Steep Elevation Gradients Research Articles

Elevational trends of biodiversity and plant traits do not converge—a test in the Helan Range, NW China

When elevational gradients include combinations of different climatic gradients, such as a decline in temperature combined with an increase in moisture, vegetation and plant trait responses are difficult to explain. Here, we used plant species richness and morphometric traits data across steep elevational gradients in the Helan Range of Northwestern China in an attempt to separate general trends (temperature related) from regional peculiarity (moisture related). Based on the floristic data of the Helan Range as well as plot-based data, both drought (at low elevation) and low temperature (high elevation) are associated with low species richness presumably also explaining the peak in diversity at mid-elevation, where climatic conditions are moderate. However, this mid-elevation peak in diversity is not mirrored in trends of plant traits such as leaf size and inflorescence size, which show either unidirectional trends or no change with elevation (with impacts of drought and low temperature perhaps gradually replacing each other). Our analysis illustrated the taxonomic and plant functional type (PFT)-related biases in functional trait studies and showed that consistent patterns only emerge after careful data stratification, with taxonomy (family level) holding more promises than PFTs. Inflorescence size increased with elevation in major insect-pollinated families, a trend not seen in wind-pollinated (graminoid) families. However, the reproductive effort expressed as inflorescence/leaf length ratio increases with elevation in the majority of plant families, irrespective of their pollination system. The fact that these biometric responses to elevation do not correlate with responses in species richness (which peaks at mid-elevation) may reflect contrasting drivers of trait selection and biodiversity. Based on our plot-based data, this analysis also confirmed the usefulness of floristic archive data for testing ecological theory related to elevational gradients.

A tale of two species: Extirpation and range expansion during the late Quaternary in an extreme environment

Death Valley, California is today the hottest hyperarid area in the western Hemisphere with temperatures of 57 °C (134 °F) recorded. During the late Quaternary, pluvial Lake Manly covered much of the Valley and contributed to a much more moderate climate. The abrupt draining of Lake Manly in the mid-Holocene and coincident dramatic shifts in temperature and aridity exerted substantial selection pressure on organisms living in this area. Our research investigates the adaptive response of Neotoma (woodrats) to temperature change over the late Quaternary along a steep elevational and environmental gradient. By combining fieldwork, examination of museum specimens, and collection of paleomiddens, our project reconstructs the divergent evolutionary histories of animals from the valley floor and nearby mountain gradients (− 84 to > 3400 m). We report on recent paleomidden work investigating a transition zone in the Grapevine Mountains (Amargosa Range) for two species of woodrats differing significantly in size and habitat preferences: N. lepida, the desert woodrat, and N. cinerea, the bushy-tailed woodrat. Here, at the limits of these species' thermal and ecological thresholds, we demonstrate dramatic fluctuations in the range boundaries over the Holocene as climate shifted. Moreover, we find fundamental differences in the adaptive response of these two species related to the elevation of the site and local microclimate. Results indicate that although N. cinerea are currently extirpated in this area, they were ubiquitous throughout the late Pleistocene and into the middle Holocene. They adapted to climate shifts over this period by phenotypic changes in body mass, as has been demonstrated for other areas within their range; during colder episodes they were larger, and during warmer intervals, animals were smaller. Their presence may have been tied into a much more widespread historical distribution of juniper ( Juniperus sp.); we document a downward displacement of approximately 1000 m relative to juniper's modern extent in the Amargosa Range. These results suggest a cooler and more mesic habitat association persisting for longer and at lower elevations than previously reported.

Forest reproduction along a climatic gradient in the Sierra Nevada, California

To elucidate broad-scale environmental controls of coniferous forest reproduction in the Sierra Nevada, California, we monitored reproduction for 5 years in 47 plots arrayed across a steep elevational (climatic) gradient. We found that both absolute seedling densities (stems < 1.37 m) and seedling densities relative to overstory parent tree basal area declined sharply with elevation. Rates of seedling turnover (the average of birth and death rates) also declined with elevation. In contrast, seed production was not predicted by elevation and was highly variable from year to year. During a mast year of seed production, the intensity of masting was uneven among plots. Seedling densities were elevated only during the single year immediately following the mast year, suggesting reproduction in our forests may be primarily limited by abiotic factors such as the availability of suitable sites and weather. Disturbance also clearly affected reproduction; plots that had recently burned had significantly higher seedling to parent tree ratios for Abies species, suggesting that even though established Abies concolor may be relatively susceptible to fire, the species can recover rapidly through prolific reproduction. Since reproductive failures may be our earliest signal of changing forest conditions, seedling dynamics could provide a sensitive, if variable, indicator of environmental changes.

