Quaking aspen short-term regeneration response to mechanical roller-felling

Quaking aspen (Populus tremuloides Michx.) is a keystone species in the western US, and typically requires high-severity disturbance (historically stand-replacing fire) to regenerate and maintain population health when coexisting with conifers. Aspen forests are declining due to fire suppression, herbivory, and drought, and restoration is a priority for many forest managers. Prescribed fire and harvest are currently the only fuel reduction treatments in practice, imposing limitations on land managers. A mechanical treatment method, called ‘roller-felling,’ has been developed to mimic stand-replacing fire by reducing fuel loading and resetting succession of late-seral stage, conifer-dominated, aspen communities. We examined the ecological impact of roller-felling by investigating factors contributing to post-treatment aspen regeneration, ultimately determining the feasibility of this method as an alternative, stand-replacing disturbance treatment. Specifically, I quantified aspen regeneration stem densities to determine if the result emulated stand-replacing fire. I also measured other metrics of treatment “success”, with additional factors influencing aspen regeneration, including the ratio of suckers to true seedling establishment, ungulate browsing pressure, and herbaceous understory diversity before and after treatment. Preliminary, single-growing season, results indicate densities exceed threshold-related objectives, while related ecological impacts remain less conclusive. A second year of sampling in this upcoming field season will allow for more definitive, short-term results on the comprehensive, ecological impact of roller-felling. This research could allow for widespread application in remote areas where logging is unfeasible and in Wildland Urban Interface areas, where prescribed fire can pose a risk to communities. Additionally, this will set groundwork for long-term monitoring of roller-felled areas, furthering understanding of aspen regeneration dynamics, and will apply to forest and fire management regionally, where goals are to reduce fire risk and maintain aspen communities across the western US.

Alternate silviculture systems, such as small patch clearcuts, may become important in the development of sustainable forest management strategies in aspen forests. A study was initiated in a 64-yr-old trembling aspen (Populus tremuloides Michx.) stand in Meadow Lake Provincial Park, Saskatchewan to determine changes in nutrient dynamics and secondary succession following patch clear-cutting. One hectare replicated patch cuts were logged in the winter of 1993–1994. There was no site preparation following harvest. Annual vegetation, regeneration and soil and plant nutrient data were collected annually, 1 yr prior to, and 3 yr following harvest. Aspen regeneration was 48 375 stems ha−1 2 yr after treatment. Stand (alpha) diversity of the understory was not affected by harvesting, although annual species turnover (beta diversity) increased slightly after harvest. There was an increase in aspen foliar nitrogen (N) for 2 yr following harvest. Soil nutrient concentrations did not differ between patch cut and unharvested plots except total potassium (18% lower in the LFH of the harvested treatments 3 yr after harvesting). Small patch cuts with the litter horizon kept intact may be appropriate for aspen-dominated stands in the boreal mixed wood to minimize ecological disturbance while maintaining aesthetics within a park setting. Key words: Ammonium, aspen regeneration, diversity, nitrate, plant community structure, soil productivity

Prescribed fire is commonly used for restoration, but the effects of reintroducing fire following a century of fire exclusion are unknown in many ecosystems. We assessed the effects of three prescribed fires, native ungulate browsing, and conifer competition on quaking aspen (Populus tremuloidesMichx.) regeneration in four small groves (0.5 ha to 3.0 ha) in Lassen Volcanic National Park, California, USA, over an 11 yr period. The effects of fire on aspen regeneration density and height were variable within and among sites. Post-fire aspen regeneration density generally decreased with greater conifer basal area (rs= −0.73), but there was a wide range of aspen regeneration densities (4000 to 36 667 stems ha−1) at transects with no live conifers post-fire. The height of aspen regeneration increased as a function of increasing years-since-fire (1 yr to 11 yr), but heavy browsing by mule deer (Odocoileus hemionusRafinesque) may alter future growth trajectories. Median percent of aspen regeneration browsed was high in burned (91 %) and unburned (81 %) transects. Only 7 % (282 stems ha−1to 333 stems ha−1) of post-fire aspen regeneration in 11-year old burns exceeded the height necessary to escape mule deer browsing (150 cm). Browsing may also be altering aspen growth form, such that multi-stemmed aspen regeneration was positively associated with proportion of aspen regeneration browsed. These four case studies indicate that the effects of prescribed fires on quaking aspen in the southern Cascade Range of northern California were highly variable and, when coupled with biotic factors (such as deer browsing and competing vegetation) and varying fire severity, fire may either benefit or hasten the decline of small aspen groves.

