Aspen Mixedwood Biodiversity Project

Resource managers are concerned that intensive clearcut logging of Alberta’s aspen-dominated boreal mixedwood forests at 60–70 year rotations may alter forest age class structure of the forest landscape and result in a change in forest structure and biota. In response to these concerns, we studied forest structure and biotic composition in young (20–30 years) mature (50–65 years) and old (120+ years) aspen mixedwood stands of fire origin in Alberta. 

Stand and Soil Structure

Old stands were structurally distinct from younger seral stages. Their uniqueness was related to a combination of canopy break-up and release of understory plants, emergence of secondary canopy species (white spruce and paper birch), deadwood accumulation, and the nonvascular communities that develop on deadwood. Relative to younger seral stages, old stands had trees of many ages and had more large-canopy trees, large snags, and advanced rot-class large deadwood. 

Old stands had a deeper organic soil layer and greater microtopographic relief than young and mature stands. The accumulation of organic material in the upper soil horizon may be related to greater input of deadwood from the canopy and subcanopy or to slower decomposition in cooler soils found in old stands. Young stands had features in common with old stands because of residual materials from the previous stand cohort that escaped combustion or because light conditions similar to old stands. 

Relative to young and old stands, mature stands were simple in structure with a closed canopy of even-age aspen, height. and diameter. In addition, mature stands had a more open understory with fewer shrubs and saplings. 

We found successional changes in stand heterogeneity. Fires produced spatially heterogeneous patterns in tree and deadwood density in young stands. The degree of spatial heterogeneity within and among stands was lowest in mature stands. In old forests, tree and deadwood density was relatively uniform within stands, but variable among stands. 

Microclimate varied among stand age by season. Among forests, young stands were colder during the day and warmer at night in summer, and warmer at night during winter. Old stands were warmer during the day during winter. Soil temperatures below ground varied by season and stand age. Old stands had lower soil temperatures than other stand ages in summer while young stands had the coldest soils in winter. Photosynthetically active radiation (PAR) varied seasonally and was related to canopy and subcanopy leaf growth and fall. Mature stands had lower PAR and higher wind speeds than did young and old stands. Herb cover during summer was positively correlated with temperatures above and below ground. Shrub/sapling cover and depth of organic material (DOM) were negatively correlated with summer air and soil temperatures. Soil temperature was highest beneath tall, sparse forest canopies and decreased as shrub/sapling cover and DOM increased. PAR was positively correlated with tall sparse trees, shrub/sapling cover, and DOM. PAR was negatively correlated with grass and herb cover. 

Compositional changes in understory plants depended on the existing seed bank as opposed to seed dispersal. Species turnover (introduction/extinctions) from young to old stands were relatively uncommon. Turnover occurred mostly among uncommon species and was independent of stand age. About half of species differed in abundance among stand age. Of the species that changed, most either increased or decreased linearly with stand age. Successional changes and patterns in understory plants were the result of changes in the relative frequency of core species. The physical characteristics of core species were associated with different stand ages. 

Downed Woody Debris

Old stands had the greatest variety of woody substrates available at different decay stages and thus supported the greatest number of moss, liverwort, and fungi species. High habitat variability in old stands may also explain why they had more rare, nonvascular species than either young or mature stands. These species fell into three ecological groups: epiphytes (commonly found on live trees); epixylics (typical on rotting wood). and terricolous (typically found on the ground). Different deadwood decay stages did not have unique nonvascular species due to high species intergradation. Epiphytes were more abundant early, epixylics were more abundant on intermediate decay stages, and terricolous species were more abundant during late decay. The sensitive liverwort group was restricted to intermediate stages. Nonvascular species assemblages on deadwood in similar decay stages differed among stand ages, indicating that time, as well as forest structure, was important in determining species assemblages. 

We found a rich biota in aspen mixedwood forests including 47 lichen species, 39 mosses, 7 liverworts, 24 fungi, 11 pteridiophytes (horsetails. club mosses, and ferns), 65 herbs, 27 shrubs, 6 trees, 3 amphibians, 76 birds, and 33 mammals. Old stands had greater bird species richness than young stands, which had greater richness than mature stands. Variation in abundance among stand age was evaluated for 33 bird species that were detected ten or more times. Twenty-one species had their highest abundance in old stands, nine species had their highest abundance in young stands, and only three species had their highest abundance in mature stands. 

Wildlife

Variation in bird abundance among stand age may have been caused by vegetation change. During succession, density of live trees decreased, height of canopy trees increased, and density of large live trees, large dead trees, and large deadwood increased. As a secondary pattern during succession, canopy and understory heterogeneity were greatest in old stands, intermediate in young stands, and lowest in mature stands. Many attributes of deadwood (snags and downed woody debris) and understory plant communities were correlated with variation in canopy heterogeneity. For most bird species, abundance was related to variation in one, or both, deadwood or understory plants. Abundance of 20 bird species was correlated with successional stage, and abundance of 14 bird species was correlated with canopy heterogeneity. Ten of the 21 species that were most abundant in old stands preferred conifer- dominated mixedwood forests during the breeding season; they may have been most abundant in old stands because those stands contained many conifer trees. 

Mammal species richness in summer was significantly higher in old stands than in young or mature stands. During winter, mammal species richness was higher in old and young stands than in mature stands. Variation in relative abundance among stand age was evaluated for 19 mammal species that were detected ten or more times. During summer, most species were more abundant in old stands (9), followed by young stands (4), and mature stands (3). Differences in mammal abundance during winter were less obvious (2, 3, 1 species had their highest abundances in old, young, and mature stands, respectively). 

