Fire ecology of Scots pine in North-West Europe

In this thesis the ecological consequences of forest fire are studied in North-west European Scots pine {Pinus sylvestris) forests. The focus is on post-fire succession, and the factors and mechanisms that influence the successional pathways after fire. Fuel load and fuel moisture determine the intensity of forest fire and thus the degree of humus consumption. In a controlled laboratory' experiment humus consumption was determined for different moisture levels. Experimental fires showed evidence that variation in precipitation throughfall causes spatial variation in humus consumption in the stand through differences in humus moisture with respect to tree crowns. Humus consumption influences tree mortality, growth of remaining trees and re-colonization. Surface fires in Scots pine plantations caused a partial reduction of the litter and humus layers and a high mortality in the smaller trees. Reduction in radial growth after surface fire was variable, and was less in large diameter trees and in trees that experienced less humus consumption around their stem bases. Experimental burning of the humus layer showed that increased removal of organic material by fire resulted in an increase in seedling numbers. Earlier studies have suggested that the charcoal produced by fire improves germination conditions by absorbing phytotoxins produced by ericaceous species. All such studies have used activated carbon as a standardized model for charcoal. Bioassays with pine seeds in aqueous extracts of Vaccinium myrtillus and Calluna vulgaris showed toxic effects of the two species, but charcoal reduced toxicity less than activated carbon. Therefore, those previous studies have overestimated the effect of charcoal on germination, likely because of the considerably higher active surface area of activated carbon. The post-fire tree cohort after severe but small-scaled fires in Scots pine stands mainly consisted of Scots pine, but also birch and aspen. Compared to succession after other disturbance types in Scots pine stands, such as windthrow or soil scarification, seedling numbers are higher after small-scale fires by a magnitude often.

Based on the good regeneration and for the purpose of fuel load reduction in areas with increased fire hazard, the prescribed burning of Scots pine stands should be reconsidered. Controlled forest fires could be used as an additional silvicultural technique to regenerate and transform single-species pine stands into mixed and more natural forests.

Keywords: biodiversity, fire ecology, fuel modelling, succession, tree regeneration

Marco Hille died in a tragic accident on December 4, 2004. At the time his thesis was almost completed, with essenstially mainly editorial work remaining.
Based on the draft thesis the Rector Magnificus of Wageningen Univeristy decided to award the doctoral degree posthumuously to recognize the work done by Marco Hille and as a tribute to an extraordinary PhD student. On April 20, 2006 the doctoral degree was handed to Marco's family.

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