Craig G. Lorimer

Tall understorey vegetation as a factor in the poor development of oak seedlings beneath mature stands

Replacement of oaks (Quercus spp.) by other species after natural and humancaused disturbance is recognized as a common problem on average and productive sites in many parts of eastern North America. Oak seedlings are often numerous beneath mature stands, but seedlings are usually too small to compete effectively with tall saplings of other species. Reasons for the poor development of oak seedlings are not well understood. An experiment was designed to evaluate the impact of tall and low understorey vegetation on oak seedling development beneath mature stands on two sites in southwestern Wisconsin. Understorey stems taller than 1.5 m and scattered small canopy trees were removed on half of the main plots, with the other plots retained as controls (...)

Age Structure and Disturbance History of a Southern Appalachian Virgin Forest

The frequency and intensity of natural disturbances in a montane old-growth forest of coniferous (Tsuga) and broad-leaved deciduous trees (Liriodendron, Betula, Castanea, Fagus, Acer) were investigated by means of diameter frequency distributions, diameter-age regressions, and anal- ysis of radial growth patterns. The seven species investigated are represented by all-aged populations regardless of the degree of shade tolerance. The age distributions of most species, however, are irregular with several prominent peaks. Apparent coincidence of these peaks among some species suggests a common underlying influence. Radial growth patterns reveal higher than average numbers of abrupt and sustained increases in growth rate in most decades of peak recruitment which, along with other evidence, suggest that disturbance was the principal causal factor. There is evidence for eight partial disturbances in the past 250 yr, each of which generally removed < 10%o of the overstory trees. These fairly light disturbances appear to be sufficient to cause major deviations from the theoretical equilibrium diameter distribution of the forest.

Natural Disturbance Regimes in Hemlock-Hardwood Forests of the Upper Great Lakes Region

The frequency of natural disturbances and their influence on the forest landscape mosaic were investigated on three large tracts of primary forest in Upper Michigan. Seventy 0.5—ha plots were randomly distributed in a total forest area of 23 000 ha dominated by sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis). Radial increment patterns were used to estimate canopy accession dates for each of a number of randomly selected overstory trees on each plot. From these data a disturbance chronology, representing the percentage of stand area occupied by cohorts originating during each decade over the last 130 yr, was compiled for each plot. Average rates of disturbance or canopy mortality are estimated at 5.7 to 6.9% per decade. The corresponding average canopy residence time of a tree is 145—175 yr. No significant differences were detected in average disturbance rates among the three study areas, between plots near the coast of Lake Superior and inland plots, among several different aspects, and among several different slope positions. Natural rotation periods increase exponentially with increasing disturbance intensity, which is defined as the approximate percentage of the plot area converted to gaps during a disturbance episode. Estimates of rotation periods range from 69 yr for ≥10% canopy removal to 1920 yr for ≥60% canopy removal. Spatial autocorrelation analysis indicated that plots with light and medium disturbances (<40%) are randomly distributed over the landscape. Plots wit heavy disturbances (≥40%) are clustered with a patch radius of ≈2 km, consistent with the sizes of thunderstorm downbursts. The data indicate that light and medium disturbances dominate the disturbance regime. The majority of stands on the landscape are composed of several major and many minor age classes. Even—aged stands with one predominant age class are uncommon. The age distribution of individual patches or cohorts in the two larger study areas (14 500 and 6073 ha) follows a nearly uniform distribution. None of the three study areas had more than 15% of the forest area converted to gaps in a single decade. The two larger areas meet most of the criteria that have been proposed for equilibrium landscapes.

THE PRESETTLEMENT FOREST AND NATURAL DISTURBANCE CYCLE OF NORTHEASTERN MAINE

Land survey records of 1793-1827 containing forest data for 1.65 X 1Off ha of northern Maine were analyzed for species composition, successional status, and frequency of large-scale disturbance. Quantitative data consists of 1,448 sample trees spaced 1.6 km apart along a 9.7- X 9.7-km grid. Species which each comprised > 10% of the total were Picea spp., Fagus grandifolia, Abies balsamea, Thuja occidentalis, and Betula lutea. These forests appeared to be largely in a climax state as indicated by the dominance of shade-tolerant species and the small percentage (8% ) of intolerant or early successional species. However, 9.3% of the tract was burned land and birch-aspen forest at the time of the survey, mostly the result of large fires in 1803 and 1825. Windfalls occurred along 2.6% of the surveyed distance. If the amount of disturbed forest at this time was typical of the natural disturbance regime, then the average recurrence interval of fire and large-scale windthrow for a given site would be 800 and 1,150 years, respectively. Data on the structure of remnant virgin stands in the region likewise suggest that the time interval between severe disturbances was much longer than that needed to attain a climax, all-aged structure.

