Andrew J. Hansen

Conserving biodiversity in managed forests: Lessons from natural forests

In this article, the authors review patterns of disturbance and succession in natural forests in the Coastal Northwest and compare structure and composition across an age gradient of unmanaged stands. Stand and landscape patterns in managed forests are then examined and compared with those in natural forests. They draw on the results to offer guidance on the management of Coastal Northwest forests that are dedicated to both wood production and conservation of biodiversity. Finally, the authors suggest that the lessons learned from natural forests here may be useful in other biomes, where unmanaged forests are rare and standards for designing seminatural forests are not available.

Global Change in Forests: Responses of Species, Communities, and Biomes

G change is often perceived as human-induced modifications in climate. Indeed, human activities have undeniably altered the atmosphere, and probably the climate as well (Watson et al. 1998). At the same time, most of the world’s forests have also been extensively modified by human use of the land (Houghton 1994). Thus, climate and land use are two prongs of human-induced global change. The effect of these forces on forests is mediated by the organisms within forests. Consideration of climate, land use, and biological diversity is key to understanding forest response to global change. Biological diversity refers to the variety of life at organizational levels from genotypes through biomes (Franklin 1993). The responses of ecological systems to global change reflect the organisms that are within them. While ecologists have sometimes not seen the forest for the trees, so to speak, it is also true that forests cannot be understood without knowledge of the trees and other component species. It is the responses of individual organisms that begin the cascade of ecological processes that are manifest as changes in system properties, some of which feed back to influence climate and land use (Figure 1). Beyond its role in ecosystems, biodiversity is invaluable to humans for foods, medicines, genetic information, recreation, and spiritual renewal (Pimentel et al. 1997). Thus, global changes that affect the distribution and abundance of organisms will affect future human well-being and land use, as well as, possibly, the climate. This article serves as a primer on forest biodiversity as a key component of global change. We first synthesize current knowledge of interactions among climate, land use, and biodiversity. We then summarize the results of new analyses on the potential effects of human-induced climate change on forest biodiversity. Our models project how possible future climates may modify the distributions of environments required by various species, communities, and biomes. Current knowledge, models, and funding did not allow these analyses to examine the population processes (e.g., dispersal, regeneration) that would mediate the responses of organisms to environmental change. It was also not possible to model the important effects of land use, natural disturbance, and other factors on the response of biodiversity to climate change. Despite these limitations, the analyses discussed herein are among the most comprehensive projections of climate change effects on forest biodiversity yet conducted. We conclude with discussions of limitations, research needs, and strategies for coping with potential future global change.

ECOLOGICAL MECHANISMS LINKING PROTECTED AREAS TO SURROUNDING LANDS

Land use is expanding and intensifying in the unprotected lands surrounding many of the worlds protected areas. The influence of this land use change on ecological processes is poorly understood. The goal of this paper is to draw on ecological theory to provide a synthetic framework for understanding how land use change around protected areas may alter ecological processes and biodiversity within protected areas and to provide a basis for identifying scientifically based management alternatives. We first present a conceptual model of protected areas embedded within larger ecosystems that often include surrounding human land use. Drawing on case studies in this Invited Feature, we then explore a comprehensive set of ecological mechanisms by which land use on surrounding lands may influence ecological processes and biodiversity within reserves. These mechanisms involve changes in ecosystem size, with implications for minimum dynamic area, species-area effect, and trophic structure; altered flows of materials and disturbances into and out of reserves; effects on crucial habitats for seasonal and migration movements and population source/sink dynamics; and exposure to humans through hunting, poaching, exotics species, and disease. These ecological mechanisms provide a basis for assessing the vulnerability of protected areas to land use. They also suggest criteria for designing regional management to sustain protected areas in the context of surrounding human land use. These design criteria include maximizing the area of functional habitats, identifying and maintaining ecological process zones, maintaining key migration and source habitats, and managing human proximity and edge effects.

