Mycorrhizal responses in wheat: shading decreases growth but does not lower the contribution of the fungal phosphate uptake pathway

Effects have been investigated of reduced C supply (induced by shade) on arbuscular mycorrhizal (AM) colonisation, mycorrhizal growth responses (MGRs) and on AM-mediated and direct uptake of phosphate (Pi) (using ³²P) in wheat, a plant that does not usually respond positively to AM colonisation. Shading markedly reduced growth and shoot/root dry weight ratios of both AM and non-mycorrhizal wheat, indicating decreased photosynthetic C supply. However, shading had very little effect on percent root length colonised by Rhizophagus irregularis or Gigaspora margarita or on MGRs, which remained slightly positive or zero, regardless of shade; there were no growth depressions under shade. By 6 weeks, when the contributions of the AM pathway were measured with ³²P supplied in small hyphal compartments, R. irregularis had supplied 23 to 28 % of shoot P with no significant effect of shading. Data show that reduced C availability did not reduce the contribution of the AM pathway to plant P, so the fungi were not acting physiologically as parasites. These results support our previous hypothesis that lack of positive MGR is not necessarily the outcome of excessive C use by the fungi or failure to deliver P via the AM pathway.     

Impacts of a native parasitic plant on an introduced and a native host species: implications for the control of an invasive weed

Background and Aims: While invasive species may escape from natural enemies in thenew range, the establishment of novel biotic interactions withspecies native to the invaded range can determine their success.Biological control of plant populations can be achieved by manipulationof a species' enemies in the invaded range. Interactions weretherefore investigated between a native parasitic plant andan invasive legume in Mediterranean-type woodlands of SouthAustralia. Methods: The effects of the native stem parasite, Cassytha pubescens,on the introduced host, Cytisus scoparius, and a co-occurringnative host, Leptospermum myrsinoides, were compared. The hypothesisthat the parasitic plant would have a greater impact on theintroduced host than the native host was tested. In a fieldstudy, photosynthesis, growth and survival of hosts and parasitewere examined. Key Results: As predicted, Cassytha had greater impacts on the introducedhost than the native host. Dead Cytisus were associated withdense Cassytha infections but mortality of Leptospermum wasnot correlated with parasite infection. Cassytha infection reducedthe photosynthetic rates of both hosts. Infected Cytisus showedslower recovery of photosystem II efficiency, lower transpirationrates and reduced photosynthetic biomass in comparison withuninfected plants. Parasite photosynthetic rates and growthrates were higher when growing on the introduced host Cytisus,than on Leptospermum. Conclusions: Infection by a native parasitic plant had strong negative effectson the physiology and above-ground biomass allocation of anintroduced species and was correlated with increased plant mortality.The greater impact of the parasite on the introduced host maybe due to either the greater resources that this host providesor increased resistance to infection by the native host. Thisdisparity of effects between introduced host and native hostindicates the potential for Cassytha to be exploited as a controltool.     

Response to grazing after nine years of cattle exlusion in a Flooding Pampa grassland,Argentina

This paper reports on changes induced by the introduction of cattle in a grassland that had remained ungrazed for 9 yr, in comparison with two adjacent grasslands: one that remained enclosed and one that has been continuously subject to grazing. Basal cover was measured on 25 interception lines, each 1 m long, three times during one year. The variables studied were: total cover, cover of grasses and dicots, cover of creeping grasses, floristic composition, and dissimilarity among sites. At the first sampling, 2 yr after cattle re-introduction, the newly grazed site was more similar to the ungrazed than to the grazed site. The newly grazed site had very low cover of dicots; the species of dicots present were different from those found in the continuously grazed area. Creeping grasses had higher cover in the newly grazed site than in the other sites, and continued to increase. At the last sampling, one year later, the newly grazed site had become more similar to the contiuously grazed site. Only after 5 yr of cattle grazing the exotic dicots that were dominant in the continuously grazed site, were recorded in the re-opened site. The absence of propagules of these species or the absence of safe sites may account for this delayed invasion.     

