Summary

The level of influence various physical factors have upon plant growth, plant community structure and the composition of that community are naturally of great interest to plant ecologists. One such factor is soil nutrient heterogeneity (SNH). SNH can come in several forms but a major factor identified through experiment and in theories is the role of the homogeneity versus the heterogeneity of nutrient supply, in particular, spatially.

Decreasing species number with increasing fertility has been repeatedly observed in a wide variety of plant communities. One such community is Calthion palustris as found in the Drentsche Aa Nature Reserve river valley system, the Province of Drenthe, The Netherlands, but it occurs in various forms across North, West and Central Europe, and as far south as Northern Spain. Mesotrophic Calthion meadows are endangered due to drainage work in The Netherlands and are mainly restricted to nature reserves where artificial fertilization has been stopped for decades. This vegetation is subject to further human influence as it is usually cut for haymaking each year in July. Along with human management practices, SNH may contribute to the increased species numbers at lower fertilities.

Many studies focus on species from contrasting habitats. The majority of species occur in the range between the extremes. Plant responses in these extreme conditions provide a valuable insight into the adaptive mechanisms of individual species, resulting from evolutionary processes among species. However, a wider evaluation of the importance of such phenomena like root plasticity, precision and scale of foraging, biomass allocation and SNH on plant community structuring and plant growth, can only be made by addressing conditions suited to species that co-occur in communities at intermediate conditions. This approach emphasises within-species adaptations to changing environmental conditions in a plant community. Hence, species from a mesotrophic environment were selected, ranging in dominance from small stature, slow growing dicotyledons (e.g. Lychnis flos-cuculi) to large stature, fast growing monocotyledons (e.g. Holcus lanatus), to determine the extent to which dominant plants with extensive roots and shoots, use a coarse-grained, high-scale foraging strategy, whereas subordinate species with smaller leaves and roots use a finegrained, high-precision foraging strategy to acquire and utilise resources, and to what extent these traits can be attributed to a species, functional group or are a general plant response level.

The Introduction (Chapter 1) lays out an array of competition theories and explores the role of coexistence theory and identifies why SNH is an area of focus. In this thesis, plant species are grown in multispecies mesocosms and grown singly in order to assess the influence of a type of SNH (spatial soil nutrient homogeneity vs. heterogeneity, using a controlled release fertiliser) upon plant and community growth and structure, and to determine the relative level or scale of the plant response and how it may contribute to the coexistence of competing plant species. Also, to assess the influence of other relevant factors, such as human management practices like cutting and mycorrhizal infection, upon the species and the community, as well as to generally explore the influence and impact of the experimental system used upon the results obtained and the difficulties of studying and simulating wild vegetation using mesocosms and artificial growing environments with their inherent horticultural implications.

Chapter 2 describes the responses of a seven species mixture to spatial SNH and two nutrient levels in terms of the species performances of the component species, in order to set a framework for later work and to address how nutrient level in conjunction with smallscale spatial supply of mineral nutrients affects the development of the multi-species mixture, and what are the potential influences of the other environmental factors. The main result was that despite the small scale of the spatial arrangement of nutrient supply, the biomass of the community under more homogeneous supply was greater at the low nutrient level than that of the community under more heterogeneous supply at over 5 times the level of nutrient supply, over a three month period. Issues raised, such as nutrient supply level, natural vs. artificial light conditions and true replication of samples are addressed in Chapter 3 and modifications incorporated into the experimental design. In addition to the continuing assessment of methods, the focus of the study is as in Chapter 2. There is also an expansion to include factors of additional important influence in the field, namely, management of the growth of dominant species and the presence of mycorrhiza, and how they affect the growth of the 7 species mixture at lower nutrient levels with natural light, under various forms of spatially patterned nutrient supply. A low heterogeneity nutrient supply results, in the short term, in a higher biomass community. Windows of opportunity for increased growth are species dependent and may enable the persistence and possible increase in size and number of subordinate species. These opportunities occur due to a high heterogeneity nutrient supply reducing aboveground biomass accumulation and management practices such as controlling of the dominant species. However, some species remain relatively unaffected. The experiments in Chapters 2 and 3 address the more complex end of the experimental scale, deliberately incorporating new factors and variability, in order to take a step towards a less reductionist experimental system. The following two chapters are at the other end of this experimental scale involving more simple experiments, performed with single plants to assess growth performance and to address the role and effect of spatial SNH especially upon root growth.