Biological invasions in Europe and the Mediterranean Basin

One. Introduction.- 1. On invading species and invaded ecosystems: the interplay of historical chance and biological necessity.- Two. Plant invasions.- 2. Plant invasions in Central Europe: historical and ecological aspects.- 3. History of the impact of man on the distribution of plant species.- 4. Recent plant invasions in the Circum-Mediterranean region.- 5. The invading weeds within the Western Mediterranean Basin.- 6. Widespread adventive plants in Catalonia.- 7. History and patterns of plant invasion in Northern Africa.- 8. Invasions of adventive plants in Israel.- 9. Man and vegetation in the Mediterranean area during the last 20,000 years.- 10. Plant invasions in Southern Europe from the Paleoecological point of view.- 11. Mediterranean weeds: exchanges of invasive plants between the five Mediterranean regions of the world.- Three. Animal invasions.- 12. The invasion of Northern Europe during the Pleistocene by Mediterranean species of Coleoptera.- 13. Migratory Phenomena in European animal species.- 14. The bean beetle (Acanthoscelides obtectus) and its host, the French bean (Phaseolus vulgaris): a two-way colonization story.- 15. Some recent bird invasions in Europe and the Mediterranean Basin.- 16. Of mice and men.- 17. Invasions by parasites in continental Europe.- 18. Human activities and modification of ichtyofauna of the Mediterranean sea: effect on parasitosis.- 19. Influence of environmental factors on the invasion of molluscs by parasites: with special reference to Europe.- Four. Mechanisms of invasions.- 20. In search of the characteristics of plant invaders.- 21. Biogeographical and physiological aspects of the invasion by Dittrichia (ex Inula) viscosa W. Greuter, a ruderal species in the Mediterranean Basin.- 22. Invaders and disequilibrium.- 23. Species-specific pollination: a help or a limitation to range extension?.- 24. Genetic differentiation in beech (Fagus sylvatica L.) during periods of invasion and regeneration.- 25. Invasion of natural pastures by a cultivated grass (Dactylis glomerata L.) in Galicia, Spain: process and consequence on plant-cattle interactions.- 26. Introduced and cultivated fleshy-fruited plants: consequences of a mutualistic Mediterranean plant-bird system.- 27. Fire as an agent of plant invasion? A case study in the French Mediterranean vegetation.- List of contributors.- Index of Genera and Species.- General index.

Ecological Causes and Consequences of Demographic Change in the New West

R areas in the American West are undergoing a dra m a tic tra n s i ti on in dem ogra phy, econ om i c s ,a n d eco s ys tem s . Long known as the “Wi l d ” We s t , the regi on has been ch a racteri zed by low human pop u l a ti on den s i ties and vast tracts of u n s et t l ed or undevel oped land (Wi l k i n s on 1993, Power 1998). For most of the 1900s, the pop u l a ti on of m a ny ru ral areas in the West grew very slowly or even dec re a s ed . Because local econ omies were based on natu ral re s o u rce indu s tries su ch as mining, l oggi n g, f a rm i n g, and ra n ch i n g, m a ny re s i dents of the regi on con s i dered con s erva ti on stra tegies on public lands detri m ental to local econ omic devel opm en t . E f forts to establish natu re re s erves and to pre s erve p u blic lands from com m ercial devel opm ent were seen as res tri cting the use of vital natu ral re s o u rce s . In recent dec ade s ,p a rts of that Wild West have given way to the “ New ” West (Ri ebsame et al. 1 9 9 7 ) . People from t h ro u gh o ut the Un i ted States have been migra ting to the Rocky Mountains and the inland We s t . With a pop u l a ti on growth ra te of 2 5 . 4 % , the mountain West was the faste s t growing regi on of the co u n try du ring the 1990s. Su rpri si n gly, rapid pop u l a ti on increases are occ u rring not on ly in urban areas su ch as Denver and Salt Lake Ci ty but also in ru ra l co u n ti e s ,m a ny of wh i ch are gaining pop u l a ti on even faster than urban areas (Th eobald 2000). Some 67% of the co u nties in the Rocky Mountains grew faster than the nati onal avera ge du ring the 1990s (Beyers and Nel s on 2000). Con s equ en t ly, s m a ll cities su ch as Bozem a n , Mon t a n a , and Moa b, Ut a h , a re beginning to ex peri en ce traffic con ge s ti on and s prawl . Some of the ru ral pop u l a ti on growth in the New We s t repre s ents an intra regi onal red i s tri buti on of people from the h i gh plains, wh i ch con ti nue to lose pop u l a ti on (Jo h n s on 1 9 9 8 ) , to more mountainous are a s . Ma ny of the new re s i den t s , h owever, a re in-migrants from other regi ons thro u gh o ut the Un i ted States (Ri ebsame et al. 1 9 9 7 ) . The re s i dents of a ru ra l su b d ivi s i on in a boom co u n ty in Montana might inclu de recent arrivals from big East Coast citi e s ,m i dwe s tern farm s ,a n d the nearest small town . Am ong the in-migrants are reti ree s , we a l t hy young adu l t s , and profe s s i onals in com p uter techn o l ogy, real estate , and other servi ce indu s tries (Nel s on 1999).

