Seed germination

The seeds of tropical rain-forest trees show a large interspecific range of time taken to germinate. For a sample of 330 species from the forests of Malaysia, 65% of species showed germination within 20 weeks of sowing fresh seed in a lightly shaded nursery (Ng 1980). The 35% of species that took longer than 20 weeks to germinate often had hard, thick seed coats or endocarps around the seed. Studies of the physiology of germination have shown that a number of factors can cause delayed germination in tropical rain-forest tree seed (Vazquez-Yanes & Orozco-Segovia 1993). These include low water content of seed at maturity, presence of a hard seed coat, small size and early stage of development of the embryo and the presence of chemical germination inhibitors. Rapid germinators tend to have high seed water content at maturity and soft seed coats. Many such species are difficult to store. They lose viability if dried and germinate if stored at high water content. This has led seed physiologists to develop a classification system based on the potential of seeds to be stored. Orthodox seeds (Roberts 1973) are those that can be stored in a dormant state for a relatively long period (many months), usually at relatively low seed water content. Recalcitrant seeds are the rapid germinators that cannot withstand drying. Many tropical tree species produce large, soft-coated seeds of high water content that are recalcitrant.

The seed dormancy of more orthodox species can be brought about in a number of ways. Hard and impermeable seed coats can physically prevent germination, the embryo may need to develop before germination can take place, or environmental conditions may induce dormancy. Seeds with hard coats, or persistent endocarps, may need scarification from animals or environmental factors before germination will take place. Heat may be another factor involved. The tiny seeds of balsa, Ochroma pyramidale, remain dormant because of an impermeable testa. The heat of superficial fires or the temperature fluctuations associated with the microclimate of large gaps can cause rupture of the testa, allowing germination to proceed. Shade appears to be the most important inducer of dormancy in the seeds of tropical trees. Photoblastic seeds are responsive to the spectral composition of the impinging radiation. It is generally accepted that phytochrome is the chemical that mediates photoblasty in seeds (Vazquez-Yanes & Orozco-Segovia 1996a).

Brain Cancer Rates Over Time
Figure 5.4 Germination of Cecropia obtusifolia along a red : far red (R/FR) ratio gradient. Bars represent ±1 SD. After Vazquez-Yanes et al. (1990).

Red light of wavelengths around 660 nm stimulates the formation of a physiologically active form of phytochrome that triggers germination in seeds. Far-red light around the 730 nm waveband initiates the conversion of the active to the inactive form of phytochrome, and causes dormancy. The relative fluence rate of the red and far-red wavebands (R: FR) is the critical spectral quality trigger for dormancy or germination in photoblastic seeds. The germination response of Cecropia obtusifolia seeds to variation in R: FR is shown in Fig. 5.4. Different microsites in the forest can differ substantially in their R: FR. Canopy leaves absorb red light, leaving the forest understorey relatively enriched in far-red wavelengths. Thus forest shade induces dormancy in photoblastic seeds, but canopy gaps can trigger germination. Leaf litter and soil also have a selective spectral absorptance that tends to reduce the R: FR of light transmitted through them. Sunflecks, brief periods of direct illumination by sunlight, occur at irregular intervals at most sites on the forest floor. These may be sufficient to trigger germination because of their high R: FR, but photoreversion due to the re-establishment of the low R: FR of the shade light in the forest understorey will probably re-implement dormancy before germination takes place. Tropical trees with photoblastic seeds show considerable variation between species, and sometimes within species, in the germination responses to the natural range of R: FR occurring in the forest (Vazquez-Yanes & Orozco-Segovia 1996a; Metcalfe 1996). Light may not be the sole trigger to germination in dormant seeds. There is evidence of an interaction of light with temperature in some species. For instance, Orozco-Segovia et al. (1987) found that seeds of Urera caracasana required exposure to 4 h of white light at 25 °C for high rates of germination, but at 35 °C 30 min of light was sufficient to achieve comparable germination percentages.

Potentially, environmentally induced dormancy can allow seeds to remain viable in the rain-forest soil for long periods. Fully imbibed seeds of several 'pioneer' species from Mexico retained high viability for more than 5 years when stored in darkness in the laboratory (Fig. 5.5). Seeds of Mallotus paniculatus showed no loss of viability after 3 years buried in mesh bags in a Malaysian rain forest (Kanzaki et al. 1997). There is some evidence that seeds may change in their environmental requirements for germination with time

Rain And Seeds Germination

Figure 5.5 Germination after different storage periods of fully imbibed seeds in darkness. After 3 years the germinability of dry seeds of the same species was near zero in all of them. The arrow indicates when the seeds of all the species in dry storage at room temperature lost all germinability. SD indicated by vertical bars. After Vazquez-Yanes & Orozco-Segovia (1996b).

Figure 5.5 Germination after different storage periods of fully imbibed seeds in darkness. After 3 years the germinability of dry seeds of the same species was near zero in all of them. The arrow indicates when the seeds of all the species in dry storage at room temperature lost all germinability. SD indicated by vertical bars. After Vazquez-Yanes & Orozco-Segovia (1996b).

buried in the soil (Vazquez-Yanes & Orozco-Segovia 1996a), tending to become more prone to germination in conditions that would have previously maintained dormancy.

