Eco-physiological adaptation strategies of dominant tree species in response to canopy and understory simulated nitrogen deposition in a warm temperate forest

Abstract

Nitrogen (N) is an indispensable element for plant growth and survival. The increased global N deposition due to human activities has aroused attention to forest ecosystems, but most studies have not considered the process of N interception, assimilation, and leaching in forest canopy. To investigate the responses and adaptive strategies of dominant tree species in a warm temperate forest to atmospheric N deposition, this study implemented five treatments: control treatment, canopy simulated low N deposition, canopy simulated high N deposition, understory simulated low N deposition, and understory simulated high N deposition. The morphological and physiological traits and nutrient uptake strategies of three dominant tree species (Liquidambar formosana, Quercus acutissima, and Quercus variabilis) in warm temperate deciduous broad-leaved forest were studied under different forms and levels of stimulated N deposition. The results showed that the specific leaf area (SLA) of the three dominant tree species decreased under high N application. To optimize resource absorption and improve nutrient transport rates, the specific root length (SRL) and specific root area (SRA) of the three tree species tended to decrease, while the average root diameter (ARD) increased. Under canopy N deposition (CAN), the contents of carbon (C), N, total chlorophyll (Tchl), and total amino acids in Q. variabilis leaves were lower than those under understory N deposition (UAN) treatment. Furthermore, principal component analysis (PCA) revealed the differences between different genera (L. formosana and Q. variabilis, L. formosana and Q. acutissima) under various N treatments, and there was no significant difference between the same genus (Q. variabilis and Q. acutissima). The results indicated that the eco-physiological characteristics of different tree species under N deposition treatment were species-specific, and traditional UAN perhaps exaggerated the effect of atmospheric N deposition on tree growth characteristics. Our study considers the retention and redistribution of forest canopy, which is more in accordance with the natural N deposition, and helps us to understand the eco-physiological responses of dominant tree species in forest ecosystem to atmospheric N deposition.