Hardwood Abundance Research Articles

Relationship between the vertical distribution of fine roots and residual soil nitrogen along a gradient of hardwood mixture in a conifer plantation

In forest ecosystems, understanding the relationship between the vertical distribution of fine roots and residual soil nitrogen is essential for clarifying the diversity-productivity-water purification relationship. Vertical distributions of fine-root biomass (FRB) and concentrations of nitrate-nitrogen (NO3 -N) in soil water were investigated in a conifer plantation with three thinning intensities (Control, Weak and Intensive), in which hardwood abundance and diversity were low, moderate and high, respectively. Intensive thinning led to the lowest NO3 -N concentration in soil water at all depths (0-100 cm) and highest FRB at shallow depths (0-50 cm). The NO3 -N concentration at a given depth was negatively correlated with total FRB from the surface to the depth at which NO3 -N concentration was measured, especially at shallow depths, indicating that more abundant fine roots led to lower levels of downward NO3 -N leaching. FRB contributed positively to nitrogen content of hardwood leaves. These findings demonstrate that a hardwood mixture in conifer plantations resulted in sufficient uptake of NO3 -N from soil by well developed fine-root systems, and translocation to canopy foliage. This study suggests that productivity and water purification can be achieved through a hardwood mixture in conifer plantations.

Complementary airborne LiDAR and satellite indices are reliable predictors of disturbance-induced structural diversity in mixed old-growth forest landscapes

In old-growth forests, natural disturbances form a complex mosaic of structures, providing a wide diversity of habitats and functions of great importance. Old-growth forests are still often seen as a homogeneous whole and few remote-sensing approaches have been tested to identify their structural diversity, especially in boreal forests. The aim of this study is to use a combination of airborne LiDAR and satellite imagery to identify and discriminate old-growth forest structures resulting from different disturbance histories. The study area, which was located in the mixed boreal forest of Quebec (Canada), is Monts Valin National Park and adjacent managed territories. Balsam fir (Abies balsamea (L.) Mill) is the dominant species in the study area, but hardwood species such as white birch (Betula papyrifera Marsh.) and trembling aspen (Populus tremuloides Michx.) can also be abundant in the early succession stages. Four forest classes were studied: second-growth (logged between 1970 and 1980); transition old-growth (burned in 1920); undisturbed old-growth (unburned for at least 125 years); and disturbed old-growth forest (unburned for at least 125 years, but severely disturbed by an insect outbreak around 1980). A multivariate Random Forest model was used to discriminate the classes on 6466 1 ha tiles, based on 11 complementary LiDAR and satellite-derived indices describing stand vertical and horizontal structure, together with “greenness” and disturbance history over the last 30 years. This model had high predictive efficiency (AUC = 94.2), with 81.8% of the tiles accurately classified. Interestingly, undisturbed old-growth forests exhibited intermediate characteristics compared to transition and disturbed old-growth forests. This emphasizes that some structural attributes recognized as important for the classification of temperate and tropical old-growth forests, such as high vertical complexity, are of lesser relevance for boreal old-growth forests. In comparison to undisturbed old-growth forests, transition old-growth forests had a taller canopy of high “greenness” due to a greater hardwood abundance; disturbed old-growth forests had a higher gap fraction and heterogeneity in tree size; second-growth forests exhibited a lower and more even canopy. Misclassified tiles were explained by spatial variation in disturbance severity or different levels of forest resistance and resilience to disturbance. These misclassifications are also of ecological interest, as they highlight the nuances in structural diversity that are rarely identified by disturbance mapping. A reasonable combination of LiDAR and satellite indices was effective not only in discriminating old-growth forests from second-growth forests, but also identifying their different structures, which result from specific disturbance histories. This method could contribute to effective monitoring of changes in the areas and characteristics old-growth forest that are caused by anthropogenic and natural disturbances.

