A comparison of lodgepole pine responses to varying levels of trembling aspen removal in two dry south-central British Columbia ecosystems

We used manual cutting to manipulate trembling aspen (Populus tremuloides Michx.) density and spatial arrangement in relation to crop lodgepole pine (Pinus contorta Dougl. Ex Loud. var. latifolia Engelm.) on two sites in contrasting dry, cool to cold ecosystems of south-central British Columbia. In the dry, cool interior Douglas-fir ecosystem (IDFdk3), we reduced the density of tall aspen (aspen at least as tall as target pine) to 0 (broadcast removal), 1000, 2500, or 4000 stems/ha when the planted lodgepole pine was 6 years old. Eight years later, pine height/diameter ratio (HDR) was significantly lower in the broadcast removal and 1000 stem/ha treatments than in the control. There were no other significant growth responses and pine survival and vigour were good regardless of treatment. In contrast, in a dry, cold sub-boreal pine spruce ecosystem (SBPSxc) where treatments were applied at a stand age of 11 years, naturally regenerated lodgepole pine stem diameter increased significantly in the broadcast removal treatment relative to the untreated control within 2 years. After 4 years, HDR had declined significantly relative to the control where tall aspen density was ≤1000 stems/ha. There were no significant pine responses where 2500 tall aspen stems/ha were retained or where tall aspen were removed only within a 1-m radius around pine. The greater difference in height (height differential) between aspen and pine at the SBPSxc than the IDFdk3 site may partly explain the differing response of lodgepole pine to treatment. Trends of decreasing sucker density with increasing aspen retention were evident at both sites, but differences were significant (p ≤ 0.05) only at the SBPSxc site.     

The increasing scarcity of red oaks in Mississippi River floodplain forests: Influence of the residual overstory

Red oaks – cherrybark oak (Quercus pagoda Raf.), willow oak (Quercus phellos L.), water oak (Quercus nigra L.), and Nuttall oak (Quercus texana Buckley; aka: Quercus nuttallii Palmer) – are not regrowing in Mississippi Delta river floodplain forests in the southeastern United States in sufficient numbers to sustain the former species composition and timber and wildlife values. Even if vigorous red oak reproduction becomes established, partial harvesting that does not remove the taller trees will suppress understory red oak height growth more than it will suppress height growth of such other species as sugarberry (Celtis laevigata Willd.), American elm (Ulmus americana L.), cedar elm (Ulmus crassifolia Nutt.), swamp dogwood (Cornus foemina Mill.), green ash (Fraxinus pennsylvanica Marshall), and sweetgum (Liquidambar styraciflua L.). Consequently, the red oaks in these partially harvested stands become increasingly suppressed and probably die; and there is a shift in species composition to the other species. In addition to ensuring vigorous oak reproduction, silvicultural clearcutting or rapid removal of the residual trees following shelterwood or seed tree harvesting to provide full sunlight is needed to ensure red oaks become a dominant part of these future river floodplain stands.     

Forest regeneration 50 years following partial cutting in mixedwood ecosystems of southern Quebec,Canada

The impact of winter harvesting on regeneration 50 years after an experimental diameter-limit cutting was examined in mixed deciduous–coniferous ecosystems of southern Quebec, Canada. The study was conducted in La Mauricie National Park, Quebec, Canada. Regeneration data in two balsam fir (Abies balsamea (L.) Mill.), red spruce (Picea rubens Sarg.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt.) ecosystem types were analyzed. Comparisons between uncut and cut stands were obtained from a total of 63 sample plots. For both ecosystems, there were no significant differences between uncut and cut plots for regeneration density and stocking. The most abundant regeneration species were balsam fir, red spruce, sugar maple, red maple (Acer rubrum L.), yellow birch and American beech (Fagus grandifolia Ehrh.). The type of diameter-limit cutting described in the study did not affect regeneration density and stocking but its impact on productivity, timber quality and genetics is still unknown.     

Sugar maple and yellow birch regeneration in response to canopy opening,liming and vegetation control in a temperate deciduous forest of Quebec

We examined how the density, growth and survival of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) regeneration are influenced by gap size, soil nutrient availability and understory vegetation. We used a factorial combination of (1) three gap sizes (small: <100 m²; medium: 100–300 m²; large: ∼1000 m²); (2) presence/absence of liming (92% CaCO₃ at 500 kg ha⁻¹, 1st year post-harvest); and (3) presence/absence of vegetation control (weeding twice a year; 1st to 3rd year post-harvest). We monitored height increment and survival of 1500 seedlings and saplings of both species from the 3rd to the 6th year post-harvest, and assessed density 6 years post-harvest. Both species exhibited a complex set of density, growth and survival responses across the combination of treatments. Compared to sugar maple, yellow birch had an overall lower density, greater growth, and similar survival rate; the two species attained maximum values in different gap size for density, and similar gap size for growth and survival. Liming had very little or no effect on the species. The growth of yellow birch was slightly but significantly greater when understory vegetation was controlled, particularly in medium and large gaps. These results suggest that a variety of canopy gap sizes can provide the right combination of understory conditions for regenerating these two functionally different tree species.     

