Old-growth Stands Research Articles

Forest Floor Bryophytes of Pseudotsuga menziesii-Tsuga heterophylla Stands in Oregon: Influences of Substrate and Overstory

Species richness and abundance of bryophytes inhabiting forest floor substrates were assessed at two sites in western Oregon. Bryophyte diversity, abundance, and community composition were compared between sites, and between young forest stands (-55 yrs) and old-growth stands (400 + yrs) within each site. Relationships of stand structural features to and community composition were assessed by stratifying sampling between diversity plots placed in areas of greater structural diversity, such as hardwood openings and remnant old-growth trees, and plots situated within the remaining more homogeneous conifer-dominated forest matrix. Richness, particularly for liverworts, was significantly higher in old-growth than young stands, and the two ages differed significantly in community composition. Substrate (ground versus coarse woody debris) and overstory (conifers versus hardwoods) were most strongly correlated with variation in community composition. Relatively open hardwood-dominated plots differed in composition from matrix plots. Bryophyte abundance was lower in denser stands and plots, and positively correlated with canopy gaps, percentage of hardwoods, and incident solar radiation. These results suggest that availability of light may limit bryophyte productivity in these

Tree mortality after prescribed burning in an old-growth Scots pine forest in northern Sweden

Tree mortality and input of dead trees were studied after a prescribed burning in a forest reserve in northern Sweden. The stand was a multi-layered old-growth forest. The overstorey was dominated by Scots pine (Pinus sylvestris L.) and the understorey consisted of mixed Scots pine and Norway spruce (Picea abies L. Karst.). Ground vegetation was dominated by ericaceous dwarf-shrubs and feathermosses. The stand has been affected by six forest fires during the last 500 years. The prescribed burning was a low intensity surface fire that scorched almost 90% of the ground. Tree mortality for smaller pines and spruces (DBH < 10 cm) was over 80% in the burned parts of the reserve. For larger pines, 10–50 cm DBH, mortality showed a decreasing trend with increasing diameter, from 14% in class 10–20 cm DBH to 1.4% in class 40–50 cm DBH. However, pines with DBH ≥ 50 cm had a significantly higher mortality, 20%, since a high proportion of them had open fire scars containing cavities, caused by fungi and insects, which enabled the fire to burn inside the trunks and hollow them out. The fire-induced mortality resulted in a 21 m3 ha–1 input of dead trees, of which 12 m3 ha–1 consisted of trees with DBH ≥ 30 cm. An increased mortality among larger trees after low-intensity fires has not previously been described in Fennoscandian boreal forests, probably owing to a lack of recent fires in old-growth stands. However, since large pines with open fire scars were once a common feature in the natural boreal forest, we suggest that this type of tree mortality should be mimicked in forestry practices aiming to maintain and restore natural forest biodiversity.

Influence of aspect and stand age on ground flora of southeastern Ohio forest ecosystems

The species composition and diversity of the ground flora (vascular plants less than one meter in height excluding tree seedlings) were compared between sixteen mesic northeast-facing and sixteen dry southwest-facing middle slopes in southeastern Ohio. On each aspect, eight of the plots were located in second-growth forest stands (70 to 90 years in age) and eight of the plots were located in old-growth forest stands (greater than 150 years in age) in order to determine how the ground flora on opposing slopes varies with time since major disturbance. Physiographic and A horizon characteristics were measured to select comparable plots. Multi-response permutation procedures (MRPP) detected significant differences in the ground-flora species composition of second-growth and old-growth plots on both northeast-facing and southwest-facing slopes. In addition, the ground-flora species composition of northeast-facing plots differed significantly from that of southwest-facing plots. Northeast-facing plots were significantly higher in species richness and Hill's diversity (N1) than southwest-facing plots. Northeast-facing second-growth plots were also significantly higher in species richness and Hill's diversity than northeast-facing old-growth plots. No significant differences in these measures were found between southwest-facing second-growth and old-growth forests plots. Twenty-one species were identified as potential indicators of age, based on their distribution across the plots. Certain species with similar ecological characteristics were found to have similar patterns of occurrence. Further research is needed to determine the extent to which differences in the ground flora of second-growth old-growth reflect the sensitivity of individual species to past disturbance.

