Effects of a heather beetle attack on soil moisture and water balance at a Danish heathland

Abstract

The effects of reclamation and high atmospheric nitrogen deposition have for long threatened the existence of heathlands in Denmark. A high nitrogen input increases the frequency and intensity of heather beetle attacks. However, any indirect effects of these attacks on the soil water balance are seldom investigated. In autumn 1994 a 2000-year old Danish inland heath was struck by a severe heather beetle attack and the effects on the soil moisture and the water balance were studied. Soil water content, gross precipitation and throughfall were measured continuously from 1993 to 1998 at the heath. The first signs of the attack on the water balance were seen in the dry summer of 1995 when the soil water content was relatively high. Four years after the beetle attack, new heather plants covered the area again and during summer the soil water seemed to be depleted to the same degree as before the beetle attack. In the years after the beetle attack a high coefficient of variation between individual soil moisture measurements was seen. It is proposed that the inhomogeneous wetting was caused by heterogeneous throughfall, water-repellent soil and break down of the structure of the organic top-horizons due to the beetle attack. The effect of the beetle attack was examined using a simple water balance model. Model simulations showed that evapotranspiration was reduced by respectively, 14, 29 and 5% in the three years following the beetle attack. From 1993 to 1998 percolation was on average 62% of precipitation with very little variation from year to year. Evapotranspiration was on average 38%, but in the years affected most by the beetle attack transpiration was relatively low whereas evaporation from soil was increased. A comparison between the modelled and measured throughfall, as well as percolation estimated by the chloride mass balance method, showed that the water balance parameters were estimated well in the two years which were most affected by the beetle attack, i.e. 1995 and 1996.