CAM variations in the leaf-succulent Delosperma tradescantioides (Mesembryanthemaceae), native to southern Africa

Drought responses of diurnal gas exchange, malic acid accumulation and water status were examined in Delosperma tradescantioides , a succulent that grows in drought-prone microenvironments in summer rainfall and all-year rainfall regions of southern Africa. When well-watered, this species exhibited Crassulacean acid metabolism (CAM)-cycling, but its carbon fixation pattern changed during the development of drought, shifting to either low-level CAM or to CAM-idling. The rate and pattern of this change depended on environmental conditions, duration of water stress and leaf age. At the onset of drought, diurnal malate fluctuation increased, but was strongly depressed (by ca 70%) as drought continued, and when leaf water content and water potential were low (ca 35 and 50% of the initial levels, respectively). When rewatered, rates of growth and photosynthesis, gas exchange and water status recovered fully to pre-stressed values within two days. Whole-shoot carbon uptake rates suggested that leaf growth had continued unabated during a short-term (≅ one week) drought. This emphasises that CAM-idling allows the maintenance of active metabolism with negligible gas exchange when soil water is limiting. It is possible that old or senescent leaves may provide water for the expansion of developing leaves during initial periods of drought. Regardless of the water regime and environmental conditions, leaf nocturnal malate accumulation and water content were positively correlated and increased with leaf age. Thus the gradual loss of water from older mature leaves may induce CAM-idling, which reduces water loss. An important ecological consequence of this combination of CAM modes is the potential to switch rapidly between fast growth via C₃ gas exchanges when well-watered to water-conserving CAM-idling during drought.     

Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Summary Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C₃ photosynthesis. ¹⁴CO₂ uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C₃ during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO₂ uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO₂ uptake was absent. The shift from CAM to CAM-idling or C₃ in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO₂ uptake. Whereas during the summer, rewatering quickly resulted in early morning CO₂ uptake, but nocturnal CO₂ uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C₃ pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.     

CAM variations in the leaf-succulent Delosperma tradescantioides (Mesembryanthemaceae), native to southern Africa

Drought responses of diurnal gas exchange, malic acid accumulation and water status were examined in Delosperma tradescantioides , a succulent that grows in drought-prone microenvironments in summer rainfall and all-year rainfall regions of southern Africa. When well-watered, this species exhibited Crassulacean acid metabolism (CAM)-cycling, but its carbon fixation pattern changed during the development of drought, shifting to either low-level CAM or to CAM-idling. The rate and pattern of this change depended on environmental conditions, duration of water stress and leaf age. At the onset of drought, diurnal malate fluctuation increased, but was strongly depressed (by ca 70%) as drought continued, and when leaf water content and water potential were low (ca 35 and 50% of the initial levels, respectively). When rewatered, rates of growth and photosynthesis, gas exchange and water status recovered fully to pre-stressed values within two days. Whole-shoot carbon uptake rates suggested that leaf growth had continued unabated during a short-term (∼ one week) drought. This emphasises that CAM-idling allows the maintenance of active metabolism with negligible gas exchange when soil water is limiting. It is possible that old or senescent leaves may provide water for the expansion of developing leaves during initial periods of drought. Regardless of the water regime and environmental conditions, leaf nocturnal malate accumulation and water content were positively correlated and increased with leaf age. Thus the gradual loss of water from older mature leaves may induce CAM-idling, which reduces water loss. An important ecological consequence of this combination of CAM modes is the potential to switch rapidly between fast growth via C₃ gas exchanges when well-watered to water-conserving CAM-idling during drought.     

Growth and tissue mass cycles in the infaunal bivalve Yoldia eightsi at Signy Island, Antarctica

