A topographic moisture index explains understory vegetation response to retention harvesting

To inform biodiversity conservation efforts in managed forest landscapes, we explore if a topographic moisture index (depth-to-water, based on remotely-sensed (LIDAR) data) can provide insight into responses of understory vegetation to retention harvesting in the boreal mixedwood forests of northwestern Alberta, Canada. Sample plots were placed along the depth-to-water moisture gradient in three forest types: coniferous, mixedwood, and deciduous (broadleaf), and in four retention harvesting treatments: unharvested (control), 50% (dispersed green-tree) retention, 20% retention, and clearcut (2% retention). Understory diversity, abundance, and composition were assessed 15 years after harvest. Harvesting affected the relationships between understory variables and the depth-to-water index, with the effects differing between forest types. Coniferous stands showed the most dramatic responses to harvesting, in that most relationships between understory attributes and the depth-to-water index changed due to harvesting. For instance, harvested coniferous stands had higher diversity on wetter sites, rather than on drier sites as was seen in the unharvested stands. In mixedwood stands only the relationship between composition and depth-to-water was affected by harvesting. Broadleaf stands were intermediate; abundance and composition showed a significant depth-to-water by harvesting treatment interaction. Abundance and depth-to-water relationships were weaker in harvested, as compared to unharvested, broadleaf stands. Within each forest type, the effects of harvesting also varied along the depth-to-water gradient. In coniferous and mixedwood forest types, wetter sites were most sensitive to harvesting while in broadleaf stands drier sites were more sensitive. Our study shows that the depth-to-water index can be used to better understand and predict the response of understory vegetation to harvesting and can be useful for guiding the placement of retention.