Herman H. Shugart

Tree Death as an Ecological Process

the patterns and causes of tree death typically are complex, and we are only beginning to appreciate the complexities. Understanding and predicting tree mortality is critical in both applied and basic ecology. Practically speaking, information on mortality is essential in calculating forest stand yields and allocating efforts in tending and protecting forests. A thorough knowledge of tree death is also necessary to interpret correctly the dying back of forests. Yet, despite its long history, forest husbandry lacks a comprehensive understanding of tree mortality. In basic ecology, tree death is relevant to a broad array of topics. Ecolo-

Surface lidar remote sensing of basal area and biomass in deciduous forests of eastern Maryland, USA

A method of predicting two forest stand structure attributes, basal area and aboveground biomass, from measurements of forest vertical structure was developed and tested using field and remotely sensed canopy structure measurements. Coincident estimates of the vertical distribution of canopy surface area (the canopy height profile), and field-measured stand structure attributes were acquired for two data sets. The chronosequence data set consists of 48 plots in stands distributed within 25 miles of Annapolis, MD, with canopy height profiles measured in the field using the optical-quadrat method. The stem-map data set consists of 75 plots subsetted from a single 32 ha stem-mapped stand, with measurements of their canopy height profiles made using the SLICER (Scanning Lidar Imager of Canopies by Echo Recovery) instrument, an airborne surface lidar system. Four height indices, maximum, median, mean, and quadratic mean canopy height (QMCH) were calculated from the canopy height profiles. Regressions between the indices and stand basal area and biomass were developed using the chronosequence data set. The regression equations developed from the chronosequence data set were then applied to height indices calculated from the remotely sensed canopy height profiles from the stem map data set, and the ability of the regression equations to predict the stem map plot’s stand structure attributes was then evaluated. The QMCH was found to explain the most variance in the chronosequence data set’s stand structure attributes, and to most accurately predict the values of the same attributes in the stem map data set. For the chronosequence data set, the QMCH predicted 70% of variance in stand basal area, and 80% of variance in aboveground biomass, and remained nonasymptotic with basal areas up to 50 m2 ha−1, and aboveground biomass values up to 450 Mg ha−1. When applied to the stem-map data set, the regression equations resulted in basal areas that were, on average, underestimated by 2.1 m2 ha−1, and biomass values were underestimated by 16 Mg ha−1, and explained 37% and 33% of variance, respectively. Differences in the magnitude of the coefficients of determination were due to the wider range of stand conditions found in the chronosequence data set; the standard deviation of residual values were lower in the stem map data set than on the chronosequence data sets. Stepwise multiple regression was performed to predict the two stand structure attributes using the canopy height profile data directly as independent variables, but they did not improve the accuracy of the estimates over the height index approach.

A systems analysis of the global boreal forest

List of contributors 1. Introduction Herman H. Shugart, Rik Leemans and Gordon B. Bonan Part I. Processes in Boreal Forests Gordon B. Bonan: 2. Silvics of the circumpolar boreal forest tree species Nedialko Nikolov and Harry Helmisaari 3. The reproductive process in boreal forest trees John C. Zasada, Terry L. Sharik and Markku Nygren 4. Soil temperature as an ecological factor in boreal forests Gordon B. Bonan 5. Fire as a controlling process in the North American boreal forest Serge Payette 6. The role of forest insects in structuring the boreal landscape C. S. Holling Part II. Patterns in Space and Time in Boreal Forests Herman H. Shugart: 7. The transition between boreal forest and tundra Luc Sirois 8. The southern boreal-northern hardwood forest border John Pastor and David J. Mladenoff 9. Transitions between boreal forest and wetland F. Z. Glebov and M. D. Korzukhin 10. Remote sensing technology for forest ecosystem analysis K. Jon Ranson and Darrel L. Williams 11. The nature and distribution of past, present and future boreal forests: lessons for a research and modeling agenda Allen M. Solomon Part III. Computer Models for Synthesis of Pattern and Process in the Boreal Forest Rik Leemans: 12. Individual-tree-based models of forest dynamics and their application in global change research Herman H. Shugart and I. Colin Prentice 13. Population-level models of forest dynamics M. D. Korzukhin and M. Ya. Antonovski 14. A spatial model of long-term forest fibre dynamics and its applications to forests in western Siberia M. Ya. Antonovski, M. T. Ter-Mikaelian and V. V. Furyaev 15. A simulation analysis of environmental factors and ecological processes in North American boreal forests Gordon B. Bonan 16. The biological component of the simulation model for boreal forest dynamics Rik Leemans 17. Role of stand simulation in modeling forest response to environmental change and management interventions Peter Duinker, Ola Sallnas and Sten Nilsson 18. Concluding comments Herman H. Shugart, Rik Leemans and Gordon B. Bonan References Index.

Global tests of biodiversity concordance and the importance of endemism

Understanding patterns of biodiversity distribution is essential to conservation strategies, but severe data constraints make surrogate measures necessary. For this reason, many studies have tested the performance of terrestrial vertebrates as surrogates for overall species diversity, but these tests have typically been limited to a single taxon or region. Here we show that global patterns of richness are highly correlated among amphibians, reptiles, birds and mammals, as are endemism patterns. Furthermore, we demonstrate that although the correlation between global richness and endemism is low, aggregate regions selected for high levels of endemism capture significantly more species than expected by chance. Although areas high in endemism have long been targeted for the protection of narrow-ranging species, our findings provide evidence that endemism is also a useful surrogate for the conservation of all terrestrial vertebrates.

