Density-dependent germination response by seeds of desert annuals

Summary Field observations and experiments indicate that the presence of seedlings at high densities inhibits subsequent germination of desert annuals. Since plants growing at high densities face severe competition for limited resources, this response by seeds is interpreted as an adaptation to avoid an unfavorable competitive climate where growth and survivorship are likely to be low.     

Nurse‐plant effects on the seed biology and germination of desert annuals

Nurse‐plants generally have positive effects on understorey species by creating more suitable conditions for stress‐intolerant plants relative to open micro‐habitats. However, long‐term effects of this plant–plant facilitation system have been rarely examined. Seeds of five desert annual species from Atiquipa coastal desert in Southern Peru were used to examine whether different microenvironmental conditions under the nurse‐plants Caesalpinia spinosa Molina (Kuntze) lead to differences in seed biology and germinability of annual plants relative to open, canopy‐free conditions. Seeds collected from plants associated with nurse‐plants were predicted to be (i) larger due to more favourable growing conditions, (ii) more viable and with greater germination rates, (iii) less variable in size and viability due to reduced environmental heterogeneity, and (iv) to germinate faster to avoid apparent competition with other annuals. Seed attribute measurements and germination trials in growth chambers were used to test these predictions. Although the plant abundance of only 2 of 5 species was strongly facilitated by the nurse‐plant, no significant differences were found in seed mass, viability or relative variability between understorey and open micro‐habitats for any of the species. Contrary to our predictions, final seed germination rates of seeds from open micro‐habitats were higher, and the open micro‐habitat treatment was more favourable for germination of seeds from both open and understorey environments. Taken together, these results suggest that plant–plant facilitation does not necessarily affect seed biology traits. Further studies addressing larger distribution ranges and/or density gradients of understorey species will illuminate the potential evolutionary effects of nurse‐plants.     

Seed germination and seedling emergence of three annuals growing on desert sand dunes in China

BACKGROUND AND AIMS: Information on the initial growth characteristics of annuals found in Chinese deserts is very limited. The aim of this study was to investigate seed germination and interactive effects of irrigation and seed burial depth in sand on seedling emergence and seedling survival in three annuals (Agriophyllum squarrosum, Bassia dasyphylla and Aristida adscensionis) commonly growing on sand dunes in these regions. METHODS: Effects of temperature, light and polyethylene glycol-6000 on seed germination were examined by irrigating seeds sown on filter paper in Petri dishes. Seedling emergence was examined for seeds sown on the surface of, or at different depths (5, 10, 20, 30, 40 and 50 mm) in, sand-filled pots, which were irrigated under different regimes. For seeds buried at a depth of 50 mm, seed viability was examined after irrigation of the pots. KEY RESULTS: Seeds of three species germinated at most temperatures recorded between spring and autumn in their native habitats. No seed dormancy was found in any species. For all three species, seedling emergence was most favoured when seeds were buried at a depth of 10 mm. When seeds sown on the sand surface were irrigated, seed germination was considerably suppressed due to water deficiency, but many seeds remained viable. For A. squarrosum and B. dasyphylla, many seeds that were deeply buried and irrigated remained ungerminated but viable, while for A. adscensionis deeply buried seeds germinated, but the seedlings did not emerge due to unfavourable seedling growth in deep sand. CONCLUSIONS: Precipitation is the most crucial factor in determining the seasonal emergence of seedlings of the three tested species in the field. The vertical distribution of seeds in sand determines the proportion of seeds that germinate after precipitation and acts to maintain seed banks over multiple years.     

Delayed seed dispersal and rapid germination as survival mechanisms of the desert plant Blepharis persica (Burm.) kuntze

Summary Experiments are described which demonstrate that the seeds of the desert species Blepharis persica are capable of germinating within 50 min of being exposed to moisture. Their radicles reach a length of about 49 mm within the first 24 h from wetting. The seeds can germinate in light and dark at temperatures ranging from less than 10° C to more than 40° C. The importance of these facts for survival are discussed.     

