Quantifying the impact of coarse woody debris on soil and vegetation in a cleared temperate woodland.

Abstract

Broadscale clearing and continued agricultural use of former temperate Australian woodlands has resulted in widespread land degradation and loss of biodiversity. Tree clearing has not only removed key ecological structures and broken an important nutrient cycling pathway, but has exposed surface soils to extreme temperature fluctuations and evaporation. Land-use intensification for cropping and livestock grazing has altered soil nutrients, compacted surface soils and reduced the cover and richness of the native plant understorey. These modifications pose a threat to the long-term survival of species sensitive to extreme surface-soil conditions as well as the maintenance of species diversity. As such, there is considerable interest in the restoration of temperate woodlands at both Federal and State levels, with the return of trees to the landscape the most common focus. However, an important structural element removed and now missing from these lands is coarse woody debris (CWD), boles and branches greater than 10 cm in diameter. In the absence of source trees, CWD cannot naturally re-accumulate. Restocking of CWD is proposed to address this problem because of the positive impact of CWD on the abundance and richness of threatened fauna. However, the impact of CWD on modified woodland surface soils and understorey plants is unknown. Hence, the merits of restocking initiatives are difficult to assess.

This thesis aims to quantify the likely impacts of CWD on surface-soil temperature, moisture content and soil characteristics as well as the effect on understorey plants. Specifically, this thesis examines the impacts of eucalypt CWD on i) diurnal surface-soil summer maximum and winter minimum temperature, ii) the magnitude of the likely reduction in the rate of surface-soil moisture content, iii) surface-soil characteristics and iv) adjacent understorey plant moisture content, growth and species composition, in a typical former temperate eucalypt woodland.

The results of this thesis indicate that CWD has both rapid and long-lasting physical and structural impacts on surface soils and understorey plants. CWD appears to be effective in protecting surface soils from long-term exposure to extreme temperatures and moisture loss during dry periods, with the impact over distance scaling with CWD diameter. Such CWD sites could form refugia better suited to soil-dependent biological activity, such as the germination and growth of the plant understorey. At a landscape scale these refugia may be vital for the establishment and survival of drought sensitive species. This potential role of CWD has not been previously reported and is particularly important in the context of the current and future predicted climate conditions.

This thesis has also revealed that CWD returns surface soils to conditions that are more representative of intact woodlands by increasing soil C and labile P and reducing soil bulk density. As such, CWD can be used to rapidly return missing soil heterogeneity to woodlands. Understorey plants would have greater access to available nutrients and soil moisture and this could result in an increase landscape productivity and a change in plant diversity. However, this may also discourage native grasses and encourage exotic forb species that are already present by accelerating their establishment and growth. Such an outcome is likely to be context-dependent, with the outcome varying to reflect different land management histories and ecosystem conditions. Thus, strategies such as weed management should be applied to address the establishment of exotic species in association with CWD application. This thesis makes a significant contribution not only in understanding the role of CWD in modifying surface soils and understorey plants, but in potentially guiding its application in restoring degraded woodlands. Show more