Comparison of communities of ectomycorrhizal fungi in old-growth and mature stands of Douglas-fir at two sites on southern Vancouver Island

Comparison of communities of ectomycorrhizal fungi in old-growth and mature stands of Douglas-fir at two sites on southern Vancouver Island

Stand dynamics during a 12-year period in a second-growth stand in a cool temperature forest in northern Japan

In 1981 and 1993, trees over 2 m high were measured and mapped to clarify stand dynamics in two permanent plots of 0.1 ha in a secondary, cool temperate, mixed broadleaf/conifer forest after logging in the Tomakomai Experiment Forest, central Hokkaido, northern Japan. The species could be placed in two groups according to the change in basal area and density, and annual height growth:Quercus mongolica var. grosseserrata, Phellodendron amurense, Prunus sargentii and Tilia japonica (Group I: GI) showed episodic regeneration and/or fast height growth;Acer mono, Acer palmatum var. matsumuae and Sorbus alnifolia (Group II: GII) showed continuous regeneration and/or slow height growth. The age distribution of stems over 10 cm in d.b.h. suggests synchronous regeneration of GI and GII species. Vertical stratification was promoted during the 12-year period by the difference in annual height growth between the two species groups. Additional tree censuses in both remnant old-growth stands and second-growth stands after large scale blowdowns demonstrated that Q. mongolica var. grosseserrata is most dominant in stands varying in the stages of development. Disturbance history and successional trends in dominant species implied that stand-devastating disturbances were responsible for the regeneration of Q. mongolica var. grosseserrata in the study forest.

Truffle production in old-growth and mature fir stands in northeastern California

Few studies have examined fruiting patterns of hypogeous fungi, and relationships between sporocarp production of hypogeous fungi and forest habitat components such as organic soil depth and amounts of decayed wood are poorly understood. We sampled sporocarps of hypogeous fungi (truffles) in four old-growth (>200 years) and four paired, mature (ca 100 years) fir ( Abies spp.) stands during four sample periods in 1993 and three sample periods in 1994 in the Lassen National Forest in northeastern California. Truffles were collected from 4-m 2 circular plots systematically located at 36 grid points per stand during each sample period. Habitat characteristics were measured in 50.3-m 2 circles centered at each grid point in 1993. We found a total of 46 truffle species in 30.4% of the 2016 total plots, and the total standing dry weight of truffles was equivalent to 2.43 kg ha −1. Total frequency and biomass of truffles and number of truffle species did not differ significantly between stand types in 1993 or 1994, but species composition did. We found no significant associations between measures of total truffle abundance and measures of habitat structure and composition at the 0.25-ha grid scale or at the 50.3-m 2 habitat plot scale. At the scale of the 4-m 2 truffle plot, plots with decayed wood were more likely to have truffles than plots without decayed wood during the final sample period of each year, but the association was significant only in 1993. Mean organic soil depth was greater in plots with truffles than plots without truffles in each sample period in both years, but ranked values were only marginally significant in one sample period. Goodness-of-fit tests to the Poisson distribution indicated that individual truffles had clumped distributions, but we could not reject the null hypothesis of random distribution of truffle collections. Our results indicate that total truffle production had recovered from stand-replacement wildfire in the mature stands, and that total truffle abundance was not strongly associated with habitat characteristics within the range of habitat variation exhibited in these stands. Individual species, however, were associated with old-growth stands and others with mature stands.

