Eucalyptus plantations were introduced in southwestern Louisiana to produce pulpwood on short rotations. Eucalyptus species are highly intolerant of competing vegetation, so multiple herbicide applications are used during early stand establishment. Understory vegetation pattern and composition are important indicators of sustainability and are often correlated with faunal diversity in intensively managed forest plantations. However, species richness and diversity alone provide incomplete information about the function of the community. A functional ecology-based approach allows assessment of community functions and services in response to human activities. In 2016 we studied understory vegetation of Eucalyptus benthamii and slash pine (Pinus elliottii), which was the primary land use replaced by eucalyptus in the area, in southwestern Louisiana. The objective of the study was to investigate understory functional diversity and net primary productivity in five slash pine sites established in 2008 (S08), five slash pine sites established in 2013 (S13), and five eucalyptus sites established in 2013 (E13). We characterized five understory traits (height, specific leaf area, leaf nitrogen, leaf carbon, leaf C/N) for each site and, for each trait, calculated community-weighted mean (CWM) and functional-divergence (FDvar). Understory vegetation structure, overstory tree diameter at breast height, height, and canopy coverage were also measured.
Results/Conclusions
We found no significant differences in CMW and FDvar between S08 and E13 plantations, which were also similar in understory vegetation structure. On the other hand, values for CMW, FDvar, and understory vegetation structural characteristics were significantly greater in younger S13 plantations than in S08 and E13 plantations, indicating a possible influence of the overstory. These results suggest that declines in functional diversity indices values and understory vegetation structural characteristics were substantially correlated with the degree of overstory development. Fast-growing E. benthamii and older slash pine plantations appeared have a more developed overstory in terms of diameter, height, basal area, and a higher degree of canopy coverage, which affected the understory biomass and consequently the functional diversity in respect of net primary productivity. The faster height and canopy development of E. benthamii relative to slash pine is an important consideration relative to management interventions, such as tree removals, that would foster understory vegetation development and forest net primary productivity.