Determining productivity gains from herbaceous vegetation management with 'age-shift' calculations

Gains in stand volume that result from competition control andfertilization are sometimes reported as ‘percentage gains’.Because percentage gains arithmetically decline over time asstand volume increases, plantation managers have difficultyin using percentage gains to project growth and revenues. The‘age-shift’ method quantifies the year advancementsin stand growth due to silvicultural treatments and, for herbaceousvegetation management, it has been proposed that this metricis less likely to change after the juvenile growth phase. Totest the sensitivity of the ‘age-shift’ method totime and hardwood competition, we used 20-year volume data from11 loblolly pine (Pinus taeda L.) studies that had early completeherbaceous and woody competition control. Volume growth gainswere expressed in terms of percentages and ‘age-shifts’.On all sites with no woody competition, percentage gains declinedfrom age 8 years to age 20 years. In contrast, age-shift estimateson these plots either remained constant or increased over time.However, in four cases where woody basal areas were greaterthan 4 m² ha^–1 at age 15 years, age-shift gains due toherbaceous control decreased and eventually resulted in volumelosses. When evaluating the response to early herbaceous competitioncontrol, age-shift calculations have promise as a useful predictivetool on sites with low levels of hardwood competition. Fivemethods for calculating age-shift are presented.