Seeing the forest for the trees: Fertiliser increases tree growth but decreases understorey diversity in the northern jarrah forest, southwest australia

Forestry science and practice suggests thinning and fertiliser increase the growth rates of individual trees. In a recent paper reporting on a long-term experiment in the Northern Jarrah Forest, Bhandari et al. (2021) found positive effects of both thinning and fertilisation, and suggested these management practices will result in a shorter return interval for large trees within the population, thereby providing significant benefits at an ecosystem scale. We argue that whereas thinning alone may be beneficial, the application of fertiliser to native ecosystems within the South West Australian Floristic Region requires caution due to impacts on understorey plant diversity. Not only are soils in this region generally deeply-weathered and highly nutrient-deficient, but the evolution of a suite of adaptations for nutrient-acquisition is implicated in both speciation and the maintenance of plant species diversity. Furthermore, long-term experiments in restored jarrah forest indicate that fertiliser both reduces species diversity and increases fine fuel loads. Therefore thinning, but not fertiliser application, is an appropriate management strategy to improve tree growth in this global biodiversity hotspot.