Relationships between plant nitrogen economy and life history in three deciduous-forest herbs

Abstract

1. We compared nitrogen (N) uptake and whole-plant N dynamics in three deciduous-forest herbs of contrasting life histories: the spring ephemeral Allium tricoccum, the summergreen Viola pubescens and the semievergreen Tiarella cordifolia. We predicted that differences in above-ground physiology would translate into differences in N acquisition and partitioning, such that nitrogen-use efficiency (NUE) would increase from Allium to Viola to Tiarella. 2. Patterns of N uptake were generally the opposite of our predictions. Allium had the lowest N uptake capacity in both laboratory and field experiments whereas roots of Tiarella had the highest specific N uptake capacity. 3. Viola was the only species in which the specific uptake capacity of roots was related to photosynthetic activity of leaves, both decreasing by a factor of two from spring to summer. In contrast, Tiarella consistently had the lowest photosynthetic capacity and the highest specific uptake capacity whereas Allium maintained substantial root uptake capacity throughout the summer when it had no photosynthetic activity. 4. There were no significant differences between species in overall NUE. However, there were differences in the components of NUE: nitrogen productivity (A) and mean residence time of N in the plant (MRT). Nitrogen productivity increased, and MRT decreased, from Allium to Viola to Tiarella. 5. In all three species, there was a balance between acquisition of N and building of biomass over the annual growth cycle, despite dramatic disjunctions between the tissue-specific rates of carbon and N acquisition in Allium and Tiarella. The variation in A and MRT we observed among co-occurring species of a single N-rich habitat was comparable with that observed by other researchers studying plants adapted to habitats of widely varying N availability.