Michael C. Grant

Genetic variation and the natural history of quaking aspen

In the fall, sightseers take to the highways of North America to enjoy the brilliant colors that are revealed as deciduous trees recycle the dominant greens of chlorophyll. In the western United States and Canada, the most colorful tree in the fall is the quaking aspen, Populus tremuloides. Brilliant yellows, rich golds, and shimmering shades of red shine, contrasting with the various green shades of the conifers. Quaking aspen earns its name for the distinctive fluttering of its leaves, even in the most gentle breezes. Early French-Canadian trappers called the tree an aspen because of its similarity to Populus tremula, a closely related species in Europe and Asia. One of the legends attached to the aspen reflects Judeo-Christian influences. The aspen quake in fear today, according to folklore, because Jesus Christ was crucified on a cross of aspen. In addition to the esthetics of quaking leaves and brilliant fall colors, extraordinary features of the natural history and genetics of aspen lend it special appeal for naturalists and laboratory scientists. Quaking aspen merits a variety of superlatives: It is North Americas most widely distributed native tree species and the second most widely distributed in the world (Barnes and

ELEVATIONAL GRADIENTS IN ADULT SEX RATIOS AND SEXUAL DIFFERENTIATION IN VEGETATIVE GROWTH RATES OF POPULUS TREMULOIDES MICHX.

Since Fishers (1930) early work on the effects of selection on the primary sex ratio which showed that mixtures should tend toward equal numbers of males and females, there have been several theoretical studies which have pointed out conditions in which deviations from 1:1 could be expected (e.g., Bodmer and Edwards, 1960; Emlen, 1970; Eshel, 1975). Empirical studies, usually of adult ratios, have shown strong imbalances in several cases (e.g., Putwain and Harper, 1972; Freeman et al., 1976; Bawa and Opler, 1977). The extent to which sex dependent mortality among adults influences changes in the primary ratio is still under theoretical investigation (Trivers and Willard, 1973; Werren and Charnov, 1978). The number of empirical studies which may be used to test these theories is still small, especially in plants (Godley, 1964; Putwain and Harper, 1972; Lloyd, 1973; Freeman et al., 1976; Bawa and Opler, 1977). We here report the results of field studies on adult sex ratios and concomitant patterns of vegetative growth rates for quaking aspen (Populus tremuloides Michx.) in the Colorado Front Range. Quaking aspen is the most widespread tree in North America, extending from Alaska to Mexico and from Maine to the state of Washington. Members of the genus are of substantial commercial, recreational and ecological significance (Little, 1971), as well as the subject of a large literature of physiological, silvicultural and general ecological attributes (Barnes, 1966, 1969, 1975; Reed, 1971; Anon., 1976). The extensive, beautiful stands or groves of quaking aspen familiar to travelers in the northern or western parts of Revised February 2, 1979

Effects of Bird Predation on Grasshopper Densities in an Arizona Grassland

In a 4-yr field experiment, we tested the hypotheses that insectivorous birds (1) controlled densities of herbivorous grasshoppers in an ungrazed semiarid grassland in southeastern Arizona, and (2) functioned as keystone predators, by limiting abundances of grasshoppers that otherwise might change vegetation cover and species composition, and/or by mediating the effects of otherwise competitively superior members of the grass- hopper assemblage. We measured grasshopper densities and vegetation on 32 464-m2 grassland plots for 1 yr, then enclosed 16 of these plots with bird exclosures and continued data collection for 3 yr. Eight of the 16 experimental plots were further modified in the last 2 yr of the study by installing fine-mesh 1 m high barriers designed to retard grasshopper dispersal. Micro- climates of caged plots differed only slightly from open plots. Lizards and rodents increased inside the exclosures, but they were removed and released elsewhere such that their average abundances did not differ among treatments. By the final year of the study, mean annual adult grasshopper density was > 2.2 times higher on plots from which birds were excluded, and where grasshoppers were enclosed by dispersal barriers, than on unmanipulated control plots. Mean nymph density was > 3.0 times higher in the same comparison. Grasshoppers were significantly more abundant in bird enclosures with insect dispersal barriers, indicating that experimental plots were dis- persal sources rather than sinks. Seven of 12 common grasshopper species were more abundant inside the bird exclosures, while none was less abundant. Among the more abundant taxa, those responding most positively were grass feeders: Eritettix simplex, Opeia obsciira, Paropomala Tyomingensis, and Phoetaliotes nebrascensis. We found no evidence that grasshoppers competed with one another under increased densities inside the bird exclosures. Although the amount of insect herbivory was somewhat higher inside the bird exclosures, and was positively correlated with grasshopper density across all 32 plots (r = 0.87), overall vegetation cover and species composition did not differ among treatments by the end of the study. Dactylotum variegatut, an aposematic species apparently immune to avian predation, showed no significant responses to the experiment. Birds clearly limited grasshoppers in this grassland ecosystem, but they failed to qualify as keystone predators, at least in the short term, for two reasons: (1) in their absence, increased grasshopper densities had no appreciable impact on vegetation cover or species composition; and (2) there was no evidence that birds mediated competition among grass- hoppers.

