Facilitation of plant species richness and endangered species by a tussock grass in a moist tall grassland revealed using hierarchical Bayesian analysis

Abstract

Using hierarchical Bayesian analysis, we tested the hypothesis that a perennial tussock grass, Ischaemum aristatum var. glaucum, facilitates the species diversity of vegetation and the regeneration of individual native vascular plants in Ukishima Marsh, a moist tall grassland of eastern Japan. We analyzed microscale distribution patterns of sympatric plant species in response to ground height. Both the species richness of the whole plant assemblage and the occurrence of many native vascular plants, including two endangered species, were positively correlated with ground elevation, which was directly mediated by I. aristatum var. glaucum tussocks and/or the occurrence of mosses on tussocks. Susceptibility to late spring inundation of microsites on the tussocks was significantly lower than that on bare ground. These results suggest that the facilitation by I. aristatum var. glaucum contributes to maintaining the high species richness of the marsh. Consideration of the facilitation among native vascular plants is necessary for establishing sound conservation plans of wetland vegetation.

Direct plant–plant facilitation in coastal wetlands: A review

“…On the other hand, transplant experiments can only be conducted on densely populated species and are inappropriate for species with a scattered population distribution population (except when cultivating seedlings). Approaches such as spatial point process, Hierarchical Bayesian analysis which was designed to investigate community-level (larger than only one or a few pairs of species in the community) consequences of species interactions (Raventos et al, 2010;Wang et al, 2011) are rarely used in coastal wetland studies. Additionally, many studies on species facilitation are too short term to achieve equilibrium in species interactions, thus longer observations are necessary.…”

Regeneration of native vascular plants facilitated by Ischaemum aristatum var. glaucum tussocks: an experimental demonstration

“…Phytogenic ground elevation, which can effectively alleviate the high stress and water disturbance of inundation, facilitates the distribution of various plant species (Fogel et al 2004;Peach and Zedler 2006;Ervin 2007;Wang et al 2011). However, such ground elevation has also been argued to work differently depending on the life stage of plants (Crain and Bertness 2005).…”

“…This region is characterized by a temperate monsoon climate with a mean air temperature of 14.8°C and annual precipitation of 1,233.6 mm (recorded at the Tsuchiura Meteorological Station, the nearest meteorological station of the Japan Meteorological Agency; average for 2000-2009; http://www.data.jma.go.jp/obd/stats/etrn/in dex.php). The marsh is frequently inundated after heavy rainfalls, as the current water level of Lake Kasumigaura is almost as high as the marsh ground (both are approximately 1.1 m above the mean sea level of Yedogawa River's estuary; Wang et al 2011).…”

“…glaucum dominates the middle layer and mosses co-occur on the ground (Nozoe et al 2010 (Fig. 1a), form patches about 3.6-cm thick on the tussock base (Wang et al 2011). The study area has also been managed for the annual harvest of traditional thatching material through winter mowing and burning since at least the 1950s.…”

“…glaucum is a keystone facilitator species of freshwater marshes, with at least two facilitation paths; directly via the ground elevation of tussock formation, and indirectly through moss occurrence on tussocks. We presumed that both paths act on seed germination and/or seedling establishment of sympatric species through the provision of safe sites against the high stress and water disturbance of inundation (Wang et al 2011), which impedes the emergence of freshwater wetland plant seedlings (Scarano et al 1997;Peterson and Baldwin 2004;Neff et al 2009), including rare species (Griffith and Forseth 2003). Although some studies of freshwater marshes have suggested that the higher species richness on tussocks results, in part, from the shorter hydroperiod (Peach and Zedler 2006), it has yet to be validated that germination and early survival are stages that benefit from the presence of tussocks.…”