Estimating root lifespan of two grasses at contrasting elevation in a salt marsh by applying vitality staining on roots from in-growth cores

Contrasting soil conditions caused by different inundation frequencies require different root growth strategies along the elevational gradient of coastal salt marshes. The objective of this study was to examine (1) if root lifespan was shorter in Elymus pycnanthus, a relatively fast-growing competitive species dominating high marshes, than in Spartina anglica, a relatively slow-growing stress-tolerating species dominating low marshes, and (2) if the species with longer lifespan had higher tissue density (g cm^-3) and lower specific root length (m g^-1) than the species with shorter lifespan. Root production and mortality rates were established by sampling roots in in-growth cores, and using triphenyltetrazolium chloride (TTC) staining to distinguish vital from dead roots. Root lifespan was estimated by dividing the living root biomass (Elymus: 36 g m^-2, Spartina: 100 g m^-2) by root production (Elymus: 0.28 g day^-1 m^-2, Spartina: 0.25 g day^-1 m^-2) or root mortality rates (Elymus: 0.42-0.53 g day^-1 m^-2). Spartina did not exhibit substantial mortality. Despite the present method only yielding rough estimates of average root lifespan, it is evident that root longevity is much shorter in Elymus than in Spartina. Root lifespan ranged between 10-19 weeks for Elymus but was close to 1 year in Spartina, indicating that Elymus replaces it's roots continuously throughout the growing season, whereas Spartina maintains its roots over the growing season. Fine roots of Elymus had slightly lower tissue density (0.094) than those of Spartina (0.139), whereas coarse roots of Elymus and Spartina had similar tissue density (0.100 g cm^-3). Fine roots of Elymus and Spartina had similar specific root length (195 m g^-1). However, coarse roots of Elymus (50 m g^-1) had higher specific root length than those of Spartina (20 m g^-1) due to having smaller root diameter (Elymus: 548 μm, Spartina: 961 μm). We conclude that present observations on Elymus and Spartina support our first hypothesis that the 'competitive' species from the high marsh had short-lived roots compared to the 'stress-tolerating' species from the low marsh. However, our result provide only weak support for the existence of a positive correlation between root longevity and tissue density and a negative correlation between root longevity and specific root length.