Shell growth in Yoldia eightsi was measured over an austral summer and winter in 1992. In specimens < 12 mm length, growth was not significantly different between summer and winter periods, and the fastest recorded rate, 6.3 μm day⁻¹ was for 5-mm individuals during the winter. In summer, specimens of all lengths grew significantly, but in winter bivalves > 27 mm length did not increase in length. Tissue dry and ash-free dry mass (AFDM) cycles were assessed at monthly intervals between December 1988 and January 1991. ANCOVA indicated significant interannual and seasonal effects on this cycle. Tissue mass increased in the summer, coinciding with the phytoplankton bloom and the period of maximum sedimentation of organic material from the water column. A standard 20-mm-length animal reached a maximum AFDM of 114 mg in February 1990. The minimum value (68 mg AFDM) throughout the 2 years of measurements was in early December 1988, at the end of the austral winter. Periods of tissue mass increase were, therefore, decoupled from shell growth, at least in juveniles. Tissue mass was significantly higher in 1990 than 1989, which was mainly due to high organic contents in the summer (January to May). This was not consistent with the pattern of organic content in the sediments at the study site, but was in phase with the cycle in sediment chlorophyll a content. Tissue mass increase depended on major resource input during the summer, but Y. eightsi was capable of maintaining winter condition from stocks of benthic microalgae in years of poor ice cover. Tissue mass declined between April and July each year. This was accompanied by large falls in tissue ash content, and coincided with the spawning period in early June. These are the first monthly tissue mass data collected over a 2-year period for an Antarctic mollusc. They are the first such data indicating seasonal variation in tissue mass and showing a decoupling of shell and tissue growth in a polar bivalve. The P/B ratio calculated from these data was 0.106, which is slightly lower than previous values found for this species, but is in line with general values for Antarctic marine benthos. Accepted: 6 December 1999     

Nutrient release from decomposing leaf litter of Banksia ornata,Dark Island heathland,South Australia

Distinct O₁ and O₂ layers, representing annual litter fall, enabled the sequential loss of biomass and nutrients (phosphorus and nitrogen) to be reconstructed in undisturbed litter layers of Banksia ornata in the Dark Island heathland, South Australia. Apart from an initial loss in biomass and nitrogen, the dry weight and nutrient content of the O₁ layer, exposed to the desiccating influence of the atmosphere, remained relatively constant until covered by the following year's leaf fall. Under the blanket of newly fallen leaves, biomass decomposition proceeded continuously through autumn, winter, spring, into the dry summer season. Even though the biomass of the decomposing leaf (O₂) layer decreased continuously, its nutrient content remained relatively constant until the summer season was reached when total decomposition and nutrient loss occurred. During spring, fine rootlets invaded the decomposing litter layer (O₂) and, together with decomposer fungi, bacteria and soil fauna, maintained the total nutrient content of the decomposing leaf at a constant level. By late spring-early summer shoot growth of the dominant heath species was initiated, inducing the mobilization of the nutrients stored in the decomposing litter layer.     

Water relations of Picea abies. I. Comparison of water relations parameters of spruce shoots examined at the end of the vegetation period and in winter

Seasonal changes of some water relations parameters of Norway spruce shoots ( Picea abies [L.] Karst.) were studied during two experiments using the pressure-volume analysis. For each experiment only shoots of a single tree were used.
During the first study, the course of the turgor loss point (as bulk osmotic pressure when turgor first reaches zero, πp) of shoots developed in late 1986 vegetation period, were measured in 1987. The turgor loss point decreased temporarily from –2.5 MPa at the beginning of the year to –3.3 MPa at the end of March, but then increased to the original level for the rest of the year.
During the second study, water relations parameters were measured in late summer 1987 and in late winter 1988. Winter shoots at full water saturation contained up to 20% less water than in late summer. Accordingly, the bulk osmotic pressure at full water saturation (π_p) decreased from –1.7 MPa in late summer to –1.9 MPa in winter, π_p decreased also from –2.2 MPa to –2.8 MPa. However, the amount of osmotically active substances (mOsmol, N) remained unchanged. The relative amount of apoplastic water in the total shoot water content appeared to drop insignificantly from 17% to 15%.
The results show that the decrease in π_o and π_p in late winter is not due to an accumulation of osmotically active substances in the vacuoles but is due to a decrease in tissue water content. The temporary reduction of the symplastic volume by deposition of osmotically inert substances seems to be the most probable cause of this phenomenon.     

