The state and transition model and the ball and cup analogy are used to organize the vegetation dynamics knowledge base for California's annual-dominated Mediterranean grasslands. These models help identify irreversible transitions and alternate stable states. Mechanisms that facilitate movement between successional stable states are categorized as demographic inertia, seedbank and germination, grazing impacts, establishment and competition, fue feedback, and irreversible changes in soil conditions. While theoretical work needs to continue to further describe states and transitions, managers can begin to use existing knowledge to develop management plans with realistic species composition objectives and to select the appropriate tools for reaching objectives.

No abstract

On Caiilomia's winter a~ud r8ngdanda precipitation controls the beginning and end of the growing season while temperature Lirgely controls se8sonai growth tmtes within the growing season. Post-germination accumulated degree-days (ADD) account for the length of the growing season and variation of daily temperature. Simple correlations of ADD and herbage yield or resultant livestock gains were determined at 5 locations in ammal type range in northern California. Degree day values were determined by summing daily degree-days from the beginning of the growing season after germinating rainfail until the ciipphrg or weigh dates. Accumulated degree-days accounted for 74 to 91% of the variation in seasonal herbage yield while accumuiated days (AD) accounted for 64 to 86% of the variation. Together, ADD and AD accounted for 94 and 8696, respectively, of the variation in stocker cattle weights. Regression coefficients relating ADD to herbage yield appear to predict maximum site productivity. A procedure for estimating a seasonai herbage yield profile based on key growth curve intlection points and using shnple field observations with 3 clipping dates and ADD is proposed.

Wide yearly fluctuations in peak standing crop on California annuai-type range arc largely cxpiained by temperature and precipitation patterns. The objective of this study is to improve the predictability of functions relating weather patterns and peak standing crop by including degree-days, dry periods, evaporation, season start dates, and lengths and precipitation as independent variables. Beak standing crop was regressed on these independent variables for the University of California Hopiand Field Station (I-IFS) and San Joaquin Experimental Range (SJER). Fall and winter precipitation, winter degree-days, and longest winter dry period were related to peak standing crop at HFS (Rz=O.61). Spring precipitation, growing season degree-drys, winter evrporation, md winter and spring &rt dates were related to peak standIng crop at SJER (R*=.72). The relationship of peak standing crop to accumulated precipitation on 20 November using 33 years of data (&0.34) was weaker than previously reported for the first 16 years (r2~0.49). This study suggests that timely prediction of peak standing crop may be possible at HFS but more difficult at SJER.

No abstract