Modelling the niche space of desert annuals needs to include positive interactions

The niche is a necessary consideration when estimating habitable area and geographic range of a species. Modellers often examine the fundamental niche and the environmental requirements for plant species, ignoring interactions among species. In deserts, positive plant interactions are important drivers of biodiversity and structure communities through many mechanistic pathways including modifying environmental conditions. Thus, we tested the hypothesis that desert shrubs increase the geographical extent of some annual species because, through modifying the microclimate, they match the niche requirements of beneficiary species. We used the database of the Global Biodiversity Information Facility to construct MaxEnt species distribution models (SDM) with and without reported benefactor species within the Mojave Desert in California. We chose 20 annual species to be modeled including 10 species that had been previously reported in the literature as being facilitated (beneficiary) and 10 that had no record of being facilitated (unreported). Beneficiary annuals co‐occurred significantly more with benefactor shrubs than the unreported annual species. The inclusion of shrubs into SDMs significantly improved model predictability and geographic range for all the beneficiary annual species, but not for the unreported annual species. Thus, positive interactions are species specific and it is possible to determine annual species dependency on benefactor shrubs at the regional scale. The co‐occurrence of benefactor shrubs and annual species can be used as a proxy for facilitation and recent developments in SDM techniques encourage the inclusion of biotic interactions. Species distribution models should include estimates of facilitation because biotic interactions determine the niche of species and can have implications with a changing climate.     

Saprotrophic cord systems: dispersal mechanisms in space and time

In natural terrestrial environments, nutrients are often patchily and sparsely distributed, and the microclimate is constantly changing both temporally and spatially. To survive, fungi must be able to transfer to a new resource before the nutrient supplies in their current food base are exhausted. While the majority of fungi propagate as spores, some basidiomycetes can grow out of a resource as mycelium in search of new resources. The mycelium of these fungi typically aggregates to form linear organs, termed cords or rhizomorphs, that ramify at the soil-litter interface in forests, interconnecting disparate litter components to form extensive (many square meters or even hectares), long-lived (many years) systems. These mycelial systems form effective dispersal mechanisms in space and time. This article reviews the two main, but not mutually exclusive, mycelial dispersal (resource capture) strategies: (1) a “sit and wait” strategy, whereby a large mycelial network waits for resources to land on it and then actively colonises those resources; and (2) growing and searching actively for new resources. The way in which mycelia balance exploration and nutrient transport, and robustness to damage, against “cost” of production and speed with which an area can be colonised, is explored using techniques borrowed from graph theory and statistical mechanics.     

Phenotypical temperature adaptation of protein turnover in desert annuals

Protein synthesis and protein degradation rates were measured in three desert annual species at four different experimental temperatures. The taxa chosen for this study were the C₃ winter annuals, Bowlesia incana Ruiz & Pavon and Plantago insularis Eastw., and a C₄ summer annual, Atriplex elegans (Moq.) D. Dietr. Peak rates of protein synthesis correlated well with the preferred habitat temperatures of B. incana and A. elegans; optima occurred at 25 and 35°C, respectively. Plants of P. insularis showed an optimum protein synthesis rate at 35°C; however, this optimum rate was considerably lower than for the other two species. Higher activation energies for protein synthesis tended to parallel adaptation to higher temperature habitats. Responses of protein degradation to temperature in A. elegans and B. incana were consistent with their natural thermal regimes, when evaluated for the transition from 25 to 35°C. Again, protein degradation in P. insularis shows an intermediate response to temperature during the 25 to 35°C transition.     

Life‐history syndromes: Integrating dispersal through space and time

Recent research has highlighted interdependencies between dispersal and other life‐history traits, i.e. dispersal syndromes, thereby revealing constraints on the evolution of dispersal and opportunities for improved ability to predict dispersal by considering suites of dispersal‐related traits. This review adds to the growing list of life‐history traits linked to spatial dispersal by emphasising the interdependence between dispersal through space and time, i.e. life‐history diversity that distributes individuals into separate reproductive events. We reviewed the literature that has simultaneously investigated spatial and temporal dispersal to examine the prediction that traits of these two dispersal strategies are negatively correlated. Our results suggest that negative covariation is widely anticipated from theory. Empirical studies often reported evidence of weak negative covariation, although more complicated patterns were also evident, including across levels of biological organisation. Existing literature has largely focused on plants with dormancy capability, one or two phases of the dispersal process (emigration and/or transfer) and a single level of biological organisation (theory: individual; empirical: species). We highlight patterns of covariation across levels of organisation and conclude with a discussion of the consequences of dispersal through space and time and future research areas that should improve our understanding of dispersal‐related life‐history syndromes.     

