No biotic homogenization in the understory of Cupressus sempervirens long-established plantations

Non-native species spreading naturally are associated with losses of native diversity and biotic homogenization. Conversely, the effect of non-native tree species used in plantations may vary, particularly according to their biogeographical origin. We investigated whether the canopy cover of Cupressus sempervirens long-established plantations affects alpha and beta taxonomic diversity of understory plant communities, alters their functional composition, or promotes disturbance-related species. We surveyed 74 plots in cypress plantations along a latitudinal gradient in Italy. We used beta-dispersion analysis and regression models to test the effect of cypress cover and environmental conditions on understory vegetation. We found no influence of cypress cover on soil conditions and no evidence of cypress-driven taxonomic homogenization. Understory diversity and composition were unrelated to cypress cover but formed three site-specific groups reflecting local environmental conditions. Likewise, cypress cover did not affect understory functional composition, although it varied across sites. Disturbance indicator values showed no significant variation along cypress cover or among sites, indicating no ruderalization. Long-established non-native plantations of C. sempervirens function as unmanaged, naturalized forests in which cypress exerts little to no influence on understory plant diversity and composition, even at high cover. This could be attributed to the vertical crown structure of C. sempervirens that allows light to reach the forest floor, to the biogeographic origin of Mediterranean cypress, and to the presence of native tree species in the canopy. Our results indicate that long-established C. sempervirens plantations can retain ecological value. However, this should not be taken as a justification for the indiscriminate establishment of new plantations, as their ecological effects may vary depending on ecosystem complexity.