Variation in microclimate and early growth of planted pines under dispersed and aggregated overstory retention in mature managed red pine in Minnesota

Retention harvests are proposed as mechanisms for introducing two-aged structure into even-aged red pine (Pinus resinosa Ait.) stands, yet little is known about seedling responses to overstory abundance and resource availability under potential harvesting treatments. We related spatially explicit measurements of overstory abundance, proportional diffuse radiation, and soil temperature and moisture to first-year mortality and 2-year growth of underplanted jack pine (Pinus banksiana Lamb.), eastern white pine (Pinus strobus L.), and red pine seedlings under four harvest treatments: no-harvest control, single- tree (dispersed) thinning, small (0.1 ha) gap group harvesting, and large (0.3 ha) gap group harvesting. Growth of all species was greater in all cut treatments than in the control, but the only difference among cut treatments was greater jack pine height growth in large-gap harvests than in small-gap harvests. Proportional diffuse radiation and soil moisture varied considerably by location within the gaps and were highest in gap centers. Seedling growth responses increased from the forest edge into gap centers where overstory influence was least. All three cut treatments could be expected to result in the development of a two-aged stand structure in mature red pine stands such as these. When an aggregate retention harvest is implemented, higher initial mortality and growth could be anticipated for seedlings near the center of the gap.