We established a greenhouse experiment based on replicated mini-ecosystems to evaluate the effects of defoliation of different plant species combinations on soil food-web structure in grasslands. Plant communities, composed of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and plantain (Plantago lanceolata), were subjected to the following defoliation treatments: no defoliation of any species (control) and selective trimming of all possible one-, two- and three-way combinations of the species either to 27cm height (weak defoliation) or to 15cm height (strong defoliation) above the soil surface three times over a 10-week period. Successive defoliations removed the largest amounts of shoot mass from systems in which T. repens was included among the defoliated species because T. repens dominated aboveground plant biomass. At the final harvest shoot mass was lowest in treatments that included defoliation of T. repens, while total root mass was on average lower in strongly than in weakly defoliated systems and did not differ between the control and defoliation treatments. Total shoot production was not affected by defoliation. Microbial basal respiration and soil NO3-N concentration differed between the combinations of defoliated species; e.g. microbial respiration was on average 32% lower in systems in which only L. perenne was defoliated than in systems in which only T. repens was defoliated. Microbial biomass and soil NH4–N concentration were not significantly affected by defoliation treatments. Enchytraeid abundance differed significantly between the combinations of defoliated species: in systems in which only L. perenne was defoliated enchytraeid abundance was on average 88% lower than in systems in which all species or only T. repens were defoliated. Enchytraeid abundance was also positively associated with total defoliated shoot mass. Abundances of both bacterial-feeding and fungal-feeding nematodes were affected by the combination of defoliated species; e.g. the abundance of bacterial feeders was on average 52% lower in systems in which only T. repens was defoliated than in systems in which both P. lanceolata and T. repens were defoliated. Fungal-feeding nematodes were also more numerous in strongly than in weakly defoliated systems and positively associated with total defoliated shoot mass. Herbivorous nematode abundance was not significantly affected by defoliation treatments. The results show that the response of soil food webs to defoliation can be affected by which combination of species in a plant community is defoliated. Further, it seems that the role of the combination of species that are defoliated may for some components of the soil biota (e.g. fungal-feeding nematodes) be explicable simply in terms of the total mass of foliage removed. However, for other components of the soil biota (e.g. bacterial-feeding nematodes and enchytraeids) species-specific properties of different plant species in the combination of defoliated species are also clearly important, over and above simple mass removal effects of defoliation.
No abstract is provided for this article.
The relative importance of predation and competition (resource limitation) in influencing the components of a below-ground food-web consisting of three tro
Boreal forests are characterized by an extensive moss layer, which may have both competitive and facilitative effects on forest regeneration. We conducted a greenhouse experiment to investigate how variation in moss species and biomass, in combination with precipitation frequency, affect Pinus sylvestris seedling growth. We found that moss species differed in their effects on seedling growth, and moss biomass had negative effects on seedlings, primarily when it reached maximal levels. When moss biomass was maximal, seedling biomass decreased, whereas height and above- relative to below-ground mass increased, due to competition for light. The effect that moss biomass had on seedling performance differed among the moss species. Hylocomium splendens and Polytrichum commune reduced seedling growth the most, likely because of their taller growth form. Seedlings were not adversely affected by Sphagnum girgensohnii and Pleurozium schreberi, possibly because they were not tall enough to compete for light and improved soil resource availability. Reduced precipitation frequency decreased the growth of all moss species, except P. commune, while it impaired the growth of seedlings only when they were grown with P. commune. Our findings suggest that changes in moss species and biomass, which can be altered by disturbance or climate change, can influence forest regeneration.
The research (performed both locally and abroad) on the allelopathic potential of most common New Zealand pasture plants (including important weeds) is reviewed. The bioassay techniques usually used for demonstrating allelopathic influences have several drawbacks, often making the observed inhibitory effect of the donor plant a possible artefact of the method used. Other criteria required for the demonstration of allelopathy in the field have been sparingly satisfied, including the chemical identification of toxins, demonstration of concentration of toxins in the field, and the elimination of other possible factors explaining interference in the field. It is concluded that, to unequivocally prove allelopathy, one must first demonstrate that toxins from one plant are taken up by neighbouring plants and produce an inhibitory response. Studies of interference phenomena (including allelopathy) between plants in pastures might be potentially useful for weed control, or for determining mutual compatibility of different species in pasture mixes.
This chapter illustrates detritus-based food webs responding to various disturbance regimes occurring in conventionally tilled (CT) and non-tilled (NT) systems. The detritus food web is based largely in the soil system, and is important in regulating nutrient cycling and energy flow, it is reasonable to expect that this food web may be a viable and meaningful indicator of disturbance exerted by tillage and alternative weed management strategies. The principal differences in disturbance regimes involve cultivation, herbicide application, residue management and manipulation of weed levels, therefore, emphasis is placed on these factors. The chapter also reviews the results of previous studies, determines how different components of the detritus food web respond to the disturbances relative to each other, and examines the consequences of these disturbances for overall food web structure. Furthermore, while disturbances in CT systems may exert predictable negative effects on most groups of soil organisms, the responses of species assemblages are less predictable. Individual taxonomic species are undoubtedly a more sensitive indicator of ecosystem disturbance than are entire functional groups, and is therefore a more appropriate unit for biomonitoring purposes.
