Karen A. Harper

Structure and composition of edges next to regenerating clear-cuts in mixed-wood boreal forest

Abstract We investigated the spatial and temporal pattern of edge influence on primary (forest structure) and secondary (understory structure and composition) responses at forest edges next to regenerating clear-cuts in Populus-dominated boreal forest in Alberta, Canada. We used before/after-harvest comparisons to determine whether there were significant effects of edge creation in the first and second year after harvest. We also investigated 5- and 16-yr-old edges to determine whether their structure and composition were significantly different from interior forest. Distance of edge influence (20 - 60 m) was not extensive compared to findings from studies conducted in other forest types. In the first two years after harvest, there were significant primary responses to edge creation including increased tree mortality, snag breakage and amount of downed coarse woody debris; as well as secondary responses including prolific vegetative regeneration of Populus spp. and a decrease in total shrub cover. Significant edge effects were still detectable 5 and 16 yr after edge creation, despite the rapid regeneration of Populus- dominated forest in the adjacent clear-cuts. These older edges showed evidence of past primary and secondary responses, including a more heterogeneous canopy, along with additional changes in understory composition. At both younger and older edges the distance of edge influence was greater for secondary responses (up to 60 m) than for primary responses (10 - 20 m). We hypothesize that secondary responses to edge creation start later than primary responses and that the distance of edge influence is greater for secondary (vs. primary) responses. Nomenclature: Moss (1983). Abbreviations: CWD = Downed coarse woody debris; DEI = Distance of edge influence; DEIc = Distance of edge influence for change from the previous year.

Gap disturbances in northern old-growth forests of British Columbia, Canada

Abstract We characterized the abundance, size and spatial patterning of canopy gaps, as well as gap-forming processes and light availability in boreal, sub-boreal, northern temperate and subalpine old-growth forests of northwestern British Columbia. The proportion of area in canopy gaps ranged from 32% in northern temperate forests to 73% in subalpine forests. Evenly distributed developmental gaps were dominant but permanent openings created by edaphic components and by shrub communities were also common, particularly in sub-boreal forests. Abundant gaps, large gap sizes, high numbers of gap makers per gap and frequent gap expansion events suggest that gaps have long tenure in these forests. Snapped stems and standing dead mortality were the most common modes of mortality in all forest types resulting in little forest floor disturbance, creating few germination sites for seedling establishment. We found high mean light levels (16–27% full sun) and little difference between non-gap and gap light environments. Our results suggest that gap dynamics in these forests differ fundamentally from those in temperate and tropical forest ecosystems. Nomenclature: Hitchcock & Cronquist (1994).

Edge influence on forest structure in large forest remnants, cutblock separators, and riparian buffers in managed black spruce forests

ABSTRACT Remnants of old forests left on the landscape following forest harvesting, especially corridors, provide benefits of connectivity and facilitation of movement or dispersal, which may be hindered by the presence of edges. Our objective was to determine the extent of edge influence on forest structure in these forest remnants in black spruce boreal forest. We sampled canopy cover and the density of trees, snags, and logs along clearcut edge–forest gradients in large forest patches, cutblock separators, and riparian buffers. The distance-of-edge influence was determined by comparing values at different distances from the edge to values in interior forest using randomization tests. Forest remnants had lower live tree density and canopy cover and higher mortality and windthrow than interior forest. Distance-of-edge influence on forest structure extended 10–30 m from the edge, and was slightly more extensive into cutblock separators where two edges are in close proximity, but was less extensive in riparian buffers, possibly due to the presence of stable internal edges near the stream. Because of edge influence, structure near the edges of forest remnants and across narrow corridors is modified; wider corridors would be required to provide a core habitat of interior forest conditions.

Edge influence on vegetation at natural and anthropogenic edges of boreal forests in Canada and Fennoscandia

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 ...

