Effects of cropland abandonment on bush encroachment and herbaceous vegetation composition in semi-arid savanna rangelands

Abstract

Cropland abandonment and bush encroachment significantly influence land transition in the semi-arid savannas of Limpopo Province, South Africa. Nevertheless, their long-term dynamics and ecological repercussions are inadequately comprehended. This study used remote sensing and ground-based vegetation assessment to examine the extent of cropland abandonment over 30 years (1994-2024) and how it influences vegetation structure, composition, and herbaceous production of both abandoned croplands and natural rangelands in Sencherere village, Limpopo Province. The study revealed that cropland area initially declined from 37.7% in 1994 to 15.1% in 2014 before increasing to 35.8% in 2024. In contrast, bush and shrub cover expanded consistently from 3.9% in 1994 to 26.8% in 2024. Land-cover classifications’ overall accuracy of 80-85% and kappa of 0.73-0.80 revealed highly dynamic transitions over time. Additionally, natural rangeland contracted (4.92%) markedly over the period but showed slight recovery (8.71%) in 2024. On the other hand, rainfall exhibited high interannual variability, but no long-term trends were found, indicating that climatic variability alone does not account for observed vegetation changes. The ground-based vegetation assessments showed that abandoned croplands remain ecologically distinct from natural rangelands even decades after cultivation ceases. Abandoned croplands were dominated by bush encroaching species such as Vachellia robusta (45.71%) and Dichrostachys cinerea (27.71%), with lower tree density (600 ± 210 trees ha¹) (p < 0.05), but taller (mean = 1.82), wider-crowned trees (mean = 2.27) relative to natural rangeland. Herbaceous species composition in abandoned cropland was dominated by Aristida congesta (28.83%), Aristida rhiniochloa (19.93%), and Pogonarthria squarrosa (14.41%), while natural rangeland was dominated by Chloris virgata (37.72%), Eragrostis plana (13.75%), and Cynodon dactylon (9.85%). Abandoned croplands exhibited higher species richness but substantially lower biomass, driven by stress-tolerant pioneer grasses including Aristida congesta, Aristida rhiniochloa, and Pogonarthria squarrosa. Natural rangelands, by contrast, supported more productive and palatable grasses such as Chloris virgata and Cynodon dactylon, resulting in far higher biomass (305.33 kg DM ha-1) and lower basal cover (34.58 kg DM ha-1). Overall, the findings show that cropland abandonment initiates persistent shifts toward woody-dominated, low-productivity states and does not lead to passive recovery of natural rangeland. The study highlights the need for continued long-term monitoring and targeted management interventions to control bush encroachment, restore herbaceous production, ground cover and maintain resilient semi-arid savanna ecosystems.