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Seed dispersal, seed attributes and edaphic factors : their role and impact on the regeneration of antelope bitterbrush (purshia tridentata, rosaceae)

University of British Columbia

The purpose of this research was to collect detailed demographic information concerning the early life stages of antelope bitterbrush Purshia tridentata and elucidate those biotic and abiotic factors limiting or enhancing seed dispersal, seedling emergence and survival. Agricultural and urban development have seriously reduced the size and continuity of natural areas in the Okanagan valley, British Columbia. Antelope bitterbrush has its most northern distribution in this setting and this study was initiated to address questions concerning the ability of this shrub to persist in the face of rapid change. The research was undertaken in three parts represented by the separate chapters of this manuscript; Chapter 1 a survey of juvenile plants, Chapter 2 a seed removal experiment and Chapter 3 a reciprocal planting experiment. In 1994 I estimated the density of juvenile Purshia shrubs at ten separate sites. This indicated significant variation in seedling establishment between sites and provided the basis for 2 separate lines of research. First, I placed Purshia seeds into the field at feeding stations to determine what class of seed eating animals, insects, birds or rodents, were responsible for seed predation. Seed predation was mostly due to nocturnal rodents some of which, through caching behaviour, also acted as effective agents of seed dispersal. Secondly, 8,000 seeds were planted individually 2.25 cm beneath the soil surface. Environmental factors explained 40 % of the variation in seedling emergence while variation between maternal families accounted for 16 %. Seedling emergence correlated negatively with silt content of soils and positively with moss cover. Shrubs that on average produced seeds with a higher nitrogen content also produced seeds that were less likely to emerge. In silty soils initial seedling emergence appeared to be limited by crusting at the soil surface. However, soils with higher silt content contained more seedlings at the end of the first year, presumably due to greater moisture availability. The correlation between moss cover and seedling emergence provides some evidence that cryptogamic crusts confer conditions favourable to seedling establishment. Knowledge of factors limiting seedling establishment is required to manage and where appropriate to restore Purshia shrubs in the highly impacted landscape of the south Okanagan valley.

Thesis/Dissertation

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2009-03-11

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http://hdl.handle.net/2429/5898

Forestry

University of British Columbia

UBCV

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