Community and species responses by birds to group retention in a coastal temperate forest on Vancouver Island,British Columbia

Group retention is a forest harvesting technique designed to sustain biodiversity and mitigate concerns regarding clearcut logging. It is characterized by retained forest patches that vary in number, size, spatial arrangement, and habitat attributes. We used birds to compare community composition and species abundance among clearcut, group retention, and uncut control forest treatments, and evaluated species’ responses to percentage retention. The bird community in group retention was more similar to that in control forests than it was to the community in clearcuts. The probability of occurrence for many bird species typical of uncut control forests was related positively to percentage retention. A preliminary analysis of plot-level effects (i.e., amount of forest sampled) suggests that patch size may be more important than total amount of retention.     

A 10-year evaluation of the functional basis for regeneration habitat preference of trees in an African evergreen forest

The spatial distribution of tree juveniles in relation to light environments may reflect species differences in growth, survival, and functional traits and will shape the nature of forest regeneration. Long-term field experiments are important to evaluate this issue because of the potentially very long juvenile period in trees. Here, we combine a 10-year seedling survival–growth data with the results of community ordination and multivariate analyses of functional traits to ask how observed juvenile light guilds are related to species functional traits and seedling performance. We transplanted seedlings at a standardized height of 11 cm into the shaded understory and quantified their growth and survival for 10-years. Using the community-wide stem distribution data, we categorized 33 species including the focal 11 species to understory vs. gap/edge guilds. Then, we determined differences between the two guilds in seedling survival, growth, as well as seed size, adult height, and a series of leaf traits, including toughness and chemical traits (fiber, protein, phenolics, tannins, alkaloids, saponins). Among the 11 non-pioneer species whose seedlings were planted into the understory, there was no significant difference in 10-year survival between light guilds, but species in gap/edge guild tended to achieve greater height than species in the understory guild. The leaf chemical traits of 33 species did not differ between the two juvenile light guilds, but gap/edge species had smaller seeds, taller adults, and tougher leaves than understory species. We used logistic regression as a complementary approach to assess the extent to which plant traits varied between light guilds and the most parsimonious model based on AIC_c ranking included only leaf toughness and had an Akaike weight of 0.52. In addition, across the 11 species planted as seedlings, these traits were not significantly related to survivorship or growth over 10 years. A Principle Components Analysis illustrated associations among traits. We conclude that light guilds in terms of juvenile stem distribution could not be explained by long-term field performance of post-establishment seedlings alone. Earlier seedling stage or later sapling stage may be more important in differentiation of light guilds. For the species examined difference in growth rates could be linked to seed size and adult stature, but not to the adult leaf chemical traits considered. These results suggest the importance of examining ontogenetic shifts and relationships among functional traits for a better understanding of regeneration strategies of tropical trees.     

Growth responses of Lobaria retigera to forest edge and canopy structure in the inland temperate rainforest, British Columbia

The foliose cyanolichen Lobaria retigera is a sensitive old-growth forest indicator in British Columbia's inland rainforest. These forests are increasingly being fragmented by harvesting, raising concerns about edge effects and the maintenance of canopy lichen communities in remaining forest patches. We have examined the response of L. retigera to edge effects, using transplant experiments to compare growth responses between abrupt (hard) edges created by clearcut harvesting and more buffered (soft) edges created by variable-retention harvesting. L. retigera thalli placed along hard edges showed high mortality rates, with most thalli (from 64 to 100%) losing biomass at the end of each of the 3-year's measurement periods. Biomass loss along soft edges was much lower, occurring in fewer than 7% of thalli in the best year, up to 42% in the worst year. When growth rates were expressed against light availability at each branch location, thalli along variable-retention transects showed a positive response to increasing light availability. In contrast, thalli along the hard edge transects showed an abrupt decline in growth rates at equivalent levels of high light availability (canopy openness values >30%), where presumably the benefits of greater light availability were offset by heat or dessication stress. These results show a much greater sensitivity to edge effects in L. retigera, compared to earlier measurements at this site with the more common lichen Lobaria pulmonaria, a species that has a dominant green-algal photobiont partner. We suggest that increased use of variable-retention harvesting systems in which a substantial number of residual trees are left along cutblock edges has the potential to reduce the impacts of forest harvesting on canopy lichen biodiversity.