Changes in forest vegetation and arbuscular mycorrhizae along a steep elevation gradient in Arizona

We assessed species composition, richness and abundance of understory vegetation, as well as arbuscular mycorrhizal (AM) inoculum potential on the San Francisco Peaks, tallest mountains in Arizona, crossing a steep, south-facing elevational gradient. These mountains have a high conservation value due to their rare habitats but previous vegetation studies have been limited. Because mature trees in the Pinaceae do not form associations with AM fungi, there may be more variation in plant community and AM fungal associations in coniferous forest than in ecosystems where all species associate with AM fungi. Differences in species composition between forest types reflected differences in the historical disturbance regimes. Species richness was highest in ponderosa pine forest (32.6 ± 1.4 per 1000 m 2 plot), although plant abundance was highest in aspen forest (49.4 ± 3.8%). Ponderosa pine and bristlecone pine forest were both high in species richness and contained species which were tolerant of frequent, low-intensity fire. Exotic species richness and abundance were highest in the lower elevations, which were also areas of high species richness and greater anthropogenic disturbance. Arbuscular mycorrhizal inoculum potential varied widely (1.2–80.1%), decreasing with increases in tree cover. We suggest indicator species that may be of use in monitoring these forests under changing climate and fire regimes.

Adrenocortical sensitivity to stress in Dark-eyed Juncos (Junco hyemalis oregonus) breeding in low and high elevation habitat

We compared the adrenocortical stress responses of a songbird, the Dark-eyed Junco, across a steep elevation gradient. Concentrations of the stress hormone corticosterone were compared at 3, 30, and 60 minutes after exposure to a standardized stressor in juncos breeding in low- (1,000 m a.s.l.) versus high- (2,000 m a.s.l.) elevation habitat in Jasper National Park, Alberta, Canada. Baseline corticosterone (3 minutes post-capture) in juncos did not differ between elevations. Stress-induced corticosterone in juncos did not differ markedly between elevations during the initial stage of the stress response (30 minutes post-capture). However, by 60 minutes post-capture, corticosterone was higher at low elevations, but with borderline statistical significance. Females had a lower adrenocortical response to stress than males, and breeding individuals had a lower response to stress than non-breeders. The influence of gender and breeding stage may correspond to an interaction between testosterone and corticosterone.

Simultaneous estimation of daily solar radiation and humidity from observed temperature and precipitation: an application over complex terrain in Austria

Using daily observations of temperature, precipitation, radiation, and humidity from 24 stations spanning a large elevation gradient in Austria, we tested several previously defined algorithms for estimating daily radiation and humidity. The estimation algorithms were first tested independently, and then combined, resulting in a combined algorithm for estimating both radiation and humidity that relies only on temperature and precipitation inputs. Mean absolute errors (MAE) for joint radiation and humidity estimates were 2.52 MJ m −2 per day and 85.6 Pa, respectively, close to values reported for the algorithm development studies. Biases were low: +0.02 MJ m −2 per day and +28.2 Pa for radiation and humidity, respectively. Initial results showed biases in estimated radiation related to horizon obstruction and snowpack. We amended the original algorithm, successfully eliminating these effects. Annual prediction MAE was weakly correlated with elevation, and annual bias was not correlated with elevation. Analysis of seasonal patterns in error-elevation relationships showed several periods with significant trends. Radiation MAE was slightly higher in mid-summer for higher elevations, and radiation biases were in general closer to zero throughout the spring and summer at higher elevations. Humidity estimates showed an increased MAE and positive bias at higher elevations in winter. We concluded that the effect of different temperature lapse rates for daily maximum and minimum temperature on the relationship between diurnal temperature range and atmospheric transmittance does not seriously impair predictions over steep elevation gradients in complex terrain.

The physical environment and major plant communities of the Karoo National Park, South Africa

The major plant communities of the Karoo National Park are described using the methods of the Zurich-Montpellier school of phytosociology, to assist with the formulation of a management strategy for the park. The vegetation physiognomy consists of Montane Karoo grassy shrublands. Karoo grassy dwarf shrublands. Karoo succulent dwarf shrublands and riparian thicket. Steep elevation and precipitation gradients within the study area have a direct impact on gradients in the vegetation. High elevation (1 800 m), and relatively high rainfall (406 mm) montane grasslands occupy communities dominated by grasses (Merxmuellera disticha, Themeda triandra) and woody species (Diospyros austro-africana, Elytropappus rhinocerotis, Euryops annae, Passerina montana). The increasing aridity away from the escarpment edge in a northerly direction is steep, and Montane Karoo dwarf shrublands replace these mesic communities. Species such as Eriocephalus ericoides, Rosenia oppositifolia and Pteronia tricephala dominate. At lower elevation (800 m) the precipitation is very low (175 mm) and uncertain (coefficient of variation of 78 ). The substrata influence the vegetation, with the sandy substrata of the drainage lines supporting more woody taxa (Acacia karroo, Lycium cinereum) and grasses (Hyparrhenia hirta, Stipagrostis namaquensis, Cenchrus ciliaris). Moving away from the mesic environment of the riparian zone, rapid desiccation occurs and the most xeric communities are encountered, dominated by Stipagrostis obtusa, S. ciliata and Pent-da incana. This document provides descriptions of the general communities and their associated landscape, lithology and soils.

