Shade-tolerant Conifers Research Articles

DO REMNANT OLD-GROWTH TREES ACCELERATE RATES OF SUCCESSION IN MATURE DOUGLAS-FIR FORESTS?

Biological legacies left by natural disturbances provide ecological functions throughout forest stand development, but their influences on processes of ecological succession are not completely understood. We investigated the successional role of one type of biological legacy: remnant old-growth trees persisting in mature Pseudotsuga menziesii (Douglas-fir) forests in the U.S. Pacific Northwest. We tested the hypothesis that remnant old-growth Tsuga heterophylla (western hemlock) and Thuja plicata (western red cedar) trees enhance the reestablishment of shade-tolerant conifers by increasing the availability of seed. Reestablishment of shade-tolerant conifers is a key process in late-successional forest development because it leads to vertical differentiation of the canopy and eventual codominance of shade-tolerant species. Two study areas were selected in the southern Washington Cascade Range, USA. Both had an unfragmented, mature forest cover that was regenerated naturally following wildfire. Twelve study sites were selected, including sites with and without remnant T. plicata and T. heterophylla. Overstory structure and composition, microsite variables, and conifer regeneration were systematically sampled using nested belt transects and quadrats. Sites with remnant T. heterophylla and T. plicata had significantly higher densities of conspecific seedlings. Multivariate analyses showed remnant T. heterophylla and T. plicata presence and density to be the strongest predictors of seedling densities, although the basal area of mature conspecific trees, relative density, aspect, stand age, and microsite characteristics were important secondary predictors. Microsite variations explained regeneration patchiness. Seedling densities were strongly correlated with proximity to remnant trees, exhibiting a negative exponential decline with distance. Shade-tolerant conifers are likely to reestablish faster at sites with remnant seed trees, but canopy disturbances are probably necessary for subsequent height growth. Remnant shade-tolerant conifers are an important biological legacy and seed source influencing rates of ecological succession in mature P. menziesii stands. Successional and stand development models should explicitly incorporate this dynamic.

Population structure and growth acclimation of mountain maple along a successional gradient in the southern boreal forest

:A range of stands was sampled in the southern boreal forest of eastern Canada to determine the impact of forest development stages and light availability on the dynamics of an abundant understory shrub: mountain maple (Acer spicatum). Mountain maple was studied at both the population and individual stem levels. At the population level, a total of 190 1-m2 quadrats were sampled in five forest types representing a successional gradient (young aspen, mature aspen, mixedwood, shade-tolerant conifer, and old spruce budworm-affected conifer forests). At the individual stem level, a total of 100 stems of different sizes were harvested in a sub-sample of the quadrats. Mountain maple stem biomass, density, and population structure were found to be affected by forest composition, the coniferous forest being the least favourable. At the individual stem level, light availability and stem size were found to have an important impact on stem morphology and growth, suggesting a high level of plasticity in relation to canopy opening. From these results, four developmental phases of mountain maple population dynamics were identified: 1) following a drastic disturbance, a first phase of rapid growth, and potentially intense competition with aspen suckers, when present, takes place; 2) following the development of a closed aspen canopy, the overall density, biomass, and presence of mountain maple in the understory increase; 3) following recruitment of conifer trees into the overstory canopy, mountain maple enters a suppression phase associated with a decrease in light; and 4) following the occurrence of small scale disturbances, mountain maple quickly responds to rapidly dominate the openings, which further ensures its maintenance in the understory.

Benefits to biodiversity from developing old-growth conditions in British upland spruce plantations: a review and recommendations

