Lee E. Frelich

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.

Earthworm invasion into previously earthworm-free temperate and boreal forests.

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.

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.

Patch Formation and Maintenance in an Old‐Growth Hemlock‐Hardwood Forest

Cause of patch formation was investigated on a 7.2 ha study area in Sylvania Wilderness Area, a primary forest remnant in Upper Michigan comprising a mosaic of hemlock, sugar maple, and mixed-forest patches. Spatial autocorrelation analysis of the stem map indicates that, although most species pairs have a neutral association between canopy trees and understory trees of other species, hemlock and sugar maple canopy trees both have strong positive self association and negative reciprocal association with each other. No species pairs have a positive reciprocal association on regeneration with each other. MOSAIC, a Markov simulation model in which transition probabilities depend on neighborhood species composition, shows that the negative reciprocal association between hemlock and sugar maple of the intensity observed in this study, could lead to spatial separation into monodominant patches over long time periods (3000 yr). The mixed-forest patches are along spatial continua of varying steepness between sugar maple and hemlock patches. Interactions between sugar maple and hemlock overstory and understory trees, along with the pattern of invasion of hemlock, provide a reasonable explanation for the patch structure. Pedological, topographical, and disturbance history differences do not coincide with the location of patches within upland forests on the study area.

Are large, infrequent disturbances qualitatively different from small, frequent disturbances?

ABSTRACT In this article, we develop a heuristic model of ecosystem-disturbance dynamics that illustrates a range of responses of disturbance impact to gradients of increasing disturbance extent, intensity, or duration. Three general kinds of response are identified and illustrated: (a) threshold response, (b) scale-independent response, and (c) continuous response. Threshold responses are those in which the response curve shows a discontinuity or a sudden change in slope along the axis of increasing disturbance extent, intensity, or duration. The response threshold occurs at a point where the force of the disturbance exceeds the capacity of internal mechanisms to resist disturbance, or where new mechanisms of recovery become involved. Within this conceptual framework, we find that some unusually large or intense disturbances, but not all, produce qualitatively different responses compared with similar disturbances of lesser magnitude. If disturbance impact does not increase with increasing disturbance extent, intensity, or duration, or if the response curve changes monotonically, then large disturbances are not qualitatively different from small ones. For example, jack pine tends to become reestablished after stand-replacing fire in boreal forests, regardless of fire size, because its serotinous cones provide an adequate seed source throughout the burned area. Thus, large fires are not qualitatively different from small fires in terms of jack pine reproduction. However, if disturbance impact does increase abruptly at some point with increasing disturbance extent, intensity, or duration, often because of thresholds in the capacity of internal mechanisms to resist or respond to disturbance impact, then large disturbances are qualitatively different from small ones, at least for some parameters of ecological response. For example, balsam fir and white cedar can recolonize a small burned patch of boreal forest in close proximity to surviving individuals of these species, but they will be eliminated from a large burn because of their susceptibility to fire-caused mortality and their inability to disperse their seeds over long distances. The conceptual framework presented here permits some new insights into the dynamics of natural systems and may provide a useful tool with which managers can assess the potential for catastrophic damages resulting from large, infrequent disturbances.

Effects of European Earthworm Invasion on Soil Characteristics in Northern Hardwood Forests of Minnesota, USA

European earthworms are colonizing worm-free hardwood forests across North America. Leading edges of earthworm invasion in forests of northern Minnesota provide a rare opportunity to document changes in soil characteristics as earthworm invasions are occurring. Across leading edges of earthworm invasion in four northern hardwood stands, increasing total earthworm biomass was associated with rapid disappearance of the O horizon. Concurrently, the thickness, bulk density and total soil organic matter content of the A horizon increased, and it’s percent organic matter and fine root density decreased. Different earthworm species assemblages influenced the magnitude and type of change in these soil parameters. Soil N and P availability were lower in plots with high earthworm biomass compared to plots with low worm biomass. Decreases in soil nitrogen availability associated with high earthworm biomass were reflected in decreased foliar nitrogen content for Carex pensylvanica, Acer saccharum and Asarum canadense but increased foliar N for Athyrium felix-femina. Overall, high earthworm biomass resulted in increased foliar carbon to nitrogen ratios. The effects of earthworm species assemblages on forest soil properties are related to their feeding and burrowing habits in addition to effects related to total biomass. The potential for large ecosystem consequences following exotic earthworm invasion has only recently been recognized by forest ecologists. In the face of rapid change and multiple pressures on native forest ecosystems, the impacts of earthworm invasion on forest soil structure and function must be considered.

INFLUENCE OF LOGGING, FIRE, AND FOREST TYPE ON BIODIVERSITY AND PRODUCTIVITY IN SOUTHERN BOREAL FORESTS

The effects of logging on ecosystem sustainability are controversial. Sur- prisingly, existing data are inadequate to allow a comprehensive evaluation of logging effects on biodiversity, composition, and productivity since appropriate comparisons of stands of similar ages and differing disturbance histories are rare. We addressed this issue using a study of 2000 plots in 80 southern boreal forest stands in northern Minnesota, USA, wherein we contrasted naturally regenerated aspen (Populus tremuloides), jack pine (Pinus banksiana), and black spruce (Picea mariana) stands established following logging or the dominant natural disturbance, wildfire, for stands of two age classes (25-40 and 70-100 yr old). For young stands, those established postlogging had higher vascular plant diversity than those postwildfire. Otherwise, we found no evidence of differing species diversity (including canopy tree, shrub, herbaceous, and bryophyte species), composition, produc- tivity, or nitrogen cycling, in forest stands of comparable age and forest type that originated after logging compared to after wildfire. These variables, however, differed significantly among forest types, with aboveground net primary productivity and plant species diversity generally higher in aspen than jack pine stands, even when growing on comparable soils, and lowest in black spruce. Although there is evidence that logging has increased the proportional landscape dominance by aspen, a forest type with higher diversity, nutrient cycling, and productivity than other types, our evidence refutes the idea that disturbance by logging has diminished stand-scale productivity or plant diversity in comparison to the common natural disturbance, wildfire.