Alpine marmot (Marmota marmota) distribution evolution under climate change: The use of species distribution models at a local scale in the western Pyrenees massif (France)

The Alpine marmot (Marmota marmota) is a social mammal living in mountainous grassland areas and has the particularity to hibernate in winter. Recent studies on a population in the French Alps found that climate change is affecting Alpine marmot population dynamics and might impact their overall distribution in the future. Using Species Distribution Models (SDMs), the effect of climate change on Alpine marmot's future distribution was investigated at a local scale, in the western part of the Pyrenean massif (New-Aquitaine region, France). This scale was chosen as an appropriate action scale for the conservation strategy for the species. Three climatic scenarios were used (RCP 2.6, RCP 4.5, and RCP 8.5) over three future 30-year periods (2021–2050, 2040–2070, 2071–2100) to predict the short- to long-term potential distribution of the target species. The results are consistent with naturalistic knowledge of the species´ ecological needs in terms of variable importance and response type. Mean maximum temperature in winter, standard-deviation of daily temperature in winter, along with the median rainfall amount in summer were the three most important climatic variables. Predictions under the two most pessimistic climate scenarios showed potential large habitat loss. In the long term, for RCP 4.5, an estimated habitat loss of 18% was predicted. In the case of RCP 8.5, a higher impact was predicted, with a 54% habitat loss. Our results show that high impact due to climate change can be expected at a long term. In addition, if winter climatic conditions are important for marmot survival through hibernation, drought in summer might be one of the drivers of future population dynamic and distribution. Our findings can be applied for other species living in grassland mountainous environments and for which access to food resources in summer is essential, facilitating the conservation of target areas.     

Seasonal biochemical plasticity of a flight muscle in a bat, Murina leucogaster

Cellular and biochemical responses of the pectoral muscle to variation in seasonal activity were studied in the bat, Murina leucogaster ognevi. We collected bats in mid-hibernation (February), end-hibernation (April), and mid-summer (August) to track major activity periods in their annual cycle. Our findings indicated that myofiber cross-sectional area decreased to 68% between mid- and end-hibernation, but returned to the winter level in mid-summer. Total soluble protein and total RNA concentrations were not altered over these sampling periods. Oxidative potential gauged by citrate synthase activity increased 1.47-fold from mid- to end-hibernation and then remained at the similar level in mid-summer. Glycolytic potential gauged by lactate dehydrogenase activity changed little between mid- and end-hibernation but increased 1.42-fold in summer, compared with the winter level. Thus, the myofibers underwent disuse atrophy during hibernation, while enzymatic catalytic function recovered towards the level of mid-summer.     

Population dynamics and regulation in the coypu Myocastor coypus in central Italy

A capture-recapture design was applied between January 1989 and June 1991 in a 37 5 ha area located in the central valley of the Tiber River (Latium), aimed at estimating the demographic parameters of a coypu population in the Mediterranean climate and at preliminarily investigating the factors that regulate its numbers in this region The population fluctuated between 27 and 137 individuals and followed a seasonal pattern It decreased after the winter and increased between summer and winter Density remained almost stable during a particularly mild winter Survival rates remained relatively high over the entire study period, reproductive activity and additions to the population from in situ reproduction, however, showed minimum peaks following colder winters Evidence of immigration was detected during periods of increasing density Key factor and preliminary density dependence analyses showed that pregnancy failure and newborn losses are cumulatively the most important factor contributing to varation in total mortality, and are positively related to coypu numbers in previous periods The failure to realize maximum potential fecundity is inversely related to previous coypu abundance We show that post-recruitment losses play only a minor role in determining population decline     

Energy reserves during food deprivation and compensatory growth in juvenile roach: the importance of season and temperature