Standing Crop and Animal Consumption of Fungal Sporocarps in Pacific Northwest Forests

Although fungal fruiting bodies are a common food supplement for many forest animals and an important dietary staple for several small mammals, changes in their abundance and consumption with forest succession or disturbance have not been quantified. Above- and belowground fungal fruiting bodies (epigeous and hypogeous sporocarps) were sampled for 46 mo in managed-young, natural-mature, and old-growth western hemlock (Tsuga heterophylla) stands in Washington State. Screen exclosures were placed over the surface of half of the sample plots to prevent aboveground predation of sporocarps. Standing crop of epigeous sporocarps was low in most seasons and then increased 30-fold to a mean of 2.28 kg/ha in the fall. Epigeous biomass varied little between stand types, and animal consumption of these sporocarps was low. Standing crop of hypogeous sporocarps was 0.78 kg/ha in managed-young stands, compared to 4.51 and 4.02 kg/ha in natural-mature and old-growth stands. In all stands, standing crop peaked in the summer and was lowest in the winter. Mean animal consumption of hypogeous sporocarps was 0.64 kg/ha, a value that exceeded the available standing crop quantity of 0.36 kg/ha in managed-young stands during the winter. In natural-mature and old-growth stands, truffle biomass remained high year-round and exceeded consumption in all seasons. Low hypogeous sporocarp biomass in the managed-young stands resulted from the general absence of large clusters of Elaphomyces granulatus, which made up >90% of the biomass in older stands. This absence in managed-young stands may be associated with the thin organic layer that has developed following harvest and burning 60 yr ago. The consistent level of animal consumption indicates that truffles may be an important and readily available year-round food source, compared to the ephemeral fruiting of epigeous sporocarps. Changes in forest composition and age due to natural disturbance or human management influence fungal sporocarp productivity and diversity and, consequently, affect food availability for animals dependent on hypogeous sporocarps.

STANDING CROP AND ANIMAL CONSUMPTION OF FUNGAL SPOROCARPS IN PACIFIC NORTHWEST FORESTS

Although fungal fruiting bodies are a common food supplement for many forest animals and an important dietary staple for several small mammals, changes in their abundance and consumption with forest succession or disturbance have not been quantified. Above- and belowground fungal fruiting bodies (epigeous and hypogeous sporocarps) were sampled for 46 mo in managed-young, natural-mature, and old-growth western hemlock (Tsuga heterophylla) stands in Washington State. Screen enclosures were placed over the surface of half of the sample plots to prevent aboveground predation of sporocarps. Standing crop of epigeous sporocarps was low in most seasons and then increased 30-fold to a mean of 2.28 kg/ha in the fall. Epigeous biomass varied little between stand types, and animal consumption of these sporocarps was low. Standing crop of hypogeous sporocarps was 0.78 kg/ha in managed-young stands, compared to 4.51 and 4.02 kg/ha in natural-mature and old-growth stands. In all stands, standing crop peaked in the summer and was lowest in the winter. Mean animal consumption of hypogeous sporocarps was 0.64 kg/ha, a value that exceeded the available standing crop quantity of 0.36 kg/ha in managed-young stands during the winter. In natural-mature and old-growth stands, truffle biomass remained high year-round and exceeded consumption in all seasons. Low hypogeous sporocarp biomass in the managed-young stands resulted from the general absence of large clusters of Ela- phomyces granulatus, which made up >90% of the biomass in older stands. This absence in managed-young stands may be associated with the thin organic layer that has developed following harvest and burning 60 yr ago. The consistent level of animal consumption indicates that truffles may be an important and readily available year-round food source, compared to the ephemeral fruiting of epigeous sporocarps. Changes in forest composition and age due to natural disturbance or human management influence fungal sporocarp pro- ductivity and diversity and, consequently, affect food availability for animals dependent on hypogeous sporocarps.

