Alan S. White

Natural disturbance regimes in northeastern North America—evaluating silvicultural systems using natural scales and frequencies

Many scientists and foresters have begun to embrace an ecological, natural disturbance paradigm for management, but lack specific guidance on how to design systems in ways that are in harmony with natural patterns. To provide such guidance, we conducted a comprehensive literature survey of northeastern disturbances, emphasizing papers that studied late-successional, undisturbed, or presettlement forests. Evidence demonstrates convincingly that such forests were dominated by relatively frequent, partial disturbances that produced a finely patterned, diverse mosaic dominated by late-successional species and structures. In contrast, large-scale, catastrophic stand-replacing disturbances were rare, returning at intervals of at least one order of magnitude longer than gap-producing events. Graphing the contiguous areas disturbed against their corresponding return intervals shows that these important disturbance parameters are positively related; area disturbed increases exponentially as the return interval lengthens. This graph provides a convenient metric, termed the natural disturbance comparability index, against which to evaluate both single and multi-cohort silvicultural systems based on their rotations or cutting-cycles and stand or gap sizes. We review implications of these findings for silvicultural practice in the region, and offer recommendations for emulating natural disturbance regimes.

Presettlement Regeneration Patterns in a Southwestern Ponderosa Pine Stand

Tree stems 2 106 yr old (i.e., established before significant European influence in this area) in a 7.3-ha old-growth ponderosa pine (Pinus ponderosa) forest in northern Arizona were aged and mapped. Age structure analysis showed that successful establishment of ponderosa pine was infrequent. The periods without successful establishment could be quite long, as suggested by four consecutive decades in which only two surviving trees were established. The stems were strongly aggregated, as measured with nearest neighbor analysis, and groups were visually distinct in the field. Most of the stems occurred in groups of three or more, with group size ranging from 3 to 44 stems and area occupied by a group ranging from 0.02 to 0.29 ha. Ages of stems within groups were variable, the most homogeneous group having a range of 33 yr and the least having a range of 268 yr. The data are not consistent with the commonly held view that southwestern ponderosa pine occurs in even-aged groups and that each group became established following the demise of the group previously occupying the site. Instead, it seems more likely that seedlings became established when one or two trees within the group died, the additional fuel surrounding the dead trees causing an intensely burned spot in the otherwise low-intensity fires that were frequent in the area. The hot spot would create a potential seedbed for pine by eliminating, at least temporarily, the competing grasses on that small area. This decreased competition, in conjunction with adequate seed production and favorable moisture conditions in the spring and early summer, may well have been critical for pon- derosa pine establishment. The relative infrequency of all these events occuning in the necessary sequence could explain the erratic age structure data from this area.

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.

The effects of harvest-created gaps on plant species diversity, composition, and abundance in a Maine oak-pine forest

Abstract In forests where large, stand-replacing disturbances are infrequent, small-scale disturbances associated with the mortality and replacement of individual trees are a primary source of heterogeneity in forest composition and structure. The disturbances considered in this study were canopy gaps created by a partial harvest in the winter of 1987–1988 in an oak–pine ( Quercus–Pinus ) forest that is part of a long-term ecosystem study at the Holt Research Forest in coastal Maine. This study examined the gap phase of the forest turnover cycle: the brief episodes of rapid change when processes determining the structure and composition of a forest community occur. The objective was to evaluate the effects of harvest-created gaps and soil moisture (as reflected by soil drainage classes) on woody and herbaceous species diversity, composition, and abundance relative to unharvested control areas. Because the vegetation was sampled in both 1993 and 1998, detection of short-term changes was possible. Harvest gaps had higher total plant species richness in the understory (vegetation Diversity measures calculated for sapling density showed a significant difference among drainage classes for species richness, but no significant difference between treatments for species richness, the Shannon–Wiener index, or the evenness index. Gap size was positively correlated with the total density of saplings, the density of several species, and species richness. There were significant differences between treatments for the density of all species tested in at least one of the four height classes. DCA of sapling data revealed some separation of harvest gaps and controls but there was considerable overlap. Axis 1 tended to separate early successional species in the harvest gaps from shade tolerant species in the controls. The understory plant community at the HRF appears to be resilient to the partial harvest that created a variety of gap sizes. Silvicultural objectives were met; the harvest gaps created sites for establishment of seedlings and sprouts of tree species, particularly early successional species.

