Forest Ecosystem Dynamics in a Non-Linear World

Forest ecosystems across North America are under increasing stress from the accelerating pace of global change which involves simultaneous changes in resource availability (temperature, moisture, nutrients), disturbance regimes (fire, insects, diseases, extreme weather, logging, urbanization) and (3) species distributions (invasive organisms, threatened species). Interactions among the agents of global change can generate emergent or unexpected ecosystem behaviour. Complex systems science provides a strong theoretical foundation for understanding these factor interactions and provides many new mathematical and simulation modeling tools that can generate complex, non-linear behaviour and provide improved understanding of ecosystem response to global change. I present an updated version of Jenny’s (1941) classic state factor model of soils and ecosystems that allows three variables (resources, disturbance and plant-soil functional groups) to interact to generate higher orders of complexity through self-organizing plant-soil feedback switches. An interactive agent-based model allows the user to vary the degree of change in resource availability, disturbance frequency or severity and the strength of negative and positive plant-soil feedbacks and to measure the effect of these changes on ecosystem resilience, diversity and landscape complexity. The model is currently being used to simulate the dynamics of lodgepole pine-Cladina-lichen dominated landscapes in central British Columbia that has been massively affected by interactions among changing fire regimes, mountain pine beetle and a warming climate

Viewing forests through the lens of complex systems science

Complex systems science provides a transdisciplinary framework to study systems characterized by (1) heterogeneity, (2) hierarchy, (3) self‐organization, (4) openness, (5) adaptation, (6) memory, (7) non‐linearity, and (8) uncertainty. Complex systems thinking has inspired both theory and applied strategies for improving ecosystem resilience and adaptability, but applications in forest ecology and management are just beginning to emerge. We review the properties of complex systems using four well‐studied forest biomes (temperate, boreal, tropical and Mediterranean) as examples. The lens of complex systems science yields insights into facets of forest structure and dynamics that facilitate comparisons among ecosystems. These biomes share the main properties of complex systems but differ in specific ecological properties, disturbance regimes, and human uses. We show how this approach can help forest scientists and managers to conceptualize forests as integrated social‐ecological systems and provide concrete examples of how to manage forests as complex adaptive systems

Germination and first-year survival of red alder seedlings in the central Coast Range of Oregon

The role of disturbance in seed germination and first-year survival of red alder (Alnus rubra) was studied over two growing seasons at four sites representing a climatic gradient within the central Coast Range of Oregon. Disturbance affected red alder seed germination and seedling establishment by altering the temperature and moisture properties of the seedbed, improving light conditions, and disrupting the activities of predators and pathogens. Seedling emergence did not differ significantly between recent clearcuts and adjacent unlogged forests but was higher on disturbed mineral soil seedbeds than on undisturbed organic seedbeds. In clearcuts, mean emergence on disturbed seedbeds ranged from 7 to 65 percent of viable seed sown and was positively correlated (r2 = 0.70) with spring soil moisture conditions across the climatic gradient. On disturbed forest seedbeds, emergence ranged from 23 to 57 percent and did not appear to be limited by soil moisture. Emergence on undisturbed seedbeds averaged below 10 percent except when seedbeds remained near saturation levels and light was not limiting. In the absence of forest disturbance, light conditions play an important role in controlling seed germination. Laboratory and field experiments demonstrated that germination is inhibited by light conditions in the forest understory. Broad-leaved trees such as red alder inhibit germination more than conifers because they filter out red light and increase the proportion of far-red light. Understory vegetation and litter layers have an additional inhibitory effect. Alder seed is subject to heavy losses from seed predators and pathogens. Predation by small mammals and birds averaged 54 to 77 percent. Although vertebrate predation was higher on disturbed seedbeds, losses to invertebrates and pathogens averaged more than three times higher on undisturbed seedbeds. A small percentage of seeds remained viable for more than one year, but longer-term seed storage seems unlikely. Rates of first-year seedling establishment observed in the study reflected natural patterns of red alder abundance in the Coast Range. During the two years of the study, no seedlings survived at the two sites on the Interior slopes of the Coast Range. At the two Coastal sites, survival averaged between 3 and 30 percent on clearcuts but was 2 percent or less in forests. Best survival occurred on a sheltered north-facing clearcut. Survival on undisturbed seedbeds was equal to or greater than that on disturbed seedbeds

Silvicultural disturbance severity and plant communities of the southern Canadian boreal forest

