Jerry F. Franklin

Ecology of Coarse Woody Debris in Temperate Ecosystems

Publisher Summary This chapter reviews the rates at which Coarse Woody Debris (CWD) is added and removed from ecosystems, the biomass found in streams and forests, and many functions that CWD serves. CWD is an important component of temperate stream and forest ecosystems and is added to the ecosystem by numerous mechanisms, including wind, fire, insect attack, pathogens, competition, and geomorphic processes. Many factors control the rate at which CWD decomposes, including temperature, moisture, the internal gas composition of CWD, substrate quality, the size of the CWD, and the types of organisms involved. The mass of CWD in an ecosystem ideally represents the balance between addition and loss. In reality, slow decomposition rates and erratic variations in input of CWD cause the CWD mass to deviate markedly from steady-state projections. Many differences correspond to forest type, with deciduous-dominated systems having generally lower biomass than conifer-dominated systems. Stream size also influences CWD mass in lotic ecosystems, while successional stage dramatically influences CWD mass in boat aquatic and terrestrial settings. This chapter reviews many of these functions and concludes that CWD is an important functional component of stream and forest ecosystems. Better scientific understanding of these functions and the natural factors influencing CWD dynamics should lead to more enlightened management practices.

Disturbances and structural development of natural forest ecosystems with silvicultural implications, using Douglas-fir forests as an example

Forest managers need a comprehensive scientific understanding of natural stand development processes when designing silvicultural systems that integrate ecological and economic objectives, including a better appreciation of the nature of disturbance regimes and the biological legacies, such as live trees, snags, and logs, that they leave behind. Most conceptual forest development models do not incorporate current knowledge of the: (1) complexity of structures (including spatial patterns) and developmental processes; (2) duration of development in long-lived forests; (3) complex spatial patterns of stands that develop in later stages of seres; and particularly (4) the role of disturbances in creating structural legacies that become key elements of the post-disturbance stands. We elaborate on existing models for stand structural development using natural stand development of the Douglas-fir—western hemlock sere in the Pacific Northwest as our primary example; most of the principles are broadly applicable while some processes (e.g. role of epicormic branches) are related to specific species. We discuss the use of principles from disturbance ecology and natural stand development to create silvicultural approaches that are more aligned with natural processes. Such approaches provide for a greater abundance of standing dead and down wood and large old trees, perhaps reducing short-term commercial productivity but ultimately enhancing wildlife habitat, biodiversity, and ecosystem function, including soil protection and nutrient retention. # 2002 Elsevier Science B.V. All rights reserved.

Widespread increase of tree mortality rates in the western United States.

Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

PRESERVING BIODIVERSITY: SPECIES, ECOSYSTEMS, OR LANDSCAPES?'

Efforts to preserve biological diversity must focus increasingly at the ecosystem level because of the immense number of species, the majority of which are currently unknown. An ecosystem approach is also the only way to conserve processes and habitats (such as forest canopies, belowground habitats, and hyporheic zones) that, with their constituent species, are poorly known. Continued concern with species is essential, however. Landscape-level issues also need much greater attention. Designing an appropriate system of habitat reserves is one landscape-level concern. Understanding and appropriately manipulating the landscape matrix is at least equal in importance to reserves issues, however, since the matrix itself is important in maintaining diversity, influences the effectiveness of reserves, and controls landscape connectivity.

Creating landscape patterns by forest cutting: Ecological consequences and principles

Landscape structural characteristics, such as patch size, edge length, and configuration, are altered markedly when management regimes are imposed on primeval landscapes. The ecological consequences of clearcutting patterns were explored by using a model of the dispersed patch or checkerboard system currently practiced on federal forest lands in the western United States. Thresholds in landscape structure were observed on a gradient of percentages of landscape cutover. Probability of disturbance,e.g., wildfire and windthrow, and biotic components,e.g., species diversity and game populations, are highly sensitive to these structural changes. Altering the spatial configuration and size of clearcuts provides an opportunity to create alternative landscapes that differ significantly in their ecological characteristics. Both ecosystem and heterogeneous landscape perspectives are critical in resource management.