Relationships between Fish Assemblage Structure and Stream Order in South Carolina Coastal Plain Streams

Forty-seven sample sites were electrofished in 22 streams on the South Carolina coastal plain. Average species numbers adjusted to a constant stream surface area were 12.7, 17.5, 21.4, and 22.0 in first- through fourth-order streams, respectively. Species addition and replacement led to large changes in species composition among stream orders. Relatively small fishes, most of which were generalized insectivores, numerically dominated headwater (first- and second-order) streams. Relatively large fishes, many of which were piscivores or benthic insectivores, were most common in fourth-order streams. Headwater species richness was higher and longitudinal species replacement was greater than often observed in other geographic regions of the United States. A comparative assessment of long-term temperature and precipitation records suggested that high species richness at headwater sites was related to mild climate and lack of steep elevation gradients. The presence of numerous small headwater species created the potential for multiple species replacements as downstream increases in habitat volume permitted the establishment of larger fish with predatory and competitive advantages. Because they support many species uncommon in larger streams, headwater streams in the southeastern coastal plain contribute importantly to biodiversity.

Plant water stress and gall formation (Cecidomyiidae: Asphondylia spp.) on creosote bush

Abstract. 1. Populations of creosote bush (Larrea tridentata (DC) Coville), were studied in Arizona to determine whether associated gallformers (Cecidomyiidae: Asphondylia spp.) were more abundant on water‐stressed or nonstressed plants. Gall densities were measured along a steep elevational gradient that extended from mesic, higher elevations to lower elevations in the desert; and in the Grand Canyon where severely water‐stressed and relatively unstressed plants occurred adjacently. At the Grand Canyon site, the responses of creosote bush to water stress were also studied.2. The number and densities of Asphondylia species increased both at lower elevations and locally on water‐stressed plants in the Grand Canyon, indicating that climatic and local conditions influence gallformer abundance in the same way.3. Five of the eight Asphondylia species studied at the Grand Canyon site were more abundant on stressed plants, two species were more abundant on nonstressed plants and one species showed no preference for either plant type.4. Densities of most species on stressed plants were positively correlated with the number of meristematic terminals per branch, which were more numerous on stressed plants, due to a bushier architecture. Flower gallformers were more abundant on nonstressed plants, which produced more flowers. Gall densities did not correlate with chemistry measurements, although these parameters also varied with level of stress.5. These results suggest that gallforming species respond variably to plant stress, even within a closely‐related lineage, and that there are effects of stress on plants, including architectural changes, that may be more important to herbivores than biochemical effects emphasized by White (1984) and others.

ECOLOGY OF AFRICAN GRAZING AND BROWSING MAMMALS

Africa is the earth's second largest continent, comprising 20% of its surface. Largely tropical, Africa extends as well into temperate zones to 37?N and 35?S. Eastern and southern Africa display steep elevation gradients due to the prevalence there of volcanic orogeny and rifting (29). Local landscapes are distinguished by substantial geological heterogeneity, dissected land forms, and resultant steep gradients of precipitation and vegetation. The consequent pronounced fragmentation of habitats and sharp juxtaposition of distinct vegetation types, combined with climatic oscillations in geological time, contributed to major adaptive radiations of the mammalian fauna (102, 120). Early zoological expeditions recorded that habitat fragmentation and wide spatial variation of animal densities and diversities were distinctive features of African ecosystems (92, 138, 162, 226). Those early records provided the bases of natural history information on animal distributions, habitat preferences, feeding habits, and general ecology; scientific research followed only much later (201). Modern scientific study of African savanna-grassland mammals began in the 1950s (23, 24, 107, 108, 148, 149, 197, 203, 204, 210, 230), long after the distributions and densities of the major game animals had been affected by growing human populations, colonial land and hunting policies, and virulent exotic diseases that affected the animals both directly and indirectly (57). The mammalian fauna has been increasingly isolated and fragmented within game reserves of varying size, habitat diversity, and animal species diversity; the ability to sustain it in the absence of active management is increasingly questioned (112, 187). For species with population sizes greater than 100 individuals, game reserve area (A) and faunal