Winter Annual Plant Research Articles

Differences in the timing of germination and reproduction relate to growth physiology and population dynamics of Sonoran Desert winter annuals

Trait differences can promote distinct survival and fecundity responses to environmental fluctuations. In a Sonoran Desert winter annual plant community, we have identified a tradeoff between relative growth rate (RGR) and water-use efficiency (WUE) that predicts interannual variation in reproductive success. Here we test the hypothesis that traits underlying RGR and WUE differences are linked to seasonal phenology. We use long-term demographic data and finer-scale, short-term data to investigate timing of germination, reproduction, and death of several winter annual species in multiple years in open and under-shrub habitats. We hypothesized that species with high WUE and less interannual demographic variability would have life cycle transitions early in the winter to spring growing season. This would be due to an ability to use small amounts of rain and photosynthesize at low temperatures. By contrast, we hypothesized that species with low WUE whose survival and reproductive rates vary greatly from year to year would have life cycle transitions later in the season. In any given year, species with high WUE germinated and reproduced earlier in the season than species with low WUE, whereas low-WUE species germinated later and had shorter reproductive phases. Our results demonstrate a direct relationship between phenology and physiological trait differences. This link between phenology and physiology is of interest because it clarifies the mechanism by which trait differences determine species' relative abundances.

Ecosystem response to nutrient enrichment across an urban airshed in the Sonoran Desert

Rates of nitrogen (N) deposition have increased in arid and semiarid ecosystems, but few studies have examined the impacts of long-term N enrichment on ecological processes in deserts. We conducted a multiyear, nutrient-addition study within 15 Sonoran Desert sites across the rapidly growing metropolitan area of Phoenix, Arizona (USA). We hypothesized that desert plants and soils would be sensitive to N enrichment, but that these effects would vary among functional groups that differ in terms of physiological responsiveness, proximity to surface N sources, and magnitude of carbon (C) or water limitation. Inorganic N additions augmented net potential nitrification in soils, moreso than net potential N mineralization, highlighting the important role of nitrifying microorganisms in the nitrate economy of drylands. Winter annual plants were also responsive to nutrient additions, exhibiting a climate-driven cascade of resource limitation, from little to no production in seasons of low rainfall (winter 2006 and 2007), to moderate N limitation with average precipitation (winter 2009), to limitation by both N and P in a season of above-normal rainfall (winter 2008). Herbaceous production is a potentially important mechanism of N retention in arid ecosystems, capable of immobilizing an amount equal to or greater than that deposited annually to soils in this urban airshed. However, interannual variability in precipitation and abiotic processes that limit the incorporation of detrital organic matter into soil pools may limit this role over the long term. In contrast, despite large experimental additions of N and P over four years, growth of Larrea tridentata, the dominant perennial plant of the Sonoran Desert, was unresponsive to nutrient enrichment, even during wet years. Finally, there did not appear to be strong ecological interactions between nutrient addition and location relative to the city, despite the nearby activity of nearly four million people, perhaps due to loss or transfer pathways that limit long-term N enrichment of ecosystems by the urban atmosphere.

Parental overwintering history affects the responses of Thlaspi arvense to warming winters in the North

