Ratio Of Grass Research Articles

The use of food waste-based diets and Napier grass to culture grass carp: growth performance and contaminants contained in cultured fish.

The present study used commercial feeds, food waste feeds, Napier grass, and mixed feeds (food waste feed to Napier grass ratio, 1:10) to feed grass carp (Ctenopharyngodon idellus). The results indicated that grass carp fed with food waste feeds and mix feeds achieved growth performance (based on specific growth rate and feed conversion ratio) that was similar to commercial feeds (p > 0.05). Concentrations of metalloid/metals in food waste feeds and polycyclic aromatic hydrocarbons (PAHs) in Napier grass were relatively higher than other types of fish feeds (p < 0.05). However, most of the metalloid/metals and PAH levels in fish fed with four types of fish feeds were not significantly different (p > 0.05). These findings show that food waste feeds are suitable for using in the production of fish feed and Napier grass can be served as supplemental feeds for grass carp, and hence reducing the production cost.

Merging trait-based and individual-based modelling: An animal functional type approach to explore the responses of birds to climatic and land use changes in semi-arid African savannas

Climate change and land use management practices are major drivers of biodiversity in terrestrial ecosystems. To understand and predict resulting changes in community structures, individual-based and spatially explicit population models are a useful tool but require detailed data sets for each species. More generic approaches are thus needed. Here we present a trait-based functional type approach to model savanna birds. The aim of our model is to explore the response of different bird functional types to modifications in habitat structure. The functional types are characterized by different traits, in particular body mass, which is related to life-history traits (reproduction and mortality) and spatial scales (home range area and dispersal ability), as well as the use of vegetation structures for foraging and nesting, which is related to habitat quality and suitability. We tested the performance of the functional types in artificial landscapes varying in shrub:grass ratio and clumping intensity of shrub patches. We found that an increase in shrub encroachment and a decrease in habitat quality caused by land use mismanagement and climate change endangered all simulated bird functional types. The strength of this effect was related to the preferred habitat. Furthermore, larger-bodied insectivores and omnivores were more prone to extinction due to shrub encroachment compared to small-bodied species. Insectivorous and omnivorous birds were more sensitive to clumping intensity of shrubs whereas herbivorous and carnivorous birds were most affected by a decreasing amount of grass cover. From an applied point of view, our findings emphasize that policies such as woody plant removal and a reduction in livestock stocking rates to prevent shrub encroachment should prioritize the enlargement of existing grassland patches. Overall, our results show that the combination of an individual-based modelling approach with carefully defined functional types can provide a powerful tool for exploring biodiversity responses to environmental changes. Furthermore, the increasing accumulation of worldwide data sets on species’ core and soft traits (surrogates to determine core traits indirectly) on one side and the refinement of conceptual frameworks for animal functional types on the other side will further improve functional type approaches which consider the sensitivities of multiple species to climate change, habitat loss, and fragmentation.

Replacement of aruana grass by gliricidia (Gliricidia sepium) on silage quality

Our objective was to investigate the effect of the replacement of aruana grass by gliricidia on the fermentative losses, chemical composition, and aerobic stability of silages. For ensiling, whole-crop aruana grass (75 d of growth) was chopped (275 g kg-1 dry matter - DM) and ensiled alone or associated with gliricidia (270 g kg-1 DM; 150 d of growth). For gliricidia, we used only the leaves and stalks to ensile. The evaluated treatments were different ratios of aruana grass to gliricidia (100:0, 75:25, 50:50, 25:75, and 0:100), with four replicates (mini-silos). The silage composed only of gliricidia exhibited a lower pH than the other silages. Dry matter recovery was not affected by the treatments, although effluent losses were affected. The replacement of aruana grass by gliricidia 50% at ensiling resulted in a higher lactic acid bacteria count. The aruana grass silages displayed lower protein contents and a higher neutral detergent fiber content than the gliricidia silages after 40 d of fermentation. Consequently, the silage consisting of 100% gliricidia showed higher in vitro DM digestibility, presenting an increase of 8.13% after 40 d of fermentation compared with 100% of aruana grass silage. After silo opening, the gliricidia silage was very stable (>72 h). The low quality of aruana grass silage is improved by replacing this grass with significant amounts of gliricidia (approximately 75%).

