Technical University of Munich, Department of Ecology, Landscape by yew20072

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Evidence for long-term cohort dynamics in a monodominant tropical rainforest
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H.J. Boehmer , H.H. Wagner , G.C. Gerrish , J.D. Jacobi , D Mueller-Dombois
1
 Technical University of Munich, Department of Ecology, Landscape Ecology (LOEK),
                   2
Munich, Germany, University of Toronto, Department of Ecology and Evolutionary Biology,
                                3
Mississauga, Ontario, Canada, University of Hawaii at Hilo, Biology Department, Hilo, HI,
                 4
United States, U.S. Geological Survey, Pacific Island Ecosystems Research Center,
                            5
Honolulu, HI, United States, University of Hawaii at Manoa, Department of Botany, Honolulu,
HI, United States

Population dynamics of the dominant species Metrosideros polymorpha (Myrtaceae) are of
key importance for the future of Hawaii’s montane rainforests. This ecosystem was affected
by a serious decline (‘ōhi‘a dieback’) of the forest canopy in the early 1970s. Based on ten
years of research, a conceptual model was presented in 1987, predicting the full recovery of
the rainforest. This hypothesis was based on the assumption that the forest decline was a
natural demographic phenomenon due to a postulated widespread cohort structure of the
Metrosideros populations.



We used detailed field data from 25 permanent plots for analyzing regeneration success over
6 time steps from 1976-2003 in order to assess evidence for the long-term cohort dynamics
theory of Metrosideros polymorpha as postulated in 1987. Plots represented a variety of
different substrates ranging from ca. 200-10,000 years in age. Data collected include the total
number of i) Metrosideros polymorpha seedlings, ii) saplings in five stem height classes, and
iii) the DBH for all trees. Tree vigor was quantified in five classes.



Seedling and sapling development was significantly different between dieback and non-
dieback plots. On dieback plots, the forest decline of the 1970’s was followed by a
regeneration wave that by 2003 led to a new cohort of Metrosideros polymorpha trees. On
non-dieback plots, no such saplings wave could be observed. We conclude that populations
dynamics of Metrosideros polymorpha since 1976 confirm the population structure model of
1987 in main parts. Cohort establishment is widespread even thousands of years after
volcanic disturbance. We conclude that long-term cohort dynamics of Metrosideros
populations are likely to be determined not by catastrophic physical disturbance but by
extreme climatic fluctuations.

								
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