OVERSTORY-UNDERSTORY RELATIONSHIPS FOR BROOM SNAKEWEED-BLUE GRAMA GRASSLANDS

Data collected over a ll-year period at 2 study areas near Vaughn and Roswell, N.M. were used to define equations that relate grass biomass to the amount of broom snakeweed (Gutierreza sarothrae [Pursh] Britt. and Rusby) occupying blue grama (Bouteloua gracilis [H.B.K. Lag]) rang eland over time. A 5 parameter sigmoidal growth equation and a negativ e exponential equation best expressed the relationship between underst ory grass biomass and overstory broom snakeweed biomass. Explanatory v ariables included realized precipitation during the second (April to J une) and third (July to September) quarters, which coincides primarily with warm-season grass growth. Minimum suppression of grass biomass o ccurred with complete elimination of broom snakeweed, suggesting contr ol strategies with high overstory mortality will likely be most benefi cial to understory production.