The role of the circumarctic forest–tundra ecotone for Arctic biodiversity

The arctic forest–tundra ecotone (FTE), which links species communities of the boreal forest with those of the arctic tundra, is expected to respond swiftly to climate change with a profound reduction of tundra as the dominating scenario. With its circumarctic expanse and up to several hundred kilometres in width, the zone occupies a large part of the vegetated surface at high latitudes. Relocation and structural changes of the ecotone vegetation will affect not only plant but also animal and other biological diversity. A large number of arctic species are dependent on the FTE in terms of food and habitat during parts of their life cycle or annual migration. In the ‘Arctic Species Trend Index’, developed to provide trends in arctic vertebrates, more than half of the species and data are from the FTE. However, in assessments of arctic biodiversity, only the northernmost tundra-dominated areas of the ecotone are included. This is unfortunate and somewhat problematic since the treed part that serves as a source of seeds for new seedlings and saplings in the tundra-dominated part is excluded. This inconsistency hampers monitoring efficiency and biodiversity conservation efforts. During the International Polar Year, a large international research project on the FTE established numerous sites around the circumpolar north where causes and consequences of vegetation change were analysed. This network of sites and data forms an excellent basis for necessary monitoring of the spatial and temporal complexity of forest encroachment into tundra and its relation to arctic biodiversity.

Changes in spatial pattern of trees and snags during structural development in Picea mariana boreal forests

Abstract Questions: How do gap abundance and the spatial pattern of trees and snags change throughout stand development in Picea mariana forests? Does spatial pattern differ among site types and structural components of a forest? Location: Boreal forests dominated by Picea mariana, northern Quebec and Ontario, Canada. Methods: Data on the abundance, characteristics and spatial location of trees, snags and gaps were collected along 200 m transects at 91 sites along a chronosequence. Spatial analyses included 3TLQV, NLV and autocorrelation analysis. Non-parametric analyses were used to analyse trends with time and differences among structural components and site types. Results: Gaps became more abundant, numerous and more evenly distributed with time. At distances of 1–4 m, tree cover, sapling density and snag density became more heterogeneous with time. Tree cover appeared to be more uniform for the 10–33 m interval, although this was not significant. Patch size and variance at 1 m were greater for overstorey than for understorey tree cover. Snags were less spatially variable than trees at 1 m, but more so at intermediate distances (4 - 8 m). Few significant differences were found among site types. Conclusions: During stand development in P. mariana forest, gaps formed by tree mortality are filled in slowly due to poor regeneration and growth, leading to greater gap abundance and clumping of trees and snags at fine scales. At broader scales, patchy regeneration is followed by homogenization of forest stands as trees become smaller with low productivity due to paludification. Abbreviations: 3TLQV = Three-term local quadrat variance; NLV = New local variance.

Interaction of edge influence from multiple edges: examples from narrow corridors

The simultaneous influence from multiple edges on remnant forest patches (such as wildlife corridors, protection buffers, small unharvested remnants or corners of larger patches) in harvested forest landscapes could impair, or possibly enhance, their effectiveness. When multiple edges are in close proximity, there may be interactions of edge influence such that the observed response is greater or less than would result from the influence of either edge alone. We examined possible ways in which two nearby forest edges of similar or different types might interact in terms of their influence on forest structure. We present an ‘edge interaction’ model for three possible types of interaction of edge influence: (1) no interaction, edge influence is limited to the strongest influence from either edge, (2) positive interaction, observed edge influence is greater than from either edge alone; (3) negative interaction or resistance, the influence from both edges is less than from a single edge (e.g., an older edge is resistant to effects from a younger edge). Empirical data for forest structure at the edges of cutblocks (harvested areas) and water bodies were entered into the models to predict edge influence in narrow forest corridors assuming the null hypothesis of no interaction. Randomization tests were used to compare predictions to observed edge influence on recently-fallen logs and Populus spp. (P. tremuloides Michx. and P. balsamifera L.) sapling density in lakeshore buffers in boreal mixedwood forest as well as on canopy cover and log, tree and snag abundance in riparian buffers and forested corridors separating cutblocks in Picea mariana (Mill.) BSP. forest. In lakeshore buffers, there was evidence of both positive and negative interaction at different locations within the buffer or at different times since buffer creation for both the abundance of logs and Populus sapling density. Trends suggested positive interaction for canopy cover and live tree density in riparian buffers and cutblock separators, and for snag density near the cut edges of riparian buffers. Testing hypotheses arising from our model of interaction of edge influence could lead to a clearer understanding of edge influence in fragmented landscapes.

