Phellinus Weirii and Other Native Root Pathogens As Determinants of Forest Structure and Process in Western North America1

P1: FHA August 1, 2000 13:14 Annual Reviews AR107-21 Annu. Rev. Phytopathol. 2000. 38:515–39 PHELLINUS WEIRII AND OTHER NATIVE ROOT PATHOGENS AS DETERMINANTS OF FOREST STRUCTURE AND PROCESS IN WESTERN NORTH AMERICA1 E.M. Hansen Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon 97331; e-mail: [email protected] Ellen Michaels Goheen USDA Forest Service, SW Oregon Forest Insect and Disease Service Center, Central Point, Oregon 97502; e-mail: [email protected] Key Words Phellinus weirii, laminated root rot, Douglas-fir, forest ecology, forest succession ■ Abstract The population structure and ecological roles of the indigenous pathogen Phellinus weirii, cause of laminated root rot in conifer forests of western North America, are examined. This pathogen kills trees in slowly expanding mortality centers, creating gaps in the forest canopy. It is widespread, locally abundant, and very long-lived. It is among the most important disturbance agents in the long intervals between stand-replacing events such as wildfire or harvest in these ecosystems and shapes the structure and composition of both wild and managed forests. Trees are infected and killed regardless of individual vigor. Management of public lands is changing dramatically, with renewed emphasis on natural forest structures and processes but pathogens, especially root rot fungi, remain a significant challenge to “ecosystem management.” Annu. Rev. Phytopathol. 2000.38:515-539. Downloaded from www.annualreviews.org CONTENTS Access provided by U.S. Department of Agriculture (USDA) on 09/02/16. For personal use only. INTRODUCTION ................................................ 516 THE PRIMEVAL FOREST ......................................... 517 PHELLINUS WEIRII .............................................. 519 Pathology .................................................... 519 Population Structure ............................................. 521 Impacts on Forest Structure ........................................ 522 Impacts on Succession and Diversity ................................. 524 1The US Government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper. 515 P1: FHA August 1, 2000 13:14 Annual Reviews AR107-21 516 HANSEN GOHEEN Impacts on Nutrient Cycling ....................................... 526 Effects of Tree Vigor ............................................ 527 PATHOGENS AND OLD-GROWTH FORESTS ..........................529 THE FOREST TODAY AND TOMORROW ............................. 530 Recent Changes in the Forest....................................... 530 Trends in Forest Management ...................................... 531 Ecosystem Management .......................................... 532 Modeling Root Disease ........................................... 534 INTRODUCTION In agricultural ecosystems pathogens are considered “pests” interfering with the production of a healthy, valuable crop. Pathogens first evolved, however, free of human expectations in much more complex natural ecosystems, and the destruction and loss we ascribe to them today are just one interpretation of their successful evolutionary strategy. Tree pathogens are integral components of forest ecosystems around the world, altering forests in many ways, both subtle and profound. In forests managed for economic value, pathogens force changes in management practices, reduce profitability, or even threaten economic viability. Here, however, we take an ecological perspective on the interactions between plant pathogens and the forest communities they inhabit. We focus on the population structure and ecological roles of one indigenous root rot pathogen, Phellinus weirii (Murr.) Gilbertson, in Douglas-fir and mountain hemlock forests of the Pacific Northwest. This and similar root decay fungi play similar roles in the other forest types of western North America, but we draw our main examples from the forests we know best. Our views of forest pathogens are inevitably dominated by a few exotic pathogens in vulnerable forests and a strong professional legacy—the idea that a goal of forest management is regulated, disease- and decay-free forests on the nineteenth- century European model (34). Even today there is little appreciation for the significant effects that indigenous pathogens have on natural forests. Recent reviews do a poor job of distinguishing native from exotic pathogens, and wild from disturbed Annu. Rev. Phytopathol. 