The Vegetation: Pattern and Succession 199 



wet end of the moisture gradient is clearly apparent (Table 6-3). In wet 

 meadows, single graminoids make up most of the vascular vegetation. 

 Underlying the graminoid canopy is a discontinuous layer of moss. These 

 two growth forms are the most important in primary production and nu- 

 trient cycling and have been studied the most intensively. 



The importance of these growth forms, which is calculated from the 

 relative cover plus relative frequency, varies in the different vegetation 

 types and in the different microtopographic units, reflecting the different 

 productivities of the growth forms along environmental gradients (Fig- 

 ure 6-6). Evergreen and deciduous shrubs, caespitose graminoids, cush- 

 ion forbs, and lichens are more important on the drier microsites. The 

 single graminoids, grasses, sedges and rushes, and the bryophytes are 

 more important on the wetter sites. Caespitose graminoids occur with 

 lower moisture, moderate soil anaerobicity, and moderate concentra- 

 tions of soluble phosphorus (Figure 6-6). The importance of the single 

 graminoids increases with moisture and is independent of soil aeration 

 and phosphorus. Mat and erect forbs are most abundant in moist sites 

 with moderate to high levels of phosphate and hydrogen sulfide, while 

 rosette forbs prefer drier sites where phosphate and sulfide levels are 

 moderate. Evergreen shrubs occur with low moisture, moderate concen- 

 trations of hydrogen sulfide and moderate phosphorus. Deciduous 

 shrubs occur with slightly higher soil moisture, slightly higher hydrogen 

 sulfide, and higher phosphorus. The importance of lichens is highest 

 with moderately low moisture, low hydrogen sulfide, and moderate 

 phosphorus. Bryophytes, by contrast, are most important with moder- 

 ately high soil moisture, somewhat anaerobic soils, and low phosphorus. 



Species Diversity Within Growth Forms 



The number of species within a growth form, as well as overall cover 

 and frequency, varies along the environmental gradients (Figure 6-7). 

 The diversity of species within a given growth form does not always cor- 

 respond with the importance of the form. The importance of single 

 graminoids is highest in wet sites, and shows little relation to soil anaer- 

 obicity and phosphate. The species diversity is greatest with moderate 

 moisture and phosphate and relatively high soil anaerobicity. Deciduous 

 shrubs are more diverse with low soil hydrogen sulfide, but show high 

 cover with higher soil hydrogen sulfide. 



Production and diversity are related. Stands with lower diversity 

 have high productivity; stands with moderate diversity have lower pro- 

 ductivity. The highest diversity occurs on the drier sites, with growth 

 forms of low productivity. The lowest diversity occurs at stream and 

 pond edges, in monospecific graminoid stands with high production. 



