Peter J. Weisberg

Forest dynamics and ungulate herbivory: from leaf to landscape

Due primarily to anthropogenic influences such as land use change, exotic species introductions, livestock grazing, altered hunting regimes, and predator control, wild ungulate populations have undergone tremendous shifts in recent decades. Although the result has been endangerment or extirpation in some regions, ungulate numbers have increased to locally or regionally high levels in other areas, causing shifts in plant species composition, problems for forest regeneration, and conflict with humans and domestic livestock. Our ability to find solutions to these problems is limited because we lack sufficient understanding of how ungulate species interact with predators, habitat, forage, competing species, and humans at multiple scales from small foraging patches to large regions. To bring together new findings in this area, encompassing various scales and foci of investigation, an international conference on ‘‘Forest Dynamics and Ungulate Herbivory’’ was held from 3 to 6 October 2001 in Davos, Switzerland. We provide a synthetic overview of the papers contained in this Special Issue, arising from that conference. A companion Special Issue of the Journal for Nature Conservation is devoted to the more management-oriented aspects of forest‐ungulate interactions, entitled ‘‘Forest‐Ungulate Interactions: Monitoring, Modeling and Management.’’ From the papers in this issue, a number of important generalizations emerge to guide our understanding and further research. Our predictive understanding of foraging ecology needs to be generalized and scaled up, if it is to become useful for predicting the consequences of herbivory for broad-scale vegetation dynamics. Further, it is important to move beyond single-factor studies, to embrace the complexity of ungulate‐vegetation interactions. The direction and magnitude of ungulate influences can be difficult to gauge because of complex interactions among species, structural units of vegetation or landscapes, ecosystem processes, and natural disturbances. Researchers need to look beyond the simplistic concept of ‘‘game damage’’, and comprehensively address the direct and indirect effects of ungulates on communities, ecosystems and landscapes. This may require a more systems-oriented, and less species-oriented approach. Ungulate‐vegetation interactions need to be better understood over multiple scales, and particularly at the coarser scales that are of interest to managers and policy-makers. Finally, a long-term view of ungulate‐vegetation interactions, where past, present, and future effects are considered in their appropriate temporal context, is absolutely essential. # 2003 Elsevier Science B.V. All rights reserved.

Abiotic and biotic influences on Bromus tectorum invasion and Artemisia tridentata recovery after fire.

Native sagebrush ecosystems in the Great Basin (western USA) are often invaded following fire by exotic Bromus tectorum (cheatgrass), a highly flammable annual grass. Once B. tectorum is established, higher fire frequencies can lead to local extirpation of Artemisia tridentata ssp. vaseyana (mountain big sagebrush) and have cascading effects on sagebrush ecosystems and the species that depend on them. We conducted a landscape-scale observational study to examine the distribution and cover of B. tectorum and A. tridentata 6 years after a large wildland fire. We used structural equation models to quantify the interacting influences of pre-fire tree canopy cover, perennial species cover, distance from potential seed source, and site environment on post-fire cover of B. tectorum and A. tridentata. Results confirmed a hypothesised negative effect of pre-fire tree canopy cover on post-fire cover of A. tridentata. Site- and landscape-level abiotic factors influenced pre-fire tree canopy cover, which, in turn, influenced the probability of rapid recovery to A. tridentata. However, B. tectorum cover was primarily influenced by a positive effect of incident solar radiation and a negative effect of perennial herbaceous species cover. Restoration efforts to reduce tree canopy cover should be limited to productive sites with sufficient cover of perennial herbaceous species to facilitate site recovery.

Spatial Variation in Tree Seedling and Krummholz Growth in the Forest-Tundra Ecotone of Rocky Mountain National Park, Colorado, U.S.A.

We measured tree seedling and krummholz leader growth in various environments within the alpine forest-tundra ecotone of Rocky Mountain National Park to assess the potential pattern of response to ...