2000.38:515-539. Downloaded from www.annualreviews.org ecosystems (4). Access provided by U.S. Department of Agriculture (USDA) on 09/02/16. For personal use only. Pathogens affect forests most dramatically by killing trees. Plants, of course, are not defenseless against pathogens, and in ecosystems where plants and pathogens have evolved together, the evolutionary success of both is assured. Because evolution acts on populations, not individuals, however, some indigenous pathogens can and do kill single trees or even groups of trees in natural ecosystems with- out threatening the forest as a whole. They kill big trees and in the process change the diversity of the forest community, and they kill small and young and weak trees, maintaining the fitness of the ecosystem. Many forest pathogens do not kill trees directly, but still affect the forest community by altering competitiveness and reproductive success of trees, and nutrient cycling and primary productivity of the P1: FHA August 1, 2000 13:14 Annual Reviews AR107-21 PHELLINUS WEIRII AND FOREST STRUCTURE 517 forest ecosystem. They change local population structure of individual species and landscape-scale patterns of forest succession. Some pathogens play critical roles in determining range limits and habitat occupancy. In some ecosystems, including the vast coniferous forests of western North America, pathogens are among the most important disturbance agents in the long intervals between stand-replacing events such as wildfire or harvest; the wild forests we know today look and function as they do because of pathogens. Pathogens have been invisible to or misunderstood by most forest ecologists, unless entire forests are destroyed by exotic diseases such as chestnut blight. There is a widespread presumption that indigenous pathogens kill trees only if they are physiologically stressed or weakened by other agents or in response to mismanagement of forest lands. Some pathogens fill these scavenger roles in forest ecosystems, but others kill vigorous dominant trees, altering the very structure and composition of the forest. In the coniferous forests of western North America, Phellinus weirii has this effect. Phellinus weirii kills trees in slowly expanding mortality centers, creating gaps in the forest canopy. Gap or patch dynamics is an important component in current studies of forest ecosystem dynamics (37), with gaps in the canopy created by “disturbance agents.” Pathogens differ from other disturbance agents such as lightning or hurricanes in fundamental ways, however, and pathogen-induced gaps have dif- ferent consequences to forest communities than other types of gaps. Pathogens usu- ally affect species differentially, that is they exhibit host specificity, and pathogens act slowly. A lightning strike or a tornado kills all the tree species in a discrete patch instantaneously, but Phellinus weirii, for example, kills Douglas-fir but not western hemlock in a patch that slowly increases in size throughout the life of the stand. THE PRIMEVAL FOREST Conifers dominate the temperate forest ecosystems of western North America. Forests extend nearly continuously about 1000 km across mountainous terrain from the eastern slopes of the Rocky Mountains to the Pacific Ocean, broken in places by the Columbia Plateau, the Great Basin, and large and small river valleys. Annu. Rev. Phytopathol. 2000.38:515-539. Downloaded from www.annualreviews.org Coniferous forests cover 82,000,000 ha in Oregon, Washington, Idaho, and British Access provided by U.S. Department of Agriculture (USDA) on 09/02/16. For personal use only. Columbia, about 52% of this region. Two main north-south mountain ranges, the Coast Ranges and the Cascade Mountains, dissect the area and through their influence on maritime temperature and precipitation, geology, and soils, delimit several very distinctive ecosystems (11). Forests west of the Cascade Mountains in Oregon and Washington and the Coast Mountains in British Columbia (westside forests) are strongly influenced by the Pacific Ocean. Annual precipitation in the Douglas-fir forests ranges from 150 to more than 500 cm a year. The maritime climate, with mild, wet winters and dry summers, favors evergreens over deciduous tree species. Conifer forests of the Northwest are more productive and accumulate greater standing biomass P1: FHA August 1, 2000 13:14 Annual Reviews AR107-21 518 HANSEN GOHEEN than other forests. Great size and age of dominant tree species are typical of western coastal forests and make them unique among the forests of the world (51). Western hemlock [Tsuga heterophylla (Raf.) Sarg.], a shade-tolerant tree, is the principal late-successional species in most westside ecosystems, but Douglas- fir [Pseudotsuga menziesii (Mirb.) Franco] dominates most forests. In the drier forests of southwestern Oregon and northern California, ponderosa pine (Pinus ponderosa Dougl. ex. Loud.), sugar pine (P. lambertiana Dougl.), and hardwood species such as Pacific madrone (Arbutus menziesii Pursh), California black oak (Quercus

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