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  • 1. Angelstam, Per
    et al.
    Roberge, Jean-Michel
    Axelsson, Robert
    Elbakidze, Marine
    Bergman, Karl-Olof
    Dahlberg, Anders
    Degerman, Erik
    Eggers, Sönke
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hjältén, Joakim
    Johansson, Therese
    Müller, Jörg
    Paltto, Heidi
    Snäll, Tord
    Soloviy, Ihor
    Törnblom, Johan
    Evidence-Based Knowledge Versus Negotiated Indicators for Assessment of Ecological Sustainability: The Swedish Forest Stewardship Council Standard as a Case Study2013In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 42, no 2, p. 229-240Article in journal (Refereed)
    Abstract [en]

    Assessing ecological sustainability involves monitoring of indicators and comparison of their states with performance targets that are deemed sustainable. First, a normative model was developed centered on evidence-based knowledge about (a) forest composition, structure, and function at multiple scales, and (b) performance targets derived by quantifying the habitat amount in naturally dynamic forests, and as required for presence of populations of specialized focal species. Second, we compared the Forest Stewardship Council (FSC) certification standards' ecological indicators from 1998 and 2010 in Sweden to the normative model using a Specific, Measurable, Accurate, Realistic, and Timebound (SMART) indicator approach. Indicator variables and targets for riparian and aquatic ecosystems were clearly under-represented compared to terrestrial ones. FSC's ecological indicators expanded over time from composition and structure towards function, and from finer to coarser spatial scales. However, SMART indicators were few. Moreover, they poorly reflected quantitative evidence-based knowledge, a consequence of the fact that forest certification mirrors the outcome of a complex social negotiation process.

  • 2.
    Ekström, Magnus
    et al.
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Westerlund, B.
    Grafström, A.
    Jonsson, B. G.
    Ståhl, G.
    Logistic regression for clustered data from environmental monitoring programs2018In: Ecological Informatics, ISSN 1574-9541, E-ISSN 1878-0512, Vol. 43, p. 165-173Article in journal (Refereed)
    Abstract [en]

    Large-scale surveys, such as national forest inventories and vegetation monitoring programs, usually have complex sampling designs that include geographical stratification and units organized in clusters. When models are developed using data from such programs, a key question is whether or not to utilize design information when analyzing the relationship between a response variable and a set of covariates. Standard statistical regression methods often fail to account for complex sampling designs, which may lead to severely biased estimators of model coefficients. Furthermore, ignoring that data are spatially correlated within clusters may underestimate the standard errors of regression coefficient estimates, with a risk for drawing wrong conclusions. We first review general approaches that account for complex sampling designs, e.g. methods using probability weighting, and stress the need to explore the effects of the sampling design when applying logistic regression models. We then use Monte Carlo simulation to compare the performance of the standard logistic regression model with two approaches to model correlated binary responses, i.e. cluster-specific and population-averaged logistic regression models. As an example, we analyze the occurrence of epiphytic hair lichens in the genus Bryoria; an indicator of forest ecosystem integrity. Based on data from the National Forest Inventory (NFI) for the period 1993-2014 we generated a data set on hair lichen occurrence on > 100,000 Picea abies trees distributed throughout Sweden. The NFI data included ten covariates representing forest structure and climate variables potentially affecting lichen occurrence. Our analyses show the importance of taking complex sampling designs and correlated binary responses into account in logistic regression modeling to avoid the risk of obtaining notably biased parameter estimators and standard errors, and erroneous interpretations about factors affecting e.g. hair lichen occurrence. We recommend comparisons of unweighted and weighted logistic regression analyses as an essential step in development of models based on data from large-scale surveys.

  • 3.
    Eriksson, Amanda
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gauslaa, Yngvar
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ekström, Magnus
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Morphology drives water storage traits in the globally widespreadlichen genus Usnea2018In: Fungal ecology, ISSN 1754-5048, E-ISSN 1878-0083, Vol. 35, p. 51-61Article in journal (Refereed)
    Abstract [en]

    Links between lichen morphology, internal/external water storage and distribution patterns are poorly known. We compared mass- (WC, % H2O) and area-based (WHC, mg H2O cm−2) hydration traits in seven pendent or shrubby Usnea species from oceanic to continental climates. All species held more external than internal water. Internal WHC and WC increased with specific thallus mass (STM, mg cm−2), while external WC decreased. Shrubby species had higher STM and total WHC than pendent ones. The continental Usnea hirta (shrubby) had the highest total and external storage; the suboceanic Usnea longissima (pendent) had the lowest internal storage. Morphology drives hydration traits and explains distributions of some Usnea species, but such traits did not distinguish oceanic from widespread species. Shrubby species maximize water storage and thus prolong hydration after rainfall events and/or hydration by dew. The low internal WHC in pendent species is likely an adaptation to frequent hydration in humid air.

