Chris J. Peterson

Climate Change and Forest Disturbances

tudies of the effects of climate change on forestshave focused on the ability of species to tolerate tem-perature and moisture changes and to disperse,but they haveignored the effects of disturbances caused by climate change(e.g.,Ojima et al.1991).Yet modeling studies indicate the im-portance of climate effects on disturbance regimes (He et al.1999). Local, regional, and global changes in temperatureand precipitation can influence the occurrence, timing, fre-quency,duration,extent,and intensity of disturbances (Baker1995, Turner et al. 1998). Because trees can survive fromdecades to centuries and take years to become established,climate-change impacts are expressed in forests, in part,through alterations in disturbance regimes (Franklin et al.1992, Dale et al. 2000).Disturbances,both human-induced and natural,shape for-est systems by influencing their composition,structure,andfunctional processes.Indeed,the forests of the United Statesare molded by their land-use and disturbance history.Withinthe United States,natural disturbances having the greatest ef-fects on forests include fire,drought,introduced species,in-sect and pathogen outbreaks, hurricanes, windstorms, icestorms, and landslides (Figure 1). Each disturbance affectsforests differently. Some cause large-scale tree mortality,whereas others affect community structure and organizationwithout causing massive mortality (e.g., ground fires). For-est disturbances influence how much carbon is stored intrees or dead wood. All these natural disturbances interactwith human-induced effects on the environment,such as airpollution and land-use change resulting from resource ex-traction, agriculture, urban and suburban expansion, andrecreation.Some disturbances can be functions of both nat-ural and human conditions (e.g., forest fire ignition andspread) (Figure 2).

Factors Influencing Succession: Lessons from Large, Infrequent Natural Disturbances

ABSTRACT Disturbance events vary in intensity, size, and frequency, but few opportunities exist to study those that are extreme on more than one of these gradients. This article characterizes successional processes that occur following infrequent disturbance events that are exceptional in their great intensity or large size. The spatial variability in disturbance intensity within large, infrequent disturbances (LIDs) often leads to a heterogeneous pattern of surviving organisms. These surviving organisms dictate much of the initial successional pattern on large disturbances where the opportunities for seeds to disperse into the middle of the disturbance are limited. The traditional distinction between primary and secondary succession is insufficient to capture the tremendous variability in succession following LIDs. Disturbance size influences succession where long-distance colonization by propagules is important. Observations from LIDs suggest the following interrelated hypotheses about trends in succession with increasing distance from seed sources when disturbanceintensity is high: (a) initial densities of organisms will be lower; (b) nucleation processes, in which recovering patches serve as foci for additional colonization and expand spatially, will be more important; (c) competitive sorting will be less important relative to chance arrival in determination of community composition, and (d) community composition will be initially less predictable; and (e) the rate of recovery of community composition will be slower. Prediction of succession following LIDs without considering contingencies such as the abundance, types, and spatial distribution of residuals, and distance to seed sources is likely to be unsuccessful for large portions of the landscape. Abundance and spatial arrangement of survivors and arrival patterns of propagules may be the pivotal factors determining how succession differs between intense disturbances of large and small extent.

The role of litter in an old-field community: impact of litter quantity in different seasons on plant species richness and abundance.

SummaryWe studied the effect of removing and adding plant litter in different seasons on biomass, density, and species richness in a Solidago dominated old-field community in New Jersey, USA. We removed all the naturally accumulated plant litter in November (658 g/m2) and in May (856 g/m2) and doubled the amount of litter in November and May in replicated plots (1 m2). An equal number of plots were left as controls. Litter removal and addition had little impact on total plant biomass or individual species biomass in the growing season following the manipulations. Litter removal, however, significantly increased plant densities but this varied depending upon the season of litter removal, species, and life history type. Specifically, the fall litter removal had a much greater impact than the spring litter removal suggesting that litter has its greatest impact after plant senescence in the fall and prior to major periods of early plant growth in spring. Annual species showed the greatest response, especially early in the growing season. Both spring and fall litter removal significantly increased species richness throughout the study. Litter additions in both spring and fall reduced both plant densities and species richness in June, but these differences disappeared near the end of the growing season in September. We concluded than in productive communities where litter accumulation may be substantial, litter may promote low species richness and plant density. This explanation does not invoke resource competition for the decline in species richness. Finally, we hypothesize that there may be broad thresholds of litter accumulation in different community types that may act to either increase or decrease plant yield and diversity.

