Although forest edges have been studied extensively as an important consequence of fragmentation, a unifying theory of edge influence has yet to be developed. Our objective was to take steps toward the development of such a theory by (1) synthesizing the current knowledge of patterns of forest structure and composition at anthropogenically created forest edges, (2) developing hypotheses about the magnitude and distance of edge influence that consider the ecological processes influencing these patterns, and (3) identifying needs for future research. We compiled data from 44 published studies on edge influence on forest structure and composition in boreal, temperate, and tropical forests. Abiotic and biotic gradients near created forest edges generate a set of primary responses to edge creation. Indirect effects from these primary responses and the original edge gradient perpetuate edge influence, leading to secondary responses. Further changes in vegetation affect the edge environment, resulting in ongoing edge dynamics. We suggest that the magnitude and distance of edge influence are a direct function of the contrast in structure and composition between adjacent communities on either side of the edge. Local factors such as climate, edge characteristics, stand attributes, and biotic factors affect patch contrast. Regional factors define the context within which to assess the ecological significance of edge influence (the degree to which the edge habitat differs from interior forest habitat). Our hypotheses will help predict edge influence on structure and composition in forested ecosystems, an important consideration for conservation. For future research on forest edges in fragmented landscapes, we encourage the testing of our hypotheses, the use of standardized methodology, complete descriptions of study sites, studies on other types of edges, synthesis of edge influence on different components of the ecosystem, and investigations of edges in a landscape context. Harper et al. Edge Influence in Fragmented Landscapes 769 Resumen: Aunque los bordes de bosque han sido extensivamente estudiados como una importante consecuencia de la fragmentación, aún no se ha desarrollado una teoría unificadora de la influencia del borde. Nuestros objetivos fueron acercarnos al desarrollo de tal teoría mediante (1) la síntesis del conocimiento actual de los patrones de estructura y composición del bosque en bordes de bosque creados antropogénicamente; (2) el desarrollo de hipótesis relacionadas con la magnitud y distancia de la influencia del borde considerando procesos ecológicos que influyen sobre esos patrones; y (3) la identificación de futuras necesidades de investigación. Recopilamos datos de 44 estudios publicados sobre la influencia del borde sobre la estructura y composición de bosques boreales, templados y tropicales. Los gradientes bióticos y abióticos creados cerca de los bordes de bosque generan una serie de respuestas primarias a la creación del borde. Los efectos indirectos de estas respuestas primarias y el borde ori...

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Summary 1.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. 2. 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. 3. 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. 4. 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 < 20 m) 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. 5. 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.

We studied the spatial and temporal response of three epiphytic lichens (Bryoria spp., Evernia mesomorpha Nyl., and Usnea spp.) to edge effects in conifer forests of northwestern Quebec. Lichen abundance and substrate variables were sampled at four distances (5, 25, 50, and 100 m) from the edge in 15 edge–interior transects in managed black spruce (Picea mariana (Mill.) BSP) forests. Responses to edge effects were assessed for short-term (1–3 years) and long-term (8 and 16–23 years) periods. We also assessed the effect of forest fragment size on lichen abundance in 27 forest fragments ranging in size from 0.03 ha to >4 ha. We found a significant effect of distance to edge on mass of E. mesomorpha and Usnea spp. Lichen mass was lower in the first 50 m of edge compared with the forest interior (100 m). Size of forest fragments had no significant effect in any of the three lichens. However, mass of Usnea spp. was low in the seven smallest (<0.5 ha) fragments. Our results suggest that the response of epiphytic lichens to edge conditions could be used by forest managers as an indicator in determining the size of remnant stands that should be left to increase the proportion of interior forest habitat in harvested forest landscapes.

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.

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