  • 4.
    Esseen, P-A
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Edge influence on the old-growth forest indicator lichen Alectoria sarmentosa in natural ecotones2006In: Journal of Vegetation Science, Vol. 17, p. 185-194Article in journal (Refereed)
  • 5.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ecology of lichens in boreal coniferous forests with reference to spatial and temporal patterns1983Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The thesis deals with the ecology of lichens in two contrasting types of forest, epiphytic lichens in old Picea abies forest of the fi re-refugia type and epigeic as well as epixylic lichens in a successional sequence of fire- susceptible Pinus sylvestris forests. Results in five separate papers form the basis for a discussion of general patterns of dispersal, succession and life strategies in lichens. The study sites were located in Medelpad and Väs­terbotten, in the central and northern part of Sweden, respectively.

    Special attention has been paid to the rare, pendulous, spruce-1ichen Usnea longissima and the coexisting lichen species. U. longissima is largely restricted to north-facing hill slopes covered with old, mesic spruce forest that is characterized by a very long continuity not disturbed by fire. A mar­ked decline in the number of sites with U. longissima was found. The decline was mainly due to the effect of different forestry practices as the species is very sensitive to environmental disturbances.

    The epiphytic lichen vegetation of six tree species occurring in the spruce forest is described. Clear successional trends with increasing tree size were • obtained for Alectoria sarmentosa, Bryoria fuscescens coll., B. nadvornikiana, Usnea filipendula' and U. subflorida'na, to a lesser extent for Bryoria capii-“" laris while U. longissima had no relationship to tree size or age.

    TTstudy of the litterfall of macrofragments of epiphytic lichens showed that thallus fragments were dispersed throughout the year with late autumn, winter and early spring as the most critical periods. It is suggested that dispersal through thallus fragmentation is more important in fruticose than in foliose species and that U. longissima has a shorter range of propagule transport than the other species of Alectoria, Bryoria and Usnea studied. The latter proposition was supported through a study of the horizontal patterns of lichen occurrence in the spruce forest.

    It is shown that the diversity in ground vegetation, after an initial in­crease, declines with succession in the pine forests. A mechanism of succes­sion in ground vegetation is presented which suggest that variations in habi­tat heterogeneity, i.e. the diversity of substrates caused by the initial dis­turbance and the stand development, largely determines diversity changes dur­ing succession. Trends of increasing thallus size, increasing size of asexual reproductive propagules and increased competitive ability with succession formed the basis for recognizing three types of strategies in Cladonia»

    It is concluded that lichens have features that are compatible with the r-K continuum and that they are variously adapted to both the stability of the substrates and that of the forest as a whole.

  • 6.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Strong influence of landscape structure on hair lichens in boreal forest canopies2019In: Canadian Journal of Forest Research, ISSN 0045-5067, E-ISSN 1208-6037, Vol. 48, no 8, p. 994-1003Article in journal (Refereed)
    Abstract [en]

    This study examines how island size, isolation, and orientation influence epiphytic hair lichens in old-growth boreal spruce forests within a naturally heterogeneous landscape with approximately 1000 forest islands distributed in open wetland matrix. Forest structure, length of Alectoria sarmentosa (Ach.) Ach., Bryoria spp., and Usnea spp., and mass of Alectoria in the lower canopy (0–5 m) of Picea abies (L.) Karst. were quantified in 30 islands (0.11–10.9 ha). Length and mass of Alectoria were also studied in 25 edges with different orientation and fetch (wind exposure). Island area had a strong positive effect on length of Alectoria but a minor effect on Bryoria and Usnea. Edge orientation influenced length and mass of Alectoria, with the strongest reduction in wind-exposed western edges, whereas fetch size had no effect. Edge influence on microclimate drives hair lichen response to landscape configuration. The gradient from Bryoria in small islands to Alectoria in large islands is caused by the same mechanisms that influence vertical canopy gradients in large homogeneous stands, with Bryoria in the upper canopy and Alectoria in the lower canopy. Genus-specific, sun-screening pigments contribute to this niche differentiation, but thallus fragmentation by wind and water storage are also important. Our findings imply that lichen conservation must consider the spatial structure of the landscape.

  • 7.
    Esseen, Per-Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Coxson, Darwyn S
    Lichens in forest ecosystems2015In: Routledge handbook of forest ecology / [ed] Kelvin S.-H. Peh, Richard T. Corlett and Yves Bergeron, Taylor & Francis, 2015, p. 250-263Chapter in book (Refereed)
  • 8.
    Esseen, Per-Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ekström, Magnus
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Westerlund, Bertil
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Jonsson, Bengt Gunnar
    Grafström, Anton
    Ståhl, Göran
    Broad-scale distribution of epiphytic hair lichens correlates more with climate and nitrogen deposition than with forest structure2016In: Canadian Journal of Forest Research, ISSN 0045-5067, E-ISSN 1208-6037, Vol. 46, no 11, p. 1348-1358Article in journal (Refereed)
    Abstract [en]