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-

Effects on carbon storage of conversion of old-growth forests to young forests.

Simulations of carbon storage suggest that conversion of old-growth forests to young fast-growing forests will not decrease atmospheric carbon dioxide (CO2) in general, as has been suggested recently. During simulated timber harvest, on-site carbon storage is reduced considerably and does not approach old-growth storage capacity for at least 200 years. Even when sequestration of carbon in wooden buildings is included in the models, timber harvest results in a net flux of CO2 to the atmosphere. To offset this effect, the production of lumber and other long-term wood products, as well as the life-span of buildings, would have to increase markedly. Mass balance calculations indicate that the conversion of 5 x 109 to 1.8 x 109 megagrams of carbon to the atmosphere.

Coarse Woody Debris in Douglas-Fir Forests of Western Oregon and Washington

Amounts and structural characteristics of coarse woody debris (CWD) were examined in relation to stand age and site moisture condition in 196 Pseudotsuga menziesii stands in western Oregon and Washington. Stands ranged from 40 to 900 yr old, and most if not all, originated after fire. In a chronosequence from the Cascade range, the amount of CWD followed a U—shaped pattern for stands 200 yr old) among site moisture classes. Dry sites averaged 72 Mg/ha moderate sites 137 Mg/ha, and moist sites 174 Mg/ha. The dynamics of CWD were modeled for three fire histories, each beginning with an initial fire in an old—growth stand but differing in number and severity of subsequent fires. All three models exhibited low values of CWD between 80 and 200 yr. The lowest and most prolonged minimum in CWD between 80 and 200 yr. The lowest and most prolonged minimum in CWD during succession occurred when additional fires burned early in succession, which probably happened preceding many stands in the southern Coast Range. The results of the study indicate that a steady—state condition in CWD may not be reached for >1000 yr, and that the nature and timing of disturbance play a key role in the dynamics of CWD in the dynamics of CWD in the region.

Microclimate in Forest Ecosystem and Landscape Ecology

Microclimate is the suite of climatic conditions measured in localized areas near the earths surface (Geiger 1965). These environmental variables, which include temperature, light, windspeed, and moisture, have been critical throughout human history, providing meaningful indicators for habitat selection and other activities. For example, for 2600 years the Chinese have used localized seasonal changes in temperature and precipitation to schedule their agricultural activities. In seminal studies, Shirley (1929, 1945) emphasized microclimate as a determinant of ecological patterns in both plant and animal communities and a driver of such processes as the growth and mortality of organisms. The importance of microclimate in influencing ecological processes such as plant regeneration and growth, soil resperation and growth, soil repiration, nutrient cycling, and wildlife habitat selection has became an essential component of current ecological research (Perry 1994). plant regeneration and growth, soil respiration, nutrient cycling, and

Evergreen Coniferous Forests of the Pacific Northwest

The massive, evergreen coniferous forests in the Pacific Northwest are unique among temperate forest regions of the world. The regions forests escaped decimation during Pleistocene glaciation; they are now dominated by a few broadly distributed and well-adapted conifers that grow to large size and great age. Large trees with evergreen needle- or scale-like leaves have distinct advantages under the current climatic regime. Photosynthesis and nutrient uptake and storage are possible during the relatively warm, wet fall and winter months. High evaporative demand during the warm, dry summer reduces photosynthesis. Deciduous hardwoods are repeatedly at a disadvantage in competing with conifers in the regional climate. Their photosynthesis is predominantly limited to the growing season when evaporative demand is high and water is often limiting. Most nutrients needed are also less available at this time. The large size attained by conifers provides a buffer against environmental stress (especially for nutrients and moisture). The long duration between destructive fires and storms permits conifers to outgrow hardwoods with more limited stature and life spans.