Vegetation of the Santa Catalina Mountains, Arizona. V. Biomass, Production, and Diversity along the Elevation Gradient

Measurements were taken in 15 communities along the elevation gradient from fir forest at high elevations, through pine forest, woodlands, and desert grassland, to deserts at low evelations in the Santa Catalina Mountains, Arizona, and in a Cercocarpus shrubland on limestone. Eight small—tree and shrub speices of woodlands and deserts were subjected to dimension analysis by the Brookhaven system. Aboveground biomass decreased along the elevation gradient from 36—79 dry kg/m² in fir and Douglas—fir forest to 0.26—0.43 kg/m² in the desert grassland and two desert samples. Net aboveground primary productivity similarly decreased from 1,050—1,150 g/m²° yr in mesic high—elevation forests to 92—140 g/m²° yr in desert grassland and deserts. Both biomass and production show a two—slope relation to elevation (and, probably, to precipitation), with a steeper decrease from the high—elevation forests to the mid—elevation woodlands, and a less steep decrease from dry woodlands through desert grassland into desert. The two groups of communities at higher vs. lower elevations also show different relations of leaf area index and chlorophyll to elevation and to productivity. The two groups may represent different adaptive patterns: surface—limiting, with low productivity in relation to precipitation but high production efficiency in relation to surface in the more arid lower elevations, vs. surface—abundant, with high productivity relative to precipitation based on high community surface area, but lower production efficiency in relation to this area, in the more humid higher elevations. Vascular plant species diversity shows no simple relation to productivity, but decreases from high—elevation fir frests to the pine forests, increases from these to the open woodlands, and decreases from dry woodlands through the desert grassland and mountain slope desert to the lower bajada (creosotebush) desert.