Plant Functional Types

Biomes or ecosystem types have long been recognized as occupying domains in an environmental space defined by temperature and moisture gradients. Within particular biomes, the factors governing structure and dynamics may vary considerably. For example, species composition and age structure in plant communities are largely controlled by asymmetric competition for an above-ground resource (light) and symmetric competition for below-ground resources (moisture and nutrients). In mesic forests, the principal constraint seems to be the availability of light. As a forest environment tends from mesic to xeric conditions, the effective constraint shifts from above-ground to below-ground (Tilman 1988; Smith and Huston 1989). In still drier environs, the forest yields to grassland, where the principal constraint is below-ground. This suggests that there are general trends or patterns in the relative influence of environmental constraints in structuring ecosystems.

Insights into nitrogen and carbon dynamics of ectomycorrhizal and saprotrophic fungi from isotopic evidence

Abstract The successful use of natural abundances of carbon (C) and nitrogen (N) isotopes in the study of ecosystem dynamics suggests that isotopic measurements could yield new insights into the role of fungi in nitrogen and carbon cycling. Sporocarps of mycorrhizal and saprotrophic fungi, vegetation, and soils were collected in young, deciduous-dominated sites and older, coniferous-dominated sites along a successional sequence at Glacier Bay National Park, Alaska. Mycorrhizal fungi had consistently higher δ15N and lower δ13C values than saprotrophic fungi. Foliar δ13C values were always isotopically depleted relative to both fungal types. Foliar δ15N values were usually, but not always, more depleted than those in saprotrophic fungi, and were consistently more depleted than in mycorrhizal fungi. We hypothesize that an apparent isotopic fractionation by mycorrhizal fungi during the transfer of nitrogen to plants may be attributed to enzymatic reactions within the fungi producing isotopically depleted amino acids, which are subsequently passed on to plant symbionts. An increasing difference between soil mineral nitrogen δ15N and foliar δ15N in later succession might therefore be a consequence of greater reliance on mycorrhizal symbionts for nitrogen supply under nitrogen-limited conditions. Carbon signatures of mycorrhizal fungi may be more enriched than those of foliage because the fungi use isotopically enriched photosynthate such as simple sugars, in contrast to the mixture of compounds present in leaves. In addition, some 13C fractionation may occur during transport processes from leaves to roots, and during fungal chitin biosynthesis. Stable isotopes have the potential to help clarify the role of fungi in ecosystem processes.

Forest Succession Models

Studies in succession attempt to determine the changes in species composition and other ecosystem attributes expected to occur over periods of time. Mathematical models developed in forestry and ecology to study ecological succession are reviewed. Tree models, gap models and forest models are discussed. Model validation or testing procedures are described. Model applications can involve evaluating large-scale and long-term changes in the ambient levels of pollutants and assessing the effects of climate change on the environment. (RJC)

Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests

We analyzed half-hourly tower-based flux measurements of carbon dioxide (CO 2 ) from a boreal aspen forest and a temperate mixed deciduous forest in Canada to examine the influences of clouds on forest carbon uptake. We showed that the presence of clouds consistently and significantly increased the net ecosystem exchanges (NEE) of CO 2 of both forests from the level under clear skies. The enhancement varied with cloudiness, solar elevation angles, and differed between the two forests. For the aspen forest the enhancement at the peak ranged from about 30% for the 20°-25° interval of solar elevation angles to about 55% for the 55°-60° interval. For the mixed forest the enhancement at the peak ranged from more than 60% for the 30°-35° interval of solar elevation angles to about 30% for the 65°-70° interval. Averaged over solar elevation angles >20°, the aspen and mixed forests had the maximal NEE at the irradiance equivalent to 78 and 71% of the clear-sky radiation, respectively. The general patterns of current sky conditions at both sites permit further increases in cloudiness to enhance their carbon uptake. We found that both forests can tolerate exceedingly large reductions of solar radiation (53% for the aspen forest and 46% for the mixed forest) caused by increases in cloudiness without lowering their capacities of carbon uptake. We suggest that the enhancement of carbon uptake under cloudy conditions results from the interactions of multiple environmental factors associated with the presence of clouds.

Habitat Selection of Breeding Birds in an East Tennessee Deciduous Forest

Bird populations were sampled between May 30 and July 20, 1972, on twenty- four 0.08-ha plots on Walker Branch Watershed, a primarily deciduous forest located in Anderson County, Tennessee. Univariate analysis of variance was used to test for differences in abundance categories of each bird species with respect to 28 habitat variables. Differences in habitat preferences within major bird families (Picidae, Parulidae, Paridae, and Thraupidae) were apparent from this analysis. Discriminant function analysis was therefore used to order the variables according to their strength in separating abundance categories for 13 of the more abundant bird species. This analysis indicated that some bird species were distributed ac- cording to specific habitat variables. For example, Downy Woodpecker abundance was highly correlated with the number of saplings on a plot. Distributions of other species (e.g., the Scarlet Tanager) were not strongly related to any single variable but were related weakly to a large number of variables. The results form a basis for predicting avifaunal composition changes resulting from alteration of habitat structure.

Trends in savanna structure and composition along an aridity gradient in the Kalahari

Abstract The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m2.ha−1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by either the Combretaceae (Combretum or Terminalia) or Colophospermum mopane of the Caesalpinaceae between 400 and 600 mm MAP, and by other representatives of the Caesalpinaceae above 600 mm MAP. The vegetation is largely deciduous up to 1000 mm MAP, except for species that apparently have access to groundwater, which may be locally dominant above about 600 mm MAP. Abbreviation: MAP = Mean annual precipitation.