A Structurally Based Analytic Model of Growth and Biomass Dynamics in Single Species Stands of Conifers

A theoretically based analytic model of plant growth in single species conifer communities based on the species fully occupying a site and fully using the site resources is introduced. Model derivations result in a single equation simultaneously describes changes over both, different site conditions (or resources available), and over time for each variable for each species. Leaf area or biomass, or a related plant community measurement, such as site class, can be used as an indicator of available site resources. Relationships over time (years) are determined by the interaction between a stable foliage biomass in balance with site resources, and by the increase in the total heterotrophic biomass of the stand with increasing tree size. This structurally based, analytic model describes the relationships between plant growth and each species’ functional depth for foliage, its mature crown size, and stand dynamics, including the self‐thinning. Stand table data for seven conifer species are used for verification of the model. Results closely duplicate those data for each variable and species. Assumptions used provide a basis for interpreting variations within and between the species. Better understanding of the relationships between the MacArthur consumer resource model, the Chapman–Richards growth functions, the metabolic theory of ecology, and stand development resulted

Genetic aspects of the biodiversity of rangeland plants

Biodiversity is the variety of life and its processes. Diversity cannot be described unless the differences between organisms can be detected and measured. The concept of genetic diversity is usually confined to individual organisms, populations, and species and may be considered as heritable differences among taxa capable of gene exchange. New macromolecular methods together with traditional morphological, cytogenetic, hybridization, and breeding-system analytical methods are providing greater detail that allows a finer resolution of genetic diversity. Rangeland plant biodiversity studies of shrub, forb, grass, and tree taxa are demonstrating genetic diversity data available from rangelands and, in general, rangeland plant genetic diversity studies are in the beginning stages. The influences of past climatic changes on plant genetic diversity studies are in the beginning stages. The influences of past climatic changes on plant genetic diversity are also only just beginning to be understood. Both conservation and use of rangeland plant resources have genetic bases. Genetic diversity studies are important for discovering and documenting the sources and patterns of variation. That information is vital if genetic diversity is to be protected and preserved so that rangeland plant resources can be effectively used and sustained to maintain future options

The Economics of Fuel Management: Wildfire, Invasive Plants, and the Dynamics of Sagebrush Rangelands in the Western United States

In this article we develop a simulation model to evaluate the economic efficiency of fuel treatments and apply it to two sagebrush ecosystems in the Great Basin of the western United States: the Wyoming Sagebrush Steppe and Mountain Big Sagebrush ecosystems. These ecosystems face the two most prominent concerns in sagebrush ecosystems relative to wildfire: annual grass invasion and native conifer expansion. Our model simulates long-run wildfire suppression costs with and without fuel treatments explicitly incorporating ecological dynamics, stochastic wildfire, uncertain fuel treatment success, and ecological thresholds. Our results indicate that, on the basis of wildfire suppression costs savings, fuel treatment is economically efficient only when the two ecosystems are in relatively good ecological health. We also investigate how shorter wildfire-return intervals, improved treatment success rates, and uncertainty about the location of thresholds between ecological states influence the economic efficiency of fuel treatments

Response of Conifer-Encroached Shrublands in the Great Basin to Prescribed Fire and Mechanical Treatments

In response to the recent expansion of piñon and juniper woodlands into sagebrush-steppe communities in the northern Great Basin region, numerous conifer-removal projects have been implemented, primarily to release understory vegetation at sites having a wide range of environmental conditions. Responses to these treatments have varied from successful restoration of native plant communities to complete conversion to nonnative invasive species. To evaluate the general response of understory vegetation to tree canopy removal in conifer-encroached shrublands, we set up a region-wide study that measured treatment-induced changes in understory cover and density. Eleven study sites located across four states in the Great Basin were established as statistical replicate blocks, each containing fire, mechanical, and control treatments. Different cover groups were measured prior to and during the first 3 yr following treatment. There was a general pattern of response across the wide range of site conditions. There was an immediate increase in bare ground and decrease in tall perennial grasses following the fire treatment, but both recovered by the second or third growing season after treatment. Tall perennial grass cover increased in the mechanical treatment in the second and third year, and in the fire treatment cover was higher than the control by year 3. Nonnative grass and forb cover did not increase in the fire and mechanical treatments in the first year but increased in the second and third years. Perennial forb cover increased in both the fire and mechanical treatments. The recovery of herbaceous cover groups was from increased growth of residual vegetation, not density. Sagebrush declined in the fire treatment, but seedling density increased in both treatments. Biological soil crust declined in the fire treatment, with no indications of recovery. Differences in plant response that occurred between mechanical and fire treatments should be considered when selecting management options

Developing a Model Framework for Predicting Effects of Woody Expansion and Fire on Ecosystem Carbon and Nitrogen in a Pinyon-Juniper Woodland