Research Highlights: Black spruce (Picea mariana Mill.) and trembling aspen (Populus tremuloides Michx.) both regenerated vigorously after wildfire. However, pure semi-upland black spruce stands are at increasing risk of changing successional trajectories, due to greater aspen recruitment. Background and Objectives: Black spruce and aspen are found across the boreal forest with black spruce dominating lowlands and aspen being common in uplands. Both species are well adapted to wildfire with black spruce holding an aerial seedbank while aspen reproduce rapidly via root suckering. In the summer of 2016, the Horse River wildfire burned 589,617 hectares of northern Alberta’s boreal forest. Methods: We assessed early regeneration dynamics of both pure aspen and pure black spruce forests. For black spruce, 12 plots were established in both bog and semi-upland habitats to assess seedling regeneration and seedbed availability. For aspen, 12 plots were established in each of the low, moderate, and high burn severities, as well as 5 unburned plots. Results: Post-fire black spruce regeneration densities did not differ between bog and semi-upland habitats, but were positively correlated with forb cover and charred organic matter seedbeds. Aspen regeneration within pure black sprue stands was substantial, particularly in semi-upland habitats, indicating a potential shift in successional trajectory. Fire severity did not significantly affect aspen regeneration in pure aspen stands, but regeneration density in all severity types was >90,000 stems ha−1. Aspen regeneration densities were negatively related to post-fire forb and shrub cover, likely due to competition and cooler soil temperature.

Post-fire aspen (Populus tremuloides) regeneration varies in response to winter precipitation across a regional climate gradient

AimFire suppression, elk (Cervus elaphusErxleben) browsing and drought have been suggested as possible explanations for low aspen (Populus tremuloidesMichx.) ramet regeneration in elk winter range of the Greater Yellowstone Area (GYA) during the twentieth century. This study analyses the relationship between the rates of aspen regeneration, biophysical factors and human land use since 1830. This approach reveals the importance of indirect human impacts, especially through fire and elk management strategies, on forest structure.LocationThis study was conducted in and around the winter range of the Jackson Hole elk herd in western Wyoming, USA. Aspen stands in this region represent the ecotone between coniferous forest and sagebrush‐grassland vegetation.MethodsAge structures of aspen stands were reconstructed from tree‐rings in order to determine how variation in drought, elk populations and fire occurrence may have affected aspen ramet regeneration since 1830. The effects of recent prescribed fires and elk browse on aspen regeneration were also evaluated by measuring stem height, browse intensity and age of ramets <2 m tall in prescribed burns and in unburned areas.ResultsAspen ramet regeneration has occurred consistently but at low frequencies since 1830, with three peaks of regeneration: 1860–85, 1915–40 and 1955–90. Periods of frequent ramet regeneration coincided with low to moderate elk populations and aspen regenerated only sporadically when elk populations were high. Based on a comparison of the age structures with a tree‐ring reconstruction of Palmer drought severity index (PDSI), observed PDSI and annual precipitation, drought variability appears unrelated to episodes of aspen regeneration. Recent regeneration patterns suggest that fire does not enhance the recruitment of tree‐sized aspen stems in the presence of elk browse, although sample sizes were small. Since 1900, elk hunting and spatially clustered elk feeding in the study area has facilitated low but consistent regeneration of aspen ramets in the twentieth century. In contrast, extremely low ramet regeneration has been observed in national parks of the Rocky Mountains, where elk have been managed according to the `natural regulation' policy (no hunting or feeding) since 1969.Main conclusionsOver time, different management strategies have altered the interactions between fire, herbivory and aspen regeneration suggesting that management history and the causes of change in management, should be explicitly included in ecological studies and future management strategies. These results also point to the value of using the spatial and temporal variation in human interactions with ecological systems as a method for understanding ecological relationships.

Recent concern regarding the potential decline of quaking aspen (Populus tremuloides Michx.) forests in the western United States has sparked concern over whether the species can be effectively regenerated. Using a retrospective approach, we quantified the response of regenerating aspen stems to an ordinary set of silvicultural treatments conducted over approximately the past decade in southern Utah, USA. A suite of variables describing stand structure and composition, stand vigor, physiographic factors, herbivore pressure, and treatment types were measured to predict the possible controls, as well as their relative importance, on aspen regeneration. Results suggested that aspen regeneration was most strongly related to browsing pressure, site preparation technique, and the presence of advance reproduction before treatment, which is a novel finding. Secondary predictors included elevation, site index, and overstory conditions, which are generally characteristics of stand vigor. Management recommendations based on our results should recognize the strong primary control that browsing pressure exerts on regeneration. First, the height of advance reproduction is inherently dependent on antecedent herbivory and also indicative of present browsing and should be assessed before treatment. Second, the most effective site preparation techniques, namely broadcast burning and browsing reduction, will directly reduce browsing pressure, assuming ungulate populations are not too large. Any management targeting timely and effective aspen regeneration should incorporate monitoring and (or) controlling browsing pressure, both before and after treatment.

