Brian J. Palik

Retention Forestry to Maintain Multifunctional Forests: A World Perspective

The majority of the worlds forests are used for multiple purposes, which often include the potentially conflicting goals of timber production and biodiversity conservation. A scientifically validated management approach that can reduce such conflicts is retention forestry, an approach modeled on natural processes, which emerged in the last 25 years as an alternative to clearcutting. A portion of the original stand is left unlogged to maintain the continuity of structural and compositional diversity. We detail retention forestrys ecological role, review its current practices, and summarize the large research base on the subject. Retention forestry is applicable to all forest biomes, complements conservation in reserves, and represents bottom-up conservation through forest manager involvement. A research challenge is to identify thresholds for retention amounts to achieve desired outcomes. We define key issues for future development and link retention forestry with land-zoning allocation at various scales, expanding its uses to forest restoration and the management of uneven—age forests.

Can retention forestry help conserve biodiversity? A meta-analysis

Industrial forestry typically leads to a simplified forest structure and altered species composition. Retention of trees at harvest was introduced about 25 years ago to mitigate negative impacts on biodiversity, mainly from clearcutting, and is now widely practiced in boreal and temperate regions. Despite numerous studies on response of flora and fauna to retention, no comprehensive review has summarized its effects on biodiversity in comparison to clearcuts as well as un-harvested forests. Using a systematic review protocol, we completed a meta-analysis of 78 studies including 944 comparisons of biodiversity between retention cuts and either clearcuts or un-harvested forests, with the main objective of assessing whether retention forestry helps, at least in the short term, to moderate the negative effects of clearcutting on flora and fauna. Retention cuts supported higher richness and a greater abundance of forest species than clearcuts as well as higher richness and abundance of open-habitat species than un-harvested forests. For all species taken together (i.e. forest species, open-habitat species, generalist species and unclassified species), richness was higher in retention cuts than in clearcuts. Retention cuts had negative impacts on some species compared to un-harvested forest, indicating that certain forest-interior species may not survive in retention cuts. Similarly, retention cuts were less suitable for some open-habitat species compared with clearcuts. Positive effects of retention cuts on richness of forest species increased with proportion of retained trees and time since harvest, but there were not enough data to analyse possible threshold effects, that is, levels at which effects on biodiversity diminish. Spatial arrangement of the trees (aggregated vs. dispersed) had no effect on either forest species or open-habitat species, although limited data may have hindered our capacity to identify responses. Results for different comparisons were largely consistent among taxonomic groups for forest and open-habitat species, respectively. Synthesis and applications. Our meta-analysis provides support for wider use of retention forestry since it moderates negative harvesting impacts on biodiversity. Hence, it is a promising approach for integrating biodiversity conservation and production forestry, although identifying optimal solutions between these two goals may need further attention. Nevertheless, retention forestry will not substitute for conservation actions targeting certain highly specialized species associated with forest-interior or open-habitat conditions. Our meta-analysis provides support for wider use of retention forestry since it moderates negative harvesting impacts on biodiversity. Hence, it is a promising approach for integrating biodiversity conservation and production forestry, although identifying optimal solutions between these two goals may need further attention. Nevertheless, retention forestry will not substitute for conservation actions targeting certain highly specialized species associated with forest-interior or open-habitat conditions.

Modeling silviculture after natural disturbance to sustain biodiversity in the longleaf pine (Pinus palustris) ecosystem: balancing complexity and implementation