In Chapter 4, the same growth environment and nutrient regime is used (as in Chapter 2) to address: How does the level of nutrient supply in conjunction with the small-scale spatial supply of mineral nutrients affect the development of single plants of different species? Under this system nutrient level is of greater importance than heterogeneity. Possible growth advantage of plants, particularly of the larger species (Holcus lanatus) under high heterogeneity is identified and the apparent transient nature of this growth advantage is elucidated. As in Chapter 2, growth responses are more evident at the lower nutrient level. Holcus lanatus displays growth, suggesting scale foraging with growth of copious roots exploring the soil environment and is responsive to the level of nutrient supply. Whereas, Lychnis flos-cuculi shows growth more in keeping with precision foraging, where this small stature species attains similar biomass regardless of the nutrient supply.

However, the ability of species to detect patches and utilise the nutrients is greatly dependent upon the ability of the species to take up the nutrient and then apportion the resource to the relevant organs. Uptake and foraging of species is addressed in the final experimental chapter (Chapter 5). Holcus lanatus and Lychnis flos-cuculi were studied using a split-root technique to ascertain whether the root investment (biomass allocation and placement) and nitrogen tissue content of such species, are affected by the presence of, and spatial pattern of soil nutrients. The growth of the two species would appear to be similar in respect of utilising resources obtained under low heterogeneity to produce larger plant growth. The small stature species, Lychnis flos-cuculi with its lower nitrogen requirement does however, like its larger counterpart, try to maximise uptake by exploiting low heterogeneity. The methods used in this chapter have allowed the mode by which each species makes adjustment for high heterogeneity to be explored. Holcus lanatus displays changes in its allocation of biomass between root and shoot whereas Lychnis flos-cuculi maintains its root to shoot biomass balance altering its allocation within its roots.

Experimental results are discussed (Chapter 6) along with a comparison of the two approaches of single plant and community experiments. SNH shows considerable influence in the species mixtures and is the most prevalent factor, however the single plant growth experiment (Chapter 4) reveals the greater role of the nutrient level used especially as the experiment proceeds. Finally, in Chapter 7, the advantages and disadvantages of both experimental methods are discussed in terms of the interpretation of results, the methodology required and issues to be considered when designing more complex experimental systems.

These experiments show that fine-scale spatial soil nutrient heterogeneity influences the growth of plants. This influence is species dependent. Its effect upon plant species coexistence occurs by increasing the opportunities for some subordinate plant species to increase their biomass and hence reduce their chances of being excluded from the plant community. This is due in part to successful exploitation of patches by some species but in large part, at least in the species mixture under study here, in short-term reduction of growth of the dominant species, Holcus lanatus.

Several models on resource competition, discussed in Chapter 1, strongly suggest that heterogeneity, both in resources and in environment, enhance the chances for coexistence, as compared to emphasis on competitive exclusion under homogeneous (equilibrium) conditions. The present study demonstrates that a range of mesotrophic species can have many subtle effects or weak interactions, partly resulting from the strong interaction with Holcus lanatus, which when combined are of great potential importance for community structure. Heterogeneity of nutrient supply provides opportunities for species, whilst also in some cases, increased heterogeneity may deprive some individuals or even species of access to sufficient resources, and exclusion may result. The ecological relevance of environmental heterogeneity largely depends on the scale of perception by the individual plants. The scale of this heterogeneity will influence plant growth and the species present, facilitating the coexistence of plant species by adding to variation and complexity within the environment. Small-scale spatial soil nutrient heterogeneity can contribute to the coexistence of species.