March/April 2005 CA Grassland Newsletter
Eviner, Valerie and F. Stuart Chapin. 2005. Selective gopher disturbance influences plant species effects on nitrogen cycling. Oikos 109: 154-166.
ABSTRACT: Plant species effects on ecosystem processes are mediated by traits such as litter quality and exudation. These same traits also influence the activity and distribution of animals that play key roles in regulating ecosystem dynamics. We planted monocultures of eight plant species commonly found in California grasslands to investigate the relative importance of plant species direct effects on nitrogen cycling, versus their indirect effects mediated by plant interactions with gophers. Plant species differed in their litter C:N ratio, which closely related to species effects on rates of net mineralization and nitrification in undisturbed soil. However, the effect of selective gopher disturbance on N cycling greatly altered these species effects.
Plant species differed in their effects on the type and timing of gopher disturbance. Small feeding holes were formed in late spring in plots containing species with high tissue quality. These feeding holes minimally disturbed the soil and did not alter N cycling rates over the short term. Large gopher mounds were formed in the winter and early spring, primarily in plots containing the grass, Aegilops triuncialis, and to a lesser extent in plots containing Avena barbata. These large mounds significantly disturbed the soil and greatly increased net nitrification rates, but had no consistent effects on net N mineralization. In undisturbed soil, Aegilops had the highest litter C:N ratio and one of the lowest rates of net nitrification. However, gophers preferentially built large mounds in Aegilops plots. Once the effects of gopher burrowing were considered, Aegilops had one of the highest rates of net nitrification, indicating that the indirect effects of plant species on N cycling can be more important than the direct effects alone. This experiment indicates that it is vital to consider interactions between plants and other organisms in order to predict the ecosystem effects of plant communities.
Hopkinson, Peter and L. Huntsinger. 2005. Are East Bay hills grasslands a historical artifact? Phytolith evidence and a potential candidate for the true East Bay vegetation type. Grasslands Winter 2005.
No Abstract. (You may remember Peterís presentation at the 2004 CA Grasslands conference)
Kennedy, PeterG. and Jessica M. Diaz. 2005. The influence of seed dispersal and predation on forest encroachment into a California grassland. Madrono 52: 21-29.
ABSTRACT: Forest encroachment into grasslands is a widespread phenomenon with significant implications for land management. We examined the influence of seed dispersal and predation on the encroachment of two tree species, Lithocarpus densiflora and Pseudotsuga menziesii, into a coastal California grassland. We mapped the distributions of L. densiflora and P. manziesii seedlings and saplings and L. densiflora acorns across the forest-grassland ecotone as well as quantified the fates of seeds of both species experimentally located at five distances across the ecotone. Species distributions varied, with P. menziesii being most abundant in the grassland and at the edge and forest. Lithocarpus densiflora acorns were absent from the grassland but equally abundant at the edge and forest. Seed predation was similar for both species, being low in the grassland, variable at the edge, and high in the forest. It appears that L. densiflora encroachment into the grassland is strongly influenced by seed dispersal, while P. menziesii encroachment is more likely influenced by seed predation than dispersal.