Recruitment of dominant native Hawaiian tree, Metrosideros polymorpha, limited by alien invaders

Background/Question/Methods

The native tree ‘ôhia (Metrosideros polymorpha) occupies a variety of habitats and is the dominant species in Hawaiian forest ecosystems. M. polymorpha is one of the first colonizers on new lava flows and dominant in much older, boggy habitats. M. polymorpha reproduce rapidly from widely dispersed windblown seeds, however recruitment limitations posed by alien plants or animals across a range of habitats are not well known. We used data collected from the on-going Forest Inventory & Analysis (FIA) program to determine what factors affect ohia distribution and recruitment. Presently, the FIA program has installed approximately 160 plots on three Hawaiian Islands. Each plot consisted of four nested circular plots where large trees (DBH > 60.9 cm) are measured on the outer macroplot, smaller trees were measured (DBH > 12.7 cm) on the middle subplot, and seedlings (> 0.3 m tall and DBH < 2.54 cm) and saplings (DBH from 2.54 to 12.45 cm) were measured on the inner microplot. Vegetation profiles and feral pig disturbance were also noted on each subplot. To examine current trends, we analyzed tree, sapling, and seedling basal area/hectare and stem density/hectare along with elevation, rainfall, substrate age, and invasive species.

Results/Conclusions

Our preliminary inventory results indicate that M. polymorpha basal area and stem density were greatest at elevations between 800 to 2,000 m, with mean annual rainfall ranging from 600 to 5,000 mm, and on young substrates from 200 to 120,000 years old. We did not find a relationship between live or dead M. polymorpha trees and seedling or saplings; this is likely due to our skewed sample favoring younger substrates where live M. polymorpha have not formed closed canopies and few M. polymorpha deaths were observed. However, our results show that M. polymorpha recruitment appeared limited to elevations above 900 m. At elevations below 900 m, the sapling density for the invasive strawberry guava (Psidium cattleianum) outnumbered M. polymorpha by 13x for saplings and 38x for seedlings. High P. cattleianum seedling densities also corresponded to pig damage on plot, where seedling densities were 4.5x greater on plots with pig damage compared to plots without. Our preliminary results suggest that while M. polymorpha remains the dominant tree species, feral pigs may be facilitating the spread of the aggressive invader P. cattleianum, which in turn may be prohibiting the regeneration of M. polymorpha.