    Hair lichens are strongly influenced by forest structure at local scales, but their broad-scale distributions are less understood. We compared the occurrence and length of Alectoria sarmentosa (Ach.) Ach., Bryoria spp., and Usnea spp. in the lower canopy of > 5000 Picea abies (L.) Karst. trees within the National Forest Inventory across all productive forest in Sweden. We used logistic regression to analyse how climate, nitrogen deposition, and forest variables influence lichen occurrence. Distributions overlapped, but the distribution of Bryoria was more northern and that of Usnea was more southern, with Alectoria's distribution being intermediate. Lichen length increased towards northern regions, indicating better conditions for biomass accumulation. Logistic regression models had the highest pseudo R-2 value for Bryoria, followed by Alectoria. Temperature and nitrogen deposition had higher explanatory power than precipitation and forest variables. Multiple logistic regressions suggest that lichen genera respond differently to increases in several variables. Warming decreased the odds for Bryoria occurrence at all temperatures. Corresponding odds for Alectoria and Usnea decreased in warmer climates, but in colder climates, they increased. Nitrogen addition decreased the odds for Alectoria and Usnea occurrence under high deposition, but under low deposition, the odds increased. Our analyses suggest major shifts in the broad-scale distribution of hair lichens with changes in climate, nitrogen deposition, and forest management.

  • 9.
    Esseen, Per-Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hedstrom Ringvall, Anna
    Harper, Karen Amanda
    Christensen, Pernilla
    Svensson, Johan
    Factors driving structure of natural and anthropogenic forest edges from temperate to boreal ecosystems2016In: Journal of Vegetation Science, ISSN 1100-9233, E-ISSN 1654-1103, Vol. 27, no 3, p. 482-492Article in journal (Refereed)
    Abstract [en]

    Questions What factors control broad-scale variation in edge length and three-dimensional boundary structure for a large region extending across two biomes? What is the difference in structure between natural and anthropogenic edges? Location Temperate and boreal forests across all of Sweden, spanning latitudes 55-69 degrees N. MethodsWe sampled more than 2000 forest edges using line intersect sampling in a monitoring programme (National Inventory of Landscapes in Sweden). We compared edge length, ecosystem attributes (width of adjacent ecosystem, canopy cover, canopy height, patch contrast in canopy height, forest type) and boundary attributes (profile, abruptness, shape) of natural edges (lakeshore, wetland) with anthropogenic edges (clear-cut, agricultural, linear disturbance) in five regions. Results Anthropogenic edges were nearly twice as abundant as natural edges. Length of anthropogenic edges was largest in southern regions, while the abundance of natural edges increased towards the north. Edge types displayed unique spectrums of boundary structures, but abrupt edges dominated, constituting 72% of edge length. Anthropogenic edges were more abrupt than natural edges; wetland edges had the most gradual and sinuous boundaries. Canopy cover, canopy height, patch contrast and forest type depended on region, whereas overall boundary abruptness and shape showed no regional pattern. Patch contrast was related to temperature sum (degree days5 degrees C), suggesting that regional variability can be predicted from climate-controlled forest productivity. Boundary abruptness was coupled with the underlying environmental gradient, land use and forest type, with higher variability in deciduous than in conifer forest. Conclusions Edge origin, land use, climate and tree species are main drivers of broad-scale variability in forest edge structure. Our findings have important implications for developing ecological theory that can explain and predict how different factors affect forest edge structure, and help to understand how land use and climate change affect biodiversity at forest edges.

  • 10.
    Esseen, Per-Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olsson, Therese
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Coxson, Darwyn
    Gauslaa, Yngvar
    Morphology influences water storage in hair lichens from boreal forest canopies2015In: Fungal ecology, ISSN 1754-5048, E-ISSN 1878-0083, Vol. 18, p. 26-35Article in journal (Refereed)
    Abstract [en]

    Hair lichens (Alectoria, Bryoria, Usnea) with high surface-area-to-mass ratios rapidly trap moisture. By photography and scanning we examined how internal water storage depended on morphological traits in five species. Specific thallus mass (STM, mg DM cm(-2)) and water holding capacity (WHC, mg H2O cm(-2)) after shaking and blotting a fully hydrated thallus increased with thallus area. STM was approximate to 50% higher in Alectoria and Usnea thalli than in Bryoria. WHC was highest in Alectoria while percent water content of freshly blotted thalli was lowest in Usnea. Thallus area overlap ratio (TAO), assessing branch density of the thallus, was highest in the two thinnest Bryoria; lower in the thicker Usnea. Within species, hair lichens increased their water storage by increasing branch density rather than branch diameter. The taxonomically related genera Alectoria and Bryoria shared water storage characteristics, and differed from Usnea. Hair lichens in lower canopies have among the lowest water storage capacity reported in lichens. 