The overwintering conditions of northern plants are expected to change substantially due to global warming. For perennial plants, winter warming may increase the risk of frost damage if the plants start dehardening prematurely. On the other hand, evergreen plants may remain photosynthetically active and thereby benefit from milder winters. The positive and negative effects of mild winters on annual plants remain, however, largely unknown. We postulated that summer annuals may be susceptible to frost damage if the seeds germinate during a mild spell in winter. Winter annuals may utilize a warm period for photosynthesis. These questions were addressed in two consecutive experiments in which pot-grown individuals of Thlaspi arvense that overwintered in the field were exposed to an elevated temperature for 8 days in growth chambers in mid-winter. No premature germination was observed in summer annuals. However, in accordance with our hypothesis, winter annuals started photosynthesising very rapidly upon exposure to elevated temperature. The winter warming treatment affected neither the total number of seeds produced nor the mean seed weight. These seeds, possessing divergent parental overwintering histories, were used as starting material for the second experiment. Seeds originating from both summer and winter annual plants germinated both in the autumn and in the following spring. We observed a major parental effect associated with the winter warming treatment. The warm spell experienced by the mother plant (either as a winter annual rosette or as a summer annual seed) reduced the proportion of autumn germination in the next generation. Only 43% of the seeds of summer annuals possessing a parental warming history germinated before the winter, whereas the germination percentage of seeds with no previous winter warming history was 71%. In the case of seeds collected from winter annual plants, 4% of the seeds germinated in autumn if the mother plants experienced the warming treatment during the previous winter, whereas the corresponding value was 37% if the mother plants did not experience warming. Our results show that summer and winter annual individuals show diverse responses to warm spells in winter. Since the responses are not limited only to the generation that actually experiences the warm spell, but also appear in their offspring, long-term studies consisting of several generations are called for.

The effect of geographic range position on demographic variability in annual plants

Summary1. The abundant centre model predicts that species abundance will decline from the centre towards the periphery of the geographic range. Thus, we expect to find decreases from the centre towards the edge in variables related to population dynamics such as population density and reproductive output. However, evidence for this pattern is contradictory, suggesting that geographically peripheral sites may not be ecologically peripheral. Populations may thrive in pockets of suitable habitat at the edge of the range or may be locally adapted to peripheral conditions.2. This study examines how the position of a site within geographic and climatic ranges of 13 species is related to the population dynamics at one common location, The Desert Laboratory at Tumamoc Hill, Tucson, AZ, USA.3. We used data on survival, fecundity, germination fraction and population density from a 25‐year long‐term data set on winter annual plants to determine whether there was a relationship between distance to the centre of the range and population dynamics. Geographic distance was calculated by determining the distance from the Desert Laboratory to the centre of the observed range determined from locality records. Climatic distance was calculated using the niche modelling software,maxent, and subtracting the mean climatic profile for the species range from that of the Desert Laboratory.4. There was no relationship between mean population metrics and distance metrics. We found significant relationships between some geographic distance metrics and variance in fecundity, survival and per‐germinant fecundity, but not germination fraction or population density. We did not find a relationship with any metric of population dynamic variation and climatic distance.5.Synthesis. Our results indicate that geographic distance from the centre of the range of 13 annual plant species more strongly predicts their population dynamics than climatic distance. This study reinforces the importance of examining vital rates and their variation in order to properly capture the effect of position within a range on population dynamics.

Effects of the inhibition of weed communities by winter-flooding

Winter-flooded paddy field is an agricultural method which putting a water among the winter, it is paid attention as an environmental friendly agriculture. Especially, it is said, winter-flooding is control paddy weeds and there is the farming potential that the annual rice yield is high. However, there is no detailed research about the effects of weed communities by winter-flooding, as the purpose of this study, we investigated the effects of the inhibition of weed communities (life cycle, harmful weed) by winter-flooding at a shore of Inba Lake, Chiba Prefecture, Japan, and compared them to paddy weeds in a dry paddy field. Methods examined the plant height (cm) and the cover degree class of all appearance species in each quadrant frame (1 m2). Quadrant in the two type paddies were 10 frames (D1-D10) at the control, and were 10 frames (W1-W10) at the treatment (Figure 1). In a winter-flooded paddy field, species number and plant volume of winter-annual plants decreased in the third winter after winter-flooding, In particular, the plant volume of Alopecurus aequalis, Cardamine flexuosa decreased significantly. We considered that the cause of this decrease was due to the depth of flooding (more than 10 cm). Eleocharis kuroguwai and Echinochloa oryzoides, which are perennial plants, Sagittaria trifolia, which is an annual plant, increased in a winter-flooded paddy field in the third summer after winter-flooding. We considered that these species grew thicker at places where the seeds germinated, and grew easily, because the conservation situation of the seeds fitted well under the winter-flooding conditions. They started to grow immediately after the drainage of water. The period of drainage coincided with increasing light intensity and temperature. In addition, the annual rice yield of the third year after winter-flooding was higher than that of the habitual practice rice field．