Forest Reorganization: A Case Study in an Old‐Growth Forest Catastrophic Blowdown

We studied the patterns and mechanisms of regeneration of a 400-ha wind- throw in an old-growth beech-hemlock forest caused by a tornado on 31 May 1985. Starting in 1986, and over a period of six growing seasons, we recorded percent cover and density of woody stems, and monitored seedling demography of nearly 5000 seedlings in the windthrow and adjacent forest. Plant community response to the disturbance was dramatic: by August of 1986, species richness, tree seedling density and total percent cover were significantly greater in the windthrow than in the adjacent forest. Shade-intolerant herbs (e.g., Erechtites hieracifolia) and shrubs (e.g., Rubus allegheniensis) established and rapidly increased in abundance during the first 3 yr, but began declining by the 5th yr of the study. Tree seedlings established in decreasing amounts through the 6 yr of the study, and the young tree canopy was dominated in 1991 by seedlings and sprouts that established prior to 1987. Fagus grandifolia, a shade-tolerant species that established via advanced regeneration, was dominant the first 3 yr, but was surpassed in the 5th yr by Betula alleghaniensis, a species of intermediate tolerance that established from seed germination just before or shortly after the disturbance. Tsuga canadensis seedling densities were initially high, but deer browsing prevented substantial growth and a drought in 1988 caused heavy mortality of browsed seedlings. Regeneration thus differed from the predictions of the gap and Hubbard Brook models of forest regeneration (which predict dominance by shade-intolerant species), and the severity model (which predicts dominance by shade-tolerant species). The differences point out important influences of availability of propagules and the impact of herbivory; and the need for more attention to models that incorporate multiple contingencies.

Microsite variation and soil dynamics within newly created treefall pits and mounds.

We studied early soil dynamics, environmental conditions and plant colonization of microsites within 28 recently created treefall pits and mounds in a catastrophic windthrow. Pit and mound sizes were proportional to the size of the fallen trees, and deposition of eroded soil in the center of pits occurred at a decreasing rate over 2 yr. Instability of the substrate contributed to low plant abundance in the pits resulting in lower total plant cover than in the intact soil microsites adjacent to the pits. Larger pits revegetated more slowly than small ones. We recognized four microsites associated with the exhumed roots of each treefall: mound, pit, wall, and intact forest floor (no soil disturbed). Both growth forms and individual species differed in their colonization among microsites, resulting in lower species diversity and total cover on mounds relative to other microsites. Small-seeded, wind-dispersed species colonized the center of pits more readily than species that relied on vegetative spread. Species that are rare in intact forest were common on the disturbed soil of pits and mounds. During two successive years, we documented significant differences among microsites in light, soil moisture, and soil temperature. The species of fallen canopy trees had little influence on the plant community after disturbance, except in the intact soil microsite. We conclude that differential colonization of microsites within forest disturbances occurs at a finer scale than previously recognized and that this facilitates the maintenance of species diversity in the plant community.

Catastrophic wind damage to North American forests and the potential impact of climate change.

Catastrophic winds from tornadoes and downbursts are a major cause of natural disturbance in forests of eastern North America, accounting for thousands of hectares of disturbed area annually. Wind disturbance shows substantial regional variation, decreasing from the mid-west to the east and from the south-east to New England. In terms of the relative importance among these types of storms, more forest damage results from tornadoes in the south-east and mid-west, while downbursts are the most important type of wind disturbance in the Great Lakes area. Downbursts vary widely in size, but large ones can damage thousands of hectares, while tornadoes are much smaller, seldom affecting more than several hundred hectares. Tornadoes cause the most severe wind disturbances. Site characteristics such as physiography, soil moisture, and soil depth; stand characteristics like density and canopy roughness; and tree characteristics such as size, species, rooting depth, and wood strength, are the factors most recognized as influencing damage patterns. The consequences of wind damage to forests, such as change in environmental conditions, density, size structure, species composition, and successional status, occur on both immediate (hours-to-days) and long-term (months-to-decades) time scales. Most wind disturbances result in the post-disturbance vegetation being comprised of surviving canopy trees, and varying amounts of sprouts, released understory stems, and new seedlings. Stand size structure is usually reduced, and successional status of a forest is often advanced. Diversity can be either increased or decreased, depending on the measure of abundance used to calculate diversity. Because tornadoes and downbursts are in part products of thermodynamic climatic circumstances, they may be affected by anticipated changes in climatic conditions as the 21st century progresses. However, the current understanding of tornado and downburst formation from supercell storms is very incomplete, and climate-change model predictions sufficiently coarse, that predictions of changes in frequency, size, intensity, or timing of these extreme events must be regarded as highly uncertain. Moreover, retrospective approaches that employ tree demography and dendrochronology require prohibitively large sample sizes to resolve details of the relationship between climate fluctuations and characteristics of these storms. To improve predictions of changes in the climatology of these storms, we need improved understanding of the genesis of tornadoes and downbursts within thunderstorms, and greater resolution in global climate models. To improve coping strategies, forest scientists can contribute by giving more attention to how various silvicultural actions influence stand and tree vulnerability. Finally, increased focus on the dynamics of forest recovery and regrowth may suggest management actions that can facilitate desired objectives after one of these unpredictable wind disturbances.