Heavy and repeated ungulate browsing on reproductive suckers has limited quaking aspen (Populus tremuloides Michx.) regeneration on many Western landscapes. However, little is known about the specific effects of season and intensity of browsing. The objectives of this study were to determine the effects of season and intensity of clipping (simulated browsing) on density and growth characteristics of suckers. Three randomly selected stands were clear-felled in mid-July 2005 and fenced. Simulated browsing intensities of 0, 20, 40, and 60% removal of the current year's growth on aspen suckers were randomly applied in early, mid, and late summers of 2006 and 2007 on permanently demarcated quadrats. Sucker density and sucker height were monitored in each quadrat. Changes in annual sucker numbers for the ends of 2006 and 2007 were compared with declines found that reflected both winter and summer (2007 only) mortality. Mortality was not found for suckers clipped in early summer compared with mortalities of 48 and 46% for mid and late summer clipped suckers, respectively. However, even at the highest mortality, there were still ample numbers (∼25,000/ha) of suckers for stand regeneration. At the end of the study, sucker height was reduced by all summer treatment intensities. After adjustment for the controls, clipping (of current year's growth) at 20 and 40% in mid and late summer resulted in suckers that were 47 and 41 cm tall, respectively. However, clipping at 60% resulted in suckers that were only 20 cm tall. Application of information found in this study to new areas under similar circumstances suggests that browsing of terminal leaders at intensities ≤40% in midsummer for 1 or 2 years would not have a substantial impact on aspen regeneration.

The paucity of aspen (Populus tremuloides) regeneration in the western United States and on Yellowstone National Park's (YNP) northern range has been of concern to managers and scientists for much of the 20th century, with the effects of ungulate browsing, climate fluctuation, and fire suppression being vigorously debated. We analyzed the interaction of fire and elk (Cervus elaphus) browsing in YNP, specifically the role of coarse woody debris as a mechanism for assisting aspen regeneration. We hypothesized that fallen conifers killed in the 1988 YNP fires would provide refugia, allowing a limited amount of aspen regeneration under the current levels of heavy ungulate browsing. We located burned sites on YNP's northern range and searched for “jackstraw piles,” where fallen conifers provided aspen refugia from ungulate browsing. We discovered that aspen suckers protected by fallen conifer barriers were on average over two times the height of adjacent unprotected suckers. Paired t-tests showed a highly significant difference between the aspen heights within the protected jackstraw sites (>0.8 m high) and those in the open that were subjected to elk browsing (P = 0.000). These results illustrate the role that fallen conifers can play in aspen regeneration as well as the interaction of the ecological processes of wildfire, ungulate browsing, and seral stage development. West. J. Appl. For. 16(2):61–64.

The increasing commercial demand for quaking aspen (Populustremuloides Michx.) has sparked renewed interest in aspen silvics and management. While aspen generally regenerates vigorously by root suckering following clear-cutting, recent reports of unsuccessful regeneration suggest a linkage to harvesting activities. Accordingly we evaluated the effects of harvest season and equipment trafficking on aspen regeneration in a glaciolacustrine landform in northern Minnesota. Regeneration vigor, as measured by stem density, stem height, and crown closure was greater in winter-harvested areas. Increased equipment trafficking significantly reduced regeneration in stands harvested during the summer. We detected no significant increases in soil density associated with equipment trafficking; however, reduced regeneration was correlated with increased amounts of rutting and soil scarification. For a given level of trafficking, aspen regeneration increased along the annual chronosequence of harvesting later and later in the summer.