Predictors of fire-tolerant oak and fire-sensitive hardwood distribution in a fire-maintained longleaf pine ecosystem

The longleaf pine (Pinus palustris) ecosystem has been reduced to a fraction of its original extent, and where this ecosystem does occur, it is often degraded by hardwood encroachment. The reduction of hardwood tree cover is often a desirable longleaf pine community restoration outcome, though hardwood midstory and overstory trees have been recognized as an important natural component of the communities. Moreover, the appropriate amount of hardwood tree cover in a restored longleaf pine community is debated, as more hardwood tree cover can benefit mixed forest and mast-dependent wildlife (e.g., fox squirrels [Sciurus niger], white-tailed deer [Odocoileus virginianus]), and less hardwood tree cover is critical to the federally endangered red-cockaded woodpecker (Leuconotopicus borealis). To inform the debate, we assessed the environmental (e.g., topography, edaphic conditions, and pine basal area) and management (e.g., distance to firebreaks, prescribed fire history) factors that influenced abundance of upland hardwood trees in xeric longleaf pine communities on a site where frequent growing-season fire has been ongoing since 1991. We counted upland hardwoods ≥5 cm diameter at breast height (DBH) at 307 random field plots (0.04 ha) and categorized all hardwood trees as belonging to either a guild of fire-tolerant oaks or a guild of fire-sensitive hardwood species. We used generalized linear models (GLM) to determine the most important predictors of abundance for both guilds. The predictors of abundance differed between the two guilds, with fire-tolerant oak abundance increasing with greater slope and proximity to ignition sources and decreasing with greater pine basal area. Fire-sensitive hardwood abundance increased with mesic site conditions and decreased with the number of growing-season fires and greater pine basal area. Although seasonality in fire history was an important predictor of fire-sensitive hardwood abundance, variables related to long-term fire-history were not important predictors of fire-tolerant oak abundance in longleaf pine communities. However, with limited variation in fire return interval across the study area, our ability to draw inferences regarding the role of fire return interval was limited. Where hardwood encroachment is not a problem, and hardwood levels are below desired, balanced target levels, hardwood abundance in longleaf pine communities can be increased by reducing pine basal area and reducing prescribed fire intensity.

Initial effects of thinning and concomitant disturbance on the understory woody community in Japanese cedar plantation

ABSTRACTRestoring planted forests to more natural forests has become an important goal of recent forest management strategies. Thinning is an effective method of restoration that improves understory light conditions and encourages the growth and establishment of future canopy tree species. However, ambiguous results have been reported depending on location and circumstances. In this study, we examined the trend in the initial response after thinning across a region to consider understory disturbance accompanied by thinning and overgrowth of shrub species that prevent the growth of tree species. Field surveys were implemented in 49, 48, and 31 Japanese cedar plantations at different times before and after thinning (before thinning and at 2 and 5 years after thinning). Compared to before thinning, understory hardwood abundance was significantly lower at 2 years after thinning, whereas it was higher at 5 years. The increase in the abundance of the understory community was conspicuous for tree species with a rapid regeneration strategy after disturbance. These results suggest that understory disturbance resets the understory community structure by increasing light levels, thereby allowing the establishment of pioneer tree species. However, the abundance of understory hardwoods after thinning was correlated with that before thinning. Thus, we suggest that the possibility of colonization of tree species are largely affected by the abiotic or biotic environmental conditions with no direct relationship to the present thinning treatment (i.e. is the factor that remains essentially intact even after thinning).

Silvicultural Rehabilitation of Cutover Mixedwood Stands

We investigated rehabilitation of mixedwood stands degraded by exploitative cutting on the Penobscot Experimental Forest in Maine. Three precommercial rehabilitation treatments were applied: control (no rehabilitation), moderate rehabilitation (crop tree release [CTR]), and intensive rehabilitation (CTR, timber stand improvement [TSI], and red spruce fill planting). Crop trees (primarily red maple, paper birch, spruce, aspen, and eastern hemlock) were selected and released based on their potential for improved growth and value, spacing, and species composition. Rehabilitation reduced sapling basal area, density, and hardwood abundance and increased crop tree diameter increment. Fill planting increased spruce stocking, but many planted seedlings were browsed. Long-term projections suggested that future stand value will repay costs of moderate rehabilitation (CTR); intensive rehabilitation (CTR-TSI-planting) as applied in this study requires greater investment than can be repaid through quality and growth improvements of low-value hardwoods and softwoods. Although simulations suggested no difference in future stand value between treated and untreated stands, improvements in composition, growth, and quality after rehabilitation will facilitate later commercial thinning and shelterwood regeneration in stands which otherwise have few management options.

