Identifying physiological and growth-allocation strategies among varying resource availabilities enabling Scotch broom (Cytisus scoparius (L.) Link) dominance in recently clearcut Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) forests of the western Pacific Northwest

Abstract

Scotch broom is an aggressive, N-fixing invader of recently harvested Douglas-fir forests in the western Pacific Northwest. We expected Scotch broom’s ability to dominate a site and displace planted Douglas-fir to be mediated by available resources and the susceptibility of a recently harvested stand to Scotch broom dominance to vary by site. To further understand the role of N-fixation in Scotch broom’s competitive advantage of Douglas-fir we measured differences in total N-fixation with P fertilization, irrigation, and their interactions. We planted Scotch broom seedlings (n=20) in pots using N-deficient sand as the soil medium in a completely randomized 2x2 factorial. All N in the soil and plant tissue at the conclusion of the study was deemed atmospherically sourced. Although serving different roles within the plant and in the N-fixation process, increased-irrigation and P-fertilization positively affected N-fixation approximately equally. In a controlled nursery setting, Scotch broom (n=46) and Douglas-fir (n=46) seedlings were planted and monitored over two years to test the effects of irrigation and fertilization on the physiology and growth of these oft-conflicting species. Scotch broom consistently showed greater assimilation and transpiration rates and plant water potentials than Douglas-fir under all treatments. Douglas-fir had greater water-use efficiency rates than those of Scotch broom throughout the experiment, however. Douglas-fir growing under optimal conditions did not surpass the growth of Scotch broom. We selected ten sites throughout the western Pacific Northwest that spanned a gradient of soil textures and other factors to test the site-specific susceptibility of Douglas-fir (n=15/site) being overtopped by Scotch broom (n=15/site). We found Scotch broom and Douglas-fir were differentially affected by site-level factors. Douglas-fir absolute height growth rates equaled and eventually surpassed those of Scotch broom as the percent-silt increased in the top 15 cm of the soil profile. In general, Douglas-fir absolute height growth rates were also more competitive with those of Scotch broom on fine-textured soils than more coarsely textured soils. We also found Douglas-fir to have a more dramatic height growth response to increasing DWD than Scotch broom and Scotch broom relative growth had a positive correlation with increasing concentrations of phosphorus in the soil.