Food Limitation and Habitat Preference of Glaucomys sabrinus and Tamiasciurus hudsonicus

Food limitation and habitat preference of northern flying squirrels ( Glaucomys sabrinus ) and red squirrels ( Tamiasciurus hudsonicus ) were examined in British Columbia, Canada. Using mark-recapture techniques to estimate population demographics, we tested the hypotheses that populations of G. sabrinus and T. hudsonicus were limited by availability of food and that old-growth stands provided higher-quality habitat than second-growth stands. Populations were monitored on six study areas; two second-growth lodgepole pine ( Pinus contorta ) stands (controls), two second-growth pine stands with food supplementation (treatments), and two old-growth lodgepole pine stands. Sunflower seeds were applied (20 kg ha−1 month−1) to treatment stands by helicopter in a uniform distribution for 3 summers, and mark-recapture of G. sabrinus and T. hudsonicus was initiated in the second summer of food supplementation. There were significantly more G. sabrinus and T hudsonicus in treatment stands than in control stands. The mean density of G. sabrinus and T. hudsonicus in treatment stands was ca. 1.8 times greater than that in control stands. Estimates of population sizes for G. sabrinus in control stands were significantly lower than those in old-growth stands. Estimates of population sizes for T. hudsonicus in control and old-growth stands were not significantly different. Adult male G. sabrinus in control and treatment stands were significantly heavier than those in old-growth stands. Body mass for T. hudsonicus and the proportion of adults breeding, recruitment, and survival rates for G. sabrinus and T. hudsonicus did not vary among control, treatment, and old-growth stands. Consequently, populations of G. sabrinus and T. hudsonicus appeared to be limited by availability of food, but individuals did not appear to benefit from food supplementation. Second-growth stands were not suboptimal habitat for G. sabrinus and T. hudsonicus .

COARSE WOODY DEBRIS AS A FUNCTION OF AGE, STAND STRUCTURE, AND DISTURBANCE IN BOREAL NEWFOUNDLAND

We examined the structural attributes of coarse woody debris (CWD) in balsam fir and mixed balsam fir-black spruce forests in western and central Newfoundland. The purpose of the study was to document CWD abundance and structure within various stages of stand development, and to generate simple models to predict CWD availability as habitat for forest wildlife. Our objectives were threefold: (1) determine the temporal patterns of CWD following clear-cut disturbance, and compare these second-growth stands to old sites (>80 yr) of natural origin; (2) relate the temporal patterns of CWD to stand development; and (3) demonstrate how factors such as site conditions and disturbance may influence these temporal patterns. Our chronosequence included 19 second-growth stands with a mean tree age ranging from 33 to 80 yr, and 7 old-growth stands ranging from 87 to 110 yr. The volume of coarse woody debris (CWD), here defined as downed wood, was relatively low to intermediate early in the chronosequence (e.g., 32.3 m3/ha in a 36-yr-old stand), lowest in a 58-yr-old stand (15.2 m3/ha), and highest in an 80-yr-old stand (78.1 m3/ha). Results indicated that CWD volume followed the general U-shaped temporal trend observed in other forest systems. The presence of CWD early in the sequence was strongly influenced by residual hardwoods (i.e., birch) left by the original logging operation. The highest observed volume of CWD corresponded with stand senescence and appeared to be maintained within old- growth stands. Snag (standing dead wood) density was low within the youngest stands ( 50-60 yr of age is not residual but generated primarily from regenerating tree structure. Factors affecting the rate of stem growth, e.g., site quality and initial stocking levels, should influence the accumulation and overall abundance of CWD within later stages of forest development. We therefore applied yield-density relationships as a method of examining stand-level dynamics of CWD.

