Responses of understory plant physiological traits to a decade of nitrogen addition in a tropical reforested ecosystem

Abstract

Reforested ecosystems are becoming a dominant forest cover type in the tropics where anthropogenic nitrogen (N) deposition has been increasing greatly. Although understory plants play an important role in ecosystem structure and functioning, it remains unclear how long-term N deposition affects eco-physiological traits of understory species. To address this question, we expanded a ten-year simulated N deposition experiment in a tropical reforested ecosystem, with variable N addition levels as Control (no addition, N0), 50kgNha−1yr−1 (N50), and 100kgNha−1yr−1 (N100). Leaf physiological traits associated with photosynthetic capacity, foliar elements and N storage of seven dominant understory species were measured. Results showed that both medium-light species and shade-tolerant species had little physiological responses to N addition. However, one high-light species (Alchornea trewioides) responded significantly to N addition, showing decreases foliar N and Pmax. We further found that Pmax of Alchornea trewioides was negatively correlated with the canopy closure which increased under long-term N addition. Our findings suggest that understory plant growth may be more limited by light rather than N availability, particularly under chronic high N deposition; and N deposition can suppress understory high-light species through accelerating canopy closure in some tropical reforested ecosystems.