Phenology and temporal niche overlap differ between novel, exotic- and native-dominated grasslands for plants, but not for pollinators

The temporal dynamics of ecosystem functioning and services are largely regulated by seasonal patterns, or phenology, of Earth’s biota. Recent evidence suggests that species in exotic-dominated systems may exhibit altered phenology relative to species in native-dominated systems. However, whether phenology of plants and pollinator communities differ between existing novel, exotic-dominated ecosystems and native-dominated systems remains poorly understood. We tested whether exotic- and native-dominated grasslands differed across a growing season for plant production, species and functional diversity, and bee pollinator abundances and diversity in the Northern Great Plains tallgrass prairie region, Iowa, USA. We found that niche overlap and some aspects of phenology in exotic-dominated systems were different from native systems for plants, but not for pollinators. Exotic dominated grasslands peaked earlier in biomass production, and exhibited consistently lower levels of species and functional diversity, including higher levels of proportion of C3 biomass, than native grasslands. These results were related to higher levels of temporal niche overlap in exotic grasslands, where species were similar across the growing season in exotic, but not in native, grasslands. Surprisingly, bee pollinator communities did not differ between exotic and native grasslands despite higher forb:grass ratios in native sites, implying that factors other than exotic-native status may be important in structuring pollinator communities at this scale in highly fragmented landscapes. Our results imply that phenological differences between communities in novel and native grasslands can have important consequences to diversity and ecosystem functioning, and that exotic or native status of plant species should be considered in future studies of phenology.

Cranial features and feeding types of subspecies forms of Moschus moschiferus L. (Moschidae, Cetartiodactyla)

The relationship between the cranial features and feeding types in the musk deer has been studied. According to the results of dispersion and cluster analyses, the six modern subspecies were divided into four trophic (mixed-feeders) subcategories--by the ratio of grass and leaves with a significant fraction of lichens (from 25 to 75%) in their diet. It was established that some morphological features of the skulls of subspecies forms correlate with their feeding specialization and habitat and can thus be useful in the reconstruction of adaptations of extinct Moschidae taxa.

Bazı Yabancı Ot Mücadele Yöntemlerinin Akdeniz Bölgesi Meralarında Ot Verimi, Botanik Kompozisyon ve Ot Kalitesi Üzerine Etkisi

Araştırma, Çukurova Bölgesi’ndeki makiliklerin tahribi sonucu ortaya çıkan kıraç meralarda yabancı otlarla etkili mücadele yöntemlerinin belirlenmesi amacıyla 2006-2009 yılları arasında Mersin ili Toroslar ilçesi Resul köyü doğal merasında yürütülmüştür. Araştırma, tesadüf blokları deneme desenine göre 3 tekrarlamalı olarak yürütülmüştür. Denemede biçme, gübreleme, 2.4-D, Picloram+2.4-D, Paraquat ve Glyphosate uygulamaları incelenmiştir. Araştırma sonuçları, gübreleme uygulamasının meranın kuru madde ve sindirilebilir kuru madde verimlerini artırdığını göstermiştir. Tüm uygulama parsellerinde kuru madde verimi, kontrol parsellerindekine göre daha yüksek olmuştur. Picloram+2.4- D+gübreleme uygulaması buğdaygillerin botanik kompozisyondaki oranını artırırken baklagillerin oranını azaltmıştır. Paraquat+gübreleme uygulaması botanik kompozisyonda diğer familya bitkilerinin oranını artırmıştır. Picloram+2.4- D+gübreleme uygulaması otun NDF oranını artırmıştır. Araştırmadan elde edilen bulgulara dayanarak, Çukurova bölgesindeki makilerin tahribi sonucu ortaya çıkmış meralarda çoğunluğu geniş yapraklı bitkilerden oluşan yabancı otların Picloram+2,4-D uygulamasıyla etkin bir şekilde kontrol edilebileceği, bu tip meralarda yabancı ot kontrolü ile birlikte yapılacak gübre uygulamasının meranı ot verimi ve kalitesinde artışa neden olacağı sonucuna varılmıştır.

Phytoliths as indicators of plant community change: A case study of the reconstruction of the historical extent of the oak savanna in the Willamette Valley Oregon, USA

The Oregon white oak savanna, once common in Oregon's Willamette Valley, has been reduced to less than 1% of its former extent. For ecological restoration purposes, we used phytolith analysis to establish both historical vegetation composition and structure at the Jim's Creek research site in Oregon, USA. We sampled the soil at 47 selected plots, extracted the phytoliths, and determined the composition of the phytolith assemblages. Based on the ratios of grass and tree phytoliths in the samples, each site was assigned to one of three ecotopes that, through phytoliths, describe the historic vegetation mosaic. By reconstructing the spatial distribution of historic vegetation types, we were able to confirm the pattern of conifer encroachment on former savanna as suggested by other studies.

