The ameliorating influence of neighbouring plants on aboveground microclimate depends on their number, size and proximity

Plants can have a facilitative effect on co-occurring neighbours by moderating microclimate conditions. In forested areas, vegetation can ameliorate macroclimate extremes relative to open areas, yet the neighbourhood characteristics that contribute to this buffering effect, particularly during early stand development, remain unclear. Here, we examined how the local neighbourhood affects aboveground temperature and relative humidity in the canopy layer of 76 mapped neighbourhood plots in an 8-year-old experimental forest in Tasmania, Australia. We assessed which aspects of the neighbourhood – species identity, density, summed basal area and spatial arrangement – best described the microclimatic conditions over a year-long period, and examined how seasonal and extreme climatic conditions influenced the strength of neighbourhood effects on microclimate. Microclimate conditions varied substantially across neighbourhoods, with a significant proportion explained by neighbourhood metrics (NMs). Daytime temperature and relative humidity (RH) were strongly associated with the community neighbourhood index (NI), which reflects the number, size and proximity of neighbouring trees rather than their species identity. Higher NI values – indicating more, larger or closer neighbours – led to cooler, moister microclimates, with the strongest effects observed in summer, where the NI explained 57% of the variation in temperature and 33% in RH. Although nighttime microclimate relationships with NMs were generally weaker, they were positively associated with the number and size of neighbours (summed basal area). The buffering effect of high NI and basal area neighbourhoods was most evident under extreme climatic conditions, with temperatures 3.5°C cooler on the hottest recorded day and 1.2°C warmer during the coldest. The amelioration in microclimatic conditions due to neighbouring plants has important implications for tree performance and forest regeneration, particularly in projected hotter and drier conditions.