Knowledge of post-dispersal seed fates and other regeneration characteristics is
crucial for predicting abundances and distributions of populations and, ultimately,
community species composition and diversity. Seed fate studies, however, are rare
primarily due to the difficulty of determining seed fates and causes of mortality.
This thesis investigated post-dispersal seed fates for four species common to
western Oregon native prairies: Bromus carinatus Hook and Am. var. carinatus,
Cynosurus echinatus L., Daucus carota L., and Prunella vulgaris var. lanceolata
(Barton) Fern. The general approach was to sow seeds of these species into
experimentally manipulated field plots for each of two years, and to recover these seeds
from the soil one year later to determine their fates (persistence, death, or establishment
as seedlings). The effect of mowing on seedling establishment was also addressed.
Additional studies focused on the effects of a single mortality factor, fungal disease, on
seed and seedling deaths.
The fate of most seeds was death (44%-80%). Few seeds established as seedlings
(4%-17%), and mowing did not significantly increase seedling establishment. Only
Daucus carota formed a persistent seed bank.
Fungal disease generally caused less than 10% mortality. Pot studies
corroborated these field results. Other investigators have suggested higher levels of
disease in natural vegetation.
Vertebrate predation significantly reduced seed numbers for only Bromus
carinatus (21%). The largest cause of death for all species for both years was the
combined group of other mortality causes (invertebrate predation, interference, and
abiotic factors) (52%-73%). The components of this combined group, however, differed
among species. The most likely components for Bromus carinatus and Cynosurus
echinatus were interference (competition plus allelopathy) and abiotic factors, although
invertebrate predation cannot be ruled out for Bromus carinatus. Seedling death due to
abiotic factors was most likely the largest component for Daucus carota. The most
probable components for Prunella vulgaris were invertebrate predation and abiotic
Implications of these findings for population patterns and for restoration of native
prairies are discussed.
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