Fatty Acid Profiles of Western Canadian Bison (Bison bison) Meat

&lt;p&gt;Western Canadian bison meat is renowned for its natural healthfulness; however, studies on the dietary effects on the fatty acid (FA) profile are limited. Herein, we evaluated the FA profiles of retail bison (&lt;em&gt;longissimus dorsi&lt;/em&gt;) representing grain-fed (Grain), grass-fed (Grass) and grass-fed plus oat and pea screening supplement from early (Early-con) or late (Late-con) season harvested finishing regimes. Bison meat contained less than 30 mg fat/g meat, and was lowest for Early-con bison. Proportions of polyunsaturated FA (PUFA) were greatest in Early-con and lowest for Late-con bison. Early-con bison had the greatest proportion of omega-6 (n-6) FA and Late-con bison the lowest, yet as mg/g meat, total n-6 content did not differ. In contrast, Grass and Early-con bison had greater proportions of 18:3n-3, 20:5n-3, 22:5n-3, 22:6n-3 and total omega-3 (n-3) FA. The n-3 content for Grain, Grass, Early-con and Late-con bison were 38, 90, 69 and 69 mg/100 g meat, respectively. The 3:1 n-6/n-3 ratios of Grass, Early-con and Late-con bison were superior to the 7:1 ratio of Grain bison. Proportions of potentially beneficial biohydrogenation intermediates (BI), including &lt;em&gt;t&lt;/em&gt;11-18:1 and &lt;em&gt;c&lt;/em&gt;9, &lt;em&gt;t&lt;/em&gt;11-conjugated linoleic acid, were greater for Early-con and Late-con bison. Proportions of &lt;em&gt;cis&lt;/em&gt;-monounsaturated FA were similar for both Grain and Grass bison; however, Late-con was greater than Early-con bison. Cumulatively, Grass, Early-con and Late-con bison were more desirable compared to Grain on account of greater proportions of n-3 FA and a lower n-6/n-3 ratio. Furthermore, seasonal supplementation enhanced the BI proportions with potential beneficial bioactivity in Early-con and Late-con bison.&lt;/p&gt;

Enhancement of anaerobic digestion of shredded grass by co-digestion with sewage sludge and hyperthermophilic pretreatment

Anaerobic co-digestion of shredded grass with sewage sludge was investigated under various temperature conditions. The conversion of grass to methane was difficult to achieve under mesophilic conditions, while its methane yield was 0.19NL/g VS-grass under thermophilic conditions. The mixture ratio of grass to sludge affected the methane yield, and the highest synergistic effect was obtained at a C/N ratio of around 10. In a continuous experiment, hyperthermophilic (80°C) pretreatment promoted a methane yield of 0.34NL/g VS-mixture, higher than that under mesophilic and thermophilic conditions (0.20 and 0.30NL/g VS-mixture, respectively). A batch experiment with hyperthermophilic pretreatment showed that 3days of treatment was sufficient for subsequent methane production, in which the highest dissolution of particulate COD, carbohydrate and protein was 25.6%, 33.6% and 25.0%, respectively.

Producción de metano y digestibilidad de mezclas kikuyo (Pennisetum clandestinum) - papa (Solanum tuberosum).

The objective of this study was to evaluate the effect of the mixture of different ratios of kikuyu grass (Pennisetum clandestinum) (K) – potato (Solanum tuberosum) (P) on methane production in vitro, during the second semester of 2012 in the University Research Center (Medellín, Colombia). The treatments consisted of mixtures K/P in ratios of 100/0 (T1), 75/25 (T2) and 50/50% (T3). The results were processed through a repeated measures analysis over time and nonlinear regression. At the end of incubation period (between 0 and 48 hours), the cumulative gas production (ml/g MSi), the cumulative methane production (ml/g MSi) and the dry matter digestibility were statistically differents (p&lt;0.05) among all treatments, increasing with higher levels of potato in the mixture. In the same time interval, the relationship between dry matter digestibility and cumulative production of gas and methane (mg/ml) was statistically equivalent (p&gt;0.05) among all treatments. The final volume of gas and methane (Vf) and its rate of production throughout the fermentation process (k), estimated from the Gompertz model, were also higher (p&lt;0.05) for treatments including potato. Increased methane production is the result of higher dry matter digestibility according CNE level increases in the mixture. Under the in vitro model and in contrast to literature reports, the greater inclusion of CNE did not reduce methane production.