Summary European forestry strategies place emphasis on developing alternative management practices to clearfelling within commercial forests as a means of increasing the non-market benefi ts of sustainable forestry. In the UK, many thousands of hectares of forest are being transformed to continuous cover forestry and a number of minimum intervention natural reserves are being created to encourage the development of old-growth conditions. This paper defi nes the term ‘old growth’ in the context of upland spruce-dominated plantations in Britain and evaluates different options for the location, design and management of old-growth areas to enhance biodiversity. Evidence outside of Britain from semi-natural analogues of upland spruce plantations suggests that old growth can develop 100–200 years after stand initiation in those parts of the landscape not subject to frequent catastrophic disturbance by wind and fi re. Old-growth stands in these forests are characterized by a high proportion of large, old trees, multiple age classes and high volumes of fallen and standing deadwood. Studies of old spruce stands in the British uplands suggest that old-growth features can begin to develop after 80–100 years, conferring substantial benefi ts to species-groups such as hole-nesting birds, mammals (e.g. red squirrel), bryophytes, lichens and fungi. Based on the likelihood of wind damage it is suggested that ~50 per cent of the current land area in upland Britain could support large patches (50–100+ ha) of old growth. To enhance colonization by woodland species, these old-growth patches should be within 2 km of existing semi-natural woodland, managed ideally on a minimum intervention basis. If production of timber was also an objective, old-growth stands could be managed by single tree selection or small group-fell silviculture, provided that over-prolifi c regeneration of shade-tolerant conifers was controlled and some deadwood and veteran trees were maintained. In surrounding areas subject to normal patch-clearfelling, small enclaves of old-growth forest (0.25–2 ha) could be retained to provide temporary habitat for species and facilitate dispersal through the landscape. The planning and design of old-growth areas needs to be considered at the landscape scale to ensure an appropriate balance between old growth and other types of woodland and nonwoodland habitats. An imaginative approach to incentives will be required to encourage positive management for old growth.

Late Quaternary dynamics of forest vegetation on northern Vancouver Island, British Columbia, Canada

Pollen analysis of radiocarbon-dated lake sediment from northern Vancouver Island, southwest British Columbia reveals regional changes in forest vegetation over the last 12,200 14C yr (14,900 cal yr). Between at least 12,200 and 11,700 14C yr BP (14,900–13,930 cal yr BP), open woodlands were dominated by Pinus contorta, Alnus crispa, and various ferns. As P. contorta decreased in abundance, Alnus rubra and more shade-tolerant conifers (i.e., Picea and Tsuga mertensiana) increased. Increases in T. mertensiana, P. contorta, and A. crispa pollen accumulation rates (PARs) between 10,600 and 10,400 14C yr BP (11,660–11,480 cal yr BP) reflect a cool and moist climate during the Younger Dryas chronozone. Orbitally induced warming around 10,000 14C yr BP (11,090 cal yr BP) allowed the northward extension of Pseudotsuga menziesii, although Picea, Tsuga heterophylla, and A. rubra dominated early Holocene forests. By 7500 14C yr BP (8215 cal yr BP), shade-tolerant T. heterophylla was the dominant forest tree. Cupressaceae ( Thuja plicata and Chamaecyparis nootkatensis) was present by 7500 14C yr BP but reached its maximum after 3500 14C yr BP (3600 cal yr BP), when a cooler and wetter regional climate facilitated the development of temperate rainforest. The highest rates of vegetation change are associated with Lateglacial climate change and species with rapid growth rates and short life spans.

Understory vegetation in young Douglas-fir forests: does thinning help restore old-growth composition?

We compared the understory herb and shrub communities of pre-commercially thinned and unthinned young- and old-growth Douglas-fir ( Pseudotsuga menziesii) stands. Sites were located in the Cascade Mountains of western Oregon. Young stands were approximately 40 years old at the time of sampling; thinning occurred 20 years earlier. Although tree spacing was wider in thinned than in unthinned stands, faster tree growth in thinned stands meant that thinning had no long-term effect on basal area of P. menziesii. The very large, widely spaced P. menziesii individuals in old-growth stands contributed similar basal area as the smaller trees in young stands; however, old stands had much higher basal area of shade-tolerant conifers. Although low shrubs responded positively to thinning, the difference in cover between thinned and unthinned stands was too small to be statistically significant. Ordination of sample plots in terms of their understory shrub and herb species composition identified a strong gradient in community composition from young to old-growth stands. Understory species associated with old growth included both gap specialists and those that may be dependent on the litter depth and mycorrhizal fungi of old-growth forest floors. Although both thinned and unthinned young stand composition were quite distinct from that of old-growth stands, thinned stands were more similar to old growth than were unthinned stands. Thinned stands had higher frequency of late-seral herbs than did unthinned stands. Forest generalist and release species showed mixed responses to thinning. We conclude that pre-commercial thinning may be a useful management tool to encourage old growth associated understory herbs without precipitating dominance of the understory by low shrubs and weedy species. Although thinning did accelerate tree growth, changes in the understory appear to have been precipitated by the transient increase in resource levels following thinning rather than by any long-term changes in stand structure.

