Advances in North American Aspen Ecology chaired by Paul Rogers

Research on Aspen Ecology From a Canadian Perspective

S. Landhausser — Tue, 23 Jun 2009 08:40:00 PDT

Trembling aspen has its core distribution in the boreal mixedwood region of Canada, where it also achieves its highest productivity. As a result there has been significant interest and research activity in Canada focusing on aspen in the past and present. While early on, most aspen research dealt with basic ecology and mostly with the management of aspen as an undesirable species, the direction of aspen research has undergone a significant change since the mid and late eighties. At that time research on aspen gained significant momentum, as technological advances allowed for the processing of aspen fiber for pulp and engineered wood products. In the mid-nineties, the increasing impact of harvesting in the boreal forest and public pressure to develop sustainable forest management practices resulted in a further shift of aspen research to include areas such as aspen regeneration, mixedwood management, and the exploration of the role of aspen in boreal forest biodiversity. In the last 10 years, there has been an increase in research on the use of aspen as a plantation species. Overall, significant advances have been made in a broad range of areas such as aspen ecology and physiology, population ecology, stand dynamics, and genetics in order to address issues applicable to silviculture, mixedwood management, and forest reclamation and restoration in the boreal forest zone. This presentation will give a brief overview over recent research directions in aspen ecology and management and will explore research needs particular to aspen throughout its range in Canada.

Aspen Ecology in the United States: Recent Advances and Future Needs

Paul C. Rogers — Tue, 23 Jun 2009 08:00:00 PDT

This presentation (along with a similar Canadian perspective) will offer a context for subsequent papers in the Advances in North American Aspen Ecology session. Collectively, we hope to outline major areas of research in North America, especially as they relate to perceived management needs. There is a long and rich history of ecological research in aspen systems of the United States. Probably the most striking long-term change, from a managerial perspective, has been a shift toward biological benefits of aspen as opposed to past avoidance, or even active denudation, of these systems in favor of more commercial species. This talk will focus on important contributions of the past decade as it explores vital areas of needed research. Sources for this presentation include a review of salient literature and summation of researcher and manager conclaves that have taken place over the past year in the western U.S. The discussion will be structured around these broad topics which encompass recent advances and expected needs: 1) genetics and phytochemistry; 2) management practices; 3) wildlife interactions and diversity; 4) measuring and monitoring cover change; and 5) adaptive approaches to disturbance and climate. Though groupings present a convenient structure for discussion, they should not be construed as exclusive of one another. The overlap of ecological and social elements elevates the need for strategic and adaptive systems thinking in this field. The Western Aspen Alliance provides a mechanism for facilitating communication among researchers and managers, as well as a center for interdisciplinary fusion.

Landscape Assessment of Browse and Fire Effects on Aspen Woodland Habitat in the Centennial Valley, Gye, Montana

N. Korb et al. — Tue, 23 Jun 2009 09:20:00 PDT

In the Greater Yellowstone Ecosystem, aspen woodlands represent a low proportion of total cover, but these habitats support disproportionately high biological diversity. The extent of aspen woodlands in the Centennial Valley has declined significantly over the past century due to increased conifer density and extent, both associated with fire exclusion. The primary management response is to restore fire through “Appropriate Management Response” policies or prescribed fires. If aspen sprouts are subjected to intense and chronic browsing by wildlife, a fire that kills overstory aspen trees may actually result in loss of resilience and even death of aspen clones. Prior to a series of planned, prescribed fires across the Centennial landscape, we examined browse effects, stand structure, and historic stand conditions at 205 randomly sampled sites across eight study areas, including the 2003 Winslow Burn. Within the burned area, the mortality of conifer trees and overstory aspen released prolific sprouting, but ubiquitous browsing by wild ungulates prevented sprouts from exceeding browse height or replacing overstory trees, except adjacent to major roads. Browse effects were similarly intense across several of the study areas where prescribed fires are proposed. Stand structure within aspen woodlands demonstrated continued replacement by Douglas-fir in 75% of stands sampled. We also documented aspen coarse wood where no living above-ground growth could be found, suggesting death of clones at 6% of sampled sites. Subsequent measures suggest browse effects change over time and may be linked to seasonal weather patterns. The distribution and behavior of wildlife has changed due to land use and wolf removal; managers must recognize the risks and uncertainties associated with restoring fire to systems that are fundamentally different from the past. Continued monitoring will be a critical component for implementing fire plans in the Centennial Valley to ensure management activities meet ecological objectives.

