Although native clovers Trifolium species are highly diverse and often frequent in California grasslands, they are seldom seeded in grassland restoration attempts within the State. A trial was undertaken near Winters town (38°38″01′N, 122°03″05′W) to identify practical management options that may enhance growth of native bunchgrasses plugs and seeded native clovers, and to assess if native and non-native clovers respond differently to management.
The study was undertaken on grassland dominated by two non-native annuals, Italian ryegrass Lolium multiflorum and soft brome Bromus hordeaceus. In 2001, a late-spring burn (a common restoration preparation practice to reduce non-native annual grass seed) was conducted.
In autumn, 35 (1.5 x 1.5 m) plots were established and fenced to exclude rabbits and cattle. Five native bunchgrass species were planted (random plots) in February at 22 and 48 plugs/m2.
Native clovers (4 species) and non-native commercial varieties (4) were sown in November 2002 and 2003. Treatments (plus controls) were applied (randomized design; 7 treatment combinations, 5 replicates each):
1) spring clipping (by brush-cutter) twice each spring (February-April) to 6-10 cm height;
2) broadleaf herbicide (2,4-D) applied twice (December 2001; March 2002).
In 2001, data were collected on cover of native and non-native grasses, and non-native long-beaked stork’s-bill Erodium botrys (almost the only forb present), and in 2002-2003, including clovers.
Of planted bunchgrass species, only the two Nasella spp. established well, accounting for 98% of native grass cover in autumn 2003. Failure of the other species was attributed to a dry spring subsequent to planting. Clipping did not influence bunchgrass cover, but it enhanced growth of both native and non-native clovers.
E. botrys was more-or-less eradicated in herbicide-treated plots. Although sprayed prior to sowing, a strong negative residual effect was apparent on native clovers (density 55% less in no-clip/herbicide vs. no-clip/no herbicide treatment in spring 2003). Native and non-native clovers responded similarly to clipping, establishing at similar densities (around 27-30 individual/m² in no clip/herbicide; and 55-60 in clip/herbicide treatments).
There was no effect of bunchgrass plug-planting density (which declined to an average to 9 and 25 individuals/m2 by autumn 2003)on clovers but the higher density reduced non-native grass cover.