Effects comparison of co-occurring Vachellia tree species on understory herbaceous vegetation biomass and soil nutrient: Case of semi-arid savanna grasslands in southern Ethiopia

Abstract

The spread of woody plant encroachment is a major concern as it adversely affects herbaceous plant community composition in a savanna ecosystem. Assessing the canopy effects of co-occurring encroaching and non-encroaching woody plant species on the understory herbaceous vegetation biomass and the soil nutrient status can improve our understandings of the link between biological invasions and herbaceous plant community ecology. We examined the canopy and adjacent inter-canopy effects of encroaching and non-encroaching Vachellia species which was previously known as Acacia on herbaceous vegetation biomass in Southern Ethiopia. A total of 12 encroaching (Vachellia drepanolobium) and 12 non-encroaching (Vachellia tortilis) tree species were selected both at an upland and a bottomland site. A 1 m × 1 m quadrat was used to investigate the canopy effects of individual tree species and inter-canopy habitats on herbaceous vegetation biomass. Soil samples were collected under the canopy of selected tree species and the adjacent inter-canopy habitat at a depth of 0–10 cm and 10–20 cm both at the upland and bottomland site. With fixed effect ANOVA, we evaluated the effects of landforms (upland or bottomland), tree species (Vachellia drepanolobium or Vachellia tortilis) and habitats (canopy or inter-canopy habitat) on total herbaceous biomass, perennial herbaceous species biomass and annual herbaceous species biomass. There was no significant (P > 0.05) difference between the canopy and inter-canopy habitat of V. drepanolobium and V. tortilis in terms of the total herbaceous biomass and annual herbaceous vegetation at the bottomland position. The canopy of V. drepanolobium had a negative effect on the biomass production of herbaceous perennial species both at the bottomland and upland site. On the contrary, the canopy of V. tortilis had a significantly positive (P < 0.05) effect on the biomass production of herbaceous perennial plants at the bottomland site. Generally, more soil organic matter accumulation was recorded under the canopy of V. tortilis than it was either under the canopy of V. drepanolobium or in the open savanna grasslands. Our findings indicated that encroaching tree species canopy and co-occurring non-encroaching tree species canopy had a predominantly different influence on herbaceous vegetation biomass. It seemed that encroaching tree species in savannas had a high acquisition and competition for resources. The findings can help improve our understanding on the herbaceous biomass predictions of different life forms in a community of encroached grasslands and non-encroached grasslands in southern Ethiopia.