Effects of mixedwood canopies on conifer advance regeneration in a subalpine old-growth forest in central Japan

:We investigated the effects of mixed conifer-deciduous canopies on the spatial patterns and growth of understorey saplings of dominant subalpine evergreen conifers. The distributions of Abies mariesii and of Abies veitchii saplings were significantly clumped at the microsites north of canopy trees of the deciduous Betula ermanii. Because irradiance comes from the south, such microsites experienced more light during the leafless period of B. ermanii, which improved the growth of Abies saplings. Moreover, the shading intensity of B. ermanii was less than that of conifers even in the season when the canopy was entirely closed. Therefore, the light conditions of such microsites appear to have a favourable influence on the growth and survival of Abies saplings, determining the spatial location of shade-tolerant Abies saplings in the understorey. Consequently, this study suggests that deciduous-canopy trees favour the advance regeneration of shade-tolerant conifers by providing the preferable growing microsites in the understorey of a mixedwood subalpine forest.

Human influence on the abundance and connectivity of high-risk fuels in mixed forests of northern Wisconsin, USA

Though fire is considered a “natural” disturbance, humans heavily influence modern wildfire regimes. Humans influence fires both directly, by igniting and suppressing fires, and indirectly, by either altering vegetation, climate, or both. We used the LANDIS disturbance and succession model to compare the relative importance of a direct human influence (suppression of low intensity surface fires) with an indirect human influence (timber harvest) on the long-term abundance and connectivity of high-risk fuel in a 2791 km2 landscape characterized by a mixture of northern hardwood and boreal tree species in northern Wisconsin. High risk fuels were defined as a combination of sites recently disturbed by wind and sites containing conifer species/cohorts that might serve as “ladder fuel” to carry a surface fire into the canopy. Two levels of surface fire suppression (high/current and low) and three harvest alternatives (no harvest, hardwood emphasis, and pine emphasis) were compared in a 2×3 factorial design using 5 replicated simulations per treatment combination over a 250-year period. Multivariate analysis of variance indicated that the landscape pattern of high-risk fuel (proportion of landscape, mean patch size, nearest neighbor distance, and juxtaposition with non fuel sites) was significantly influenced by both surface fire suppression and by forest harvest (p > 0.0001). However, the two human influences also interacted with each other (p < 0.001), because fire suppression was less likely to influence fuel connectivity when harvest disturbance was simultaneously applied. Temporal patterns observed for each of seven conifer species indicated that disturbances by either fire or harvest encouraged the establishment of moderately shade-tolerant conifer species by disturbing the dominant shade tolerant competitor, sugar maple. Our results conflict with commonly reported relationships between fire suppression and fire risk observed within the interior west of the United States, and illustrate the importance of understanding key interactions between natural disturbance, human disturbance, and successional responses to these disturbance types that will eventually dictate future fire risk.

Effect of harvest intensity on development of natural regeneration and shrubs in an Ontario boreal mixedwood stand

Resource managers in northern Ontario need guidelines for establishing productive species mixtures and maintaining a viable conifer component through natural regeneration on upland boreal mixedwood (BMW) sites. The hypothesis that partial cutting favours conifers by inhibiting development of hardwood reproduction and associated competitors was tested. Harvesting to nominal basal area (BA) reductions of 0, 36, 68, and 100% was conducted in a northeastern Ontario mixedwood stand dominated by trembling aspen ( Populus tremuloides Michx.) in the overstory and balsam fir ( Abies balsamea [L.] Mill.) and shrubs in the understory. A positive linear relationship existed between photosynthetically active radiation (PAR) transmittance and initial BA reduction at 1, 3, and 5 years after harvesting. However, the relationship weakened over the 4-year period. Although BA reduction level did not account for significant variation in height growth of conifer advance regeneration, it explained 31% of the variation in conifer sapling diameter growth, 61% of the variation in hardwood regeneration height growth, and 21% of the variation in shrub height growth. Based on post-harvest height comparisons among conifers, hardwoods, and shrubs, partial cutting to promote conifer advance regeneration on BMW sites cannot be recommended. Maintaining a viable understory of shade-tolerant conifers for the next rotation may be more feasible than promoting conifer–hardwood overstories in the current rotation. The increase in sapling growth with BA reduction level implies that the entire overstory should be removed if the site is adequately stocked with conifer saplings. Similarly, harvesting should remove the entire overstory if the objective is to maximize growth of hardwood regeneration. Clearcutting promoted growth of both hardwood crop trees and shrubs, but hardwoods remained dominant after 5 years. Height development patterns indicate that regeneration and competitors should be monitored long enough following harvesting to determine whether additional interventions may be necessary to achieve stand structure objectives. Further studies are required to investigate the effects of other factors on advance regeneration, such as pattern and season of harvest and degree of soil disturbance.

