As elevation and rainfall increase along many tropical mountains, different forest types occur. In these forests, tree species richness, diversity, and productivity typically decrease, while stem density increases with increasing elevation and rainfall. Within forest types along elevation (longitudinal) gradients, riparian to upland environmental (lateral) gradients may also influence vegetation characteristics and ecosystem processes. Ecosystem properties such as productivity, measured by litterfall, can be greater in lower elevation riparian areas compared to adjacent non-riparian upland areas, presumably due to increased water availability.
However, upland and riparian areas of headwater streams at higher elevation may have increased hydrological connectivity due to frequent surface water flow. Therefore, differences between the upland and riparian environment may be minimal in headwater streams, and there may not be distinct riparian areas.
We hypothesized that differences in tree community characteristics and litterfall between upland and riparian areas would decrease as elevation and precipitation increased in the Luquillo Experimental Forest (LEF), because of increased soil saturation and hydrological connectivity of headwater streams and upland areas. Within each forest type (Tabonuco 450 m, Palo Colorado 600 m, and Elfin 900 m) three riparian plots were randomly established < 2 m from stream edge (riparian) and three plots 40 m away from stream edge (upland). Litterfall production was sampled and characterized for one year by placing five baskets along riparian zones within each forest type (<3 m of the stream edge) and five baskets along upland areas (> 40 m away from riparian areas) for a total of ten baskets in each forest type.
Results/Conclusions
There was no interaction between forest type (Tabonuco, Palo Colorado or Elfin) and riparian or upland site for stem density, species richness or diversity (H’). There was also no difference in stem density, species richness or diversity between riparian and upland sites, or among the forest types.
We observed a trend of higher values of total litter in the riparian areas of the Tabonuco forest, while in the higher elevation Palo Colorado and Elfin forest, the trend was inversed with higher values of total litter in upland areas.
Within each forest type, leaf litterfall species richness was higher in the riparian areas compared to upland areas, and greater in the lower elevation forest site.
In this tropical montane wet forest landscape, as elevation increases, upland and riparian differences decrease, as forests at higher elevation are more similar to wetlands and lower elevation riparian areas.