Morphology and Biomass Allocation of Current-year Shoots of Ten Tall Tree Species in Cool Temperate Japan

Abstract

Variations in morphology and biomass allocation of current-year shoots were investigated in saplings of ten cooltemperate tall tree species. In all the species, the ratio (leaf mass)/(stem mass) was greater in shorter current-year shoots than in longer current-year shoots. This indicates the presence of shoot differentiation, where shorter currentyear shoots are oriented to light-capture or leaf-display and longer current-year shoots are oriented to crown-expansion or space-acquisition. However, two types of differentiation were found. In type I, shorter current-year shoots had greater ratios of (leaf area)/(stem length) than longer current-year shoots and a ratio of (stem mass)/(stem length) comparable to that of longer current-year shoots. In type II, shorter current-year shoots had disproportionately smaller ratios of (stem mass)/(stem length) than longer currentyear shoots and a ratio of (leaf area)/(stem length) comparable to that of longer current-year shoots. Type I corresponds to the apparent differentiation of dwarf shoots. Type II was newly found in this study. I hypothesize that shorter current-year shoots in type I have stout stems for supporting offspring shoots in spite of severe self-shading, whereas shorter current-year shoots in type II avoid selfshading at the expense of stout stems. Implications of the two types of shoot differentiation are discussed in relation to foliage-crown dynamics.Key words: Biomass allocation -Frequency distribution of shoot length -Leaf display index -Shoot differentiation -Stem length -Stem mass per unit stem lengthCurrent-year shoots are fundamental units for tree crown development, and the relative importance of light-capture (i.e., leaf-display) and crown-expansion (i.e., space-acquisition) for current-year shoots is not uniform even within a single tree. Therefore, the differentiation of current-year shoots for light-capture or crown-expansion is an effective Present address:Abbreviations: LDI, the ratio of (leaf area)/(stem length) of a currentyear shoot (leaf display index); LSR, the ratio of (leaf mass)/(stem mass) of a current-year shoot (leaf-stem ratio); LSRPo,, the ratio of (total leaf mass)/(total stem mass) of the sampled current-year shoots (be., leaf-stem ratio of a population of current-year shoots) tactic for constructing an efficient assimilation system (Halle et a/, 1978, Steingraeber and Waller 1986, Seino 1998. Crown-expansion is more important than light-capture for current-year shoots facing an open space with high light availability; the opposite is true for current-year shoots in the inner and lower portions of a crown and for current-year shoots facing the crown of neighboring trees (Remphrey et a/. 1983, Borchert and Tomlinson 1984, Jones and Harper 1987. The differentiation of current-year shoots is likely to be universal among tree species.Patterns of differentiation of current-year shoots are closely related to the productive strategies of trees. For example, the total amount of foliage and the leaf-stem balance of an assimilation system...

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