Fallen logs: creating patchiness in chenopod shrublands of South Australia.

Abstract

Patchiness of resources strongly affects productivity and diversity of arid systems. Patchiness associated with trees, shrubs, grasses, and animal diggings are well documented but there is very little information on the effect of fallen logs. Fallen logs are one of few documented non-living structures in arid lands that contribute to resource heterogeneity. They can be particularly important in grazed systems, where most resource patches associated with living plants are reduced or destroyed. Further, logs provide a unique opportunity to study aspects of resource patchiness without them having biotic effects on annual plants. My Ph.D. thesis consists of four chapters: Long term influence of fallen logs on patch formation and their effects under contrasting grazing regimes Here I studied the long term dynamics of patch formation and the effect of fallen logs in heavily grazed areas. I studied soil nutrient content and soil seed banks associated with the presence of logs in the long ungrazed Koonamore Vegetation Reserve and in adjacent heavily grazed paddock in chenopod shrublands of South Australia. I studied patches associated with fallen logs of unknown age, and others known to have persisted for up to 78 years. Logs acted as traps for soil nutrients and seeds. Organic carbon and total nitrogen were higher in soils next to logs, and also higher inside the reserve than in the grazed paddock. Propagule number and species richness were higher next to logs than in open spaces and viable seeds were in much higher abundance next to logs in the grazed paddock than any other site. Increased time in situ of a log had some effect on soil organic carbon, total nitrogen and available potassium. Logs act as resource traps which may enhance the diversity of the system, and may be particularly important for maintaining patches of resources in areas of severe grazing damage. Microclimate or accumulation of resources: which is the main driver of annual plant communities in a patchy system? Here I decoupled microclimate and accumulation components of a patch to determine their relative importance for plant communities. I measured soil temperature and soil volumetric water content next to logs and in adjacent open spaces. I then conducted a soil core swapping experiment to separate these soil microclimate conditions from seed and resource accumulation to assess their importance in determining annual plant communities. Soils next to logs had lower maximum and higher minimum temperatures than open space soils. Also, patches next to logs had lower soil volumetric water content and dried faster after a rainfall event than open spaces. The soil core swapping experiment showed complex interactions on the annual plant community suggesting that each factor has its own impact on annual plant communities. Spatial and temporal heterogeneity of resources drive changes in community structure of arid annuals Here I studied the effects of spatial and temporal heterogeneity on density dependence and community structure of annual plant communities. To achieve this I used fallen logs and open spaces coupled with high and low watering regimes in both the field and the glasshouse conducted over two consecutive years, encompassing two growing seasons. I found some evidence of competition among annual plant communities, but community structure of annual plants were strongly influenced by spatial heterogeneity of resources as well as watering regime. My results varied greatly across the two growing seasons, suggesting that both temporal and spatial heterogeneity of resources in arid lands are important determinants of annual plant communities. Patch formation and patch dissipation associated with fallen logs To study patch formation I introduced logs and fake logs (large PVC pipes) to open spaces at multiple orientations at two levels of grazing degradation. I assessed soil properties and any changes to annual plant communities over the three following years. I found that three years was not sufficient for creating changes to soil nutrients or annual plant communities when introducing logs, but the destocking which occurred during my experiment had a strong effect on soil nutrient contents. To look at patch dissipation I selected logs in pairs and one of each fallen log was removed. At each of log, open space and removed log I deployed soil temperature and moisture probes. I also collected soil samples to determine soil nutrient contents and soil seed bank across a one year period I found the removal of the log had immediate changes to the microclimate associated with the log, but that one year was not long enough to remove accumulated nutrients or seeds in the seed bank.