Collembola feeding habits and niche specialization in agricultural grasslands of different composition

The introduction of white clover in grassland is a common practice to improve the quality of the pasture and to limit the use of industrial sources of inorganic fertilizer N inputs. However, little is known about the extent to which the introduction of different crop compositions affects soil quality in general, and soil biota in particular. Recent studies have shown that epedaphic and euedaphic Collembola can have distinct differences in feeding strategy that suggests trophic niche differentiation according to soil habitat. Combining fatty acid (FA) biomarkers with the carbon isotopic ratios technique, we investigated belowground trophic interactions. We compared the FA composition of the three most predominant groups of Collembola (Isotoma spp., Lepidocyrtus cyaneus and Poduromorpha) and bulk soil in white clover-only, a grass-clover mixture, and grass-only plots. Differences between FA-δ13C of Collembola groups and the FA-δ13C signatures of potential food sources (plant material and soil microorganisms) were calculated to explore trophic links in the three cropping combinations. Collembola showed difference in FA compositions between groups. We observed a lower amount of neutral lipid fatty acids (NLFAs) in Isotoma spp. compared to L. cyaneus. Poduromorpha showed a higher relative percentage of the NLFAs 16:1ω7 and 20:4 compared to L. cyaneus, and of 18:3 compared to Isotoma spp. The NLFA oleic/linoleic ratio (18:1ω9/18:2ω6,9) ratio was higher in Poduromorpha compared to L. cyaneus in all the treatments. Soil FA analysis highlighted a larger fungal/bacterial ratio in grass compared to clover plots, while in Collembola no differences in the FA pattern were detected in relation to the treatments. Nevertheless, results of δ13C fractionation in Collembola disclosed differences in the food sources used depending on the crop composition. Apparently, Isotoma spp. and L. cyaneus fed on clover-related diet in clover plots and more on grass-related diet in mixture treatments while Poduromorpha fed more on a plant-related diet, such as leaf litter, in the mixture treatment. Our results suggest differentiation in Collembola feeding strategy and in the diet in relation to food availability and palatability.

Introduction of Coastcross-1 under phosphorus doses and densities of seedlings in area with legumes

The aim of this study was to evaluate strategies for establishing consortium pastures with the predominance of the legume species Arachis pintoi using doses of phosphorus and density of seedlings Coastcross-1. The experimental design was a randomized block with four replications in a factorial 4x4, using four doses of phosphorus (50; 80; 110 and 140kg ha-1) and four densities of seedlings (3.1, 4.2, 6.3 and 12.5 seedlings/m²). The effects of seedlings densities and phosphorus doses were analyzed through regression testing at 10% probability. The variables evaluated were: production of total forage mass, botanical and morphology composition. There was significant effect of phosphorus doses in leaf/stem ratio of grass; of the densities of seedlings in the production of total forage mass, in botanical and morphological composition of forage and harvest dates for the production of forage mass and botanical composition. It might be inferred that the use of higher densities of seedlings Coastcross-1 benefited the establishment of grass in areas which there is the predominance of legumes. However, it has not achieved sufficient population to be considered ideal for the recovery of the grass pasture.

Performances of Some Perennial Warm Season Grasses Alfalfa (Medicago sativa L.) Mixtures under Mediterranean Conditions

