The Vegetation: Pattern and Succession 203 



Leaf expansion cannot take place, since the ground may still be frozen 

 and water unavailable, but photosynthesis is still possible (Larcher et al. 

 1973). In such a situation evergreen leaves containing photosynthetic en- 

 zymes stored through the winter are advantageous (Miller 1978). Thin 

 snow cover also results in low soil water and the possibility of periodic 

 summer drought, at least in the surface soil layers. The summer drought 

 should select for leaves with structural thickenings and for plants with 

 deep roots to exploit deeper, moister soil layers (Oberbauer 1978, Ober- 

 bauer and Miller 1979). Thus, on exposed sites the vegetation should 

 comprise plants of low stature with long-lived leaves, low photosynthesis 

 rates, high leaf resistance, buds protected by scales, hairs or persistent 

 plant parts, rigid leaves and deep roots. These trends are exemplified in 

 the vegetation turnover rate, growth form composition, and above- to 

 belowground biomass ratios (Table 3-2, Table 6-2, Figure 6-5) of the 

 heath vegetation types. 



More protected sites have lower wind speeds, which lead to deeper 

 snow cover, greater protection from winter abrasion, high soil water, 

 higher temperatures in the active season, and increased litter deposition 

 and nutrients. The decreased abrasion loss makes possible taller growth 

 forms and the storage of nutrients above ground. The increased snow 

 cover results in later exposure of the surface. Air and ground tempera- 

 tures, when the surface is finally exposed, are higher than for the sites 

 that are exposed earlier. Early in the season the ground is warmed by the 

 influx of meltwater. Thus leaf expansion becomes possible as soon as 

 photosynthesis is possible. Deciduous leaved plants, with nutrients 

 stored in the aboveground stems, can occur. The vegetation of these sites 

 includes many deciduous shrubs. With still greater snow cover the short 

 snow-free season can favor evergreen forms which can regain the carbon 

 cost of leaf construction through several seasons. 



Increasing protection or impeded drainage leads to soils saturated 

 with water, low soil aeration, and low soil temperatures because of the 

 high heat capacity of the soil (Chapters 3 and 7). Low temperatures de- 

 crease mineralization and uptake ability, leading to reduced nutrient 

 availability. The plant may compensate by increasing the number and 

 biomass of absorbing roots. The lower soil oxygen can lead to greatly in- 

 creased respiration demands to support the root biomass, unless aeren- 

 chyma are present. Thus, selection may be strong for plants with aeren- 

 chyma, which may have deep roots, or for plants with shallow roots, 

 such as the evergreen shrub, or no roots, such as mosses. Saturated con- 

 ditions are found in the Dupontia and Carex-Eriophorum meadows and 

 in the Arctophila pond margins (Table 3-2, Table 6-2, Figure 6-5). Slight- 

 ly better aeration allows plants without aerenchyma to occur and vegeta- 

 tion similar to Salix heath develops. 



