Historical and contemporary distributions of carnivores in forests of the Sierra Nevada, California, USA

Aim Mammalian carnivores are considered particularly sensitive indicators of environmental change. Information on the distribution of carnivores from the early 1900s provides a unique opportunity to evaluate changes in their distributions over a 75‐year period during which the influence of human uses of forest resources in California greatly increased. We present information on the distributions of forest carnivores in the context of two of the most significant changes in the Sierra Nevada during this period: the expansion of human settlement and the reduction in mature forests by timber harvest. Methods We compare the historical and contemporary distributions of 10 taxa of mesocarnivores in the conifer forests of the Sierra Nevada and southern Cascade Range by contrasting the distribution of museum and fur harvest records from the early 1900s with the distribution of detections from baited track‐plate and camera surveys conducted from 1996 to 2002. A total of 344 sample units (6 track plates and 1 camera each) were distributed systematically across c. 3,000,000 ha area over a 7‐year period. Results Two species, the wolverine (Gulo gulo) and the red fox (Vulpes vulpes), present in the historical record for our survey area, were not detected during the contemporary surveys. The distributions of 3 species (fisher [Martespennanti], American marten [M. americana], and Virginia opossum [Didelphisvirginiana]) have substantially changed since the early 1900s. The distributions of fishers and martens, mature‐forest specialists, appeared to have decreased in the northern Sierra Nevada and southern Cascade region. A reputed gap in the current distribution of fishers was confirmed. We report for the first time evidence that the distribution of martens has become fragmented in the southern Cascades and northern Sierra Nevada. The opossum, an introduced marsupial, expanded its distribution in the Sierra Nevada significantly since it was introduced to the south‐central coast region of California in the 1930s. There did not appear to be any changes in the distributions of the species that were considered habitat generalists: gray fox (Urocyon cinereoargenteus), striped skunk (Mephitis mephitis), western spotted skunk (Spilogale gracilis), or black bear (Ursus americanus). Detections of raccoons (Procyon lotor) and badgers (Taxidea taxus) were too rare to evaluate. Contemporary surveys indicated that weasels (M. frenata and M. erminea) were distributed throughout the study area, but historical data were not available for comparison. Main conclusions Two species, the wolverine and Sierra Nevada red fox, were not detected in contemporary surveys and may be extirpated or in extremely low densities in the regions sampled. The distributions of the mature forest specialists (marten and fisher) appear to have changed more than the distributions of the forest generalists. This is most likely due to a combination of loss of mature forest habitat, residential development and the latent effects of commercial trapping. Biological characteristics of individual species, in combination with the effect of human activities, appear to have combined to affect the current distributions of carnivores in the Sierra Nevada. Periodic resampling of the distributions of carnivores in California, via remote detection methods, is an efficient means for monitoring the status of their populations.     

Topographic mediation of growth in high elevation foxtail pine (Pinus balfouriana Grev. et Balf.) forests in the Sierra Nevada, USA

Aim Climate variability is an important mediating agent of ecosystem dynamics in cold, semi‐arid regions such as the mountains of western North America. Climatically sensitive tree‐ring chronologies offer a means of assessing the impact of climate variability on tree growth across temporal scales of years to centuries and spatial scales of metres to subcontinents. Our goal was to bring practices from landscape ecology that highlight the impact of landscape heterogeneity on ecological pattern and processes into a dendroclimatic study that shows that the biophysical setting of target trees affects ring‐width patterns. Location This study was conducted at two sites near alpine treeline in the Sequoia National Park, USA (36°30′ 00′ N, 118°30′ 00′ W). Methods We collected stand information and increment cores from foxtail pines (Pinus balfouriana Grev. et Balf.) for eight tree‐ring chronologies in four extreme biophysical settings at two sites using proxies for soil moisture and radiation derived from a digital elevation model. Results Biophysical setting affected forest age–class structure, with wet and bright plots showing high recruitment after 1900 ad , but had no obvious effect on immature stem density (e.g. seedlings). Biophysical setting strongly affected ring‐width patterns, with wet plots having higher correlation with instrumental temperature records while dry plots correlated better with instrumental precipitation records. Ring‐width chronologies from the wet plots showed strong low‐frequency variability (i.e. hundreds of years) while ring‐width chronologies from the dry plots showed strong variability on multidecadal scales. Main conclusions There was a strong association between biophysical setting and age‐class structure, and with ring‐width patterns in foxtail pine. The mediation of ring widths by biophysical setting has the potential to further the understanding of the expression of synoptic‐scale climate across rugged terrain. When combined with remotely sensed imagery, a priori GIS modelling of tree growth offers a viable means to devise first‐order predictions of climatic impacts in subalpine forest dynamics and to develop flexible and powerful monitoring schemes.     

