Short-term vs. long-term effects of understory removal on nitrogen and mobile carbohydrates in overstory trees

Understory management in forest ecosystems has been applied to improve the wood production for hundreds of years worldwide. The carbon-physiological mechanisms underlying these positive effects of understory management on the growth of overstory trees have received less attention. We studied the non-structural carbohydrate (NSC) and total nitrogen (N) concentrations in tissues (needles, stem sapwood, and fine roots) of three tree species (two evergreen and one deciduous species) grown in the presence or absence (understory cut) of understory shrubs in plantations in southwestern China, to test whether understories affect the carbon and nitrogen status in the overstory trees. The concentrations of N, NSC (= soluble sugars + starch) in overstory trees varied significantly with understory treatments during the dry season rather than the wet season. Trees grown without understory shrubs had higher levels of N and NSC compared to trees grown with understories. The present study provides insight to explain the functional mechanisms for understory effects on growth of overstory trees, and indicates that the nitrogen and carbon status in overstory trees may be more strongly negatively affected by understory in stressful conditions rather than in optimal growth conditions. Moreover, the present study provides ecophysiology-based knowledge for dealing with understory vegetation management in forest ecosystems.