Relationships Between Stream Discharge And Yield Of Dissolved Substances From A Colorado Mountain Watershed

The dependence of concentration and yield of dissolved substances on water discharge was studied over a 150-week period in the watershed of Como Creek, Colorado, near the Continental Divide. Substances were divided into three groups on the basis of relationship between concentration and discharge: (1) substances whose concentrations decrease with increasing discharge (HCO3−, NO3−, Ca2+, Mg2+, Na+); (2) substances whose concentrations show no trend with discharge (NH4+, dissolved organic phosphorus and nitrogen, K+, SO42-); (3) substances whose concentrations increase with increasing discharge (dissolved organic carbon, H+,PO43-). The concentration-discharge relationships were compared between years and generally showed evidence of a common slope between years, even though discharge and weather varied considerably between years. Mean concentrations between years were compared statistically at a common discharge and showed no significant differences between years, suggesting common control mechanisms between years for a given substance. Substances were also compared with respect to yield (product of concentration and discharge). Three categories of yield response to increasing discharge were identified. All showed increasing yield with increasing discharge but with: (A) yield increasing slower than discharge (HCO3−, NO3−, Ca2+, Mg2+, Na+); (B) yield increasing at the same rate as discharge (NH4+, DON); (C) yield increasing faster than discharge (DOP, K+,SO42-, DOC, H+, PO43-). Simple groundwater-precipitation mixing models are not adequate to explain these differences in concentration and yield responses to discharge. A satisfying mechanism to explain the differences between responses of dissolved substances to discharge must take into account not only mixing but also the chemical reaction of incoming precipitation with soil and soil water and the intensity of biological demands for the substances.

Acid Precipitation in the Western United States

Precipitation chemistry data from a rural area at an elevation of 2900 meters near the Continental Divide in Colorado show unexpectedly low pH values for bulk precipitation. A significant downward trend in pH over the last 3 years is associated with increasing amounts of nitric acid in precipitation.

Variation in allozymes and stomatal size in pinyon (Pinus edulis, Pinaceae), associated with soil moisture

Microgeographic allozyme variation was examined in pinyon pine, Pinus edulis, among five collection sites in Owl Canyon, Colorado. Relatively dry and moist sites were identified by associated plant communities and the sizes and densities of trees. Three moist sites and two dry sites were compared, and because all sites were within 600 m of one another, isolation by distance was not considered as a viable explanation of genetic differentiation between sites. Allelic frequencies at glycerate dehydrogenase (Gly) differed by 14% between moist and dry areas, and the pattern of microgeographic variation found here-allele 3 higher in frequency on dry sites-was consistent with previous studies of microgeographic variation in contrasting moist and dry sites. Trees within one of the dry sites were examined to test the hypothesis that stomata sizes and densities are heterogeneous among Gly genotypes. Heterozygotes had the longest and widest stomata; the stomatal area of heterozygotes was 28% greater than the stomatal area of homozygotes. Whereas the stomatal areas of the two homozygotes were similar, their shapes did not overlap when projected on a bivariate plot of length and width. These results suggest that stomatal shape may play a role in adapting pinyon to heterogeneity in soil moisture.

