Elizabeth A. John

TOWARD UNDERSTANDING THE CONSEQUENCES OF SOIL HETEROGENEITY FOR PLANT POPULATIONS AND COMMUNITIES

Several recent studies demonstrate that yield of individual plants, and their allocation of biomass between roots and shoots, can be profoundly affected by the pattern of supply of soil-based resources. Patchy provision of soil-based resources can affect the location of root biomass, as roots often proliferate in nutrient-rich patches. Root system size is important in determining whether plants access nutrient-rich patches, and the proportion of root systems located within such patches. This proportion will alter as growth proceeds. Species with small root systems have a limited ability to place roots in nutrient-rich patches even when they are very close. Of four species with different root system sizes, the growth of the species with the smallest root system was significantly limited by being located in nutrient-poor substrate even when nutrient-rich substrate was only 3.5 cm away, whereas three species with larger root systems were not disadvantaged. Both in the laboratory and in the field, root density is higher in nutrient-rich patches, and such patches can contain roots of many plants. Recent work showing that plants can respond to non-self roots sharing the same nutrient supply suggests that competition will be more severe in nutritionally patchy substrates than in homogeneous environments with the same overall nutrient supply. Taken together, these facts lead to the prediction that inter- and intraspecific plant interactions will be influenced by patterns of nutrient supply. We present evidence supporting this prediction, and indicating that population and community structure are also affected by patterns of nutrient supply. Significant differences in population yield, plant size distribution, and mortality have been recorded between populations growing under patchy and uniform conditions. Plant communities grown from identical seed inocula, with the same overall nutrient supply, provided in different spatial and temporal patterns, differed by up to 44% in total biomass, up to 70% in root biomass, and differed in species composition. These significant effects of heterogeneous resource supply on plants merit further detailed study. We present a framework of predictions of the impacts of different types of spatial heterogeneity in nutrient supply on the performance of single plants, and on plant interactions, plant populations, and plant communities.

Distribution of Roots in Soil, and Root Foraging Activity

A knowledge of the pattern of root distribution in soil is critical to a number of areas of ecology. For example, our ability to model the interactions between climate and vegetation depends in part on our knowledge of the global pattern of distribution of belowground biomass at various soil depths, and how it will change as one vegetation type replaces another (Jackson et al. 1996). Similarly, our understanding of ecosystem processes is currently limited by poor understanding of the distribution, quantity and productivity of fine roots within a variety of ecosystems, even though, for example, the annual production of fine roots may be twice that of leaves in Northern American hardwood forests (Fahey and Hughes 1994), and up to 80 % of the biomass of some ecosystems is underground (Jackson et al. 1996). At a more local scale, we are limited in our ability to accurately model the processes involved in plant competition by our inadequate knowledge of the fine-scale distributions of roots of individual plants and of the distribution of their associated symbionts (Mou et al. 1995; Casper and Jackson 1997; Casper et al. 2000).

The biotic and abiotic changes associated with Brachypodium pinnatum dominance in chalk grassland in south-east England

Detailed vegetation surveys were made at three East Sussex ancient chalk grassland sites, and soil nutrient and light levels were compared within and outside dominant stands of tor grass Brachypodium pinnatum. The presence of B. pinnatum had a detrimental effect on species diversity with forbs most sensitive to its dominance, followed by cryptogams and graminoids. Within each group, low growing species were generally most vulnerable to high B. pinnatum cover but certain species, typical of more productive habitats, were able to persist. Although the general response in the three communities was similar, there were important differences, with forb diversity affected less, and coarse grasses reaching higher cover values under B. pinnatum at a previously under-grazed site than at the other two sites. The levels of soil nitrate, but not other nutrients, were significantly higher under B. pinnatum stands than under the surrounding community. This may have significant implications for the control of B. pinnatum, as elevated nitrates are likely to enhance its ability to out-compete typical chalk grassland species.

High phenotypic plasticity of Suaeda maritima observed under hypoxic conditions in relation to its physiological basis