  • 11.
    Esseen, Per-Anders
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Rönnqvist, Marie
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gauslaa, Y.
    Coxson, D. S.
    Externally held water: a key factor for hair lichens in boreal forest canopies2017In: Fungal ecology, ISSN 1754-5048, E-ISSN 1878-0083, Vol. 30, p. 29-38Article in journal (Refereed)
    Abstract [en]

    Lichens hold water inside (internal pool) and outside their body (external pool). Yet, external pool size is not known in hair lichens dominating boreal forest canopies. Here we quantify morphological traits and internal/external water in two widespread Bryoria species along Picea abies canopy-height gradients: Bryoria fuscescens at 5-20 m and Bryoria capillaris at 15-20 m. Dry mass and specific thallus mass (STM) of intact B. fuscescens increased with height, while STM of individual branches did not. Maximum water holding capacity (mg H2O cm-2) increased with height, but did not differ between the species. Bryoria had much larger external (79-84% of total) than internal water pools, trapping water by dense clusters of thin, overlapping branches. They thus increase water storage in boreal forest canopies and influence hydrology. High external water storage extends hydration periods and improves lichen performance in upper canopies, and thereby contributes to the success of these hair lichens. 

  • 12. Farber, Leonie
    et al.
    Solhaug, Knut Asbjorn
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bilger, Wolfgang
    Gauslaa, Yngvar
    Sunscreening fungal pigments influence the vertical gradient of pendulous lichens in boreal forest canopies2014In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 95, no 6, p. 1464-1471Article in journal (Refereed)
    Abstract [en]

    Pendulous lichens dominate canopies of boreal forests, with dark Bryoria species in the upper canopy vs. light Alectoria and Usnea species in lower canopy. These genera offer important ecosystem services such as winter forage for reindeer and caribou. The mechanism behind this niche separation is poorly understood. We tested the hypothesis that species-specific sunscreening fungal pigments protect underlying symbiotic algae differently against high light, and thus shape the vertical canopy gradient of epiphytes. Three pale species with the reflecting pigment usnic acid (Alectoria sarmentosa, Usnea dasypoga, U. longissima) and three with dark, absorbing melanins (Bryoria capillaris, B. fremontii, B. fuscescens) were compared. We subjected the lichens to desiccation stress with and without light, and assessed their performance with chlorophyll fluorescence. Desiccation alone only affected U. longissima. By contrast, light in combination with desiccation caused photoinhibitory damage in all species. Usnic lichens were significantly more susceptible to light during desiccation than melanic ones. Thus, melanin is a more efficient light-screening pigment than usnic acid. Thereby, the vertical gradient of pendulous lichens in forest canopies is consistent with a shift in type and functioning of sunscreening pigments, from high-light-tolerant Bryoria in the upper to susceptible Alectoria and Usnea in the lower canopy.

  • 13. Fraver, S
    et al.
    Jonsson, B-G
    Jönsson, M
    Esseen, P-A
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Demographics and disturbance history of a boreal old-growth Picea abies forest2008In: Journal of Vegetation Science, ISSN 1100-9233, E-ISSN 1654-1103, Vol. 19, no 6, p. 789-798Article in journal (Refereed)
    Abstract [en]

    Question: To what extent do tree growth, mortality, and long-term disturbance patterns affect stand structure and composition of an old-growth Picea abies forest?

    Location: Boreal Sweden.

    Methods: We linked data from three 50 m × 50 m permanent plots established in 1986 with dendrochronology data to evaluate tree growth and mortality over an 18-year period and to describe a several-hundred-year disturbance history for this forest type.

    Results: Averaged over all diameters, P. abies trees had an annual mortality rate of 0.60%; however, diameter had a striking effect on both growth and mortality, with trees of intermediate diameters (ca. 20–30 cm) showing faster growth and lower mortality. Their increased vigor gave rise to a diameter distribution resembling the ‘rotated sigmoid’ (not reverse-J) proposed for such conditions, and it led to a deficit of snags of intermediate diameters. Slow-growing trees had an increased likelihood of dying. Although recruitment occurred in most decades over the past 400 years, two prominent recruitment peaks occurred (mid 1700s and 1800s), neither of which appeared to cause a shift in tree species composition. The lack of fire evidence suggests that fire was not responsible for these recruitment peaks.

    Conclusions: Taken together, these results depict a rather impassive system, where canopy trees die slowly over decades. Field observations suggest that fungal infections, mediated by wind, account for much of the mortality during these periods of relative quiescence. However, these periods are at times punctuated by moderate-severity disturbances that foster abundant recruitment.

    Nomenclature: Mossberg & Stenberg (2003).

  • 14. Fraver, Shawn
    et al.
    D'Amato, Anthony W.
    Bradford, John B.
    Jonsson, Bengt Gunnar
    Jonsson, Mari
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Tree growth and competition in an old-growth Picea abies forest of boreal Sweden: influence of tree spatial patterning2014In: Journal of Vegetation Science, ISSN 1100-9233, E-ISSN 1654-1103, Vol. 25, no 2, p. 374-385Article in journal (Refereed)
    Abstract [en]