This research was conducted to determine forage yield and botanical composition of duo (alfalfa + one grass) and trio ( alfalfa + two grasses) mixtures of some warm season perennial grass species such as dallis grass (Paspalum dilatatum Poir.), Rhodes grass (Chloris gayana L.), Bermuda grass (Cynodon dactylon (L.) Pers.) Guineae grass (Panicum maximum Jacq.), blue couch grass (Digitaria didactyla Willd) and finger grass (Digitaria milanjiana (Rendle) Stapf) with alfalfa (Medicago sativa L.) as well as their pure sowings under irrigated conditions of Mediterranean during the years of 2011 and 2012. In the study, green herbage yield, dry matter yield, alfalfa proportion in dry matter yields and land equivalent ratio (LER) were determined. The experimental design was completely randomized block design with three replications. According to the results, it was determined that the ratios of warm season grasses in the botanical composition of the mixtures declined with the establishment age while rate of alfalfa in the botanical composition of the mixtures increased. Increasing the rate of alfalfa in the botanical composition of the mixtures also increased hay quality of the mixtures. The mixtures gave higher dry matter yield than pure sowings of the species. The highest green herbage yield (98.14 t ha1) was obtained from the mixture of Rhodes grass + alfalfa. The highest dry matter yield (22.46 t ha1) was obtained from the mixture of dallis grass + Guinea grass + alfalfa. The highest value of land equivalent ratio (LER) (1.49) was obtained from the mixture of bermuda grass +Rhodes grass + alfalfa. From the results of the study, it was concluded that Guinea grass and finger grass did not show good adaptaion to the Cukurova conditions due to low temperatures during the winter season. Bermuda grass, Rhodes grass, blue couch grass and dallis grass could be used to establish pasture mixtures with alfalfa, having long grazing season and giving high hay yield and quality. Before the establishment of such pasture mixtures, it was needed to research proper mixture ratios and proper management techniques of the mixtures

Forage Yield and Botanical Composition of Mixtures of Some Perennial Warm Season Grasses with Alfalfa (Medicago sativa L.) Under Mediterranean Condi

This research was conducted to determine forage yield and botanical composition of duo (alfalfa + one grass) and trio ( alfalfa + two grasses) mixtures of some warm season perennial grass species such as dallis grass (Paspalum dilatatum Poir.), Rhodes grass (Chloris gayana L.) and Bermuda grass (Cynodon dactylon (L.) Pers.) with alfalfa as well as their pure sowings under irrigated conditions of Mediterranean during the years of 2009-2011. In the study, green herbage yield, dry matter yield, botanical composition and land equivalent ratio (LER) were determined. The experimental design was completely randomized block design with three replications. According to the results, it was determined that the ratios of warm season grasses in the botanical composition of the mixtures declined with the establishment age while rate of alfalfa in the botanical composition of the mixtures increased. The mixtures gave higher dry matter yield than pure sowings of the species. The highest green herbage yield (68.44 t ha-1) was obtained from the mixture of alfalfa + Rhodes grass. The highest dry matter yield (15.40 t ha-1) was obtained from the mixture of alfalfa + Bermuda grass + Rhodes grass. The highest alfalfa proportion (50.0%) was obtained from the mixture of alfalfa + Bermuda grass. The highest LER (1.63) was obtained from mixture of alfalfa + dallis grass

Spatially explicit modeling of lesser prairie‐chicken lek density in Texas

ABSTRACTAs with many other grassland birds, lesser prairie‐chickens (Tympanuchus pallidicinctus) have experienced population declines in the Southern Great Plains. Currently they are proposed for federal protection under the Endangered Species Act. In addition to a history of land‐uses that have resulted in habitat loss, lesser prairie‐chickens now face a new potential disturbance from energy development. We estimated lek density in the occupied lesser prairie‐chicken range of Texas, USA, and modeled anthropogenic and vegetative landscape features associated with lek density. We used an aerial line‐transect survey method to count lesser prairie‐chicken leks in spring 2010 and 2011 and surveyed 208 randomly selected 51.84‐km2 blocks. We divided each survey block into 12.96‐km2 quadrats and summarized landscape variables within each quadrat. We then used hierarchical distance‐sampling models to examine the relationship between lek density and anthropogenic and vegetative landscape features and predict how lek density may change in response to changes on the landscape, such as an increase in energy development. Our best models indicated lek density was related to percent grassland, region (i.e., the northeast or southwest region of the Texas Panhandle), total percentage of grassland and shrubland, paved road density, and active oil and gas well density. Predicted lek density peaked at 0.39 leks/12.96 km2 (SE = 0.09) and 2.05 leks/12.96 km2 (SE = 0.56) in the northeast and southwest region of the Texas Panhandle, respectively, which corresponds to approximately 88% and 44% grassland in the northeast and southwest region. Lek density increased with an increase in total percentage of grassland and shrubland and was greatest in areas with lower densities of paved roads and lower densities of active oil and gas wells. We used the 2 most competitive models to predict lek abundance and estimated 236 leks (CV = 0.138, 95% CI = 177–306 leks) for our sampling area. Our results suggest that managing landscapes to maintain a greater percentage of grassland and shrubland on the landscape with a greater ratio of grasses to shrubs in the northeast Panhandle should promote greater lek density. Furthermore, increases in paved road and active oil and gas well densities may reduce lek density. This information will be useful for future conservation planning efforts for land protection, policy decisions, and decision analyses. © 2013 The Wildlife Society

