Graduate Thesis Or Dissertation
 

Recruitment of ponderosa pine seedlings beneath partial overstories

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  • The objective of this project was to investigate the fate of ponderosa pine regeneration from seed to established seedling, and to determine the relative influences of several important stand elements on those fates. The project was carried out in a series of observational and experimental recruitment studies in central Oregon. In an analysis of the temporal and spatial patterns of seedfall during one season, viable seeds accounted for 62-73 percent of all seeds, and seed density averaged more than 1 million per hectare. More than half of all viable seeds fell within the first 30 days. Seedfall occurred more rapidly at the more mesic site. The less mesic site produced more seed, and the denser stand within each site yielded more seed. Seedfall patterns within each stand were not spatially uniform. At the two stands of moderate densities, the relationship of seed density to distance from trees followed a negative binomial distribution. In an observational study of germinant emergence and survival, about 3 percent of viable seed resulted in emergents. A small fraction of germinants survived the first summer, with most mortality occurring immediately after emergence. Most emergents were from caches, and germinants from caches remained a substantial portion of the cohort after two seasons. Mineral soil substrates produced fewer emergents and were associated with higher mortality rates than litter substrates. Understory cover produced fewer germinants than open locations, but mortality rates were lower. After one year, most survivors were beneath understory cover. In a seed-sowing experiment, less than 30 percent of seeds sown in fall resulted in live germinants the following spring, and just 5 percent resulted in live germinants by the following fall. Shrubs had no effect on germinant emergence rates, but germinants under shrubs succumbed to desiccation more slowly. Nearly 4 of 5 germinants that survived the first summer were beneath shrubs. Shrubs reduced solar radiation and reduced soil temperatures, especially during midday, but did not affect relative humidity or air temperature. Shallow burying of seeds to mimic rodent caching greatly improved emergence rates, with nearly 90 percent of all germinants emerging from buried seed.
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