The effect of 21 days of starvation, followed by a period of compensatory growth during refeeding, was studied in juvenile roach Rutilus rutilus during winter and summer, at 4, 20 and 27° C acclimation temperature and at a constant photoperiod (12L : 12D). Although light conditions were the same during summer and winter experiments and fish were acclimated to the same temperatures, there were significant differences in a range of variables between summer and winter. Generally winter fish were better prepared to face starvation than summer fish, especially when acclimated at a realistic cold season water temperature of 4° C. In winter, the cold acclimated fish had a two to three‐fold larger relative liver size with an approximately double fractional lipid content, in comparison to summer animals at the same temperature. Their white muscle protein and glycogen concentration, but not their lipid content, were significantly higher. Season, independent of photoperiod or reproductive cycle, was therefore an important factor that determined the physiological status of the animal, and should generally be taken into account when fish are acclimated to different temperature regimes. There were no significant differences between seasons with respect to growth. Juvenile roach showed compensatory growth at all three acclimation temperatures with maximal rates of compensatory growth at 27° C. The replenishment of body energy stores, which were utilized during the starvation period, was responsible for the observed mass gain at 4° C. The contribution of the different energy resources (protein, glycogen and lipid) was dependent on acclimation temperature. In 20 and 27° C acclimated roach, the energetic needs during food deprivation were met by metabolizing white muscle energy stores. While the concentration of white muscle glycogen had decreased after the fasting period, the concentrations of white muscle lipid and protein remained more or less constant. The mobilization of protein and fat was revealed by the reduced size of the muscle after fasting, which was reflected in a decrease in condition factor. At 20° C, liver lipids and glycogen were mobilized, which caused a decrease both in the relative liver size and in the concentration of these substrates. Liver size was also decreased after fasting in the 4° C acclimated fish, but the substrate concentrations remained stable. This experimental group additionally utilized white muscle glycogen during food deprivation. Almost all measured variables were back at the control level within 7 days of refeeding.     

Winter severity limits red fox populations in Eurasia

Aim  We investigated geographical variation in the density of the red fox in relation to climatic variables, habitat productivity and seasonality to identify those factors that were the best predictors of fox density.
Location  Published data on red fox abundance were collated from 69 locations over Europe and Asia.
Methods  Using generalized linear models and the information-theoretic approach, we analysed the contribution of climatic measures (winter and summer temperature, mean snow depth and duration), primary productivity and seasonality indices [based on the fraction of photosynthetically active radiation (FPAR) index] to account for variation in red fox density.
Results  Red fox density in winter ranged from 0.001–2.8 individuals km⁻²; the average density was 0.21 individuals km⁻². Variation in red fox density was best explained by the winter temperature and seasonality. Density decreased with declining winter and summer temperatures, increasing degree of seasonality and increasing duration of snow cover. There was no relationship with habitat productivity.
Main conclusions  Our results indicate that winter climatic conditions and seasonality, but not habitat productivity, may limit red fox density in Eurasia. One explanation for the limitation of the red fox population may be the fox's physiological capability to cope with abiotic conditions. Concurrently, the severity of winter may lead to reduced availability of the fox's prey. That, together with a shorter reproductive period may result in lower reproductive output as well as lower survival of adults and cubs.     

Rewilding the Scottish Highlands: Do Wild Boar,Sus scrofa,Use a Suitable Foraging Strategy to be Effective Ecosystem Engineers?

Ecosystem engineers are increasingly being reintroduced to restore ecological processes in restoration and rewilding projects. To predict and adaptively manage the impact of such species their behavioral ecology must be understood and quantified. Rooting behavior by wild boar qualifies them as ecosystem engineers due to their impact on vegetation disturbance regimes. The behavioral foraging ecology of wild boar was quantified in a fenced area in the Scottish Highlands in order to provide some of the understanding necessary to predict their ability to affect ecosystem restoration. Five wild boar were monitored within a 125 ha fenced area using Global Positioning System (GPS) collars and behavioral monitoring over a 12‐month period. Their activity budget, the relationship between foraging behavior and vegetation communities, and how these relationships vary between seasons was investigated. The results indicate that wild boar invested approximately four more hours daily to rooting during the autumn and winter than the spring and summer. During the spring and summer, grazing was the dominant foraging behavior (approximately 28% of foraging period) while rooting dominated in autumn and winter (approximately 76% of foraging period). Deep rooting behavior is particularly associated with bracken‐dominated communities. Associations between rooting, vegetation community, and season will have a strong influence on the spatial and temporal distribution of rooting behavior. This variation could have important implications for the impacts of boar on vegetation community dynamics. These results detail some of wild boar's ecosystem engineering behaviors; however, further research is required to consider the wider impacts of a full reintroduction.     