Influence of climate and environment on post-fire recovery of mountain big sagebrush

In arid and semi-arid landscapes around the world, wildfire plays a key role in maintaining species diversity. Dominant plant associations may depend upon particular fire regime characteristics for their persistence. Mountain shrub communities in high-elevation landscapes of the Intermountain West, USA, are strongly influenced by the post-fire recovery dynamics of the obligate-seeding shrub, mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle). This species is a short-distance disperser with a short-lived seedbank, leading to highly variable post-fire recovery times (15-100 years). We investigated the relative importance of site productivity and seasonal climate in explaining the variance in recovery time for 36 fires, comprising a fire chrono-sequence (from 1971 to 2007) for the Great BasinandColoradoPlateau.A.t.vaseyanarecoverywaspositivelyrelatedtoprecipitation inthecoolseasonimmediately following fire, likely because deep soil-water recharge that persists throughout the growing season enhances first-year seedling survival. Percentage sand fraction positively correlated with recovery rate yet negatively correlated with live cover in unburnt stands. Our data support the hypothesis that post-fire recovery rate of A. t. vaseyana depends on the climatically controlled ephemerality of the regeneration niche, as is likely true for many arid-land shrub species. Additional keywords: Artemisia tridentata ssp. vaseyana, Colorado Plateau, fire effects, Great Basin, precipitation variability, succession.

Influences of climate, fire, and topography on contemporary age structure patterns of Douglas-fir at 205 old forest sites in western Oregon.

Knowledge of forest development is basic to understanding the ecology, dynamics, and management of forest ecosystems. We hypothesized that the age structure patterns of Douglas-fir at 205 old forest sites in western Oregon are extremely variable with long and (or) multiple establishment periods common, and that these patterns reflect variation in regional-scale climate, landscape-scale topography, and landscape-scale fire history. We used establishment dates for 5892 individual Douglas-firs from these sites to test these hypotheses. We identified four groups of old forest sites with fundamentally different Douglas-fir age structure patterns. Long and (or) multiple establishment periods were common to all groups. One group described old forests characterized by substantial establishment from the early 1500s to the mid-1600s, with decreasing establishment thereafter. Another group was characterized by peaks of establishment in the middle to late 1600s and in the late 1800s and early 1900s. A third group was...

Longitudinal- and transverse-scale environmental influences on riparian vegetation across multiple levels of ecological organization

Riparian vegetation is distinct from adjacent upland terrestrial vegetation and its distribution is affected by various environmental controls operating at the longitudinal scale (along the river) or transverse scale (perpendicular to the river). Although several studies have shown how the relative importance of transverse or longitudinal influences varies with the scale of observation, few have examined how the influences of the two scales vary with the level of ecological organization. We modeled vegetation-environment relationships at three hierarchically nested levels of ecological organization: species, plant community, and vegetation type. Our hierarchically structured analyses differentiated the spatial extent of riparian zones from adjacent upland vegetation, the distribution of plant community types within the riparian zone, and the distribution of plant species within community types. Longitudinal gradients associated with climate and elevation exerted stronger effects at the species level than at the community level. Transverse gradients related to lateral surface water flux and groundwater availability distinguished riparian and upland vegetation types, although longitudinal gradients of variation better predicted species composition within either riparian or upland communities. We concur with other studies of riparian landscape ecology that the relative predictive power of environmental controls for modeling patterns of biodiversity is confounded with the spatial extent of the study area and sampling scheme. A hierarchical approach to spatial modeling of vegetation-environment relationships will yield substantial insights on riparian landscape patterns.

Ecosystem Modeling Adds Value to a South African Climate Forecast

Livestock production in South Africa is limited by frequent droughts. The South African Weather Service produces climate forecasts estimating the probability of low rainfall three and six months into the future. We used the ecosystem model SAVANNA applied to five commercial farms in the Vryburg region of the North-West Province, and five communal areas within the Province, to assess the utility of a climate forecast in refining drought coping strategies. Rainfall data from 1970 to 1994 were modified to represent a drought (225 mm of rainfall) in 1977/1978, and used in simulations. In a simulation on an example commercial farm we assumed a forecast was available in 1977 portending an upcoming drought, and that the owner sold 490 cattle and 70 sheep prior to the drought. Over the simulation period, the owner sold 31% more cattle when the forecast was used,versus when the forecast was ignored. Populations of livestock on both commercial and communal farms recovered more quickly following the drought when owners sold animals in response to the forecast. The economic benefit from sales is being explored using optimization techniques. Results and responses from South African livestock producers suggest that a real-time farm model linked with climate forecasting would be a valuable management tool.

Do beavers promote the invasion of non-native Tamarix in the Grand Canyon riparian zone?

