Understanding limitations to invasions by non-native plants at high latitudes

非本地植物的入侵是北美温带地区的一个严重问题。相比之下，北方寒带森林相对来说没有受到入侵，冻土带几乎没有入侵植物。很少有研究试图确定导致北方生态系统排斥外来物种的因素，或者了解未来的环境变化是否会增加它们对入侵的脆弱性。非本地植物向极地的分布范围可能受到诸如恶劣环境条件等障碍的限制，也可能受到竞争或草食能力下降的限制，或者仅仅是由于有限的传播。城镇选址可以潜在地缓解这些因素。频繁的引进和繁殖体的本地迁移可能有助于维持非本地种群。土壤可以通过养分的富集和土壤生物群的变化而得到改善。人类的活动和结构可能会提供庇护所，并创造更温暖的小气候。反复的干扰可能会为本地植物的建立创造场地，并减少本地植物的竞争。最后，城镇可以为本地食草动物提供避难所。马尼托巴省的丘吉尔（北纬58.5度），在北方/苔原的边界上，为研究北方入侵提供了一个独特的机会。城镇和相关地点记录了一百多种非本地植物，反映了该地区欧洲人居住的悠久历史及其作为交通枢纽的作用，特别是小麦和其他种子作物。许多物种能够存活数十年，但至关重要的是，几乎没有物种入侵过周围的自然区域。我建议调查限制在定居地区出现的外来物种的因素，并将它们排除在附近的冻土带和树线栖息地之外。为了做到这一点，我将在丘吉尔镇及其附近的被入侵地区以及附近未被入侵的北方和苔原环境中进行一系列实地观察和实验。我的实验室将使用移植物、普通花园和实验操作来测试入侵是否受到当地建立的入侵者成功分散到新地点的失败的限制，包括温度和土壤养分在内的环境因素是否可能阻止扩散后的成功建立，以及包括草食、竞争和土壤反馈在内的生物相互作用是否可能阻止建立，即使物理条件有利。我们还将调查城镇和其他居民区的条件是否能够消除这些障碍。结果将有助于确定北方入侵是否是一个新出现的问题，并预测气候变化是否会消除入侵者进一步传播的障碍。

Invasions by non-native plants are a serious problem throughout temperate North America. In contrast, northern boreal forests are relatively uninvaded, and the tundra remains almost free of invasive plants. Few studies have attempted to identify the factors responsible for the exclusion of non-natives from northern ecosystems, or to understand whether future environmental changes may increase their vulnerability to invasion. Poleward range limits of non-native plants may be set by barriers such as harsh environmental conditions, by reduced ability to tolerate competition or herbivory, or simply by limited dispersal. Townsites potentially can alleviate many of these factors. Frequent introduction and local movement of propagules may help to maintain populations of non-natives. Soils may be improved by nutrient enrichment and changes in soil biota. Human activities and structures may provide shelter and create warmer microclimates. Repeated disturbance may create sites for establishment and reduce competition by native plants. Finally, towns may provide refuges from native herbivores. Churchill, Manitoba (58.5º N), at the boreal/tundra boundary, provides a unique opportunity to study northern invasions. Over a hundred non-native plants have been recorded in town and associated sites, reflecting the area's long history of European occupancy and its role as a transportation hub, particularly for wheat and other seed crops. Many species have been able to persist for decades, but crucially almost none have invaded surrounding natural areas. I propose to investigate factors restricting locally-occurring non-natives to settled areas, and excluding them from nearby tundra and treeline habitats. To do this, I will combine a series of field observations and experiments in invaded areas in and near the Churchill townsite and in nearby uninvaded boreal and tundra environments. My lab will use transplants, common gardens, and experimental manipulations to test whether invasion is limited by failure of locally established invaders to successfully disperse to new sites, whether environmental factors including temperature, and soil nutrients may prevent successful establishment following dispersal, and whether biotic interactions including herbivory, competition, and soil feedback may prevent establishment even if physical conditions are favourable. We also will investigate whether conditions in town and other inhabited areas can remove these barriers. Results will help to determine whether northern invasions are an emerging problem, and to predict whether climate change will remove barriers to further spread of invaders.