SCOTCH BROOM (CYTISUS SCOPARIUS) AND SOIL NITROGEN: ECOLOGICAL IMPLICATIONS by Jacqueline Shaben B.Sc, University of Victoria, 1999 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (ZOOLOGY) THE UNIVERSITY OF BRITISH COLUMBIA October 2006 © Jacqueline Shaben, 2006 ABSTRACT Scotch broom (Cytisus scoparius), a leguminous shrub with nitrogen-fixing rhizobial root associations, is an invasive plant in the endangered Garry oak ecosystem (GOE) of Southern British Columbia. Broom frequently spreads from disturbed areas such as power line right-of-ways, and thus is a threat to native ecosystems. To unravel the ecological relationship between Cytisus scoparius and soil nitrogen, I conducted two independent studies in 2004-2005. The first study assessed broom's effect on nutrient availability and plant community composition in two Garry oak sites. The second evaluated the impact on seedling recruitment of broom following fertilization with biosolids obtained from sewage treatment. To determine if broom increased soil nitrogen availability, adjacent broom-invaded and non-invaded plots at two GOE sites (Rocky Point and Bamberton) were investigated. Here, soil-nutrient availability was compared using PRS™ ion-exchange membrane probes and concurrent plant surveys. No differences were observed in nitrogen availability between broom-present and -absent plots except for a weak trend of higher NHV" at Rocky Point. By contrast, phosphorus availability at Rocky Point was significantly lower in the broom-present plots. Plant richness was independent of broom presence, however multivariate analysis showed that species identity and abundance differed, with native species declining and introduced species increasing in the broom- invaded plots. I conducted a complementary bioassay in which a native grass and an exotic grass were grown individually either with Cytisus or the native Holodiscus discolor, to test if grasses benefited from the presence of Cytisus. No difference in grass growth between treatment combinations was observed. The second study tested a novel method of broom control. With fertilization trials, I compared the efficacy of sewage biosolids and ammonium nitrate on the suppression of broom seedlings following broom removal. In addition, I conducted a greenhouse germination experiment to explore the mechanism of broom suppression. Broom seedlings were fewest and vegetation biomass tended to be highest in the biosolids plots. The germination experiment indicated no difference in broom germination rate when II grown without plant competition. This suggests that the negative effects of biosolids on broom are due to increased competition from other plants for light and water. The management implications of these two studies are that Cytisus scoparius, though not correlated with dramatic increases in nitrogen availability, may alter phosphorus availability in GOE and thus influence plant composition. Addition of sewage biosolids to disturbed sites however, causes broom seedlings to lose their competitive advantage in dense vegetation that is facilitated by complete nutrient addition from biosolids. Ill TABLE OF CONTENTS ABSTRACT II TABLE OF CONTENTS IV LIST OF TABLES VI LIST OF FIGURES VII ACKNOWLEDGEMENTS . VIII 1. GENERAL INTRODUCTION 1 1.1 INVASIVE PLANTS 1 1.2 STUDY SPECIES - SCOTCH BROOM 2 1.3 CONTROL OF SCOTCH BROOM 2 1.4 SCOTCH BROOM AND THE GARRY OAK ECOSYSTEM 3 1.5 OBJECTIVES 4 1.6 REFERENCES 6 2. IMPACT OF SCOTCH BROOM (CYTISUS SCOPARIUS) ON PLANT COMMUNITY STRUCTURE IN THE GARRY OAK ECOSYSTEM: UNRAVELING THE RELATIONSHIP BETWEEN BROOM, SOIL NUTRIENTS AND PLANT DIVERSITY 7 2.1 INTRODUCTION 7 2.2 METHODS 10 2.2.1 Sites 10 2.2.2 Soil nutrient measurements 13 2.2.3 Broom densities 14 2.2.4 Plant diversity 15 2.2.5 Bioassay 16 2.2.6 Statistical analyses 19 2.3 RESULTS 20 2.3.1 Broom densities/biomass 20 2.3.2 Soil nutrient measurements 23 2.3.3 Plant diversity 31 2.3.4 Bioassay 35 2.4 DISCUSSION 38 2.4.1 Soil nutrient measurements 38 2.4.2 Plant Diversity......; 39 2.4.3 Bioassay : 40 2.5 CONCLUSION 42 2.6 REFERENCES 45 3. POTENTIAL OF FERTILIZER APPLICATION TO SUPPRESS SCOTCH BROOM RE-ESTABLISHMENT 47 3.1 INTRODUCTION 47 3.2 METHODS 50 3.2.1 Sites 50 3.2.2 Site Preparation 51 3.2.3 Biosolids and Application Rate 54 IV 3.2.4 Monitoring broom re-establishment 55 3.2.5 Broom germination experiment 58 3.2.6 Statistical analyses 59 3.3 RESULTS 60 3.3.1 Soils 60 3.3.2 Monitoring broom re-establishment 66 3.3.3 Broom germination experiment 72 3.4 DISCUSSION 73 Caveat 73 3.4.1 Soils 73 3.4.1 Plant Monitoring 75 3.4.3 Germination experiment 77 3.5 CONCLUSION 78 3.6 REFERENCES 79 4. CONCLUSION 83 4.1 REFERENCES 84 APPENDICES 85 APPENDIX A - BIOSOLIDS APPLICATION