Phenotypic plasticity and precipitation response in Sonoran Desert winter annuals

Temporal environmental variation has profound influences on population dynamics and community structure. Examination of functional traits that influence resource uptake and allocation can illuminate how co-occurring species translate environmental variation into different demographic outcomes, yet few studies have considered interspecific differences in trait plasticity. We experimentally manipulated soil moisture to test the hypothesis that differences in morphological plasticity contribute to species differences in demographic response to unpredictable precipitation in Sonoran Desert winter annual plants. We compared plasticity of leaf traits and biomass allocation between Pectocarya recurvata (Boraginaceae) and Stylocline micropoides (Asteraceae), co-occurring species that differ in long-term demographic patterns. The species with highly variable population dynamics, Stylocline, had striking increases in leaf area and root biomass in response to an experimental increase in soil moisture. In contrast, the species with buffered long-term population dynamics, Pectocarya, did not differ in leaf morphology or biomass allocation between soil moisture treatments. Regardless of water treatment, Pectocarya had earlier reproductive phenology and greater fecundity than Stylocline, suggesting that differences in the timing of the phenological transitions from vegetative to reproductive growth may affect species' responses to precipitation pulses. Combining long-term observations with experimental manipulations provides a window into the functional underpinnings and demographic consequences of trait plasticity.

Spatial patterns of plant isotope tracers in the Los Angeles urban region

Plant–environment interactions are difficult to study in urban areas, in part due to the confounding factors that affect physiology, including alterations to atmospheric composition and climate. We wished to determine whether the spatial distribution of urban plant isotope and chemical tracers could be used to distinguish among the many environmental factors that may influence plant physiological processes. We extensively sampled winter annual plants in the region in and surrounding Los Angeles, USA, and analyzed plant material for stable carbon, nitrogen and oxygen isotopes as well as carbon and nitrogen content; and radiocarbon composition. We then overlay maps of the spatial distribution of pollutant, climatic, geographic, and population variables to determine if there were significant relationships. Multiple regression analysis indicated that the radiocarbon content of winter annual plants was strongly related to ozone and carbon monoxide concentrations. Nitrogen isotopes and leaf nitrogen content were related to atmospheric NO2 and ozone concentrations. Oxygen isotope ratios were correlated with atmospheric vapor pressure deficit and ozone concentrations. These relationships suggest that plant processes are influenced by anthropogenic N uptake and ozone damage in this region. For stable carbon isotopes, spatial variability was correlated with temperature and the distribution of pollutants and point sources, but the functional relationships were less clear. While further studies are needed to confirm the mechanisms, these results highlight the potential for mapping of plant isotopes as a tool for studying complex plant–environment interactions in urban landscapes.

Functional tradeoffs determine species coexistence via the storage effect

How biological diversity is generated and maintained is a fundamental question in ecology. Ecologists have delineated many mechanisms that can, in principle, favor species coexistence and hence maintain biodiversity. Most such coexistence mechanisms require or imply tradeoffs between different aspects of species performance. However, it remains unknown whether simple functional tradeoffs underlie coexistence mechanisms in diverse natural systems. We show that functional tradeoffs explain species differences in long-term population dynamics that are associated with recovery from low density (and hence coexistence) for a community of winter annual plants in the Sonoran Desert. We develop a new general framework for quantifying the magnitude of coexistence via the storage effect and use this framework to assess the strength of the storage effect in the winter annual community. We then combine a 25-year record of vital rates with morphological and physiological measurements to identify functional differences between species in the growth and reproductive phase of the life cycle that promote storage-effect coexistence. Separation of species along a tradeoff between growth capacity and low-resource tolerance corresponds to differences in demographic responses to environmental variation across years. Growing season precipitation is one critical environmental variable underlying the demographic decoupling of species. These results demonstrate how partially decoupled population dynamics that promote local biodiversity are associated with physiological differences in resource uptake and allocation between species. These results for a relatively simple system demonstrate how long-term community dynamics relate to functional biology, a linkage scientists have long sought for more complex systems.

