172 F. S. Chapin III et al 

 1200 



T^ 1000 



^ 800 



.? 600 

 Q 400 



200- 







0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 



Phosphorus Concentration in Blades, % 



0.9 



FIGURE 5-14. Tiller weight of aboveground material o/Dupontia fish- 

 eri in relation to the phosphorus concentration of the leaf blades (after 

 Ulrich and Gersper 1978) and the range of concentration measured in the 

 field in 1972 and 1973 (Chapin et al. 1975, Tieszen, unpubl. data). In the 

 laboratory, experimental plants (%) were grown in solution culture with 

 different phosphorus concentrations and the total leaf phosphate was 

 estimated from measurements of acid-soluble phosphate. Field 

 measurements of biomass and nutrient concentration (o) were made in 

 the moisture-nutrient gradient. 



always higher than the critical level necessary for maximum growth in the 

 laboratory (Figure 5-14). Moreover, weights of field plants were consis- 

 tently below those of laboratory plants when either nitrogen or phos- 

 phorus was tested in the laboratory as the only factor limiting growth 

 (Ulrich and Gersper 1978). Laboratory studies on plant critical levels 

 suggest two hypotheses: 1) nitrogen and phosphorus never act as the sole 

 limiting factor in the field but are both among a complex of limiting fac- 

 tors, and 2) tundra graminoids never produce nutrient-deficient tissues, 

 but rather limit growth rate. Graminoids at Barrow produce new tissue 

 only if an adequate quantity of nitrogen and phosphorus is available for 

 maximal development and presumably for optimal function. Environ- 

 mental factors such as nutrients restrict growth rather than compromise 

 the effectiveness of new tissues that are produced. Agricultural crops and 

 species from fertile habitats differ substantially in this regard, respond- 

 ing to nutrient stress with deficiency symptoms and reduced respiration 

 and photosynthesis (Chapin 1980a). 



