Physiological and production responses of plant growth forms to increases in limiting resources in alpine tundra: implications for differential community response to environmental change

Abstract

Physiological and growth measurements were made on forbs and graminoids following additions of water and N+water in a graminoid-dominated dry meadow and a forb-dominated moist meadow, to determine if the community-level response was related to differential responses between the growth forms. Graminoids had higher photosynthetic rates and lower transpiration rates and foliar N concentrations than forbs, and consequently maintained higher photosynthetic N- and water-use efficiencies. Photosynthetic rates, stomatal conductance, and transpiration rates increased significantly only in response to N fertilization and only in moist meadow species. The increase in photosynthetic rates was unrelated to variation in foliar N concentration, but instead correlated with variation in stomatal conductance. Growth based N-use efficiency was higher in moist meadow graminoids than in moist meadow forbs, but did not differ between the growth forms in the dry meadow. The moist meadow community had higher biomass and N standing crops, but the relative increase in these factors in response to N fertilization was greater in the dry meadow. Graminoids had a greater relative increase in biomass and N accumulation than forbs following N fertilization, but moist meadow graminoids exhibited a greater response than dry meadow graminoids. The difference in the growth response between the dry meadow and moist meadow graminoids to N fertilization was correlated with more conservative leaf gas exchange responses in dry meadow species, presumably related to a higher frequency of soil water deficits in this community. Community-level response to the resource additions was therefore mediated by the plant growth form response, corresponding with differences between the growth forms in physiological factors related to resource acquisition and use.

Ecological issues related to N deposition to natural ecosystems: research needs

“…An increase in nitrophilic species, which are often tall and densely growing, can result in decreased competitive ability of small and slow-growing species (Falkengren-Grerup, 1986) and eventual declines in diversity. A similar response was observed when tundra was fertilized (Bowman et al 1995). They reported a shift from forb-dominated to grassdominated communities, due to the greater capacity of alpine graminoids to sequester N. A reduction in species number and the increasing dominance of a few grass species, in response to N fertilization, have been well documented in the Rothamsted Park Grass experiment (UK) (Tilman et al, 1994) and at the Cedar Creek Long-Term Ecological Research Site in Minnesota (US) (Inouye and Tilman, 1995).…”

“…Tundra ecosystems have received more research attention (Williams et al, 1996;Bowman et al, 1995;Baron et al, 1994), but much of the research on these ecosystems is in its early stages and comes from very few sites. There is a notable paucity of research evaluating N deposition effects on North American wetlands, although much fine research has been done on European wetlands.…”

“…Other elemental deficiencies or toxicities may be exacerbated by atmospheric N deposition or may moderate the response. For example, high N deposition in the Netherlands apparently induced a phosphorus deficiency in Douglas-fir (Pseudotsuga menziesii) stands (Mohren et al, 1986), andBowman et al (1995) found N and P colimited production in Colorado wet meadows.…”

Nitrogen Excess in North American Ecosystems: Predisposing Factors, Ecosystem Responses, and Management Strategies

“…Notwithstanding elevated leaching, alpine tundra Ϫ NO 3 at Niwot Ridge responded to N fertilization, and responded greatly to N and P fertilization (Bowman et al 1995). The discrepancy between export from Ϫ NO 3 the watershed and N limitation in vegetation, is probably due to short growing seasons and a temporal disjunct between the times when most N is available (early spring) and when plants need it for growth (late spring and summer) (Baron et al 1994).…”

“…The discrepancy between export from Ϫ NO 3 the watershed and N limitation in vegetation, is probably due to short growing seasons and a temporal disjunct between the times when most N is available (early spring) and when plants need it for growth (late spring and summer) (Baron et al 1994). While N fertilization of tundra produced a moderate growth increase, the most dramatic response was a shift to from forb-dominated to grass-dominated communities (Bowman et al 1993(Bowman et al , 1995. Alpine graminoid communities have a greater capacity than forb communities to sequester N, and because of greater N use efficiency, graminoids also produce litter with lower N concentration, and probably slower decomposition rates.…”