High risk of plant invasion in the understory of eucalypt plantations in South China

Patterns of understory invasion in invasive timber stands of a tropical sky island

Current climate and land cover change threaten global mountaintops with increased spread of invasive species. Long-established plantations of invasive trees on these mountaintops can alter their surroundings, further increasing invader-facilitated invasion. Identifying the ecological conditions promoting such associations can help develop better management interventions. The Western Ghats's Shola Sky Islands (>1400 m MSL) host vast stretches of invasive tree plantations that sustain the colonization of other invasive woody, herbaceous, and fern species in their understories. Here, we analyzed vegetation and landscape variables from 232 systematically placed plots in randomly selected grids using non-metric multidimensional scaling and Phi coefficient approaches to examine patterns of association (positive interactions) between understory invasive species with specific invasive overstory species. We also conducted GLMM with zero inflation to determine the influence of environmental variables where such associations occur. We find that understory invasion of multiple species under the canopy of other invasives is widespread across the Shola Sky Islands. Stands of Eucalyptus host the colonization of 70% of non-native invasive species surveyed across the Shola Sky Islands. In particular, the Lantana camara invasion is strongly associated with Eucalyptus stands. We also found that climatic variables affect the colonization of understory woody invasive species, while invasion by exotic herbaceous species is associated with the density of road networks. Canopy cover impacts all invasives negatively, while fire incidence was negatively associated with invasion by Lantana spp. and the Pteridium spp. While the restoration of natural habitats primarily targets the highly invasive Acacia, less invasive Eucalyptus and Pinus are often not included. Our study suggests that retaining such invasive species in natural habitats, particularly protected areas, can hinder ongoing grassland restoration efforts by facilitating further invasions by multiple woody and herbaceous species.

Different antioxidant regulation mechanisms in response to aluminum-induced oxidative stress in Eucalyptus species

Forest ecosystems play an important role in environmental protection and maintaining ecological balance. Understanding the physiological mechanisms of tree species response to aluminum (Al) toxic is crucial to reveal the main causes of plantation decline in acid rain area. As an important afforestation tree species in tropical and subtropical areas, Eucalyptus has high economic value and plays crucial ecological roles. However, continuous fertilization and acid precipitation can exacerbate soil acidification and increase soil active Al, which has a significant negative impact on Eucalyptus growth. Hence, species and genotypes with high Al resistance are required to solve the problem of Al toxicity of acidic soils for sustainable forest production. In this study, E. urophylla was better adapted to Al stress than E. grandis or E. tereticornis; its high Al resistance was attributed to greater antioxidant enzyme activity and non-enzymatic antioxidant content, and a lower degree of membrane lipid peroxidation than E. grandis or E. tereticornis. The differences in adaptability among the three pure species were attributed to their distinct habitats. Eucalyptus urophylla × E. grandis inherited the outstanding adaptability to Al stress from its maternal species (E. urophylla), indicating that Al tolerance is highly heritable and can be selected in Eucalyptus breeding. Our results indicated that the response of Eucalyptus to Al stress may fluctuate according to the time under stress, and might be related to dynamic changes in ROS elimination and accumulation.

Non-Additive Effects of Mixing Eucalyptus and Castanopsis hystrix Trees on Carbon Stocks under an Eco-Silviculture Regime in Southern China

