Eucalyptus (Eucalyptus spp.) plantations are an emerging forest management practice in the Southeast U.S. for producing pulpwood on short rotations of five to eight years. Eucalyptus species grown in these plantations are highly intolerant of competing vegetation, so herbicides are used intensively in the first year of the plantations to facilitate establishment and growth. Understory vegetation patterns and composition are important indicators of overall biodiversity and sustainability and are often highly correlated with faunal diversity in intensively managed forest plantations. A study was carried out from 2014 through 2016 in southwestern Louisiana within the mid-South U.S. to understand the effects of intensive management of eucalyptus plantations on understory vegetation diversity relative to that of slash pine plantations, which were replaced by Eucalyptus benthamii plantations. The objective of the study was to assess species richness of understory vegetation in these plantations. The study was conducted in five slash pine sites established in 2008 (S08), five slash pine sites established in 2013 (S13), and five sites of E. benthamii established in 2013 (E13). The species richness of understory vegetation was investigated and compared in the three types of plantations through species accumulation curves and five other non-parametric richness estimators: Chao 2, ICE, Jackknife 1, Jackknife 2, and bootstrap. Overstory tree diameter at breast height (DBH) and height were also measured.
Findings indicated no significant differences in understory species richness between S08 and E13 plantations, which were also not significantly different in tree DBH and height. On the other hand, S13 had significantly smaller trees and higher species richness than S08 and E13. Number of observed species reported by non-parametric estimators was consistent with species accumulation curves. Understory species richness was not significantly influenced by overstory tree species. However, understory species richness was higher in the younger S13 plantations, indicating that the development of canopy architecture and structure may play a fundamental role in understory species richness. These results suggest that the decrease of understory vegetation richness is substantially correlated with the degree of canopy closure. Fast-growing E. benthamii appears to reach a closed-canopy condition earlier than slash pine, altering the species composition pattern in a very short time period.