Stand age related differences in forest microclimate

Quantifying spatio-temporal variation in forest microclimate is important to understand the habitat associations of some species, identify potential refugia, and manage forests in a changing climate. We quantified relationships between microclimatic conditions and stand age using a detailed case study of the Mountain Ash (Eucalyptus regnans) forests in south-eastern Australia. We deployed sensor buttons to capture data on day-time and night-time seasonal (Julian Day) temperature profiles in even-aged stands that were 12-years-old, 30–40-years-old, ∼82–95-years-old, and 200+ -years-old. We constructed Bayesian generalised linear models (GLMs) to quantify the main and interacting effects of stand age and topographic factors and foliage cover on seasonal profiles for: median, maximum and variability in day-time and night-time temperature. The most parsimonious model for median day-time temperature included a main effect of midstory and overstory cover, both of which were inversely related to median day-time temperature. The most parsimonious model for median night-time temperature measures included an additive effect of forest age and seasonality. Temperatures for the youngest forests (that were 12-years-old) were different from other age classes and were characterised by the lowest median and minimum night-time temperatures. The most parsimonious model for day-time maximum temperature and day-time interquartile range included an interaction between forest age and Julian day. Day-time maximum temperatures were warmest in 12-year-old forest. The coolest maximum day-time temperatures in summer were generally in old growth stands (200+ -year-old). The greatest variability in maximum day-time temperatures was in 12-year-old forest whereas, in general, old growth stands experienced the least variable day-time temperatures. Our sensor data also showed that old growth forests were characterised by fewer days above 30, 35 and 40 degrees than 12-year old forest. Our empirical data contain compelling evidence that the youngest forests were characterised by the hottest day-time temperatures, the coldest night-time temperatures, and the most variable day-time and night-time temperature regimes. Age class differences in stand structure (e.g. canopy height, prevalence of large old trees and dense wet understorey) may explain stand age related seasonal differences in local microclimatic conditions.