Modeling silviculture after natural disturbance to maintain biodiversity is a popular concept, yet its application remains elusive. We discuss difficulties inherent to this idea, and suggest approaches to facilitate implementation, using longleaf pine (Pinus palustris) as an example. Natural disturbance regimes are spatially and temporally variable. Variability leads to a range of structural outcomes, or results in different pathways leading to similar structures. In longleaf pine, lightning, hurricanes, surface fires, and windthrow all lead to similar structures, but at different rates. Consequently, a manager can select among various natural disturbance patterns when searching for an appropriate silvicultural model. This facilitates management by providing flexibility to meet a range of objectives. The outcomes of natural disturbances are inherently different from those of silviculture, for example, harvesting always removes boles. It is instructive to think of silvicultural disturbances along a gradient in structural outcomes, reflecting degree of disparity with natural disturbance. In longleaf pine this might involve managing for two-cohort structure, instead of multi-cohort structure characteristic of old growth stands. While two-cohort structure is a simplification over the old growth condition, it is an improvement over single-cohort management. Reducing structural disparity between managed and unmanaged forests is key to sustaining biodiversity because of linkages that exist between structural elements, forest biota, and ecosystem processes. Finally, interactions of frequency, severity, intensity, seasonality, and spatial pattern define a disturbance regime. These components may not have equal weight in affecting biodiversity. Some are easier to emulate with silviculture than are others. For instance, ecologists consider growing-season fire more reflective of the natural fire regime in longleaf pine and critical for maintenance of biodiversity. However, dormant season fire is easier to use and recent work with native plants suggests that seasonality of fire may be less critical to maintenance of species richness, as one component of biodiversity, than is generally believed. Science can advance the goal of modeling silviculture after natural disturbances by better illustrating cause and effect relationships among components of disturbance regimes and the structure and function of ecosystems. Wide application requires approaches that are adaptable to different operational situations and landowner objectives. A key point for managers to remember is that strict adherence to a silvicultural regime that closely parallels a natural disturbance regime may not always be necessary to maintain biodiversity. We outline examples of silvicultural systems for longleaf pine that demonstrates these ideas.

Relationships between environmental characteristics and macroinvertebrate communities in seasonal woodland ponds of Minnesota

Abstract We related macroinvertebrate communities and environmental variables in 66 small seasonal woodland ponds of northern Minnesota, USA. These wetlands were relatively pristine, being embedded in 50- to 100-y-old 2nd-growth forests. Macroinvertebrate taxon richness in ponds increased as hydroperiods lengthened, tree canopies opened, water pH declined, and litter input decreased. Eighteen macroinvertebrate taxa were widespread (occurred in >50% of ponds), and hydrology, water chemistry, geomorphology, vegetation, occurrence of other macroinvertebrate taxa, and presence of amphibian larvae each explained some variation in relative abundance of widespread macroinvertebrates. The first 4 axes of a canonical correspondence analysis explained 37% of total variation in relative abundance of widespread macroinvertebrate taxa. Overall, however, macroinvertebrates were remarkably unresponsive to environmental variables. Most relationships between macroinvertebrates and environmental variables were nonsignificant, and the few significant relationships observed were weak (<20% of variation). We suggest that this lack of response occurs because most macroinvertebrates in seasonal woodland ponds are habitat generalists. These species routinely endure pronounced and unpredictable environmental changes; hence, they possess a durability that makes them resistant to most natural variation in habitat conditions.

Effects of thinning on drought vulnerability and climate response in north temperate forest ecosystems

Reducing tree densities through silvicultural thinning has been widely advocated as a strategy for enhancing resistance and resilience to drought, yet few empirical evaluations of this approach exist. We examined detailed dendrochronological data from a long-term (> 50 years) replicated thinning experiment to determine if density reductions conferred greater resistance and/or resilience to droughts, assessed by the magnitude of stand-level growth reductions. Our results suggest that thinning generally enhanced drought resistance and resilience; however, this relationship showed a pronounced reversal over time in stands maintained at lower tree densities. Specifically, lower-density stands exhibited greater resistance and resilience at younger ages (49 years), yet exhibited lower resistance and resilience at older ages (76 years), relative to higher-density stands. We attribute this reversal to significantly greater tree sizes attained within the lower-density stands through stand development, which in turn increased tree-level water demand during the later droughts. Results from response-function analyses indicate that thinning altered growth-climate relationships, such that higher-density stands were more sensitive to growing-season precipitation relative to lower-density stands. These results confirm the potential of density management to moderate drought impacts on growth, and they highlight the importance of accounting for stand structure when predicting climate-change impacts to forests.