Adventitious rooting: examining the role of auxin in an easy- and a difficult-to-root plant

Therooting responses of cuttings of difficult-to-root lilac (Syringavulgaris) and easy-to-root forsythia(Forsythia×intermedia)were compared. The rooting ability of lilac cuttings declined over the growingseason (May–June). There was also a decline in the initial concentrationof free IAA at the base of the cuttings, but there was not a tight relationshipbetween basal IAA concentration and rooting ability. Polar auxin transportability was measured in lilac and forsythia during the period of maximum growthby [³H]IAA application to stem internodal tissue. Transport abilitydeclined in lilac over this time period, particularly in terms of transportintensity and percentage of [³H]IAA transported. In contrast thechanges in polar auxin transport ability in forsythia were less marked. Thisdifference between species was maintained in winter hardwood cuttings, withforsythia tissue showing greater polar auxin transport ability than lilac. Theimportance of polar auxin transport for adventitious rooting was demonstratedinboth lilac and forsythia softwood cuttings by use of the polar transportinhibitor 2,3,5-triiodobenzoic acid (TIBA). Overall the results indicate thatdifferences in polar auxin transport ability between lilac and forsythiacontribute to differences in rooting ability.     

Effects of climate on population fluctuations of ibex

Predicting the effects of the expected changes in climate on the dynamics of populations require that critical periods for climate‐induced changes in population size are identified. Based on time series analyses of 26 Swiss ibex (Capra ibex) populations, we show that variation in winter climate affected the annual changes in population size of most of the populations after accounting for the effects of density dependence and demographic stochasticity. In addition, precipitation during early summer also influenced the population fluctuations. This suggests that the major influences of climate on ibex population dynamics operated either through loss of individuals during winter or early summer, or through an effect on fecundity. However, spatial covariation in these climate variables was not able to synchronize the population fluctuations of ibex over larger distances, probably due to large spatial heterogeneity in the effects of single climate variables on different populations. Such spatial variation in the influence of the same climate variable on the local population dynamics suggests that predictions of influences of climate change need to account for local differences in population dynamical responses to climatic conditions.     

Trigonella arcuata-associated rhizobia��an Ensifer (Sinorhizobium) meliloti population adapted to a desert environment

Long-term monitoring of soil properties reveals site-specific ecosystem shifts in soil processes due to land use and climate changes. This paper aims to study the effects of physical landscape changes associated with grazing on soil thermal and moisture regime at the plot scale in a semiarid Leymus chinensis steppe of Inner Mongolia, China. The investigated sites were subjected to three grazing intensities: ungrazed since 1979 (UG79), moderately grazed only in winter time (WG), and heavily grazed (HG). At each plot, we recorded the soil moisture and temperature over a 6-year period that spanned between June 2004 and September 2009 and experienced a large range in precipitation (162 to 362 mm). Based on these monitoring data, we divided a year into four hydric periods: (1) growing period (late April to August); (2) transitional period from summer to winter (September?COctober); (3) winter time (November?Cfirst March); and (4) transitional period from winter to summer (March?CApril). In general, soil moisture in grazed sites was lower than in the ungrazed site, particularly for the 30?C50 cm soil layer. Seasonal fluctuation of the soil moisture, due to variable precipitation and atmospheric demands, was most significant in the topsoil (0?C10 cm) and was less pronounced in deeper soil. Regardless of hydric seasons, soil moisture was significantly influenced by grazing intensity, whereas soil temperature was slightly influenced. With increasing grazing intensity, soil water storage decreased remarkably. Consequently, grazing reduced plant available water and therefore grassland productivity, which are linked to a great extent with the trampling-induced soil structure change and soil moisture regime.     

Seasonal activity and distribution of cereal aphid parasitoids in Belgium

Activity of antagonists in winter and early spring as well as in late summer and autumn plays an important role in the control of cereal aphid populations. Indeed, parasitoids active early in the year are important to avoid high aphid densities. Late summer and autumn correspond to the transition period between two wheat cultures, and parasitoid activity would thus reduce aphid infestation. During these critical periods, other crops like rye-grass and red clover containing alternative hosts could play the role of parasitoid reservoirs. During 1996, parasitoid activity was measured by trap planting in wheat, rye-grass, red clover and fallow grassland. In each plot, early spring was characterized by an important parasitoid activity, while aphids remained at a low level. Later in spring, activity decreased, and aphid development began even during this period. During the summer, when aphids reached their population peak, parasitoid activity rose again. Percentage of parasitism of aphids sampled was only important during summer in red clover and rye-grass. In wheat and fallow grassland, percentage of parasitism remained at a low level. The role of synchronization between parasitoid activity and aphid development is discussed.     