Effect of harvest intensity on development of natural regeneration and shrubs in an Ontario boreal mixedwood stand

Resource managers in northern Ontario need guidelines for establishing productive species mixtures and maintaining a viable conifer component through natural regeneration on upland boreal mixedwood (BMW) sites. The hypothesis that partial cutting favours conifers by inhibiting development of hardwood reproduction and associated competitors was tested. Harvesting to nominal basal area (BA) reductions of 0, 36, 68, and 100% was conducted in a northeastern Ontario mixedwood stand dominated by trembling aspen ( Populus tremuloides Michx.) in the overstory and balsam fir ( Abies balsamea [L.] Mill.) and shrubs in the understory. A positive linear relationship existed between photosynthetically active radiation (PAR) transmittance and initial BA reduction at 1, 3, and 5 years after harvesting. However, the relationship weakened over the 4-year period. Although BA reduction level did not account for significant variation in height growth of conifer advance regeneration, it explained 31% of the variation in conifer sapling diameter growth, 61% of the variation in hardwood regeneration height growth, and 21% of the variation in shrub height growth. Based on post-harvest height comparisons among conifers, hardwoods, and shrubs, partial cutting to promote conifer advance regeneration on BMW sites cannot be recommended. Maintaining a viable understory of shade-tolerant conifers for the next rotation may be more feasible than promoting conifer–hardwood overstories in the current rotation. The increase in sapling growth with BA reduction level implies that the entire overstory should be removed if the site is adequately stocked with conifer saplings. Similarly, harvesting should remove the entire overstory if the objective is to maximize growth of hardwood regeneration. Clearcutting promoted growth of both hardwood crop trees and shrubs, but hardwoods remained dominant after 5 years. Height development patterns indicate that regeneration and competitors should be monitored long enough following harvesting to determine whether additional interventions may be necessary to achieve stand structure objectives. Further studies are required to investigate the effects of other factors on advance regeneration, such as pattern and season of harvest and degree of soil disturbance.

Comparative growing space efficiency of four tree species in mixed conifer–hardwood forests

The influence of shade tolerance, canopy position, and tree size on growing space efficiency (GSE) in mixed stands of co-occurring conifer and hardwood species was investigated in hemlock–northern hardwood forests. Three alternative measures of two-dimensional growing space—total crown area (TCA), exposed crown area (ECA), and a projection of the total available growing space (AGS)—were investigated to clarify the comparative importance of shaded and illuminated crown regions and unoccupied space in the forest canopy. GSE was expressed as ratios of stem volume increment and biomass increment per unit of growing space. Late-successional, shade-tolerant species have often been portrayed as slow growing, inefficient users of their growing space; however, hemlock ( Tsuga canadensis), which is one of the most shade-tolerant conifers in North America, was the most efficient canopy tree in our sample across all measures of GSE. Likewise, the mid-tolerant yellow birch ( Betula alleghaniensis) tended to be less efficient than the more shade-tolerant maples ( Acer rubrum and Acer saccharum). For all species, volume increment per unit of growing space increased with increasing tree height and canopy position, but within a given stratum decreased with increasing crown size. The relative efficiency of each species did not appear to be influenced by the measure of growing space employed. In most cases, volume and biomass increments per unit of ECA and AGS were significantly greater ( p<0.05) for intermediate than dominant crown class trees. However, for a given level of ECA or AGS, efficiency did increase with increasing relative height, which suggests that efficiency is influenced by the relative vertical position of growing space in the forest canopy. In general, the shaded area of a crown (i.e., TCA−ECA) was not a significant predictor of volume increment once height and ECA were known, suggesting that once 100% canopy closure is reached, packing trees more tightly may not increase stand-level production. However, mean volume increment per unit of TCA scaled more accurately to the stand-level than mean volume increment per unit of ECA. Potential scaling problems associated with mixed-species stands are discussed.