Aspen Condition on Cedar Mountain, Utah: Ecological Indicators of Decline

A. J. Leffler et al. — Tue, 23 Jun 2009 09:40:00 PDT

The condition of quaking aspen (Populus tremuloides) ecosystems in the western United States is of rising concern with several studies reporting considerable loss of aspen from the landscape in the past 150 years. Potentially more alarming are accelerated losses, often termed 'sudden aspen decline', in recent decades. Although there is vigorous debate in the literature regarding region-wide aspen condition, local aspen loss can negatively affect individual stakeholders and local indicators of decline are poorly understood. We examined a large (~ 275 km2) landscape dominated by pure aspen communities in southern Utah with the goals of describing the existing stand structure in detail and determining if local indicators of decline exist. We sampled over 80 aspen-dominated forest plots on Cedar Mountain, Utah using standard forest health monitoring protocols. Major findings include: (1) uniform age of adults within the study area, (2) approximately 10% crown dieback on half of the plots sampled, (3) roughly 50% of the study plots had greater than 50% of the trees with damage to the bole, (4) about 25% of the adult basal area was dead, and (5) over half the plots had few sub-canopy individuals or inadequate regeneration. Physiographic variables including elevation, slope, and aspect were generally not strong indicators of aspen condition, typically explaining less than 15% of the variation in basal area, mortality, dieback, or damage. While healthy stands were rarely observed in the most drought prone physiographic settings, the inverse was not necessarily true; healthy and unhealthy stands were found in more mesic settings. Principle components analysis identified two clusters of plots, those with regeneration and those without; however, no other variables were found to differ between these groupings. We suggest exogenous factors such as land-use history or altered disturbance regimes influence aspen condition on Cedar Mountain, Utah.

Developmental Shifts in Functional Traits of Quaking Aspen

E. Smith et al. — Tue, 23 Jun 2009 10:30:00 PDT

Quaking aspen is a clonal species that can produce suckers throughout its life cycle, resulting in multiple-age classes within a clone. Although each ramet in a clone is genetically identical, we hypothesize that there is physiological variation between the different age classes of the clone. This variation leads to phenotypic diversity, which may increase clonal fitness in response to selection forces. We conducted a study in seven pure aspen clones with 10 identifiable age classes that ranged from first year suckers to overstory trees >150 years old. We measured specific leaf area, leaf defense chemistry (tannins and phenolic glycosides), carbohydrate and leaf nutrients status, stem water potential and leaf photosynthesis. Each tree was aged by coring and then counting annual rings and clonal relationships were confirmed using microsatellite marker analysis. Phenolic glycosides decreased linearly with age, while condensed tannins increased. Photosynthesis and stem water potential declined markedly after age 40. The data suggests that aspen switch their defense strategy from resistance to tolerance once they grow above the mammal browse line, which may allow them to re-allocate resources from defense to growth. Reductions in photosynthesis and leaf water potential as clones age, may result in greater sensitivity to abiotic and biotic stresses.

Fine Root Dynamics of Trembling Aspen in Boreal Forest and Aspen Parkland in Central Canada

B. Pinno et al. — Tue, 23 Jun 2009 11:10:00 PDT

Fine roots constitute a relatively small component of ecosystem carbon pools but they are vitally important in carbon and nutrient cycles because of their role in water and nutrient uptake and high turnover rates. The objective of this study was to compare trembling aspen fine root (<2mm) dynamics, including standing crop, productivity, and turnover rates, between two forest regions. Sites were located in boreal forest near Prince Albert in central Saskatchewan and 500 km south in aspen parkland near Regina. The parkland site is a proxy for the expected future climate of the boreal site. Mini-rhizotrons were established in mature trembling aspen stands at both sites to a depth of 40 cm and were monitored throughout the 2004 growing season. The parkland site had significantly greater fine root biomass (23.2 vs 5.4 Mg/ha, p=0.018) and root length productivity (234.9 vs 29.8 m/m2 image/year, p=0.004) than the boreal site. Fine roots in the parkland site also made up a larger component of total tree biomass than in the boreal site and accounted for a larger percentage of the total net biomass production (50% vs 38%). Root size distribution differed between sites with the parkland site having significantly more standing crop and yearly production of the smallest root sizes (<0.2 mm). Fine root length turnover was similar between sites (41 vs 49 %/year, p=0.385). This study indicates that there are significant differences in trembling aspen fine root dynamics between these forest regions which raise questions about future C cycling and tree productivity in the boreal forest depending on how the fine roots respond to the future climate.

Adverse Influence of Radio Frequency Background on Aspen Seedlings and the Possible Relation to Increasing Atmospheric CO2