Sagebrush-steppe ecosystems are one of the most threatened ecosystems in North America due to woodland expansion, wildfire, and exotic annual grass invasion. Some scientists and policy makers have suggested that woodland expansion will lead to increased carbon (C) storage on the landscape. To assess this potential we used data collected from a Joint Fire Sciences Program demonstration area to develop a Microsoft Excel™ based biomass, carbon, and nitrogen (N) spreadsheet model. The model uses input for tree cover, soil chemistry, soil physical properties, and vegetation chemistry to estimate biomass, carbon, and nitrogen accumulation on the landscape with woodland expansion. The model also estimates C and N losses associated with prescribed burning. On our study plots we estimate in treeless sagebrush-steppe ecosystems, biomass accounts for 4.5 Mg ha−1 C and 0.3 Mg ha−1 N this is \u3c10% of total estimated ecosystem C and N to a soil depth of 53 cm, but as tree cover increases to near closed canopy conditions aboveground biomass may account for 62 Mg ha−1 C and 0.6 Mg ha−1 N which is nearly 53% of total estimated ecosystem C and 13% of total estimated ecosystem N to a soil depth of 53 cm. Prescribed burning removes aboveground biomass, C and N, but may increase soil C at areal tree cover below 26%. The model serves as a tool by which we are able to assess our understanding of the system and identify knowledge gaps which exist for this ecosystem. We believe that further work is necessary to quantify herbaceous biomass, root biomass, woody debris decomposition, and soil C and N with woodland expansion and prescribed fire. It will also be necessary to appropriately scale these estimates from the plot to the landscape

Dynamics of Western Juniper Woodland Expansion into Sagebrush Communities in Central Oregon

Western juniper (Juniperus occidentalis) woodlands in Oregon have expanded four-fold from 600,000 ha in 1930 to \u3e 2.6 million ha, often resulting in the reduction and fragmentation of sagebrush (Artemisia spp.) communities. We documented dynamics of western juniper across the John Day Ecological Province in central Oregon by recording size class and growth form at 178 sites. We used stratified random sampling, with strata based on vegetation association (sagebrush, juniper, other) and distance from juniper stands. Only 26 percent of sites contained pre-settlement trees (in other words, \u3e 140 years old), and \u3c 5 percent of the 2,254 junipers tallied were pre-settlement trees. Mean densities of pre-settlement trees by stratum ranged from 0 to 18 trees/ha, suggesting that historically, juniper was widely scattered across the landscape. Current densities of post-settlement trees ranged from 75 to 211 trees/ha in non-woodland strata to 457 trees/ha in the juniper stratum. Juniper in non-woodland strata was most abundant in sites adjacent to juniper stands and in sagebrush communities. Mean densities of post-settlement trees were greatest in the \u3e 2.0-m tall size class (82 trees/ha), followed by the 0.3 to 1-m tall size class (52 trees/ha). These densities pose substantial risk to sagebrush communities in central Oregon. Questions remain about the extent of western juniper woodlands across the species’ range that have replaced or are expanding into sagebrush communities versus sites that historically supported woodlands. However, our findings suggest that within sagebrush communities of the John Day province, intensive management through removal of western juniper may be prudent, while retaining pre-settlement trees

A Management-Oriented Classification of Pinyon-Juniper Woodlands of the Great Basin

Pinyon-juniper woodlands occupy about 18 percent (7.1 million ha, 17.6 million acres) of the land area of the Great Basin (Tueller and others 1979). The associated tree species are found over a wide range of environmental conditions extending from communities representative of the upper fringes of the Mohave Desert to communities found at the lower fringes of high mountain forests. Over this spatial and elevational range, communities associated with pinyon-juniper woodlands are highly variable, with complex distribution and compositional patterns. This variability is due to climatic changes occurring over the last 10,000 years and to variation in current environmental conditions (Nowak and others 1994a; Tausch and others 1993). While juniper has been present somewhere in the area for over 30,000 years (Nowak and others 1994a,b), pinyon is a relatively recent addition with a presence ranging from less than 2,000 to about 8,000 years depending on location. Over the last century many changes have occurred in these woodlands and both the types and the pace of change could potentially increase into the future. In order to successfully inventory, plan, manage, and monitor complex wildlands like the pinyon-juniper woodlands, ecological classification is required. Ecological classifications result in several benefits. The resulting hierarchy of strata can provide guidelines for the collection and retrieval of both factual and interpretive information. Results and experiences from particular sites can be compared to other unstudied sites that are shown to be relatively similar by classification. This can increase the chances of the repetition of successful management actions and reduce the chances of failure. Research, particularly that research attempting to refine interpretations of actual data, can also be better focused if sites are related to an existing classification scheme. Creation of a hierarchy of ecological strata of increasing similarity enhances interpretation through both extrapolation and interpolation of survey data, research results, and management experiences

Woodland Expansion\u27s Influence on Belowground Carbon and Nitrogen in the Great Basin U.S.