Dependence of needle architecture and chemical composition on canopy light availability in three North American Pinus species with contrasting needle length.

Morphology and chemical composition of needles of shade-intolerant southern conifers (Pinus palustris Mill. (mean needle length +/- SD = 29.1 +/- 4.1 cm), P. taeda L. (12.3 +/- 2.9 cm) and P. virginiana Mill. (5.1 +/- 0.8 cm)) were studied to test the hypothesis that foliage acclimation potential to canopy light gradients is generally low for shade-intolerant species, and in particular, because of mechanical limitations, in species with longer needles. Plasticity for each needle variable was defined as the slope of the foliar characteristic versus irradiance relationship. A novel geometrical model for needle area and volume calculation was employed for the three-needled species P. palustris and P. taeda. Needle thickness (T) strongly increased, but width (W) was less variable with increasing daily integrated quantum flux density averaged over the season (Q(int)), resulting in changes in cross-sectional needle shape that were manifested in a positive relationship between the total to projected needle area ratio (A(T)/A(P)) and Q(int) in the three-needled species. In contrast, cross-sectional needle geometry was only slightly modified by irradiance in the two-needled conifer P. virginiana. Needle dry mass per unit total needle area (M(T)) was positively related to Q(int) in all species, leading to greater foliar nitrogen contents per unit area at higher irradiances. Separate examination of the components of M(T) (density (D) and the volume (V) to A(T) ratio; M(T) = DV/A(T)) indicated that the positive effect of light on M(T) resulted solely from increases in V/A(T), i.e., from increases in the thickness of foliage elements. Foliar chlorophyll content per unit mass increased with increasing Q(int), allowing an improvement in light-harvesting efficiency in low light. The variables characterizing needle material properties (D, the dry to fresh mass ratio, and needle carbon content per unit mass) were generally independent of Q(int), suggesting that needles were less stiff and had greater tip deflections under their own weight at lower irradiances because of smaller W and T. Comparisons with the literature revealed that plasticity in foliar characteristics tended to be lower in the studied shade- intolerant species than in shade-tolerant conifers, but plasticity among the investigated species was unaffected by needle length. However, we argue that, because of mechanical limitations, plastic changes in needle cross section in response to low irradiance may decrease rather than increase light-interception efficiency in long-needled species.

The Effect of Multinutrient Fertilization on Understory Vegetation Annual Production

Abstract This study quantified the effects of operational multinutrient fertilization on understory shrub, forb, and grass production in inland Northwest forests. Understory vegetation annual production increased at some sites while other sites showed no change or decreases following fertilizer treatments. The greatest understory response to fertilization occurred in low density, shade-intolerant, overstory-dominated stands. Understory vegetation response was lower under overstories dominated by more shade-tolerant conifers. Multinutrient fertilization produced increases in annual understory production of up to 2,400 lb/ac for combined understory growth forms over the entire growing season. West. J. Appl. For. 17(3):147–153.

The potential effects of sexual reproduction and seedling recruitment on the maintenance of red maple (Acer rubrum L.) populations at the northern limit of the species range

