Eric K. Zenner

Maintaining a pine legacy in Itasca State Park.

ABSTRACT: Itasca State Park, located in the northern central portion of Minnesota, is challenged with maintaining a pine forest covertype, yet regeneration failures may allow much of the park to succeed to northern hardwoods. Efforts to improve pine regeneration and growth have included deer exclosures and prescribed burning to reduce competing vegetation. We revisited Itascas Mary Lake deer exclosure 66 years after establishment to compare stand development, structure, and white pine (Pinus strobus L.) regeneration and growth with that of adjacent plots that have been: (1) untreated and (2) recently repeatedly under-burned. Overstory structure and composition was similar among all three treatments, and mid- and understory structures were similar in the treatments subject to deer browse. Sapling and midstory white pine were only present in the exclosure. White pine regeneration was present in all treatments and most abundant in the burned treatment, but was restricted to the smallest height class and consistently overtopped by the shrub layer. Regeneration, sapling, and midstory layer tree densities were highest within the deer exclosure, as was white pine height growth. The untreated plot lacked young pine and will likely succeed to northern hardwoods with a shrub understory. The three-fire sequence of the burned treatment increased the abundance of white pine regeneration at this site, but may require additional measures to control competing vegetation to allow that regeneration to ascend into the sapling layer.

Tree size distribution and abundance explain structural complexity differentially within stands of even-aged and uneven-aged structure types

Abstract Characterizations of physical structural complexity are an important surrogate for the potential of forested stands to provide desired ecosystem services such as biodiversity. Distinguishing between stands with different structural conditions is not only a necessary feature of useful structural metrics and indices, but how such measures vary among stands can reveal clues to the ecological processes driving structure. We used stand inventory metrics and indices of structural complexity to differentiate between even-aged and uneven-aged structure types using 10 stem-mapped coniferous stands of each type distributed across Switzerland. Within each structure type, we further explored relationships among stand inventory metrics and structural indices over a roughly 10-year period of management intervention. The even-aged and uneven-aged structure types were clearly differentiated using both stand inventory metrics and spatially explicit structural complexity indices. Overall, structural complexity within even-aged stands was strongly related to, and best predicted by, metrics including the distribution of basal area among canopy layers, while complexity in the uneven-aged stands was most strongly related to, and best predicted by, metrics including measures of abundance. Although predictive models could be developed for canopy position mixture, diameter differentiation, and small-scale structural complexity (but not spatial aggregation) using only stand inventory metrics, the prediction success after only a single management intervention was lower than expected. These results indicate that research to explore small-scale structural complexity requires detailed spatially explicit inventory data and that management to enhance structural complexity may require the manipulation of different attributes in stands of even-aged (diameter distribution) and uneven-aged (total abundance) structure types.

Reconstructing the competitive dynamics of mixed-oak neighborhoods

The disparity between the potential for latent oak dominance within a stand and their region-wide decline in importance value raises questions about the competitiveness of oaks in early stand dynamics. We reconstructed tree height growth dynamics in mixed-species neighborhoods to determine if currently dominant oaks were ever shorter than their competitors and at what age currently subordinate oaks fell behind. In 23-36 year old mixed-oak stands in two physiographic provinces of Pennsylvania, we identified dominant and subordinate northern red oaks (Quercus rubra L.) and chestnut oaks (Quercus prinus L.) competing with adjacent non-oaks (red maple (Acer rubrum L.), sweet birch (Betula lenta L.), and black cherry (Prunus serotina Ehrh.)) of equal crown class. In each stand, we randomly selected 20 tree neighborhoods and collected stem cross sections every metre from the base to the tip of each tree. In the Allegheny Plateau province, dominant northern red oaks never averaged more than 2 m shorter than their competitors, while by age 20, subordinate oaks were commonly more than 2 m behind. Dominant chestnut oaks in the Ridge and Valley province were never more than 1 m shorter than their competitors; subordinate oaks, however, were generally always at least a metre behind. In both regions, growth dynamics of currently subordinate trees were indistinguishable from those of currently dominant trees during their first decade. Because oaks that were considerably behind at age 15 were likely to be subordinate by age 30, the window of opportunity for release to prevent oaks from becoming permanently overtopped and unable to recover may thus be relatively early (age 10-20).