English

Annual bromes, primarily cheatgrass (Bromus tectorum) and Japanese brome (Bromus japonicus) are Eurasian winter annual plants that have come to be considered invasive. As such they are not included purposely in seed mixes used for mine reclamation. Examination of long-term vegetation monitoring data from reclaimed and reference areas at the Big Sky Mine in Rosebud County, MT offer insight into the dynamics of the presence of these weeds in a Northern Great Plains setting. Over the past 22 years, annual brome cover as a percent of total vegetation cover in the native open ponderosa pine (Pinus ponderosa) woodland vegetation at the Big Sky Mine has varied from as low as 0.3% in 2002 to as high as 50.7% in 1991. In reclaimed areas over the same period, annual bromes ranged from as little as 1.5 % to as much as 25.1 % of total vegetation cover. As of 2008 Native Reference Area vegetation was 37.0% annual bromes while reclaimed areas had declined to 6.5% annual bromes. Biomass production in reclaimed areas has exceeded reference area levels by 50 to 75% on most years. Precipitation in these adjacent areas can be presumed to have been very similar and grazing of both areas has been light to non-existent over the period. These data suggest that strong perennial herbaceous growth suppresses annual brome abundance especially during periods favorable precipitation. Native area vegetation periodically experiences outbreaks of annual bromes as drought stresses increase and then annual bromes decline as the competitive strength of the native perennials returns. High abundance of annual bromes is a symptom of reduced or lagging competitive strength of perennial plants: it is suggested that at least in the Northern Great Plains, annual bromes constitute a litmus test for community competitive strength, and are not of themselves a conquering invasion. Additional Keywords: Invasibility, reclamation biomass production, cheatgrass, downy brome, Japanese brome, invasive plants, cycles of invasions.

PHOTOSYNTHETIC RESOURCE-USE EFFICIENCY AND DEMOGRAPHIC VARIABILITY IN DESERT WINTER ANNUAL PLANTS

We studied a guild of desert winter annual plants that differ in long-term variation in per capita reproductive success (lb, the product of per capita survival from germination to reproduction, l, times per capita reproduction of survivors, b) to relate individual function to population and community dynamics. We hypothesized that variation in lb should be related to species' positions along a trade-off between relative growth rate (RGR) and photosynthetic water-use efficiency (WUE) because lb is a species-specific function of growing-season precipitation. We found that demographically variable species have greater RGR and greater leaf carbon isotope discrimination (Delta, a proxy inversely related to WUE). We examined leaf nitrogen and photosynthetic characteristics and found that, in this system, variation in Delta is a function of photosynthetic demand rather than stomatal regulation of water loss. The physiological characteristics that result in low Delta in some species may confer greater photosynthetic performance during the reliably moist but low temperature periods that immediately follow winter rainfall in the Sonoran Desert or alternatively during cool periods of the day or early growing season. Conversely, while species with high Delta and high RGR exhibit low leaf N, they have high biomass allocation to canopy leaf area display. Such trait associations may allow for greater performance during the infrequent conditions where high soil moisture persists into warmer conditions, resulting in high demographic variance. Alternatively, high variance could arise from specialization to warm periods of the day or season. Population dynamic buffering via stress tolerance (low RGR and Delta) correlates negatively with buffering via seed banks, as predicted by bet-hedging theory. By merging analyses of population dynamics with functional trait relationships, we develop a deeper understanding of the physiological, ecological, and evolutionary mechanisms involved in population and community dynamics.

