Alterations in the microenvironment of mixed oak stands to improve regeneration success in Pennsylvania's forests

Background/Question/Methods

Across central hardwood forests, oaks fail to regenerate on high quality sites primarily due to low light and competition from other species. When present, oak seedlings often do not survive. A better understanding of the effects of silvicultural treatments on microenvironment is needed before developing prescriptions to promote oak establishment and regeneration. A long-term study was initiated on three 17 ha replicates on the Moshannon State Forest in Pennsylvania using fenced and unfenced shelterwood blocks to alter light and understory treatments (herbicide and fire) to reduce competing herbaceous and woody vegetation. Overstory/midstory treatments were used to create three light levels: Control - untreated stands with 2-5% full sunlight (>80% relative density); Moderate - midstory tree removal with 10-20% full sunlight (60-70% relative density); and High - midstory and canopy tree removal with 30-40% full sunlight (40-50% relative density). Future late springs prescribed fires were planned. Herbicides were applied to treat understory vegetation with both broadcast mistblower (glyphosate and sulfometuron methyl) and basal bark (triclopyr) treatments. Continuously logged soil and air temperature and biweekly soil moisture were collected in growing seasons immediately preceding and following treatments within 72 0.3 ha plots. Photosynthetically-active radiation (PAR) was measured mid-growing season.

Results/Conclusions

No significant differences in volumetric soil moisture (12 cm depth), soil and air temperatures were detected during the pretreatment growing season. After thinning, PAR was 8%, 11% and 24% full sunlight in the control, moderate and high light treatments, respectively. Soil moisture was significantly higher in the high light plots by late July, compared with either the control (lowest) or moderate light plots. While soil moisture content did not differ between herbicide and non-herbicide plots, PAR did increase with herbicide treatments. PAR was 1.40, 1.09, 1.13 times higher in control, moderate, and high light plots, respectively, compared to non-herbicide plots. Air and soil temperature (10 cm depth) did not significantly differ among the thinning treatments. However, soil temperatures were highest in the high light and lowest in control plots. In July, mean monthly air temperatures were 0.26°C and 0.57°C higher and soil temperatures were 0.31°C and 0.73°C higher in moderate and high light, respectively, compared with control plots. These observed changes in microenvironment have important implications for success of oak regeneration through direct effects on acorn desiccation, germination, sprouting capacity, photosynthesis, early seedling growth and survival, and frost and heat damage.