BACKGROUND AND AIMS Phenotypic plasticity, the potential of specific traits of a genotype to respond to different environmental conditions, is an important adaptive mechanism for minimizing potentially adverse effects of environmental fluctuations in space and time. Suaeda maritima shows morphologically different forms on high and low areas of the same salt marsh. Our aims were to examine whether these phenotypic differences occurred as a result of plastic responses to the environment. Soil redox state, indicative of oxygen supply, was examined as a factor causing the observed morphological and physiological differences. METHODS Reciprocal transplantation of seedlings was carried out between high and low marsh sites on a salt marsh and in simulated tidal-flow tanks in a glasshouse. Plants from the same seed source were grown in aerated or hypoxic solution, and roots were assayed for lactate dehydrogenase (LDH) and alcohol dehydrogenase, and changes in their proteome. KEY RESULTS Transplanted (away) seedlings and those that remained in their home position developed the morphology characteristic of the home or away site. Shoot Na(+), Cl(-) and K(+) concentrations were significantly different in plants in the high and low marsh sites, but with no significant difference between home and away plants at each site. High LDH activity in roots of plants grown in aeration and in hypoxia indicated pre-adaptation to fluctuating root aeration and could be a factor in the phenotypic plasticity and growth of S. maritima over the full tidal range of the salt marsh environment. Twenty-six proteins were upregulated under hypoxic conditions. CONCLUSIONS Plasticity of morphological traits for growth form at extremes of the soil oxygenation spectrum of the tidal salt marsh did not correlate with the lack of physiological plasticity in the constitutively high LDH found in the roots.

Hemiparasitic plant impacts animal and plant communities across four trophic levels

Understanding the impact of species on community structure is a fundamental question in ecology. There is a growing body of evidence that suggests that both subdominant species and parasites can have disproportionately large effects on other organisms. Here we report those impacts for a species that is both subdominant and parasitic, the hemiparasite Rhinanthus minor. While the impact of parasitic angiosperms on their hosts and, to a lesser degree, coexisting plant species, has been well characterized, much less is known about their effects on higher trophic levels: We experimentally manipulated field densities of the hemiparasite Rhinanthus minor in a species-rich grassland, comparing the plant and invertebrate communities in plots where it was removed, present at natural densities, or present at enhanced densities. Plots with natural and enhanced densities of R. minor had lower plant biomass than plots without the hemiparasite, but enhanced densities almost doubled the abundance of invertebrates within the plots across all trophic levels, with effects evident in herbivores, predators, and detritivores. The hemiparasite R. minor, despite being a subdominant and transient component within plant communities that it inhabits, has profound effects on four different trophic levels. These effects persist beyond the life of the hemiparasite, emphasizing its role as a keystone species in grassland communities.

The effectiveness of glyphosate for controlling Brachypodium pinnatum in chalk grassland

Where Brachypodium pinnatum forms dominant, low diversity stands with distinct boundaries, the herbicide glyphosate is sometimes used, in the UK, to control the species. The effectiveness of glyphosate in reducing B. pinnatum dominance and restoring a species rich chalk grassland community was assessed here by treating B. pinnatum stands at two ancient chalk grassland sites, and monitoring the subsequent development of the plant community, over several years. The rate of re-colonisation of treated stands varied between sites. At both sites many of the species initially colonising treated areas were ruderal or uncharacteristic of the surrounding chalk grassland. Where colonisation proceeded rapidly, ruderal species declined, and chalk grassland species characteristic of the site increased in cover. However, B. pinnatum re-colonised all treated stands even though glyphosate inhibited the germination of its seeds. It is predicted that B. pinnatum will regain dominance following glyphosate treatment, and we suggest that glyphosate should only be used to control B. pinnatum as part of an integrated control programme.

Responses of insect herbivores to sharing a host plant with a hemiparasite: impacts on preference and performance differ with feeding guild

1. Root hemiparasites are common components of many ecosystems and can affect both the biomass and the nutritional quality of the plants they infect. The consequences of these modifications for the preference and performance of three herbivore feeding guilds sharing a host with the hemi‐parasite were examined.

Impact of the invader Ipomoea hildebrandtii on grass biomass, nitrogen mineralisation and determinants of its seedling establishment in Kajiado, Kenya

The invasive coloniser Ipomoea hildebrandtii aggravates the problem of inadequate grass forage in Kajiado district, Kenya. To test its impact on grass biomass, nitrogen (N) mineralisation and seedling establishment, grazing and coloniser density were controlled using experimental exclosures and weeding treatments separately in a factorial design. Grass biomass increased 47% in weeding treatments and 117% with protection from grazing. Ipomoea hildebrandtii removal also led to decline in soil moisture at at a depth of 5 cm and an increase at 30 cm, and lower soil compaction. Grazing lowered soil moisture and increased soil compaction. Mineralisation of N was highest under the dominant grass Chloris roxburghiana followed by I. hildebrandtii and bare ground. Weeding increased N mineralisation whereas grazing lowered it. Multiple regression showed that I. hildebrandtii seedling establishment was significantly higher with low grass biomass, high soil moisture at a depth of 30 cm and higher soil N nitrification. Thus weeding, which caused an increase in the latter two factors, led to increased seedling establishment. Grazing, which lowered soil moisture and mineralisation, led to lower seedling establishment despite reduced competition from lowered grass biomass. This shows invasibility by I. hildebrandtii in rangelands increases when lowered competition is accompanied by an increase in soil resources such as moisture and nitrogen, thus supporting the resource fluctuation theory.