No. 1 Feburary 1998
THE POTENTIAL FOR SNOW GUM TO RESPOND TO CHANGING PRECIPITATION REGIMES.
J.C.G. Banks and D.W.K. Smith, Department of Forestry, Australian National University, Acton, ACT 0200
Treeline forests in the Australian Alps are dominated by Snow Gum, Eucalyptus pauciflora, an evergreen broad-leaved species of Gwandanian origin. This species has characteristically opportunistic growth, and as such is never truly dormant. In predicting the response of this species to climate change it is important to determine relationships between tree growth and precipitation regimes. This study investigated tree growth and climate for the period 1911-1992. Precipitation is an ideal climate factor for study as it fluctuates both within and between growing seasons and for which long term data are available. The seasonal growth or tree rings provide long term records of growth performance and represent an integration of all growth factors, climatic, edaphic and biotic.
A tree ring site index, or SI, was derived from 26 tree boles sampled from the upper subalpine forest zone in the Perisher and Blue Cow ski fields in Kosciuszko National Park. The SI showed a marked increase in amplitude during the second half of this century, i.e. a 4-fold increase in the variance, indicating environmental factors influencing tree growth have both favoured and suppressed seasonal growth to a greater extent than before. Typically these extreme SI values persist for only one growing season indicating the capacity of the species to respond rapidly to changing environmental conditions.
Ring width per se was not correlated with seasonal rainfall, indicating that complex relationships exist between ring width and rainfall and that other factors are variously involved. Wide rings were more often correlated with low spring rainfall and above average autumn rainfall. The suggested scenario for this relationship is that in a warm dry spring tree foliage retention is maximised and the tree then capitalises on this foliar array when favourable soil moisture conditions persist through the autumn months; water is not limiting to growth in spring as the snow melt compensates for any rainfall deficit; leaf maturation may occur more quickly under these conditions limiting the immature leaf period when foliage is most susceptible to insect grazing, therefore restricting insect populations for the remainder of the growing season.
Narrow rings are produced in growing seasons when one or more growth factors fall below critical levels for normal growth. Rainfall deficit periods were not correlated with narrow rings, soil water storage compensates for low rainfall periods. Snow persistence is associated with narrow rings in some years with a two in three chance for narrow rings to occur in seasons when snow cover persists into December, suggesting that snow cover limits the length of the effective growing season by suppressing spring soil temperatures and prolonging soil waterlogging. Heavy insect foliage grazing, crown damage by winter glaze conditions, and even heavy flowering and fruiting may be other factors associated with narrow ring years.
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