Foliar nutrients and Sr isotopes along a climate gradient on Mt. Haleakala, Maui

On the northeast, windward side of Mt. Haleakala, Maui, a trade-wind inversion layer at 1900 m controls the uppermost extent of a tropical montane cloud forest. Treeline marks the boundary between the cool and moist cloud forest below and the drier and colder locations above. Within the cloud forest, Metrosideros polymorpha is the dominant canopy tree. Above treeline, the predominant species are a xerophytic shrub and a non-native grass; M. polymorpha exists only as isolated individuals within the drier, cooler shrub and grasslands. We sampled surface soils and vegetation of the three species along four elevational transects to observe how nutrient availabilities and sources change across treeline. Transects were comprised of locations within the upper extent of the cloud forest, at treeline, and in the shrub and grassland communities. Soils were analyzed for C, N, net N mineralization rates, and plant-available P. Foliar samples were analyzed for C, N, P, base cations, micronutrients, and the isotopic ratio of 87Sr/86Sr. Total soil N and net mineralization rates decrease with increasing elevation across treeline. N content of the grass and shrub reflect soil characteristics and also significantly decrease above treeline, although this trend is not observed in M. polymorpha. Preliminary isotopic results suggest for all three species a change in source of nutrients from predominately rock-derived in the upper extent of the cloud forest to partially precipitation- derived in the shrub and grasslands.