Beavers (Castor canadensis Kuhl) can influence the competitive dynamics of plant species through selective foraging, collection of materials for dam creation, and alteration of hydrologic conditions. In the Grand Canyon National Park, the native Salix gooddingii C.R.Ball (Goodding’s willow) and Salix exigua Nutt. (coyote willow) are a staple food of beavers. Because Salix competes with the invasive Tamarix ramosissima Ledeb., land mangers are concerned that beavers may cause an increase in Tamarix through selective foraging of Salix. A spatial analysis was conducted to assess whether the presence of beavers correlates with the relative abundance of Salix and Tamarix. These methods were designed to detect a system-wide effect of selective beaver foraging in this large study area (367 linear km of riparian habitat). Beavers, Salix, and Tamarix co-occurred at the broadest scales because they occupied similar riparian habitat, particularly geomorphic reaches of low and moderate resistivity. Once the affinity of Salix for particular reach types was accounted for, the presence of Salix was independent of beaver distribution. However, there was a weak positive association between beaver presence and Salix cover. Salix was limited to geomorphic settings with greater sinuosity and distinct terraces, while Tamarix occurred in sinuous and straighter sections of river channel (cliffs, channel margins) where it dominated the woody species composition. After accounting for covariates representing river geomorphology, the proportion of riparian surfaces covered by Tamarix was significantly greater for sites where beavers were present. This indicates that either Tamarix and beavers co-occur in similar habitats, beavers prefer habitats that have high Tamarix cover, or beavers contribute to Tamarix dominance through selective use of its native woody competitors. The hypothesis that beaver herbivory contributes to Tamarix dominance should be considered further through more mechanistic studies of beaver foraging processes and longterm plant community response.

Modeling fire and landform influences on the distribution of old-growth pinyon-juniper woodland

Expansion of Pinus and Juniperus species into shrub steppe in semi-arid regions of the western United States has been widely documented and attributed in part to fire exclusion. If decreased fire frequency has been an important cause of woodland expansion, one would expect to find age structures dominated by younger trees on more fire-prone sites, with old-growth pinyon-juniper woodland limited to sites with lower fire risk. We compared current old-growth distribution with spatial models for fire risk in a 19-km2 watershed in central Nevada, USA. Multiple GIS models were developed to represent fire susceptibility, according to abiotic factors representing fuels and topographic barriers to fire spread. We also developed cellular automata models to generate fire susceptibility surfaces that additionally account for neighborhood effects. Rule-based GIS models failed to predict old-growth distribution better than random models. Cellular automata models incorporating spatial heterogeneity of site productivity predicted old-growth distribution better than random models but with low accuracy, ranging from 58% agreement at the single-pixel (0.09-ha) scale to 80% agreement for 20-pixel neighborhoods. The best statistical model for predicting old-growth occurrence included the negative effect of topographic convergence index (local wetness), and the positive effects of solar insolation and proximity to rock outcrops. Results support the hypothesis that old-growth woodlands in the Great Basin are more likely to occur on sites with low fire risk. However, weak relationships suggest that old-growth woodlands have not been confined to fire-safe sites. Conservation efforts should consider the landscape context of old-growth woodlands across a broad landscape, with an emphasis on conserving landscape variability in tree age structure.

Influences of Climate and Land Use History on Forest and Timberline Dynamics in the Carpathian Mountains During the Twentieth Century

The forest-tundra mountain ecotone, associated with the alpine timberline and treeline, is reported to have risen in elevation in many parts of the world as a result of global warming. The main goal of this study was to analyze the changes in timberline position over the Carpathian mountain range with respect to observed climate change and other global change issues, such as land use change. Global climate change has resulted in periods of significant warming in the Carpathian region, with the most significant of these occurring over 1975–2000. Forest cover change was analyzed for elevations above 1,000 m between 1880 and 2000 by comparing military maps of the Austro-Hungarian Empire and Landsat imagery. The whole region had a forest cover ratio of 72.8 % in 1880, decreasing to 71.2 % in 2000. There has been considerable forest increase at timberline (34 % of the total afforestation), indicating that substantial changes are happening at this ecotone, similarly to other European mountains; 12 % of total deforestation was observed also at timberline. Forest cover changes varied among regions, increasing by 8 % in the West Carpathians, by 1.4 % in the Ukrainian Carpathians, and decreasing in the North Romanian and South Carpathians by 6.1 and 0.7 %, respectively. Forest cover in the Ukrainian, North Romanian, and South Carpathians has declined at lower elevations, which can be attributed to widespread illegal logging in post-socialist times. At higher, less accessible elevations forest cover has mostly increased. In the West Carpathians forest cover has evenly risen at all elevations—a result of the declining importance of agriculture and increasingly sustainable forestry practices. We consider these observed changes to be a result of favorable climate change coupled with ongoing land use change.