USING LANDSCAPE HIERARCHIES TO GUIDE RESTORATION OF DISTURBED ECOSYSTEMS

Reestablishing native plant communities is an important focus of ecosystem restoration. In complex landscapes containing a diversity of ecosystem types, restoration requires a set of reference vegetation conditions for the ecosystems of concern, and a predictive model to relate plant community composition to physical variables. Restoration also requires an approach for prioritizing efforts, to facilitate allocation of limited institutional resources. Hierarchy theory provides a conceptual approach for predicting plant communities of disturbed ecosystems and, ultimately, for prioritizing restoration efforts. We demonstrate this approach using a landscape in southwestern Georgia, USA. Specifically, we used an existing hierarchical ecosystem classification, based on geomorphology, soil, and vegetation, to identify reference plant communities for each type of ecosystem in the landscape. We demonstrate that ecosystem identity is highly predictable using: only geomorphic and soil variables, because these upper hierarchical levels control the development of vegetation, a lower hierarchical level. We mapped the potential distribution of reference ecosystems in the landscape and used GIS (geographic information systems) to determine relative abundance of each ecosystem, as a measure of its historical rarity. We joined the reference ecosystem map with a current cover map to determine current abundance of each reference ecosystem, and percentage conversion to different disturbance classes. We show that over half of the landscape supports something other than reference plant communities, but degree of rarity varies widely among ecosystems. Finally, we present an index that integrates information on historical and current rarity of ecosystems, and disturbance levels of individual polygons, to prioritize restoration efforts. The premise of the index is that highest priority be given to restoring (1) currently rare ecosystems that were also historically rare and (2) the least disturbed examples of these ecosystems, as these will require the least effort to restore. We found that 80% of high-priority sites occur within just three (of 21) ecosystems. Moreover, the high-priority ecosystems all occur within stream valleys. Our approach provides managers with a straightforward methodology for determining potential distribution of reference ecosystems and for allocating efforts and resources for restoration in complex landscapes. Development of a priority index for a specific landscape requires an understanding of the hierarchical relationships among geomorphology, soil characteristics, and plant communities, in addition to well-defined restoration objectives.

SPATIAL DISTRIBUTION OF OVERSTORY RETENTION INFLUENCES RESOURCES AND GROWTH OF LONGLEAF PINE SEEDLINGS

Increasingly, overstory retention is being used in forests traditionally managed for single-cohort structure. One rationale for retention is that residual stand structure better resembles the complex structure of forests after natural disturbance, helping to perpetuate ecosystem fuctions dependent on that structure. The benefits of retention come at the cost of reduced survival and growth of regeneration because of competition with residual trees. We argue that inhibition of regeneration depends not only on the number and size of residual trees, but also on their spatial arrangement, which ranges from dispersed to aggregated. We use a model of competition at the scale of seedlings to hypothesize that maximum stand-level resource availability, seedling growth, and seedling survival occur with aggregate retention, rather than dispersed retention, even with constant residual basal area. We test our hypothesis with a silvicultural experiment in longleaf pine (Pinus palustris) in Georgia, USA. Replicated treatments included an uncut control, dispersed retention, small-aggregate retention, and large-aggregate retention. We measured light, soil nitrogen, soil moisture, and growth of longleaf pine seedlings across the full range of overstory conditions in each treatment. Postharvest basal areas in the cut treatments were similar. Gap light index increased from the control to large-aggregate retention, as did nitrogen availability, measured on exchange resins. Nitrogen mineralization did not differ among treatments, nor did soil moisture or temperature. Seedling biomass increment increased significantly from the control to large-aggregate retention. Survival did not differ among treatments. We argue that these results are a consequence of exponential relationships between overstory competition intensity, resource availability, and seedling growth. Given this relationship, resources and seedling growth are low across a wide range of decreasing overstory competitor abundance but increase exponentially only at very low competitor abundance. This seedling-scale model translates into maximum stand scale resource availability and seedling growth with large-aggregate retention, compared to dispersed retention, because the probability of a seedling occupying a site free of overstory competition is greater with the former. Our research shows that one can improve competitive environnments for regeneration by manipulating spatial distribution of residual trees without sacrificing the ecological benefits of overstory retention.