Short-term changes in Pinus strobus sapling height/diameter ratios following partial release: testing the acclimative stem-form development hypothesis

Following release, mature trees can acclimatively shift aboveground growth allocation from height to diameter at breast height (DBH) growth to adjust their stem form (reducing height/diameter ratio (HDR)) to better withstand increased wind stress. The purpose of this study was to determine if, over a 6-year period, 7- to 12-year-old eastern white pines (Pinus strobus L.) also responded to different levels of partial release with (i) stem-form adjustments (reduced HDR) through growth-allocation shifts from height to DBH growth and (ii) lower HDR values (shorter heights) than for unreleased trees of the same DBH (“acclimative stem-form development hypothesis”). Over the 6-year postrelease period, juvenile white pine seedlings exhibited unsynchronized height and DBH growth response patterns that depended on their prerelease height growth. Height growth of faster growing white pines was temporarily reduced. Concurrently, DBH growth was enhanced with increasing release intensity. HDR reductions followed a fair...

A Bayesian modeling of wildfire probability in the Zagros Mountains, Iran

Abstract The preparation of probability distribution maps is the first important step in risk assessment and wildfire management. Here we employed Weights-of-Evidence (WOE) Bayesian modeling to investigate the spatial relationship between historical fire events in the Chaharmahal-Bakhtiari Province of Iran, using a wide range of binary predictor variables (i.e., presence or absence of a variable characteristic or condition) that represent topography, climate, and human activities. Model results were used to produce distribution maps of wildfire probability. Our modeling approach is based on the assumption that the probabilities reflect the observed proportions of the total landscape area occupied by the corresponding events (i.e., fire incident or no fire) and conditions (i.e., classes) of predictor variables. To assess the effect of each predictor variable on model outputs, we excluded each variable in turn during calculations. The results were validated and compared by the receiver operating characteristic (ROC) using both success rate and prediction rate curves. Seventy percent of fire events were used for the former, while the remainder was used for the latter. The validation results showed that the area under the curves (AUC) for success and prediction rates of the model that included all thirteen predictor variables that represent topography, climate, and human influences were 84.6 and 80.4%, respectively. The highest AUC for success and prediction rates (86.8 and 84.6%) were achieved when the altitude variable was excluded from the analysis. We found slightly decreased AUC values when the slope-aspect and proximity to settlements variables were excluded. These findings clearly demonstrate that the probability of a fire is strongly dependent upon the topographic characteristics of landscapes and, perhaps more importantly, human infrastructure and associated human activities. The results from this study may be useful for land use planning, decision-making for wildfire management, and the allocation of fire resources prior to the start of the main fire season.

Toward managing mixed-species stands: from parametrization to prescription

A better understanding and a more quantitative design of mixed-species stands will contribute to more integrative and goal-oriented research in mixed-species forests. Much recent work has indicated that the structure and growth of mixed species forests may fundamentally differ from monocultures. Here we suggest how to progress from the present accumulation of phenomenological findings to a design of mixed-species stands and advanced silvicultural prescriptions by means of modelling. First, the knowledge of mixing effects on the structure and growth at the stand, species, and individual tree level is reviewed, with a focus on those findings that are most essential for suitable modelling and silvicultural designs and the regulation of mixed stands as opposed to monocultures. Then, the key role of growth models, stand simulators, and scenario assessments for designing mixed species stands is discussed. The next section illustrates that existing forest stand growth models require some fundamental modifications to become suitable for both monocultures and mixed-species stands. We then explore how silvicultural prescriptions derived from scenario runs would need to be both quantified and simplified for transfer to forest management and demonstrated in training plots. Finally, we address the main remaining knowledge gaps that could be remedied through empirical research.