Evaluation of sampling methods to quantify abundance of hardwoods and snags within conifer-dominated riparian zones

& Aims Six sampling alternatives were examined for their ability to quantify selected attributes of snags and hardwoods in conifer-dominated riparian areas of managed headwater forests in western Oregon. & Methods Each alternative was simulated 500 times at eight headwater forest locations based on a 0.52-ha square stem map. The alternatives were evaluated based on how well they estimated the number of hardwoods and snags per hectare and their basal area per hectare using root mean square error and percent bias. & Results In general, 3.6-m wide systematic strips oriented perpendicular to the stream outperformed the other alternatives. However, the variance of all six sampling alternatives was quite high and further research is needed to determine an optimal sampling method for quantifying hardwoodandsnagattributesinforestsdominatedbyliveconifers. & Conclusion When sampling snag and hardwood as a minor component of the overall forest composition within a riparian area, we suggest using 3.6-m strips perpendicular to the stream.

Influence of post-harvest silviculture on understory vegetation: Implications for forage in a multi-ungulate system

Natural disturbance emulation has emerged as a key management approach to maintaining biodiversity in logged boreal forests. Forest managers’ success in emulating understory forest ecosystem functions, e.g., for the provision of habitat even for large mammals, has not been tested due, in many cases, to incomplete records of silviculture. We examined regenerating areas of previously conifer-dominated forests in northwestern Ontario, Canada, 10 and 30 years after logging and 10 and 30 years after fire to test if understory development and moose ( Alces alces) forage abundance differed between the two disturbance types and artificial or natural regeneration approaches. In addition, we counted moose pellet groups as a measure of moose use of the region. Specific treatments included: (1) naturally regenerating, fire-origin forests, (2) post-harvest, regenerating forests with natural establishment of trees, and (3) post-harvest, regenerating forests with mechanical or chemical site preparation and planting and/or herbicide spraying. We hypothesized that the understory in post-harvest stands would support higher forage availability for moose compared to similarly-aged, fire-origin stands. Abundance of hardwoods, shrubs, and herbaceous plants was greater in naturally-regenerated post-harvest stands than in fire-origin and artificially regenerated post-harvest stands at both 10 and 30 years post-disturbance. However, post-harvest, naturally regenerating stands were not significantly associated with higher moose use, rather evidence of moose use increased as a function of the amount of naturally regenerating logged forest in the surrounding landscape. This study suggests that, relative to fire, the intensity of post-harvest silviculture influences habitat suitability for moose. The effect likely cascades to other ungulates, such as woodland caribou ( Rangifer tarandus caribou), and vegetation management needs to be considered at scales greater than the stand level in order to achieve habitat management for large mammals.

Hardwood recruitment into conifer plantations in Japan: Effects of thinning and distance from neighboring hardwood forests