Hybridization between two parapatric ranid frog species in the northern Sierra Nevada,California, USA

Contact zones between species provide a unique opportunity to test whether taxa can hybridize or not. Cross‐breeding or hybridization between closely related taxa can promote gene flow (introgression) between species, adaptation, or even speciation. Though hybridization events may be short‐lived and difficult to detect in the field, genetic data can provide information about the level of introgression between closely related taxa. Hybridization can promote introgression between species, which may be an important evolutionary mechanism for either homogenization (reversing initial divergence between species) or reproductive isolation (potentially leading to speciation). Here, we used thousands of genetic markers from nuclear DNA to detect hybridization between two parapatric frog species (Rana boylii and Rana sierrae) in the Sierra Nevada of California. Based on principal components analysis, admixture, and analysis of heterozygosity at species diagnostic SNPs, we detected two F1 hybrid individuals in the Feather River basin, as well as a weak signal of introgression and gene flow between the frog species compared with frog populations from two other adjacent watersheds. This study provides the first documentation of hybridization and introgression between these two species, which are of conservation concern.     

The savannization of moist forests in the Sierra Nevada de Santa Marta, Colombia

In the Rio Ranchería watershed of the Sierra Nevada de Santa Marta, between 500 and 1500 m, savanna vegetation is interspersed with moist forests. The savannas are composed of native savanna grasses like Aristida adscensionis L., Arundinella sp., Panicum olyroides Kunth, and Schyzachyrium microstachyum (Desv.) Roseng., Arrill & Izag and the African Melinis minutiflora P. Beauv. There is also Curatella americana L. and Byrsonima crassifolia (L.) H.B.K., two typical tree species of the neotropical savannas. Although moist forest patches occur more often on lower slopes and narrow valley bottoms, they can also be found on mid- and upper-slopes and less often on ridges. Thus, these forest patches are not gallery forests as are found throughout the neotropics, but the result of deforestation and fractionation of a continuous forest. A comparison of soil profiles between the savannas and remnant forest patches on the same slope, showed the disappearance of the A and B horizons (approx. 50 cm) under savanna vegetation. The sharp difference between the savanna and forest soils at the Rio Ranchería does not appear to be due to a change in soil water status along a toposequence or differences in the underlying bedrock. We hypothesize that the savannas of the Rio Ranchería watershed, are the result of deforestation and land practices on infertile soils derived from granite. The savannization process was likely initiated by Amerindians by means of the frequent use of fire or clearing lands for the cultivation of maize. The introduction of cattle by Spaniards (c. 1530) and the frequent use of fire to maintain grazing fields, contributed to further degradation of the habitat. While some tropical landscapes recovered their forest cover when human pressure was removed approximately 500 years ago, areas such as the Rio Ranchería watershed have suffered permanent damage. The savannas of this region are likely to remain unless fire is suppressed and soil restoration practices implemented.     

