Klaus J. Puettmann

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.

Overstory composition and stand structure influence herbaceous plant diversity in the mixed aspen forest of northern Minnesota

Abstract The relationship of herbaceous plant diversity to overstory composition and stand structure in the mixed aspen forest of northern Minnesota was investigated on 23 study sites that contained aspen in monoculture or in mixture with boreal conifers or northern hardwood species. On each site overstory species were placed into species groups: conifers, aspen and hardwoods other than aspen. Each site was then placed in one of three cover-type groups based on proportion of the overstory species groups: Aspen (>0.9 basal area in aspen), Aspen-Conifer (>0.15 basal area in conifer species) and Aspen-Hardwood (>0.15 basal area in hardwood species other than aspen). The relationships between diversity of herbaceous vegetation and the following factors were tested: (1) overstory composition, defined as the proportion of basal area by species group in the overstory and (2) stand structure. Stand structure was described by the vertical position and horizontal arrangement of balsam fir within the stand. In addition, relationships were tested with respect to other stand structural features such as shrub height and cover, average amount of plant material intercepted within the vertical profile and an index of plant occupancy within the vertical profile (modified Foliage Height Diversity Index-FDH). Understory herbaceous diversity (H′) and proportion of aspen basal area were significantly positively related whereas understory herbaceous diversity was significantly negatively related to proportion of hardwood basal area and not related to proportion of conifer basal area. Mixtures of overstory tree species provided a range of stand structures that can be represented by shrubs, subcanopy trees or the overstory trees. In the three cover-type groups different structural components were related to herbaceous diversity indicating that (1) overstory composition and stand structure interactively influence understory diversity patterns and (2) it is difficult to characterize stand structure for the range of stand conditions with a single measure. Increased diversity of structure (modified FHD) in the Aspen-Conifer group is negatively related with diversity (H′) of the herb layer. Composition of herbaceous species varied depending on presence or absence of conifers in the overstory. These patterns may be influenced by the interaction of a variety of resource levels and climate conditions, which, in turn, are controlled by factors such as tree architecture or shade tolerance of overstory trees.

Silvicultural alternatives to conventional even-aged forest management - what limits global adoption?

BackgroundThe development of forestry as a scientific and management discipline over the last two centuries has mainly emphasized intensive management operations focused on increased commodity production, mostly wood. This “conventional” forest management approach has typically favored production of even-aged, single-species stands. While alternative management regimes have generally received less attention, this has been changing over the last three decades, especially in countries with developed economies. Reasons for this change include a combination of new information and concerns about the ecological consequences of intensive forestry practices and a willingness on the part of many forest owners and society to embrace a wider set of management objectives. Alternative silvicultural approaches are characterized by a set of fundamental principles, including avoidance of clearcutting, an emphasis on structural diversity and small-scale variability, deployment of mixed species with natural regeneration, and avoidance of intensive site-preparation methods.MethodsOur compilation of the authors’ experiences and perspectives from various parts of the world aims to initiate a larger discussion concerning the constraints to and the potential of adopting alternative silvicultural practices.ResultsThe results suggest that a wider adoption of alternative silvicultural practices is currently hindered by a suite of ecological, economic, logistical, informational, cultural, and historical constraints. Individual contexts display their own unique combinations and relative significance of these constraints, and accordingly, targeted efforts, such as regulations and incentives, may help to overcome specific challenges.ConclusionsIn a broader context, we propose that less emphases on strict applications of principles and on stand structures might provide additional flexibility and facilitate the adoption of alternative silvicultural regimes in a broader set of circumstances. At the same time, the acceptance of alternative silvicultural systems as the “preferred or default mode of management” will necessitate and benefit from the continued development of the scientific basis and valuation of a variety of ecosystem goods and services. This publication is aimed to further the discussion in this context.

Conservation implications of browsing by Odocoileus virginianus in remnant upland Thuja occidentalis forests

Regeneration success of canopy dominants is linked to multiple factors, including the ability of their seedlings to survive browsing and to utilize available resources in the understory. In remnant upland northern white cedar Thuja occidentalis L. forests, eAects of browsing on recruitment of cedar seedlings were evaluated at 7 sites, including known deer yards, located on a portion of the Lake Superior Highlands in northeastern Minnesota. Experimental plantings and vegetation surveys were conducted inside and outside large deer exclosures constructed between 1937 and 1997. Objectives were to determine severity and intensity of browsing by Odocoileus virginianus Zimmerman, the ability of seedlings to survive browsing in environments beneath T. occidentalis and adjacent paper birch (Betula papyrifera) Marshall canopies, and potential long-term sapling recruitment under diAerent browsing scenarios. From 1994 to 1997, 76% of unexclosed planted T. occidentalis seedlings had been browsed at least once, compared with 0% of exclosed seedlings. Increased browsing intensity and decreased light availability increased T. occidentalis mortality rates. Simulation models and vegetation surveys demonstrated that the best recruitment rates occurred for seedlings planted under Betula canopy and subjected to low browsing severity, but no recruitment occurred under high browsing pressure under either canopy type. The current level of browsing in these forests has the potential to alter the future composition of canopy tree species through sustained prevention of T. occidentalis recruitment. # 2000 Published by Elsevier Science Ltd. All rights reserved.

