Post-fire Chondrilla juncea and biocontrol at Boise River Wildlife Management Area 2018-2019

Top-down and bottom-up factors affecting invasive populations are rarely considered simultaneously, yet their interactive responses to disturbances and management interventions can be essential to understanding invasion patterns. We evaluated post-fire responses of the exotic perennial forb Chondrilla juncea (rush skeletonweed) and its biocontrol agents to landscape factors and a post-fire combined herbicide (imazapic) and bacteria (Pseudomonas fluorescens strain MB906) treatment that targeted invasive annual grasses in a sagebrush steppe ecosystem. Biocontrol agents released against C. juncea in previous decades included Cystiphora schmidti (gall midge), Aceria chondrillae (gall mite), and Puccinia chondrillina (rust fungus). C. juncea abundance was greater in sprayed than unsprayed plots, and where soils were coarser, slopes faced southwest, solar heat loads and topographic water accumulation were greater, and cover of deep-rooted native perennials was lower. Mite infestation was greater in unsprayed plots, midge infestation was greater at higher elevations on steeper slopes, and midges were more abundant while rust was less abundant on gravelly soils. Biocontrol infestation levels varied considerably between years and could not be predicted in 2019 from 2018 infestation levels. Multiple biocontrol species were often present at the same plots but were rarely present on the same C. juncea individuals. These results suggest that spatial patterns of invasion by C. juncea are related to deep-soil water availability, warmer conditions, and alleviation of competition. Treatments designed to reduce invasive annual grasses may inadvertently release C. juncea by both reducing plant competition for soil resources and affecting biocontrol agent (mite) abundance.

针对入侵种群的自上而下（top-down）与自下而上（bottom-up）调控因子极少被同时纳入考量，然而二者对干扰与管理干预的交互响应，对于解析入侵格局而言至关重要。本研究在蒿灌丛草原（sagebrush steppe）生态系统中，评估了外来多年生杂类草糙毛鸦葱（Chondrilla juncea，俗称rush skeletonweed）及其生物防治因子（biocontrol agents）对火烧干扰的响应，并测试了针对入侵一年生草本的火烧后联合处理方案：除草剂咪唑烟酸（imazapic）与荧光假单胞菌MB906菌株（Pseudomonas fluorescens strain MB906）。此前数十年间针对糙毛鸦葱释放的生物防治因子包括施密特瘿蚊（Cystiphora schmidti，gall midge）、鸦葱瘿螨（Aceria chondrillae，gall mite）以及鸦葱锈菌（Puccinia chondrillina，rust fungus）。糙毛鸦葱的种群多度在喷施处理样地中更高，且在土壤质地较粗、坡向为西南、太阳热负荷与地形集水量更高、深根本土多年生植物盖度更低的样地中更为丰富。瘿螨的寄生率在未喷施样地中更高；瘿蚊的寄生率在陡坡的高海拔区域更高；砾石质土壤中瘿蚊种群更为丰富，而锈菌的寄生率则更低。生物防治因子的寄生水平年际差异显著，无法通过2018年的寄生水平预测2019年的寄生情况。同一样地常可检测到多种生物防治因子，但极少有多种生防天敌同时侵染同一株糙毛鸦葱个体。本研究结果表明，糙毛鸦葱的入侵空间格局与深层土壤水分有效性、温度较高的生境以及竞争缓解密切相关。旨在抑制入侵一年生草本的治理措施，可能通过两方面间接促进糙毛鸦葱的入侵：一是降低植物间对土壤资源的竞争，二是影响生防天敌（瘿螨）的种群多度。