Accur...