Plant community distribution along water table and grazing gradients in montane meadows of the Sierra Nevada Range (California, USA)

This study aimed to identify dominant plant communities across five wet and mesic meadows in the Sierra Nevada Range (California, USA) and examine the impacts of environmental and grazing gradients on plant community distribution and diversity. Species composition and environmental conditions were recorded in 100 plots over two years. Classification and ordination analyses were used to classify plant communities and identify relationships between community types and both environmental and grazing gradients. We identified the following six plant community types: Carex jonesii, Carex leporinella, Carex nebrascensis, Carex utriculata, Eleocharis pauciflora, and Veratrum californicum. We found strong connections between plant communities and water table variables, with low water table (r ²?=?0.56) and mean water table (r ²?=?0.30) significantly correlated with Axis 1 while high water table (r ²?=?0.29) and elevation were correlated with Axis 3 (r ²?=?0.49). We found significant differences among community types for all three water table variables and for elevation. We found no correlation between grazing and community type classification, but there was a significant difference in grazing levels among community types. The plant communities and relationships to water table found in this work may aid managers in understanding present conditions and identifying future changes in meadow ecosystems.     

Climatic influences on fire regimes in the northern Sierra Nevada mountains, Lake Tahoe Basin, Nevada, USA

Aim The goal of this study was to understand better the role of interannual and interdecadal climatic variation on local pre‐EuroAmerican settlement fire regimes in fire‐prone Jeffrey pine (Pinus jeffreyi Grev. & Balf.) dominated forests in the northern Sierra Nevada Mountains. Location Our study was conducted in a 6000‐ha area of contiguous mixed Jeffrey pine‐white fir (Abies concolor Gordon & Glend.) forest on the western slope of the Carson Range on the eastern shore of Lake Tahoe, Nevada. Methods Pre‐EuroAmerican settlement fire regimes (i.e. frequency, return interval, extent, season) were reconstructed in eight contiguous watersheds for a 200‐year period (1650–1850) from fire scars preserved in the annual growth rings of nineteenth century cut stumps and recently dead pre‐settlement Jeffrey pine trees. Superposed epoch analysis (SEA) and correlation analysis were used to examine relationships between tree ring‐based reconstructions of the Palmer Drought Severity Index (PDSI), Southern Oscillation Index (SOI), Pacific Decadal Oscillation (PDO) and pre‐EuroAmerican fire regimes in order to assess the influence of drought and equatorial and north Pacific teleconnections on fire occurrence and fire extent. Results For the entire period of record (1650–1850), wet conditions were characteristic of years without fires. In contrast, fire years were associated with drought. Drought intensity also influenced fire extent and the most widespread fires occurred in the driest years. Years with widespread fires were also preceded by wet conditions 3 years before the fire. Widespread fires were also associated with phase changes of the PDO, with the most widespread burns occurring when the phase changed from warm (positive) to cold (negative) conditions. Annual SOI and fire frequency or extent were not associated in our study. At decadal time scales, burning was more widespread during decades that were dryer and characterized by La Niña and negative PDO conditions. Interannual and interdecadal fire–climate relationships were not stable over time. From 1700 to 1775 there was no interannual relationship between drought, PDO, and fire frequency or extent. However, from 1775 to 1850, widespread fires were associated with dry years preceded by wet years. This period also had the strongest association between fire extent and the PDO. In contrast, fire–climate associations at interdecadal time scales were stronger in the earlier period than in the later period. The change from strong interdecadal to strong interannual climate influence was associated with a breakdown in decadal scale constructive relationships between PDO and SOI. Main conclusions Climate strongly influenced pre‐settlement pine forest fire regimes in northern Sierra Nevada. Both interannual and interdecadal climatic variation regulated conditions conducive to fire activity, and longer term changes in fire frequency and extent correspond with climate‐mediated changes observed in both the northern and southern hemispheres. The sensitivity of fire regimes to shifts in modes of climatic variability suggests that climate was a key regulator of pine forest ecosystem structure and dynamics before EuroAmerican settlement. An understanding of pre‐EuroAmerican fire–climate relationships may provide useful insights into how fire activity in contemporary forests may respond to future climatic variation.     

