Acer extent of invasion in France & Canada

Quantitative comparisons of distribution and abundance of exotic species in their native and non-native ranges represent a first step when studying invaders. However, this approach is rarely applied ô°¶ particularly to tree species. Using biogeo- graphical contrasts coupled with regional dispersal surveys, we assessed whether two exotic maple tree species, Acer negundo and Acer platanoides, can be classified as invasive in the non-native regions surveyed. We also examined the importance of biogeography in determining the degree of invasion by exotic species using this reciprocal approach. Local-scale surveys were conducted in a total of 34 forests to compare density, relative abundance, age structure of native and introduced populations, and whether the two introduced maple species negatively affected native tree species density. Regional-scale surveys of a total of 136 forests were then conducted to assess distribution in the introduced regions. Introduced popula- tions of A. negundo were denser than populations measured in their native range and negatively related to native tree species density. Age structure did not differ between regions for this species. At the regional scale, this species has invaded most of the riparian corridors sampled in France. Conversely, the density of A. platanoides introduced populations was similar to that of native populations and was not related to native tree species density. Although seedling recruitment was higher away than at home, this species has invaded only 9% of the forests sampled in southern Ontario, Canada. Although reported invasive, these two exotic maple species differed in their relative demographic parameters and regional spread. Acer negundo is currently invasive in southern France while A. platanoides is not aggressively invasive in southern Ontario. Importantly, this study effectively demonstrates that biogeography through structured contrasts provide a direct means to infer invasion of exotic species.针对外来物种在原生分布区与非原生分布区的分布格局与种群丰度开展定量对比，是研究外来入侵物种的首要步骤。然而该方法极少被应用，尤其针对木本植物类群。本研究通过生物地理对比结合区域扩散调查，评估了复叶槭（Acer negundo）与挪威槭（Acer platanoides）这两种外来槭树在调查的非原生分布区内是否可被界定为入侵物种，同时采用这种双向对比方法探究生物地理因素在外来物种入侵程度判定中的重要性。研究团队共在34处森林开展局域尺度调查，用以对比两种外来槭树的原生种群与引入种群的密度、相对丰度及年龄结构，并探究这两种引入槭树是否会对本地树木的种群密度产生负面影响。随后，研究团队又针对136处森林开展区域尺度调查，以评估这两种物种在引入分布区的分布情况。复叶槭的引入种群密度高于其原生分布区的种群密度，且与本地树木种群密度呈显著负相关关系；该物种的年龄结构在原生与非原生分布区间并无显著差异。在区域尺度上，该物种已入侵法国境内绝大多数被调查的河岸廊道生境。与之相反，挪威槭的引入种群密度与原生种群密度相近，且与本地树木种群密度无显著相关关系。尽管挪威槭在非原生分布区的幼苗更新率高于原生分布区，但该物种仅入侵了加拿大安大略省南部被调查森林中的9%。尽管这两种外来槭树均有入侵相关报道，但二者的相对种群动态参数与区域扩散程度存在显著差异。目前复叶槭在法国南部已形成入侵，而挪威槭在安大略省南部并未表现出强入侵性。值得注意的是，本研究有效证明了：通过标准化的生物地理对比方法，可以直接推断外来物种的入侵情况。