The effects of nutrients and disturbance on dry grass-dominated vegetation

PhD thesis

By Erik Aude

Summary

While the development objective of my Ph.D.-study was to investigate the importance of disturbance and productivity for the shaping of dry grass-dominated plant communities, the specific objectives were to test a number of hypotheses regarding vegetation development in hedgerow vegetation and experimental vegetation in response to experimental treatments. I combined field studies with experiments. Firstly, field vegetation data and explanatory variables were collected in organic and conventional hedgerows (Paper I and II). Secondly, an outdoor factorial microcosm experiment was carried out in order to test the influence of nutrients, defoliation and vascular plant species composition on the recruitment and diversity of bryophytes (Paper III). Finally, a full factorial experiment was established in order to test the separate and interaction effect of nutrient enrichment and herbicide disturbance on biomass, species diversity and frequency of bryophytes (Paper IV). While nutrient gradients were simulated by differential nutrient addition, disturbance took the form of herbicide application and repeated defoliation (simulated grazing).

Nutrients
Result from the mesocosm experiment showed that nutrient enrichment resulted in a significant change in soil N, conductivity and pH (Paper IV). Both experiments showed furthermore that nutrient enrichment resulted in an increase in productivity and a subsequent following negative influence on plant diversity (Paper III and IV). In particular, nutrient enrichment resulted in litter accumulation at least at low disturbance levels, and litter amounts explained more variation in species diversity than living biomass (Paper IV). In comparison to disturbance, nutrient enrichment explained more of the variation in species diversity (Paper III and IV). This indicates that a reduction in nutrient levels is of highest priority in the conservation and restoration of species diverse plant communities dominated by grasses.

Disturbance
Herbicide disturbance affected species diversity and litter accumulation significantly (paper IV). Herbicide disturbance moreover explained the largest part of the variation in total biomass (Paper IV). There was however a difference in the effects of disturbance on biomass and litter. Herbicide disturbance showed significantly negative impact on litter accumulation but no significant impact on living biomass (Paper IV). The classical hypothesis of maximum species diversity at intermediate disturbance level was questioned in this study because results showed the relationship between disturbance and species diversity was highly depended on nutrient level (Paper IV).

Interactions
Statistically significant interactions were found between herbicide disturbance and nutrient enrichment according to productivity, species diversity and bryophyte abundance (Paper IV). The interaction was for example expressed in a negative disturbance influence on species diversity at infertile conditions, whereas at intermediate nutrient levels the disturbance affect was unclear. At the highest nutrient level disturbance had a positive influence on species diversity (Paper IV). The relationship between productivity and species diversity varied along the disturbance gradient (Paper IV).

Bryophytes
Nutrient enrichment demonstrated a negative impact on both abundance (Paper III and IV) and species diversity (Paper III) of bryophytes. High nutrient levels without disturbance resulted in the lowest species diversity and to complete bryophyte extinction in up to 50 % of sample plots (Paper III). Vascular plant biomass showed an inverse relationship to bryophyte abundance (Paper III). Litter explained a larger part of the variation in bryophyte abundance compared to living biomass (Paper IV). Nutrient enrichment level did, however, explain more of the variation in frequency of bryophytes than litter and biomass together (Paper IV). Disturbance prevented complete bryophyte extinction (Paper III and IV), but disturbance did not fully compensate for the negative effect of nutrient enrichment. Five out of 11 acrocarp bryophytes were unable to persist/colonise at high nutrient levels even with biomass removal (Paper III).

Hedgerow
This Ph.D.-study reinforces and illustrates the problems of low habitat quality and poor dispersal abilities in the hedgerows of the Danish landscape (Paper II). An ordination of almost all Danish hedgerow vegetation sample plots (n = 687) together with relevant reference data from plant communities of the Danish landscape revealed that nutrient levels were the most important gradient in the data. Hedgerow vegetation sample plots were similar to plant communities of low naturalness (Paper II).

Organic farming
There were no differences in soil nutrient levels and structural parameters between organic and conventional hedgerows. In spite of this the species diversity were found to be significantly higher in organic hedgerows. The higher diversity resulted from higher numbers of bryophytes, weeds, ruderals, and plant species characteristic of semi natural habitats (Paper I & II). Thus organic farming seems to support a better hedgerow habitat for some species. However, the experiment could not confirm that spray drift doses alone were responsible for the lower diversity in conventional hedgerows (Paper IV). The additional plausible explanations of a higher species diversity in organic hedgerows was: 1) A larger species pool, 2) Use of larger herbicide doses in conventional hedgerow than the tested doses, and 3) Higher rates of dispersal vectors. More research is however needed to pinpoint the mechanisms behind the higher species diversity in organic farmlands.

Conclusions
It was concluded that a successful conservation and restoration of botanical diversity in the Danish landscape requires:

1. Cessation of nutrient pollution of small biotopes and permanent grassland, e.g. by establishing "buffer zones" around fields.
2. Biomass removal is important but not enough (e.g. in road verge management) to maintain and conserve all species.
3. Conversion to organic farming will have a beneficial effect on some species.

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