    Question What factors best characterize tree competitive environments in this structurally diverse old-growth forest, and do these factors vary spatially within and among stands? Location Old-growth Picea abies forest of boreal Sweden. Methods Using long-term, mapped permanent plot data augmented with dendrochronological analyses, we evaluated the effect of neighbourhood competition on focal tree growth by means of standard competition indices, each modified to include various metrics of trees size, neighbour mortality weighting (for neighbours that died during the inventory period), and within-neighbourhood tree clustering. Candidate models were evaluated using mixed-model linear regression analyses, with mean basal area increment as the response variable. We then analysed stand-level spatial patterns of competition indices and growth rates (via kriging) to determine if the relationship between these patterns could further elucidate factors influencing tree growth. Results Inter-tree competition clearly affected growth rates, with crown volume being the size metric most strongly influencing the neighbourhood competitive environment. Including neighbour tree mortality weightings in models only slightly improved descriptions of competitive interactions. Although the within-neighbourhood clustering index did not improve model predictions, competition intensity was influenced by the underlying stand-level tree spatial arrangement: stand-level clustering locally intensified competition and reduced tree growth, whereas in the absence of such clustering, inter-tree competition played a lesser role in constraining tree growth. Conclusions Our findings demonstrate that competition continues to influence forest processes and structures in an old-growth system that has not experienced major disturbances for at least two centuries. The finding that the underlying tree spatial pattern influenced the competitive environment suggests caution in interpreting traditional tree competition studies, in which tree spatial patterning is typically not taken into account. Our findings highlight the importance of forest structure - particularly the spatial arrangement of trees - in regulating inter-tree competition and growth in structurally diverse forests, and they provide insight into the causes and consequences of heterogeneity in this old-growth system.

  • 15. Grafström, Anton
    et al.
    Ekström, Magnus
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics. Department of Forest Resource Management, Swedish University of Agricultural Sciences, SE-90183 Umeå, Sweden.
    Jonsson, Bengt Gunnar
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ståhl, Göran
    On combining independent probability samples2019In: Survey Methodology, ISSN 0714-0045, E-ISSN 1492-0921, Vol. 45, no 2, p. 349-364Article in journal (Refereed)
    Abstract [en]

    Merging available sources of information is becoming increasingly important for improving estimates of population characteristics in a variety of fields. In presence of several independent probability samples from a finite population we investigate options for a combined estimator of the population total, based on either a linear combination of the separate estimators or on the combined sample approach. A linear combination estimator based on estimated variances can be biased as the separate estimators of the population total can be highly correlated to their respective variance estimators. We illustrate the possibility to use the combined sample to estimate the variances of the separate estimators, which results in general pooled variance estimators. These pooled variance estimators use all available information and have potential to significantly reduce bias of a linear combination of separate estimators.

  • 16. Harper, K A
    et al.
    MacDonald, S E
    Burton, P J
    Chen, J
    Brosofske, K D
    Saunders, S C
    Euskirchen, E S
    Roberts, D
    Jaiteh, M S
    Esseen, P-A
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Edge influence on forest structure and composition in fragmented landscapes2005In: Conservation Biology, ISSN 0888-8892, E-ISSN 1523-1739, Vol. 19, p. 768-782Article in journal (Refereed)
  • 17. Harper, Karen A.
    et al.
    Macdonald, S. Ellen
    Mayerhofer, Michael S.
    Biswas, Shekhar R.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hylander, Kristoffer
    Stewart, Katherine J.
    Mallik, Azim U.
    Drapeau, Pierre
    Jonsson, Bengt-Gunnar
    Lesieur, Daniel
    Kouki, Jari
    Bergeron, Yves
    Edge influence on vegetation at natural and anthropogenic edges of boreal forests in Canada and Fennoscandia2015In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 103, no 3, p. 550-562Article in journal (Refereed)
    Abstract [en]

    Although anthropogenic edges are an important consequence of timber harvesting, edges due to natural disturbances or landscape heterogeneity are also common. Forest edges have been well studied in temperate and tropical forests, but less so in less productive, disturbance-adapted boreal forests. We synthesized data on forest vegetation at edges of boreal forests and compared edge influence among edge types (fire, cut, lake/wetland; old vs. young), forest types (broadleaf vs. coniferous) and geographic regions. Our objectives were to quantify vegetation responses at edges of all types and to compare the strength and extent of edge influence among different types of edges and forests. Research was conducted using the same general sampling design in Alberta, Ontario and Quebec in Canada, and in Sweden and Finland. We conducted a meta-analysis for a variety of response variables including forest structure, deadwood abundance, regeneration, understorey abundance and diversity, and non-vascular plant cover. We also determined the magnitude and distance of edge influence (DEI) using randomization tests. Some edge responses (lower tree basal area, tree canopy and bryophyte cover; more logs; higher regeneration) were significant overall across studies. Edge influence on ground vegetation in boreal forests was generally weak, not very extensive (DEI usually <20m) and decreased with time. We found more extensive edge influence at natural edges, at younger edges and in broadleaf forests. The comparison among regions revealed weaker edge influence in Fennoscandian forests.Synthesis. Edges created by forest harvesting do not appear to have as strong, extensive or persistent influence on vegetation in boreal as in tropical or temperate forested ecosystems. We attribute this apparent resistance to shorter canopy heights, inherent heterogeneity in boreal forests and their adaptation to frequent natural disturbance. Nevertheless, notable differences between forest structure responses to natural (fire) and anthropogenic (cut) edges raise concerns about biodiversity implications of extensive creation of anthropogenic edges. By highlighting universal responses to edge influence in boreal forests that are significant irrespective of edge or forest type, and those which vary by edge type, we provide a context for the conservation of boreal forests.