Conservation of plant species diversity in managed and natural forests is a concern worldwide. To clarify the effects of thinning and distance from neighboring hardwood forests on hardwood establishment in conifer plantations, we measured the abundance and diversity of hardwoods at different life stages (seed-fall, soil seed bank, seedling, sapling) and environmental conditions (light, soil temperature, litter accumulation, and composition) from −2 to 40 m from the hardwood-coniferous forest boundary into the coniferous forest interior in five unthinned and four thinned stands in Cryptomeria japonica plantations in Japan. In unthinned stands, hardwood abundance and diversity decreased with increasing distance into the coniferous forest interior for all life stages, perhaps because of a distance-dependent decrease in seed-fall and the subsequent soil seed bank for wind- and bird-dispersed seeds. Higher light and soil temperatures and shallower litter accumulation were observed at the hardwood-coniferous forest boundary compared to the coniferous forest interior, suggesting favorable environmental conditions for germination, emergence, and seedling growth for hardwood species, and resulting in higher species diversity near the boundary. In thinned stands, hardwood abundance and diversity also decreased with increasing distance into the coniferous forest interior for seed-fall and the soil seed bank, but did not decrease for seedlings and saplings, probably because of distance-independent improvement of environmental conditions due to thinning. Higher light availability and soil temperature and lower litter accumulation in thinned stands enhance seedling establishment for not only shade-tolerant but also shade-intolerant and light-demanding species. Our results suggest that distance-dependent diversity of juvenile hardwoods in dense coniferous plantations is closely related to distance-dependent gradients in both seed-fall and environmental conditions, but thinning enhances diversity, even in the forest interior.

Efficacy of herbicides and fire to improve vegetative conditions for northern bobwhites in mature pine forests

Declining northern bobwhite (Colinus virginianus) populations during the past 30 years have prompted managers to seek ways to improve habitat quality for this species. Reductions in frequency of prescribed fire throughout considerable expanses of mature pine (Pinus spp.) forests have resulted in closed-canopy conditions, predominantly woody understory conditions, and a loss of early-successional habitats needed by bobwhites. Herbicides, particularly in conjunction with prescribed fire, may be useful for managing these pine forests to benefit the bobwhite and other early-successional species, but effects of herbicides in combination with fire are not well understood. Therefore, we used 3 similar-aged, mature pine stands to evaluate vegetative response to selective herbicides with prescribed fire with respect to bobwhite nesting and broodrearing habitats. Our treatments were imazapyr with fire, imazapyr combined with glyphosate with fire, and dormant-season prescribed fire only. Plant diversity tended to decline on herbicide treatments during the first year but increased substantially on imazapyr plots during the second growing season following a burn. Bobwhite food plants increased following application of imazapyr during the first growing season and were greater for both herbicide treatments than burning alone during the second growing season. Abundance of hardwoods declined on both herbicide treatments. However, no treatments produced bare-ground percentages known to be selected by bobwhites, and only prescribed fire alone created and maintained suitable escape cover. Overall, imazapyr with fire provided the greatest net improvement in vegetative conditions for bobwhites and retained floristic diversity. We recommend that managers target areas in which vegetative conditions have progressed to where burning alone is incapable of restoring early-successional plant communities needed by the bobwhite and other species and apply imazapyr with fire to create diverse, patchy habitat for bobwhites.

Plant species diversity and composition along an experimental gradient of northern hardwood abundance in Picea mariana plantations

Maintenance of biodiversity in managed forest ecosystem is an increasing concern, particularly in plantation forests. To test the two hypotheses that (i) past vegetation treatments modify species diversity and that (ii) a trade-off exists between plantation productivity and diversity, we measured plant species diversity in the upper, intermediate and lower strata of 16–17-year-old Picea mariana plantations along a gradient of hardwood abundance. The gradient was obtained with various intensities of non-chemical release and thinning treatments. P. mariana productivity linearly decreased as the proportion of hardwoods within the canopy increased. As the proportion of P. mariana increased, total species richness and plant diversity (Shannon index [ H′]) first increased up to an intermediate level, and then decreased. Proportion of non-crop to crop tree species explained 66% of the variation in species diversity of the upper strata, 0% of the intermediate strata, and 20% of the lower strata. Intensity of past vegetation treatments slightly affected species composition, as evaluated by the Sørensen’s index of similarity. Maintaining an almost exclusive cover of P. mariana had marginal effects on species richness and understory diversity. But, a large increase in hardwoods proportion occurred at the expense of species richness and diversity of the understory stratum. Differences in canopy light transmission between covers could not explain this result. Variation in species richness and diversity along the gradient of hardwood abundance had a hump shape, a characteristic of the classic disturbance-diversity hypothesis. But, the intensity of past vegetation control had a weak predictive capability for plant species richness and diversity. Thus, vegetation control to maximise crop tree productivity in P. mariana plantations is not likely to affect understory plant diversity or composition. It may however contribute to an increase in stand structural diversity.