Quantitative Evidence for Increasing Forest Fire Severity in the Sierra Nevada and Southern Cascade Mountains, California and Nevada, USA

Recent research has concluded that forest wildfires in the western United States are becoming larger and more frequent. A more significant question may be whether the ecosystem impacts of wildfire are also increasing. We show that a large area (approximately 120000 km²) of California and western Nevada experienced a notable increase in the extent of forest stand-replacing (“high severity”) fire between 1984 and 2006. High severity forest fire is closely linked to forest fragmentation, wildlife habitat availability, erosion rates and sedimentation, post-fire seedling recruitment, carbon sequestration, and various other ecosystem properties and processes. Mean and maximum fire size, and the area burned annually have also all risen substantially since the beginning of the 1980s, and are now at or above values from the decades preceding the 1940s, when fire suppression became national policy. These trends are occurring in concert with a regional rise in temperature and a long-term increase in annual precipitation. A close examination of the climate–fire relationship and other evidence suggests that forest fuels are no longer limiting fire occurrence and behavior across much of the study region. We conclude that current trends in forest fire severity necessitate a re-examination of the implications of all-out fire suppression and its ecological impacts. Author Contributions: Jay Miller designed the study, performed research, analyzed data, and wrote the article. Hugh Safford performed research, analyzed data, and wrote the article. Michael Crimmins performed research and analyzed data. Andi Thode designed the study and performed research.     

Avian population trends in the vulnerable montane forests of the Northern Appalachians,USA

Declines in bird populations are an important issue facing conservationists. Although studies have documented bird declines in a variety of lowland habitats, montane habitats are generally under represented in these investigations. Nevertheless, montane habitats are vulnerable because of their restricted geographic distribution as well as their exposure to environmental stressors such as atmospheric deposition and climate change. We surveyed birds at 768 points on 42 transects in montane spruce-fir forests the White Mountains of New Hampshire from 1993–2003. We detected 17,479 individuals of 73 species during this period, of which 10 were abundant enough for analyses. Of these 10 species, three exhibited significant population declines during the survey period: Yellow-bellied Flycatcher (Empidonax flaviventris), Bicknell’s Thrush (Catharus bicknelli) and Magnolia Warbler (Dendroica magnolia). Two of these species (Yellow-bellied Flycatcher and Bicknell’s Thrush) are considered ecological indicator species for montane spruce-fir forest. Declines in these species are an indication that recent concern on the part of conservationists about montane spruce-fir forest and the birds that inhabit them are justified. Our observation that these trends were not reflected in the National Breeding Bird Survey (BBS) analyses, and that one high priority species, the Bicknell’s Thrush, did not occur on BBS routes in New Hampshire during the survey period, argues for the importance of continued efforts to monitor these habitats.     

Present and past old‐growth forests of the Lake Tahoe Basin,Sierra Nevada,US

Abstract. We described 38 relictual old‐growth stands – with data on the mortality, regeneration, floristic richness, fuel load and disease incidence in our study area in the Tahoe Basin of California and Nevada. The stands are within the lower and upper montane zones (1900–2400 m a.s.l.) and they are rare, occupying < 2% of the land in the Basin's watershed. Correlation matrices and ANOVAs of forest types and conifer species with environmental gradients revealed significant relationships with elevation, distance east of the Sierran crest, slope aspect, annual precipitation, date of complete snow melt, litter depth and degree of soil profile development. Pathogens, parasites and wood‐boring insects were present on 23% of living trees; 16% of all trees were dead. We compared these stands to a reconstruction of pre‐contact Basin forests and to ecologically analogous old‐growth forests of Baja California that have never experienced fire suppression management. Currently, overstorey trees (> 180 yr old) in the Basin stands have ca. 33% cover, 54 m².ha^‐1 basal area and 107 individuals.ha^‐1, values very similar to reconstructions of pre‐contact Basin forests and to modern Baja California forests. Understorey trees (60–180 yr old), however, are several times more dense than historic levels and species composition is strongly dominated by A. concolor, regardless of the overstorey composition. The ratio of Pinus: Abies has increased – and the age structure of extant stands predicts that it will continue to increase – from approximately 1:1 in pre‐contact time to 1:7 within the next century. Disease incidence and mortality in Baja forests were lower. Although we quantitatively defined current Basin old‐growth forests – in terms of stand structure – we realize that our definition will differ from that of both past and future old‐growth forests unless management protocols are changed.     