BLOWDOWN HISTORY AND LANDSCAPE PATTERNS IN THE ANDES OF TIERRA DEL FUEGO, ARGENTINA

The effects of periodic gales on Nothofagus (southern beech) stand devel- opment and landscape dynamics were studied in a 10.4-km2 study area in the Sierra de las Pinturas, part of the Andes in Argentine Tierra del Fuego. We reconstructed blowdown history (patch sizes, dates of origin, return intervals) since the late 1700s to assess how periodic large-scale wind disturbance influenced landscape pattern in a relatively simple system. Most previous studies have focused on single storms in more diverse forests and in landscapes influenced by several disturbance types and human activities. Boundaries of post-blowdown stands were digitized from aerial photographs and analyzed within a geo- graphic information system. Ages of blowdowns and return intervals were determined from scars, growth releases, and maximum tree ages. Discrete blowdown patches (0.1 to >100 ha) covered two-thirds of the study area and ranged from 19 to -200 yr of age, with 20- 30 yr between major events. The meteorology of these storms was unknown, but we suspect they were caused by intense low-pressure systems originating in Antarctica. The mean return interval for 34 sites was 145 yr, with a range of 103-218 yr. Based on treefall size distributions, most stands blown over in the past 100 yr were even-aged, with mean tree dbh (diameter at breast height) between 20 and 32 cm. Aerial photographs taken in 1970 were used for before-and-after comparisons of a 1972 blowdown. Seventy-one percent of the area blown over in 1972 was even-aged, and 35% of the boundaries from this storm exactly coincided with previous stand boundaries. Damage patterns from storms in 1924 and 1972 suggested that forests located on valleys parallel to the wind, windward side slopes, and possibly upper leeward slopes were most vulnerable to blowdown, but few landscape positions escaped being hit by repeated storms. Return intervals were not sig- nificantly related to slope, elevation, or aspect; but surprisingly, shorter return intervals were associated with deeper soils. The landscape pattern of blowdown and recovery shifted over time because of variation among individual storms and because a small proportion of old-growth stands were converted to blowdowns and vice versa. Browsing by guanacos (Lama guanaco), a large native camelid, has severely inhibited tree regeneration during the past 75 yr in small blowdowns and the perimeters of larger ones, converting some stands to open meadows and incipient alpine communities. In the relatively simple Noth- ofagus forests of Tierra del Fuego, periodic gales are the main determinants of forest structure and pattern across a range of scales from small patches to entire landscapes.

Blowdown History and Landscape Patterns in the Andes of Tierra del Fuego, Argentina

The effects of periodic gales on Nothofagus (southern beech) stand development and landscape dynamics were studied in a 10.4-km2 study area in the Sierra de las Pinturas, part of the Andes in Argentine Tierra del Fuego. We reconstructed blowdown history (patch sizes, dates of origin, return intervals) since the late 1700s to assess how periodic large-scale wind disturbance influenced landscape pattern in a relatively simple system. Most previous studies have focused on single storms in more diverse forests and in landscapes influenced by several disturbance types and human activities. Boundaries of post-blowdown stands were digitized from aerial photographs and analyzed within a geographic information system. Ages of blowdowns and return intervals were determined from scars, growth releases, and maximum tree ages. Discrete blowdown patches (0.1 to >100 ha) covered two-thirds of the study area and ranged from 19 to ≈200 yr of age, with 20–30 yr between major events. The meteorology of these storms was unknown, but we suspect they were caused by intense low-pressure systems originating in Antarctica. The mean return interval for 34 sites was 145 yr, with a range of 103–218 yr. Based on treefall size distributions, most stands blown over in the past 100 yr were even-aged, with mean tree dbh (diameter at breast height) between 20 and 32 cm. Aerial photographs taken in 1970 were used for before-and-after comparisons of a 1972 blowdown. Seventy-one percent of the area blown over in 1972 was even-aged, and 35% of the boundaries from this storm exactly coincided with previous stand boundaries. Damage patterns from storms in 1924 and 1972 suggested that forests located on valleys parallel to the wind, windward side slopes, and possibly upper leeward slopes were most vulnerable to blowdown, but few landscape positions escaped being hit by repeated storms. Return intervals were not significantly related to slope, elevation, or aspect; but surprisingly, shorter return intervals were associated with deeper soils. The landscape pattern of blowdown and recovery shifted over time because of variation among individual storms and because a small proportion of old-growth stands were converted to blowdowns and vice versa. Browsing by guanacos (Lama guanaco), a large native camelid, has severely inhibited tree regeneration during the past 75 yr in small blowdowns and the perimeters of larger ones, converting some stands to open meadows and incipient alpine communities. In the relatively simple Nothofagus forests of Tierra del Fuego, periodic gales are the main determinants of forest structure and pattern across a range of scales from small patches to entire landscapes.

