Response of Metrosideros Polymorpha Seedlings to Experimental Canopy Opening

Twenty 100-m2 plots in a forest dominated by tree fern (Cibotium spp.) and ohia (Metrosideros polymorpha) on the island of Hawaii were subjected to different degrees of canopy removal. Seedlings growing in the control plots received an average of 10% full irradiance, whereas those growing in completely cleared plots received an average of 43% full irradiance. Invasion of Metrosideros tree seedlings into the completely cleared plots averaged more than three times that in the undisturbed forest. These open-born seedlings grew in height by an average of 3.5 cm/yr, 80% more than comparably sized seedlings that had started life in the shade and were then subjected to higher irradiances. The average height increment of all shade-born seedlings was 2.5 cm/yr in the control plots and 6.6 cm/yr in open plots; in terms of relative growth rates, this represents an increase from 0.19 to 0.47 cm -cm- yr-1. Height growth was optimal (10.5 cm/yr) at microsites receiving 55- 60% relative irradiance. During the course of monitoring the growth and irradiance of 1130 seedlings that were tagged before canopy disturbance, it was found that these shade-born seedlings suffered mortality at a rate of 57%/yr when growing at 5 50% indicate that many of these Metrosideros seedlings cannot be readily conditioned to open habitats. It therefore appears that shade-born seedlings of this variety of Metrosideros are intermediate in shade tolerance. Although seedlings may survive in the shade for a year or two, dense canopies of tree ferns or older Metrosideros trees are able to suppress their growth effectively, so that the Metrosideros population may have to depend on canopy gaps or canopy diebacks to maintain itself in mature rain forests. Some individuals were nevertheless able to acclimate successfully to very high light intensities, and this same species is a successful colonizer of barren volcanic deposits, suggesting that Metrosideros polymorpha exhibits broad acclimation ability and/or a wide range of genetic variability.