Competition and regeneration in quaking aspen–white fir (Populus tremuloides–Abies concolor) forests in the Northern Sierra Nevada,USA

Question: How does competition between quaking aspen (Populus tremuloides) and white fir (Abies concolor) affect growth and spatial pattern of each species? Location: The northern Sierra Nevada, California, USA. Methods: In paired plots in mixed aspen‐ (n=3) or white fir‐dominated (n=2) stands, we mapped trees and saplings and recorded DBH, height, species, and condition and took increment cores. We tallied seedlings by species. Tree ring widths were used as a measure of basal area change over the last decade, and canopy openness was identified using hemispherical photographs. Linear mixed models were used to relate neighborhood indices of competition, stand, and tree‐level variables to diameter increment. Spatial patterns of stems were identified using the Neighborhood Density Function. Results: White fir radial growth was higher in aspen‐ than white fir‐dominated plots. Individual‐level variables were more important for white fir than for aspen growth, while variables representing competitive neighborhood were important only for aspen. The forest canopy was more open in aspen‐ than white fir‐dominated stands, but ample aspen seedlings were observed in all stands. Canopy stems of aspen and white fir were randomly distributed, but saplings and small trees were clumped. Aspen saplings were repelled by canopy aspen stems. Conclusions: Variation in canopy openness explained more stand–stand variation in white fir than aspen growth, but high light levels were correlated with recruitment of aspen seedlings to the sapling class. Radial growth of aspen was predicted by indices of neighborhood competition but not radial growth of white fir, indicating that spacing and stem arrangement was more important for aspen than white fir growth. Fire suppression has removed a major disturbance mechanism that promoted aspen persistence and reduced competition from encroaching conifers, and current forests favor species that regenerate best by advance regeneration (white fir).     

Plant species richness and diversity along an altitudinal gradient in the Sierra Nevada, Mexico

This article presents an analysis of plant species richness and diversity and its association with climatic and soil variables along a 1300‐m elevation gradient on the Cerro Tláloc Mountain in the northern Sierra Nevada in Mexico. Two 1000‐m² tree sampling plots were created at each of 21 selected sampling sites, as well as two 250‐m² plots for shrubs and six 9‐m² plots for herbaceous plants. Species richness and diversity were estimated for each plant life form, and beta diversity between sites was estimated along the gradient. The relationship between species richness and diversity and environmental variables was modelled using simple linear correlation and regression trees. Species richness and diversity showed a unimodal pattern with a bias towards high values in the lower half of the elevation gradient under study. This response was consistent for all three life forms. Beta diversity increased steadily along the elevation gradient, being lower between contiguous sites at intermediate elevations and high – the species replacement rate was nearly 100%– between sites at the extremes of the gradient. Few species were adapted to the full spectrum of environmental variation along the elevation gradient studied. The regression tree suggests that differences in species richness are mainly influenced by elevation (temperature and humidity) and soil variables, namely A₂ permanent wilting point, organic matter and horizon field capacity and A₁ horizon Mg²⁺.     

Cattle grazing and conservation of a meadow-dependent amphibian species in the Sierra Nevada

World-wide population declines have sharpened concern for amphibian conservation on working landscapes. Across the Sierra Nevada''s national forest lands, where almost half of native amphibian species are considered at risk, permitted livestock grazing is a notably controversial agricultural activity. Cattle (Bos taurus) grazing is thought to degrade the quality, and thus reduce occupancy, of meadow breeding habitat for amphibian species of concern such as the endemic Yosemite toad (Anaxyrus [ = Bufo] canorus). However, there is currently little quantitative information correlating cattle grazing intensity, meadow breeding habitat quality, and toad use of meadow habitat. We surveyed biotic and abiotic factors influencing cattle utilization and toad occupancy across 24 Sierra Nevada meadows to establish these correlations and inform conservation planning efforts. We utilized both traditional regression models and Bayesian structural equation modeling to investigate potential drivers of meadow habitat use by cattle and Yosemite toads. Cattle use was negatively related to meadow wetness, while toad occupancy was positively related. In mid and late season (mid July–mid September) grazing periods, cattle selected for higher forage quality diets associated with vegetation in relatively drier meadows, whereas toads were more prevalent in wetter meadows. Because cattle and toads largely occupied divergent zones along the moisture gradient, the potential for indirect or direct negative effects is likely minimized via a partitioning of the meadow habitat. During the early season, when habitat use overlap was highest, overall low grazing levels resulted in no detectable impacts on toad occupancy. Bayesian structural equation analyses supported the hypothesis that meadow hydrology influenced toad meadow occupancy, while cattle grazing intensity did not. These findings suggest cattle production and amphibian conservation can be compatible goals within this working landscape.     