RATE 85 APPENDIX B - NATIVE SEED MIXES USED 88 APPENDIX C - TRACE ELEMENT APPLICATION RATE 89 APPENDIX D - AMMONIUM NITRATE APPLICATION RATE 92 APPENDIX E - SPECIES LISTS 93 V LIST OF TABLES Table 2-1 Treatment combinations for bioassay and corresponding description of species 17 Table 2.2 Initial characteristics of soil used in bioassay and of reference soil obtained from an intact Garry oak meadow 18 Table 2.3 Scotch broom densities and biomass/m2 at each site for both Rocky Point & Bamberton 21 Table 2.4 Selected initial soil characteristics and nutrient values from composite soil cores taken at Rocky Point and Bamberton in February, 2005 24 1 Table 2.5 Mean values of N03~, NFL," " and P in broom-invaded and un-invaded plots- four sample periods and entire growing season 30 Table 2.6 Average species richness and evenness values at Bamberton and Rocky Point 31 Table 2.7 Pre-bioassay TKN values, and pre- and post-bioassay soil NFL/ and NO3" values in ppm for all four bioassay combinations with native GOE soil values for reference 37 Table 3.1 Initial soil analysis results for the three sites prior to treatment 53 Table 3.2 Percent cover and corresponding cover class as adapted from the Braun- Blanquet percent cover scale 57 1 Table 3.3 Results of soil analyses for NFL;" " and N03" taken 3 to 5 months after initial establishment of site and one year after establishment 61 Table 3.4 Summary of comparisons of soil NFI/, NO3" and TKN changes between pretreatment sampling and the first and second years following fertilization 63 Table 3.5 Cation exchange capacity (meq/lOOg) by treatment at all three sites 1 year following site establishment 66 Table 3.6 Mean percent-cover class of broom for each treatment at three sites 68 Table 3.7 Average relative abundance of grasses, non-native grasses & native shrubs- all three sites, each treatment 69 Table 3.8 Average plant species richness by treatment at each of the three sites 70 VI LIST OF FIGURES Figure 2.1 Map showing locations of two study sites on Southern Vancouver Island. ..11 Figure 2.2 Relationship between average Cytisus biomass (g/m2) and NFLt+ and NO3" availability rates at Rocky Point 22 Figure 2.3 Relationship between average Cytisus density (stems/m2) and nitrogen availability at Rocky Point 22 + Figure 2.4 NH4 availability in broom-invaded and un-invaded plots for each of 4 sample periods at Bamberton and Rocky Point 26 Figure 2.5 NO3" availability in broom-invaded and un-invaded plots for each of 4 sample periods at Bamberton and Rocky Point 27 Figure 2.6 Phosphorus availability in broom-invaded and un-invaded plots for each sample period at Bamberton and Rocky Point 28 Figure 2.7 Average nutrient supply rates for the period of January to June 29 Figure 2.8 Multidimensional scaling ordinations of plant diversity by quadrat at Bamberton 32 Figure 2.9 Multidimensional scaling ordinations of plant diversity by quadrat at Rocky Point 33 Figure 2.10 Summed percent dissimilarity of plant species between un-invaded and broom-invaded plots at Rocky Point. 34 Figure 2.11 Summed percent dissimilarity of plant species between un-invaded and broom-invaded plots at Bamberton 34 Figure 2.12 Mean mass of grasses grown with Cytisus and Holodiscus 36 Figure 3.1 NH/ and N03" availability in ammonium nitrate, biosolid and un-treated plots at Iona Beach, Burnaby Mountain and Duncan 65 Figure 3.2 Mean number of broom seedlings by treatment at all three sites, sampled in July 2005 67 Figure 3.3 Mean percent-cover class of broom for each treatment at all three sites 68 Figure 3.4 Dried vegetation biomass for all three treatments at each of the three sites...71 Figure 3.5 Fates of broom seeds and seedlings under different soil fertilizer treatments 72 VII ACKNOWLEDGEMENTS As solitary as these innumerable hours of thesis writing have been, they have consistently served to remind me of how immensely collaborative my project has actually been. First and foremost, I would like to thank my supervisor, Judy Myers, for accepting me into her lab, encouraging me to stay despite my rough start to grad school and showing confidence in me the whole way through. I also truly appreciate the advice and editing effort that Drs. Art Bomke and Sue Grayston provided me these past three years. Though I am still no entomologist, there is no denying that, under the tutelage of my lab mates, I have come a long way in understanding what goes on in the lab and greenhouses frequented by the Myers crew. Thanks guys, for making me feel welcome. For her great sense of humour, field-work stamina, plant i.d. tenacity and problem- solving genius, I thank Kristen Stevenson for postponing her trip to New Zealand in order to work with me all summer, 2005.