Understory light regimes following silvicultural treatments in central hardwood forests in Kentucky, USA

Manipulation of the light regime is a primary goal of many silvicultural treatments, but the specific light conditions created remain poorly documented for many forest types and geographic locations. To help quantify effects of silvicultural treatments on light conditions, measurements of basal area, canopy cover, and photosynthetically active radiation (PAR), measured both instantaneously and across time, were collected in central hardwood forests in Kentucky, USA following silvicultural treatments. These measurements were used to: (1) Investigate the magnitudes of differences in understory percent ambient PAR following implementation of shelterwood with reserves and thinning treatments; (2) document the spatial and temporal distribution and variability of understory percent ambient PAR in shelterwood with reserves treatments (mean residual basal area = 5.2 m2/ha), thinning treatments (18.5 m2/ha), and untreated controls (27.1 m2/ha); and (3) examine relationships between: basal area and canopy cover; basal area and measured percent ambient PAR; and canopy cover and measured percent ambient PAR.Mean light levels from instantaneous measurements were 78% of ambient in the shelterwood with reserves, 33% of ambient in the thinning and 9% of ambient in the control. Similarly, only 1.3% of the approximately 140 h of PAR measurements in the controls indicated high light conditions (>60% of ambient), 15.9% in the thinning treatment and 65.4% in the shelterwood with reserves treatment. There were only 32 periods of high light found across all plots in the control, 176 periods of high light in the thinning treatment, and 441 periods of high light in the shelterwood with reserves treatment. Indexes of variability in light across time and among sampling locations within a stand did not differ statistically between the shelterwood with reserves and thinning treatments but both treatments were statistically more spatially and temporally variable than the uncut control. Simple linear regression relationships were observed between stand basal area and mean relative PAR (r2 = 0.8784 for instantaneous measurements, r2 = 0.9697 for continuous measurements), and basal area and canopy cover (r2 = 0.8479). Such relationships provide a means for including light management in forest planning and application of silvicultural treatments. The results for the distribution of light also suggest, however, that treated stands may have similar mean light levels, but differ substantially in the spatial and temporal distribution of light.

► Effects of silvicultural treatments on structure and light were measured.
► Silvicultural treatments increased both temporal and spatial variability in light.
► Regressions suggested basal area can guide creation of desired mean light levels.
► Managers may need to address variability in light as well as mean amounts of light.
► Further study of the effects of temporal variability in light on trees is needed.