Production: Biomass Relationships and Element Cycling in Contrasting Arctic Vegetation Types

Abstract

Primary production, plant biomass, plant element content, and various measures of turnover and element cycling were compared among four contrasting tundra vegetation types near Toolik Lake, Alaska. The study sites were selected to represent extreme examples of the wide variation in plant growth form composition that is typical of northern ecosystems. The aim of the research was to determine whether vegetation types that differ in their dominant plant growth form also differ in their production: biomass relationships and overall patterns of element use. The four sites included tussock tundra, a deciduous shrub—dominated riparian tundra, an evergreen heath tundra, and wet sedge tundra. Biomass and element content (N, P, K, Ca, and Mg) were determined for both vascular and nonvascular plants, and production estimates were obtained for vascular plants. Production and biomass of most tissues were determined by quadrat harvest methods, with additional, separate determinations of stem secondary growth and belowground rhizome growth as components of net primary production (NPP). Production, biomass, and element content of roots were not determined. Vascular plant biomass (excluding roots) varied by nearly 9 × among sites, from 217 to 1877 g/m 2. At least 50% of the vascular biomass at all four sites was belowground stems, but the relative allocation to leaves vs. aboveground stems varied greatly. When mosses and lichens were included, total biomass varied by only 5 × among sites, and lichens were in fact the most abundant plant form at the evergreen heath site. The element content in vascular biomass of these communities varied by 8—21 ×, depending upon the element; including mosses and lichens, element content varied by 6—12 ×. Primary productivity of vascular plants also varied sharply among sites, from 32 to 305 g · m—2 · yr—1 (not including root production). Leaves were the largest single component of NPP at all sites, but the relative importance of above— and belowground stem growth varied considerably. The element requirements of vascular NPP varied by 9—17 ×, depending upon the element. Despite these order—of—magnitude differences among sites in biomass, production, and element requirements, and the dramatic variation in allocation patterns, there was remarkably little difference among sites in the overall production: biomass relationships and element turnover. These overall similarities were due to the dominant effect of stems and rhizomes in the whole—plant and whole—vegetation biomass and element budgets. The wide range of leaf turnover rates and leaf production efficiencies was compensated at the whole—plant level by stem processes, especially element storage. Thus, we conclude that plant growth form composition is not a good indicator of whole—vegetation biomass turnover rates and patterns of element use, although growth form composition is related to total production and biomass.