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

Abstract

Contrasting soil conditions caused by different inundation frequenciesrequire different root growth strategies along the elevational gradient ofcoastal salt marshes. The objective of this study was to examine (1) if rootlifespan was shorter in Elymus pycnanthus, a relativelyfast-growing competitive species dominating high marshes, than inSpartina anglica, a relatively slow-growingstress-tolerating species dominating low marshes, and (2) if the species withlonger lifespan had higher tissue density (g cm−3) and lowerspecific root length (m g−1) than the species with shorterlifespan. Root production and mortality rates were established by samplingrootsin in-growth cores, and using triphenyltetrazolium chloride (TTC) staining todistinguish vital from dead roots. Root lifespan was estimated by dividing theliving root biomass (Elymus: 36 gm−2, Spartina: 100 gm−2) by root production (Elymus:0.28 g day−1 m−2,Spartina: 0.25 g day−1m−2) or root mortality rates(Elymus: 0.42–0.53 g day−1m−2). Spartina did not exhibitsubstantial mortality. Despite the present method only yielding rough estimatesof average root lifespan, it is evident that root longevity is much shorter inElymus than in Spartina. Rootlifespanranged between 10–19 weeks for Elymus but was closeto 1 year in Spartina, indicating thatElymus replaces it's roots continuously throughout thegrowing season, whereas Spartina maintains its roots overthe growing season. Fine roots of Elymus had slightlylowertissue density (0.094) than those of Spartina (0.139),whereas coarse roots of Elymus andSpartina had similar tissue density (0.100 gcm−3). Fine roots of Elymus andSpartina had similar specific root length (195 mg−1). However, coarse roots ofElymus (50 m g−1) had higherspecific root length than those of Spartina (20 mg−1) due to having smaller root diameter(Elymus: 548 μm,Spartina: 961 μm). We conclude thatpresentobservations on Elymus and Spartinasupport our first hypothesis that the ‘competitive’ species fromthehigh marsh had short-lived roots compared to the'stress-tolerating'species from the low marsh. However, our result provide only weak support forthe existence of a positive correlation between root longevity and tissuedensity and a negative correlation between root longevity and specific rootlength.