The effects of thirteen years of annual prescribed burning on a Quercus ellipsoidalis community in Minnesota

A Quercus ellipsoidalis community in central Minnesota has been prescribed burned annually since 1965 in an attempt to restore the area to its presettlement oak savanna structure and composition. By 1979 density and basal area of the overstory were significantly lower in the burned area than in an adjacent unburned area but were still higher than estimated savanna values because of the persistence of stems @>25 cm diameter at breast height (dbh). A tall—shrub/small—tree layer was totally lacking in the burned area but averaged 19% cover in the unburned area. Understory richness was significantly higher in the burned area than in the unburned area. Most of the species that showed a significant difference between the two areas peaked in the burned area; this was especially true for grasses and forbs. These results indicate that annual prescribed burning is gradually restoring the area to savanna but that the restoration is not yet complete. Complete restoration may not be possible with annual burning because such burning seems to have little effect on large—tree (@>25 cm dbh) mortality.

Disturbance dynamics of old-growth Picea rubens forests of northern Maine

Abstract Question: How have the spatial and temporal aspects of past disturbance shaped the current structure and composition of old-growth Picea rubens forests? Location: Northern Maine, USA. Methods: We established three 50 m × 50 m plots in old-growth Picea rubens forests and mapped the location of trees and saplings. We extracted increment cores from canopy trees, and recorded growth releases indicating past disturbance. By linking spatial data (tree positions) and temporal data (dated growth releases), we reconstructed the location and size of former canopy gaps back to 1920, and determined a more general disturbance chronology extending as far back as 1740. Results: We found no evidence of stand-replacing disturbances. The disturbance dynamic includes pulses of moderate-severity disturbances caused by wind storms and host-specific disturbance agents (spruce budworm, spruce bark beetle) interposed upon a background of scattered smaller canopy gaps. Consequently, rates of disturbance fluctuated considerably over time. Reconstructed canopy gaps were temporally and spatially scattered; during disturbance peaks, they were both larger and more numerous. Conclusions: Despite peaks in disturbance, several of which created relatively large gaps, this system has experienced no significant change in species composition. Instead, the shade-tolerant Picea rubens has maintained canopy dominance. The patch dynamics described here consist of dramatic structural, not compositional, changes to the forest. The persistence of Picea rubens is attributed to a combination of traits: (1) abundance of advance regeneration; (2) ability to endure suppression and respond favourably to release; and (3) longevity relative to ecologically similar species. Nomenclature: Gleason & Cronquist (1991) for vascular plants; Crum (1983) for mosses. Abbreviation: DWD = Down woody debris.

Structure and spatial patterns of trees in old-growth northern hardwood and mixed forests of northern Maine

Stand structure including spatial patterns was studied in northern hardwood and mixed forest types in the 2000-ha old-growth Big Reed Forest Reserve in northern Maine using complete stem mapping, dendrochronology, and spatial analyses on ∼0.5 plots. The inclusion of saplings, dead wood, age distributions, spatial pattern, and interactions provided some idea of underlying processes and temporal change. Structural characteristics were most comparable to spruce-northern hardwood forests of northern New England and New York, and most characteristics matched expected patterns for old-growth forests of the region.Results indicated smaller maximum-tree sizes, lower basal areas (26–34 ) and downed-wood volumes (29–64), higher densities (475–649), but similar species longevities compared to other mesic old-growth forests further south and in the Lake States. The stands were dominated by very shade-tolerant tree species, including Fagus grandifolia Ehrh., Acer saccharum Marsh.,Picea rubens Sarg. and Abies balsamea(L.) Mill, with each species found in many crown positions and age and size classes. The sapling layer was dominated by Fagus grandifolia followed by Picea rubens. Most species had reverse-J shaped diameter distributions, but age distributions were indicative of synchronous, episodic recruitment. In most plots, Acer saccharum diameter distributions were skewed towards the mid-larger size classes. Lack of young and small Acer saccharum stems suggested change in forest composition towards Fagus grandifolia dominance. Most species formed small-scale clusters (≤15 ) perhaps in response to small gap disturbances. Snags were the dominant dead wood type and were randomly to regularly distributed in most plots. Logfall directions were unrelated to hurricane paths. Recent small-scale disturbance events and topographic position appear to be important in explaining current structure and dynamics of the hardwood and mixed forests of Big Reed Forest Reserve in northern Maine. The continued effects of beech bark disease had a greater effect on hardwood plots, whereas a recent spruce budworm outbreak had a greater effect on plots with higher conifer density.The dominance of very shade tolerant tree species in small-scale clusters, and randomly distributed snags rather than clustered uproots were indicative of the prevalence of small scale gap disturbance regimes in the hardwood and mixed forests of Big Reed Forest Reserve in northern Maine. Varying topographic position may allow for slight changes in disturbance regime leading to consequent variation in structure and dynamics. H1, a more open plot on upper exposed slopes, had distinctly different characteristics such as lower live and dead tree and sapling densities than the other plots, but more uprooted trees and Acer saccharum saplings. Such small scale gap disturbance regimes operating on an episodic basis, and effects of slight variations in this regime on stand composition and structure have significant implications for silvicultural interventions and management of these forest types.

