4 - Clonal biology of caespitose grasses

Summary

Introduction

Graminoids comprise one of the largest subgroups of clonal plants among terrestrial angiosperms (Tiffney & Niklas, 1985). Caespitose graminoids represent a unique growth form that is characterized by the compact spatial arrangement of ramets within individual clones and the absence of rhizomes or stolons. Caespitose graminoids occur on all continents from the high Arctic to the Sub-Antarctic and are distributed over a wide range of precipitation zones (Leith, 1978; Walter, 1979). This growth form is particularly dominant in the grassland biome which occupies 24 million km² including tropical and temperate grasslands, savannas and shrub steppe (Leith, 1978).

The wide distribution and dominance expressed by caespitose graminoids is somewhat surprising given that the majority of modern monocot families are rhizomatous (Tiffney & Niklas, 1985). Rhizomes are modified horizontal ramets located belowground that represent a mode of clonal growth well suited for ramet dispersal, effective resource acquisition, and resource storage (Grace, 1993). In contrast, caespitose graminoids possess several attributes that could potentially limit their success, including intense intraclonal competition for nutrients (Hartnett, 1993) and for photosynthetically active radiation (Caldwell et al., 1983; Ryel, Beyschlag & Caldwell, 1993,1994), and a limited ability to access heterogeneously distributed resources (Van Auken, Manwaring & Caldwell, 1992). This poses the question, ‘what structural and/or functional attributes contribute to the ecological success of caespitose graminoids without the benefits conferred by rhizomes?’ The adaptive value of caespitose clones must be sufficient to offset the benefits associated with rhizomes or this growth form would not be so successful (e.g. Pedersen & Tuomi, 1995).