Pattison Robert R, Mack Richard N
School of Biological Sciences, Washington State University, Pullman, WA, 99164, USA.
Oecologia. 2009 Jan;158(4):591-602. doi: 10.1007/s00442-008-1187-7. Epub 2008 Oct 25.
Identifying the environmental constraints that affect the distribution of an invasive species is fundamental to its effective control. Triadica sebifera (Chinese tallow tree) has invaded the southeastern United States, but its potential for further range and habitat extension has been unresolved. We explored experimentally environmental factors in macro- and microhabitats that affect its persistence at five widely separated sites along the Atlantic seaboard of the United States and at two sites inland; three sites occur well beyond the tree's current range. At each site, seeds and young vegetative plants (0.5-0.65 m tall) of T. sebifera were placed in four microhabitats (closed-canopy upland, closed-canopy lowland, open-canopy upland, and open-canopy lowland). Plant growth, leaf CO(2) assimilation rates, leaf N concentrations and delta(13)C ratios, and stem water potential were measured for two growing seasons. Percent seed germination was consistently higher in open-canopy microhabitats and lowest at northern and inland sites. T. sebifera grew in all open-canopy microhabitats, even 300-500 km beyond its current distribution. Plant growth in closed-canopy habitats was lower, attributable to lower carbon gain per unit leaf area in shaded compared with open-canopy environments, especially at northern and inland sites. Neither competition, other than canopy shade, nor grazing was a key constraint on distribution at any scale. Our results demonstrate that T. sebifera is dispersal limited at landscape scales but limited locally by dispersal and overstory shade; it has yet to occupy the full extent of its new range in North America. Quantifying environmental factors both within and well beyond a species' current range can effectively highlight the limits on its distribution.
确定影响入侵物种分布的环境限制因素是对其进行有效控制的基础。乌桕已入侵美国东南部，但它进一步扩大分布范围和栖息地的潜力仍不明确。我们通过实验探究了宏观和微观栖息地中的环境因素，这些因素在美国大西洋沿岸五个相距甚远的地点以及两个内陆地点影响着乌桕的存活情况；其中三个地点位于该树种目前分布范围之外。在每个地点，将乌桕的种子和幼龄营养植株（高0.5 - 0.65米）放置在四种微观栖息地中（郁闭冠层高地、郁闭冠层低地、开阔冠层高地和开阔冠层低地）。在两个生长季节中测量了植物生长、叶片二氧化碳同化率、叶片氮浓度和δ¹³C比值以及茎水势。开阔冠层微观栖息地中的种子发芽率始终较高，而在北部和内陆地点最低。乌桕在所有开阔冠层微观栖息地中都能生长，甚至在其当前分布范围以外300 - 500公里的地方也能生长。郁闭冠层栖息地中的植物生长较慢，这是由于与开阔冠层环境相比，遮荫环境下单位叶面积的碳获取量较低，尤其是在北部和内陆地点。除了冠层遮荫外，竞争和放牧在任何尺度上都不是分布的关键限制因素。我们的结果表明，乌桕在景观尺度上受扩散限制，但在局部受扩散和上层遮荫限制；它尚未占据其在北美新分布范围的全部区域。量化物种当前分布范围内外的环境因素能够有效地突出其分布的限制因素。