LAND USE CHANGE AROUND PROTECTED AREAS: MANAGEMENT TO BALANCE HUMAN NEEDS AND ECOLOGICAL FUNCTION

Protected areas throughout the world are key for conserving biodiversity, and land use is key for providing food, fiber, and other ecosystem services essential for human sustenance. As land use change isolates protected areas from their surrounding landscapes, the challenge is to identify management opportunities that maintain ecological function while minimizing restrictions on human land use. Building on the case studies in this Invited Feature and on ecological principles, we identify opportunities for regional land management that maintain both ecological function in protected areas and human land use options, including preserving crucial habitats and migration corridors, and reducing dependence of local human populations on protected area resources. Identification of appropriate and effective management opportunities depends on clear definitions of: (1) the biodiversity attributes of concern; (2) landscape connections to delineate particular locations with strong ecological interactions between the protected area and its surrounding landscape; and (3) socioeconomic dynamics that determine current and future use of land resources in and around the protected area.

Avian response to landscape pattern: The role of species' life histories

We suggest that the life histories of species within communities may differ among geographic locations and that communities from distinct biomes may respond uniquely to a given trajectory of landscape change. This paper presents initial tests relevant to these hypotheses. First, the representation of various life-history guilds in avifaunas from the Eastern Deciduous (EDF) and Pacific Northwest (PNW) forests were compared. Three guilds contained more species in the EDF community (large patch and/or habitat interior guild, small patch and/or edge guild, and fragmentation-sensitive guild). The guild of predators requiring large forest tracts was better represented in the PNW. Next, the relative sensitivity of each community to habitat change was ranked based on the life-history traits of their species. The EDF avifauna had a significantly higher index of sensitivity to both forest fragmentation and to landscape change in general. Among the birds with high scores for sensitivity to landscape change were several species that have received little conservation attention thus far including some associated with open-canopy habitats. Lastly, the validity of using life histories to predict community response to landscape change was supported by the fact that the sensitivity scores for PNW species correlated significantly with independent data on species population trends. While more rigorous analyses are suggested, we conclude that knowledge of life histories is useful for predicting community response to landscape change and that conservation strategies should be uniquely tailored to local communities.

Bird Habitat Relationships in Natural and Managed Forests in the West Cascades of Oregon

Ecologists have advocated retaining various densities of canopy trees in harvest units in Pacific Northwest forests. In contrast to clear-cutting, this practice may better emulate the patterns of disturbance and structural complexity typical of natural forests in the region. Several ecological attributes, including vertebrate habitat diversity, are thought to be associated with stands of complex structure. The goal of this study was to determine bird abundance in canopy retention sites relative to other common stand types in the Pacific Northwest and to develop habitat functions for extrapolating bird abundance across current and future landscapes. We used data from five previous studies in the west central Cascades of Oregon to compare bird abundance and to develop habitat functions for forest birds across a wide range of natural and managed stand structures and ages. The 67 stands included clearcuts, retention sites, young closed-canopy plantations, mature stands, and old-growth stands. ANOVA revealed that 18 of the 23 species included in the analysis differed significantly in abundance among the stand types, with some species being primarily associated with each of the stand types. The habitat variables used to build habitat functions included tree density by size class, mean tree diameter, and variation in tree diameter. Linear, polynomial, and various nonlinear regression models were evaluated for each bird species. Significant habitat functions were generated for 17 of the 23 bird species. The analyses identified four habitat-use guilds among the 17 bird species: open-canopy; open-canopy with dispersed large trees; structurally complex closed-canopy; and structurally simple closed-canopy guilds. This study is the first in the Pacific Northwest to compare bird abundances across natural stands, traditionally managed plantations, and stands managed under ecological forestry approaches. The results suggested that canopy tree retention benefits many, but not all, of the bird species we studied, Moreover, the nonlinear responses of bird abundance revealed thresholds in tree density at which bird abundance changed dramatically. Knowledge of these thresholds allow managers to design stands for specific biodiversity objectives. The habitat functions presented here can be used to predict bird abundance based on habitat measurements derived from field data, remotely sensed data, or output from computer models of forest dynamics.