The role of edge contrast and forest structure in edge influence: vegetation and microclimate at edges in the Brazilian cerrado

The effect of the adjacent non-forested environment on the forest near the edge, edge influence (EI), is an important impact in fragmented landscapes and is believed to vary with factors such as forest structure and edge contrast. In order to improve our understanding of the factors governing the variability in EI, we studied microclimate and vegetation at cerrado edges surrounded by variable land uses in southeastern Brazil, a system with both forest and savanna fragments. We determined the significance, magnitude and distance of EI on microclimate, vegetation structure and grass biomass which we measured along five transects perpendicular to fourteen edges in forest or savanna next to different land uses. We introduce a quantitative measure of edge contrast that considers land uses at different distances from the same edge (e.g., a firebreak between a forest edge and a plantation) and verified whether edge contrast is correlated with EI in this system. Notwithstanding the large variation in EI among variables and study sites, there were some similarities in the patterns of EI between forest and savanna edges. Edge contrast was successfully quantified by our measure but was only correlated with EI on moisture and grass biomass. Our results point to the high variability in EI within a region. Our quantitative measure of edge contrast may be useful in explaining variability in EI. However, much unexplained variation remains in the highly fragmented cerrado system which is affected by EI in both forest and savanna fragments.

Spatial pattern in the organic layer and tree growth: A case study from regenerating Picea mariana stands prone to paludification

Abstract Questions: 1. How does the spatial structure of the organic layer affect tree sapling physiology? 2. Are the organic layer and Picea mariana height growth spatially structured at different scales? 3. Does microtopography influence the accumulation of organic matter and does organic layer thickness affect height growth? Locations: Picea mariana forests, northwestern Quebec, Canada. Methods: We assessed the spatial pattern of each variable in one wildfire site and one harvest site using semivariograms and correlograms. We measured the cross-correlation between relative elevation and organic layer thickness, and between organic layer thickness and growth using cross-correlograms. Results: Picea mariana height growth was autocorrelated to a greater extent in the wildfire site (103 m) than in the harvest site (43 m). The spatial structure of organic layer thickness was similar in both sites. Deeper depressions in the harvest site, as illustrated by spatial variance in relative elevation at short distances (ca. 50 m), and by high autocorrelation values, increased the accumulation of organic matter within 20 m. Conclusions: The interaction between microtopography and organic matter accumulation led to paludification and poor growth of Picea mariana at the harvest site. Paludification at the wildfire site was independent of microtopography and was probably a result of stand development.

Old-Growth Forests in the Canadian Boreal: the Exception Rather than the Rule?

Old-growth boreal forest is considered rare since large fires or insect disturbances occur too frequently to allow forest stands to reach old ages. Recent research on historical fire regimes in the circumboreal region have shown that this assertion is false for several reasons. First, fire frequency varies greatly among regions and natural fire cycles are often longer than the normal lifespan of the trees, especially in northeastern Canada, allowing stands of fire-adapted species to be replaced by self-maintaining late successional species. Second, even in landscapes characterised by short fire intervals, the stochastic occurrence of fires allows some parts of the landscape to escape fire for long periods. Old-growth boreal forests possess compositional, structural and functional characteristics, such as a legacy of deadwood and greater structural diversity, that differ significantly from those of post-fire forests. Current forestry practices in boreal forests only partly emulate natural stand dynamics and lead to an important decrease in the proportion of old-growth forests.