Significance of xeromorphic features in humid subalpine environments in New Zealand

Abstract

In the vegetation of New Zealand, xeromorphic characters of various kinds are present to an extent that seems out of keeping with the predominantly moist, mild climates. The subalpine belt in the North Island, west of the Main Divide in the South Island, and in Stewart Island surely experiences one of the wettest and cloudiest climates in the world, yet even here there is a predominance of plants with features that would generally be regarded as xeromorphic. This leads mc to suggest that at least some of these features have evolved, not in response to drought, but in response to the need to make the most effective use of radiant energy received in short spells of fine weather. This hypothesis is developed as a corollary to the principle, clearly described by Gates (1965), that direct solar radiation warms a leaf until it reaches a temperature at which incoming heat is balanced by release of heat through re-radiation, convection from both surfaces of the leaf, and loss of latent heat in transpiration. The temperature of the leaf at this point of equilibrium is always greater than that of the surrounding air, commonly by more than 10°c. Gates stresses the advantages of reducing this heat load, but in environments where metabolism is slowed down by insulficient warmth, it may be more advantageous to possess a leaf structure which restricts release of heat, so that equilibrium with incoming radiation is reached at higher temperatures.