Nitrogen addition alters nutrient allocation and functional traits of early spring herbaceous plants in a temperate forest

Abstract

Early spring herbaceous plants play a crucial role in nutrient cycling within temperate forest ecosystems, however, whose response of nutrient absorption and allocation strategies to increased atmospheric nitrogen (N) deposition remains unclear. Based on this, we conducted a 15-year N addition field experiment with three different N treatments (0, 2.5, 5.0 g N m² yr⁻¹). We studied the effects of N addition on leaf and fine root functional traits, and allocation strategies, in seven dominant early spring herbaceous plants within a natural secondary forest. The results showed that (1) After N addition, the aboveground organs of early spring herbaceous plants exhibited a "rapid response" strategy, reducing leaf expansion, which lowered specific leaf area (SLA) and photosynthetic nitrogen use efficiency (PNUE) (2) The belowground organs displayed a conservative strategy, characterized by an increase in specific root length and root tissue density, while root diameter decreased. This suggests that early spring plants can enhance nutrient and water uptake capabilities by expanding root traits in high-nutrient environments. (3) Under high N conditions, early spring herbaceous plants tended to allocate more resources to aboveground organs to cope with competitive pressure, allowing them to occupy ecological niches more rapidly. (4) Early spring herbaceous plants responded to N addition through a mechanism of "leaf dominance with root trait regulation", altering photosynthetic efficiency to influence plant growth. However, this shift also exacerbates phosphorus limitation, which may become a critical factor limiting future growth. Overall, N addition drove a rapid resource utilization strategy in early spring herbaceous plants within nutrient-rich environments, highlighting their ecological adaptability in the context of global environmental change.