  • 18. Jansson, K. Ulrika
    et al.
    Nilsson, Mats
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Length and classification of natural and created forest edges in boreal landscapes throughout northern Sweden2011In: Forest Ecology and Management, ISSN 0378-1127, E-ISSN 1872-7042, Vol. 262, no 3, p. 461-469Article in journal (Refereed)
    Abstract [en]

    Forest edges have numerous implications for structure and function of forest ecosystems. Previous studies on edge quantity have used broad classifications. However, edge influence is driven by the contrast in vegetation structure between adjoining ecosystems, and thus we need detailed site-specific data to assess the role of edges in forests. We studied the variability of sharp edges in 28 boreal landscapes (4 km × 4 km) across an 830 km gradient throughout northern Sweden. Our objectives were: (1) to compare the length of natural and created edges, (2) to classify edges in detail by edge origin, maintenance and forest attributes, and (3) to examine relationships between length of edge and landscape variables. Data were collected using stereo-interpretation of high spatial resolution colour infrared aerial photographs, in combination with line intersect sampling and plots. The length of edge varied from 12 to 102 m ha−1 among landscapes, with an overall mean of 54 m ha−1. Created edges dominated most landscapes (mean 33 m ha−1) and had greater variability than natural edges (mean 21 m ha−1). Maintained edges (e.g. roads, agricultural land) were more abundant than regenerating edges caused by logging. Thirty percent of total edges adjoined narrow linear features. Seventy percent adjoined wider patches and showed high variability (35 classes). Overall, high-contrast edges towards mature forest dominated, i.e. ones that may experience strong edge influence. The amount of edge increased with percent of landscape affected by disturbance, and decreased with latitude and elevation. This study shows that edges are both abundant and highly variable in boreal forests and that forestry is the main driver behind edge creation. Detailed classification of edges based on site-specific forest and patch attributes may help to estimate edge influence at landscape level, and can guide experimental design for examining the impact of edges on structure and function of forest ecosystems.

  • 19.
    Jansson, Ulrika
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Edge, canopy and neighbourhood attributes as predictors of potential edge influence in boreal forestsManuscript (Other academic)
    Abstract [en]

    Forest edges play a key role in fragmented ecosystems and may strongly influence biodiversity. Edge influence is complex and site specific due to interactions among several local and regional factors. Data on these factors are crucial to predict potential edge influence. We assessed the variability in edge, canopy and neighbourhood attributes among 1008 sharp forest edges in northern Sweden. The edges represented three edge types: natural (wetland, water), maintained (e.g. roads, agriculture) and regenerating (clearcuts, young forest). Data was collected from 28 landscapes (each 1600 ha) using line intersect sampling and stereo interpretation of colour infrared aerial photographs in a digital photogrammetric system. Due to the topography, natural edges were predominantly oriented towards SW or NE while created edges had a more uniform pattern. Created edges had denser canopies and higher contrast in tree height than natural edges. Regenerating edges had the largest fetch, followed by natural and maintained edges. The regenerating edges thus experience the highest risk of wind damage. The majority of edges in mature productive conifer forest had high contrast and large fetch. Overall, edges showed high variability with respect to potential for microclimatic edge influence. This suggests that biodiversity in boreal forests may have a broad range of responses to edges. Our analysis shows that predictions of edge influence on biodiversity in forest ecosystems can be improved by combining edge, canopy and neighbourhood attributes.

  • 20.
    Jansson, Ulrika
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Mats
    Forest Resource Management, SLU, Umeå.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Forest edge density in a gradient of boreal landscapes in SwedenManuscript (Other academic)
    Abstract [en]

    The boreal forests in Fennoscandia are strongly fragmented by intensive forestry and other land use resulting in high amount of edge habitat. Detailed data on edge quantity is critical to assess fragmentation effects on biodiversity. We estimated the density of sharp forest edges in 28 landscapes, each 4 km × 4 km, along a 830 km gradient in northern Sweden. Data was collected from colour infrared aerial photographs using line intersect sampling and 20-m radius plots. Forest edge density was 54 ± 4.6 m ha-1 land area (mean ± SE) but variation was high among landscapes (12-102 m ha-1). Natural edges constituted 38% but created edges dominated (36% maintained and 26% regenerating). Thirty percent of edges adjoined narrow (5-19 m) open landscape elements (corridor edges) and seventy percent adjoined more exposed patches (≥20 m, patch edges). We found 34 types of patch edges, with the highest density recorded for edges in mature forest. Edge density increased with proportion of landscape disturbed by forestry and agriculture, and decreased with latitude and altitude, but natural and created edges showed contrasting patterns. The high density of sharp edges and the variability among landscapes implies that edges may have strong and diverse effects on the structure, function and biodiversity of boreal forest ecosystems. This variability must be taken into account when formulating strategies for sustainable forest management.