Core Ideas Multivariate statistics and fuzzy logic analysis were used to assess aspen regeneration. Vegetation indices were the dominant factors in determining regeneration suitability. Vegetation indices were significantly higher in high suitability areas. Skidder traffic was significantly lower on high regeneration suitability land. The percentage of slash coverage was significantly lower on high suitability land. Vigorous aspen ( Populus tremuloides Michx.) regeneration immediately following a harvesting event is important to ensuring the continued health and productivity of the future forest. This study aimed to examine the potential of using unoccupied aerial vehicle, multispectral remote sensing, and GIS mapping techniques to develop a comprehensive approach for predicting aspen regeneration success at the harvest block scale. Three winter harvested blocks were studied at Duck Mountain Provincial Park in east‐central Saskatchewan, Canada. Ten regeneration predictor variables (number of skidder passes, percentage slash coverage, topographic wetness index, slope, aspect, slope position, and four vegetation indices: green normalized vegetation index [GNDVI], normalized red‐edge index [NDRE], simple RED to NIR ratio [SR], and chlorophyll index green [CIG]) were determined for 168 measurement plots 1 yr after harvest. Principal component analysis, principal component regression, fuzzy logic analysis, and GIS mapping techniques, were combined for the first time in this study to determine cumulative effects on aspen regeneration. On average, low suitability areas had significantly more skidder traffic (34 passes) compared to below average (17), above average (10), and high (7) suitability areas. Low suitability areas also had significantly more slash coverage (13.1%) compared to below average (8.49%) or high suitability land (7.18%). High suitability areas had significantly higher GNDVI, NDRE, SR, and CIG indices, compared to low and below average suitability land. Not only does this method of analysis help to assess how a combination of factors may influence aspen regeneration, it can also act as a decision support system tool for industry, or government, to improve aspen regeneration assessments.

Using Forest Health Monitoring to assess aspen forest cover change in the southern Rockies ecoregion

Aspen (Populus tremuloides) and lodgepole pine (Pinus contorta var. latifolia) co-occur in the southern Rocky Mountains (USA), where mountain pine beetle (MPB, Dendroctonus ponderosae) has caused extensive lodgepole pine mortality since the late 1990s. Both species excel in post-disturbance high-light environments, but lodgepole pine has generally been thought to establish poorly on undisturbed seedbeds, and aspen suckering may be inhibited by intact aspen overstory. We ask whether lodgepole pine and aspen will regenerate in sufficient quantities to revegetate these forests. We visited a random sample of aspen and lodgepole pine stands across the affected landscape in northern Colorado and southern Wyoming to measure regeneration and overstory mortality. Lodgepole pine regeneration is occurring in 85% of stands, and most stands have >550 stems ha−1. The median aspen sucker density was 6175 stems ha−1. Surprisingly, neither lodgepole pine nor aspen regeneration density was related to overstory mortality level. Animal damage is currently affecting aspen in these forests. Over 50% of stands had damage to 60% or more of their suckers, but 30% of stands had <20% of their stems damaged. Browsed stems were significantly shorter for their ages and were shorter than the 2.5-m height threshold for possible elk browsing. However, the results suggest that sufficient quantities of down lodgepole pine may protect aspen from damage and allow aspen to successfully recruit to the overstory. Multiple regression analysis showed that down lodgepole pine basal area, followed by browsing pressure, were the most important predictors of sucker height and the proportion of suckers browsed. Although 15% of stands had no lodgepole pine regeneration, aspen and lodgepole pine forests are generally regenerating despite animal browsing on aspen. This study is the first to present a regional perspective on regeneration in MPB-affected lodgepole pine and aspen forests, and overall, intervention does not seem necessary to ensure a mix of both species in the future.

The consequences of interactions among genetic, ontogenetic, and environmental factors for the quality of winter-dormant tissues as food for browsing herbivores is poorly understood. We conducted two sequential common garden studies to assess the impacts of intraspecific genetic variation, nutrient availability, prior defoliation, and ontogenetic stage on the chemical quality of winter-dormant tissue in quaking aspen (Populus tremuloides Michx.). In the first study, saplings of 12 aspen genotypes were grown under low and high soil nutrient conditions, with or without two successive seasons of defoliation. Quantity and quality of current year's twig growth were assessed. Twig production varied among genotypes and declined under low nutrient availability, but showed little response to prior defoliation. Chemical quality of sapling twigs varied substantially among genotypes, and in response to nutrient availability and prior defoliation. Overall, browse quality improved (nitrogen levels increased while phenolic glycoside and condensed tannin levels decreased) after defoliation. Growth and chemical variables exhibited low to moderate clonal repeatability (broad sense heritability) values. Our second study employed the same 12 genotypes, grown under high-nutrient conditions and with or without two seasons of defoliation. The trees were coppiced to produce root sprouts, which were chemically assessed 1 yr later. Rejuvenation via coppicing led to increased levels of nitrogen, phenolic glycosides (salicortin), and tannins in root sprouts, and the magnitude of change varied among aspen genotypes. Signatures of defoliation nearly 2 yr earlier persisted in terms of elevated levels of phenolic glycosides in root sprouts of previously defoliated trees. Aspen forests likely present browsing herbivores with chemically heterogeneous environments because of the interactions of genetic, ontogenetic, and environmental factors that vary over space and time.

Learning from spatial variability: Aspen persistence in the Centennial Valley, Montana