Canopy feedbacks and microtopography regulate conifer seedling distribution in two Minnesota conifer-deciduous forests

Successful regeneration of some species may be linked to microsite availability, which in turn may be influenced by dominant canopy species. We investigated the relationship between canopy feedback...

The dynamics of mixed stands of Alnus rubra and Pseudotsuga menziesii: extension of size−density analysis to species mixture

The monoculture self-thining concept was expanded to polycultures by describing a self-thinning surface in a mixed-species size-density space. The size- density relationships of pure and mixed populations of Alnus rubra and Pseudotsuga menziesii in the Pacific Northwest, USA were investigated. For polycultures of A. rubra and P. menziesii, the average size-density surface showed a near-planar region for stands consisting principally of A. rubra. Thus, A. rubra dominates stand dynamics in this region. At high proportions of P. menziesii, the surface showed a curvilinear increase towards the relative density level of pure P. menziesii stands (...)

Canopy type, forest floor, predation, and competition influence conifer seedling emergence and early survival in two Minnesota conifer-deciduous forests

There is concern that the conifer component of mixed conifer–deciduous forests in the Great Lakes region is on the decline, possibly the result of insufficient conifer regeneration. Limitations on conifer regeneration that occur during the first 18 months of seedling emergence and establishment were examined for Abies balsamea (L.) Mill. and Pinus strobus L. in two mixed conifer–deciduous forest types in northeastern Minnesota. We hypothesized that the removal of potential barriers to regeneration, including forest floor, seed predation, and competition with understory vegetation, will enhance conifer regeneration. We also hypothesized that the importance of the above regeneration barriers, and early regeneration success of each species, will differ beneath deciduous and conifer canopy types. The study was conducted under conifer and deciduous canopy types for two sites of each forest type: Abies balsamea – Populus tremuloides Michx. forest and Thuja occidentalis L. – Betula papyrifera Marsh. Germination experiments were conducted with seeds of A. balsamea and P. strobus. Experimental treatments included forest floor removal, caging to exclude seed predators and herbivory, and weeding to study the effects of understory competition. Abies balsamea and P. strobus responded differently to experimental treatments at different developmental stages. The best conditions for early regeneration of both A. balsamea and P. strobus included reduction of seed predation (p < 0.0001) and competition with understory vegetation (p ≤ 0.001). Abies balsamea additionally benefited from a conifer overstory (p < 0.0001) and a reduction in thickness of the forest floor (p < 0.01). Résumé: Le déclin de la composante coniférienne des forêts mixtes conifériennes–décidues dans la région des Grands Lacs devient préoccupant. Il pourrait être dû à une régénération coniférienne insuffisante. Les obstacles à la régénération des conifères, qui se manifestent pendant les premiers 18 mois de l’émergence et de l’établissement des semis, ont été examinés pour Abies balsamea (L.) Mill. et Pinus strobus L., dans deux types forestiers mixtes conifériens–décidus du nord-est du Minnesota. Les auteurs ont émis l’hypothèse que l’élimination des barrières potentielles à la régénération, incluant la litière, la prédation des semences et la compétition de la végétation du sous-bois, améliorera la régénération des conifères. Ils ont aussi émis l’hypothèse que l’importance des barrières mentionnées, ainsi que la réussite de chaque espèce, tôt au cours de la régénération, seront différentes sous les types de couvert décidu et coniférien. L’étude a été réalisée sous les types de couvert décidu et coniférien dans deux sites de chacun des types forestiers suivants : forêt d’Abies balsamea – Populus tremuloides Michx. et de Thuja occidentalis L. – Betula papyrifera Marsh. Les essais de germination ont été réalisés avec les semences de A. balsamea et de P. strobus. Les traitements expérimentaux comprenaient l’enlèvement de la litière, l’utilisation de cages pour exclure les prédateurs des semences et les herbivores, ainsi que le sarclage pour étudier les effets de la compétition du sous-bois. Abies balsamea et P. strobus ont répondu différemment à ces traitements expérimentaux, à différents stades de leur développement. Les meilleurs conditions, tôt au cours de la régénération, aussi bien d’A. balsamea que de P. strobus, comprenaient la réduction de la prédation (p < 0,0001) et la compétition de la végétation du sous-étage (p ≤ 0,001). En plus, A. balsamea a bénéficié du couvert coniférien (p < 0,0001) et de la réduction de l’épaisseur de la litière (p < 0,01). [Traduit par la Rédaction]

