Relationships between Pinus ponderosa forest structure, light characteristics, and understory graminoid species presence and abundance

Productivity of graminoid species is known to vary in response to different Pinus ponderosa densities. Specific forest structural characteristics rela ted to these differences remain elusive, yet are important for management o bjectives that aim at a diverse understory vegetation composition. In this study, we have addressed the effects of P. ponderosa forest structure and l ight environments on individual graminoid species presence and abundance. G raminoid species presence was positively related to mean diameter of P. pon derosa trees (including seedlings), but presence was poorly related to ligh t characteristics. Larger tree diameters may relate to graminoid species pr esence through shade intensity because shade under larger diameter trees is less dense. The abundance of small diameter trees indicates past disturban ce and exposed mineral soil which may favor some graminoid species over oth ers. Graminoid species abundance measured as basal area cover and plant num bers were more closely related to direct sunlight. Cool-season species perf ormed better on sites with spring/autumn or morning summer sun while the la rger plants of warm-season species were on sites with midday summer sun. Th ese results suggest that manipulation of forest structure to maintain stand s of large diameter trees in a patchy distribution will create a variety of direct sun regimes needed to maintain a diverse grass understory. This typ e of forest structure is also consistent with pre-European southwestern P. ponderosa forest conditions. (C) 1999 Elsevier Science B.V. All rights rese rved.