AimFor this study, we wanted to evaluate the reproductive potential of northern red maple (Acer rubrum L.) populations to identify the possible factors responsible for the scattered distribution pattern of these northern populations.LocationSamara production and long‐term establishment of seedlings were observed along a north–south transect crossing the transition zone between continuous and discontinuous stands of red maple (47°80′–49°27′ N) in western Quebec.MethodsEleven populations of red maple were selected along a latitudinal gradient extending to the northern limit of the species. Seed traps were placed in each stand and distributed under the canopy of mature red maple trees. Seed abundance was tracked for 6 years from 1988 to 1993. Phenological observations were made in 1992 and 1993 at Roquemaure (Roq), a site located at the centre of the latitudinal gradient. Red maple trees were randomly selected within the population; counts of flower buds, pollinated buds and samaras produced were made in 1992–93. Samaras were collected from each branch immediately before dispersal and counted. During the summer of 1987, seedlings (&lt; 1 cm d.b.h.) were collected and aged at each site in twenty 1 m2 quadrants and age of the seedlings (&lt; 1 cm d.b.h.) was determined by counting the annual scars left by terminal buds.ResultsSamaras were produced even at the northern limit but large yearly variations were observed. Over the 6‐year period we counted 3 years (1989, 1990, 1993) when samara production was high, and 3 years (1988, 1991, 1992) when production was low. Phenological observations indicate that the occurrence of spring frosts at the time of flower bud flushing could contribute to decreasing the abundance of seeds. The age structure of southern localities had a relatively constant production of seedlings, as indicated by an inverse J‐shaped distribution. However, the five northernmost localities show sporadic recruitment.Main conclusionsPopulations at the northern limit are maintained essentially through vegetative reproduction and infrequent sexual recruitment. Our results indicate that regeneration within established stands through sexual recruitment is possible in all of the populations we studied. This potential becomes very low at more northerly sites and sexual reproduction alone would be unlikely to ensure successful stand regeneration. Without major disturbances in those stands, shade tolerant conifer species such as balsam fir (Abies balsamea) or black spruce (Picea mariana) would readily dominate the canopy. The discontinuous distribution of red maple stands at the northern limit is the consequence of either a random colonization of few sites during a better climatic period or remnants of a much larger distribution that has been constrained because of climatic deterioration.

Growth limitations for conifer regeneration under alternative silvicultural systems in a coastal montane forest in British Columbia, Canada

In coastal montane British Columbia (BC), Canada, alternative silvicultural systems that retain varying levels of overstorey structure and employ different gap sizes may provide means to promote regeneration of shade-tolerant conifers. In this study, part of an operational trial of Montane Alternative Silvicultural Systems (MASS), foliage morphology and physiology of planted Abies amabilis and Tsuga heterophylla were measured in conjunction with shade, soil and tree water status, and soil temperature to compare above- and below-ground growth limitations under Clearcut (CC), Green Tree (GT), Patch Cut (PC) and Shelterwood (SW) systems. Three years after planting, both A. amabilis and T. heterophylla seedlings in the SW had smaller height increments (mm) than in any of the other treatments. Acclimation of specific leaf area (SLA, cm 2 g −1) indicated that there were above-ground limitations on foliage growth and development in T. heterophylla in the SW. In A. amabilis, despite an increase in photochemical efficiency ( F v/ F m), above-ground growth limitations in the SW were not alleviated. There was no differential acclimation in the maximum light saturated rate of photosynthesis ( A max) or in foliage nitrogen concentrations (%N) in either A. amabilis or T. heterophylla among the silviculture systems. This, combined with the lack of differences in soil water or soil temperature, may indicate that nutrient availability was not different among the silvicultural systems. Foliar morphological and physiological acclimation showed that the source of seedling height growth limitations in the SW system 3 years after planting was above- and not below-ground. Therefore, retention of 25% of the pre-harvest overstorey stand structure in a dispersed pattern can limit the early growth of regenerating montane conifer seedlings presumably as a result of a 47% reduction in available light and not as a result of reduced nutrient availability.

Controls on conifer regeneration in managed riparian forests: effects of seed source, substrate, and vegetation

Red alder (Alnus rubra Bong.) commonly dominates the overstory in managed riparian forests of the Pacific Northwest. Although it is assumed that competition with a dense shrub layer inhibits succession from hardwoods to shade-tolerant conifers within these forests, this assumption has not been explicitly tested. We hypothesized that seed availability rather than competitive interactions largely shape patterns of conifer regeneration in managed riparian forests. At nine locations in western Washington, we established paired transects in riparian sites adjacent to abundant versus few or no sources of seed to examine the importance for conifer regeneration of dispersal limitation, rooting substrate, and vegetation cover. Frequency and density of regeneration were significantly greater in transects within 100 m of remnant forest patches than in paired sites at greater distances (mean frequencies of 59 vs. 18% and densities of 0.23 vs. 0.16 trees/m2, respectively). Where seed sources were present, regeneration was positively associated with coarse woody debris and negatively associated with fine litter. Regeneration was most abundant in plots with &lt;10% herb or shrub cover; however, for most species, recruitment occurred across the full range of understory plant cover. We detected no relationship between regeneration density and overstory conifer or hardwood cover. Our results suggest that, in managed forests, conifer regeneration is largely limited by seed availability and only secondarily by competitive interactions or substrate conditions. In managed landscapes, conventional strategies of vegetation control are not likely to be as effective in increasing conifer regeneration in riparian forests as managing for seed sources through green-tree retention.

