208 P.J. Webber et al. 



the Carex meadow from drained lake sediments forms a link with the al- 

 luvial succession. Trough formation in the Carex-Oncophorus meadow 

 can reverse the trend by producing wetter areas and vegetation types in 

 troughs and centers of low-centered polygons. As polygonization contin- 

 ues, the diversity of surfaces increases. Vegetation on the drier rims of 

 low-centered polygons and tops of low-relief, high-centered polygons of 

 the Carex-Oncophorus meadow can change to that of the Carex-Poa 

 meadow. Further depression of centers and impounding of water pro- 

 duces ponds and Arctophila pond margin vegetation. Some ponds may 

 breach their impounding rims and drain while others may fill with sedi- 

 ments. In this manner, the Dupontia meadow may be re-formed. The cy- 

 cle may also be closed by the coalescence of ponds as their margins or 

 rims are eroded by thermokarst activity, and large ponds and even thaw 

 lakes may be re-formed. 



When the presence of various species is plotted on this successional 

 sequence of vegetation types, orderly patterns of species change emerge. 

 Some species occur in only one type or successional stage, such as Hiero- 

 chloe alpina, Cassiope tetragona, Caltha palustris, Phippsia algida and 

 Saxifraga rivularis. Others occupy several types, such as Poa arctica, Du- 

 pontia fisheri, Saxifraga foliolosa, Carex aquatilis and Eriophorum an- 

 gustifolium. Those species occurring in more than one type usually occur 

 sequentially in the postulated successional sequences and this occurrence 

 is not interrupted or haphazard. The single exception is Arctophila fulva, 

 which dominates the pond margin vegetation, and also occurs in the Du- 

 pontia meadow and in the Cochlear ia meadow. 



Orderly patterns are also formed when the numbers of the species of 

 different growth forms in each vegetation type and various productivity 

 measures are plotted in the successional sequence (Figure 6-10). In the al- 

 luvial sequence from Cochlearia meadow to the Luzula heath on the tops 

 of high-centered polygons, the total number of species increases from the 

 Cochlearia meadow through the Carex-Poa meadow to the Salix heath 

 but decreases abruptly from the Salix heath to the Luzula heath. In the 

 thaw-lake cycle from the Dupontia meadow through the Carex- 

 Oncophorus meadow, Carex-Eriophorum meadow and Arctophila pond 

 margin, the total number of species first increases from the Dupontia 

 meadow to the Carex-Oncophorus meadow and then decreases through 

 the Carex-Eriophorum meadow to the Arctophila pond margin. The 

 Arctophila pond margin has the lowest diversity in total number of spe- 

 cies and in the number of species in each growth form. The highest pro- 

 portion of graminoids is found in the Carex-Eriophorum meadow and 

 the smallest in the Salix heath. The vegetation types found on dry sites 

 have the highest proportion of dicotyledons. The Cochlearia meadow 

 has several typical pioneer species which are dicotyledons, for example 

 Cochlearia officinalis, Saxifraga rivularis and Stellaria humifusa. 