Boreal forest ecosystems are adapted to periodic disturbance, but there is widespread concern that conventional forest practises degrade plant communities. We examined vegetation diversity and composition after clearcut logging, mechanical and chemical site preparation in eight 5- to 12-yr old studies located in southern boreal forests of British Columbia and Quebec, Canada to find useful indicators for monitoring ecosystem integrity and to provide recommendations for the development and testing of new silvicultural approaches. Community-wide and species-specific responses were measured across gradients of disturbance severity and the results were explained in terms of the intermediate disturbance hypothesis and a simple regeneration model based on plant life history strategies. Species richness was 30 to 35% higher 5 to 8 years after clearcut logging than in old forest. Total and vascular species diversity generally peaked on moderately severe site treatments, while non-vascular diversity declined with increasing disturbance severity. On more-or-less mesic sites, there was little evidence of diversity loss within the range of conventional silvicultural disturbances; however, there were important changes in plant community composition. Removing soil organic layers caused a shift from residual and resprouting understory species to ruderal species regenerating from seeds and spores. Severe treatments dramatically increased non-native species invasion. Two important challenges for the proposed natural dynamics-based silviculture will be 1) to find ways of maintaining populations of sensitive non-vascular species and forest mycoheterotrophs, and 2) to create regeneration niches for disturbance-dependent indigenous plants without accelerating non-native species invasion

Silvicultural disturbance severity and plant communities of the southern Canadian boreal forest

Boreal forest ecosystems are adapted to periodic disturbance, but there is widespread concern that conventional forest practises degrade plant communities. We examined vegetation diversity and composition after clearcut logging, mechanical and chemical site preparation in eight 5- to 12-yr old studies located in southern boreal forests of British Columbia and Quebec, Canada to find useful indicators for monitoring ecosystem integrity and to provide recommendations for the development and testing of new silvicultural approaches. Community-wide and species-specific responses were measured across gradients of disturbance severity and the results were explained in terms of the intermediate disturbance hypothesis and a simple regeneration model based on plant life history strategies. Species richness was 30 to 35% higher 5 to 8 years after clearcut logging than in old forest. Total and vascular species diversity generally peaked on moderately severe site treatments, while non-vascular diversity declined with increasing disturbance severity. On more-or-less mesic sites, there was little evidence of diversity loss within the range of conventional silvicultural disturbances; however, there were important changes in plant community composition. Removing soil organic layers caused a shift from residual and resprouting understory species to ruderal species regenerating from seeds and spores. Severe treatments dramatically increased non-native species invasion. Two important challenges for the proposed natural dynamics-based silviculture will be 1) to find ways of maintaining populations of sensitive non-vascular species and forest mycoheterotrophs, and 2) to create regeneration niches for disturbance-dependent indigenous plants without accelerating non-native species invasion

Site Preparation Severity Influences Lodgepole Pine Plant Community Composition, Diversity and Succession Over 25 Years

Lodgepole pine ecosystems of central British Columbia face cumulative stresses and management practices are increasingly scrutinized. We addressed tradeoffs between â light-on-the-landâ versus more aggressive silvicultural approaches by examining plant communities and indicator species (non-natives, berry-producers, epiphytes, mycotrophs, pine rust alternate hosts) across a gradient of 5-6 site preparation treatments at the Bednesti trial (established 1987). We tested whether more severe site preparation (1) caused plant community composition to diverge from a 35-46-yr-old reference forest; (2) accelerated succession by hastening crown closure; or (3) delayed succession by promoting seral species. Non-metric multi-dimensional scaling (NMDS) ordination showed all treatments converging toward the reference forest composition. At 10 yr, succession was incrementally delayed by more severe treatments; at 25 yr, only burn windrows were still delayed. Mixed effects models based on site preparation severity were better than crown closure models for 11 of 13 variables tested, suggesting belowground processes mostly drive succession in these infertile ecosystems. Invasive hawkweeds persisted on all treatments at 25 yr. Limited, contradictory data did not support using mechanical or fire treatments to reduce alternate hosts of pine stem rusts. Long-term trials like Bednesti highlight the need for ecosystem-specific strategies and diverse approaches to accommodate conflicting benefits and risks of disturbance in forests.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Plant Community Responses to Mechanical Site Preparation in Northern Interior British Columbia

Ten-year response of plant communities to disk trenching, plowing, roto clearing and windrow burning was studied on two contrasting sites to address concerns that mechanical site preparation reduces structural and species diversity. Cover and height of all species on randomly located subplots within 0.05- to 0.075-ha treatment plots were used to develop indices of volume, structural diversity, and species diversity; to ordinate the plots; and to correlate species diversity with crop-tree performance. At both sites, community response was strongly influenced by the severity of site preparation. On a boreal site dominated by willow (Salix L. spp.), green alder (Alnus crispa (Ait.) Pursh ssp. crispa) and trembling aspen (Populus tremuloides Michx.), site preparation increased structural diversity and had little effect on species diversity. High-severity treatments increased non-native species abundance 10- to 16-fold while only marginally enhancing growth of planted white spruce (Picea glauca (Moench) Voss) over medium-severity treatments. On a nutrient-poor sub-boreal site, species diversity declined with increasing treatment severity and with increasing lodgepole pine (Pinus contorta var. latifolia Engelm.) stem volume. Velvet-leaved blueberry (Vaccinium myrtilloides Michx.) was highly sensitive to mechanical disturbance. Moderate mechanical treatments appear to improve conifer performance while causing little change to plant communities, but high severity treatments can cause substantial change