K. Haggerty — Tue, 23 Jun 2009 10:50:00 PDT

An experiment investigating possible effects of exposure to the ambient radio frequency (RF) background on aspen seedlings was conducted in rural Colorado at an altitude of 1,700 meters, on a north-facing slope in the Little Thompson River Valley. The location was 0.5 km distant from any RF source, including electrical power lines. In the spring 2007, three treatment enclosures were established: RF shielded (Faraday-caged), mock-shielded, and unshielded. Nine aspen seedlings in gallon pots were placed in each treatment enclosure. All factors: light level, humidity, airflow and temperature were monitored and maintained at equal levels between the shielded and mock-shielded environments. The un-caged seedlings were exposed to higher light level (full sun), had higher airflow, and generally lower humidity, since they were not in a screened enclosure. After two months, the RF-shielded group had produced 74% more active lead length and 60% more leaf area than the mock-shielded group, though both groups produced nearly the same number of leaves. Differences in growth and morphology between the mock-shielded and unshielded groups were slight. In early October 2007, the shielded group’s leaves turned a range of bright fall colors, and those leaves were substantially free of leaf fungus. At the same time, in the two exposed groups, leaves were green-yellow and a high percentage of leaf area was affected by leaf fungus. These results suggest that the manmade RF environment may be adversely affecting growth, dormancy, and resistance to fungus in aspen seedlings. Inhibition of growth in aspen caused by exposure to the RF background would limit their ability to take up CO2 and sequester carbon. This RF interaction with plants may, therefore, be indirectly contributing to the ongoing increase in atmospheric CO2.

Environmental and Physiological Drivers of Aspen to Conifer Succession

S. St. Clair et al. — Tue, 23 Jun 2009 11:30:00 PDT

An important knowledge gap in aspen ecology is a poor understanding of factors that define aspen to conifer succession dynamics. Environmental conditions in the understory change dramatically as conifers establish in aspen stands. Light levels decrease and soil chemistry (reductions in soil N and C) and structure are modified. These two changes likely have important impacts on regeneration success of aspen suckers and conifer seedlings, the outcome of which will influence shifts in stand composition. We conducted a field and greenhouse study to examine how changes in light environment and soil chemistry that occur along aspen-conifer overstory transition zones (pure conifer, aspen-conifer mix, pure aspen, gap) affect the growth, photosynthesis and leaf defense chemistry of regenerating aspen and subalpine fir. Results from the greenhouse and the field study were consistent. Aspen had substantially greater reductions (>50%) in photosynthesis and growth rates than subalpine fir when grown under low light or on conifer modified soils. Maintenance of subalpine fir growth and photosynthesis on conifer modified soils was a function of greater nitrogen acquisition resulting from an increase in their root:shoot ratio. Leaf defense chemistry of aspen was significantly reduced under mixed and pure conifer stands as a result of reduction in light levels. The results suggest that subalpine fir seedlings have higher fitness than aspen suckers as conifer establishment in aspen stands modify light and soil conditions.

Using Root Carbohydrates Reserves as an Indicator of Vulnerability to Defoliation in Trembling Aspen

S. Landhausser et al. — Tue, 23 Jun 2009 11:50:00 PDT

Tent caterpillar (Malacosoma disstria [H¸bner]) and large aspen tortrix (Choristoneura conflictana [Walker]) are native defoliators of trembling aspen (Populus tremuloides Michx.) in the boreal forests of North America. Defoliation events can be sporadic and localized but there can be large scale outbreaks covering hundreds of square kilometers. Large outbreaks are thought to occur in 10-year cycles and can last several years. Generally it is thought that defoliation events have short-term effects on aspen productivity but do not result in significant mortality, as aspen reflushes after these spring defoliation events. However, in combination with other stressors such as drought, it has been observed that aspen clones can be weakened and are more susceptible to stem dieback or even clone mortality. Over a period of 8 years we determined seasonal carbohydrate reserves of different tissues in aspen clones. Non-structural carbohydrate reserves were determined in twig, stem and root samples from 9 different clones. During the collection period some of the aspen clones were defoliated in 2000 and/or 2007. After defoliation, tissue carbohydrate reserves in stems and twigs recovered by the end of the same summer. In contrast, in roots, carbohydrates reserves (particularly starch) were still depressed the second summer after defoliation, relative to clones that were not defoliated. After only one defoliation event starch reserves in the roots were close to zero, suggesting that repeated defoliations could have significant impacts on the survival of aspen clones. The research indicates that root reserves are severely impacted by defoliation and that clones with already low carbohydrate reserves are likely at a higher risk of dieback and mortality and could function as a valuable indicator to assess risks of clonal dieback in aspen.

The Impact of Cattle Grazing on Aspen Regeneration on Crown Lands in Western Manitoba

J. Renton et al. — Tue, 23 Jun 2009 13:30:00 PDT

In North America there has been an increasing appreciation for the value of trembling aspen as a source of timber (Populus tremuloides Michx). Moreover, aspen stands and the understory vegetation that they support also provide valuable forage for livestock and wildlife. Timber harvest and cattle grazing may occur on the same land base, although usually not simultaneously. The purpose of this study was to determine the affects of cattle grazing in post-harvest aspen stands in western Manitoba. In this study, grazed and non-grazed sites are compared to assess the effects of cattle grazing on stem density, tree health, and species diversity in the understory plant community across a seven-year chronosequence of harvests. Environmental data were collected to complement the biological data including soil compaction, soil texture, moisture regime and grazing pressure. Non-grazed sites in the oldest harvests had taller stems and significantly higher stem densities of aspen and all other tree species (p<0.1). Trees in grazed plots also exhibit poorer tree health characteristics than those in the non-grazed plots.