Microclimate and land surface temperature in a biodiversity enriched oil palm plantation

Agroforestry options such as mixed-species tree planting and natural regeneration in oil palm plantations may alleviate negative effects of forest loss on biodiversity and ecosystem functioning. The effects of agroforestry on microclimate and land surface temperatures (LST) remain largely unknown despite their central role in controlling abiotic and biotic factors and in buffering climate at a larger scale. We assessed spatial and temporal microclimate and LST variability in a biodiversity enrichment experiment, in which tree islands have been planted in an oil palm plantation in Sumatra (Indonesia). Four years after establishment of the experiment, we measured microclimate and LST using mini microclimate sensors and drone-recorded thermal images. We examined experimental effects of tree species richness (0, 1, 2, 3 or 6), plot size (25 m2, 100 m2, 400 m2, 1600 m2) and stand structural complexity on microclimate and LST. Diurnal patterns showed ambient air temperature peaks and relative humidity (RH) minima at 3 pm, whereas diurnal soil temperatures peaked around 6 pm. The lowest LST were observed from oil palm canopy leaves and the highest from bare soils and understorey vegetation (including trees). Spatial and temporal ranges of ambient air temperature were smaller than LST ranges, and average ambient air temperature and LST were positively correlated. Tree species diversity had no overall significant effect neither on microclimate nor LST, but humidity was higher in planted tree islands compared to natural regeneration only. Smaller plots were characterized by higher mean air, soil and LST, compared to larger plots. Structurally complex plots were associated with low mean and maximum values of ambient air temperature, soil temperature and LST and high mean and minimum RH. Still, conditions were hotter and drier in several experimental plots compared to conventional oil palm plantations, considering a higher transpiration in the latest. We conclude that stand structural complexity and tree island size control microclimate and LST in the experimental oil palm agroforests, but alleviating the harsh microclimate conditions in oil palm plantations might take longer to occur.