Eucalyptus plantations harbor great potential for supporting ecosystem services, but this prospect is weakened under long-term traditional silviculture regimes. To reform these traditional silviculture regimes, we carried out a long-term Eucalyptus eco-silviculture experiment. However, the derived benefits and mechanisms that arise in mixed species stands under the eco-silviculture regime are not fully understood. Here, we evaluated tree carbon storage (TCS), understory vegetation carbon storage (UCS), floor litter carbon storage (FLCS), soil organic carbon storage (SOCS), and ecosystem carbon storage (ECS) in seven-year-old mono-specific plantations of a Eucalyptus hybrid (E. urophylla × E. grandis) and Castanopsis hystrix, as well as mixed plantations of these two trees under an eco-silviculture regime in southern China. The results showed that the tree height, diameter at breast height (DBH), volume, and biomass of eucalypt trees and C. hystrix in the mixed plantation were significantly higher than that of the trees in the corresponding single-species plantations. The mixed-species plantation had the largest TCS (84.33 Mg ha−1), FLCS (4.34 Mg ha−1), and ECS (313.31 Mg ha−1), as well as a higher SOCS (233.98 Mg ha−1), but the lowest UCS (0.96 Mg ha−1), among the three plantation types. The mixture effects analysis revealed significant synergistic effects (non-additive effect, NAE > 0) on TCS, SOCS, and ECS, and significant antagonistic effects (NAE < 0) on UCS. These synergistic effects were mainly due to the complementary ecological niches of the two species in the mixed-species plantation, which could potentially enable them to maximize the use of local resources, and to increase stand productivity and litter production. These results imply that beyond the gains in timber production obtained by having both Eucalyptus and C. hystrix trees growing in the same plantation stand, such mixed-species plantations enhance carbon sequestration to a greater extent than mono-specific plantations of either Eucalyptus or C.hystrix trees. In conclusion, we suggest planting mixed plantations of species with complementary ecological niches under an eco-silviculture regime, to effectively resolve the contradiction between timber production and ecosystem services, and, thereby, also promote the sustainable development of Eucalyptus plantations.

Changes in taxonomic and functional diversity of plants in a chronosequence of Eucalyptus grandis plantations

Tree plantations have become one of the fastest-growing land uses and their impact on biodiversity was evaluated mainly at the taxonomic level. The aim of this study was to analyze environmental changes after the Eucalyptus plantation in an area originally covered by natural grasslands, taking into account the alpha and beta (taxonomic and functional) diversity of plant communities. We selected nine plantation ages, along a 12 years chronosequence, with three replicates per age and three protected grasslands as the original situation. At each replicate, we established three plots to measure plant species cover, diversity and environmental variables. Results showed that species richness, and all diversity indices, significantly declined with increasing plantation age. Canopy cover, soil pH, and leaf litter were the environmental drivers that drove the decrease in taxonomic and functional diversity of plants through the forest chronosequence. Based on the path analyses results, canopy cover had an indirect effect on plant functional diversity, mediated by leaf litter depth, soil pH, and plant species richness. The high dispersal potential, annual, barochorous, and zoochorous plant species were the functional traits more affected by the eucalypt plantations. We recommend two management practices: reducing forest densities to allow higher light input to the understory and, due to the fact that leaf litter was negatively associated with all diversity facets, we recommend reducing their accumulation or generate heterogeneity in its distribution to enhance biodiversity.

Effects of enriched planting of native tree species on surface water flow, sediment, and nutrient losses in a Eucalyptus plantation forest in southern China

Enriched planting of native tree species in monoculture plantation forests is a commonly recommended forest practice. However, its effect on various ecological processes is generally lacking. Here, we carried out an experiment in a 16-year-old Eucalyptus plantation in South China to assess the effects of enriched planting of native tree species on surface water, soil erosion and nutrient losses. Two treatments were conducted in 2008: (1) enriched planting of native broadleaved tree species with uniform thinning of 60% of Eucalyptus trees (TEP); and (2) enriched planting of native broadleaved tree species without thinning (NEP). The original Eucalyptus plantation stands was used as the control (CK). Runoff plots (total n=9, 3 for each treatment or CK) were established in 2009, and surface water flow, sediment, nitrogen (N) and phosphorus (P) losses were monitored from major rainfall events in 2010-2012. Results showed that enriched planting in Eucalyptus plantation significantly reduced surface water flow and soil erosion. Compared with CK, TEP and NEP reduced annual surface water flow by 29-43% and 11-16%, and reduced annual soil erosion by 38-54% and 20-33% throughout the study period, respectively. TEP and NEP had significantly lower annual mean concentrations of N and P in surface water. Compared with CK, TEP reduced annual N and P losses through surface water by 42-60% and 44-64%, respectively, while NEP reduced them by 25-28% and 24-34%, respectively. N and P losses were significantly related to surface water flow. Between the two treatments, TEP was better for retaining water and soil, and for preventing nutrient loss. These results clearly demonstrated that the enriched planting of native tree species effectively retained surface water and nutrients.