Growth and abundance of desert annuals in an arid woodland in Oman

Given the role of Scaevola plumieri as a major pioneer species in the dune environment, as well as the need forindigenous sand stabilisers for South Africa’s coastal dunes, the reproductive phenology was unravelled as a firststep to establish the potential usefulness of the species in dune stabilisation programmes. In the highly variablecoastal environment the plants must cope with many problems for successful reproduction to take place. Theplants of S. plumieri overcome these difficulties through vegetative reproduction, but sexual reproduction is byno means insignificant. Numerous peduncles, each carrying multiple buds, were produced per stem, but only fewdeveloped into ripe seeds. Several of the flowers were unfertilised and of the seeds produced many were infectedand/or aborted. There was a strong effect of dune position on the reproductive phenology of S. plumieri. Thestems situated at the landward face of the foredunes showed an overall higher reproductive performance numberof buds, flowers, unripe seeds, ripe seeds. The phenogram showed that the reproduction of stems at the landwardside of the foredunes started on average 34 days earlier and showed a tendency for a longer reproductionperiod. For the duration of the different stages in the reproduction sequence, the bud stage’ took the shortesttime and the ‘ripe seed stage’ longest time. Between the different sample years, the production of the number ofpeduncles, buds, flowers and seeds was the similar; only differences in reproduction time (duration) of the differentstages were observed for the bud, flower an unripe seeds stage between the sample years.     

Photosynthetic capacity in relation to leaf position in desert versus old-field annuals

Summary Desert annuals of Death Valley, California have higher average light-saturated photosynthetic capacities and leaf nitrogen contents than do early-successional annuals of Illinois. The leaves of annuals in the light-unlimited Death Valley environment change little in specific weight, nitrogen, or photosynthetic capacity with age. In contrast, these properties decrease markedly with age in the leaves of the Illinois annuals even in leaves not exposed to the usual shading that accompanies canopy development. These results are interpreted in a carbon-gained-per-nitrogen-invested context.This study is dedicated to Professor M. Evenari for his continuing strong contributions to desert ecology     

Differences in seed dormancy and germination in amphicarpic legumes: manifold bet-hedging in space and time

两型结实豆科植物种子休眠和萌发的差异：空间和时间上多样性的两头下注策略通过休眠而实现的在空间或时间上的传播是生物体对它们所经历的环境施加影响的主要过程之一。在植物中,由于这两种传播都是通过种子进行的,而且两者在适应性方面发挥的作用也是类似的,因此预测它们之间存在较强的进化相关性。在本研究中,我们使用了两型结实的植株来研究这些进化相关性,这类植株能同时产生具有高度空间传播力的种子以及不会传播的地下种子。我们对经不同组合的休眠过程破坏处理(即干燥后熟、冷积层和物理划伤)后的种子的萌发百分比进行了评估,以研究两种两型结实豆科植物——野豌豆(Vicia amphicarpa L.)和山黧豆(Lathyrus amphicarpos L.),以及同它们亲缘关系很近的两种单型结实类群——救荒野豌豆(Vicia sativa L.)和红山黧豆(Lathyrus cicera L.)的飞播种子和地下种子在休眠和萌发方面的变化情况。研究结果表明,空间传播和时间传播之间存在着复杂的相互作用。在刚完成传播时,空播种子比地下种子更具休眠性,但当种子经后熟处理而开始物理休眠时,这种趋势也随之逆转。单型结实植株种子的萌发百分比高于其两型结实的同属植株,并在后熟处理后都同样失去了休眠性。相反,两型结实植株的种子则表现出同时受生理休眠和物理休眠调节的不同休眠性策略,预计这可能使出苗时间发生变化,从而提供了多层次多样化的两头下注策略(bet-hedging)。这一策略能够使植株依赖于丰产季中的历史有利区域而不阻碍空间和时间上的迁移,从而有可能在不可预测性很高的条件下具有适应性。     

Subsocial spiders in space and time: A fine scale approach to the dynamics of dispersal

