Survivorship, growth and self‐thinning in Banksia ericifolia

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Abstract
High‐density (dense) and low‐density (sparse) plots were set up in naturally sown monospecific stands of Banksia ericifolia in coastal heath, 3 years after fire. This was done both in high‐growth and low‐growth areas. Plant mortality was recorded quarterly, and two harvests were made at 6 and 9 years to sample growth. Density‐independent mortality at an exponential rate was observed in the low‐growth treatments at both densities, and in the high‐growth sparse treatment. Growth level affected mortality, with the half‐life of populations in the high‐growth sparse plots being double that of populations in the low‐growth plots. Density‐dependent mortality (self‐thinning) was seen only in the high‐growth dense plots. Seasonal effects on mortality were slight; maximum mortality was observed in the spring‐summer period in plots subject to density‐independent mortality, and in the winter‐spring quarter in plots that had self‐thinned. Yields in the high‐growth plots and the low‐growth dense plots were high for heath vegetation. The self‐thinning populations did not exceed White's (1985) upper boundary for thinning lines of log intercept (K) = 5 on standardized axes. The data suggested a log intercept value in the range 4.8–4.9 in the high‐growth stands assuming a thinning‐line slope of – 1.5. Banksia ericifolia (a large shrub/small tree) has a high mean plant weight per given thinning density compared with trees, where an upper limit of log K= .4 has been suggested by White (1985). The volume of canopy space per plant in B. ericifolia is not unusual compared with other species. The amount of biomass packed into a given volume of canopy space was high in this Banksia, achieved by having leaves with a low ratio of area to weight (specific leaf area, SLA). For given values of density, leaf area index and proportion of shoot as leaf, plants with a low SLA will be several times heavier than plants with a high SLA. This achieves a high biomass to volume ratio without an erectophile canopy and may explain the high intercept seen for thinning lines of conifers.