From the Field The Tosign (Thymus)-tree trade-off: Eucalyptus and Cupressus tree plantations threaten the ecological and economic value of Thymus schimperi in the Ethiopian Highlands

It forms dense, mat-like stands of small, paired, fragrant leaves and seasonal purple flowers favoured by pollinators. Tosign is well known to Ethiopian households as a flavouring agent, a traditional medicine, and bee forage (Lulekal et al., 2013); underfoot, it holds thin mountain soils in place and shapes the plant communities around it (Friis et al., 2010). Yet in many places Tosign is disappearing as Eucalyptus globulus and Cupressus lusitanica plantations spread across the highlands, altering the light, water, and soil conditions the herb depends on (Belachew and Minale, 2025; Fikadu, 2022). This blog shares what we found when we looked closely at what happens to Tosign where these plantations take hold, and what, practically, can be done about it.

Thymus schimperi favours open, sunny habitats such as montane grasslands and rocky, well-drained slopes, spreading into low, dense mats of tiny fragrant leaves and, in season, small purple flowers. During our fieldwork, the contrast between open sites and plantation sites was impossible to miss; away from Eucalyptus and Cupressus, Tosign grows thick and green, sharing the ground comfortably with native grasses and herbs. Under the trees, as the next sections show, it is a very different story. 

None of this makes Eucalyptus and Cupressus simple villains. They grow fast, yield plenty of wood, and tolerate degraded land, which is exactly why so many farmers and land managers choose to plant them (Endalew, 2024). Families depend on them for fuelwood and building poles. The trouble starts when they are planted too densely: close-packed stands transform the ecosystem beneath them, and low, sun-loving plants like Tosign are among the first to suffer.

• Too much shade: Tosign needs sunlight, and under a dense canopy, very little reaches the ground. Growth slows, biomass and essential oil production drop, and many plants eventually die out (Neumann and DeMalach, 2024). 
• Thirsty neighbours: Eucalyptus and Cupressus roots run deep and wide, drawing heavily on soil water and even lowering the water table, leaving shallow-rooted herbs with little to drink (Scott and Prinsloo, 2008; Bayle, 2019). 
• Chemical interference: Eucalyptus leaches allelochemicals into the topsoil, while Cupressus releases phytotoxic compounds and organic acids; both suppress the germination, regrowth, and growth of surrounding plants (Nega and Gudeta, 2019; Fikadu, 2022; Lemessa et al., 2022). 
• A blanket of leaves: Fallen Eucalyptus and Cupressus leaves are rich in tannins and break down slowly. Over time, they acidify the soil, lock up phosphorus, and form a thick layer that keeps seeds from reaching bare ground; short-rotation plantations also draw heavily on the soil’s nutrient reserves (Aweto and Moleele, 2005; Demessie et al., 2012; Kobina, 2016; Dessie and Erkossa, 2011). 

The decline of Tosign can trigger a cascade of ecological and socioeconomic effects. As its population shrinks, pollinators lose a key forage source, and honey yields decline; reduced plant diversity and vegetation cover leave more soil exposed to rainfall and runoff, increasing erosion risk (Lemessa et al., 2022). These ecological changes translate directly into livelihood and cultural losses, since local communities rely on Tosign for food, traditional medicine, income, and cultural practice (Lulekal et al., 2013). A single plant’s decline can therefore ripple outward: fewer floral resources, weaker pollinator support, lower honey potential, diminished diversity and soil protection, and reduced access to a culturally important resource.

We are grateful to the Department for Environment, Food and Rural Affairs (DEFRA), United Kingdom, and its Global Centre on Biodiversity for Climate (GCBC) programme, which funded this work under the project “Deploying Diversity for Resilience and Livelihoods”.