Productivity, recovery, diversity, and function of aspen-dominated forests vary in response to biomass harvest severity

University of Minnesota Ph.D. dissertation. August 2014. Major: Natural Resources Science and Management. Advisor: Dr. Anthony W. D’Amato. 1 computer file (PDF); viii, 170 mpages.Given uncertainty surrounding future climate and disturbance regimes, balancing objectives that include continuing to provide current forest products, meeting future resource demands, and maintaining ecosystem services presents a formidable challenge to forest managers. This research explored the short- and medium-term impacts of removing harvest residues for bioenergy feedstocks on aspen-dominated forests of the Lake States region. On sandy soils the removal of residues reduced standing biomass compared with stem-only harvest (SOH) 15 years after treatment, but no negative effect on aboveground biomass was observed following whole-tree harvest (WTH) on clayey or silty loam soils. Maximum diameter and the density of stems (> 5 cm diameter at breast height) declined on silty loam and sandy soils in response to increased severity in compaction and organic matter removal, respectively, indicating that structural development may be slowed. Although three species diversity measures and four functional diversity measures were used to assess community response to harvest disturbance, only indicator species analysis detected a functionally-relevant shift in community composition and structure that followed the most severe treatment combination on silty loam. This result highlighted the importance of employing multiple measures of diversity and composition to assess harvest impacts. Observations 2 years following bioenergy harvest with retention of aggregated overstory reserve trees indicate that both residue removal and overstory retention influence understory community composition. However, species diversity measures differed only between controls and disturbed areas (aggregates, SOH, WTH). Herbaceous plants considered interior forest obligates, such as Trientalis borealis, occurred in the aggregate understory, suggesting potential for small aggregates (0.1 ha) to serve as refugia for some species, at least in the short-term. Aspen sucker densities 0-5 m from the aggregate in the adjacent harvested areas were indistinguishable from densities 20 m from the aggregate edge, indicating ecological objectives might be achieved through aggregate retention without a trade-off in initial regeneration densities. Overall, results indicate that responses to the level of disturbance associated with harvest residue removal differ among sites, even when dominated by the same overstory species, but there is potential for severe disturbance to reduce standing biomass, shift community composition, and alter function and structure

Long-Term Soil Productivity Study: 25-Year Vegetation Response to Varying Degrees of Disturbance in Aspen-Dominated Forest Spanning the Upper Lake States

Installations of the Long-Term Soil Productivity Study were established in northern Minnesota and Michigan at the Chippewa, Ottawa, and Huron-Manistee National Forests (NFs) in the early 1990s and have since provided a wealth of data for assessing the response of aspen-dominated forest ecosystems to varying levels of organic matter removal and soil compaction. An assessment of 25-year standing woody biomass indicates that neither whole-tree harvest nor whole-tree harvest combined with forest floor removal reduced forest productivity on silt-loam soils compared with conventional, stem-only harvest; however, moderate and heavy compaction did negatively impact aspen biomass and stem densities. In contrast, whole-tree harvest reduced standing biomass of aspen and all species combined on sandy soils at the Huron NF while compaction had no discernable impact. Neither treatment factor affected vegetation response at the Ottawa NF (clay soils), but reduced sample size at this site may have increased variability. Over all, the response of standing biomass and forest structure to organic matter removal and compaction treatments demonstrate that the sustainability of practices such as whole-tree harvesting and associated potential for soil impacts varies with site conditions, even when stands are dominated by the same species (e.g., Populus tremuloides)

Influence of Mature Overstory Trees on Adjacent 12-Year Regeneration and the Woody Understory: Aggregated Retention versus Intact Forest