Water relations of the freeze-tolerant New Zealand alpine cockroach Celatoblatta quinquemaculata (Dictyoptera: Blattidae)

Celatoblatta quinquemaculata is a freeze-tolerant alpine cockroach found on the Rock and Pillar Range, Central Otago, New Zealand. This study investigated seasonal changes in water content, as well as desiccation tolerance, and the relationship between desiccation and cold tolerance. Whole body water contents from field-fresh cockroaches collected over a 20 month period ranged from 69.9+/-1.0% fresh weight (FW) in February 1998 to 60.3+/-1.1% FW in July 1998. Water contents were significantly lower in winter than summer, and were positively correlated to microhabitat temperatures over the week preceding collection. Cockroaches survived the loss of up to 82% (mean: 56.7%+/-10.2) of their initial body water content, and the amount of water loss sustained was not dependent on the rate of water loss. Cockroaches did not suffer further mortality due to desiccation after removal to 99% relative humidity, but only regained lost water if given access to liquid water. Experimental dehydration did not enhance freeze-tolerance, but did slightly lower the supercooling point. It is concluded that reduction of body water content in winter may be a consequence of cold hardening responses, but desiccation does not constitute the cold hardening mechanism itself.     

The influence of climatic conditions on long-term changes in the helminth fauna of terrestrial molluscs and the implications for parasite transmission in southern England

The influence of climatic conditions on the prevalence and transmission of helminth parasites in a terrestrial mollusc population was studied in a grassland site in southern England between 1974 and 1983. Molluscs were sampled in each September of 5 years over this period (1974, 1976, 1979, 1981, 1983). Climatic conditions had a variable effect on parasite prevalence. Trematode sporocyst infections increased after wet summer and warm winter conditions and declined in hot, dry periods. Cestode infections increased after combined wet spring and summer weather and low winter temperatures, although trematode metacercariae and nematode infections were less likely to be influenced by climate. The effects on parasite transmission were undertaken by comparing parasite prevalences in the principal definitive hosts, the common shrew (Sorex araneus) and the wood mouse (Apodemus sylvaticus) in the same habitat over the period 1973-1983. Changes in parasite prevalences in the molluscan population were rarely replicated by changes in the small mammal population, except for trematode parasites in small mammals during a period of severe drought in 1976. These results suggest that only long-term persistent modifications in climate are likely to affect host-parasite dynamics.     

Fatty acids and cholesterol in the liver cell nuclei of hibernating Yakutian ground squirrels

The content of neutral lipids in tissue homogenates and liver cell nuclei of hibernating Yakutian ground squirrels was studied. In homogenates, hibernation increases the content of fatty acids and reduces the content of glycerides and cholesterol. When studying the liver cell nuclei of torpid winter ground squirrels, we detected a twofold increase in the content of fatty acids, cholesterol, and monoglycerides as compared to the “summer” ground squirrels. In the active “winter” ground squirrels, as compared to the torpid winter ones, the content of cholesterol did not change, whereas the content of fatty acids, monoglycerides, and diglycerides decreased but remained higher than in the “summer” ground squirrels.     

Long‐term dynamics of winter and summer annual communities in the Chihuahuan Desert

Abstract. Winter and summer annuals in the Chihuahuan Desert have been intensively studied in recent years but little is known about the similarities and differences in the dynamics between these two communities. Using 15 yr of census data from permanent quadrats, this paper compared the characteristics and temporal dynamics of these two distinct, spatially co‐existent but temporally segregated communities. Although the total number of summer annual species recorded during our 15 yr of observation was higher than winter annuals, the mean number of species observed each year was higher in the winter community. The winter community exhibited lower temporal variation in total plant abundance and populations of individual species, lower species turnover rate and higher evenness than the summer community. The rank abundances of species in winter were significantly positively correlated for a period of up to 7 yr while in summer significant positive correlations in rank abundance disappeared after 2 to 3 yr. The higher seasonal species diversity (i.e. number of species observed in each season) in winter rather than the overall special pool (over 15 yr) may be responsible for the greater community stability of winter annuals. The difference in long‐term community dynamics between the two communities of annual plants are likely due to the differences in total species pool, life history traits (e.g. seed size), and seasonal climatic regimes.