Low seed availability may limit recruitment in grazed Atriplex vesicaria and contribute to its local extinction

Poor recruitment in Atriplex vesicaria Hewd exBenth (bladder saltbush) under sheep grazing in the chenopodshrublands of southern Australia contributes to a decline in the shrub'spopulation growth rate. This may lead to local extinction of the species overlarge areas around watering points. This study investigated whether low seedavailability may contribute to poor recruitment. It examined the incidence offlowering and seed bank size at sites distributed across a large grazedpaddock,and examined the longevity of seed in the soil. Grazing by sheep reduced theincidence of flowering and input to the seed bank. The proportion of shrubswithflowers increased with distance from water, showing the characteristicpiosphereresponse. Shrubs on grazed sites closer to water experienced extended periodswhen they failed to flower or flowered poorly. The seed bank was negligible atthree of the sites within 1650 m of water for all three years ofsampling. In contrast, the seed bank at the most distant site sampled (2800m from water) was small in 1990 (37 ± 5.1seeds/m²) but in 1991 and 1992 seed numberswere substantial (626 ± 315.2 seeds/m²and 318 ± 169.0, respectively). Soil seed was short-lived inthis study, with only 34% and 17% of the original seed remainingas viable ungerminated seed after 12 months for the under-canopy andexposed treatments respectively. Whilst recruitment may also be limited byaltered soil conditions due to grazing and trampling and the availability ofsafe sites, the results of this study suggest that low seed availability may bean important factor contributing to poor recruitment and may limit the abilityof the population to recover from the loss of established plants. Management ofgrazing must take into account the need for A. vesicariapopulations to flower and set seed on a regular basis.     

An ecological consideration of the significance of cation-exchange capacity of roots of some Utah range plants

Summary Root cation-exchange capacities (CEC) are related to tissue nutrient content of several native Utah range plants. The root CEC values for dicotyledonous species were found to be significantly larger than for monocotyledonous species (grasses). The relative amounts of monovalent and divalent cations taken up are strongly correlated with root CEC. Dicot species tend to take up divalent ions more efficiently than monocots, but monocots take up relatively more monovalent cations than dicots. The relationship of root CEC to cation uptake helps explain differential distribution of grass lands and shrublands in common climatic zones and has important implications for range revegetation programs.     

Spatial analysis of rangeland's vegetation intensity as related to selected physical soil variables over ABQAIQ municipality of Saudi Arabia

Through most of the natural rangelands of Saudi Arabia, overgrazing, sand drifting, and off-road vehicles driving are almost the major contributing factors that lead to vegetation damage and land degradation in natural landscapes. The current study aimed at examining the dynamic nature of Abqaiq rangelands' vegetation, generating a vegetation intensity map, and investigating possible impacts caused by the spatial variability of obtained soil texture, sand content (%), and the derived available water capacity (DAW capacity (%)) on the intensity distribution of vegetation cover. Ordinal-nominal correlation type and spatial autocorrelation processes were adopted throughout the study to analyze the spatial correspondences as well as the interrelated patterns of the given variables. The resultant correlation between vegetation and soil texture (ordinal versus nominal variables) has yielded a significant (p-value < 0.001) relevancies, where, silt loam texture class, in particular, has proven to have the most correlated values to the most intensive vegetation habitats, where 5%, 35%, and 28% of the silt loam class were occupied by 80%, 60%, and 40% of vegetation intensities, respectively. Whereas, for the continuous variables, correlation outcomes have achieved a substantial negative spatial autocorrelation concerning vegetation intensity and sand content percentage, revealing a total absence of green biomass over the sandy soils. Additionally, vegetation intensity versus DAW capacity percentage significantly yielded a positive autocorrelation, revealing a high clustering of green biomass cover that associates with high clustering of high-water capacity soils. The autocorrelation strength identifier (Morn's I), produced an approximate value of 0.3 with a pseudo p-value of 0.001, for both relationships. The findings of this study would help researchers and relative authorities to grasp the reasons, consequences and behaviors of rangeland flora, considering Abqaiq's area as an example.     

Use of deciduous woody species as a diet supplement for goats grazing Mediterranean shrublands during the dry season

Nutritional stress during the dry summer period is considered as a primary factor limiting goat production in the Mediterranean region. This study was conducted to determine if the browse of deciduous woody fodder plants is useful as a supplement for goats grazing kermes oak shrublands during July and September. Browses used were of Amorpha fruticosa L., Carpinus orientalis Mill., Colutea arborescens L., Fraxinus ornus L., Ostrya carpinifolia Scop. and Robinia pseudoacacia L.. Treatments included: (1)–(6) kermes oak (Quercus coccifera L.) shrubland plus fresh branches (2 kg/animal/day) of one of the above fodder plants as browse supplement, (7) kermes oak shrubland plus 0.25 (July) or 0.5 (September) kg alfalfa pellets/animal/day and (8) grazing in kermes oak shrubland as a control. Fodder plants produced high amounts of grazable material during summer whose quality ranged from medium (Carpinus, Fraxinus, Ostrya) to high (Amorpha, Colutea and Robinia). Goats ingested higher (p ≤ 0.05) browse of Fraxinus (268 g DM) and Ostrya (285 g DM) than the remaining browses (22–177 g DM) and alfalfa pellets (170 g DM) during July, but of Robinia and alfalfa (469 and 434 g DM, respectively) in September. During September, intake of all supplement feeds increased compared to July except for Ostrya, which remained constant. Goats supplemented by alfalfa pellets and browse supplements gained weight in both these periods in contrast to the animals grazing only kermes oak shrubland, except for Amorpha and Colutea in July. It is concluded that during summer, when the forage quality of kermes oak shrubland declines, browse supplement with certain deciduous woody fodder plants allows weight gains similar to those of alfalfa pellets; thereby, improving grazing animal performance.     