Effects of growth stage and hardening conditions on the association between frost resistance and the expression of the cold-induced protein COR14b in barley

Sowing date, being determinant for growth stage, may play a decisive role in optimising freezing resistance of winter annual plants. In cereal species, in spite of the abundant literature analysing the factors responsible for the acquisition of frost resistance through the cold hardening process, the involvement of the growth stage per se, has been seldom considered, especially at the earlier vegetative phases. In this work the contribution of growth stage in determining resistance to freezing temperature has been analysed in field and growth chamber experiments using winter and spring barley cultivars exposed to different hardening conditions. Field damage was assessed twice during winter on plants sown at three different dates. In the growth chamber experiments several acclimation treatments at 11/7 and/or 3/1 °C (day/night) were simulated. In both field and laboratory experiments the development of cold acclimation was monitored by means of a COR14b specific antibody, since in previous studies the expression of COR14b was found genetically linked to frost resistance. The lowest resistance, found in the youngest plants and in spring cultivars, however, was not always associated with the lowest level of COR14b accumulation. COR14b accumulation correlated with frost resistance at the earlier field sampling date and in plants grown at 11/7 °C. In a following phase of the hardening process (second sampling in field and 4 weeks at 3/1 °C in growth chamber) the accumulation of COR14b was independent of plant stage and genotype, showing no association with freezing resistance. Results suggest that growth stage is crucial for the achievement of maximal resistance in barley, but not for COR14b expression.

Horseweed (Conyza canadensis) seed collected in the planetary boundary layer

Horseweed is a winter or summer annual plant, native to North America and distributed worldwide in temperate climates. This plant is considered an important agricultural weed because it can reduce agricultural yields by 90% at high densities and becomes problematic under low-tillage agriculture. Seed production is robust with an estimated 200,000 seeds produced per plant, and seed dispersal is wind-assisted. The confirmation of glyphosate-resistant horseweed in Delaware in 2001 and the rapid spread of the resistant biotype, currently covering more than 44,000 ha, has necessitated a change in the discussion about weed dispersal. Large radio-controlled airplanes were used to sample the lower atmosphere for the presence of horseweed seeds during a 3-d period in early September 2005 in southern Delaware. The collection of multiple seeds at heights ranging from 41 to 140 m above ground level strongly suggests that horseweed seeds are entering the Planetary Boundary Layer (PBL) of the atmosphere, where long-ranged transport of aerial biota frequently occurs. With wind speeds in the PBL frequently exceeding 20 m s−1, seed dispersal can easily exceed 500 km in a single dispersal event.

First Report of Crown Rot of Gazania rigens Caused by Sclerotinia sclerotiorum in Louisiana.

Gazania rigens (L.) (treasure flower, Asteraceae) is grown as a winter and summer annual bedding plant in Louisiana and the lower southern United States. In March 2006, cv. Kiss Mix was observed in a wholesale nursery with symptoms of leaf yellowing, wilt, crown rot, and death. White mycelia and black sclerotia were present on some infected and dead plants. The plants had been grown outdoors and approximately 2% of 1,120 plants had been lost. Petiole and crown tissue from infected plants were surface disinfected in 70% ethyl alcohol, and sections were placed on acidified potato dextrose agar (PDA). A fungus that produced white mycelia and black sclerotia consistently grew from tissue pieces. Other characteristics included production of numerous sclerotia (32 to 71 per dish from 10 dishes) that were oval to oblong and formed in a ring at the periphery of culture dishes. Sclerotia measured 3 to 7 mm long (mean = 4.4, standard error = 0.15, N = 50) × 2 to 4 mm wide (mean = 3.0, standard error = 0.06, N = 50). Cells of sclerotial rinds were globose and lacked erect tomentum hyphae (1). Growth rate of the fungus at 26°C on PDA ranged from 1.3 to 3.1 cm/day (mean = 2.2, standard error = 0.05, N = 45) and mycelia covered the dishes after 3 days (2). On the basis of these characteristics, the fungus was identified as Sclerotinia sclerotiorum (Lib.) de Bary. Fungal inoculum for pathogenicity tests was grown on twice-sterilized wheat grains and 1 g of 10-day-old inoculum, consisting of fungus mycelia and sclerotia, was placed at the base of six G. rigens cv. Daybreak Mix plants. Inoculated and noninoculated control plants were placed in a dew chamber held at 22°C for 48 h and then moved to a greenhouse where temperatures ranged from 20 to 25°C. Leaf yellowing, wilt, and crown rot developed after 3 to 4 days on all inoculated plants followed by death after 6 days. S. sclerotiorum was reisolated from all inoculated plants. Noninoculated plants remained healthy. Sclerotinia crown rot of G. rigens was first reported in the United States from California (3) and has also been reported from Italy and Argentina (4). To our knowledge, this is the first report of Sclerotinia crown rot on G. rigens in Louisiana.

