Thermal refugia in cleared temperate Australian woodlands: Coarse woody debris moderate extreme surface soil temperatures

Extensive tree clearing and agriculture has exposed surface soils of former Australian temperate woodlands to more extreme temperature fluctuations. Given the highly variable climate in these regions, and increasing temperature overall, amelioration of extreme surface soil temperatures would have implications for soil-dependent biological activity. Reintroduction of coarse woody debris (CWD) to these woodlands may moderate surface soil temperatures. Diurnal surface soil temperatures at a depth of 2.5 cm were monitored during summer and winter at different distances from CWD with varying diameters. CWD samples were located beyond the edge of neighbouring tree canopies. The result of the initial analysis showed that CWD reduced the durations of time at extremely high summer and low winter surface soil temperatures. Soil temperatures near CWD were largely maintained below the summer high of 30 degrees C and above the winter low of 5 degrees C. The duration of time above 30 degrees C in summer was reduced by 93% near CWD and the duration of time below 5 degrees C in winter was reduced by 77%, compared with reference distances. Simple nonlinear statistical models were developed to explain the observed daily summer maximum and winter minimum surface soil temperatures as functions of distance from CWD and CWD diameter. The forms of these models reflected different dominant physical processes in each season. Mean summer daily maximum surface soil temperatures were significantly cooler by 11.1 degrees C and mean winter daily minimum temperatures were significantly warmer by 1.9 degrees C near CWD compared to the respective mean maximum and minimum temperatures at reference distances. Increasing CWD diameter increased the range of the impact over distance from CWD in both summer and winter. During the summer maxima, larger diameter CWD also increased the magnitude of the reduction in surface soil temperature near CWD. The results of this study demonstrate that CWD can be effective in protecting surface soils from sustained exposure to extreme surface soil temperatures. Therefore, at a landscape scale, CWD could create thermal refuges better suited to soil-dependent biological activity, such as for the germination and growth of the plant understorey in woodlands. This impact of CWD may be valuable for supporting understorey productivity and diversity in cleared lands. (C) 2015 Elsevier B.V. All rights reserved.