Retention harvesting, an approach that intentionally retains legacy features such as mature overstory trees, provides options for achieving ecological objectives. At the same time, retained overstory trees may compete with the nearby recovering understory for resources, and much remains to be learned about potential trade-offs with regeneration objectives, particularly over extended time periods. We assessed the influence of aggregated retention (reserved mature overstory and understory patches) versus intact forest on structure and productivity (standing biomass) of the adjacent woody understory and regeneration 12 years after harvest in northern Minnesota, USA. Each site was dominated by Populus tremuloides Michx., a species that regenerates prolifically via root sprouts following disturbance. Overall, fewer differences than expected occurred between the effects of intact forest and aggregated retention on regeneration, despite the small size (0.1 ha) of aggregates. Instead, harvest status and distance from harvest edge had a greater influence on structure and standing woody biomass. Proximity to aggregates reduced large sapling biomass (all species, combined) relative to open conditions, but only up to 5 m into harvested areas. This suggests the trade-off for achieving productivity objectives might be minimal if managers use retention aggregates in this region to achieve ecological objectives and meet management guidelines

Influence of Mature Overstory Trees on Adjacent 12-Year Regeneration and the Woody Understory: Aggregated Retention versus Intact Forest

Retention harvesting, an approach that intentionally retains legacy features such as mature overstory trees, provides options for achieving ecological objectives. At the same time, retained overstory trees may compete with the nearby recovering understory for resources, and much remains to be learned about potential trade-offs with regeneration objectives, particularly over extended time periods. We assessed the influence of aggregated retention (reserved mature overstory and understory patches) versus intact forest on structure and productivity (standing biomass) of the adjacent woody understory and regeneration 12 years after harvest in northern Minnesota, USA. Each site was dominated by Populus tremuloides Michx., a species that regenerates prolifically via root sprouts following disturbance. Overall, fewer differences than expected occurred between the effects of intact forest and aggregated retention on regeneration, despite the small size (0.1 ha) of aggregates. Instead, harvest status and distance from harvest edge had a greater influence on structure and standing woody biomass. Proximity to aggregates reduced large sapling biomass (all species, combined) relative to open conditions, but only up to 5 m into harvested areas. This suggests the trade-off for achieving productivity objectives might be minimal if managers use retention aggregates in this region to achieve ecological objectives and meet management guidelines

Harvest impacts to stand development and soil properties across soil textures: 25-year response of the aspen Lake States LTSP installations

In addition to long-standing concerns about sustaining forest productivity, maintaining forest ecosystems under changing conditions and emerging threats has become increasingly important when planning forest management. With the aim of understanding effects of management on both productivity and recovery, we quantified the 25-year impact of varying degrees of organic matter (OM) removal and soil compaction on above-ground biomass, soil carbon and nutrients, soil bulk density, and stand development in aspen-dominated forests in the upper Lake States region of the US. Treatment impacts were assessed at three different sites with comparable overstory composition, but with varying soil texture, site quality, and climate. Across all sites, soil C and N generally decreased with increasing OM removal, and bulk density increased with increasing compaction; 25-year observations indicate recovery of bulk density at the surface (0–10 cm) but not at deeper portions of the soil profile. At the most productive site (loamy soils) with favorable initial soil porosity, severe compaction decreased mean aboveground biomass (-46%), particularly of trees (-73%). Biomass at 25 years did not differ among organic matter removal treatments (e.g. stem-only harvest), but a greater increase in soil C occurred with stem-only harvest relative to whole-tree harvest plus forest floor removal. In contrast, at a less productive site with sandy soils poorly buffered to nutrient and C removals, whole-tree harvest reduced biomass by 25% (tree biomass declined 35%) relative to stem-only harvest while compaction treatments did not differ in effects on biomass production, soil C or soil N. On clay soils, compaction treatments did not significantly impact biomass production, but whole-tree harvest plus forest floor removal reduced tree biomass by 47% relative to whole-tree harvest alone. Assessment of mean relative density indicates canopy closure has not yet occurred at the least productive site (clay soils) or the more severely disturbed stands at the intermediate site (sandy soils), suggesting the possibility for treatment impacts not yet discernible to become more pronounced as stands develop and nutrient uptake continues in the future. Our results align with concepts of soil quality and texture-specific limitations to growth, underlying a need to understand key soil limitations when considering forest management impacts to aboveground structure and productivity.This article is published as Curzon, Miranda T., Robert A. Slesak, Brian J. Palik, and Julia K. Schwager. "Harvest impacts to stand development and soil properties across soil textures: 25-year response of the aspen Lake States LTSP installations." Forest Ecology and Management 504 (2022): 119809. Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted

Harvest Impacts to Stand Development and Soil Properties Across Soil Textures: 25-Year Response of the Aspen Lake States LTSP Installations

In addition to long-standing concerns about sustaining forest productivity, maintaining forest ecosystems under changing conditions and emerging threats has become increasingly important when planning forest management. With the aim of understanding effects of management on both productivity and recovery, we quantified the 25-year impact of varying degrees of organic matter (OM) removal and soil compaction on above-ground biomass, soil carbon and nutrients, soil bulk density, and stand development in aspen-dominated forests in the upper Lake States region of the US. Treatment impacts were assessed at three different sites with comparable overstory composition, but with varying soil texture, site quality, and climate. Across all sites, soil C and N generally decreased with increasing OM removal, and bulk density increased with increasing compaction; 25-year observations indicate recovery of bulk density at the surface (0–10 cm) but not at deeper portions of the soil profile. At the most productive site (loamy soils) with favorable initial soil porosity, severe compaction decreased mean aboveground biomass (-46%), particularly of trees (-73%). Biomass at 25 years did not differ among organic matter removal treatments (e.g. stem-only harvest), but a greater increase in soil C occurred with stem-only harvest relative to whole-tree harvest plus forest floor removal. In contrast, at a less productive site with sandy soils poorly buffered to nutrient and C removals, whole-tree harvest reduced biomass by 25% (tree biomass declined 35%) relative to stem-only harvest while compaction treatments did not differ in effects on biomass production, soil C or soil N. On clay soils, compaction treatments did not significantly impact biomass production, but whole-tree harvest plus forest floor removal reduced tree biomass by 47% relative to whole-tree harvest alone. Assessment of mean relative density indicates canopy closure has not yet occurred at the least productive site (clay soils) or the more severely disturbed stands at the intermediate site (sandy soils), suggesting the possibility for treatment impacts not yet discernible to become more pronounced as stands develop and nutrient uptake continues in the future. Our results align with concepts of soil quality and texture-specific limitations to growth, underlying a need to understand key soil limitations when considering forest management impacts to aboveground structure and productivity

Impacts of Historic Disturbance on Drought Response of Aspen-dominated Forest

Water limitation is expected to have greater impact on forest growth in the Lake States region in the future, and management may have long-term impacts on how forests respond to this and other potential stressors. We examined the response of aspen-dominated forest growing on droughty, sandy soils to a regionwide drought that occurred in 2012. During July 2018, we stem-mapped, measured, and cored overstory trees across nine replicated treatments at the Long-Term Soil Productivity (LTSP) study installation at Huron-Manistee National Forest in lower Michigan, USA. Annual biomass growth, reconstructed from cross-dated tree ring widths, was used to calculate indices of resistance and resilience and evaluate drought responses at tree and stand scales. Our results indicate little to no direct impact of organic matter removal or compaction on stand-scale growth responses to the 2012 drought, but species, tree size, and density (neighborhood crowding) did influence resistance and resilience.This proceeding is published as Schwager, Julia; Curzon, Miranda T.; Palik, Brian J. 2023. Impacts of Historic Disturbance on Drought Response of Aspen-dominated Forest. In: Kern, Christel C.; Dickinson, Yvette L., eds. Proceedings of the first biennial Northern Hardwood Conference 2021: Bridging science and management for the future. Gen. Tech. Rep. NRS-P-211. Madison, WI: U.S. Department of Agriculture, Forest Service, Northern Research Station: 242-246. https://doi.org/10.2737/NRS-GTR-P-211-paper52.Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted

