Growth patterns and annual growth cycle ofAcacia karrooHayne in relation to water stress I. Leaf and shoot growth

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Abstract
A field study was conducted to describe the growth patterns of A. karroo at different levels of water stress in different edaphic situations. Shoots are heterophyllous and are formed by free growth. The degree of development of a shoot, relative to others in the canopy, is governed by branch and position in the canopy. At least six phenological phases were identified in the annual growth cycle. The pattern of growth and the phenological cycle are not changed by water stress. Initiation, emergence and development of shoots and leaves are governed by how favourable environmental conditions are. If there is little shoot growth early in the season, these plants can partially compensate by producing more leaf per unit of shoot if environmental conditions improve. Leaf and shoot growth at the beginning of the season took place only if there was sufficient moisture available, and if the minimum temperature had risen above a threshold amount. Where there was insufficient soil moisture, no growth was observed before rains had fallen. The growth strategy of A. karroo differs markedly from that of broadleaved African savanna tree species. Growth in A. karroo is dominated by current growing conditions, rather than those of the previous season. They are able to make opportunistic growth at any time. Soil depth had a marked influence on plant growth, presumably due to a larger available nutrient and moisture pool.