CHANGES IN HARDWOOD FOREST UNDERSTORY PLANT COMMUNITIES IN RESPONSE TO EUROPEAN EARTHWORM INVASIONS

European earthworms are colonizing earthworm-free northern hardwood forests across North America. Leading edges of earthworm invasion provide an opportunity to investigate the response of understory plant communities to earthworm invasion and whether the species composition of the earthworm community influences that response. Four sugar maple-dominated forest sites with active earthworm invasions were identified in the Chippewa National Forest in north central Minnesota, USA. In each site, we established a 30 x 150 m sample grid that spanned a visible leading edge of earthworm invasion and sampled earthworm populations and understory vegetation over four years. Across leading edges of earthworm invasion, increasing total earthworm biomass was associated with decreasing diversity and abundance of herbaceous plants in two of four study sites, and the abundance and density of tree seedlings decreased in three of four study sites. Sample points with the most diverse earthworm species assemblage, independent of biomass, had the lowest plant diversity. Changes in understory plant community composition were most affected by increasing biomass of the earthworm species Lumbricus rubellus. Where L. rubellus was absent there was a diverse community of native herbaceous plants, but where L. rubellus biomass reached its maximum, the herbaceous-plant community was dominated by Carex pensylvanica and Arisaema triphyllum and, in some cases, was completely absent. Evidence from these forest sites suggests that earthworm invasion can lead to dramatic changes in the understory community and that the nature of these changes is influenced by the species composition of the invading earthworm community.

EXOTIC EUROPEAN EARTHWORM INVASION DYNAMICS IN NORTHERN HARDWOOD FORESTS OF MINNESOTA, USA

European earthworms are invading previously worm-free hardwood forests across Minnesota and the Great Lakes region. In many of these forests, earthworm invasions have been associated with the loss of a previously thick forest floor. The ability of earth- worms to alter and control ecosystem processes has been demonstrated in agricultural systems, but the dynamics and impact of these invasions in native forest ecosystems is largely unknown. The impacts of earthworm invasion are expected to be related to the size and species composition of the earthworm population because different species have dif- ferent habitat and feeding preferences. We identified four sugar maple dominated forests in north central Minnesota in the Chippewa National Forest with active earthworm invasion. In each site a sample grid of 45 points (30 150 m) 10 m apart in three parallel transects with 15 points each was established that spanned a visible leading edge of invasion. Over four years earthworm populations and forest floor thickness were sampled across all tran- sects, thus providing both a space-for-time assessment of decadal scale successional dy- namics and a four-year window into shorter time changes. We found a succession of earth- worm species across the visible leading edge due to different patterns of colonization by different earthworm species. Marked increases in space and time in earthworm biomass were associated with the development of discrete transition zones where forest floor thick- ness decreases to zero in as little as 75 m from areas that have forest floor layers up to 10 cm thick with advancement of the visible leading edge of up to 30 m in four years at three of the study sites. The epi-endogeic species Lumbricus rubellus led to the most rapid removal of forest floor material during initial invasion. Epigeic and epi-endogeic species of earth- worms may facilitate the establishment of other species of earthworms leading to the establishment of stable populations of endogeic and anecic species, which prevent recovery of the forest floor.

Neighborhood effects, disturbance severity, and community stability in forests

ABSTRACT A theoretical framework and conceptual model for temporal stability of forest tree-species composition was developed based on a synthesis of existing studies. The model pertains primarily to time periods of several tree lifetimes (several hundred to a few thousand years) at the neighborhood and stand spatial scales (0.01–10 ha), although a few extensions to the landscape scale are also made. The cusp catastrophe was chosen to illustrate compositional dynamics at the stand level for jack pine, northern hardwood, and white pine forests in the Great Lakes Region of the United States and for tropical rainforests in the northern Amazon basin. The models feature a response surface (degree of dominance by late-successional species) that depends on two variables: type of neighborhood effects of the dominant tree species and severity of disturbances. Neighborhood effects are processes that affect the chance of a species replacing itself at the time of disturbance (they can be positive, neutral, or negative) and are of two types: overstory–undestory effects, such as the presence of advanced reproduction; and disturbance-activated effects, such as serotinous seed rain. Disturbance severity is the proportion of trees killed during a disturbance. Interactions between neighborhood effects and disturbance severity can lead to either punctuated stability (dramatic but infrequent change in composition, in those forests dominated by species with positive neighborhood effects) or succession (continuous change, in those forests dominated by species with neutral-negative neighborhood effects). We propose that neighborhood effects are a major organizing factor in forest dynamics that provide a link across spatial scales between individual trees and disturbance/patch dynamics at the stand and landscape scales.