Norman L. Christensen

Dependence of radar backscatter on coniferous forest biomass

Two independent experimental efforts have examined the dependence of radar backscatter on above-ground biomass of monospecie conifer forests using polarimetric airborne SAR data at P-, L- and C-bands. Plantations of maritime pines near Landes, France, range in age from 8 to 46 years with above-ground biomass between 5 and 105 tons/ha. Loblolly pine stands established on abandoned agricultural fields near Duke, NC, range in age from 4 to 90 years and extend the range of above-ground biomass to 560 tons/ha for the older stands. These two experimental forests are largely complementary with respect to biomass. Radar backscatter is found to increase approximately linearly with increasing biomass until it saturates at a biomass level that depends on the radar frequency. The biomass saturation level is about 200 tons/ha at P-band and 100 tons/ha at L-band, and the C-band backscattering coefficient shows much less sensitivity to total above-ground biomass. >

Fire, Global Warming, and the Carbon Balance of Boreal Forests

Fire strongly influences carbon cycling and storage in boreal forests. In the near-term, if global warming occurs, the frequency and intensity of fires in boreal forests are likely to increase significantly. A sensitivity analysis on the relationship between fire and carbon storage in the living-biomass and ground-layer compartments of boreal forests was performed to determine how the carbon stocks would be expected to change as a result of global warming. A model was developed to study this sensitivity. The model shows if the annual area burned in boreal forests increases by 50%, as predicted by some studies, then the amount of carbon stored in the ground layer would decrease between 3.5 and 5.6 kg/M2, and the amount of carbon stored in the living biomass would increase by 1.2 kg/M2. There would be a net loss of carbon in boreal forests between 2.3 and 4.4 kg/M2, or 27.1- 51.9 Pg on a global scale. Because the carbon in the ground layer is lost more quickly than carbon is accumulated in living biomass, this could lead to a short-term release of carbon over the next 50-100 yr at a rate of 0.33-0.8 Pg/yr, dependent on the distribution of carbon between organic and mineral soil in the ground layer (which is presently not well-under- stood) and the increase in fire frequency caused by global warming.

Competition and Tree Death

can flatten all the trees in their paths. More subtle events of synchronous tree mortality, such as those caused by defoliating insects or fungal diseases, may attract attention because of the ghostly skeletons that remain. But these dramatic forms of tree mortality are the exception; most tree deaths go unnoticed. At best only a few of the many thousands of seedlings produced by a typical mature tree can survive and grow to achieve canopy position and reproductive status. Most die as a direct or indirect consequence of failure to compete successfully for light, water, or soil nutrients. This form of tree death has come to be called natural thinning, or self-thinning when applied to a relatively even-aged population of a single species. These terms contrast the process with artificial thinning, where less vigorous or unwanted trees are selectively removed by man. Although individually the deaths attributable to natural

Succession: A Population Process

Recent critical reviews suggest the need for a reductionistic approach to the study of secondary plant succession. We propose viewing succession as the result of the underlying plant population dynamics. This approach is being developed using nearly 50 years of permanent sample plot records. After initial establishment Pinus taeda shows an exponential depletion with stands of various densities conforming to the reciprocal yield relationship. Uneven-aged hardwoods also show exponential depletion. Canopy disturbance can enhance the establishment process, though severe disturbance and the consequent abundant regeneration can lead again to dense, even-aged stands with low levels of establishment. These results suggest a general pattern of forest development wherein establishment is initially important, but is quickly replaced by mortality as the dominant process when the dense, even-sized stand starts to thin. Eventually, failing additional disturbance, natural mortality will again open the canopy allowing development of a balance between establishment, and mortality.

CONVERGENCE DURING SECONDARY FOREST SUCCESSION

(1) Successional convergence in community composition was examined in terms of three questions: (i) for a given site is there a continuous shift in composition toward that characteristic of climax? (ii) does variation in community composition along an environmental gradient increase or decrease with succession? (iii) to what extent is the species composition along a successional gradient determined by site characteristics rather than by chance factors? Hypotheses regarding the nature of successional convergence are reviewed in terms of these questions. (2) These hypotheses were tested using data for tree (>0.5 cm dbh) and herb (all foliage 80-yr-old pines, and uneven-aged hardwoods). For each age-group first axis ordination scores (detrended correspondence analysis) were highly correlated most consistently with soil pH as compared with other soil and site variables. This correlation was greatest in the intermediate-age (40-60and 60-80-yr-old) pines and in the hardwood stands, and lowest in old pine stands. The distribution of species in the ordination space was most similar between the intermediate-age pine stands and the hardwoods; in contrast, young and old pine age classes were dissimilar to hardwoods. Species distributions in relation to the soil pH gradient were also most similar between intermediate-age pines and hardwoods. Thus, there is a shift in species composition toward that characteristic of climax, but it is probably not monotonic. (3) Beta-diversity was highest in the hardwoods and lowest among the pines. Community differentiation along gradients increased in this chronosequence. (4) Predictability of soil pH, based on stand species composition, was highest in the intermediate-age pines and in the hardwoods. Habitat breadth in relation to soil pH decreased steadily with successional age. These results are consistent with the hypothesis that the role of chance factors (e.g. site history and seed rain) decreases with successional age.

