Regional and Historical Variation in Garlic Mustard Distribution in Western Massachusetts 2009-2010

The susceptibility of a site to invasion by nonnative species depends on its current ecological features and its historical land use. Certain environments might be more conducive to an invasive plant’s success, and several recent studies have shown that former agricultural sites are more susceptible to invasion than sites that have been continuously wooded. We studied the invasive herb garlic mustard (Alliaria petiolata), in two regions with distinct ecological characteristics (the Connecticut River Valley and the Housatonic River Valley in Massachusetts), and two historical land uses (wooded versus cleared in 1830). We surveyed the presence and absence of garlic mustard in randomly selected 100 x 25 m forested sites along road edges and determined 1830 land use from historical records. A significantly (p less than10^-10) higher proportion of the random survey sites contained garlic mustard in the Housatonic River Valley than in the Connecticut River Valley, while there was only a marginally significant (p=.057) difference in garlic mustard presence among sites with different 1830 land use. Among sites that contained garlic mustard - both random survey sites and 9 previously known sites - ANOVA showed that land use history, but not ecoregion, was a significant determining factor in the amount of garlic mustard present, with bigger populations in sites that were open in 1830. Thus, the ecoregion appears to primarily determine the number of sites colonized, while land use history determines performance within colonized sites. This suggests that for garlic mustard, the role of 1830 land use is not in facilitating initial establishment, but in serving as a cause or indicator of environmental quality for proliferation of the invasion. It also suggests that mechanisms of dispersal may differ in the two regions. Further studies should be conducted to determine what aspects of ecology and historical land use are relevant the success of garlic mustard.

一个地点被外来物种入侵的易感性，取决于其当前的生态特征与历史土地利用状况。部分环境可能更利于入侵植物成功定植，近期多项研究表明，既往农业用地相较持续覆盖林木的区域，更易遭受生物入侵。
本研究以入侵草本植物蒜芥（Alliaria petiolata）为研究对象，选取马萨诸塞州两个生态特征迥异的区域——康涅狄格河谷与霍萨托尼克河谷，以及两种历史土地利用类型（1830年为林地vs 1830年已被开垦）开展研究。我们沿道路边缘随机选取100×25米的林地样点，调查蒜芥的存在与否，并通过历史档案确定各点位1830年的土地利用类型。
结果显示，霍萨托尼克河谷的随机调查样点中，蒜芥检出率显著高于康涅狄格河谷（p<10^-10）；而不同1830年土地利用类型样点的蒜芥检出率仅存在边际显著差异（p=0.057）。在所有检出蒜芥的样点（含随机调查样点与9处已知样点）中，方差分析（ANOVA）结果表明，土地利用历史而非生态区域，是影响蒜芥种群规模的显著因素：1830年曾被开垦的样点内，蒜芥种群规模更大。
由此可见，生态区域主要决定了被入侵样点的数量，而土地利用历史则决定了入侵物种在已定植样点内的种群表现。这表明，对于蒜芥而言，1830年的土地利用并未促进其初始定植，而是作为入侵种群增殖所需环境质量的成因或指示因子发挥作用。研究同时发现，两个区域的入侵扩散机制可能存在差异。未来需开展进一步研究，以明确影响蒜芥入侵成功的生态与历史土地利用相关因子。