The Luquillo Long Term Ecological Research Program (LUQ) focuses on understanding factors driving long-term change in tropical forest ecosystems in the Luquillo Mountains. To increase our understanding of biotic and abiotic controls on ecosystems, we extended our work from the tabonuco forest to a range of forest types occurring along a climatic and forest gradient in the Luquillo Mountains. Forest sites investigated along the gradient (from the coast to the top of the mountain) include mature stands of mangroves, dry semi-deciduous woodland, Pterocarpus, lowland moist, tabonuco, sierra palm, colorado, and elfin forests that span a wide range in elevation, temperature, and precipitation conditions. Ascending the Luquillo Mountains, climate becomes cloudier, wetter (1500 to 4900 mm rain yr-1), and cooler (c. 26 to 18.5 ?C).

From the coast to the mountain tops, tree basal area, aboveground biomass, canopy heights and mean plant species richness of forest communities peaked at mid elevations. Litterfall decreased from 9 to 2 Mg/ha/y and was strongly positively correlated with a relatively small range in temperature (r2 = 0.87, n = 12 sites) along the upper portion of the gradient: 330 m elevation gradient from 635 m to 968 m. However, litter decomposition and soil CO2 effluxes follow a stepwise trend that suggests threshold effects. A common substrate decomposition experiment showed essentially two decomposition environments: (1) palm and colorado forests and (2) cloud forest types, suggesting a likely role of litter quality in controlling patterns in decomposition along the gradient. The home-court advantage of palm tissues decomposed in palm rather than under a dicotyledonous canopy suggests biotic (decomposer) effects.

Microbial communities differ among forest types based on fatty acid composition and DNA. The fungal to bacterial ratios based on fatty acids markers decrease along the elevation gradient, indicating that diversity and abundance of bacterial groups increases. Sulfate reducing and Crenarchaeota bacteria, functional groups common to coastal and extreme environments and versatile degraders, were present in the all forests types but their diversity increases along the gradient. These bacterial groups likely contribute to decomposition of complex substances from decaying trees.

The percent of the forest floor covered by white-rot basidiomycetes that form litter mats decreased with elevation, but was markedly less in the colorado forest. While mat abundance is broadly inversely related to climatic variables along the elevation gradient, the anomalous ‘saddle’ corresponding to the base of the cloud condensation zone (colorado forest) may be related to qualities of leaf litter typical of this forest type as it does not correspond to abiotic gradients.

The abundance and diversity of litter invertebrates decreased with increasing elevation. However, palm stands had higher abundance and diversity of litter invertebrates than adjacent dicotyledonous stands. In contrast, earthworm diversity in the soil was highest at the top of the mountain. Soil oxygen generally decreased along the elevation gradient from 15-21 % in the lower elevations to 0-5 % in the upper elevations. There is considerable coherence in the long term data set suggesting that sub-seasonal cycles, driven by a combination of climate and biota, are responsible for the observed trends.