Microclimate, soil nutrients and stable isotopes in relation to elevation in the Australian Wet Tropics

Microclimate, such as soil and surface air temperatures, and edaphic factors, such as soil organic matter content and nutrient availability, are important parameters of the below-canopy environment that shape vegetation communities. Yet, the literature examining how microclimate and edaphic properties vary along elevation gradients in tropical rainforests is limited, hindering our understanding of key ecological processes in the forest understory. Here we present an analysis of high-resolution (15-min frequency) microclimate data spanning approximately 3 years (December 2019–September 2022) across 20 rainforest sites, ranging from 40 to 1550 meters above sea level (a.s.l.). We also present analyses of soil chemical properties, including δ¹⁵N isotope composition from the same study sites. Our study found soils were consistently cooler than air during the day and warmer than air during the night across all sites. The difference in mean temperature between the wettest (summer) and the driest (winter) quarter for both soil and air also increased with elevation, as did the annual temperature range. Soil organic matter content and C:N ratio increased with elevation, in concert with a decline in soil pH. Together, edaphic factors displayed a strong correlation with climatic factors, suggesting temperature as an important driver of soil properties across elevation. Finally, soil δ¹⁵N was found to decline with increasing elevation, suggesting a tighter N cycle in high elevation, higher organic matter soils. These observations highlight the existing elevational trends in both microclimate and edaphic variables in the Australian Wet Tropics; understanding how these trends may shift with climate change could be important for predicting impacts on species distributions.