Environmental gradients influence plant establishment, survival, and functional traits. Along the Panama Canal Isthmus there is a strong rainfall gradient with an underlying mosaic of soil types ranging in soil nutrient availability. In this region, tree species distribution patterns are correlated with soil phosphorus availability and rainfall patterns, but how understory plant species such as palms relate to these factors is less clear. We hypothesized that due to greater resource use efficiency and optimal biomass allocation, specialist species will have greater seedling performance growing in home soil and sites compared to species not occurring there. To test this hypothesis, we used two specialist species ( Chamaedorea tepejilote and Geonoma congesta ) and two generalist species ( Geonoma cuneata var. cuneata and Chamaedorea pinnatifrons ), and for these four species we measured traits on seedlings and assessed their performance in shade house and field transplant experiments using five soils. Soils were sourced from five sites which varied in nutrient availability and rainfall, and were distributed along lowland tropical forests of the Panama Canal Isthmus. In the shadehouse experiment, leaf functional traits were determined by species rather than soil nutrient availability. However, in the shadehouse experiment, seedling biomass allocation, and relative growth rate were determined by interactions between species and soil, with weak support for home-site advantage for one of the species. In the field transplant experiment, seedling survival was strongly related to dry season water availability. However, species tended to have high survival at home sites and other sites with higher dry season rainfall. Together, results from these experiments suggest that understory palm species seedling performance are determined by species-specific responses to the combination of soil nutrient and water availability. This indicates that while soil nutrients influence seedling biomass allocation, dry season water availability determines both specialist and generalist seedling survival and therefore distributions along the soil nutrient and moisture gradient.
The influence of osmotic effects of aqueous plant extracts on the results of allelopathy bioassays was investigated for four pasture grass species. It was
While there has been much recent interest about the relationships between plant diversity and plant productivity, much remains unknown about how the diversity of mycorrhizal fungi affects plant productivity. We investigated the effects of ectomycorrhizal fungal community composition and diversity on the productivity and growth characteristics of seedlings of two tree species ( Pinus sylvetris and Betula pendula ) as well as their interactions with each other. This involved setting up a mycorrhizal fungal diversity gradient from one to eight species using a design previously demonstrated to be able to separate diversity effects from compositional effects. We found that the eight mycorrhizal fungal species differed in their effects on seedling productivity and that the nature of effects was determined by the fertility of the substrate. Fungal species richness effects were also important in affecting seedling productivity over and above what could be explained by “sampling effect” but only in some situations. For B. pendula in a low fertility substrate there were clear positive causative effects between fungal species richness and productivity with the eight species treatment having over double the productivity of any of the eight monoculture treatments; no diversity effects were, however, detected in a high fertility substrate. For P. sylvestris in a high fertility substrate there were significant negative effects of fungal diversity on productivity while in a low fertility substrate no effects were apparent. The possible mechanistic bases for these results are discussed. The growth of P. sylvestris relative to that of B. pendula when grown in combination was unaffected by mycorrhizal treatments. Our results provide clear evidence that effects of mycorrhizal fungal diversity on productivity are context dependent and may be positive, negative or neutral depending on the situation considered.
In production horticulture, it is desirable that ground management strategies are selected in such a way as to ensure that there are adequate levels of soil biota present to carry out key ecosystem processes required for long-term crop growth. We established replicated field plots in a New Zealand kiwifruit orchard of each of five ground management treatments, i.e. maintenance of pasture, planting of a dwarf fescue mulch, sawdust application, cultivation and repeated use of herbicides, and then monitored the responses of components of the soil biota to these treatments over a 5-year period. Those treatments involving enhancement of basal resource inputs (pasture, fescue, sawdust) consistently supported higher levels of microbial biomass and activity than did the others. These effects were not consistently propagated through higher trophic levels of the decomposer food web, although populations of microbe-feeding and predacious nematodes did often differ significantly across treatments. This idiosyncratic response of decomposer food web components to treatments is believed to be due to the complex interplay of top-down and bottom-up forces in soil food webs. There were also important treatment effects on nematode community structure; ordination analysis revealed that the sawdust and cultivated plots supported different species assemblages to the pasture and fescue plots. Further, treatments supporting greater basal resource inputs tended to result in a higher diversity of nematodes; on average the Shannon–Weiner diversity index for the 0–5cm depth layer was 2.80 and 2.64 for the fescue and pasture treatments, and only 2.32 and 2.45 for the cultivation and herbicide treatments. Populations of Collembola were also generally enhanced in plots with greater basal resource inputs. We utilised litterbag decomposition rates as a measure of the performance of ecosystem functioning carried out by the soil biota, and generally found that surface placed litter decomposition rates were greatest in those treatments supporting greater levels of basal resource inputs and microbial biomass (i.e. greatest for the mulched and fescue plots, least for the herbicide and cultivated plots), but were generally independent of higher trophic levels. Most of our results could be explained by the fact that treatments differed in the amounts of the basal resources that were likely to be present, rather than other components of agricultural intensification such as direct effects of cultivation-induced disturbance or herbicide toxicity. Finally, our study indicates that in order to gain a more complete picture of how agricultural intensification affects soil biota in the long-term requires experiments which simultaneously consider several trophic levels and several modes of intensification, and which run for several years.
The issue of how plant community composition affects decomposer community composition and function is considered, by reviewing recent literature and through the use of two examples.
The coupling between a resonant excitation and a nonresonant parametric process in a nonlinear system is studied experimentally under non-phase-matched conditions. Our study performed in the context of anti-Stokes stimulated Raman scattering provides a clear observation of the self-induced phase matching of a parametric process. A close agreement with theoretical predictions is observed.