Mitigating effects of slash on soil disturbance in ground-based skidding operations

Reinforcement of skid trails with slash has been shown to reduce soil disturbances, but there have not been any studies documenting the mitigating effects following traffic of harvest equipment traffic on steep mountainous skid trails. The objective of this study was to quantify potentially mitigating effects of slash cover on soil compaction and rutting on skid trails in mountainous hardwood forests. The effects of the placement of no slash (bare soil), light density slash (7.5 kg m−2) and heavy density slash (17.5 kg m−2) in a skid trail following one, five, and nine machine passes on both gentle slopes <20% and steep slopes >20% in a downhill skidding operation by a steel-tracked skidder were studied. Bulk density and rut depth increased following harvest equipment traffic on both slope gradients. Compared to bare soils, soil bulk density was not significantly reduced by light slash density; however, soil bulk density was significantly reduced by heavy slash up to five machine passes on steep slopes. Light and heavy slash significantly reduced rut depth in both slope classes. The study revealed a high protective role of slash, particularly on steep skid trails. However, benefits of slash to mitigate soil compaction were limited to five passes, after which the slash deteriorated and only provided benefits against rutting.

Identifying Restoration Opportunities for Northern White Cedar by Contrasting Historical and Modern Inventories in an Ecological Classification System Context

We propose an approach toward identifying restoration opportunities on the landscape using historical Public Land Survey (PLS) data and modern Forest Inventory and Analysis (FIA) data within an ecological classification system (ECS) context, and demonstrate this approach for northern white cedar (Thuja occidentalis) in Minnesota, U.S.A. Concern over long-term declines in the abundance of northern white cedar, particularly in younger (<40 yr) upland stands, has spurred interest in northern white cedar restoration in the Lake States region. Comparing historical PLS and modern FIA data revealed a decline in long-term northern white cedar area by 45% due to land use conversion and species shifts in extant forests in Minnesota. Changes in extent and relative abundance varied by ECS land type association (LTA): decreases occurred predominantly in mixed species stands and upland northern white cedar communities, while increases occurred in monotypic stands and lowland northern white cedar communities. We suggest that this landscape variation can guide adaptive management and restoration by focusing adaptive management on LTAs in which northern white cedar has fared well, and restoration on LTAs in which northern white cedar has dramatically declined. Where the data are available, this ECSPLS-FIA approach could facilitate the prioritization of potential restoration candidate areas by revealing population dynamics in a temporal context that match the life span of the species of interest.

Effects of harvesting on nitrogen and phosphorus availability in riparian management zone soils in Minnesota, USA

Riparian management zones (RMZs) protect streams from excess nutrients, yet few studies have looked at soil nutrients in forested RMZs or the impacts of partial harvesting on nutrient availability. We investigated the impacts of upland clearcutting in conjunction with uncut and partially harvested RMZs (40% basal area reduction) on soil nutrients in forests in Minnesota, USA. Nitrate, ammonium, and phosphorus were measured using exchange resins. Upland clearcutting increased dormant and growing season nitrate, ammonium, and total inorganic nitrogen in the upland 2 to 5 times compared with uncut upland. Upland clearcutting increased dormant and growing season nitrate and total inorganic nitrogen just inside the RMZ boundary 2 to 5 times compared with this location adjacent to uncut upland. Dormant season nitrate and total inorganic nitrogen were 2 times higher in the entire RMZ adjacent to upland clearcut. Phosphorus was not affected by treatment. Partial harvesting of the RMZ did not increase nutrients co...

Spatial patterns of natural Pinus strobus L. regeneration in a Pinus resinosa Ait. stand1

Abstract Understanding the regeneration niche of white pine (Pinus strobus L.) is necessary to promote the restoration of this formerly widespread species. We tested several hypotheses drawn from the literature relating to four features of the regeneration niche for P. strobus: distance to seed source, overstory influence, segregation into spatial neighborhoods, and deer browsing. All hypotheses were tested in a 0.36-ha stem-mapped Pinus resinosa Ait. stand with abundant naturally regenerating P. strobus. Few regenerating P. strobus were found in the immediate vicinity of the seed source tree or other overstory trees and the distribution of distances away from the seed tree differed for very young (< 5 yr) and older regeneration. The spatial arrangement of older regeneration was less associated with the overstory than that of young regeneration, although growth rates were lower in areas of higher overstory influence. Evidence was found for the segregation of regeneration on the basis of size and age classes into spatially distinct neighborhoods, which varied in spatial extent. Spatial segregation was also observed for regeneration that had been browsed, although this was not related to size. Our results supported some hypotheses gleaned from the literature and refuted others, indicating that our understanding of the germination and establishment niches of P. strobus is as yet too immature to predict the factors shaping the regeneration niche.