Biology of Dittrichia graveolens (Stinkwort): Implications for Management.

摘要

Dittrichia graveolens (stinkwort) is an annual plant in the family Asteraceae. It is native to the Mediterranean basin and has invaded other regions of the world with similar climates, including California. This plant has become a management concern in California primarily due to its rapid spread at a statewide level and unknown potential for economic and environmental impacts. Initial efforts to control the spread of D. graveolens have had variable success; resource managers have encountered some difficulty developing effective methods and timing of management. This difficulty is directly related to the absence of scientific data on basic plant biology, life cycle, and life history traits. To address the need for targeted, effective management methods and a better understanding of the potential for spread in California, the research in support of this Masters of Science thesis investigated the biology of D. graveolens at multiple scales. With the information gained through this research, we have the basic tools and knowledge to begin to develop and test effective management strategies.;Field, laboratory, and greenhouse studies were employed to examine and describe the biological characteristics of D. graveolens at all life stages. Firstly, seed and germination biology were investigated for two seed collections: 2010 and 2011. Seed viability was very high in both years and was comparable to total germination at optimal temperatures, indicating a likely absence of primary dormancy. Seedling emergence in the field corresponded with periods of precipitation from fall to spring annually, and total emergence was not significantly lower in a below average precipitation year when compared with an average precipitation year. Seedling emergence was not affected by light level and the base temperature for germination was 4-6.5°C. Secondly, plant growth and phenology were examined in the field and greenhouse. The majority of canopy growth occurred during the summer months (June-September) although total plant growth was lower in the year with minimal late spring precipitation. Total plant growth was greatly reduced in low light environments indicating that D. graveolens is not expected to be a strong competitor in shaded forest and woodland ecosystems. Bolting time was significantly delayed in low light; however, flowering occurred in early-mid September for plants in all light environments and in both field and greenhouse conditions. This result suggests that reproductive growth is a response to photoperiod, making it a predictable component of management planning. Root growth of D. graveolens occurred later in the spring than common grassland annual species Centaurea solstitialis (yellow starthistle), Holocarpha virgata (virgate tarweed), and Bromus hordeaceus (soft brome), suggesting that it may not be competitive in grassland systems when late-spring soil moisture is limited. Lastly, an estimation of the population growth rate of D. graveolens in disturbed upland areas without density dependence shows populations growing extremely rapidly with a very high stable growth rate. The adult plant life stage contributed the most to this high population growth rate in the form of very high annual seed production; other life stages contributed only minimally. Individual plants produced 4,000-90,000 seeds during the seed production period (September-December, 2011), depending on plant size. After nine months in the soil seed bank, 39% of seeds remained viable and an equal number germinated readily in the lab. Induced dormancy is therefore likely absent for seeds near the soil surface. Based on these results, we can expect D. graveolens to continue to spread rapidly in California---specifically in high light environments with late-spring soil moisture. However, equipped with the knowledge of plant biology, environmental conditions that limit growth and development, and the life stages that are most sensitive to management, our ability to develop effective management strategies is greatly improved.