Paul C. Rogers

Ecology, biodiversity, management, and restoration of aspen in the Sierra Nevada

This report was commissioned by the USDA Forest Service Lake Tahoe Basin Management Unit to synthesize existing information on the ecology and management of aspen (Populus tremuloides) in the Sierra Nevada of California and surrounding environs. It summarizes available information on aspen throughout North America from published literature, internal government agency reports, and experienced scientists and managers. The historic distribution, abundance, and ecologic role of aspen in the Sierra Nevada are discussed, along with the reproductive physiology of aspen. Issues that affect aspen health and vigor in the Sierra Nevada and elsewhere are covered, along with methodology for assessing the condition of aspen and monitoring the effects of management activities to restore and maintain aspen. Descriptions of the types of aspen that occur in the Sierra Nevada are presented along with alternative techniques to manage and restore aspen that are applicable wherever aspen is found.

Tree species diversity, richness, and similarity between exotic and indigenous forests in the cloud forests of Eastern Arc Mountains, Taita Hills, Kenya.

Biodiversity assessment for tree species was conducted in three forest fragments ofthe Taita Hills, southeastern Kenya to compare species diversity between and within three exotic forest plantations of pine, eucalyptus, cypress and the indigenous forests. The study sites were: Ngangao (120 ha), Chawia (86 ha), and Mbololo (185 ha). A Y-plot design was used to sample 32 plots comprising of 65 subplots. At each subplot, all juvenile trees of 5 cm and above in diameter at breast height (DBH) were enumerated and recorded by species. Tree regeneration (seedlings and saplings) was tallied by species. The Shannon-Weiner Index was used to calculate species diversity and evenness. The derived Shannon’s indices were further converted into effective numbers to show the magnitude of differences in species biodiversities. To evaluate differences in species diversities, a one way ANOVA was conducted and to separate the means, Tukey’s HSD and Duncan’s tests were used for even and uneven number of samples respectively. Jaccard’s similarity index was used to assess species similarities. There were more than 58 species whose stem densities varied between 10 and 2 000 trees per hectare. There were significant differences in species diversities between forest types and sites; the indigenous forests showed higher diversities than the exotic forests. Similarly, Chawia sites had higher species diversity than both Ngangao and Mbololo. Chawia also had a higher number of regenerated species than the two other sites, including species such as Xymalos monospora, Rapanea melanophloeos, and Syzygium guineense, which are associated with low levels of disturbance. These findings indicate that the indigenous forest is more diverse in species as would be expected in the tropics. The high species diversity in Chawia could be accounted for by the higher levels of disturbance it underwent, unlike the two other sites. The regeneration of species associated with low levels of disturbance found in the exotic plots of Chawia show the likelihood of presence of long-term soil seed banks. The low regeneration in the exotics plots observed in Ngangao and Mbololo are likely due to the absence of seed banks since some of the plantations were established on bare land (in Ngango), or the inherent physiology (allelopathy) of some of species repelling the regeneration of others.

Aspen in the Sierra Nevada: regional conservation of a continental species

Abstract Quaking aspen (Populus tremuloides Michx.), a common species in North America, is a minor species in the Sierra Nevada of California. However, the limited coverage of aspen in this area appears to carry a disproportionate biodiversity load: numerous species are dependent on the unique components of aspen forests for habitat. Land managers in the region believe the species is declining due to fire suppression policies of the past century. Recent research from other regions shows mixed results when assessing the extent of decline. This review focuses on the crossroads between human and natural history to describe a broader picture of aspen ecology in the Sierra Nevada. The method used here combines a review of the ecological literature with historical synthesis. A central conclusion is that the current “decline” in aspen must be placed in the context of an unusual regeneration pulse brought on by intensive Euro-American resource extraction activities of the late 19th century. We address unique features of the Sierra aspen population, the interface of climate change and human-caused disturbance, and conservation strategies for restoration of an aspen community more closely aligned with contemporary climate-disturbance cycles. Conservation recommendations include reintroduction of mixed-severity natural fires and complimentary wildlife, such as top predators, where practical, plus allowance for local flexibility where deviations are appropriate based on ecology and social concerns.