The ability of some species to remain dormant in the shade and germinate only in direct sunlight has been recognised as a characteristic uniting a group of tropical tree species of similar ecology. Swaine & Whitmore (1988) used this as the key character to define 'pioneer' species. These are the fast-growing, shade-intolerant species typically found only in gaps or other early successional sites in the forest. The more shade-tolerant species have germination that is generally not dependent on degree of shading. Swaine & Whit-more (1988) referred to these species as 'non-pioneers'. There have been few attempts to test the validity of the dichotomy proposed by Swaine & Whit-more. Raich & Gong (1990) investigated the germination of 43 Malaysian tree species in clearing, gap and forest understorey sites of 60%, 40% and 1.2% full-sun PAR. Only seven species germinated equally well in all sites. In the clearing, 16 species showed low rates of germination, or failed completely; 22 species germinated at higher rates in the understorey than the clearing and 12 species germinated better in one or other of the open sites than they did in the understorey. Some species germinated little in the understorey, but did so readily when these seeds were transferred to the clearing (Fig. 5.6). The clearing conditions killed seeds of a number of species. Raich & Gong (1990) argued that a clear dichotomy of species on germination response was not easy to discern. A more complex pattern of seed environmental responses was seen. This was also the picture to emerge from a study of 19 species from tropical West Africa (Kyereh et al. 1999). Only three species germinated in significantly lower proportions in complete darkness than a light treatment and only one species was found to have a germination response to variation in R : FR. All the species showed some germination in forest understorey conditions. The sample included several species that are strongly light-demanding such as Ceibapentandra and Ricinodendron heudelotii.

A thick shell around a seed need not necessarily delay germination. In Mezzettia parviflora the seed has a woody covering 3-4 mm thick, derived from the middle integument (Lucas et al. 1991b). This protects the large seed from most attackers. Orang utans can just about open some seeds with their teeth. Yet the Mezzettia seedling can break out of the seed and germinate very rapidly. This is because of a special band of brittle brachysclereids (stone cells) running around the shell and a small plug through the wall at one end. This provides a built-in weakness to the shell that allows the turgor pressure of the seed to open it. Mammalian seed-eaters are not assisted by this design because the plug and band are too narrow for their teeth to exploit. Seed-eating beetles are so small that they operate at a scale where shell hardness is more important than strength. The weak band does not differ appreciably in hardness from the rest of the shell so again the seed predators cannot exploit it.

The big seeds of Cavanillesia platanifolia contain large quantities (27%) of mucilage (Garwood 1985). The mucilage takes up water very readily and rapidly (7g water g~ fruit in 10 min). This probably assists the seed to germinate and the seedling to establish in the seasonally dry forest on Barro Colorado Island, Panama.

Delaying germination

Why is there this large range in time taken to germinate among species? The main advantage of waiting is to increase the likelihood of secondary dispersal and possibly to stagger germination allowing some seeds to escape unfavourable periods, such as droughts, to which they would have succumbed as seedlings. The disadvantage of waiting is seed mortality. Rapid germinators avoid seed predators by soon becoming seedlings, although some seed eaters will also attack the cotyledons of seedlings. Predator satiation may be facilitated by rapid germination because it means less time for the predators to exploit the seeds. Seeds that possess mechanisms to detect the quality of the environment and germinate or remain dormant accordingly have the benefit of both delayed and rapid germination options.

Timing of germination

Barro Colorado Island, Panama, has quite a marked dry season each year. The species in the rain forest showed three main patterns of timing of germination with respect to the climatic seasonality (Garwood 1983): 42% of species dispersed their seed during the dry season and remained dormant until the rains started and germination took place; 40% of species dispersed and germinated in the wet season; and 18% of species dispersed in one rainy season but did not germinate until the next.

Germination and litter

As already mentioned above, forest leaf litter can act as a selective filter of light, transmitting a spectral composition of reduced R : FR, which can influence the germination of seeds. Litter can also act as a physical barrier to seedling establishment. Small seedlings may not be able to push leaves out of the way and will die in the low-light conditions under the litter. For three tree species, there was a direct correlation between seed size and germination success through litter in the laboratory (Vazquez-Yanes & Orozco-Segovia 1992). Under shade-house conditions, litter layers were found negatively to influence germination and emergence of seeds of 'shade-intolerant' species more than 'shade-tolerant' ones (Molofsky & Augspurger 1992). However, there was a considerable range of susceptibility to litter suppression among the 'shade-intolerant' species. In high-light conditions, litter was actually beneficial to some species such as Gustavia superba. The seeds were probably cooler and moister than they would have been without litter present. Litter in gaps may facilitate colonisation by more shade-tolerant species. In a Puerto Rican forest, litter removal was found to increase seedling density significantly (Guzman-Grajales & Walker 1991). The increase came largely from small-seeded species such as Cecropia schreberiana and Chionanthus domingensis. Large-seeded species were either unaffected by litter manipulation, or became rarer in litter-removal sites. The influence of seed size on germination and seedling establishment has been confirmed by further experiments (Everham et al. 1996). Litter removal and disturbance of the soil surface (scarification) have been reported as increasing the density of germinants by a factor of 2.5 in gaps in lowland dipterocarp forest in Danum Valley, Sabah, Malaysia

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Weeks since sowing

Figure 5.6 Cumulative germination over time for Trema tomentosa (Ul-maceae) in a forest-understorey, gap and clearing site. One basin of seeds (F to C) was moved from the forest understorey into the clearing on week 15; a second basin was left in the understorey. All basins contained 150 seeds. Note that the y-axis extends below zero to indicate clearly that no germination occurred in the forest understorey. Germination was recorded for 28 weeks, but no germination occurred after 25 weeks. Final germination was significantly different among all treatments. After Raich & Gong (1990).

(Kennedy & Swaine 1992). These germinants represented only a small fraction of the seed bank of the uppermost layer (0-5 cm) of the soil, hardly more than 5%, even with scarification. Similar results were obtained in forest in Singapore (Metcalfe & Turner 1998).

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