Population Structure of the Hawaiian Tree Fern Cibotium chamissoi Across Intact and Degraded Forests Oʻahu, Hawaiʻi

Abstract

Because the population status of Cibotium on Oʻahu is currently unknown, this biogeographical study examines the population structure, abundance, and potential restoration importance of the species Cibotium chamissoi across a spectrum of "intact" and "degraded" forest communities in the Waiʻanae and Koʻolau Mountains of Oʻahu. The terms intact and degraded are inherently subjective, and for the purposes of this study are defined and used as follows: "intact" forests are considered to be constituted mainly of native species and are presumed to function as a healthy ecological system; conversely "degraded" forests are those heavily invaded by alien species and/or feral pigs, with low cover of native species and possibly reduced native species diversity. The island of Oʻahu is the most developed of the Hawaiian Islands, and has undergone intense development and subsequent forest degradation. Intact native forests are generally confine'd to the upper elevations. Feral ungulates and alien weed species have degraded lower forests and even pockets of higher elevation forests. Therefore the mid-elevation zone is an ideal area to study the tropical island forest landscape because here is found a spectrum of intact native and degraded alien habitat where Cibotium chamissoi is the common tree fern species. It is accepted that Cibotium are truly an integral part of Hawaiian forest communities, therefore research is needed to determine if they are indeed disappearing from Oʻahu's forests. Though this study will not assess all of the potential causes for a possible Cibotium decline, we may be able to determine baseline patterns in abundance and population structure of C. chamissoi. By isolating the impacts of feral ungulates and examining the effects of environmental gradients on C. chamissoi populations on Oʻahu, at the very least, we may be able to identify or dismiss several possible explanations for variations in C. chamissoi recruitment and abundance.