Forest Health Monitoring In the Ngangao Forest, Taita Hills, Kenya: A Five Year Assessment Of Change

ABSTRACT Forest Health Monitoring (FHM) provides a standardized detection-level survey of forest and tree characteristics for large forested areas. We have adopted FHM methods from this temperate-based program to tropical forests in the Eastern Arc Mountains (EAM) of Kenya and Tanzania. This paper reports the first assessment of trend data in the EAM over a period from 2001 to 2006. Growth and diversity statistics are presented for 11 forest plots in Ngangao Forest; only six (of 45) species constitute the predominance (68%) of basal area growth. Four endemic trees were tallied in our survey: Dasylepis integra, Leptonychia usambarensis, Macaranga conglomerata, and Polyscias stuhlmannii. We contrast diversity statistics with the nearby Chawia Forest and confirm the basic principal of species area relationships. Additionally, we examine visual crown assessments and tree damages. Tree crowns appeared healthy overall, although one species (Albizia gummifera) displayed increased dieback and foliage transparency and decreased crown density. Tree damages increased slightly over the survey period with stem decays constituting the most prominent symptom. Overall, we found no significant change in this first remeasurement of forest conditions at Ngangao Forest. The results reported here, however, may require more intense inspection of conditions. The Forest Health Monitoring approach constitutes a detection-level survey; it is dependent on follow-up by local experts to determine more precise causality where initial problems are found.

Aspen indicator species in lichen communities in the Bear River Range of Idaho and Utah

Abstract. Aspen are thought to be declining in this region due to a combination of fire suppression, grazing and wildlife management practices, and potentially cool/wet climates of the past century which favor advancing conifer succession. Many scientists are concerned that aspens related species may also be losing habitat, thereby threatening the long-term local and regional viability of this important community. To date, few studies have specifically examined the role of aspens epiphytic lichen community. This paper presents basic community research describing the application of Indicator Species Analysis for lichens growing on aspen stems in the central Rocky Mountains of North American. Results show unique lichen assemblages between conifers and aspen — the dominant hardwood of mid-elevations in this region.

Quaking Aspen in the Residential-Wildland Interface: Elk Herbivory Hinders Forest Conservation

ABSTRACT: Quaking aspen (Populus tremuloides) forests are experiencing numerous impediments across North America. In the West, recent drought, fire suppression, insects, diseases, climate trends, inappropriate management, and ungulate herbivory are impacting these high biodiversity forests. Additionally, ecological tension zones are sometimes created at residential-wildland interfaces with divergent management directives. For example, private conservation reserves bordering public land may be degraded from browsing where game species find refuge from hunting and plentiful forage. We examined putative herbivore impacts to nearly pure aspen forests at Wolf Creek Ranch (WCR), a sparsely developed residential landscape in northern Utah. Forty-three one-hectare monitoring plots were established to measure forest attributes including site characteristics, tree and vegetation condition, and herbivore use. Additionally, we tested the ability of a plot-level visual rating system to characterize objective field measures. Results suggest elk (Cervus elaphus) herbivory is currently having a strong effect on aspen in the study area, reducing many locations to nearly single-layer aspen forests dominated by aging canopy trees. Regeneration (<2 m tall stems) is experiencing moderate to high browse impacts, and recruitment (2–6 m stems) is below replacement levels on approximately half of WCRs aspen forests. The visual rating system accurately reflected significant trends in forest cover, canopy height, plot aspect, regeneration, recruitment, and tree mortality. Ordination of plot and forest data indicated a strong negative relationship between elk presence and recruitment success. We make recommendations for addressing difficult herbivore-aspen interactions where publicly managed wildlife present barriers to conservation within residential forest reserves.

Quaking Aspen in Utah: Integrating Recent Science with Management☆

On the Ground Quaking aspen is widely regarded as a key resource for humans, livestock, and wildlife with these values often competing with each other, leading to overuse of aspen in some locations and declines. Wereview trends in aspen science and management, particularly in Utah. Historically, research conducted here holds a prestigious place in international aspen circles. We highlight recent studies continuing the tradition to keep rangeland managers informed of important developments, focusing on aspen functional types, historical cover change and climate warming, ungulate herbivory, and disturbance interactions.