  • 21.
    Jansson, Ulrika
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Growth of the old forest lichen Usnea longissima in forest edges2009In: The Lichenologist, ISSN 0024-2829, E-ISSN 1096-1135, Vol. 41, no 6, p. 663-672Article in journal (Refereed)
    Abstract [en]

    The lichen Usnea longissima was used to examine how distance from forest edge and edge contrast influence growth of pendulous lichens. Thalli of two sizes (12 and 27 cm) were transplanted to the lower canopy of old Picea abies forest at 5, 25 and 100 m distance from cutovers. Sites represented three levels of edge contrast: high (clear-cut), intermediate (3 m tall saplings) and low (6-7 m tall young forest). Lichen growth was assessed as annual length and weight gain. Growth rates of intact thalli were size-dependent, with both growth variables being higher in long than in short thalli. Distance and edge contrast had significant effects on weight gain in long thalli but not in short ones. Weight gain in long thalli was twice as high near the edge (23%) compared to the forest interior (12%). The highest weight gain (31%) occurred at intermediate contrast edges with lower growth at both low (18%) and high contrast edges (20%). Chlorophyll a concentration was highest near the edge and positively correlated with weight gain, so growth was apparently stimulated by both increased photosynthetic capacity and higher light availability near the edge. The lower part of the canopy in forest edges apparently have favourable growth conditions for U. longissima with growth being influenced by vegetation on adjoining cutovers. Therefore growth responses can not explain the previously observed decline of pendulous lichens following edge creation. Our results suggest that vegetation buffers can improve conditions for pendulous lichens near forest edges.

  • 22. Jonsson, Bengt Gunnar
    et al.
    Ekström, Magnus
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Grafström, Anton
    Ståhl, Göran
    Westerlund, Bertil
    Dead wood availability in managed Swedish forests: policy outcomes and implications for biodiversity2016In: Forest Ecology and Management, ISSN 0378-1127, E-ISSN 1872-7042, Vol. 376, p. 174-182Article in journal (Refereed)
    Abstract [en]

    Dead wood is a critical resource for forest biodiversity and widely used as an indicator for sustainable forest management. Based on data from the Swedish National Forest Inventory we provide baseline information and analyze trends in volume and distribution of dead wood in Swedish managed forests during 15 years. The data are based on ≈30,000 sample plots inventoried during three periods (1994–1998; 2003–2007 and 2008–2012). The forest policy has since 1994 emphasized the need to increase the amount of dead wood in Swedish forests. The average volume of dead wood in Sweden has increased by 25% (from 6.1 to 7.6 m3 ha−1) since the mid-1990s, but patterns differed among regions and tree species. The volume of conifer dead wood (mainly from Picea abies) has increased in the southern part of the country, but remained stable or decreased in the northern part. Heterogeneity of dead wood types was low in terms of species, diameter and decay classes, potentially negatively impacting on biodiversity. Overall, we found only minor effects of the current forest policy since most of the increase can be attributed to storm events creating a pulse of hard dead wood. Therefore, the implementation of established policy instruments (e.g. legislation and voluntary certification schemes) need to be revisited. In addition to the retention of dead trees during forestry operations, policy makers should consider calling for more large-scale targeted creation of dead trees and management methods with longer rotation cycles.

  • 23. Jönsson, Mari T.
    et al.
    Fraver, Shawn
    Jonsson, Bengt Gunnar
    Dynesius, Mats
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Rydgård, Mats
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Eighteen years of tree mortality and structural change in an experimentally fragmented Norway spruce forest2007In: Forest Ecology and Management, Vol. 242, no 2-3, p. 306-313Article in journal (Refereed)
    Abstract [en]

    Long-term experimental forest fragmentation studies remain uncommon, despite their critical role in the advancement of ecological theory and conservation planning. In 1986 five circular forest fragments (1/16–1 ha) were exposed through clearcutting within an old-growth Norway spruce (Picea abies) forest in northern Sweden. Initial responses to fragmentation (1986–1991) showed very high tree mortality and structural degradation of the fragments. In the present study we re-inventoried these fragments to evaluate tree mortality patterns and structural changes occurring over a longer time period (1991–2004). The fragments can readily be viewed as harvest retention patches or ‘woodland key habitats’ (i.e., set-aside patches of high conservation value), allowing us to make inferences about the effectiveness of these novel conservation tools. Tree mortality rates dropped markedly (to 1.2–3.9%/year) compared to the initial responses, yet remained elevated over those of control plots in the nearby unfragmented forest (0.7%). Mortality increased with tree diameter, resulting in smaller-diameter, more homogenous stands. Mortality also generally increased with decreasing fragment size and was dependent of tree location within fragments. Standing death (45% of dead trees, 1991–2004) replaced uprootings (71%, 1986–1991) as the dominant mode of mortality. Numbers of dying and standing dead trees increased during the second sampling period, further adding to structural change and reduced stand density. Elevated tree mortality resulted in uncharacteristically high volumes of coarse woody debris. Results clearly show that adverse edge-related changes to forest structure and function persist up to two decades after fragmentation. Fragments of this size largely fail as remnants intended to maintain forest interior conditions and late-successional forest structure. However, when embedded within a harvested landscape, they: (1) provide abundant coarse woody debris and snags for deadwood-dependent species that risk extirpation in the surrounding matrix and (2) retain important structures for the developing stands.

