Reproductive patterns in annual legume species on an aridity gradient

Reproductive patterns are analysed in annual legumes of west Asia, and their relationships to increasing aridity determined by multivariate analysis. Dormancy, seed size, dispersal and fecundity are shown to be partially substitutable in terms of their effect on survival and population growth.The range of patterns show greatest diversity under mesic conditions in coastal, mediterranean areas with high winter rainfall, low incidence of frost and long growing season. Increasing aridity leads to increasing reproductive homogeneity, in which a subset of patterns-those composed of high levels of seed dormancy, high seed to pod ratios, restricted dispersal capabilities and early flowering-become predominant. These findings corroborate earlier theoretical and empirical evidence concerning desert annuals.The majority of widespread species are shown to possess arid-type patterns. This exerts considerable influence on both the type and degree of ecotypic differentiation within species. The only feature that responds consistently to climatic change is flowering time, particularly among species characterized by high seed dormancy. In the few widespread species that do not display arid-type characteristics, ecotypic differentiation appears more frequently in a larger suite of traits.The unequal environmental demands made on species showing different levels of dormancy (e.g. variation in length of growing season from year to year and place to place) leads to strong asymmetries governing the relationships between reproduction and ecological amplitude. In this respect arid-type strategists with high seed dormancy appear to have greater chances of expansion than others. This has implications when choosing pasture legumes to improve mediterranean grasslands.     

鄂尔多斯高原西部几种荒漠灌丛群落种间联结关系的研究

采用ＰＣ、ＰＣＣ、ＡＣ、Ｘ２和ｒ等公式对鄂尔多斯高原西部５种典型荒漠灌丛群落种间联结系数的求算,通过这些种间的相互关系,阐述了这些物种对生境要求的差异和分布特点,本文的研究结果对与建立在种群关系基础上的荒漠植被的研究具有重要的意义。     

Microhabitat differentiation in Chiuhuahuan Desert plant communities

The effects of microhabitat differentiation on small-scale plant community structure in the Chiuhuahuan Desert were studied using multivariate analysis. The results showed that microhabitats (i.e., kangaroo rat mounds, ant mounds, shrubs, half-shrubs, and open areas) played a critical role in structuring small-scale plant community structure and maintaining species diversity. Annual plants were much more sensitvive to the presence of differentiated microhabitats than perennials and winter annuals exhibited stronger microhabitat perferences than summer annuals. Species diversity was highest on ant mounds while open areas supported the lowest diversity during both winter and summer. Biomass was highest in the shrub habitats followed by kangaroo rat mounds, ant mounds, half-shrubs, and open areas. Much of the diversity of these plants could be explained by the individualistic responses of species to the biotic effect of other plants or to disturbance by animals, or individualistic responses of species to differences in microenvironments.     

Leaf expansion,net CO2 uptake,Rubisco activity,and efficiency of long-term biomass gain for the common desert subshrub Encelia farinosa

Encelia farinosa is one of the most abundant and highly studied species of the Sonoran Desert, yet characteristics of its leaf development and long-term photosynthetic capacity are relatively unknown. The net CO₂ uptake rate and the Rubisco activity per unit leaf area for E. farinosa in a glasshouse increased in parallel for about 18 days after leaf emergence (leaf area was then 5 cm²), after which both were constant, suggesting that Rubisco levels controlled net CO₂ uptake. Instantaneous net CO₂ uptake rates at noon for well-watered E. farinosa in the glasshouse at different temperatures and light levels correctly predicted differences in daily net CO₂ uptake at four seasonally diverse times for transplanted plants under irrigated conditions in the field but overpredicted the daily means by 13%. After this correction, seasonally adjusted net CO₂ uptake per unit leaf area multiplied by the estimated monthly leaf area predicted that 42% of the net carbon gain was incorporated into plant dry weight over a 17-month period. The ecological success of E. farinosa apparently reflects an inherently high daily net CO₂ uptake and retention of a substantial fraction of its leaf carbon gain.     

Mechanisms for the keystone status of kangaroo rats: graminivory rather than granivory?

Graminivory by kangaroo rats (Dipodomys spp.) was investigated as a potential mechanism for the keystone role of these rodents in the dynamics of desert grasslands. Experiments confirmed that Ord's kangaroo rats (Dipodomys ordii) cut and consumed a large proportion of the tillers of three Chihuahuan Desert tussock-forming grass species. Field observations indicated that the characteristically cut grass tillers were absent from all-rodent and medium-sized kangaroo rat exclosures, but were frequent in large-sized kangaroo rat and rabbit exclosures, indicating that the medium-sized kangaroo rats (D. ordii, D. merriami) were responsible for grass cutting. Tiller waste as a percentage of peak standing crop ranged from 7% in grassland habitats to 0.7% in Flourensia cernua shrubland. Of the 13 species of perennial, tussock-forming grasses measured, only one, Muhlenbergia porteri, had no tillers cut by kangaroo rats. This study demonstrates that the keystone role of kangaroo rats in Chihuahuan Desert grassland ecosystems is probably the result of their graminivory. Received: 28 October 1996 / Accepted: 26 February 1997     

Translocation of paclobutrazol and gibberellic acid in Sturt's Desert Pea (Swainsona formosa)

The translocation patterns of paclobutrazol and gibberellic acid were studied by applying these plant growth regulators locally to the main or lateral shoots of Sturt's Desert Pea. Paclobutrazol only reduced the growth of shoots to which it was directly applied indicating that it was readily translocated acropetally within a shoot (via xylem) but not basipetally (via phloem), although some phloem translocation has been reported. Gibberellic acid elongated the main shoot and enhanced apical dominance irrespective of the place of application suggesting it is readily translocated both through xylem and phloem. The translocation patterns did not vary between the main or lateral shoots.