Mule deer impede Pando’s recovery: Implications for aspen resilience from a single-genotype forest

Aspen ecosystems (upland Populus-dominated forests) support diverse species assemblages in many parts of the northern hemisphere, yet are imperiled by common stressors. Extended drought, fire suppression, human development, and chronic herbivory serve to limit the sustainability of this keystone species. Here we assess conditions at a renowned quaking aspen (Populus tremuloides) grove—purportedly the largest living organism on earth—with ramifications for aspen biogeography globally. The “Pando” clone is 43 ha and estimated to contain 47,000 genetically identical aspen ramets. This iconic forest is threatened in particular by herbivory, and current management activities aim to reverse the potential for type conversion, likely to a non-forest state. We set out to gauge agents affecting recent deterioration through a network of monitoring plots and by examining a chronosequence of historic aerial photos to better understand the timing of putative departure from a sustainable course. Sixty-five permanent forest monitoring plots were located in three management regimes existing within Pando: no fencing, fencing with active and passive treatments, fencing with passive-only treatment. At each sample plot we measured live and dead mature trees, stem recruitment and regeneration, forest and shrub cover, browse level, and feces counts as a surrogate for ungulate presence. Ordination results indicate that aspen regeneration was the strongest indicator of overall forest conditions at Pando, and that mule deer (Odocoileus hemionus) presence strongly impacts successful regeneration. Additionally, fencing with active/passive treatments yielded the most robust regeneration levels; however, a fence penetrable by ungulates in the passive-only treatment most likely played a role in this outcome. The aerial photo sequence depicts various human intrusions over the past seven decades, but perhaps most telling, a decline in self-replacement beginning 30–40 years ago. Aspen communities in many locations in North American and Europe are impacted by unchecked herbivory. The Pando clone presents a unique opportunity for understanding browse mechanisms in a forest where tree genotype, closely aligned with growth and chemical defense, is uniform.

The influence of the foodscape on quaking aspen stand condition and use by ungulates

In order to study the effects of herbivory on plant communities, we determined whether the types and concentrations of chemicals present in different aspen (Populus tremuloides Michx.) stands and understories, i.e., the foodscape, are associated with aspen use by elk (Cervus elaphus L.) and with aspen regeneration and recruitment. Transects were established in aspen stands with high, medium, and low regeneration levels (N=5 locations/regeneration level; ranging from 2,331 m to 2,724 m in elevation) in Wolf Creek Ranch in northern Utah. Using non-metric multidimensional scaling (NMDS) ordination and regression analyses, we examined the relationships between aspen regeneration, recruitment, elk presence, browsing, and other landscape elements with the foodscape (e.g., biomass and chemical composition of the understory and chemical defenses of juvenile aspen trees). The foodscape was affected by elevation and canopy height but it did not explain aspen use or indicators of aspen resilience. Our findings suggest that foodscapes of lower nutrient content– occurring at lower elevations under drier climatic conditions–are more likely to foster aspen stands with less forb and grass understory, and thus lower nutritional biomass. Nevertheless, the extent of the decline in the availability of nutrients in the understory did not appear to influence aspen browsing or indicators of aspen resilience. Future research should focus on exploring the influence of additional–and more contrasting–gradients of chemical availability in the landscape on aspen use by herbivores. Keywords— Browsing, Elk, Phenolic glycosides, Plant secondary compounds, Preference.

Reply to Spanhove & Lehouck “Don't Miss the Invasions! A Note on Forest Health Monitoring in the Taita Hills, Kenya”

Spanhove and Lehouck raise several excellent points in their critique of “Forest Health Monitoring in the Ngangao Forest, Taita Hills, Kenya: A Five Year Assessment of Change” (JEANH 97(1): 3-17). Their central argument, that Rogers et al. neglected monitoring of invasive alien species in their assessment of ‘forest health’, cannot be disputed. Though we briefly address the influence of endemic trees (specifically pioneer species), and contrast those influences between Chawia (a highly disturbed forest) and Ngangao (a moderately disturbed forest), we do not extend our assessment to other invasive exotic vegetation. Thus, our study constitutes an assessment of change in the condition of the Ngangao Forest (primarily trees) and misses other elements of the ecosystem, such as the invasive species component correctly pointed out by Spanhove & Lehouck. In addition to alien species, there are enumerable components critical to ecosystem ‘health’ not examined here (e.g. forest soils and erosion, nutrient cycling, epiphytes, bioindicators of air quality, understorey vegetation, invertibrates, wildlife, etc.).