Évolution de la régénération de peuplements résineux et mélangés au cours des 30 années après coupe à blanc mécanisée

This paper presents results from a 30 year monitoring of 15 clearcut areas. Study areas were located before cutting in stands having an abundant advance regeneration. Four study areas were initially dominated by black spruce, four by balsam fir, three by jack pine and the last four comprised a mix of species. Stand inventories and regeneration surveys were conducted before harvesting. Harvesting was done in regular logging operations, without any special care to protect the advance growth. Regeneration surveys were taken again after harvesting, 5 years, 10 years, 20 years and 30 years after harvesting. Black spruce stands regenerated to black spruce dominated stands. Balsam fir, which was sometimes abundant in the advance regeneration, did not invade the stands and the abundance of hardwoods was temporary. In some cases a reduction in yield is to be expected. In balsam fir stands, harwoods were more aggressive. A reduction in softwood yield is predicted for three of the four cutovers. The presence of a tall regeneration seems to lead to higher yields in black spruce and balsam fir stands. The evolution of jack pine and mixed stands is quite variable. In some jack pine stands, hardwoods will constitute a significant component of the new stand. In some mixed stands, softwoods are not very abundant whereas in others they form an important part of the stand at 30 years. This study has shown an urgent need for a better understanding of stand dynamics in young stands. Such an understanding is necessary for an adequate forecast of the yield of second growth stands.

Structure of mixed pine and hardwood stands 12 years after various methods and intensities of site preparation in the Georgia Piedmont

In the Georgia Piedmont (U.S.A.), size, abundance, and species diversity of trees were quantified in a plantation of Pinustaeda L. 12 years after various methods and intensities of site preparation. In clear-cut only versus site-prepared plots, greater hardwood abundance (27% vs. 8% of the total basal area) and size (8.6 vs. 7.4 m in height) were associated with reduced pine volume (73 vs. 123 m3/ha) and increased Simpson and Shannon diversity indices. Tree-species richness was greater in plots where residual trees from clear-cutting had been removed with a chainsaw versus large machinery (10 vs. 7 species). With increasing site-preparation intensity, reductions in basal area of volunteer pines coincided with proportionate increases (R2 = 0.80) in basal area of planted pines. As a result of this compensatory effect, total volume of all pines varied little (122–134 m3/ha) among site-preparation intensities. Research results suggest that site-preparation treatments can be selected to facilitate the development of a variety of stand structures, including those that favor evenness (clear-cut only) or richness (manual cutting) of tree species, low-cost production of pine fiber (manual cutting), and stand uniformity for management of pine sawtimber (mechanical and herbicide).

Active and Abandoned Red-Cockaded Woodpecker Habitat in Kentucky

We characterized composition, density, and structure of habitat occupied by red-cockaded wood-peckers (Picoides borealis) on the Daniel Boone National Forest (NF), Kentucky, and we compared areas surrounding active and abandoned cavity trees. Basal area of active colonies (22.7 m 2 /ha) was composed of 48% pine (chiefly Pinus echinata) and 52% nonpine (chiefly Quercus spp.); density (2,257 stems/ha) and hardwood abundance (88% of total stems) were much higher than recorded in red-cockaded habitat elsewhere. Stand characteristics did not differ between areas immediately surrounding active and abandoned trees, but areas surrounding abandoned colonies had lower percentages (P≤0.04) of habitat dominated by pine and pine-oak and greater percentages (P≤0.02) of stands that had been harvested. Damage to cavity trees by pileated woodpeckers (Dryocopus pileatus) was greater (P<0.002) on abandoned areas. The availability of suitable habitat was apparently the major limitation to red-cockaded woodpecker populations; hence management should emphasize the maintenance of, or increase in, the area occupied by mature stands of pine and pine-oak