Effects of copper on species composition of periphyton in a Sierra Nevada, California, stream

SUMMARY. 1. Changes in species composition of the periphyton of an oligotrophic. Sierra Nevada stream continuously dosed for 1 year at three concentrations of copper (2.5, 5 and 10 μg 1^?1 Cu_T; approximately 12, 25 and 50 ng 1^?1 Cu²⁺) were determined. 2. The numerically most abundant taxa were Bacillariophyceae (Achnanthes minutissima, Cocconeis placentula, Cymbella microcephala, C. sinuata, Fragilaria conslruens, F. crotonensis. Navicula spp., Synedra acus and S. rumpens), and the Cyanophyta Lyngbya spp., a co-dominant during spring and summer. 3. Population densities of Lyngbya spp. were markedly reduced at all test concentrations of copper. Population densities of the principal Chlorophyta (Spirogyra spp. and Cladophora spp.) and the diatom Amphipleura pellucida were reduced at 5 μg 1^?1 CU_T. Of the twenty-two most abundant taxa, sixteen were reduced in abundance by continuous exposure to 10 μg 1^?1 Cu_T. 4. There was no commensurate reduction in standing crop (total number of individuals of all taxa). Achnanthes minutissima, a co-dominant in the control, was the primary replacement species. Other taxa that were more abundant at 5 μg1^?1 Cu_T than in the control were Ceratoneis arcus, Cocconeis placentula, Navicula spp. and Synedra rumpens. Only A. minutissima and Calothrix spp. were more abundant at 10 μg 1^?1 than in the control. 5. Three resemblance measures (Canberra metric, Bray-Curtis and Dice) and diversity (Brillouin's) were evaluated for detecting differences in species composition among experimental stream sections. The Canberra metric, an index sensitive to proportional rather than absolute differences, was the most informative of these indices.     

The effect of different pine hosts on the sampling of Rhizopogon spore banks in five Eastern Sierra Nevada forests

Our primary goal was to determine whether detection of Rhizopogon internal transcribed spacer (ITS) groups is affected by the pine species used in seedling bioassays. Our secondary goal was to investigate composition of Rhizopogon spore banks in the Eastern Sierra Nevada of California, a previously unsampled region. We used seedlings of Pinus contorta, Pinus jeffreyi, Pinus lambertiana, and Pinus muricata as bioassay plants and identified the Rhizopogon retrieved by internal transcribed spacer (ITS) sequence analysis. We found that each of the pine species retrieved all of the abundant Rhizopogon ITS groups, but there were significant differences among pines in the richness of Rhizopogon ITS groups recovered. Pinus muricata recovered all ITS groups found in this study and was significantly better than P. lambertiana. Rhizopogon communities from the five sampled sites contained six to eight ITS groups per site, with two unique sequence groups and a higher abundance of the Rhizopogon ellenae and Rhizopogon arctostaphyli groups than at previously sampled sites. These results show high cross-receptivity between Rhizopogon and pine species, and regional patterns in spore bank composition.     

A multivariate model of plant species richness in forested systems: old-growth montane forests with a long history of fire