Recent changes (1930s–1990s) in spatial patterns of interior northwest forests, USA

We characterized recent historical and current vegetation composition and structure of a representative sample of subwatersheds on all ownerships within the interior Columbia River basin and portions of the Klamath and Great Basins. For each selected subwatershed, we constructed historical and current vegetation maps from 1932 to 1966 and 1981 to 1993 aerial photos, respectively. Using the raw vegetation attributes, we classified and attributed cover types, structural classes, and potential vegetation types to individual patches within subwatersheds. We characterized change in vegetation spatial patterns using a suite of class and landscape metrics, and a spatial pattern analysis program. We then translated change in vegetation patterns to change in patterns of vulnerability to wildfires, smoke production, and 21 major forest pathogen and insect disturbances. Results of change analyses were reported for province-scale ecological reporting units (ERUs). Here, we highlight significant findings and discuss management implications. Twentieth century management activities significantly altered spatial patterns of physiognomies, cover types and structural conditions, and vulnerabilities to fire, insect, and pathogen disturbances. Forest land cover expanded in several ERUs, and woodland area expanded in most. Of all physiognomic conditions, shrubland area declined most due to cropland expansion, conversion to semi- and non-native herblands, and expansion of forests and woodlands. Shifts from early to late seral conifer species were evident in forests of most ERUs; patch sizes of forest cover types are now smaller, and current land cover is more fragmented. Landscape area in old multistory, old single story, and stand initiation forest structures declined with compensating increases in area and connectivity of dense, multilayered, intermediate forest structures. Patches with medium and large trees, regardless of their structural affiliation are currently less abundant on the landscape. Finally, basin forests are now dominated by shade-tolerant conifers, and exhibit elevated fuel loads and severe fire behavior attributes indicating expanded future roles of certain defoliators, bark beetles, root diseases, and stand replacement fires. Although well intentioned, 20th-century management practices did not account for landscape-scale patterns of living and dead vegetation that enable forest ecosystems to maintain their structure and organization through time, or for the disturbances that create and maintain them. Improved understanding of change in vegetation spatial patterns, causative factors, and links with disturbance processes will assist managers and policymakers in making informed decisions about how to address important ecosystem health issues.

Structure and composition of unmanaged riparian forests in the coastal mountains of Oregon, U.S.A.

We characterized the structure and composition of unmanaged riparian forests in three river basins in Oregon's coastal mountains. Our objective was to evaluate stand attributes at three spatial scales: streamside (site), drainage network (stream order), and basin (subregion). Data on basal area, species composition, snag density, canopy cover, and tree regeneration were collected along transects at 124 sites. Conifer basal area increased with distance from stream, a trend similar among subregions, and was highest at sites along first-order streams. Hardwood basal area was relatively constant with distance from stream and was proportionally higher at sites along second- and third-order streams than at sites along first-order streams. Conifer and hardwood tree regeneration occurred infrequently and varied by topographic position, stream order, and subregion. Conifer regeneration was associated with basal area of shade-tolerant conifers and appeared to be limited by shrub competition. The unmanaged forests we studied were characterized by a patchy mosaic of structure and composition. Hardwoods and shrubs were major components of the near-stream environment in these forests, whereas dominance of conifers was limited to hillslopes. It appears that fine-scale patterns associated with proximity to the stream are influenced by coarser scale factors such as valley-floor width and climate.