Plant litter: Its dynamics and effects on plant community structure

We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems. Litter alters the physical and chemical environment directly and indirectly. The decomposition of litter may release both nutrients and phytotoxic substances into the soil. The physical changes produced by litter also alter the activity of decomposers, resulting in an indirect effect on the chemical environment. The accumulated litter intercepts light, shading seeds and seedlings, and reduces the thermal amplitude in the soil. By reducing maximum soil temperatures, and creating a barrier to water vapor diffusion, litter reduces evaporation from the soil. However, litter may also diminish water availability when it retains a large proportion of rainfall. Litter creates a physical barrier for seedling and sprout emergence and to seeds reaching the soil.     

Markovian chains and the role of history in succession

Marhovian chains are probabilistic matrix models used to simulate the dynamics of systems in which each transition depends upon the present state of the system, but is independent of the pathway that brought the system to its present state. Their application to ecological succession has not been especially fruitful. Appropriate Markovian models could be used to test successional hypotheses analogous to the Marhovian assumptions. However, serious mathematical and empirical difficulties thwart the development of adequate models, primarily because of the importance of historical and spatial factors in succession.     

Growth and competition of Cytisus scoparius,an invasive shrub,and Australian native shrubs

English broom (Cytisus scoparius) is an aggressive invasive shrub in native sclerophyll forests of South Australia. We studied its relative growth rate (RGR) and competitive ability in soils from invaded and uninvaded woodlands, in comparison to three native species it commonly displaces:Hakea rostrata, Acacia verniciflua, and A. myrtifolia. Hakea was the slowest growing species throughout the year. Both native species had their highest RGR during spring. The RGR of broom was higher than that of both hakea and acacia in the winter and spring. Despite losing its leaves in the summer, the RGR of broom through the year was higher than that of either of the native species. Soil from the invaded stands had higher organic C, N and soluble P than that from uninvaded sites. Broom and acacia grew better in the higher nutrient soil than in the lower nutrient soil. Competition did not decrease the final biomass of any of the species in low nutrient soil. In the higher nutrient soil the biomass of broom was reduced by competition with acacia, but not by competition with hakea. Competition by broom reduced the biomass of hakea but not that of acacia. Broom's earlier and higher RGR, high competitiveness in nutrient rich soils, and probably its ability to change nutrient availability could be important contributors to the mechanisms by which it invades native woodlands.     

Native faunal communities depend on habitat from non-native plants in novel but not in natural ecosystems

Invasive non-native plants are a major driver of native biodiversity loss, yet native biodiversity can sometimes benefit from non-native species. Depending on habitat context, even the same non-native species can have positive and negative effects on biodiversity. Blackberry (Rubus fruticosus aggregate) is a useful model organism to better understand a non-native plant with conflicting impacts on biodiversity. We used a replicated capture-mark-recapture study across 11 consecutive seasons to examine the response of small mammal diversity and abundance to vegetation structure and density associated with non-native blackberry (R. anglocandicans) in native, hybrid and blackberry-dominated novel ecosystems in Australia. Across the three habitat types, increasing blackberry dominance had a positive influence on mammal diversity, while the strength and direction of this influence varied for abundance. At a microhabitat scale within hybrid and native habitat there were no significant differences in diversity, or the abundance of most species, between microhabitats where blackberry was absent versus dominant. In contrast, in novel ecosystems diversity and abundances were very low without blackberry, yet high (comparable to native ecosystems) within blackberry as it provided functionally-analogous vegetation structure and density to the lost native understory. Our results indicate the ecological functions of non-native plant species vary depending on habitat and need to be considered for management. Comparative studies such as ours that apply a standardized approach across a broad range of conditions at the landscape and habitat scale are crucial for guiding land managers on control options for non-native species (remove, reduce or retain and contain) that are context-sensitive and scale-dependent.     

Experimental evaluation of the foliar flag hypothesis using fruits of Rhus glabra (L.)

I tested experimentally whether the presence of colorful plastic ovals (simulating foliar flags) attached to infructescences of Rhus glabra increase fruit removal by birds in a forest-oldfield border. I used a factorial experimental design testing for the effect of size (small or large) and color (yellow or red) of the flags. There was also a control, without flags. Large red flags increased the percent of fruits removed from the panicles, but yellow and small flags had no effect. My results give partial support to Stiles (1982) hypothesis that early color change of leaves close to the fruits in some plants may serve as visual signals that attract frugivorous birds and enhances seed dispersal.