Vegetation changes associated with climate shifts and anthropogenic disturbance can have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate (Rydb.) Boivin) ecosystems. Sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. and Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. offers the possibility of increased organic carbon (OC) storage on the landscape; however, little is currently known about the distribution of OC on these landscapes, or the role that nitrogen (N) plays in OC retention. We quantified the relationship between tree cover, belowground OC, and total below ground N in expansion woodlands at 13 sites in Utah, Oregon, Idaho, California, and Nevada, USA. One hundred and twenty nine soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90 cm. Soil, coarse fragments, and coarse roots were analyzed for OC and total N. Woodland expansion influenced the vertical distribution of root OC by increasing 15–30 cm root OC by 2.6 Mg ha−1 and root N by 0.04 Mg ha−1. Root OC and N increased through the entire profile by 3.8 and 0.06 Mg ha−1 respectively. Woodland expansion influenced the vertical distribution of soil OC by increasing surface soil (0–15 cm) OC by 2.2 Mg ha−1. Woodland expansion also caused a 1.3 Mg ha−1 decrease in coarse fragment associated OC from 75–90 cm. Our data suggests that woodland expansion into sagebrush ecosystems has limited potential to store additional belowground OC, and must be weighed against the risk of increased wildfire and exotic grass invasion

Resilience and Resistance of Sagebrush Ecosystems: Implications for State and Transition Models and Management Treatments

In sagebrush ecosystems invasion of annual exotics and expansion of piñon (Pinus monophylla Torr. and Frem.) and juniper (Juniperus occidentalis Hook., J. osteosperma [Torr.] Little) are altering fire regimes and resulting in large-scale ecosystem transformations. Management treatments aim to increase resilience to disturbance and enhance resistance to invasive species by reducing woody fuels and increasing native perennial herbaceous species. We used Sagebrush Steppe Treatment Evaluation Project data to test predictions on effects of fire vs. mechanical treatments on resilience and resistance for three site types exhibiting cheatgrass (Bromus tectorum L.) invasion and/or piñon and juniper expansion: 1) warm and dry Wyoming big sagebrush (WY shrub); 2) warm and moist Wyoming big sagebrush (WY PJ); and 3) cool and moist mountain big sagebrush (Mtn PJ). Warm and dry (mesic/aridic) WY shrub sites had lower resilience to fire (less shrub recruitment and native perennial herbaceous response) than cooler and moister (frigid/xeric) WY PJ and Mtn PJ sites. Warm (mesic) WY Shrub and WY PJ sites had lower resistance to annual exotics than cool (frigid to cool frigid) Mtn PJ sites. In WY shrub, fire and sagebrush mowing had similar effects on shrub cover and, thus, on perennial native herbaceous and exotic cover. In WY PJ and Mtn PJ, effects were greater for fire than cut-and-leave treatments and with high tree cover in general because most woody vegetation was removed increasing resources for other functional groups. In WY shrub, about 20% pretreatment perennial native herb cover was necessary to prevent increases in exotics after treatment. Cooler and moister WY PJ and especially Mtn PJ were more resistant to annual exotics, but perennial native herb cover was still required for site recovery. We use our results to develop state and transition models that illustrate how resilience and resistance influence vegetation dynamics and management options

Response of Conifer-Encroached Shrublands in the Great Basin to Prescribed Fire and Mechanical Treatments

In response to the recent expansion of piñon and juniper woodlands into sagebrush-steppe communities in the northern Great Basin region, numerous conifer-removal projects have been implemented, primarily to release understory vegetation at sites having a wide range of environmental conditions. Responses to these treatments have varied from successful restoration of native plant communities to complete conversion to nonnative invasive species. To evaluate the general response of understory vegetation to tree canopy removal in conifer-encroached shrublands, we set up a region-wide study that measured treatment-induced changes in understory cover and density. Eleven study sites located across four states in the Great Basin were established as statistical replicate blocks, each containing fire, mechanical, and control treatments. Different cover groups were measured prior to and during the first 3 yr following treatment. There was a general pattern of response across the wide range of site conditions. There was an immediate increase in bare ground and decrease in tall perennial grasses following the fire treatment, but both recovered by the second or third growing season after treatment. Tall perennial grass cover increased in the mechanical treatment in the second and third year, and in the fire treatment cover was higher than the control by year 3. Nonnative grass and forb cover did not increase in the fire and mechanical treatments in the first year but increased in the second and third years. Perennial forb cover increased in both the fire and mechanical treatments. The recovery of herbaceous cover groups was from increased growth of residual vegetation, not density. Sagebrush declined in the fire treatment, but seedling density increased in both treatments. Biological soil crust declined in the fire treatment, with no indications of recovery. Differences in plant response that occurred between mechanical and fire treatments should be considered when selecting management options.Keywords: Restoration, Single-needle piñon, Nonnative species, Resilience, Cheatgrass, Sagebrush, Western juniper, Utah juniper, Piñon–junipe