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

Gains in stand volume that result from competition control and fertilization are sometimes reported as ‘percentage gains’. Because percentage gains arithmetically decline over time as stand volume increases, plantation managers have difficulty in using percentage gains to project growth and revenues. The ‘age-shift’ method quantifies the year advancements in stand growth due to silvicultural treatments and, for herbaceous vegetation management, it has been proposed that this metric is less likely to change after the juvenile growth phase. To test the sensitivity of the ‘age-shift’ method to time and hardwood competition, we used 20-year volume data from 11 loblolly pine (Pinus taeda L.) studies that had early complete herbaceous and woody competition control. Volume growth gains were expressed in terms of percentages and ‘age-shifts’. On all sites with no woody competition, percentage gains declined from age 8 years to age 20 years. In contrast, age-shift estimates on these plots either remained constant or increased over time. However, in four cases where woody basal areas were greater than 4 m2 ha−1 at age 15 years, age-shift gains due to herbaceous control decreased and eventually resulted in volume losses. When evaluating the response to early herbaceous competition control, age-shift calculations have promise as a useful predictive tool on sites with low levels of hardwood competition. Five methods for calculating age-shift are presented.