Decadal shifts in the plant community of an East Maui Cloud Forest

Background/Question/Methods: Measured responses of forest ecosystems to global change drivers have been inconsistent, highlighting the need for further research across a wide range of spatial and temporal scales. Over recent decades, Hawai’i has experienced increased warming, drought, and decreased precipitation, trends that are expected to continue with global climate change. To identify temporal dynamics in plant community composition in a montane cloud forest on East Maui, we compared data from 26 vegetation plots that were surveyed six times between 1973 and 2013. Plots are arranged downslope from the ecotone with a subalpine shrubland along two transects, one wet and the other wetter. We compared temporal trends in structure to location along elevation gradients and identified trends in abundance for each taxon. Patterns of change were then compared to hypotheses gained from modern vegetation-climate associations, paleorecords, and shifting biotic interactions.

 

Results/Conclusions: Steep precipitation and elevation gradients were strongly related to plant community composition in all years surveyed. Exotic plants are still rare in this area, but six individuals were first recorded in plots in 2013. More taxa showing decreasing trends in abundance since 1973 than showed increasing trends. Decreasing taxa included biotically pollinated shrubs and were associated with wetter sites in 1973. Plant community composition and structure have also changed, but changes differed by location. We found a positive relationship between plot elevation and change in tree density on both transects (Spearman rank order correlation, r_s= 0.564 and 0.638 for the wet and wetter transects respectively, p < 0.05 for both). Despite decreased moisture availability and predictions of loss, sites near forestline actually gained trees, apparently driven by recruitment of saplings of the monodominant tree Metrosideros polymorpha into the canopy. Conversely, the lowest elevation sites do not appear to have gained trees. Overall, results from this resurvey indicate that climatic context may structure plant community response to anthropogenic drivers, even over short distances in native cloud forest.