Role of Photoinhibition of Photosynthesis and Soil Water in the Initial Establishment of Clearcut-Planted White Spruce Seedlings
Alberta Pacific Forest Industries Inc.
Lead Researcher: 
Ken Greenway, PhD, and Rongzhou Man, PhD, Alberta Research Council, Vegreville, Alberta
Maintenance of photosynthetic systems by protecting seedlings from photoinhibition and photooxidation may increase survival and growth of planted seedlings. Photoinhibition is a short-term disruption of photosynthesis that occurs when light-harvesting pigments collect more solar energy than the photosynthetic system can productively utilize. It is common under natural conditions, especially for shade-tolerant plants that have relatively low light-saturated photosynthetic capacities. The study was designed to assess how moderation of the light environment through artificial shade would affect planted white spruce survival and early growth in harvested blocks.

The study area is located about 100 km northwest of Lac La Biche. The trial was located in two adjacent cutblocks, both of which were covered by 120 year old aspen prior to being harvested in 1998. The sites were straight-bladed in the spring following harvest, and planted with one-year old container-grown hot-lifted white spruce seedlings (415B) on July 10/11, 1998. Each cutblock had 60 experimental plots, each consisting of five seedlings. Within each cutblock there were three levels of shading: no-shade control, all season shade, and summer-shade only.

Light transmission through the shade frames was between 50-60% under either sunny or cloudy conditions. The light level under the shade treatments was high enough to saturate the photosynthetic systems of white spruce. The overall treatment effect of shading on soil temperature and soil moisture was not significant. Shading decreased daylight air temperatures and relative humidity and increased nighttime temperatures. Shading treatments had minimal impact on seedling quality as measured by chlorosis, terminal damage and mortality. Seedlings of different shading treatments did not show significant difference in height increment in either the first or second growing season. The no-shade seedlings had slightly more diameter growth than shaded seedlings. Shading did not significantly change measured root response variables, however it did significantly increase fluorescence yield and chlorophyll content, indicating reduced photoinhibition and photooxidation in white spruce seedlings. The study supports the practices of partial-cut silvicultural systems, or other silviculture systems in which white spruce are regenerated under the forest canopy.