Microclimate through space and time: Microclimatic variation at the edge of regeneration forests over daily, yearly and decadal time scales

A major aim of sustainable forest management is the maintenance or recolonisation of harvested areas by species that were present pre-disturbance. Forest influence (a type of edge effect that focuses on the effect of mature forests on adjacent disturbed forest) is considered to be an important factor that contributes to the ability of mature forest species to re-colonise disturbed areas. Forest influence occurs in two main ways by: (1) by providing a source of propagules or individuals for recolonisation; and (2) by its influence on the biotic and abiotic conditions of the disturbed forest. This study focuses on forest influence's impact on microclimate conditions of adjacent disturbed areas regenerating after harvesting. In particular, the study investigates whether microclimate within a regenerating forest changes with increasing distance from a mature forest edge, and whether the magnitude of microclimatic change varies over diurnal, seasonal and successional time scales. Results of the study showed that the microclimate of regenerating forests is affected by the distance to a standing mature forest. Temperature, relative humidity, vapour pressure deficit, and the short-term fluctuations of these microclimate parameters were influenced by nearby mature forest. In addition, the study found that the magnitude of forest influence changes over diurnal, seasonal and successional time scales. For example, it was discovered that forest influence is greatest during the middle of the day, during the summer months when solar heating is greatest and on hot windy days. Critically, the impact of forest influence peaked around ~27. years after disturbance in the areas studied, with less influence shortly after disturbance. We speculate this is due to lower levels of midday shading in the ~7. year old forest. Forest influence on microclimate persisted in regeneration areas that were harvested 45. years ago, although the magnitude and importance of the effect was low. We conclude that proximity to mature forest stands (forests influence) impacts the microclimate of forests regenerating after disturbance, although the response is quite variable through time. Our results provide insight into the role of microclimate on the ability of mature forest species to successfully re-colonise after disturbance. Management practices, such as aggregated retention and other forms of retention forestry, which increase the proportion of harvested area under forest influence, may provide a mechanism to promote the re-colonisation of mature-forest species.