Search

We used the eddy covariance technique to measure exchanges of water vapor, energy, and [CO.sub.2] between California serpentine grassland and the atmosphere. Even though the system was built around an inexpensive, one-dimensional sonic anemometer and a closed-path [CO.sub.2] analyzer, energy balance closure was accurate to ± 20% at a net radiation of 500 W/[m.sup.2], and the spectra and cospectra indicated only modest information loss from incompletely resolved high-frequency turbulence. In the early and middle parts of the growing season, net radiation, latent heat, and sensible heat all had similar diurnal dynamics, with latent heat accounting for [approximately equal] 60% of the net radiation. Late in the growing season, energy dissipation by latent heat dropped dramatically, even though the vapor pressure gradient remained high. The Ω factor, an index of the role of canopy conductance in regulating transpiration (Jarvis and McNaughton 1986), decreased from 0.8 early in the growing season (indicating predominant control of transpiration by net radiation) to 0.1 late in the growing season (indicating a shift to control of transpiration by canopy conductance and vapor pressure deficit). Canopy conductance was a linear function of the product of net photosynthesis and relative humidity, divided by the [CO.sub.2] concentration, as predicted by Ball (1988). The slope of the relationship, however, was greater early in the growing season than at other times. Whole-ecosystem carbon exchange rates were modest, with midday net photosynthesis reaching maximum values of 6-8 µmol [multiplied by] [m.sup.-2] [multiplied by] [s.sup.-1] in early April. Diurnal variation in photosynthesis roughly paralleled variation in photosynthetically active photon flux density (PFD), but with the daily maximum increasing with canopy development early in the growing season and decreasing with drought at the end of the growing season. Photosynthesis did not clearly saturate at high levels of PFD. Ecosystem dark respiration increased strongly ([Q.sub.10] = 4.6) with increasing soil surface temperature. The efficiency with which absorbed radiation was used in ecosystem photosynthesis, integrated over entire days, was 0.0115 ± 0.0015 mol [CO.sub.2]/mol PFD ( [approximately equal] 20% of the values measured for healthy, single leaves under low light conditions) until late in the growing season, when the efficiency fell sharply. Using simple assumptions to extrapolate measurements from 11 d to the entire growing season, we estimate ecosystem annual gross primary production to be 11.1 mol [CO.sub.2]/[m.sup.2], yielding an annual net primary production of [approximately equal] 133 g biomass/[m.sup.2]. This is near the center of the range of published values from above-ground harvest studies., Key words: California serpentine grassland; canopy [CO.sub.2] exchange; canopy energy exchange; eddy covariance; light-use efficiency; omega factor; scaling. INTRODUCTION Relative to the state of the art at the leaf level, [...]

In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica), at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 [m.sup.2] to 32 [m.sup.2], had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species-rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species-rich plots were more invasible. Key words: Bromus diandrus: California annual grassland; disturbance; dominance: Eschscholzia californica; invasibility; plot size; species richness., INTRODUCTION Certain ecological communities seem especially prone to invasion by a new species, and determining why they are more vulnerable has been a long-standing problem in ecology (Robinson and Valentine [...]