Propagule Bank and Seedling Establishment

Seed dispersal is deemed successful only if the transported propagules germinate at a potential site or are incorporated into the soil (in which case it will become established at a later time). The stock of all dispersal units in the soil is called the propagule or seed bank, also a seed pool.

Seeds may remain viable for several years, in some cases up to a few centuries, until more favourable conditions for development occur. Seeds from Medicago lupulina have been found to be viable in the soil after 26 years, significantly lower than the seed of Spergula arvensis, which has been shown to maintain viability up to 1700 years (Urbanska 1992). Many propagules reduce metabolic activity before germination. Germination-inhibiting chemical substances may prevent further development. Sometimes temperatures must fall below a certain minimum value (vernalisation) before the seedling can develop. Such periods are called dormancy and may be determined genetically or by external conditions. Many seeds are protected by a thick pericarp, which can be impermeable to water. Dormancy acts as an environmental checklist for the seed, where until all environmental condition (typically light and temperature) cues are checked off, the seed will remain in the soil until conditions are met. The amount of time a seed will remain viable in a seed bank will vary from species to species.

Baskin and Baskin (2001) identified the differences between dormancy types; the first, endogenous dormancy, is where the properties associated with the seed embryo inhibit germination (e.g. physiological inhibiting mechanisms or an undeveloped embryo), whereas exogenous dormancy is described as where the properties of the endosperm or any other tissues of the seed or fruit inhibit germination (e.g. physical, chemical or mechanical constraints on germination and embryo growth).

The period of dormancy is different for different types of propagules. In tropical rainforests, seeds of shade-tolerant trees will tend to germinate immediately if they are found within a suitable growing site. Seeds of shade-tolerant trees often remain dormant until favourable conditions emerge, which will then allow for germination to occur, for example, after a fallen tree opens a gap in the canopy. In dry regions with very variable wet periods, species differ considerably in dormancy, even within individuals of the same species. In general, there is a clear favourability for seeds to be non-dormant in wet tropical rain forests (60% of species are non-dormant), mirroring an environmental gradient of decreasing precipitation and increasing seasonality or uncertainty of rainfall events. Generally, it may be assumed that the dormancy of propagules serves to tune germination to growth conditions that will provide a suitability bridge for a cold winter or a dry season.

A seed bank is usually located in the uppermost 10 cm of soil. Seed banks are made up of two types: temporary and long-term. Temporary propagule banks consist of the accumulation of seeds that will germinate in the short-term (next year). Long-term propagule banks are particularly important for the regeneration of plant communities. Propagules in temporary banks have only limited ability to germinate—as for tropical shade trees—and the regeneration of forests after a severe disturbance is unlikely. This also applies to rainforests in southern Chile outside the tropics. A few years after clearing of the almost natural stands and reforestations of the area with Pinus radiata, the local seed banks contained hardly any propagules of indigenous species (Scherer and Deil 1997). This underlines the importance of seed banks for the protection of species and biotopes.

The phase of seedling establishment is particularly sensitive in the life cycle of plants. Pathogens, herbivores and competitors, but also climatic abnormalities, may lead to large losses. Some seedlings require the protection of neighbouring species (e.g. Arabis hirsuta, Primula viridis), but for others germination is reduced by neighbouring species (Plantago lanceolata, Sanguisorba minor). A third group of species was able to germinate under all experimental conditions (e.g. Medicago lupulina). Large, time- dependent fluctuations have also been observed. There are close relations between rates of germination and interannual fluctuations of climatic conditions, particularly of temperatures. Most species germinate more successfully at higher temperatures, while others germinate better at lower temperatures (Espigares and Peco 1993). In some species of Acer, seed germination can commence only if it experiences a certain number of cool, moist days (known as stratification) near the freezing point to break dormancy and allow for germination to ensue (Solarik et al. 2016).

While the molecular and biochemical bases for dormancy and germination are well known for certain model species under controlled environmental conditions, we still lack a clear understanding of some of the fundamental processes under natural conditions that inhibit or promote seed germination and establishment. The successful establishment of a plant population is only secured by a permanent input of propagules, even if edaphic and climatic conditions at the growing site are suitable, pollinators (when necessary) are present, and the species is able to successfully compete with co-occurring individuals and protect itself against pathogens.

Distribution Patterns.As a result of different dispersal mechanisms, plant species develop distinct spatial patterns of distribution. These patterns are not permanently fixed in space because they depend largely on abiotic and biotic factors, which continuously fluctuate over time (daily, weekly, monthly, seasonally and yearly). Among these factors especially those related to climate, soil and mechanical impacts (flooding, thunderstorms) and—in cultural landscapes—agrochemical and agro- technical influences are important. Accurately identifying the propagule distribution pattern will depend on the spatial scale, where variability from the local-scale small patches to large-scale landscapes must be considered. A several-square- kilometre area with small forest islands, grasslands and bogs will show different patterns when compared with dispersal at finer scales (cm², m²) in the same area, which typically will yield much different results. Today, a species’ distribution is typically assessed by the use of a grid system in analysis. Although results will depend on the size of the grid chosen for analysis, the spatial scale (e.g. “patch”, “local” or “landscape”) becomes essential when considering dispersal patterns.

Distribution patterns will typically fall under three main types: clumped, regular and random distributions. The clumped distribution is the most common type in natural vegetation, mainly because of a close relationship between abiotic and biotic resources associated with plant establishment and growth. As these resources often present a mosaic structure, species and plant communities often become clumped. The introductory photo of Part IV shows an example of clumped distribution in a tropical high-mountain belt with abiotic site factors changing on a large scale. On a smaller scale, spatially aggregated clumped distribution can be the result of a restricted seed shadow, where seed is either dispersed or animals fail to travel far before excreting or dropping the seed. Fangliang et al. (1997) found in the highly diverse tropical rainforests of Malaysia a 4:1 relation of clumped to random distributed tree species, which suggests a strong influence of animal-dispersed seeds in these systems.

A regular distribution occurs when the prop- agule is dispersed evenly over an area, where a comparable amount of seeds are found at long and short distances from the parent tree. This distribution pattern is rarely found in nature. The regular distribution can occur when the environmental conditions across the dispersal distance change in a regular way: the zonation of different plant community changes with a significant change in the landscape. Vegetation along a riverside is an example. The mechanical force of the water during the rainy season and the different water storage capacities of the sediments on the low fluvial terraces with different grain sizes can result in the formation of a regular sequence of different species and communities (Fig. 17.25). Regular patterns are also sometimes observed in tropical dry woodlands (“leopard skin”) if the distance between plant individuals is the result of competition for water, the limiting resource. The introductory photo of Chap. 18 shows an example of regular vegetation patterns at the landscape scale, with an evergreen gallery riverine forest and a seasonally moist dry woodland. Most examples of regular patterns found in nature are actually artificial, for example, in fruit tree plantations or even crop cultivation (introductory photo in Chap. 17).

Although rare, regular spatial patterns are typically determined by animals and wind vectors, they are most important for random distributions. One precondition is that the plant species concerned must be generalists, that is, they have a wide resource-based spatial niche (area where species-specific requirements are met), and the spatial distribution of the environmental resources is rather homogeneous. Abiotic and biotic factors are thus spaced in an unpredictable way, as are the individuals of plant species.