Functional ecology of advance regeneration in relation to light in boreal forests

This paper reviews aspects of the functional ecology of naturally established tree seedlings in the boreal forests of North America with an emphasis on the relationship between light availability and the growth and survival of shade tolerant conifers up to pole size. Shade tolerant conifer species such as firs and spruces tend to have a lower specific leaf mass, photosynthetic rate at saturation, live crown ratio, STAR (shoot silhouette area to total needle surface area ratio), and root to shoot ratio than the shade intolerant pines. The inability of intolerant species such as the pines and aspen to survive in shade appears to be mainly the result of characteristics at the shoot, crown, and whole-tree levels and not at the leaf level. Although firs and spruces frequently coexist in shaded understories, they do not have identical growth patterns and crown architectures. We propose a simple framework based on the maximum height that different tree species can sustain in shade, which may help managers determine the timing of partial or complete harvests. Consideration of these functional aspects of regeneration is important to the understanding of boreal forest dynamics and can be useful to forest managers seeking to develop or assess novel silvicultural systems.

CANOPY GAP CHARACTERISTICS AND TREE REPLACEMENT IN THE SOUTHEASTERN BOREAL FOREST

This study identifies patterns in the gap disturbance regime along a successional gradient in the southern boreal forest and uses this information to investigate canopy composition changes. Gaps were characterized in hardwood, mixed-forest, and conifer stands surrounding Lake Duparquet in northwestern Quebec. From 39 to 80 gaps were evaluated along transects established in each of these stands. The abundance of gap makers and gap fillers and total regeneration was evaluated by species, as well as the size of each gap encountered along the transects. The percentage of the forest in canopy gap was calculated directly from the proportion of the transect in gap and by using gap area and line-intercept techniques. Changes in composition were evaluated from gap-maker and gap-filler distributions and by using transition matrices based on species mortality and regeneration in canopy gaps. The percentage of the forest in canopy gap ranges from 7.1% in a 50-yr-old forest dominated primarily by aspen to 40.4% in a 234-yr-old fir-dominated forest. Gap events are due to individual or small-group tree mortality in the early successional forest but become species-specific events controlled by spruce budworm outbreaks in the later stages of succession. Due to the high latitude, direct light only reaches the forest floor in the very largest gaps of the conifer-dominated stands. However, these gaps form slowly as budworm-caused mortality occurs over a number of years, whereas in aspen-dominated stands gaps are formed quickly by the snapping of tree stems. Balsam fir is the most abundant gap-filling species; however, its abundance is negatively correlated to gap size in all stand types. Markovian transition matrices suggest that in the young aspen-dominated forests small gaps lead to species replacment by more shade-tolerant conifers but that in the oldest forests the larger gaps will result in maintenance of the intolerant species and an increase in the abundance of cedar.

Canopy Gap Characteristics and Tree Replacement in the Southeastern Boreal Forest

This study identifies patterns in the gap disturbance regime along a successional gradient in the southern boreal forest and uses this information to investigate canopy composition changes. Gaps were characterized in hardwood, mixed-forest, and conifer stands surrounding Lake Duparquet in northwestern Quebec. From 39 to 80 gaps were evaluated along transects established in each of these stands. The abundance of gap makers and gap fillers and total regeneration was evaluated by species, as well as the size of each gap encountered along the transects. The percentage of the forest in canopy gap was calculated directly from the proportion of the transect in gap and by using gap area and line-intercept techniques. Changes in composition were evaluated from gap-maker and gap-filler distributions and by using transition matrices based on species mortality and regeneration in canopy gaps. The percentage of the forest in canopy gap ranges from 7.1% in a 50-yr-old forest dominated primarily by aspen to 40.4% in a 234-yr-old fir-dominated forest. Gap events are due to individual or small-group tree mortality in the early successional forest but become species-specific events controlled by spruce budworm outbreaks in the later stages of succession. Due to the high latitude, direct light only reaches the forest floor in the very largest gaps of the conifer-dominated stands. However, these gaps form slowly as budworm-caused mortality occurs over a number of years, whereas in aspen-dominated stands gaps are formed quickly by the snapping of tree stems. Balsam fir is the most abundant gap-filling species; however, its abundance is negatively correlated to gap size in all stand types. Markovian transition matrices suggest that in the young aspen-dominated forests small gaps lead to species replacment by more shade-tolerant conifers but that in the oldest forests the larger gaps will result in maintenance of the intolerant species and an increase in the abundance of cedar.