Residual effects of NPK fertilization on shrub growth in a Yukon boreal forest

Residual effects of nutrient additions were sustained in shrubs 4–8 years after fertilization stopped in a northern boreal forest in Yukon, Canada. We measured the growth rate of grey willow (Salix glauca L.) and bog birch (Betula glandulosa Michx.) twigs during the growing seasons of 1998, 2001, and 2002, 4–8 years after NPK fertilization from 1987 to 1994 had ceased. We also measured the nitrogen concentration of the 1998 growth tissue. Willow twigs had significantly higher growth rates in previously fertilized shrubs than control shrubs in these 3 years, even greater than differences observed during fertilization. Willow also had higher N concentration in fertilized twigs than control twigs. Birch growth was negatively correlated with hare abundance during the snowshoe hare (Lepus americanus) abundance cycle. Birch growth rate was higher in fertilized twigs than control twigs in 1998 during the hare peak, but fertilized birch twigs approached control levels by 2002 following the hare decline.