土壌種子バンクの生態

土壌種子バンク、落葉、土壌表面、または多くの生態系の土壌における種子の自然な貯蔵。これは、次の世代の植物が生息するための貯蔵庫として機能します。土壌種子バンクという用語は、特定の地域における単一の種またはすべての種の種子の保管を表すために使用できます。生態系が経験するさまざまなストレス（寒さ、山火事、干ばつ、混乱など）を考えると、多くの場合、種子銀行は多くの植物にとって重要な生存メカニズムであり、生態系の長期安定性を維持します。

種子休眠の役割

種子の休眠と発芽に対する環境の制約は、土壌種子バンクのさまざまな特性に影響を与えます。たとえば、種子の休眠は、種子が土壌中で生存可能な状態を維持できる期間を決定します。胚の未熟性、化学的阻害剤、物理的制約などの要因が種子の休眠に影響します。たとえば、植物のキャノピーでフィルタリングされた光は、一部の種の発芽を阻害する可能性がありますが、長い冬の低温は、他の種の休眠を打破する可能性があります。その結果、季節、混乱、またはその他の環境変化による種子バンクの発芽のパターンはかなり多様になります。

Variation in the characteristics of seed dormancy determine whether a species’s soil seed bank is transient (temporary) or persistent. Transient seed banks are composed of species that produce seeds with a brief or no period of dormancy. Such seeds generally germinate prior to the next round of seed production, and the seed bank is thus continually depleted and reestablished. Transient seed banks are typical for many plants, especially long-lived perennials such as trees and shrubs. Often, such species rely on other strategies or life-history stages for persistence. For example, species may depend on long-lived adults, “banks” of seedlings in a forest understory, or extensive seed dispersal. In contrast, species with persistent seed banks have seeds that can remain dormant for more than a year, meaning that there is always some viable seed in the soil as a reserve.Persistent seed banks are common in annual plants and some woody plants, in which the failure of seed to establish the next generation would mean the collapse of the population. Scientists sometimes further classify persistent seed banks based on the extent or pattern of dormancy.

The role of disturbance

In addition to dormancy, considerable variation occurs in seed bank germination because of seasonal or other environmental shifts. Disturbances such as fire, flooding, windstorms, plowing, or forest clearing are frequently strong selective forces and may increase the overall germination response of seeds. Ecosystems characterized by wildfire often have extreme cases of persistent seed banks, as is common for many areas with Mediterranean climates, such as Australia, California, and South Africa. In those ecosystems the germination of many species requires signals provided by fire, such as a heat pulse into the soil or chemicals from smoke or charred wood. Germination may not occur until after a wildfire, which then results in mass germination from the seed bank the following spring. Similarly, the seed banks of agricultural weeds are often well adapted to the almost continuous human-made disturbances of their environment. Such weeds frequently have complex dormancy patterns that reflect the agricultural practices under which they evolved.

Seed bank modeling

Researcher Dan Cohen was one of the first scientists to model soil seed banks. In the 1960s, focusing on desert annuals subject to highly irregular rainfall, he developed population-dynamics models that suggested that a reserve of some fraction of seed in the soil was essential for the plants to avoid local extinction. Cohen found that the dynamics of soil seed banks reflect the degree of ecological constraint a species or population faces in establishing the next generation. Although his work focused on annuals, the conceptual framework applies readily to any plant species. Such modeling is important to ecological research and conservation planning, as traditional demographic models and field surveys often fail to consider population reserves in the soil.