Individual study: The effect of burn season on regeneration of heather Calluna vulgaris in two different aged stands at Glen Dye Estate, Aberdeenshire, Scotland
Kayll A.J. (1966) Some characteristics of heath fires in north-east Scotland. Journal of Applied Ecology, 3, 29-40
Burning of heather Calluna vulgaris-dominated heath/moor (muirburn) in Scotland generally aims to maintain productivity for sheep and red grouse Lagopus lagopus scoticus. Managers may burn 5-10% of the heather area each year during the burn season (1 October-15 April). However, success of burns varies greatly but reasons for this are not well understood. There is disagreement as to the efficacy of spring vs. autumn burning, although general feeling in northeast Scotland is that the spring burning is better. No information has been published on the temperatures reached at or just below the surface and in and above the heather canopy. This vertical temperature profile is important in determining the effects of fire on plants. Data is also lacking on fuel loads and burning conditions associated with fires of given intensities. The research summarised here was undertaken to characterize some variables associated with moor burning practices by describing fires burnt in autumn and spring.
Study areas: Heather was burnt on the Glen Dye Estate, Aberdeenshire (National Grid ref. N0 7090). The areas comprised stands aged approximately 15 and 25 years. The fires burnt in autumn were in the 15-year-old stand, and those in spring in the 25-yr-old stand. Within the burn areas, rectangular plots measuring 15 x 20 m were establised (2 in the 15-yr-old stand; 5 in the 25-yr-old stand).
Fuel load: On 0.5 m² sample plots (six for 15-yr-old stands; 10 for 25-yr-old stands) adjacent to areas to be burnt, vegetation height and cover were described, and litter was removed to estimate fuel load. After the main areas had been burnt, samples of residual material was collected (separated into heather tops, heather stems, other shrubs, mosses and litter), dried and weighed. Estimates of fuel consumed were calculated.
Burning: All fires were headfires (burnt with the wind). Two autumn fires were conducted in the 15-year-old heather (11 and 24 October 1963). The spring fires in the 25-year-old heather were burnt on the same day (4 April 1964).
Measurements: Data were collected immediately before and during the fires: air temperature, relative humidity, wind velocity, wind direction and nature (e.g. constant or gusty etc.), slope aspect and slope. The moisture content of the surface soil was assessed subjectively. Fire temperatures were monitored with chromel-aluinel thermocouples.
Autumn burning: Burns were conducted in dry, windy conditions. Fuel load was 15,900 kg/ha of which 93% was burnt. In the second fire, temperatures in the crown (c. 500ºC) were high but surface temperatures low (60°C). Both fires removed longer heather stems and most foliage, and Calluna regeneration in the follwing year was vigorous and considered excellent.
Spring burning: Spring burning was carried out under cool, moist conditions with 23,200 kg/ha fuel load. The burns were incomplete with only 30% of the available fuel consumed, and crown and surface temperatures were low (c. 250°C, and ambient to 100°C, respectively). Many heather stems were left. Subsequent regeneration was poor. The ineffectiveness of the burn was attributed to stand structure, low wind speed, high heather moisture content, and generally wetter conditions compared to the autumn burns.
Conclusions: Whether burning in autumn is more effective than burning in spring has not been resolved by this study, and the results are not strictly comparable because of the differences in heather stand age, available fuel load and burn conditions.