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).