Overstory response to alternative thinning treatments in young Douglas-fir forests of Western Oregon

An increase in land dominated by young second-growth Douglas-fir forests in the Pacific Northwest has coincided with heightened concerns over loss of old-growth habitat. In search of options for managing young forests to provide late-successional forest structures, the Young Stand Thinning and Diversity Study was designed to test the effectiveness of modified thinning in acceleration of late-successional structural characteristics. Thinning treatments included: a control, a light thin (typical of standard commercial thins), a heavy thin (densities lower than typically prescribed), and a light thin with gaps (stands thinned lightly with the addition of 0.2 hectare patch cuts evenly spaced throughout the stand). Early response (maximum of 5–7 years post-treatment) of overstory vegetation was examined. Average growth of Douglas-fir increased in all thinned stands, but growth of the largest Douglas-fir trees was only accelerated in the heavy thin. After thinning, the canopy of all thinned treatments was initially more open than the control, but after 5–7 years the light thin was no longer significantly different from the control. The light with gaps thin had the highest variation in overstory canopy cover. Differentiation of vertical canopy structure among treatments was not evident. There was no difference in mortality among any of the treatments for most species tested; those that did had highest mortality in the control. Our results indicate that thinning can be effective in hastening development of some, but not all late-successional attributes, but such acceleration is not equivalent among the different thinning treatments.

Can Boreal and Temperate Forest Management be Adapted to the Uncertainties of 21st Century Climate Change

Considerable uncertainties remain about magnitude and character, if not general direction of anthropogenic climate change. Global mean temperature could increase by 1.5–4.5°C or more over historic levels, and extreme weather events—drought, storms, and flooding—are likely to increase greatly in frequency. Although ecologists and foresters agree that the practice of forestry will be transformed under climate change, these uncertainties compound the challenge of achieving sustainable, adaptive forest management. In this aritcle, we (i) present a multidisciplinary synthesis of current knowledge of responses of temperate and boreal tree species and forest communities to climate change, and (ii) outline silvicultural strategies for adapting temperate and boreal forests to confront climate change. Our knowledge synthesis proceeds through critical appraisals of efforts to model future tree distributions and responses to climate change, and reviews physiological, phenological, acclimation, and epigenetic responses to climate. As is the case of climate change itself, there are numerous uncertainties about tree species and provenance responses to climate change. For example, acclimation of respiration and epigenetic conditioning of seed embryos has the potential to buffer species against limited warming. Provenances within species also display idiosyncratic responses to altered climates, implying that soemm varieties will be more resilient or resistant to climate change than others. Genetically determined limits to climatic tolerance, and the limits of tree community resistance and resilience (speed of recovery from disturbance) in the face of climate-related disturbances are largely unknown. These unknowns require managers to adopt a portfolio of silvicultural strategies, which may range from minor modifications of current practices to design of novel multi-species stands that may have no historical analogue. Forest managers must be prepared to respond nimbly as they develop, incorporate new insights about climate change and species responses to warming into their practices. Marshalling all strategies and sources of knowledge should enable forest managers to mount (at least) a partially successful response to the challenges of climate change.

Seedbed and moisture availability determine safe sites for early Thuja occidentalis (Cupressaceae) regeneration

Regeneration of many late-successional tree species depends on specialized safe sites. The primary objective was to investigate the roles of seedbed and moisture retention as dimensions of safe sites for the early regeneration of drought-sensitive northern white cedar (Thuja occidentalis). We hypothesized that rates of germination, survival, and growth of T. occidentalis are unlikely to differ among seedbed types under conditions of abundant water, but that differences are likely to emerge as water becomes more limited. In a 67-d greenhouse experiment, cedar seeds were sown on logs, leaf litter, and soil of cedar and paper birch (Betula papyrifera) canopy origin. Seedbeds were subjected to three water treatments. Among the water treatments, highest germination rates occurred within the high water treatment, although germination on cedar litter was comparable to that of the low water treatment. Higher germination and survival rates occurred on decayed logs than other natural seedbeds for medium (P = 0.001) and low (P < 0.0001) water treatments. Germination on birch logs occurred at higher rates than on cedar logs within the low water treatment (P = 0.04). Seedling growth for the medium water treatment was lower on leaf litter than any other type of seedbed (P < 0.01). Results generally demonstrated that the interplay between seedbed and moisture retention is a component of safe sites for T. occidentalis regeneration.