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Grassland communities dominated by Festuca scabrella or by Stipa curtiseta and Agropyron dasystachyum were burned experimentally in spring or autumn. Forb, shrub and graminoid biomasses were greater in the unburned Festuca community than in the Stipa-Agropyron type. Spring and autumn burning delayed early season growth of graminoids in both grassland types in the 1st growing season after fire. Peak green biomass was reached later in the season in burned plots relative to reference areas. Plant growth in the spring of the 2nd yr after fire was more rapid in burned plots and peak biomass was reached earlier than in the reference plots. Autumn burning reduced forb biomass significantly early in the growing season during 1 of 3 yr that this parameter was measured. No effect of burning on shrub biomass was detected. Autumn and spring burning of Festuca grassland reduced peak, green, graminoid biomass production in the 1st and 2nd yr after fire; autumn burning had the most negative effect. Reductions in graminoid biomass after burning Stipa-Agropyron stands were smaller than in the Festuca community. Nitrogen concentrations in F. scabrella shoots regrown after fire were higher than those from the reference areas. Soil water content during the growing season, and snow trapping were reduced by burning. Peak green biomass and total graminoid biomass in Festuca grassland recovered to the level of the reference plots 2-3 yr after burning. Recovery was slower in the Stipa-Agropyron community.

The impacts of autumn or spring burning on Symphoricarpos occidentalis were studied in Fescue Prairie in central Saskatchewan. Symphoricarpos density increased two- to three-fold over preburn density in the 1st 2 growing seasons following a burn. Individual shoot weights were approximately one-third those of plants in reference (unburned) sites. Leaves that were approximately two to three times larger, combined with increased stem densities, enabled plants in burned stands to re-establish a leaf area index equal to reference sites within 2 mo of growth in the 1st growing season. Total standing crop of S. occidentalis was reduced only for the early part of the 1st growing season following burning. In the 1st growing season following burning, xylem water potentials and stomatal conductance of burned plants equalled or exceeded those of reference sites. A delayed effect of burning was expressed in the 2nd growing season, with soil moisture, xylem water potentials and turgor potentials being lower in burned plants than in the reference. Flexibility in resource allocation, with a larger proportion of biomass in leaves, and unaffected or improved water status are adaptive features that enable S. occidentalis to regain its position in the plant community the 1st growing season following disturbance by burning. One-time burning in autumn or spring should not reduce the dominance of S. occidentalis in this region.

Objectives of this research were to determine the extent of internal cycling of N in Agropyron dasystachyum (Hook.) Scribn. (northern wheatgrass) and to determine the impact of individual plant N balance on N cycling in a mixed grassland ecosystem dominated by this species. The soil-plant system contained 381 g N [m.sup.-2] , nearly all in the organic form. Plant-available N, about 1.4% of total N, was mainly in the form of NH4; [NO.sub.3] was scarce, probably because of effective plant scavenging, and inhibition of nitrificati by low soil moisture. Nitrogen retranslocation from senescing leaves to new leaves occurred over the entire growing season. The highest N concentration was observed in the newly-emerged leaves, regardless of the time of emergence. Nitrogen recycled within plants, calculated from seasonal changes of N and biomass in individual plant leaves was as much as 69% of the total N taken up by A. dasystachyum shoots. Nitrogen-use efficiency and N-conservation are well-developed in this ecosystem.

Responses to defoliation, nitrogen form ([NH.sub.4] or [NO.sub.3]) and supplemental nitrogen (0-225 mg [kg.sup.-1] dry soil weight) were studied in Agropyron dasystachyum (Hook.) Scribn., a dominant grass in Canadian mixed prairie. Plants were grown in the greenhouse in soil containing nitrapyrin to retard nitrification of ammonium. Shoot, root and crown biomasses increased with nitrogen supply; root biomass decreased at the highest nitrogen levels. Defoliation reduced all biomass components, including cumulative biomass (calculated by adding biomass removed in the clipping treatment to regrowth), but increased shoot/ root ratios. Biomass allocation to shoots increased when nitrogen was supplied as [NH.sub.4], but remained constant when supplied as [NO.sub.3]. Defoliation reduced tillering, but only in plants receiving additional nitrogen. Defoliated plants were able to compensate fully for losses in leaf area in part through increases in specific leaf area([cm.sup.2] [g.sup.-1]). Defoliation and hi of [NH.sub.4] independently shifted plant growth pattern toward enhanced production of photosyntheti tissues.