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

The effects of reclamation and high atmospheric nitrogen deposition have fo r long threatened the existence of heathlands in Denmark. A high nitrogen i nput increases the frequency and intensity of heather beetle attacks. Howev er, any indirect effects of these attacks on the soil water balance are sel dom investigated. In autumn 1994 a 2000-year old Danish inland heath was st ruck by a severe heather beetle attack and the effects on the soil moisture and the water balance were studied. Soil water content, gross precipitatio n 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 s ummer of 1995 when the soil water content was relatively high. Four years a fter the beetle attack, new heather plants covered the area again and durin g 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 through fall, water-repellent soil and break down of the structure of the organic t op-horizons due to the beetle attack. The effect of the beetle attack was e xamined using a simple water balance model. Model simulations showed that e vapotranspiration was reduced by respectively, 14, 29 and 5% in the three y ears following the beetle attack. From 1993 to 1998 percolation was on aver age 62% of precipitation with very little variation from year to year. Evap otranspiration was on average 38%, but in the years affected most by the be etle 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, showe d that the water balance parameters were estimated well in the two years wh ich were most affected by the beetle attack, i.e. 1995 and 1996.