Geographic patterns of simulated establishment of two Bouteloua species: implications for distributions of dominants and ecotones

Abstract. Our overall objective was to use a soil water model to predict spatial patterns in germination and establishment of two important perennial C₄-bunchgrasses across the North American shortgrass steppe and desert grassland regions. We also predicted changes in establishment patterns under climate change scenarios. Bouteloua gracilis dominates the shortgrass steppe from northeastern Colorado to southeastern New Mexico. Bouteloua eriopoda dominates desert grasslands in central and southern New Mexico. Germination and establishment for each species were predicted at 16 sites along the gradient using a daily time step, multi-layer soil water model (SOILWAT) to determine the percentage of years that temperature and soil water criteria for germination and establishment were met. Percentage of years with predicted establishment decreased from north to south for B. gracilis, but increased from north to south for B. eriopoda, comparable to observed dominance patterns. The 95 % confidence interval around the point at which simulated establishment were equal for the two species was near the location of the shortgrass steppe-desert grassland ecotone where both species are abundant. The intersection in percentage of years with establishment for the two species was predicted to move further north when climate was scaled using three Global Circulation Models (GCMs), indicating a possible northward expansion of B. eriopoda. Our results suggest that recruitment by seed may be an important process in determining, at least in part, the geographic distribution of these two species. Changes in climate that affect establishment constraints could result in shifts of species dominance that may or may not be accompanied by changes in species composition.     

铁线莲属研究随记(Ⅵ)

讨论了Clematis eriopoda Maxim和sect．Atragenopsis Boiss．的地位,认为这二分类群均应成立;描述了2新种,1新变种;过去长期被归并的卷萼铁线莲C.tubulosa得到恢复;Clematis heracleifolia var．ichangensis被转移改作卷萼铁线莲的变种;首次给出光叶铁线莲Clematis glabrifolia的果实的形态描述。     

Subdominant species distribution in microsites around two life forms at a desert grassland‐shrubland transition zone

Question: In the same landscape context — at a desert grassland‐shrubland transition zone, how does subdominant plant abundance vary in microsites around dominant grasses and shrubs? Location: Sevilleta LTER, New Mexico, USA (34°21’N; 106°53’W; 1650 m a.s.l.). Methods: We compared the distribution of subdominant plants in canopy, canopy edge and interspace microsites around individual shrubs (Larrea tridentata) and grasses (Bouteloua eriopoda) at a transition zone that has been encroached by shrubs within the past 50 ‐ 100 a. Plots of variable size according to microsite type and dominant plant size were sampled. Results: Subdominant abundance was higher in microsites around L. tridentata shrubs than in microsites around B. eriopoda. Furthermore, differences in species abundance and composition were higher among microsites around grasses than among microsites around shrubs. The distribution of subdominants was mostly explained by their phenological characteristics, which indicates the importance of temporal variation in resources to their persistence. Conclusions: This study of coexistence patterns around dominants revealed ecological contrasts between two dominant life forms, but other factors (such as disturbances) have to be taken into consideration to evaluate landscape‐scale diversity.     

铁线莲属研究随记(VI)(英文)

讨论了Clematis eriopoda Maxim.和sect.Atragenopsis Boiss.的地位,认为这二分类群均应成立;描述了2新种,1新变种;过去长期被归并的卷萼铁线莲C.tubulosa得到恢复;Clematisheracleifoliavar.ichangensis被转移改作卷萼铁线莲的变种;首次给出光叶铁线莲Clematisglabrifolia的果实的形态描述。     

Distribution of plant species at a biome transition zone in New Mexico

Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe — Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species? Location: central New Mexico, USA. Methods: Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled for the occurrence of subordinate species and soil texture within a 1500‐ha transitional mosaic of patches. Results: Of the 52 subordinate species analysed, 16 species were associated with B. gracilis‐dominated patches and 12 species with B. eriopoda‐dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs. Soils of B. gracilis‐dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda‐dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch‐scale variation in composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes. Conclusions: Patch types at this biome transition zone have characteristic life‐form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.     

Does shrub invasion indirectly limit grass establishment via seedling herbivory? A test at grassland‐shrubland ecotones

Question: Does shrub invasion at ecotones indirectly limit grass establishment by increasing mammalian seedling herbivory? Location: Chihuahuan Desert, New Mexico, USA. Methods: We tested the hypothesis that herbivore‐related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub‐dominated portions of grassland‐shrubland ecotones. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. Within each site we transplanted B. eriopoda seedlings to grass‐dominated, middle, and shrub‐dominated positions of replicate ecotones during the time of year (mid‐summer) when they would naturally appear and monitored seedling fates. We estimated population size/activity of putative small mammal herbivores. Results: Seedlings were killed by mammals in greater numbers in shrubland than in grassland or middle ecotone positions at the site with large herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in middle ecotone positions. The abundance patterns of herbivores did not parallel patterns of seedling herbivory across the ecotones or between sites. Conclusions: Seedling herbivory is an important process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear. We speculate that variation in small mammal foraging behavior may contribute to seedling herbivory patterns. Restoration strategies in the Chihuahuan Desert need to account for the abundance and/or behavior of native herbivores.     

气候变暖与荒漠-草原过渡带植物动态响应灰色分析--以美国新墨西哥州Sevilleta生态过渡带为例

在当前全球气候变暖的形势下,地表生态系统表现出不同的响应。干旱-半干旱地区的生态过渡带的生物反应更为敏感。美国新墨西哥州中部的荒漠-草原生态过渡带是监测全球气候变化和人类活动对生物影响的重要区域之一。本文以Sevilleta荒漠．草原生态过渡带为例,运用灰色系统分析方法．研究在当前全球气候变暖的条件下,1989～1998年10年间该生态过渡带的两种共存的植物优势种(Bouteloua eripoda)和(Bouteloua grncilis)历年的种群密度、组合形式和变化趋势,分析引起这些变化的主要气候因子。结果表明,Sevilleta生态过渡带中来自荒漠草原的优势物种Bouteloua eripoda的密度有上升的趋势,来自大草原的优势物种Bouteloua gracilis的密度有下降的趋势,它们的密度比大于1且有上升的趋势;在影响种群密度变化的降水、最高温度、最低温度、湿度的4个气候因子中,温度因子起着重要的作用。由此可以认为,随着气候的变暖,本区来自荒漠地区的优势植物种在荒漠-草原群落过渡带中将逐渐处于强优势地位,生态过渡带将有可能被荒漠草原所替代。     

Plant regulation of soil nutrient distribution in the northern Chihuahuan Desert

Vegetation throughout the southwestern United States has changed from perennial grassland to woody shrubland over the past century. Previous studies on the development of islands of fertility focused primarily on only the most limiting, plant-essential element, soil nitrogen (N). The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata, creosotebush. It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F). Within adjacent grassland and shrubland plots, surface soils were collected under and between vegetation and analyzed for a suite of soil nutrients. Soil nutrient distribution followed a uniform pattern that mirrored the spatial homogeneity of bunchgrasses in the grassland, but followed a patchy distribution that mirrored the spatial heterogeneity of individual shrubs in the shrubland. The main differences were that in the grassland, all elements were uniformly distributed, but in the shrubland the plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under the shrub canopy, and the non-limiting and non-essential elements were either concentrated in the intershrub spaces or were equally concentrated under shrubs and in the interspaces. Our results show how vegetation shifts from grassland to shrubland contribute to long-term, widespread change in the structure and function of desert ecosystems.