Are juvenile forms of New Zealand heteroblastic trees more resistant to water loss than their mature counterparts?

Abstract

One of the theories for the evolution of divaricating shrubs and juvenile forms of heteroblastic woody plants suggests that these forms were selected for resistance to water loss. We examined the water relations and carbon isotope ratios (δ13C) of excised shoots from juvenile and mature forms of one homoblastic species, seven heteroblastic species, and the divaricating shrub Coprosma rotundifolia. Shoots from the juvenile divaricate forms and C. rotundifolia showed responses (relatively high rates of water loss, high osmotic water contents, more negative δ13C, elastic cell walls and low water contents at turgor loss) characteristic of water spenders. Mature and juvenile forms of Pittosporum eugenioides and Pseudopanax crassifolius and mature Carpodetus serratus showed the opposite characteristics. The ordination of these characteristics suggested that juvenile forms of P. eugenioides, Streblus microphylllus, Sophora microphylla, and Carpodetus serratus were more water‐spending than their mature forms, whilst juvenile forms of Pseudopanax crassifolius, Hoheria angustifolia, and Plagianthus regius tended to be somewhat less water‐spending than their mature forms. Our results do not support the hypothesis that juvenile forms of heteroblastic trees are more resistant to water loss than their mature counterparts. We suggest that forms with a divaricating habit produce a micro‐environment within the shrub that favours small leaves, with relatively high leaf conductance, water loss, and CO2 assimilation rates, that also may be partially adapted to shade.