Dennis H. Knight

Landscape Patterns and Legacies Resulting from Large, Infrequent Forest Disturbances

ABSTRACT We review and compare well-studied examples of five large, infrequent disturbances (LIDs)—fire, hurricanes, tornadoes, volcanic eruptions, and floods—in terms of the physical processes involved, the damage patterns they create in forested landscapes, and the potential impacts of those patterns on subsequent forest development. Our examples include the 1988 Yellowstone fires, the 1938 New England hurricane, the 1985 Tionesta tornado, the 1980 eruption of Mount St. Helens, and the 1993 Mississippi floods. The resulting landscape patterns are strongly controlled by interactions between the specific disturbance, the abiotic environment (especially topography), and the composition and structure of the vegetation at the time of the disturbance. The very different natures of these interactions yield distinctive temporal and spatial patterns and demand that ecologists increase their knowledge of the physical characteristics of disturbance processes. Floods and fires can occur over a long period, whereas volcanic eruptions and wind-driven events often last for no more than a few hours or days. Tornadoes and floods produce linear patterns with sharp edges, but fires, volcanic eruptions, and hurricanes can affect broader areas, often with gradual transitions of disturbance intensity. In all cases, the evidence suggests that LIDs produce enduring legacies of physical and biological structure that influence ecosystem processes for decades or centuries.

A Phytosociological Analysis of Species-Rich Tropical Forest on Barro Colorado Island, Panama

Data on tree species composition and population structure are used as a source of ecological information on the species—rich forest of Barro Colorado Island, Panama Canal Zone (BCI). Thirteen stands, representing both the young and old forest on the Island, were sampled using 10 m ° 20 m quadrats for all individuals °2.5 cm dbh. Over 300 species were encountered and most were identified. The data are evaluated for interpreting late secondary succession (>60 yr), detecting soil—vegetation patterns, and for yielding autecological information. Successional status was estimated by placing species with similar population structure patterns into groups, and then calculating the abundance of species groups that seemed indicative of successional status. Five population structure patterns were recognized. The results suggest that the older BCI forest is not climax after at least 130 yr of succession. Both principal components analysis and a Bray—Curtis type ordination were used to determine whether ecological patterns on BCI are reflected by species composition. Forest age was best reflected by an ordination of the larger tree species. Species found only in the young forest or only in the older forest are identified. Stands with a distinctive gley soil were not segregated on the ordination. Species diversity increases most rapidly during the first 15 yr of succession, but continues to increase slowly after 65 yr. The Shannon—Wiener diversity measure averaged 4.8 (log base 2). Some data suggest that wind—caused canopy gaps are important for the persistence of several species in the older forest, e.g., Cecropia sp.

Fire Frequency and Subalpine Forest Succession Along a Topographic Gradient in Wyoming

Differences in fire frequency and the rate of secondary succession following fire have had a major effect on the present composition of forest vegetation in a 4500—ha undisturbed watershed in the subalpine zone of the Medicine Bow Mountains, southeastern Wyoming. USA. Periodic fire coupled with slow secondary succession has perpetuated lodgepole pine forest on the upland, while mature Engelmann spruce—subalpine fire forests have developed in sheltered ravines and valley bottoms where fire is less frequent and succession following fire is more rapid and/or more direct. A graphic model is presented showing the relationship between topographic position, fire—free interval, and the occurrence of mature forests dominated by spruce and fire. See full-text article at JSTOR

Tree Growth, Mortality, Recruitment, and Canopy Gap Formation during a 10‐year Period in a Tropical Moist Forest

All trees @>2.5 cm dbh were censused on a 1.5—ha tract of 60—yr—old tropical moist forest in 1968 and again in 1978 to determine rates of tree mortality, recruitment, dbh increment, and canopy gap formation. Species composition changed very little. The pioneer of gap species Cordia alliodora, Luehea seemanii, and Spondias radlkoferi had no recruitment and accounted for most mortality in the larger size classes. Ninety percent of all mortality was for stems <10 cm dbh. Total tree density decline 11% (from 3112 to 2781 trees/ha), but basal area increased 22% (from 25.7 to 31.4 m2/ha). Growth in diameter was highly variable, both among species and among size classes. Trees in the 30—50 cm dbh class had a mean dbh increment of 0.9 cm/yr. Gaps occurred over an area equal to 7.3% of the plot during the 10—yr period, suggesting that about 137 yr would be required for the 1.5—ha plot to be affected by tree falls.

Interpreting the Yellowstone Fires of 1988Ecosystem responses and management implications

Norman L. Christensen is a professor in the Department of Botany, Duke University, Durham, NC 27706. James K. Agee is a professor in the College of Forest Resources, University of Washington, Seattle, WA 98195. Peter F. Brussard is a professor in and the chairman of the Biology Department, University of Nevada, Reno, NV 89557. Jay Hughes is a professor in and dean of the College of Forestry and National Resources, Colorado State University, Fort Collins, CO 80523. Dennis H. Knight is a professor in the Department of Botany, University of Wyoming, Laramie, WY 82071. G. Wayne Minshall is a professor in the Department of Biology, Idaho State University, Pocatello, ID 83209. James M. Peek is a professor in the College of Forest Resources, Wildlife, and Range Science, University of Idaho, Moscow, ID 83843. Stephen J. Pyne is a professor in the Department of History, Arizona State University, West Campus, Phoenix, AZ 85017. Frederick J. Swanson is a senior research scientist in the USDA Forest Ser-

