Forest Plot Data
We used data on tree abundance and composition from forest plots sampled
across the major islands of the Hawaiian archipelago using largely
similar methods and compiled in the updated version of the Open Nahele
database (Craven et al., 2018). We specifically used data from plots
that ranged from 100 m2 - 10000 m2in area and had a minimum of 15 individuals (so that we could use
rarefaction-based tools to control for sampling effort). We included
plots from each of three main Hawaiian islands that spanned a large
precipitation gradient (Figure 1), Hawai’i (~0.5 Ma),
the Maui Nui complex (~2 Ma) (including Maui, Moloka’i
and Lana’i because they formed a single landmass during most of their
history (Clague & Sherrod, 2014; Price, 2004) and Oʻahu
(~3 Ma). We were unable to include Kaua’i, the oldest
main island (4.7 Ma), as there were too few plots available for
analyses. We also excluded species of tree ferns (Cyatheales) from the
analysis as they were not consistently sampled in all of the datasets
(Craven et al., 2018).
We selected two versions of the forest plot dataset for our analyses.
First, to evaluate the interaction between island age and the
precipitation gradient, we used plots that were relatively uninvaded by
alien tree species (i.e., a maximum of 40% of alien individuals), and
only analyzed native tree species and species introduced by Polynesians
around 800 years ago (e.g., Cordyline fruticosa ). We found that
keeping vs. excluding the Polynesian species had no qualitative
influence on the results. For these analyses, we had a total of 282
plots with 79 native species across the islands. Second, to evaluate
whether including alien species in the analysis eroded the signatures of
island age and/or environmental gradients, we included all available
forest plots (i.e., including those with >40% alien
individuals), and considered both native and alien tree species in our
analyses. For these analyses, we had a total of 375 plots with 142
species, of which 63 are alien. Species names are standardized based on
The Plant List v 1.1 (Cayuela et al., 2017) in OpenNahele, and status as
native or alien was obtained from the flora of the Hawaiian islands
(Wagner & Khan, 2023).