Seasonal Changes in Carbon Fractions in Alaskan Tundra Plants of Differing Growth Form: Implications for Herbivory

Chapin, F; McKendrick, J; Johnson, D

"(1) The seasonal patterns of carbohydrate, lipid, cellulose, hemicellulose and lignin were examined in plants of major tundra growth forms: deciduous shrub, evergreen shrub, deciduous forb, tussock graminoid, moss and lichen. (2) Stems of deciduous shrubs (Salix pulchra and Betula nana) and forb (Rubus chamaemorus) showed well-developed over-winter carbohydrate storage, with concentrations increasing in autumn and declining in spring, coincident with initiation of leaf and root growth. The evergreen shrub (Ledum palustre) showed little seasonal fluctuation in carbohydrates of stems and old leaves. The graminoid (Eriophorum vaginatum), which had an intermediate amount of over-wintering green leaf biomass, showed seasonal carbohydrate fluctuations in stems intermediate between those of evergreen and deciduous shrubs. Among mosses, Aulacomnium spp. showed greater seasonal fluctuation in carbohydrate concentration of brown material than did Polytrichum/Pogonatum spp. The chemical procedure designed and used here for vascular plants gave ambiguous results for lichens. (3) Ether-extractable lipid in stems of Salix, Rubus and Eriophorum was low and showed no consistent seasonal pattern among different categories of stems, suggesting that it was not a major over-winter energy store in these species. Stems of Betula had higher lipid contents than did other deciduous species, but lipid declined from mid-summer to autumn and was therefore unlikely to be a major over-winter energy store. Lipid showed little seasonal fluctuation in Ledum. In brown tissue of mosses it declined in summer and increased in autumn. (4) Cellulose content generally increased with age in leaves and stems of most species, reflecting its structural role. (5) Hemicellulose content was higher in graminoids than in other growth forms and fluctuated substantially through the growing season in a pattern suggesting that it acts as an energy store. The large `hemicellulose' fraction of lichens probably included particular lichen polysaccharides. (6) Of the plants examined, mosses had the highest concentrations of lignin-like materials, and Eriophorum and lichens were lowest in lignin. Lignin content was similar in stems of all dicotyledonous species examined and bore no clear relation to woodiness. (7) Chemical composition of all study species was compared with the preferences for them of herbivores studied at the same site. Preferences of each of eight generalist herbivores (vertebrate and invertebrate) for the study species correlated positively with nitrogen, phosphorus and potassium and negatively with lipid and cellulose contents. The similarity of preference ranking among generalist herbivores suggests that characteristics of the plant material (rather than animal digestive physiology) exert primary control over patterns of food choice by these generalist herbivores. Preference ranking of Lemmus (brown lemming), a specialist on graminoids and mosses, correlated positively with fibre and negatively with mineral nutrient contents."

Seasonal Changes in Carbon Fractions in Alaskan Tundra Plants of Differing Growth Form: Implications for Herbivory

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