Effect of Herbicides on Field Violet (Viola arvensis) in Four Direct-Seeded Canola Management Systems

Field violet is a winter or summer annual plant that is a serious weed of canola crops in Europe. It is a weed of increasing concern within reduced tillage fields in central Alberta, where its response to registered herbicides has not been evaluated. Two commercial fields within the Aspen Parkland ecoregion of Alberta were used to evaluate the efficacy of postemergence (POST) herbicides against field violet in conventional, imidazolinone-resistant (IMI-resistant) and glufosinate-resistant canola cultivars, as well as to evaluate the plant's response to various timings and rates of glyphosate in glyphosate-resistant canola. Control of field violet was lower in field experiments conducted in 2002 compared with 2003, probably because of abnormally low rainfall in 2002. The POST herbicides evaluated provided inadequate control of field violet in conventional canola. Glufosinate control at 500 g ai/ha was unacceptable unless the crop canopy closed shortly after application. In IMI-resistant canola, thifensulfuron did not significantly reduce plant density and biomass under the extremely dry conditions experienced in 2002, but in 2003, it conferred respective reductions of 79 and 86% relative to nontreated controls. Imazamox plus imazethapyr did not affect plant growth. Field violet was controlled by pre- and postcrop emergence glyphosate at 445 g ae/ha. Postharvest application of glyphosate provided good control throughout the following growing season when spring emergence was minimal. Herbicide activity was also evaluated on two- to four-leaf seedlings in a greenhouse experiment. Dose– response curves reflected the activity observed in field experiments. Strategies for effective field violet control with herbicides are dependent on cultivar selection and the management system, but are improved by timing application to young, actively growing plants.

Root-knot nematodes affect annual and perennial weed interactions with chile pepper

A field microplot experiment was conducted in 1996 and 1997 to determine the influence of root-knot nematodes on intra- and interspecific interactions between chile pepper (chile) and spurred anoda and between chile and yellow or purple nutsedge (or both) using a substitution design. An additional objective was to determine the influence of London rocket, a winter annual and host plant for root-knot nematodes, on the inter- and intraspecific interactions between chile and spurred anoda. Twelve plant combinations were planted into paired 76-cm-diam microplots at a density of 24 plants per microplot each year. Each pair of microplots had one root-knot nematode–infested and one uninfested plot. One randomly selected plant pair or triplet from each plot was destructively sampled in June, July, August, and September each year. Data included leaf area, plant dry weights (leaf, stem, root or root plus rhizome, chile fruit, and nutsedge tuber), and nematode egg production from the belowground biomass of the different plant species within a 2,355-cm3sampled soil volume. Chile hosted the highest population of root-knot nematodes, followed by spurred anoda, purple nutsedge, and yellow nutsedge. Few root-knot nematode eggs were recovered from London rocket before incorporation into the microplots each spring. Root-knot nematode populations were higher in 1997 and, as a result, more interactions between nematodes and plant combinations were observed for chile. Spurred anoda and root-knot nematodes reduced chile shoot and root weights to levels not significantly different from zero in 1997. Yellow and purple nutsedge shoots, except for those from the original tuber, were removed throughout the season, and these species interfered less with chile. Spurred anoda was not affected by interspecific interference. Few interactions were observed between the spurred anoda plant combinations and root-knot nematodes. In June 1996, low populations of root-knot nematodes (< 4,000 eggs per gram of root) stimulated spurred anoda growth, but higher populations in June 1997 (> 30,000 eggs per gram of root) reduced spurred anoda growth. Prior presence of London rocket had little consistent influence on spurred anoda or chile. Yellow and purple nutsedge growth variables were affected by interference from chile and the other nutsedge species. Tuber number and tuber weight were higher when plants were infected with root-knot nematodes, particularly early in the season. The enhanced tuber production may increase early-season interference from nutsedge species under production conditions. The results suggest that the presence of root-knot nematodes influences plant interference, but the effect is species specific. Annual plants are affected differently compared with perennial nutsedges, possibly because of the continuous association between the perennials and the parasite.