Current and Predicted Long-term Effects of Deer Browsing in Hemlock Forests in Michigan, USA

Remnants of virgin hemlock Tsuga canadensis (L.) Carr. forest in the Porcupine Mountains, Michigan, USA, have experienced inadequate hemlock regeneration lasting several decades. White-tailed deer Odocoileus virginianus Zimmermann browsing seems to be the major cause of the observed decline ojhemlock regeneration, rather than poor seedbed conditions or changing climate. In some areas, significant changes m the size-structure of the Jorest have already occurred, with a shift of dominanceJ?om hemlock to sugar maple Acer saccharum Marsh. taking place. A simulation of forest development is used to predict the changes inJorest structure that will occur if no action is taken to control browsing. From this simulation it is estimated that in less than 150 years, hemlock will become only a minor component of the forest over large areas where it is currently the major dominant.

Scale and frequency of natural disturbances in the northeastern US: implications for early successional forest habitats and regional age distributions

Recent declines in the amount of habitat suitable for early successional wildlife in the northeastern US have prompted public land managers to consider establishing minimum levels of young forest, based on the natural range of variation, in order to maintain viable populations of these species. In this paper, we review evidence on the frequency, severity, and scale of natural disturbances in four major forest regions of the northeastern US. Using six independent lines of evidence, we examined the influence of natural disturbances in presettlement and modern times. In situations where estimates of annual disturbance rates were available, we estimated the regional age distribution of forest stands based on the assumption of random spatial pattern of disturbance. Available evidence suggests a gradient of generally decreasing disturbance frequency from coastal regions to the interior uplands and mountains. The proportion of the presettlement landscape in seedling‐sapling forest habitat (1‐15 years old) ranged from 1 to 3% in northern hardwood forests (Fagus‐Betula‐Acer‐Tsuga) of the interior uplands to possibly >10% in coastal pine‐oak (Pinus‐Quercus) barrens. Within a region, variability in the amount of young forest is not well known, but upper slopes and ridges generally had the highest disturbance frequency and severity. Comparison of line transect data of the presettlement land surveys with modern plot surveys suggests that present-day amounts of young forests in northern hardwood and spruce‐hardwood forests in some regions may be several times higher than in presettlement times. In coastal oak forests and pine‐oak barrens, the amount of young forests and open woodlands may be less because of reduced fire frequency. # 2003 Elsevier B.V. All rights reserved.

FOREST STRUCTURE AND LIGHT REGIMES FOLLOWING MODERATE WIND STORMS: IMPLICATIONS FOR MULTI-COHORT MANAGEMENT

Moderate-severity disturbances appear to be common throughout much of North America, but they have received relatively little detailed study compared to catastrophic disturbances and small gap dynamics. In this study, we examined the immediate impact of moderate-intensity wind storms on stand structure, opening sizes, and light regimes in three hemlock-hardwood forests of northeastern Wisconsin. These were compared to three stands managed by single-tree and group selection, the predominant forest management system for northern hardwoods in the region. Wind storms removed an average of 41% of the stand basal area, compared to 27% removed by uneven-aged harvests, but both disturbances removed trees from a wide range of size classes. The removal of nearly half of the large trees by wind in two old-growth stands caused partial retrogression to mature forest structure, which has been hypothesized to be a major disturbance pathway in the region. Wind storms resulted in residual stand conditions that were much more heterogeneous than in managed stands. Gap sizes ranged from less than 10 m2 up to 5000 m2 in wind-disturbed stands, whereas the largest opening observed in managed stands was only 200 m2. Wind-disturbed stands had, on average, double the available solar radiation at the forest floor compared to managed stands. Solar radiation levels were also more heterogeneous in wind-disturbed stands, with six times more variability at small scales (0.1225 ha) and 15 times more variability at the whole-stand level. Modification of uneven-aged management regimes to include occasional harvests of variable intensity and spatial pattern may help avoid the decline in species diversity that tends to occur after many decades of conventional uneven-aged management. At the same time, a multi-cohort system with these properties would retain a high degree of average crown cover, promote structural heterogeneity typical of old-growth forests, and maintain dominance by late-successional species.