Treefall and resprouting following catastrophic windthrow in an old-growth hemlock-hardwoods forest

Abstract We surveyed the type of treefall (snapped or uprooted), and for snapped trees the height of break and sprouting tendency, in a catastrophic windthrow in an old-growth hemlock-hardwoods forest. Two-thirds of the 630 trees sampled were uprooted and one-third snapped. Trunk size (diameter at breast height) was more important than species in determining whether trees snapped or uprooted. Tree characteristics interacted with storm meteorology to determine the overall pattern of treefalls. We found that only 25% of the snapped trees sprouted in the four growing seasons after the tornado, and that the tendency to sprout varied significantly among species. Of those snapped trees that sprouted, only 68% were alive at the end of the fourth growing season. In contrast to small gaps in the tropics, where sprouted trees contribute a substantial portion of the post-disturbance canopy, sprouting will be of little importance in the re-establishment of forest in this catastrophic windthrow.

An unexpected change in spatial pattern across 10 years in an aspen-white-pine forest

1 The change in spatial pattern due to mortality over a period of 10 years was examined in a c. 60-year-old second-growth aspen-white-pine forest in northern Michigan, USA. 2 It is predicted that unitary plants undergoing competitive thinning will shift toward a more regular distribution of stems. Such a pattern has been reported for temperate jack pine (Pinus banksiana) and four tropical tree species, but was not seen in our population of canopy aspen. The distribution of aspen living in 1989 tended toward greater clumping than that expected from random mortality of aspen living in 1979. It is suggested that this contrast to theoretical predictions was due to the clonal nature of aspen. 3 White pine invading under aspen was clumped at all scales, both in 1979 and 1989, and its distribution showed significant repulsion from aspen at scales of 11 m and 14m in 1989. This suggests that the net effect of the initially dominant aspen on invading white pine was one of inhibition of establishment, and that the spatial location and therefore the abundance of white pine was constrained by the locations of aspen ramets.

Microsite and elevational influences on early forest regeneration after catastrophic windthrow

. We compared vegetation establishment in 25 treefall pits and mounds along a hillside elevational gradient in a fourth-year catastrophic windthrow in eastern North America. Plant communities differed greatly between pits and mounds, with pit microsites having significantly greater species richness, total biomass, and total tree stem density. Species richness in pits and on mounds decreased with increasing elevation from the bottom of the hillside, although the effect of elevation on mound species richness was less than that of elevation on pit species richness. Biomass of Erechtites hieraciifolia decreased significantly, while that of Betula alleghaniensis increased significantly with elevation. However, total biomass of both pit and mound microsites was unrelated to elevation. Total stem density decreased with elevation in pits, but was unaffected by elevation on mounds. This study shows that both small-scale (microsite) effects and intermediate-scale effects influence the re-establishment of plant communities within this catastrophic windthrow. Consideration of both microsite and position along intermediate-scale gradients may allow more precise prediction of plant community composition and dynamics in recovery of disturbed areas.

Tornado damage and initial recovery in three adjacent, lowland temperate forests in Missouri

. We surveyed tornado damage 14 months after disturbance in three adjacent forest sites in southeastern Missouri, USA: upland, an occasionally-flooded lowland and a frequently flooded swamp. We analysed three 0.09-ha plots in the swamp, and three 0.04-ha plots in the other sites and recorded the herbaceous-layer in five 1-m2 quadrats per plot. Overall, 30 % of the individuals and 64 % of the basal area of these sites was blown down. However, mortality was less than structural damage: 20 % of the trees were dead 14 months post-disturbance, an intermediate level of mortality relative to other wind disturbances. Damage varied widely among species, sites and tree sizes, but large trees of all species were most likely to be damaged and least likely to resprout. While damage was greatest in the wettest site, we attribute the increase to larger tree sizes and lower density there, rather than to inherent rooting instability. This study suggests that different forest composition and structure may be sufficient to cause differential damage, without topographic effects. Herbaceous-layer response was more rapid than in a previously reported tornado windthrow in Pennsylvania, and was dominated by fast-growing, shade-intolerant forbs.