Coarse Woody Debris following Fire and Logging in Wyoming Lodgepole Pine Forests

The accumulation and decomposition of coarse woody debris (CWD) are processes that affect habitat, soil structure and organic matter inputs, and energy and nutrient flows in forest ecosystems. Natural disturbances such as fires typically produce large quantities of CWD as trees fall and break, whereas human disturbances such as timber harvesting remove much of the CWD. Our objective was to compare the amount of CWD removed and left behind after clear-cutting to the amount consumed and left behind after natural fires in Rocky Mountain lodgepole pine. The masses of fallen logs, dead-standing trees, stumps, and root crowns more than 7.5 cm in diameter were estimated in clear-cut and intact lodgepole pine forests in Wyoming and compared to estimates made in burned and unburned stands in Yellowstone National Park (YNP), where no timber harvesting has occurred. Estimates of downed CWD consumed or converted to charcoal during an intense crown fire were also made in YNP. No significant differences in biomass of downed CWD more than 7.5 cm in diameter were detected between burned stands and those following a single clear-cut. However, the total mass of downed CWD plus the mass of snags that will become CWD was nearly twice as high in burned stands than in clear-cuts. In YNP, approximately 8% of the downed CWD was consumed by fire and an additional 8% was converted to charcoal, for an estimated loss of about 16%. In contrast, approximately four times more wood (70%) was removed by clear-cutting. Considering all CWD more than 7.5 cm in diameter that was either still present in the stand or removed by harvesting, slash treatment, or burning, clear-cut stands lost an average of 80 Mg ha−1 whereas stands that burned gained an average of 95 Mg ha−1. Some CWD remains as slash and stumps left behind after harvesting, but stands subjected to repeated harvesting will have forest floor and surface soil characteristics that are beyond the historic range of variability of naturally developing stands.

Mountain pine beetle outbreaks in the Rocky Mountains: Regulators of primary productivity?

We consider the hypothesis that mountain pine beetles function as cybernetic regulators of primary productivity in ecosystems of lodgepole pine forest through their selective killing of dominant trees and the subsequent redistribution of resources. Following a recent major beetle outbreak in Yellowstone and Grand Teton national parks, surviving trees did grow significantly faster (P < .1); wood production was redistributed among canopy, subcanopy, and understory trees; and annual wood production per hectare usually returned to pre-attack levels or exceeded them within 10-15 yr. However, reconstructions of annual wood production over the last 70-80 yr indicate that the beetle outbreak did not reduce the variation in productivity; rather, the beetles introduced more variation than would have existed in their absence. Hence, our results do not support the hypothesis that the beetles function as cybernetic regulators (in the strict sense). Nevertheless, the beetle-pine system that we studied shows great resilience, and the effects of beetles on primary productivity do not appear to be as severe as conventional wisdom maintains. Annual wood production per hectare returned quickly to previous levels in the stands we studied, and associated ecological changes can be considered generally benign or even beneficial.

Root Gap Dynamics in Lodgepole Pine Forest: Nitrogen Transformations in Gaps of Different Size

Belowground responses to aboveground disturbance were studied in experimental gaps created in a 95—yr—old stand of Pinus contorta in southeastern Wyoming. One—, 5—, 15—, and 30—tree clusters were felled to create a series of gaps in the root mat, and solution—phase N was monitored over two consecutive snow—melt periods via tension—tube water collectors. We hypothesized that dissolved and extractable nitrogen concentrations would not exceed predisturbance levels until a threshold canopy gap size had been achieved. As predicted, NOx—N attained significantly higher solution N concentrations (2—5 mg/L) only with the death of 15 trees or more. However, dissolved organic nitrogen decreased gradually with increasing gap size. Net mineralization and nitrification were studied using 30—d in situ incubation assays in each gap. Extractable nitrate routinely was negligible until the 30—tree gaps had been attained. Predicting the effects of disturbance on nutrient cycling, including timber—harvesting practices, requir...

The nitrogen cycle in lodgepole pine forests, southeastern Wyoming

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m−2), with aboveground detritus (37.5–48.8g · m−2) and living biomass (19.5–24.0 g · m−2) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m−2 · yr−1), and biological fixation of atmospheric N (<0.03 g · m−2 · yr−1, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m−2 · yr−1).Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m−2 to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m−2 · yr−1 apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L−1) in 02 leachate. A critical transport process was solubilization of organic N, mostly ‘fulvic acids’. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH4 for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m−2 · yr−1 with estimated root turnover of 0.37-gN · m−2 · yr−1, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m−2 · yr−1). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.

Biomass and Nutrient Accumulation During Stand Development in Wyoming Lodgepole Pine Forests

Accumulation rates of biomass and nutrients during stand development in lodgepole pine (Pinus contorta ssp. latifolia [Engelm. ex Wats.] Critchfield) forests were calculated using dendrochronological reconstructions of biomass in conjunction with information on detrital immobilization of nutrients. Dead wood, forest floor, and live tree Including root) biomass were examined. Maximum total biomass accumulation rates of 2.5-3.2 Mg°ha-1 °yr-1 were reached between ages 40 and 60 yr in even-aged stands, but an uneven-aged stand did not achieve a maximum accumulation rate (1.5 Mg°ha-1 °yr-1 ) until after 80 yr of development. The results suggest (1) a major role for forest floor detritus in the retention of N, P, Ca, and Mg; (2) the importance of living vegetation for K retention; and (3) the fundamental differences in biomass and nutrient dynamics between even-aged stands that develop after fire and stands that develop through gradual invasion of a meadow.