Transition from Heathland to Grassland: Damaging Effects of the Heather Beetle

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Abstract
(1) On the Hoorneboeg, the Netherlands, changes in heathland vegetation on 40 ha were studied from 1980 until 1982, during an outbreak of the heather beetle (Lochmaea suturalis). (2) Defoliation resulted in the death of Calluna vulgaris over areas varying from 0.01 ha to 3.0 ha. The outbreak caused a distinct damage and death pattern. The C. vulgaris was not more than 10 years old in the dying and in the surviving patches. No clear relation was found between damage intensity and vegetation type. (3) During the 3 years, the areas occupied by grass-rich vegetation expanded at the expense of heathland. These replacements were not random; the transition probabilities for the vegetation types were not constant. In 1982, the vegetation types were clearly related to the damage caused in 1980 and 1981, although there were differences according to soil type. Grass-poor vegetation was more frequent than expected on the slightly damaged areas and grass-rich vegetation was more frequent than expected on the heavily damaged areas. (4) In 1980, the relation between vegetation type and soil type was clear. The grass-rich vegetation mainly occupied the relatively fertile soils and the grass-poor vegetation the low-nutrient soils. This relation became weaker during the period of observation because the grass-rich vegetation expanded on to the less fertile soils. (5) Grass-rich vegetation is most likely to increase when grasses are present under, or in the immediate vicinity of, damaged C. vulgaris. (6) Outbreaks of heather beetle can be considered as catalysts triggering the transition from heathland dominated by C. vulgaris to grassland. Grasses cannot increase under the closed canopy of C. vulgaris, but they are able to survive in its shade. During a beetle outbreak, grasses can become established and those already present increase and eventually become dominant.