DEPRESSIONAL WETLAND VEGETATION TYPES: A QUESTION OF PLANT COMMUNITY DEVELOPMENT

When wetland restoration includes re-establishing native plant taxa as an objective, an understanding of the variables driving the development of plant communities is necessary. With this in mind, we examined soil and physiographic characterstics of depressional wetlands of three vegetation types (cypressgum swamps, cypress savannas, and grass-sedge marshes) located in a fire-maintained longleaf pine ecosystem in southwestern Georgia, USA. Our objective was to establish wether plant community development in these wetlands is controlled primarily by hydrogeomorphic features or by different disturbance histories. We did not identify physical features that uniquely separate the wetland vegetation types. Instead, we observed a range of topo-edaphic conditions that likely drive variations in hydrologic regimes, which in turn, are probable influences on fire regime. We propose that several long-term successional trajectories may be initiated in the prolonged absence of fire, altered hydrology, or both, which link the distinctive vegetation types. Thus, a range of vegetation types may be suitable as potential restoration goals for these depressional wetlands. We suggest that the opportunities or constraints for use of prescribed fire in the long-term management of restored wetlands and adjacent uplands should be a significant consideration in the development of restoration strategies targeting specific plant communities.

SEASONAL POND CHARACTERISTICS ACROSS A CHRONOSEQUENCE OF ADJACENT FOREST AGES IN NORTHERN MINNESOTA, USA

Small seasonal ponds are abundant in many forest landscapes, yet they remain poorly understood in terms of their response to disturbance of the surrounding upland forest. The potential for such a response is large because of the small size and, hence, high perimeter-to-area ratios of most ponds. High perimeter-to-area ratio may increase the importance of functional connections with the surrounding forest, via exchange of energy, organisms, and materials. To better understand this connection, we studied 19 seasonal ponds across a 100-year chronosequence of single-cohort forests in northern Minnesota. Our objective was to see if there are distinct changes over time in select pond attributes, which may reflect alteration of functional linkages with the surrounding forest. In 1998 and 1999, we sampled hydroperiod, water depth and chemistry, canopy openness, grass, sedge, shrub, and coarse woody debris cover, coarse particulate organic matter (CPOM) flux, and macroinvertebrate and amphibian populations. We related these variables to stand age through regression. Stand age explained little variation for most variables. Responsive variables included canopy openness and CPOM flux. Canopy openness, in turn, was related positively to total macroinvertebrate abundance, sensitive taxon richness, and Haliplidae beetle and Physidae snail abundances. Calling wood frogs occurred more frequently under an open canopy and low CPOM flux. An open canopy, which occurs more often over ponds in yonnger than in older forest, likely results in increases in water and air temperatures and photosynthetically active radiation, all of which may influence resource availability and habitat suitability for some macroinvertebrates and amphibian taxa. Results from our exploratory study suggest that many characteristics of small seasonal ponds are unaffected by harvest of the adjacent upland forest, at least as detected through examination of a chronosequence. However, responsive variables may include several abiotic characteristics that provide mechanistic links to pond foodwebs.

Predicting plant species diversity in a longleaf pine landscape

Abstract: In this study, we used a hierarchical, multifactor ecological classification system to examine how spatial patterns of biodiversity develop in one of the most species-rich ecosystems in North America, the fire-maintained longleaf pine-wiregrass ecosystem and associated depressional wetlands and riparian forests. Our goal was to determine which landscape features are important controls on species richness, to establish how these constraints are expressed at different levels of organization, and to identify hotspots of biological diversity for a particular locality. We examine the following questions: 1) How is the variance in patterns of plant species richness and diversity partitioned at different scales, or classification units, of the hierarchical ecosystem classification developed for the study area? 2) What are the compositional similarities among ecosystem types? 3) For our study area, what are the sites expected to harbour highest species richness? We used a spatially explicit map of biodiversity to project abundance of species-rich communities in the landscape based on a previously developed ecological classification system for a lower Gulf Coastal Plain landscape. The data indicate that high species richness in this ecosystem was found in sites with frequent fire and high soil moisture. Sites in fire-maintained landscapes with lower frequency of fire were associated with geomorphological characteristics, suggesting a dependence of the diversity-disturbance relationship with soil type. With more frequent fire on some sites, high diversity shifts from canopy component to ground flora, with an overall increase in total species richness. Our approach demonstrates how potential species richness can be identified as a restoration goal and that multiple vegetation endpoints may be appropriate vegetation objectives. We identify basic management needs for the maintenance of biodiversity in this ecosystem that can be derived from an understanding of the combination of factors that most strongly predict diverse plant communities.