Impacts of Historic Disturbance on Drought Response of Aspen-Dominated Forest

Water limitation is expected to have greater impact on forest growth in the Lake States region in the future, and management may have long-term impacts on how forests respond to this and other potential stressors. We examined the response of aspen-dominated forest growing on droughty, sandy soils to a regionwide drought that occurred in 2012. During July 2018, we stem-mapped, measured, and cored overstory trees across nine replicated treatments at the Long-Term Soil Productivity (LTSP) study installation at Huron-Manistee National Forest in lower Michigan, USA. Annual biomass growth, reconstructed from cross-dated tree rigs widths, was used to calculate indices of resistance and resilience and evaluate drought responses at tree and stand scales. Our results indicate little to no direct impact of organic matter removal or compaction on stand-scale growth responses to the 2012 drought, but species, tree size, and density (neighborhood crowding) did influence resistance and resilience

Response of natural tree regeneration to climate adaptation treatments in Pinus resinosa-dominated forests

Uncertainty and emerging threats associated with climate change necessitate the development of new approaches for managing forest ecosystems. To address this need the Adaptive Silviculture for Climate Change (ASCC) Network was established to examine the efficacy of three climate adaptation strategies in important forest types across North America: 1) resistance to change by increasing overstory tree health through reduced inter-tree competition, 2) resilience by creating conditions that allow change within the natural range of variability while encouraging greater abundance of native species considered suitable for projected future climate, and 3) transition which involves actively facilitating systems to have a more adaptive response. The present study focused on the influence of Adaptive Silviculture for Climate Change treatments on natural regeneration in a Pinus resinosa Ait. (red pine)-dominated forest in northern Minnesota, USA. We aimed to answer the following research questions: 1) How do different climate adaptation strategies (resistance, resilience, and transition) influence natural regeneration relative to passive management? 2) Do impacts on the understory woody community, including trees and shrubs, differ among treatments in terms of abundance, composition, and diversity? Naturally regenerated trees and shrubs were sampled during the 5th growing season following Adaptive Silviculture for Climate Change treatment implementation (May-August 2019). The species composition of naturally regenerated trees differed among treatments as did adaptability, quantified as a composite index that integrated disturbance response and life history traits. The transition treatment resulted in greater capacity for adaptation to future conditions in the newly regenerated cohort. All treatments increased tree species diversity and richness relative to passive management, but the greatest woody species diversity occurred in the resilience treatment. This suggests a trade-off between maximizing woody species diversity (greatest in the resilience treatment) and adaptability (greatest in transition treatment). Overall, these results affirm the potential for using silviculture to increase tree diversity and adaptability through natural regeneration in anticipation of threats posed by changing climate in a major forest type, with results serving as a model for expectations in similar ecosystems.This article is published as Wiechmann, Lewis J., Miranda T. Curzon, and Brian J. Palik. "Response of natural tree regeneration to climate adaptation treatments in Pinus resinosa-dominated forests." Forest Ecology and Management 523 (2022): 120499. doi:10.1016/j.foreco.2022.120499. Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted

Harvest residue removal and soil compaction impact forest productivity and recovery: Potential implications for bioenergy harvests

Understanding the effects of management on forest structure and function is increasingly important in light of projected increases in both natural and anthropogenic disturbance severity and frequency with global environmental change. We examined potential impacts of the procurement of forest-derived bioenergy, a change in land use that has been suggested as a climate change mitigation strategy, on the productivity and structural development of aspen-dominated ecosystems. Specifically, we tested the effects of two factors: organic matter removal (stem-only harvest, whole-tree harvest, whole-tree harvest plus forest floor removal) and soil compaction (light, moderate, and heavy) over time. This range of treatments, applied across three sites dominated by aspen (Populus tremuloides Michx.) but with different soil textures, allowed us to characterize how disturbance severity influences ecosystem recovery. Disturbance severity significantly affected above-ground biomass production and forest structural development with responses varying among sites. At the Huron National Forest (sandy soils), the removal of harvest residues reduced above-ground biomass production, but no negative effect was observed following whole-tree harvest at the Ottawa and Chippewa National Forests (clayey and loamy soils, respectively) relative to stem-only harvest. Maximum diameter and the density of stems greater than 5 cm DBH exhibited negative responses to increased disturbance severity at two sites, indicating that structural development may be slowed. Overall, results suggest that disturbance severity related to procuring harvest residues for bioenergy production may impact future productivity and development, depending on site conditions and quality