Changes in mammal species abundance were associated with forest attributes that varied in relation to stand age and canopy heterogeneity. Complex old stands supported more species at high relative abundance, structurally simple mature stands supported few species at high relative abundance, and young stands that were intermediate in structural complexity due to pre-fire residuals (snags, large live trees, deadwood) supported an intermediate number of species at high relative abundance. 

Three mammal groups were measured in greater detail because of their economic importance (ungulates) or because of their perceived sensitivity to logging practices (bats, flying squirrel). During winter, structures that offer thermal protection may attract deer, while browse availability may limit moose. For example, relative abundance of white-tailed deer during winter was significantly higher in old than in young and mature stands; a trend that may be explained by greater availability of thermal protection (white spruce) found with shrubby browse in old stands. Relative abundance of moose during winter was significantly higher in young and old stands than in mature stands; a trend that may be explained by high abundance of browse in young and old stands. For both white-tailed deer and moose, abundance did not differ among stand age during summer; thus. their abundance seems to be affected by stand age only in winter. 

Bat abundance was evaluated using echolocation calls and feeding buzzes, while bats fitted with radio transmitters were used to locate roost trees. Adult and juvenile little brown bat. silver-haired bat of both sexes, and one adult of both hoary and northern long-eared bat were captured. Nine little brown bats and five silver-haired bats were radio-tagged and followed to 23 roosts; all in Populus spp. trees. Mean roost tree height and DBH were 21.8 m and 41.4 cm, respectively, for little brown bat, and 22.4 m and 41.3 cm for silver-haired bat. 

Myotis spp., silver-haired, hoary, and big brown bats were detected in all stand ages. Myotis spp. constituted the highest proportion of total activity (#passes/hr) in both years. During 1993, more old than young stands were used by Myolis spp. and all bats in the first hour post-sunset. During 1994, more old stands than both young and mature stands were used. The distribution of bat activity in the boreal forest was related to tree density, the presence of large canopy gaps, and the abundance and distribution of roost trees in uneven age stands.

Flying squirrel abundance was higher in old stands than other stand ages and was positively related to conifer and shrub/sapling densities. Although flying squirrels frequently used cavities with small entrances in large diameter aspen snags, preference for den trees, relative to availability, appeared to be based on live trees, low decay stage, high percent of bark remaining, a single cavity in the tree, and a large cavity entrance. 

Much of the structural complexity (residual green trees, snags, deadwood) found in young stands is related to the variable manner in which fire burns through aspen mixedwood forests. Furthermore, these structures appear to be highly related to the presence and abundance of the many different species of plants and wildlife. If connection between fire, stand structure, and biodiversity exists, then resource managers must compare structures retained by natural disturbances (e.g., wildfire) to those left by human-caused disturbances (e.g., harvest). 

Current Practices and Opportunities for Change

The current and dominant form of forest management in Alberta’s mixedwood forests includes unstructured clearcut logging, a narrow range of rotation ages leading to a constant-age class merchantable forest, silvicultural enhancement for conifer regeneration, and the administrative separation of the hardwood and softwood landbase. These forestry practices are fundamentally different from fire as a stand-initiating event and, when implemented over periods as short as one rotation, will lead to a forest landscape that is both different and more uniform than those found presently. Changes to the forest landscape caused by current forestry practices include the loss of structural complexity (green trees, snags, deadwood) in young forests, loss of older forest stages, and the likely spatial separation of aspen and conifers. The transformation of the boreal forest landscape, in tum, is likely to alter the composition and abundances of biota and the nature of ecological processes. Current forest management practices, therefore, are inadequate to ensure the ecological integrity of Alberta’s boreal mixedwood forests. Results of this study offer a template for incorporating the natural dynamics of snags, DWM, and conifer regeneration into forestry operations.

Provincial government agencies and the forest industry are therefore encouraged to:

1) accept the ecological importance of fire in maintaining stand and landscape level variability, and therefore a) implement forest harvest strategies that approximate patterns produced by fire by retaining structures (green trees, snags, deadwood), b) adopt variable harvest rotation ages, and c) maintain natural disturbances in the boreal mixedwood forest to the extent permissible by society,

2) recognize the existence of a distinctive “old-growth” stage in aspen mixedwood forests, and ensure that appropriate frequencies of older seral stages are maintained in the boreal forest landscape. The practice of sequencing the harvest of old stands early in the rotation needs to be re-evaluated in the context of maintaining appropriate frequencies of all seral stages on the forest landscape. An empirical description of fire return intervals offers a preferred model for maintaining age class,

3) recognize that mixed canopies of aspen and spruce dominate the mixedwood forests and that this interspersion provides important substrate, forage, nesting, and cover for many biota, and therefore devise land tenure, harvesting, and silviculture systems that do not encourage the unmixing of hardwood and softwood species at the scale of the stand, and

4) recognize that many of the ecological effects of commercial forestry, including those guided by forest ecosystem management, are presently unknown, and therefore establish aspen mixedwood forest reserves of appropriate scales as ecological benchmarks against which other forest land-uses can be evaluated.

The implementation of these recommendations may affect future production and availability of tree fiber for harvest from Alberta’s aspen mixedwood forests. Changes to forest harvest levels will be affected by the degree to which the recommendations are adopted (i.e., how much structure is retained on cutblocks, how variable rotation ages become, how much wildfire occurs, how much area is assigned to unharvested reserves). For example, the retention of high densities of live aspen trees on cutblocks for purposes of wildlife habitat or ecological processes will likely reduce post-harvest aspen regeneration and, hence, future fiber production. A review of the social and economic consequences of the proposed recommendations is required to arrive at sustainable management goals that meet ecological, social, and economic criteria. However, resource managers must recognize that all forest-based land-uses and social benefits (i.e., commercial forestry, hunting. aesthetics, bird-watching, etc.) ultimately depend on functional forest ecosystems; hence wise forest stewardship should be the primary and overriding societal objective.