Competitive abilities of native grasses and non-native (Bothriochloa spp.) grasses

Old World Bluestems (OWB), introduced from Europe and Asia in the 1920s, recently have begun to raise concerns in the Great Plains. Despite suggestion in the late 1950s that OWB were weedy and negatively impacted biological diversity, they were widely introduced throughout the Great Plains for agricultural purposes. Anecdotal evidence suggests that OWB exhibit invasive characteristics that promote competitive exclusion of native species. The objective of our study was to quantify the competitive abilities of two OWB species (Caucasian bluestem; Bothriochloa bladhii (Retz.) S.T. Blake (= Bothriochloa caucasica (Trin.) C.E. Hubb.) and yellow bluestem; Bothriochloa ischaemum (L.) Keng) with three native grass species (big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium (Michx.) Nash), and sideoats grama (Bouteloua curtipendula (Michx.) Torr.)). A greenhouse target-neighbor study was conducted to assess both interspecific and intraspecific competition. A total of 480 pots (4.4 l) filled with native soil was used with all pair-wise combinations of species and four density treatments (six replications). Vegetative tiller height, above- and belowground biomass were measured at the end of 16 weeks. Both of the OWB significantly inhibited at least one growth parameter of the three native grass species, while most of the native species did not inhibit growth of either OWB species. Growth of B. ischaemum was enhanced when grown in association with S. scoparium. Based upon the results of our study of OWB competitive superiority and previous research, many of the characteristics possessed by OWB are found to be in common with known invasive species. Hence, we propose that two OWB are competitively superior to three common native prairie species providing them with the ability to invade and threaten the native grasslands of the Central and Southern Great Plains.     

The influence of soil depth on plant species response to grazing within a semi-arid savanna

Grassland patches within a semi-arid savanna were evaluated over 45-years for (1) local temporal dynamics of basal area for five dominant grass species within long-term heavily grazed and ungrazed treatments, (2) the influence of soil depth (resource availability) on vegetation dynamics, and (3) the applicability of community-level grazing response groups over fine-scale patterns of soil heterogeneity. Temporal patterns in species composition and basal area were dependent upon soil depth. In the heavy grazed treatment, Hilaria belangeri dominated deep soils while Erioneuron pilosum and Bouteloua trifida were restricted to shallow soils. In the ungrazed treatment, removal of grazing resulted in successional changes that were significantly different across soil depths. After 45 years without grazing, Eriochloa sericea was most abundant on deep soils while Bouteloua curtipendula was more abundant on intermediate and shallow soils. Community-level functional groups that are based on grazing were not appropriate when multiple pattern-driving variables were considered across multiple scales indicating that functional groups should only be applied to certain processes at specific scales. Within the ungrazed treatments, variable soil depths have resulted in a shifting mosaic in time and space where early- and late-successional species co-exist continuously but spatially separated within the community. In the heavily grazed treatment, species are somewhat spatially arranged by soil depths, but much of the inherent heterogeneity is eliminated and species composition is dominated by the three grazing-resistant short-grasses. Broad scale successional changes may appear linear and predictable while at finer scales, the same changes may be described as non-linear and dependent upon soil depth resulting in thresholds that are partially explained by weather patterns, seed bank limitations and competitive inhibitions.     

Spatial patterns in leaf area and plant functional type cover across chronosequences of sagebrush ecosystems

Since most studies of ecosystem dynamics after disturbance require longer durations of study than the life span of most research careers, many studies rely on chronosequence approaches to substitute space for time. We tested the chronosequence approach for assessing the change in plant functional type cover and leaf area index (L) using three replicated mountain big sagebrush (Artemesia tridentata var. vaseyana (Rydb.) Boivin) dominated ecosystems in southern Wyoming. We further tested our broader inferences of mountain big sagebrush ecosystem chronosequences by assessing whether dynamics in spatial patterning of plant functional type cover and leaf area index would compromise the chronosequence approach. We hypothesized that (1) L and total cover increase with age at similar rates across replicated chronosequences, (2) spatial autocorrelation is greatest with shrub cover, and (3) spatial autocorrelation increases with age. We failed to reject all three hypotheses. Our analyses showed that mean shrub cover, total cover, and L all increased linearly with time since disturbance across all three replicated chronosequences. While neither graminoid nor forb cover was correlated with time since disturbance, graminoid cover did show an inverse relationship with shrub cover and L. Semivariogram analysis showed that spatial patterning increased with shrub cover and time since disturbance. Thus, while we cannot yet provide a process to fit the spatial patterns, the chronosequence approach for sagebrush ecosystems recovering from disturbance has survived a rigorous test because the mean changes in shrub cover, total cover, and L were replicable across three different sites.