Microclimatic edge effects in a recently harvested forest: do remnant forest patches create the same impact as large forest areas?

Microclimatic forest influence (edge effects into open or regenerating areas) occurs within harvested forest, but the depth and magnitude may change depending on the design of the harvested area. This is an important consideration for managers, because gradients in microclimate can significant affect the ability of species to recolonise following disturbance. One harvesting method that can increase the amount of forest influence is aggregated retention. This technique involves leaving groups of trees (aggregates) within the harvested area, thus increasing the amount of regenerating forest that is near to an edge and therefore under forest influence. However, differences in the scale of forest influence generated from aggregates compared to unlogged forests surrounding harvested areas have not been tested. Understanding the ability of retained aggregates to generate forest influence is important in designing and implementing aggregated retention harvesting practices.

This study tested whether retained aggregates generated similar levels of forest influence as mature forest surrounding harvested areas. Microclimatic forest influence was examined by monitoring spatial changes in air temperature and relative humidity along transects running from within standing mature forest into harvested forest. Intact forest and aggregate transects were located in the direction of maximum expected forest influence (south-facing edges). Results showed that forest influence was mostly similar in both depth and magnitude regardless of the type of forest from which it was generated. Temporal examination of forest influence showed that it changed throughout the day and across the year, with peaks in magnitude occurring during the middle of the day, and in months close to the equinoxes. Shade derived from standing trees was a potential driver of temporal patterns in microclimatic forest influence.

At its peak magnitude, the microclimatic forest influence observed will likely have significant impacts on habitat suitability and thus, presumably, species recolonisation after disturbance. Results indicated that aggregated retention is an effective method for generating forest influence within harvested areas. Therefore, aggregated retention has the potential to be a valuable harvesting technique to alter microclimate and have beneficial impacts on the recovery of harvested forests.