REGENERATION AND ESTABLISHMENT
Seeds need gaps in vegetation to germinate. Seeds may be sensitive to quality of light, temperature fluctuations, water regime in the soil, soil consistency and other factors. Different species germinate under different sets of conditions.
Initial growth is dependent on seed size. Larger seeds will grow more quickly than small seeds and there will be selection for the larger seeds. Seedlings from species with small seeds are often light-demanding and epigeal; those from large seeds are shade-tolerant and hypogeal.
This stage is probably the most critical in a plant’s life cycle. Gaps vary enormously in size, shape and how they are formed, and different plant species are favored in particular conditions. The timing of gap formation within a year and between years can influence which species establish. Conditions in small gaps are buffered and many seedlings are likely to appear, resulting in intense competition. In large gaps conditions will be harsher and there will be fewer seedlings. Early germination and seedling growth have great advantages.
Seeds will only germinate if they reach a suitable place. Germination conditions differ greatly between different species. For a seed to germinate and begin to grow there needs to be a gap in the vegetation and in dense plant cover there is almost no regeneration by seed. But gaps are constantly present, caused by the death of established plants, by climatic factors such as frost, wind or flood and by animals through herbivory, trampling or burrowing, defecating, etc. Even in a tropical rainforest gaps are frequent (Fig. 1), one published figure being approximately one per hectare with a mean size of 89 m2. Gap size and the environmental conditions within the gap will vary.
Environmental conditions in a gap are different from conditions under a vegetation canopy and these can be detected by seeds. In a gap, light reaching the soil will not be filtered through leaves to the same extent as under a canopy, giving more light with a different quality. Leaves filter out red light and many seeds respond to the ratio between red and ‘far-red’ light, which is filtered much less, using phytochrome . They germinate when the proportion of red light increases, i.e. when not under a leaf canopy, with different species responding to different levels. Shade-tolerant species do not respond like this. Some seeds respond to daily fluctuations in temperature, which will be greater by 5ºC or more within a gap with less surrounding vegetation as buffer, although certain species will only respond to temperature under particular light regimes. The parameters will also be affected by whether the seed is buried, since soil will buffer fluctuations; there will be more germination near the surface.
Conditions in the soil vary greatly. The water regime and humidity and nutrient conditions must be favorable for each species, many seeds being particularly sensitive to nitrate concentration, only germinating where it is high enough. The type of soil and nature of the disturbance will affect how a seed lies on the soil, e.g. to allow it imbibe water for germination. If a seed is buried, there is likely to be a higher CO2 concentration than in the atmosphere and this can inhibit germination. All the factors mentioned continuously vary. Plant species vary enormously in their sensitivity to the different factors and the interaction between them, leading to great differences in germination conditions for different species.
The speed of initial growth of a seedling will depend on seed size. Larger seeds have more stored nutrients and often a greater proportion of carbohydrate in the seeds, and the seedlings will be able to grow more quickly than seedlings from small seeds. Quicker growth at this early stage can be critical for the seedling to get established ahead of others, so it is likely that there will be selection pressure for increase in seed size within any one species. Between species, seedlings from larger seeds are more shade tolerant and drought tolerant than small ones in the early stages of growth when dependent on the stored nutrients in the seed. Most small seeds are epigeal and the cotyledons are needed as the plant’s first leaves. Many of these species are light demanding pioneers. By contrast, many large seeds are hypogeal, more reliant on the seed’s food stores.
There are exceptions to the generalities outlined here and there is much still to be learned about germination and early seedling growth and the relative advantages of the differences between seeds.
The stage between initial seedling growth and establishment as an adult is the most critical stage in a plant’s life cycle. It is in this stage that it will face the greatest competition from other plants and be most vulnerable to attack by herbivores and pathogens of all kinds. It is key to understanding diversity in plant communities. Many of the dominant plants in a community such as the trees or the grasses that form the sward in a grassland are long-lived, some living for centuries once established, so study of these plants in their critical stage is often impossible.
Gaps in vegetation are diverse (Fig. 1) and which species will establish in a gap will depend on numerous properties of the gap. Most obviously, gaps differ in size. Small gaps, perhaps created by the death of a single large plant, will remain mainly surrounded by vegetation and heavily influenced by shade and by root growth of the surrounding plants. Very large gaps, normally created by human clearance or a natural disaster, may have areas in their centers in which there is no influence of any surrounding vegetation, grading to their edges in partial shade and with roots from neighboring plants. The different gap conditions may be colonized by a whole range of different species. As the gap ages, conditions will change and there will be succession with the new conditions favoring different species.
The timing of the creation of the gap will be critical. In nearly all communities the plants flower and set seed at particular seasons and the degree of dormancy of the seeds is variable. This indicates that a gap created at one time of year may be colonized by a different suite of species from a similar gap formed at another time. In addition, many of the long-lived plants of a community do not set fruit every year, or the fruits vary greatly in their abundance, so different years will differ.
For successful establishment it is vital for a plant to have seeds there at the beginning and for these to germinate and grow quickly. Small gaps are usually colonized by many different individuals and species. The conditions for growth are often favorable, without extremes of temperature or other weather conditions since they are buffered by the surrounding vegetation. There will be a period of intense competition. In a large gap, depending on its nature and formation, fewer seeds may be present and these are more susceptible to weather extremes, particularly drought, so if the seedling can survive its initial growth, competition will be less intense and the resulting survivors more dependent on which seeds were present at the start. Early growth may be critical for survival since the first to grow will overtop later seedlings and differences are likely to increase. There is evidence that the smaller seedlings are more susceptible to herbivore attack, e.g. from molluscs, and less resistant to pathogens too. Differences of one or a few days in germination time can be critical for survival over the subsequent years.