Vegetation development in a southern Maine pitch pine-scrub oak barren.

oak, mixed deciduous woodland, scrub oak, and open-canopy pitch pine. We investigated five factors that potentially influence vegetation distribution in the 856-ha preserve: soil texture, moisture, and fertility; topography; and disturbance history. Although analysis of variance revealed significant differences in soils and topography among plots in the five communities studied, multivariate analyses indicated a weak relationship between these environmental variables and species composition and structure. In contrast, disturbance history clearly influenced community and species distribution within the preserve. Effects of historic logging, charcoaling, and cultivation for blueberries persist despite a stand-replacing wildfire that swept through most of barrens in October, 1947. Pollen and charcoal analysis of sediments from a pond near the preserve suggests fire has influenced the barrens for at least the last several centuries, but that prior to European settlement of the area in the 18th century the character of the vegetation differed somewhat from that of today. Our results demonstrate that both fire and edaphic factors have influenced the vegetation of the area, but that historic land use has also played a strong role in determining the present character of the plant communities studied.

Biomass response mechanisms of understory species the first year after prescribed burning in an Arizona ponderosa-pine community

Abstract Species-specific biomass and density responses were determined for three grasses and two shrubs in a northern Arizona ponderosa-pine (Pinus ponderosa) forest one year following a prescribed burn. Sources of response were determined by monitoring changes in surviving plants as well as contributions by new seedlings and sprouts. Results are reported for four overstory types (open sawtimber sites, below-canopy sawtimber sites, and pole and sapling patches). Responses varied among species and within species by overstory type. Sitanion hystrix showed a positive biomass response to prescribed burning in the open sawtimber sites, due to increased growth of surviving plants and a large influx of seedlings. In contrast, in the pole patches, where fire severity was high, S. hystrix had a high mortality and low colonization, but maintained its pre-burn biomass levels through increased growth of surviving plants. Muhlenbergia montana decreased following burning in both the open sawtimber sites and pole patches, because of high mortality, no increased growth of surviving plants, and virtually no seedling colonization. Ceanothus fendleri experienced high mortality in the below-canopy sawtimber sites and pole patches, but often sprouted vigorously following fire.

Effects of competition from young northern hardwoods on red pine seedling growth, nutrient use efficiency, and leaf morphology

Abstract The effects of competition from three northern hardwood tree species on red pine ( Pinus resinosa Ait.) seedlings were examined on two clear-cut sites in western Maine. We examined how planted red pine seedlings altered their nutrient use efficiency and shoot morphology under changing environmental conditions and how these changes related to their ability to tolerate competition. A three-factor experimental design was used to determined the effects of species of competitors and their abundance as well as fertilization on red pine seedling growth, nutrient use efficiency, and leaf morphology. The competitors were striped maple ( Acer pensylvanicum L.), red maple ( Acer rubrum L.), and pin cherry ( Prunus pensylvanica L.) established at two densities (high and zero) with two levels of fertilization (0 and 224 g m −2 of 10-10-10 NPK commercial fertilizer). Nitrogen and phosphorus use efficiencies were calculated as total aboveground biomass divided by total nutrient content. Specific leaf area (cm 2 g −1 ), leaf area ratio (cm 2 g −1 ), and total leaf area (cm 2 ) were measured for all red pine seedlings. Plots were harvested at two time periods, when pine seedlings were 2 years old (1989) and 3 years old (1990). Total biomass, annual production, and leaf area index (m 2 leaf area m −2 ground surface area) were calculated for competitors on each plot. Red pine seedlings had higher specific leaf area, leaf area ratio, and nitrogen use efficiency on competitor plots than on plots free of competition, suggesting a phenotypic response to resource depletion of light and nutrients by competitors. Fertilization decreased growth of red pine seedlings and decreased nitrogen use efficiency. Red pine total leaf area and biomass were lower on the competitor plots. Higher competitor biomass, leaf area index, and nutrient uptake explained the lower growth of red pine seedlings on competition plots. Pin cherry was the most significant competitor eith striped maple being intermediate; red maple had the least effect on pine seedling growth. The results of this study indicate that the growth of red pine seedlings was governed by the availability of resources as influenced by the competitor species, the efficiency of nutrient use, and the ability of red pine to adjust its growth pattern in response to resource availability.