The phenological pattern of a plant species is an important adaptive trait in climates with an unfavourable season. Below-ground seasonal dynamics vary among species, but not much is known about a functional significance of such a variation. This study aims to clarify the inter-specific differences in detailed seasonal patterns of root growth and senescence and the consequences on the functional aspect of nutrient uptake, with focus on northern wetland plants known to differ in their root overwintering strategies; their roots either senesce in autumn, or survive the winter. For this purpose, a garden experiment was conducted for four ecologically contrasting wetland monocotyledonous plant species in Northern Ontario, with monthly harvests of whole plants over one year during the growing season (August to October and March to October) and measuring nutrient uptake using a 15N stable nitrogen isotope tracer.
1) Species with overwintering roots produced new roots from early April until late October, with a significant late-season growth spurt, 35-40% of the production happening after mid-September. For species with autumn-senescing roots, root production mainly occurred between mid-April to late-July, with high mortality in late September.
2) By mid-September leaves of species with autumn-senescing roots had mostly senesced, but the plants still took up the 15N tracer, indicating active root function until shortly before root senescence. Species with overwintering roots had the highest δ15N values in their rhizomes in mid-October, indicating a strong nitrogen uptake late in the season.
3) Rhizome biomass of all species decreased in early spring (mid-March to April). Roots of species with overwintering roots were 100% alive at that time, their biomass decreasing by 30-36% and specific root length increasing 25-30% at the onset of a new growing season.
The results show that the different wetland plant root overwintering strategies are associated with distinct phenological patterns of root production, and with distinct seasonal patterns of nutrient uptake. Besides their importance for late-autumn and early spring nutrient uptake, overwintering roots also seem to play an important role as storage organs for the early spring leafing-out.