Latitudinal gradient of C4 grass contribution to Black Soil organic carbon and correlation between δ13C and the melanic index in Japanese forest stands

Black Soil in Japanese forests is believed to have formed under grasslands, on the basis of pollen and stable carbon isotopic analyses. Carbon stable isotope ratios (δ13C) have indicated that the δ13C value widely ranged from −25 to −17 ‰ in various land-use soils. We measured the δ13C of soil organic carbon (SOC) in Black Soil in forests from northern (43°N) to southern (31°N) Japan. The δ13C values in topmost soils were contaminated by carbon from current C3 vegetation. Excluding these soils, the average contribution ratio of C4 grass in Black Soils was estimated to be ~44.6 % of SOC by mass balance calculation from the δ13C of SOC. The proportion of C4 plants supplying soil carbon was smaller at higher latitudes, this indicating that the δ13C values of SOC were affected by the competitiveness of C4 grass and C3 plants which might depend on the temperature. The melanic index, which is an index of humus properties and divides the humus into “Type A” (≤1.7) and other humus (>1.7), correlates negatively with δ13C values. This result indicates that C4 grass played an important role in generating the dark-colored organic matter in Japanese Black Soils. The δ13C values of soil profiles with key tephra are therefore potentially useful for the study of past climate dynamics and vegetation responses.

Tree-grass ratios in savannas – challenging paradigms

news and update ISSN 1948-6596 commentary Tree–grass ratios in savannas: challenging paradigms One of the greatest challenges for plant ecologists is to ascertain which factors allow trees and grasses to co-exist in savannas (Moustakas et al. 2010; Figure 1). Two major hypotheses that ad- dress this issue have been proposed (Figure 2). The resource-based hypothesis states that grasses and trees have different root depths, and thus occupy different competitive niches (Scholes and Archer 1997). More recently, various versions of the disturbance hypothesis have been formulated, which postulate that disturbances such as fires and herbivory are major factors restricting the growth of trees (Higgins et al. 2000, Sankaran et al. 2005). In both hypotheses, rainfall is thought to benefit trees. Higher rainfall should cause deeper water infiltration, thereby benefitting the deeper- lying roots of trees and allowing them to grow faster and outcompete grasses by shading. Distur- bances that reduce woody cover, such as fires, would explain lower woody cover in relatively high-rainfall areas. Recently, these ideas have been the focus of a number of macroecological studies, which have found some support for the positive influ- ence of rainfall, and the negative influence of dis- turbances, on woody cover. In their seminal pa- per, Sankaran et al. (2005) showed that, for 854 sites across Africa, maximum woody cover in- creased with precipitation at sites with lower lev- els of precipitation (< 650 mm/year), but showed no relationship with precipitation at higher rainfall levels. They thus suggested that arid and semi-arid savannas are ‘stable’: here, even in the absence of disturbances, trees and grasses would continue to coexist. In contrast, in higher rainfall (mesic) sa- vannas, tree–grass coexistence would be facili- tated by disturbances such as fire and herbivory; in the absence of disturbances, high-rainfall sa- vannas would be expected to become forests. Other continental studies have corroborated these findings (Bucini and Hanan 2007, Sankaran et al. 2008, Staver et al. 2011). These studies have contributed to the idea that, at continental scale, rainfall, with the mediating effect of disturbances, plays a pivotal role in the tree–grass ratio of sa- vannas. However, two studies that were recently published have challenged various aspects of these two hypotheses. February et al. (2013) set up experimental plots to assess the effect of rain- fall manipulation on tree growth in a nutrient- poor mesic and a nutrient-rich semi-arid savanna Figure 1. What de- termines tree–grass ratios in tropical savannas? (Photo- graph taken at Weenen Game Re- serve, South Africa). frontiers of biogeography 5.3, 2013 — © 2013 the authors; journal compilation © 2013 The International Biogeography Society

