Rebuilding after collapse: evidence for long-term cohort dynamics in the native Hawaiian rain forest

This is the accepted manuscript of an article published by Wiley.Questions. Do long-term observations in permanent plots confirm the conceptual model of Metrosideros polymorpha cohort dynamics as postulated in 1987? Do regeneration patterns occur independently of substrate age, i.e. of direct volcanic disturbance impact? Location. The windward mountain slopes of the younger Mauna Loa and the older Mauna Kea volcanoes (island of Hawaii, U.S.A.). Methods. After widespread forest decline (dieback), permanent plots were established in 1976 in 13 dieback and 13 non-dieback patches to monitor the population structure of M. polymorpha at approximately 5-year intervals. Within each plot of 20 x 20 m, all trees with DBH >2.5 cm were individually tagged, measured, and tree vigor assessed; regeneration was quantified in 16 systematically placed subplots of 3 x 5 m. Data collected in the subplots included the total number of M. polymorpha seedlings and saplings (5 stem height classes). Here we analyze monitoring data from 6 time steps from 1976 – 2003 using repeated measures ANOVA to test specific predictions derived from the 1987 conceptual model. Results. Regeneration was significantly different between dieback and non-dieback plots. In dieback plots, the collapse in the 1970s was followed by a 'sapling wave' that by 2003 led to new cohort stands of M. polymorpha. In non-dieback stands, seedling emergence did not result in sapling waves over the same period. Instead a 'sapling gap' (i.e. very few or no M. polymorpha saplings) prevailed as typical for mature stands. Canopy dieback in 1976, degree of recovery by 2003, and the number of living trees in 2003 were unrelated to substrate age. Conclusions. Population development 46 of M. polymorpha supports the cohort dynamics model, which predicts rebuilding of the forest with the same canopy species after dieback. The lack of association with substrate age suggests that the long-term maintenance of cohort structure in M. polymorpha does not depend on volcanic disturbance but may be related to other environmental mechanisms, such as climatic anomalies.This research was funded by the German Research Foundation (DFG) grant BO 1768 to H. J. B., the National Science Foundation grant DEB-7910993 to D. M.-D., the U.S. Geological Survey's Ecosystems Research Program, and a Discovery Grant to H. H. W. by the Natural Sciences and Engineering Research Council of Canada. Permission to use the study sites was given by the Hawai´i Volcanoes National Park, U.S. Park Service, and the Hawai'i Division of Forestry and Wildlife