  • 24. Ramezani, Habib
    et al.
    Svensson, Johan
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Landscape Environmental Monitoring: Sample Based Versus Complete Mapping Approaches in Aerial Photographs2011In: Environmental Monitoring / [ed] Ema O. Ekundayo, InTech , 2011, p. 205-218Chapter in book (Other academic)
  • 25. Ståhl, Göran
    et al.
    Allard, Anna
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Glimskär, Anders
    Ringvall, Anna
    Svensson, Johan
    Sundquist, Sture
    Christensen, Pernilla
    Gallegos Torell, Åsa
    Högström, Mats
    Lagerqvist, Kjell
    Marklund, Liselott
    Nilsson, Björn
    Inghe, Ola
    National inventory of landscapes in Sweden (NILS): scope, design, and experiences from establishing a multi-scale biodiversity monitoring system2011In: Environmental Monitoring & Assessment, ISSN 0167-6369, E-ISSN 1573-2959, Vol. 173, no 1-4, p. 579-595Article in journal (Refereed)
    Abstract [en]

    The landscape-level and multiscale biodiversity monitoring program National Inventory of Landscapes in Sweden (NILS) was launched in 2003. NILS is conducted as a sample-based stratified inventory that acquires data across several spatial scales, which is accomplished by combining aerial photo interpretation with field inventory. A total of 631 sample units are distributed across the land base of Sweden, of which 20% are surveyed each year. By 2007 NILS completed the first 5-year inventory phase. As the reinventory in the second 5-year phase (2008-2012) proceeds, experiences and insights accumulate and reflections are made on the setup and accomplishment of the monitoring scheme. In this article, the emphasis is placed on background, scope, objectives, design, and experiences of the NILS program. The main objective to collect data for and perform analyses of natural landscape changes, degree of anthropogenic impact, prerequisites for natural biological diversity and ecological processes at landscape scale. Different environmental conditions that can have direct or indirect effects on biological diversity are monitored. The program provides data for national and international policy and offers an infrastructure for other monitoring program and research projects. NILS has attracted significant national and international interest during its relatively short time of existence; the number of stakeholders and cooperation partners steadily increases. This is constructive and strengthens the incentive for the multiscale monitoring approach.

  • 26.
    Ståhl, Göran
    et al.
    Department of Forest Resource Management, SwedishUniversity of Agricultural Sciences, SE 901 83 Umeå, Sweden.
    Ekström, Magnus
    Umeå University, Faculty of Social Sciences, Umeå School of Business and Economics (USBE), Statistics.
    Dahlgren, Jonas
    Department of Forest Resource Management, SwedishUniversity of Agricultural Sciences, SE 901 83 Umeå, Sweden.
    Esseen, Per-Anders
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Grafström, Anton
    Department of Forest Resource Management, SwedishUniversity of Agricultural Sciences, SE 901 83 Umeå, Sweden.
    Jonsson, Bengt-Gunnar
    Department of Natural Sciences, Mid-Sweden University, SE 851 70 Sundsvall, Sweden.
    Informative plot sizes in presence-absence sampling of forest floor vegetation2017In: Methods in Ecology and Evolution, ISSN 2041-210X, E-ISSN 2041-210X, Vol. 8, no 10, p. 1284-1291Article in journal (Refereed)
    Abstract [en]

    1. Plant communities are attracting increased interest in connection with forest and landscape inventories due to society’s interest in ecosystem services. However, the acquisition of accurate information about plant communities poses several methodological challenges. Here, we investigate the use of presence-absence sampling with the aim to monitor state and change in plant density. We study what plot sizes are informative, i.e. the estimators should have as high precision as possible.

    2. Plant occurrences were modelled through different Poisson processes and tests were developed for assessing the plausibility of the model assumptions. Optimum plot sizes were determined by minimizing the variance of the estimators. While state estimators of similar kind as ours have been proposed in previous studies, our tests and change estimation procedures are new.

    3. We found that the most informative plot size for state estimation is 1.6 divided by the plant density, i.e. if the true density is 1 plant per square metre the optimum plot size is 1.6 square metres. This is in accordance with previous findings. More importantly, the most informative plot size for change estimation was smaller and depended on the change patterns. We provide theoretical results as well as some empirical results based on data from the Swedish National Forest Inventory.

    4. Use of too small or too large plots resulted in poor precision of the density (and density change) estimators. As a consequence, a range of different plot sizes would be required for jointly monitoring both common and rareplants using presence-absence sampling in monitoring programmes.

  • 27. Will-Wolf, S
    et al.
    McCune, B
    Esseen, P-A
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Contribution of lichen ecology to understanding and managing ecosystems2006In: Journal of Vegetation Science, Vol. 17, p. 123-124Article in journal (Refereed)
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