Structure and floristics of secondary and old-growth forest stands in lowland Costa Rica

We characterized stand structure and floristic composition of woody life forms in three, 16–18 yr old secondary stands that regenerated after pasture abandonment, and three nearby old-growth stands of tropical rain forest in lowland Costa Rica. Basal area and stem density for each of four plant size classes (seedlings, saplings, treelets, trees) were similar among stand types, but density of adult canopy palms (individuals ≥ 10 cm DBH), was lower in the secondary stands. We estimate that 15% of the basal area of stems ≥ 10 cm DBH correspond to remnant trees in our secondary stands. The observed rapid woody regrowth compared to other published studies in the lowland neotropics, can be attributed to moderate land use and possibly, to the influence of nutrient-rich volcanic soils in the study area. Overall, plant species richness was lower in the secondary stands, but this difference was less pronounced in the smallest size classes (seedlings, saplings). Median percent similarity of all pairwise stand comparisons showed that floristic composition of saplings (stems ≥ 1 m tall and ≥ 5 cm DBH) was more similar between secondary and old-growth stands than for trees (stems ≥ 10 cm DBH). Because the potential value of secondary forests in conserving woody plant diversity appears highest for the young size classes, we suggest that further studies on floristic composition, especially those addressing the dynamics of the understory component, are needed to refine our understanding of the role of this natural resource in the maintenance of plant biodiversity in disturbed landscapes.

Windthrow damage 2 years after partial cutting at the Date Creek silvicultural systems study in the Interior Cedar–Hemlock forests of northwestern British Columbia

Partial cutting that removed either 30 or 60% of the volume as single trees or small groups up to 0.5 ha had little effect on wind damage to merchantable trees (≥17.5 cm diameter). On average, 6.7 stems per hectare of windthrow occurred across unlogged and logged units, representing approximately 1.9% of the standing trees. Over 2 years, 0.63 m 2 ·ha n1 of merchantable basal area was damaged, or 1.5% of the original standing basal area. In the partial cuts, 2.2% of the trees were damaged compared with 1.1% in unlogged areas. The 1.1% increase in damage in partial cut units was well below the 10% effect size considered large enough to warrant either management intervention or to deem the partial cutting a failure. The greatest wind damage occurred in the old-growth stands. For 8 of the 9 tree species examined, no individual tree characteristics seemed to predispose them to wind damage. Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes, Populus tremuloides Michx., and Abies lasiocarpa (Hook.) Nutt. were the most susceptible species to windthrow.

Comparison of Tree Basal Area and Canopy Cover in Habitat Models: Subalpine Forest

Canopy cover and basal area are 2 common measures of tree cover used in forest wildlife habitat models and resource selection studies. When choosing between these 2 measures, it is important to recognize that they may differentially estimate relative cover of coexisting tree species due to differences in bole diameter distributions, crown overlap, and crown widths as a function of bole diameter. I found moderate agreement (multivariate Ρ = 0.62, P 17%) to differences in diameter distributions among species in mature to old-growth (>200 yr) stands. Low densities of large diameter lodgepole pine or Engelmann spruce were a greater proportion of the basal area than of the canopy cover, while the converse occurred for higher densities of small and large diameter subalpine fir. Only 3 of 14 deviations >17% were attributed to species differences in crown overlap and crown width as a function of bole diameter, which occurred for young (22 yr) and intermediate-aged (48-80 yr) stands. I recommend use of basal area rather than canopy cover to estimate tree cover when it is desirable to emphasize large, uncommon trees that are resources used by the wildlife species of interest or that are indicators of important forest disturbance and successional conditions.