Changes in sex ratios of a dioecious grass with grazing intensity: the interplay between gender traits, neighbour interactions and spatial patterns

Summary Dioecious plants frequently face sex‐related resource allocation trade‐offs. Differential traits of male and female plants can have important consequences that influence their direct and indirect interactions with neighbours and herbivores. Gender differences in traits have been considered to be inherently linked to sex‐biased ratios along environmental stress gradients. However, the mechanisms behind the variation in sex ratios with increasing biotic stress remain elusive. We linked the results of process‐based field experiments with fine‐scale spatial patterns of naturally established plants (both sexes) at different grazing intensities. We demonstrated that slow‐growing females were better defended against insects and small mammals but were more susceptible than males to competition from neighbours. In large‐herbivore exclosures, we found that sex‐biased spatial patterns at the microsite scale can be predicted from gender‐specific traits, with females more segregated from competitors (unpalatable grasses) than males. Traits related to growth/defence trade‐off in females, coupled with their higher competition costs than males, suggest that gender dimorphism in traits can directly influence species interactions and spatial organization. In the presence of large herbivores, female individuals were preferentially browsed despite their larger investment in antiherbivore compounds. The greater distance of females from unpalatable competitors was the main determinant of female‐biased consumption by domestic herbivores. Females growing close to neighbours successfully eluded browsing but also faced competition. Scaling up to a population level, population sex ratios drift from female‐ to male‐bias with increasing domestic grazing intensity. Synthesis: The sexual dimorphism in traits of a dioecious bunchgrass results in differential herbivory by natural enemies, differential competitive ability and differential spatial distribution of sexes. Domestic grazers disrupt this balance by shifting species interactions and their spatial organization. Changes in dioecious plant populations towards male‐biased sex ratios with grazing intensity could not be predicted solely by gender‐specific traits.

Miocene to Pliocene changes in South African hydrology and vegetation in relation to the expansion of C4 plants

The Miocene expansion of C4 plants (mainly tropical grasses) between 8 and 4 million years (Ma) remains an enigma since regional differences in the timing of the expansion rules out decreased CO2 (pCO2) as a dominant forcing [e.g. Tipple and Pagani, 2007. The early origins of terrestrial C4 photosynthesis. Annu. Rev. Earth Planet. Sci. 35, 435–461]. Other environmental factors, such as low-latitude aridity and seasonality have been proposed to explain the low tree versus grass ratio found in savannahs and tropical grasslands of the world, but conclusive evidence is missing. Here we use pollen and stable carbon (δ13C) and hydrogen (δD) isotope ratios of terrestrial plant wax from a South Atlantic sediment core (ODP Site 1085) to reconstruct Miocene to Pliocene changes of vegetation and rainfall regime of western southern Africa. Our results reveal changes in the relative amount of precipitation and indicate a shift of the main moisture source from the Atlantic to the Indian Ocean during the onset of a major aridification 8Ma ago. We emphasize the importance of declining precipitation during the expansion of C4 and CAM (mainly succulent) vegetation in South Africa. We suggest that the C4 plant expansion resulted from an increased equator-pole temperature gradient caused by the initiation of strong Atlantic Meridional Overturning Circulation following the shoaling of the Central American Seaway during the Late Miocene.

Mathematical Analysis of a Size Structured Tree-Grass Model in Savanna Ecosystems

Several continuous competition-based models have been developed to study the possible coexistence of trees and grass in savanna ecosystems according to environmental parameters such as climate or fire regime. In those models fire intensity is a fixed parameter. Here, we model fire intensity through an increasing function of grass biomass and let fire return-time vary accordingto climate types. (For instance, in Africa, in moist savannas fire occurs every 1-2 years, while in dry savannas it is every 3-10 years.) Following [1], we also consider a tree-grass compartmental model that distinguishes small trees (like saplings) that are sensitive to fire from tall trees that are not.On those bases, we model the savanna vegetation dynamics through three state variables (biomass of grass and of the two classes of woody plants) involved in a system of three interrelated, non-linear equations.We carry out a qualitative analysis that highlights three ecological thresholds namely, the destruction ratio of trees due to causes which aredierent of re, the survival ratio of grass after the predation of trees andthe survival ratio of trees after fire. These thresholds summarize the dynamics of the system. Finally, we develop a non-standard numerical scheme[2], and show some numerical simulations to illustrate our analytical results.