The use of dendrochronology in the interpretation of the dynamics of the snow gum forest

Abstract

The objectives of the study were to gain expertise in the dating of tree rings in snow gum, and to apply it to the interpretation of historical influences on the structure and dynamics of the snow gum forest. The subalpine forests of the Brindabella Ranges were chosen for study because their history of exploitation is reasonably well known, they contain a variety of stand structural types, and access is good. Additional stands were sampled in the Snowy Mountains and on Booths Range to provide as wide a spectrum of stand types as possible. A preliminary examination of seasonal growth found that with the exception of 'lammas rings' in early growth, false rings are uncommon. Missing rings are typically confined to zones of severely restricted growth. Anomalous zones within rings are not infrequent, being caused by drought, fire or frost and that 'marker rings' can be recognised and are useful in chronology building. The ageing of over 200 trees in 18 stands showed live trees to be up to 350 years old that regeneration after wildfire is essentially even-aged and that while uneven-aged stands may once have been more common, most stands are now even-aged, originating after wildfires since European settlement. Fire scar dating shows the frequency of fires to have changes dramatically with European settlement, e.g. from1 to 10 fires per century in Aboriginal times to as frequently as one fire every 2 to 4 years during the period 1860 to 1950. Fire frequency may have returned to approximately that of the Aboriginal Period since 1950. The period of peak fire frequency was highly destructive, with the consequence that structural and other changes are still taking place. Analyses of growth patterns suggest that where growth is not impeded, seasonal growth ring width reaches a maximum by about 60 years it subsequently declines and is minimal by 150 years. Many factors modify this growth pattern. competition and periods of short Growth is highly sensitive to and long tem suppression are common. Fire plays a major role in recovery from suppression by removing competing trees and/or understorey,and may stimulate the production of wide rings for up to 10 years, possibly as a result of greater nutrient uptake. Post-fire ash nutrients are probably important for the maintenance of health and vigour of trees growing in an otherwise nutrient poor environment. Analyses of tree ring nutrient patterns for calcium, potassium, magnesium, manganese and phosphorus suggest that the sapwood of snow gum acts as a 'store' for nutrients taken up in excess of seasonal requirements. Nutrients are opportunistically taken up when available in the soil, e.g. after fire. Under good growing conditions nutrients are withdrawn from the sapwood. Alternatively where these nutrients are not withdrawn peaks in the residual concentration of nutrients in the heartwood will reflect events in the life of a tree, which can be dated, e.g. a drought period following fire. Complex relationships between fire, tree growth and nutrient cycling are indicated. This is possibly common to the genus as a whole which has evolved in an environment where soil nutrient levels have been declining and fire is an occasional but inevitable event.