Habitat Selection by Spotted Owls during Natal Dispersal in Western Oregon

The probability of successful natal dispersal may influence the viability of northern spotted owl (Strix occidentalis caurina) populations within fragmented forests. We tested 6 null hypotheses examining the relations between habitat selection, forest fragmentation, and the probability of mortality for juvenile northern spotted owls during natal dispersal in western Oregon, 1982-85. Older forest (old-growth and mature stands) was the vegetation type used most frequently during transience (35.3%) and colonization (61.2%), and spotted owls selected closed-canopy forests over open-canopy forests during both phases of dispersal (P < 0.05). The hypotheses that spotted owl habitat selection was independent of forest fragmentation were not rejected for either transient (P = 0.51) or colonization (P = 0.43) dispersal. Likewise, net dispersal distance was independent of forest fragmentation (P = 0.92). Use of open sapling stands during transient dispersal decreased the probability of mortality (Wald test = 2.21, P = 0.03), whereas use of clearcuts during colonization dispersal increased the probability of mortality (Wald test = 1.95, P = 0.03). Net dispersal distance did not affect the probability of mortality (P = 0.67), yet a negative relation existed between dispersal distance and the amount of clearcut used during transient dispersal (P < 0.005). These results provide additional evidence of the selection of older forests by spotted owls and how their natal dispersal may be affected by harvesting such forests; particularly, that use of clearcuts may decrease the probability of successful natal dispersal.

Gap regeneration of major tree species in different forest types of Japan

Gap regeneration of major tree species was examined, based on the pattern of gap phase replacement, in primary old-growth stands of warm-temperate, cool-temperate and subalpine forests, Japan. Using principal component analysis, the gap-regeneration behavior of major tree species could be divided into three guilds and that of Fagus crenata (monodominant species of cool-temperate forests). The criteria used for this division were total abundance of canopy trees and regenerations and relative abundance of regenerations to canopy trees. The gap-regeneration behavior of species in the first guild was that canopy trees regenerate in gaps from seedlings or saplings recruited before gap formation; they had higher total abundance and more abundant regenerations relative to their canopy trees. The gap-regeneration behavior of F. crenata was same as species in the first guild, but F. crenata had less abundant regenerations relative to its canopy trees. Species in the second guild had lower total abundance and less abundant regenerations to their canopy trees. The guild contained species whose canopy trees regenerate in gaps from seedlings or saplings recruited after gap formation or regenerate following largescale disturbance. The third guild consisted of species with lower total aboundance and more abundant regenerations relative to their canopy trees. The gap-regeneration behavior of some species in this guild was that trees regenerate in gaps from seedlings or saplings recruited before gap formation, and grow, mature, and die without reaching the canopy layer, while the gap-regeneration behavior of other species was same as that of species in the first guild or F. crenata. Major tree species of subalpine forests were not present in the third guild.

Variation in canopy gap formation among developmental stages of northern hardwood stands

