Competing Vegetation Benefits Lodgepole Pine

Removing Sitka alder and herbs from cutblocks when planting lodgepole pine may do more harm than good to the crop trees, research shows.

The study's authors advise that money spent on removing this vegetation from pine plantations may not be well-spent.

Herbs, particularly pinegrass, on the very dry, cool Montane Spruce biogeoclimatic subzone study site near Kamloops, British Columbia, initially compete with pine seedlings for light and moisture. The vegetation also protects trees from drying out, sun scald and browsing by snowshoe hares. When herbs were removed, pine mortality quickly climbed, and after 15 years was 16 percent higher than on untreated sites.

Eliminating all the herbs also enabled a growth surge, producing larger trees and somewhat more stand volume. Surprisingly, the amount of alder left in a stand never affected pine survival and growth, even at 55 percent shrub cover.

The growth benefits for trees of managing competing vegetation, though, appear short-term. Over time, the lack of alder's contribution to soil nitrogen through the plant's nitrogen-fixing capabilities became more pronounced. Soil nitrogen mineralization where alder and herbs were removed declined drastically, and after 15 years these soils had 90 percent less ammonia, NH3, than untreated sites. Consequently, the differences in pine growth among vegetation treatments noticeably decreased after ten years.