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-

Fire and the Nitrogen Cycle in California Chaparral

Analysis of soils from burned and unburned chaparral indicates that high nitrate concentrations following fire are due to the addition of ammonium and organic nitrogen in the ash. Inhibition of mineralization in unburned chaparral results in low nitrate concentrations. Fluctuations in the amount of soil nitrate in unburned chaparral are the direct result of foliar leaching.

Monitoring of wildfires in Boreal Forests using large area AVHRR NDVI composite image data

Abstract Normalized difference vegetation index (NDVI) composite image data, produced from AVHRR data collected in 1990, were evaluated for locating and mapping the areal extent of wildfires in the boreal forests of Alaska during that year. A technique was developed to map forest fire boundaries by subtracting a late-summer AVHRR NDVI image from an early summer scene. The locations and boundaries of wildfires within the interior region of Alaska were obtained from the Alaska Fire Service, and compared to the AVHRR-derived fire-boundary map. It was found that AVHRR detected 89.5% of all fires with sizes greater than 2000 ha with no false alarms and that, for most cases, the general shape of the fire boundary detected by AVHRR matched those mapped by field observers. However, the total area contained within the fire boundaries mapped by AVHRR were only 61 % of those mapped by the field observers. However, the AVHRR data used in this study did not span the entire time period during which fires occurred, and it is believed the areal estimates could be improved significantly if an expanded AVHRR data set were used.

Fire and soil-plant nutrient relations in a pine-wiregrass savanna on the coastal plain of North Carolina

SummaryChanges in soil and plant nutrient conditions were evaluated following various burn and clip treatments in a longleaf pine-wiregrass savanna in Bladen Co., N.C., USA. Ground fires were found to add substantial quantities of N, P, K, Ca, and Mg to the soil, though not necessarily in forms immediately available to plants. Less than 1% of the total nitrogen in the charred residue (ash) is present as nitrate or ammonium. Considerable quantities of all nutrients examined were lost to the atmosphere during burning. Green leaf tissue in recently burned areas was consistently higher in N, P, K, Ca, and Mg compared to unburned areas. Howerver, when compared to similar tissues from clipped plots, burned area tissues were significantly higher in N, Ca, and Mg only. Data presented here suggest that tissue age significantly affects nutrient content and must be considered in any analysis of tissue nutrient content following burning. Within 4–6 months following fire, burned-area tissue nutrient content decreases to concentrations found in the unburned area. Burning resulted in initial enrichment of available soil nutrients including PO4, K+, Ca++, and Mg++, however, NO3-, and NH4+ concentrations in burned soil were not significantly different from unbruned soil. Soil and plant nutrient changes in an area burned two years in succession indicate that repeated burning may diminish nutrient availability. Plant response to various nutrient enrichment treatments of the soil indicated that nitrogen is limiting growth in both burned and unburned soils and that burning may alter some factors other than nutrients which may retard plant growth in unburned areas.

Population Dynamics in Loblolly Pine Stands: Changes in Skewness and Size Inequality

densities ranged from 618 to 28 861 stems/ha. We used these data to test four predicted trends in size structure. We found that (1) in young stands size inequality was greater at higher plant densities, (2) skewness of tree volumes was significantly greater at higher initial densities, whereas skewness of diameters was not, (3) size inequality increased significantly prior to the onset of self-thinning, then decreased during self-thinning, and (4) skewness did not increase before self-thinning or significantly decrease during self-thinning. Numerous measures of size distribution have been employed to assess competition in plant populations, including various measures of inequality and skewness, as well as parameters of fitted distributions such as the Weibull distribution. We found the two-parameter Weibull distribution only moderately effective at capturing the forms of the distributions observed. Correlations showed very close linear relationships among the Gini coefficient, the coefficient of variation, and the shape parameter of the Weibull distribution-both within and across different measures of tree size. All three statistics were correlated with initial density. Skewness coefficients for distributions of diameter, height, and volume were not closely correlated with each other or with other population characteristics. We recommend measures of inequality as more suitable for summarizing biologically important properties of tree populations than measures of skewness or skewness-sensitive parameters of probability distributions.