Maron John L, Connors Peter G
Bodega Marine Laboratory, University of California, Box 247, 94923, Bodega Bay, CA, USA.
Oecologia. 1996 Feb;105(3):302-312. doi: 10.1007/BF00328732.
Invasions by exotic weedy plants frequently occur in highly disturbed or otherwise anthropogenically altered habitats. Here we present evidence that, within California coastal prairie, invasion also can be facilitated by a native nitrogen-fixing shrub, bush lupine (Lupinus arboreus). Bush lupines fix nitrogen and grow rapidly, fertilizing the sandy soil with nitrogen-rich litter. The dense lupine canopy blocks light, restricting vegetative growth under bushes. Heavy insect herbivory kills lupines, opening exposed nitrogen-rich sites within the plant community. Eventual re-establishment of lupine occurs because of an abundant and long-lived seed bank. Lupine germination, rapid growth, shading and fertilization of sites, and then death after only a few years, results in a mosaic of nutrient-rich sites that are available to invading species. To determine the role of bush lupine death and nitrogen enrichment in community composition, we examined nutrient dynamics and plant community characteristics within a site only recently colonized by lupine, comparing patches where lupines had recently died or were experimentally killed with adjacent areas lacking lupine. In experimentally killed patches, instantaneous pool sizes of exchangeable ammonium and nitrate nitrogen were higher than in adjacent sites free of lupine. Seedlings of the introduced grass Bromus diandrus accumulated 48% greater root biomass and 93% more shoot biomass when grown in a greenhouse in soil collected under experimentally killed lupines compared to B. diandrus seedlings grown in soil collected at least 1 m away from lupines. At the end of the spring growing season, total above-ground live plant biomass was more than twice as great in dead lupine patches as in the adjacent lupine-free grassland, but dead lupine patches contained 47% fewer plant species and 57% fewer native species. Sites where lupines have repeatedly died and reestablished during recent decades support an interstitial grassland community high in productivity but low in diversity, composed of mostly weedy introduced annual plants. In contrast, at a site only recently colonized by bush lupines, the interstitial grassland consists of a less productive but more diverse set of native and introduced species. We suggest that repeated bouts of lupine germination, establishment, and death can convert a rich native plant community into a less diverse collection of introduced weeds.
外来杂草植物的入侵经常发生在受到高度干扰或其他人为改变的栖息地。在此，我们提供证据表明，在加利福尼亚沿海草原，本地固氮灌木——羽扇豆（Lupinus arboreus）也能促进入侵。羽扇豆能固定氮并快速生长，用富含氮的凋落物给沙质土壤施肥。浓密的羽扇豆树冠遮挡阳光，限制了灌木下植物的营养生长。严重的昆虫啃食会杀死羽扇豆，在植物群落中形成富含氮的裸露区域。由于丰富且寿命长的种子库，羽扇豆最终会重新建立起来。羽扇豆的萌发、快速生长、对场地的遮荫和施肥，然后仅几年后死亡，导致形成了一系列可供入侵物种利用的富含养分的斑块。为了确定羽扇豆死亡和氮富集在群落组成中的作用，我们研究了一个最近才被羽扇豆殖民的场地内的养分动态和植物群落特征，比较了羽扇豆最近死亡或被实验性杀死的斑块与相邻没有羽扇豆的区域。在实验性杀死的斑块中，可交换铵态氮和硝态氮的瞬时库大小高于相邻没有羽扇豆的场地。与在距离羽扇豆至少1米处采集的土壤中生长的二穗短柄草（Bromus diandrus）幼苗相比，在温室中，在实验性杀死的羽扇豆下采集的土壤中生长的引入草种二穗短柄草的根生物量积累多48%，地上生物量多93%。在春季生长季节结束时，死羽扇豆斑块中的地上活植物总生物量是相邻无羽扇豆草地的两倍多，但死羽扇豆斑块中的植物物种数量少47%，本地物种数量少57%。在近几十年里羽扇豆反复死亡和重新建立的场地，支持着一个生产力高但多样性低的间隙性草地群落，该群落主要由杂草性引入一年生植物组成。相比之下，在一个最近才被羽扇豆殖民的场地，间隙性草地由一组生产力较低但更具多样性的本地和引入物种组成。我们认为，羽扇豆反复的萌发、建立和死亡过程会将一个丰富的本地植物群落转变为一个多样性较低的引入杂草集合。