Recently, efforts to develop multivariate models of plant species richness have been extended to include systems where trees play important roles as overstory elements mediating the influences of environment and disturbance on understory richness. We used structural equation modeling to examine the relationship of understory vascular plant species richness to understory abundance, forest structure, topographic slope, and surface fire history in lower montane forests on the North Rim of Grand Canyon National Park, USA based on data from eighty‐two 0.1 ha plots. The questions of primary interest in this analysis were: (1) to what degree are influences of trees on understory richness mediated by effects on understory abundance? (2) To what degree are influences of fire history on richness mediated by effects on trees and/or understory abundance? (3) Can the influences of fire history on this system be related simply to time‐since‐fire or are there unique influences associated with long‐term fire frequency? The results we obtained are consistent with the following inferences. First, it appears that pine trees had a strong inhibitory effect on the abundance of understory plants, which in turn led to lower understory species richness. Second, richness declined over time since the last fire. This pattern appears to result from several processes, including (1) a post‐fire stimulation of germination, (2) a decline in understory abundance, and (3) an increase over time in pine abundance (which indirectly leads to reduced richness). Finally, once time‐since‐fire was statistically controlled, it was seen that areas with higher fire frequency have lower richness than expected, which appears to result from negative effects on understory abundance, possibly by depletions of soil nutrients from repeated surface fire. Overall, it appears that at large temporal and spatial scales, surface fire plays an important and complex role in structuring understory plant communities in old‐growth montane forests. These results show how multivariate models of herbaceous richness can be expanded to apply to forested systems.     

Effects of copper on species composition of benthic insects in a Sierra Nevada, California, stream

SUMMARY. 1. Effects of copper on species composition and production of benthic insects in an oligotrophic stream dosed at low concentrations (2.5-15 μg 1^-1 Cu_r; approximately 12-75 ng 1^-1 Cu²⁺) were determined. Dosing was initially in autumn-early winter when peak densities of many species occur. It was resumed the following summer near the time of egg hatch of most species and continued through the remaining aquatic stages of univoltine and multivoltine taxa.     

Local ecology and geographic ranges of plants in the Bishop Creek watershed of the eastern Sierra Nevada, California, USA

Aim The physiological requirements and tolerances of a species partially determine both its habitat preferences within a community and its broader geographic range. Therefore, we predicted that local ecology should be correlated with geographic distribution. We tested for a correlation between local ecology and range size, and we attempted to account for this correlation by the climate of the range. Location Bishop Creek Watershed, on the eastern side of the Sierra Nevada, California. Methods We recorded all plant species growing in each of 263 plots in the montane to alpine zones of the watershed. The local habitat preferences of 282 species were described in terms of wetness, elevation, soil, and amount of shade. The size and centre of the geographic range for each species were determined from regional floras. Results Wetness preference within the watershed was significantly correlated with range size. Specifically, plants of wet sites had larger ranges that extend to the north, whereas plants of dry sites tended to have smaller ranges centred to the east. The correlation between local wetness preference and range size was entirely explained by the location of the range centre of the species. Main conclusions A possible reason for the large ranges of mesophilic plants in our study area is that mesic habitats are continuous throughout the western Cordillera, while dry alpine habitats are isolated by valleys to the east. The correspondence between local ecology and geographic distributions implies evolutionary stasis in the niches of these plant species.     

Fire disturbance and forest structure in old‐growth mixed conifer forests in the northern Sierra Nevada,California

Question: This study evaluates how fire regimes influence stand structure and dynamics in old‐growth mixed conifer forests across a range of environmental settings. Location: A 2000‐ha area of mixed conifer forest on the west shore of Lake Tahoe in the northern Sierra Nevada, California. Methods: We quantified the age, size, and spatial structure of trees in 12 mixed conifer stands distributed across major topographic gradients. Fire history was reconstructed in each stand using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing the fire history with the age, size, and spatial structure of trees in a stand. Results: There was significant variation in species composition among stands, but not in the size, age and spatial patterning of trees. Stands had multiple size and age classes with clusters of similar aged trees occurring at scales of 113 ‐ 254 m². The frequency and severity of fires was also similar, and stands burned with low to moderate severity in the dormant season on average every 9–17 years. Most fires were not synchronized among stands except in very dry years. No fires have burned since ca. 1880. Conclusions: Fire and forest structure interact to perpetuate similar stand characteristics across a range of environmental settings. Fire occurrence is controlled primarily by spatial variation in fuel mosaics (e.g. patterns of abundance, fuel moisture, forest structure), but regional drought synchronizes fire in some years. Fire exclusion over the last 120 years has caused compositional and structural shifts in these mixed conifer forests.