Alternative Silvicultural Regimes in the Pacific Northwest: Simulations of Ecological and Economic Effects

New silvicultural strategies to sustain both ecological and human communities are being developed and implemented on federal forest lands in the Pacific Northwest (PNW) United States. Two important stand-level components of the new silviculture regimes are rotation age and retention level of live trees in harvest units. Ecologists have suggested that canopy tree retention and longer rotations will create patterns of stand structure in managed forest that are similar to those in natural forests, and promote long-term ecological productivity and biodiversity. Forest economists, however, are concerned that canopy tree retention and long rotations may reduce wood production, although the high value of large logs produced by these new silvicultural regimes may compensate for reduced growth rates. We used the forest model ZELIG to perform a factorial simulation experiment on long-term responses of ecological and economic variables to nine retention levels and four rotation lengths. ZELIG output on forest structure and composition was input to a forest economics model that calculated net value of wood products in 1989 dollars. The simulated stand data were also linked with regression equations to predict the densities of 17 bird species as a function of tree size class distribution. Five replicates of each treatment were run for the 240-yr simulation period. Results indicated that stand structure under each of the canopy tree retention levels was more similar to the pre-treatment natural forest than following clear-cutting. Variation in tree size under intermediate levels of retention, however, did not reach the level of the natural forest during the simulation period. Tree species composition was strongly related to retention level and rotation age. Shade-intolerant Douglas- fir (Pseudotsuga menziesii) lost dominance to shade-tolerant species under intermediate retention levels and longer rotations. Wood production decreased significantly with increasing retention level and rotation age, with a notable threshold between retention levels of 0 and 5 trees per hectare. Net wood products value did not decrease as rapidly with retention level, and did not differ much among rotation ages, because of the high value of large logs. Bird species responded individualistically to retention level and rotation age. Some had peak densities under short-rotation clear-cutting, but most were associated with structurally complex, closed-canopy forest. Consequently, bird species richness increased significantly with retention level and rotation age. Within the assumptions and limitations of our models, this application provided knowledge on trends and thresholds that can help land managers to choose silvicultural regimes that best balance their management objectives. We concluded that retention level and rotation age strongly influence ecological and economic responses in PNW forests; efforts are needed to reduce uncertainty about these effects.

An Approach for Managing Vertebrate Diversity Across Multiple-Use Landscapes

Land managers face the difficult challenge of maintaining biodiversity on lands also used for commodity production. We present an approach for managing the habitats of terrestrial vertebrates at the landscape scale on multiple-use lands. The approach is based on the hypothesis that animal community response to landscape change is a function of species life histories and local patterns of landscape change. Key steps are: (1) set clear objectives; (2) associate target species with specific habitat configurations; (3) assess the potential sensitivity of species by mapping habitat suitability and examining species life histories; (4) evaluate alternative management prescriptions using simulation models; and (5) implement preferred or experimental strategies and monitor the responses of habitats and species. The approach was demonstrated for a watershed in western Oregon. Management objectives were to maximize habitat diversity for early- and late-successional bird species and to produce saw timber at levels compatible with the habitat goals. Habitat associations of 51 bird species were described by four variables that encompass three spatial scales. An analysis of species sensitivity to landscape change revealed several species that may merit special attention. The landscape model LSPA and the gap model ZELIG.PNW were used to simulate four disturbance/management scenarios over a 140-yr period: natural fire, wood production, multiple use, and no action. The results indicated that 65% more saw timber would be produced under the wood production run than the multiple-use run, but the former would maintain habitats for many fewer bird species than the latter. The multiple-use scenario was selected as the preferred alternative. We suggest carrying out management experiments and rigorous monitoring during the implementation phase. While this approach has various limitations, it is an incremental step towards the effective management of species diversity on multiple-use lands.