Naturalized plants - Naturalized plants

This refers to plants that have been transferred from their original habitat to another area by human intervention and have become naturalized and reproduced there. As an island nation, Japan has many naturalized plants that have been introduced from foreign countries and become naturalized over the years for various reasons. There are also known examples of naturalization from other continents in Europe and the United States.

Naturalized plants may escape after being intentionally introduced for cultivation or may be introduced unintentionally when their seeds or fruits (seeds) attach to or become mixed with other objects. In either case, naturalized plants must have a correspondingly strong survival ability to join the population of native plants in Japan. Seeds that are fortunate enough to be introduced germinate and grow, completing their life cycle (primary naturalization; if they do not complete their life cycle, they are called provisional naturalization), and then expand their range of distribution (secondary naturalization). Primary naturalization sites resulting from unintentional seed introduction are varied, including ports, freight station premises, chicken farms, rice mills, and ranches. Naturalized plants with strong indigenous survival abilities can expand their range of distribution further by secondary naturalization in places that have been artificially influenced, such as roadsides, gardens, fallow fields, and reclaimed land.

Solidago altissima, which has grown abundantly throughout Japan in recent years, is a good example of a naturalized plant (its native place is Alabama in North America, where it is the state flower). It reproduces well locally, is classified into many species, is highly variable, its pollen is anemophilous, and its many seeds are dispersed by wind. In addition, its underground stems have a strong ability for vegetative reproduction, and release allelopathic substances that are harmful to other plants. These characteristics are likely to be advantageous for life in new lands. In general, naturalized plants tend to be higher-order polyploid plants compared to native species, cytologically speaking.

Naturalized plants that have become naturalized in new lands only reach the early stage of community succession (secondary succession), and eventually disappear when they reach the stage of perennial herbs such as Miscanthus or the stage of sun-loving trees.

[Kazuo Kotaki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

本来の生育地から、人間の媒介によって他の地域に移され、そこで野生化して繁殖する植物をいう。島国である日本には、さまざまな原因で長い年月の間に、外国から渡来し、野生化した帰化植物がある。欧米などでも他の大陸からの帰化の例が同様に知られている。

　帰化植物には、栽培などの目的で意識的に移入した植物が逸出する場合と、種子や果実（種実）が他物に付着したり混入したりして、無意識的に移入される場合とがあるが、いずれにしても、帰化植物が日本在来の植物集団のなかに仲間入りするには、それ相応の強い生活力をもたねばならない。移入の機会に恵まれた種子は、発芽成長して生活環を全うし（一次帰化。生活環を全うしない場合は仮帰化）、さらに分布域を拡大する（二次帰化）。無意識的な種子移入による一次帰化地は、港、貨物駅構内、養鶏場、精米所、牧場など、さまざまである。旺盛(おうせい)な土着能力をもつ帰化植物は、路傍、庭、休耕している田畑、造成地など、人為的な影響を受けた場所を二次帰化地として、さらに分布圏を広げていく。

　近年、日本各地に繁茂しているセイタカアワダチソウは帰化植物のよい例である（原産地は北アメリカのアラバマ州であり、州花とされている）。現地ではよく繁殖し、多くの種に分類され、変異性に富み、花粉は風媒され、多数の種子は風で散布される。また、地下茎は強い栄養繁殖の能力をもち、他の植物に有害となる他感作用物質を出す。このような性質が、新しい土地での生活に有利なのであろう。一般的に帰化植物は、細胞学的には在来種に比べて高次の倍数体植物が多いという。

　新しい土地に野生化した帰化植物も、群落遷移の初期（二次遷移）の段階までのことで、やがてススキなどの多年生草本期や陽樹期になると消滅する。

［小滝一夫］

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