Trends in gap dynamics among pole, mature, and old-growth northern hardwood stands were investigated on eight sites in the Porcupine Mountains of western upper Michigan. Recent gaps (created between 1981 and 1992) were identified using permanent plot records of tree mortality, while older gaps (1940–1981) were identified using stand reconstruction techniques. Although canopy gaps were somewhat more numerous in pole and mature stands, gaps were &lt;25% as large as those in old-growth stands because of smaller gap-maker size, and the proportion of stand area turned over in gaps was only about half as large. Gap makers in younger stands generally had mean relative diameters (ratio of gap-maker DBH to mean DBH of canopy trees) &lt;1.0 and were disproportionately from minor species such as eastern hophornbeam (Ostryavirginiana (Mill.) K. Koch). Gap makers in old-growth stands had mean relative diameters &gt;1.5 and were predominantly from the dominant canopy species. Even in old-growth forests, most gaps were small (mean 44 m2) and created by single trees. Based on the identity of the tallest gap tree in each gap, nearly all shade-tolerant and midtolerant species have been successful in capturing gaps, but gap capture rates for some species were significantly different from their relative density in the upper canopy. The tallest gap trees of shade-tolerant species were often formerly overtopped trees, averaging more than 60% of the mean canopy height and having mean ages of 65–149 years. Canopy turnover times, based on gap formation rates over a 50-year period, were estimated to average 128 years for old-growth stands dominated by sugar maple (Acersaccharum Marsh.) and 192 years for old-growth stands dominated by hemlock (Tsugacanadensis (L.) Carrière). While these estimates of turnover time are substantially shorter than maximum tree ages observed on these sites, they agree closely with independent data on mean canopy residence time for trees that die at the average gap-maker size of 51 cm DBH. The data support previous hypothetical explanations of the apparent discrepancy between canopy turnover times of &lt;130 years for hardwood species and the frequent occurrence of trees exceeding 250 years of age.

Gap Size, Within-Gap Position and Canopy Structure Effects on Conifer Seedling Establishment

1 Emergence, establishment and growth of Abies amabilis, Pseudotsuga menziesii and Tsuga heterophylla were studied for 2 years in variously sized canopy gaps created in four stands on the west slope of the Cascade Range in central Oregon and southern Washington, USA. Seedlings originating from seeds sown on controlled microsites were compared with natural seed rain. 2 Seedling establishment was greater in gaps than in closed-canopy areas, but was relatively low in portions of large gaps exposed to direct solar radiation, particularly for Tsuga. Some evidence of gap partitioning by regenerating seedlings was found, though all species were most abundant in shaded portions of gaps. 3 Seedling size increased with gap size, and was greatest at gap centres. Pseudotsuga growth was relatively low except in the largest gaps, Tsuga growth increased dramatically with gap size, and Abies growth responded the least to gap size. 4 Seedling establishment and growth differed among the four stands. Establishment in closed-canopy areas was lowest in stands with dense conifer canopies relatively close to the forest floor. Natural establishment of Tsuga in gaps was very low in mature stands but abundant in old-growth stands, reflecting differences in seed rain. 5 Silviculturally created openings may accelerate the development of multiple canopy layers in mature forests, but gap size and the availability of shade-tolerant tree seeds will also control the rate and spatial pattern of canopy development.

Climate change, tree species distributions and forest dynamics: A case study in the mixed conifer/northern hardwoods zone of northern Europe

Three bioclimate variables (growing degree days after budburst, temperature of the coldest month and a moisture index) are used in a model (STASH) to predict potential range limits of north European tree species. CO2-induced climate warming scenarios cause major changes in these limits. The dominant conifers of the mixed conifer/northern hardwoods zone, Picea abies and Pinus sylvestris, retreat from the south and west while Fagus sylvatica and other temperate hardwoods spread to the north. A gap model (FORSKA 2) is used to simulate the associated transient responses of a forest reserve within this zone. In both dry Pinus- and moist Picea- dominated forest communities, Picea continues to increase while Pinus sooner or later declines. These changes are continuing successional (non-climatic) responses to the cessation of disturbance 150 years ago. Climate warming speeds up the succession, and allows Fagus to establish and increase. The eventual decline of Picea however is delayed due to the persistence of old-growth stands. Picea forests subject to continuing disturbance show a more rapid shift to dominance by Fagus and other temperate hardwoods. Delayed immigration of new species, including Fagus, would favour earlysuccessional species such as Betula pendula and Quercus spp. in a forest with reduced biomass and diversity. The results emphasise the complex and sometimes counter-intuitive nature of transient responses, and the importance of considering disturbance history and potential migration rates when predicting the impacts of rapid climate change on forests.