Survival of Dutch heathlands

OBJECTIVES OF THE THESISHeathlands in The Netherlands are vanishing due to the invasion of trees. The transition from heathland to woodland in Dutch heathlands may either proceed directly or is preceded by the development of an intermediate grass heath. These changes are due to natural succession in the absence of management. In addition to the absence of management atmospheric pollution i.e. increased inputs of nitrogen has accelerated the replacement of heather heath by grass heath.The main hypothesis in this thesis is that the encroachment of grasses in heathlands as well as the response of the vegetation to management and environmental change depend on soil type, climatic conditions, and previous management. This thesis deals in particular with the performance of heather (Calluna vulgaris) and grasses (Molinia caerulea, Deschampsia flexuosa) on dry lowland heath as a function of environmental conditions in a site, with the purpose of matching management options with site conditions (Chapter 1).CHANGES IN DUTCH HEATHLANDSEvidence for replacement of heather by grasses in The Netherlands was obtained from sequential air photographs of heathlands. It is shown that apart from the transition of heathland to woodland, even before atmospheric pollution started in the seventies, heather was already being replaced by grasses in sites where there was no periodic management. However, in most sites the increase of grasses has been accelerated appreciably during the last decade (Chapter 2).RESTORATION OF HEATHThe results of long term field experiments (since 1976) show that a heathland taken over by grasses can be restored by turf cutting. Other treatments i.e. burning, ploughing or mowing treatments had no result, or in the case of mowing (including removal of the biomass), the result was short lived (Chapter 3).The good result of turf cutting may be due to physical environmental changes or changes in nutrients. It is shown that the establishment of heather after turf cutting has partly a physical explanation i.e. can be attributed to exposure of the bare soil, which enables heather seeds in the seed bank to germinate, while the shortlived grass seeds are absent (Chapter 4). These results also suggest that direct succession of heath to woodland (without a grassy stage) is due to the absence of grass seeds.SOIL TYPE AND LIFE SPAN OF HEATHERAlthough the establishment of heather after turf cutting has a physical explanation, the removal of nutrients by turf cutting does increase the life span of Calluna and reduces the competitive ability of the grasses. The effect of turf cutting lasts longer on poor soils. A heathland subjected to turf cutting appears to be more enduring on podzol soils than on more fertile brown podzolic soils, where the lifespan of heather plants is shorter. The advantage of a longer life span of heather is that the formation of suitable gaps for the establishment of grasses or other species is retarded (Chapter 5). Furthermore, in this chapter it is shown that the accelerated encroachment of grasses in heathland due to atmospheric pollution (Chapter 2) is likely to occur only in heathlands on brown podzolic soils. On such soils growth is limited by nitrogen, whereas on podzol soils, where growth is limited by phosphorus, nitrogen does not affect growth (Chapter 5).MANAGEMENT AND LIFE SPAN OF HEATHERThe nutrient status of a site depends not only on the soil present, but also on the management, and therefore growth rates and the life span of heather probably also depend on these. An analysis of the effects of burning on nutrient levels reveals that as much nitrogen may be depleted by fire as by turf cutting, particularly if the shorter rotation period of prescribed burning is taken into account (Chapter 6). Thus, it seems likely that the nitrogen-depleting effects of turf cutting and burning are more or less equal on brown podzolic soils, where nitrogen is the growth-limiting factor. Turf cutting, however, depletes phosphorus more effectively than burning and the phosphorus available to plants may even increase after a fire (Chapter 6). This implies that growth rates on podzol soils will be higher on burnt heath than on heath subjected to turf cutting. A comparison of growth of Calluna in Dutch heathlands confirms this (Chapter 7).CLIMATE AND LIFE SPAN OF HEATHERFinally, the effect of climatic conditions on the performance of heather is assessed. It is shown that the climate in The Netherlands - especially in the south of the countries sub- optimal for heather (Chapter 7). Periodic drought and frost frequently damage heather in Dutch heathlands and make it more susceptible to attack and damage from the heather beetle (Chapter 8).THE NEED FOR SITE SPECIFIC MANAGEMENTIt is hypothesized that heather plants in The Netherlands have a shorter life span because the climatic conditions are adverse for them. These adverse conditions can, however, be compensated to some extent by low nutrient levels, which induce higher carbohydrate levels in the heather plants. Plants with enhanced carbohydrate contents are less susceptible to plant damage and enable the plant to regrow after dying back. It is concluded that under climatic conditions that are sub-optimal for heather, as occur in The Netherlands (particularly in the South) heather can only for longer periods survive if nutrient levels are low. Turf cutting as practised in The Netherlands in the past was therefore not only an economic necessity, but seems also to be ecologically necessary if Dutch heathlands are to be kept purple (Chapter 8).