MINIMUM OPENING SIZES FOR CANOPY RECRUITMENT OF MIDTOLERANT TREE SPECIES: A RETROSPECTIVE APPROACH

Minimum opening sizes and time required for successful canopy recruitment of tree saplings are not well known because the gap capture process is slow and difficult to monitor. In this study, we investigated canopy recruitment in harvest openings created by group selection in hemlock–hardwood forests to determine if yellow birch (Betula alleghaniensis Britt.), a species with intermediate shade tolerance, could successfully reach the canopy via small openings. We measured gap trees in openings ranging from 40 to 1000 m2 and up to 55 years old. Sample trees up to 25 m tall, spanning all stages of the gap capture process, were destructively sampled to determine height–growth rates, total age, and size at time of gap formation. Results indicate that yellow birch comprised >40% of the upper stratum of gap saplings in openings as small as 100–400 m2. The ultimately successful hardwood gap trees, on average, reached crown shoulder height of the mature gap border trees (73% of total canopy height) within 28–37 yea...

VARIABILITY AND CONVERGENCE IN STAND STRUCTURAL DEVELOPMENT ON A FIRE‐DOMINATED SUBALPINE LANDSCAPE

The 1988 Yellowstone fires resulted in a complex mosaic within which post- fire lodgepole pine seedling densities varied by over five orders of magnitude. Investigators have speculated that such postfire mosaics of vegetation structure may persist until the next large disturbance, but the fate of the initial structural variability of postfire communities is currently poorly understood. We studied lodgepole pine (Pinus contorta var. latifolia Englem. ex Wats.) stands in Yellowstone National Park (Wyoming, USA) unburned by the 1988 fires to determine how variation in stand structure changes with increasing stand age. The coefficient of variation in stand density decreased from 231% in 12-yr-old stands to 91% in stands aged 50-100 yr, and to 37% for stands ages 200-250 yr. Substantial variability in age distributions both within and among age classes suggested that both gradual infilling of initially sparse stands and self-thinning of initially dense stands are important processes affecting variation in stand density. Variation in stand density was explained primarily by stand age (P < 0.0001) and by geographic location (P < 0.01). Field estimates and re- constructions of stand density trajectories suggest the importance of biotic processes and the contingent effects that produce initial patterns of stand density. Variation in stand density is substantially reduced 125 yr following fire and remains relatively unchanged beyond approximately 200 yr. These results suggest that large, infrequent fires impose a pattern of stand structural variability that may persist for centuries, but stand density likely converges within the fire-free interval in this landscape.

Predicting tree growth from crown variables in managed northern hardwood stands

Abstract The predictability of individual-tree growth rates for Acer saccharum, Fraxinus americana, and Tilia americana in northern hardwood stands in Wisconsin was studied in relation to crown dimension and crown competition variables measured in the field. Data were collected from 221 destructively sampled trees in eight second-growth stands on above-average sites. The single best independent variable for predicting basal area growth was the projection area of the exposed portion of the crown, which had higher correlations with observed growth than initial stem diameter, diameter-based competition indices, and other crown variables. Basal area growth equations containing total crown projection area, percent exposed crown area, and relative height had R2 values ranging from 0.77 to 0.88, which represent increases of 13 to 47 percentage points compared with similar equations with only diameter-based competition variables. The most accurate height growth models were functions of total tree height and percent exposed crown area, with R2 values of 0.74 for the non-linear maple equation and 0.44 for the linear white ash equation. The results appear to support the hypothesis that significant competitive stress on individual trees is induced only by the ring of competitor trees immediately surrounding the subject tree crown. By using percent exposed crown area as a crown competition variable, this competitive effect can be estimated without direct measurements of any of the competitors, potentially saving much field measurement time during the model calibration phase and eliminating the statistical lack of independence generally associated with plot competition measures.