. The plant functional types (growth forms) traditionally recognized by arctic ecologists provide a useful framework for predicting vegetation responses to, and effects on, ecosystem processes. These functional types are similar to those objectively defined by cluster analysis based on traits expected to influence ecosystem processes. Principal components analysis showed that two major suites of traits (related to growth rate and woodiness) explain the grouping of species into functional types. These plant functional types are useful because they (1) influence many ecological processes (e.g. productivity, transpiration, and nutrient cycling) in similar ways, (2) predict both responses to and effects on environment, including disturbance regime, and (3) show no strong relationship with traits determining migratory ability (so that no functional type will be eliminated by climatic change simply because it cannot migrate). Circumstantial evidence for the ecological importance of these functional types comes from the distribution of types along environmental gradients and the known ecological effects of traits (e.g., effects of litter quality on decomposition and of plant height on winter albedo) that characterize each functional type. The paleorecord provides independent evidence that some of these functional types have responded predictably to past climatic changes. Field experiments also show that plant functional types respond predictably to changes in soil resources (water and nutrients) but less predictably to temperature. We suggest that evidence for the validity of arctic plant functional types is strong enough to warrant their use in regional models seeking to predict the transient response of arctic ecosystems to global change.

Nitrogen (N) limits plant growth in many terrestrial ecosystems, potentially constraining terrestrial ecosystem response to elevated CO. In this study, elevated CO stimulated gross N mineralization and plant N uptake in two annual grasslands. In contrast to other studies that have invoked increased C input to soil as the mechanism altering soil N cycling in response to elevated CO, increased soil moisture, due to decreased plant transpiration in elevated CO, best explains the changes we observed. This study suggests that atmospheric CO concentration may influence ecosystem biogeochemistry through plant control of soil moisture.

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Abstract.Cotton (Gossypium hirsutum L. cv. Acala S J-2) seedlings were grown in modified Hoagland nutrient solution with or without 150 mM NaC1 and supplemental 10 mM CaC12. The spatial distribution of bulk-tissue osmotic potential (U?s) and total osmotica, K, Na and Ca contents were determined in the growth zone of the primary root. This information was combined with the growth-velocity data from an earlier study (Zhong and L/iuchli 1993) to estimate net deposition rates of osmoticum, water, K, Na and Ca by using the continuity equation. The u?~ was essentially uniform along the growth region for all treatments and considerably lowered by 150 mM NaC1 in the medium. Total osmotica deposition was well synchronized with growth and deposition rates were enhanced by 150 mM NaC1. Osmoregulation in the treatments with 150 mM NaC1 was indicated by an apparent solute accumulation which appeared to be due to the enhancement of osmoticum deposition rates. The presence of 150 mM NaC1 greatly reduced the deposition rates of K and Ca throughout the growth zone; 10 mM Ca mitigated this effect only on K deposition in the apical 2.5-mm region. The deposition rate of Na was increased greatly by 150 mM NaC1; the increase was reduced by 10 mM Ca. At 150 mM NaC1, selectivity of K versus Na of the root was enhanced greatly in the apical 2 mm region by the presence of 10 mM Ca; this mitigating effect by Ca declined rapidly with distance from the root tip. We conclude that one possible mechanism by which supplemental Ca alleviates the inhibitory effects of NaC1 on cotton root growth is by maintaining plasma-membrane selectivity of K over Na. *Present address:Department of Integrative Biology, University of California, Berkeley, CA 94720, USA Abbreviations and symbol: HCa, LCa = high (10 mM) and low (1 mM) CaCI2; HNa, LNa = high (150 mM) and low (1 mM) NaC1; T~ = osmotic potential Correspondence to: H. Zhong; FAX: I (510) 643 6264;