冬季一年草における未発芽種子バーナリゼーションと種子二次休眠との関係

冬季一年草における未発芽種子バーナリゼーションと種子二次休眠との関係

A mechanistic examination of diversity‐stability relationships in annual plant communities

While recent theoretical work has demonstrated several mechanisms whereby more diverse communities can exhibit greater temporal stability, empirical examinations have been few and the subject of much debate. We show that the temporal stability of natural summer and winter annual plant communities, at spatial scales of 0.25 m2 and 0.25 ha, tends to increase with community richness. Furthermore, more diverse communities exhibited greater stability because they contained a greater abundance of individuals (overyielding effect). Statistical averaging (the portfolio effect) and negative covariances between species (insurance and competition effects) did not enhance stability. Relationships between diversity and stability tended to be weak and were significant only at the smaller spatial scale. Because more diverse communities contained higher densities of individuals, the effect of diversity per se on stability was unclear and likely small. If overyielding is common in other ecological systems, the loss of individuals and biodiversity may often result in increased variation in ecological communities.

Why do some thorny plants resemble green zebras?

The rosette and cauline leaves of the highly thorny winter annual plant species of the Asteraceae in Israel ( Silybum marianum) resemble green zebras. The widths of typical variegation bands were measured and found to be highly correlated with leaf length, length of the longest spines at leaf margins and the number of spines along leaf circumference. Thus, there is a significant correlation between the spinyness and strength of variegation. I propose that this is a special case of aposematic (warning) coloration.

Isotopic composition of carbon and oxygen in desert fauna: investigations into the effects of diet, physiology, and seasonality

Carbon and oxygen isotope ratios of bone apatite were measured in 14 endothermic and ectothermic vertebrates native to the Chihuahuan Desert and collected in June and July of 1999 and 2000. The δ 8O values of most reptiles were very high, up to 44‰ (standard mean ocean water (SMOW), some of the highest δ 18 O values ever measured for an animal. The δ 18O values of rodents and birds were lower (∼32±5‰ vSMOW), and the earless lizard Holbrookia maculata were the lowest of all species analysed (∼25‰). Omnivorous grasshopper mice ( Onychomys torridus) had lower δ 18O values than granivorous rodents. Results from oxygen analysis likely reflect variation in diet and body water flux differences between endotherms and ectotherms. Carbon isotope analysis revealed a dramatic shift in diet from C 3 plants in 1999 to C 4 plants in 2000 in most rodents and birds. Kangaroo rats and reptiles did not change, having a constant δ 13 C value indicative of a C 3-based diet in both years. This suggests reliance on winter annual plant seed caches for kangaroo rats, but not other rodents. The carbon isotope data can be explained in terms of seasonal differences within and between years in the timing and intensity of the seasonal rainfall events, and the productivity of summer and winter annual plants. This study illustrates that stable isotope analysis is a powerful method for tracking dietary change and feeding behavior in desert vertebrates.

Effects of regional pool size on local diversity in small‐scale annual plant communities

The shape of the relationship between local and regional richness has been used to infer the relative strength of local vs. regional processes in structuring communities. While most empirical work has demonstrated linear relationships, recent reviews suggest that prior work has contained biases by selecting communities from different biological provinces and by defining large‐scale local communities. We evaluate the utility of inferring interaction strength from local–regional relationships by examining 11 years of data from small‐scale, annual plant communities at one site with strong local interactions. Both winter and summer annual plant assemblages exhibited linear rather than saturating relationships over a wide range of regional pool sizes. Our empirical work supports recent theory suggesting that local–regional relationships cannot be used to infer the strength of interactions in communities.