Biogeography and Landscape-Scale Diversity of the Dominant Crenarchaeota of Soil

We surveyed the diversity of soil Archaea across a large scale elevational gradient of ecosystem types, from foothills forest to alpine tundra in the Front Range of the Rocky Mountains. We used a dilution technique to sequence the single most abundant archaeal 16S rDNA sequence in each of the 40 soil cores distributed across the gradient to compare our results to those of typical 16S clone library studies.We found a greater diversity of sequences than has typically been found in clone library studies from a single site or core, identifying sequences both from the Terrestrial Group and the FFSB Group at several sites. We did not observe any significant environmental correlates with the dominant sequence type, nor was there any relationship between the spatial distance between samples and the phylogenetic similarity of the dominant sequence types. Despite using a very different methodology, our collective results are in remarkably good agreement with other studies of soil Crenarchaeota in terms of the diversity and relative abundance of sequence types identified. We are able to identify two instances of very tightly clustered sequences which we suggest are the results of global selective sweeps—one closely related to SCA1145, an abundant globally distributed group within the Terrestrial Group of Crenarchaeota, and another nested within the more basal FFSB group of sequences. We replicated our sequence results at two levels: first, by repeating the dilution and PCR processes from the same soil core DNA extraction, and second, by performing a replicate DNA extraction from the same homogenized soil core sample. Pairs of sequences produced by the dilution replicates were significantly more similar than the pairs of sequences produced by the extraction replicates, suggesting that soil Crenarchaeota exists in highly localized and discrete clonal populations.

Scaling patterns of biomass and soil properties: an empirical analysis

We argue that studies at multiple scales must necessarilychange the extent of measurements, not just the spacing, in order toeffectivelycapture information regarding processes at multiple scales. We have implementeda multi-scale sampling scheme using transects of 10 cm, 1m, 10 m, 100 m, and 1 km ateach of four sites along an elevational gradient from dry foothills forest toalpine tundra in the Front Range of Colorado; these four sites form anadditional transect of 22 km. Along each of these transects wetookten equally spaced soil cores and measured variables important in determiningboth microbial and plant community structure: soil water content, organicmattercontent, pH, and total soil biomass. With this sampling scheme we are able totreat scale as an independent variable in our analyses, and our data show thatboth particular sites and particular variables can determine whether estimatesof mean values are scale-dependent or not. A geostatistical analysis using allof our data shows common relationships between scales across ecologicallydiverse sites; biomass shows the most complex pattern of distribution acrossscales, as measured by fractal dimension. Our analyses also reveal theinadequacy of several standard geostatistical models when applied to data frommultiple scales of measurement – we recommend the use of the boundedpowerlaw model in such cases. We hypothesize that because biological communitiesmustrespond simultaneously to multiple variables with differing patterns of spatialvariation, the spatial variation of biological communities will be at least ascomplex as the most complex environmental variable at any given site.

POPULATION AND SUBSPECIFIC GENETIC DIFFERENTIATION IN THE FOXTAIL PINE (PINUS BALFOURIANA)

Abstract We performed an allozyme survey of genetic differentiation in Pinus balfouriana, a subalpine conifer endemic to California that is comprised of two allopatric subspecies, one in the Klamath Mountains and the other in the southern Sierra Nevada. Although the two subspecies are morphologically distinct and gene flow between them is virtually nonexistent, we observed much higher levels of differentiation among populations within a subspecies than between the two subspecies. Differentiation is particularly strong in the Klamath populations (multilocus FST= 0.242), which are small, isolated, and ecologically marginal. We attribute this strong differentiation to the mountain island effect, in which populations restricted to high elevations become isolated from each other on different mountains separated by unsuitable intervening habitat, with consequent reduced gene flow allowing populations to evolve independently. Populations of P. balfouriana in the Klamath region only exist scattered on the few highest ridges and peaks that rise above 2000 m, which defines the lower limit of the species elevational distribution. This pattern of distribution has allowed genetic drift and allelic sorting through historical events to produce strong population‐level differentiation, which was likely in place before the two subspecies were geographically separated. Because P. balfouriana occurs on both serpentine soils and nonserpentine soils in the Klamath Mountains, we tested for genetic differentiation between populations growing on serpentine versus nonserpentine soils and our results were equivocal. Our data, combined with several other studies of conifers, show that the mountain island effect can produce significant genetic differentiation in conifers whose life‐history traits of widely dispersed pollen, long generation times, and high outcrossing rates would lead us to predict a more homogenous population genetic structure.

Evidence that filterable phosphorus is a significant atmospheric link in the phosphorus cycle

Deposition of phosphorus from the atmosphere was measured weekly over 7 annual cycles in the montane region of Colorado, USA. Deposition of phosphorus consistently reached a sharp peak in early summer, near the beginning of maximum biological activity. The peak is associated with soluble phosphorus capable of passing through a filter. Pollen studies show that the peak cannot be accounted for by leaching of phosphorus from pollen, and the amounts of miscellaneous mineral and organic particulate materials are too small to account for the peak. The study thus indicates transport of filterable, water-soluble phosphorus substance, probably of biogenic origin, in large amounts over a short period of time. This transport mechanism, heretofore unsuspected, may account for significant long-distance phosphorus transport.