Shade responses of five grasses native to southwestern US Pinus ponderosa forests

Recent increases in Pinus ponderosa Dougl. ex Laws. forest density in the s outhwestern United States have severely reduced understory herbaceous bioma ss and altered understory species composition. To examine whether changes i n graminoid species composition are caused by increased shading, we studied the effects of shade on leaf gas exchange, biomass, and reproductive chara cteristics of five grass species native to Arizona P. ponderosa forests in a greenhouse study. Blepharoneuron tricholepis (Nash) Torr., Koeleria crist ata (L.) Pers., Festuca arizonica Vasey, Muhlenbergia montana (Nutt.) Hitch c., and Sitanion hystrix (Nutt.) J.G. Smith were grown under three light le vels representative of photosynthetic photon flux densities and red/far-red ratios that occur beneath P. ponderosa canopies. In general, all species g rew better under unshaded conditions, but all survived and flowered even un der the dense shade treatment. Reduction of net assimilation rate by shadin g was the strongest during early reproductive shoot growth for all species except K. cristata, whose assimilation rate was unaffected by shading. Biom ass allocation and reproductive responses to shading varied among species. Biomass of S. hystrix was the least affected by shading of all species, and it showed no response in biomass allocation to reproduction but increased height and weight of individual flower stalks under shade. Overall, S. hyst rix and K. cristata, species that occur in dense P. ponderosa stands, were least affected by experimental shading, which suggests that shade is a cont ributing factor to the distribution of grass species in Arizona P. ponderos a forests.