Using intraspecific variability to evaluate filters on community assembly along an elevational gradient

Background/Question/Methods: Forest understories have likely adapted to environmental conditions that make their ecologies distinct from those of canopy species. Yet, tropical understories have received surprisingly little research attention despite being the birth-place of the null theory of community assembly. As such, the rules governing community assembly in understories remain poorly studied. Traditional community assembly theory assumes species partition their niche space, which promotes coexistence. As more species are added to the community, niche space fractionates, a phenomenon known as niche packing. Conversely, null theory predicts competitive equivalence and overlap in niche-space. To understand the filters influencing community assembly, and address the paucity of research in understory ecology, we investigated niche partitioning using functional leaf and stem traits as proxies for niche-space in Piper, a diverse understory genus. To understand how environmental factors and species richness influenced niche partitioning, we measured niche-space in four communities along an elevation and species richness gradient in Southeastern Peru. Intraspecific variation was used to understand the internal and external filters influencing community assembly. A standard effect size calculated from observed values and null model values were used to determine the significance of internal and external filters. For each trait, the effect of elevation on niche packing was evaluated using linear regression.

Results/Conclusions: As expected, significant internal filters were observed but only for leaf thickness, specific leaf area, diameter at breast height, leaf toughness, and specific wood density of branches, which indicated niche packing. External filters on community assembly were only observed for chlorophyll content, and neither internal or external filters were observed for chlorophyll fluorescence. Contrary to traditional theory, there was no uniform effect of elevation on niche packing. Given the traits we measured, we conclude that species likely achieve coexistence through trade-offs in life history and resource-acquisition strategies. The external filter we observed may indicate that environmental filters act upon photosynthetic traits, which could be linked to the elevational changes in soil-nitrogen in our system. The absence of relationship between elevation and niche packing was likely due to the presence of the cloud forest ecotone. Additional sampling at different elevations is needed to better understand the effect of the elevation on community assembly in the understory.