Less than 0.2% of all spider species live in close associations with conspecifics. Among these, subsocial spiders show characteristics of both solitary spiders (e.g., individuals disperse for breeding) and social spiders (e.g., prolonged cooperative behaviours at least prior to independent reproduction). Dispersing individuals build small webs, usually with one inhabitant, whereas colonies are large webs with plant debris and harbouring multiple females. We studied the spatiotemporal dynamics of dispersal in the subsocial spider Anelosimus baeza. We followed the occupancy of all colonies and dispersal webs over the breeding season by mapping the number and sex of spiders with respect to their location in three dimensions. We studied the settlement patterns of new webs and fluctuation in web occupancy through movement between occupied and abandoned webs of colonies and dispersal webs. The occupancy of webs was highly dynamic with changes occurring at small time scales. The similarity in the patterns of web occupancy by females among dispersal webs was partially explained by their spatial and their temporal proximity. Our results suggest that dispersal webs may be used by spiders as a temporary refuge by both sexes during the breeding season. Patterns described here suggest new approaches to dispersal studies in group living spiders.     

Delayed dispersal: youth costs carry lifetime gains

An analysis of reproductive success in the green woodhoopoe Phoeniculus purpures challenges the view that delayed dispersal is costly. Females delaying dispersal for longer had more reproductive events in life and higher lifetime production of offspring.     

Spatial patterns of desert annuals in relation to shrub effects on soil moisture

Questions: What are the effects of a shrub (Haloxylon ammodendron) on spatial patterns of soil moisture in different seasons? How does productivity of understorey annuals respond to these effects? Are such effects always positive for annuals under shrubs? Location: South Gurbantunggut Desert, northwest China. Methods: Using geostatistics, we explored seasonal patterns of topsoil moisture in a 12 × 9‐m plot over the growing season. To determine spatial patterns of understorey annuals in response to H. ammodendron presence, biomass of annuals was recorded in four 0.2 × 5.0‐m transects from the centre of a shrub to the space between shrubs (interspace). We also investigated vertical distribution of root biomass for annuals and soil moisture dynamics across soil profiles in shrub‐canopied areas and interspaces. Results: Topsoil moisture changed from autocorrelation in the wet spring to random structure in the dry season, while soil moisture below 20 cm was higher in shrub‐canopied areas. Across all microhabitats, soil moisture in upper soil layers was higher than in deeper soil layers during the spring wet season, but lower during summer drought. Topsoil was close to air‐dry during the dry season and developed a ‘dry sand layer’ that reduced evaporative loss of soil water from deeper layers recharged by snowmelt in spring. Aboveground biomass of understorey annuals was lowest adjacent to shrub stems and peaked at the shrub margin, forming a ‘ring’ of high herbaceous productivity surrounding individual shrubs. To acclimate to drier conditions, annuals in interspaces invested more root biomass in deeper soil with a root/shoot ratio (R/S) twice that in canopied areas. Conclusions: Positive and negative effects of shrubs on understorey plants in arid ecosystems are commonly related to nature of the environmental stress and tested species. Our results suggest there is also microhabitat‐dependence in the Gurbantunggut Desert. Soil water under H. ammodendron is seasonally enriched in topsoil and deeper layers. Understorey annuals respond to the effect of shrubs on soil water availability with lower R/S and less root biomass in deeper soil layers and develop a ‘ring’ of high productivity at the shrub patch margin where positive and negative effects of shrubs are balanced.     

Within‐and among‐year germination in Sonoran Desert winter annuals: bet hedging and predictive germination in a variable environment

In variable environments, organisms must have strategies to ensure fitness as conditions change. For plants, germination can time emergence with favourable conditions for later growth and reproduction (predictive germination), spread the risk of unfavourable conditions (bet hedging) or both (integrated strategies). Here we explored the adaptive value of within‐ and among‐year germination timing for 12 species of Sonoran Desert winter annual plants. We parameterised models with long‐term demographic data to predict optimal germination fractions and compared them to observed germination. At both temporal scales we found that bet hedging is beneficial and that predicted optimal strategies corresponded well with observed germination. We also found substantial fitness benefits to varying germination timing, suggesting some degree of predictive germination in nature. However, predictive germination was imperfect, calling for some degree of bet hedging. Together, our results suggest that desert winter annuals have integrated strategies combining both predictive plasticity and bet hedging.     

Delayed germination of seeds: Cohen's model revisited

D. Cohen's (1966, J. Theor. Biol., 12, 110–129) model of delayed germination of seeds in a variable environment is extended to incorporate density dependence. It is shown that this has a considerable impact on the optimal germination rate, and the results are interpreted in terms of bet-hedging behavior. The effect of spatial dispersal is briefly considered.