Seedling mortality

The mortality of tree seedlings in the tropical rain forest is generally high, particularly for the new germinants. Many surveys of seedling population dynamics may well miss much of the earliest, and most intense, mortality because monitoring tiny seedlings and identifying them correctly is very difficult (Kennedy & Swaine 1992). A cohort of germinants often declines in numbers in a log-linear fashion, at least initially. That is, a straight line of negative gradient is produced by plotting seedling number on a logarithmic scale against time plotted linearly (Fig. 5.9). Li et al. (1996) have followed annual seedling cohorts at La Selva, Costa Rica, starting some 2-3 months after the peak of germination in the forest. The mean half-life of the multi-species cohorts was 2.49 months. Survivorship was highest for cohorts establishing during wet periods. Cohort half-lives for individual species varied from 0.37 to 40.7 months. Species such as Cecropia obtusifolia and C. insignis had very short half-lives. Large-seeded palms like Socratea durissima and Welfia georgii were among the most persistent seedlings. There was a negative relation between seedling recruitment and seedling persistence across the species (Fig. 5.10). In other words, there was no species that was very abundant as a new germinant that had a high seedling survival. Survivorship appears to improve as seedlings get larger. Among three common tree species on Barro Colorado Island, Panama, seedlings greater than 50 cm tall showed annual survival of more than 80% (De Steven 1994).

Small seedlings on the forest floor are open to may possible causes of death. These include the following.

Breakage due to falling debris or large animals There is a continual rain of leaves, twigs, branches and trunks onto the forest floor. A small seedling is susceptible to damage, which may often be

Figure 5.9 The fraction of all tagged seedlings of nine species surviving through one year under shaded conditions on Barro Colorado Island, Panama. The fraction is based on a summation of seedlings surviving at all distances from the parent. An equal number of seedlings were observed at each distance interval from the parent. After Augspurger (1984).

Figure 5.9 The fraction of all tagged seedlings of nine species surviving through one year under shaded conditions on Barro Colorado Island, Panama. The fraction is based on a summation of seedlings surviving at all distances from the parent. An equal number of seedlings were observed at each distance interval from the parent. After Augspurger (1984).

Figure 5.10 Relation between seedling abundance and survivorship among the 75 most abundant species (five or more germinated seedlings) at La Selva, Costa Rica. Longer half-lives are associated with lower seedling recruitment over the 18month period. After Li et al. (1996).

fatal, from many of these falling objects (Aide 1987). In addition, large mammals (elephants, deer, pigs, human beings) will crush forest-floor vegetation as they move around. One of the major advantages to a seedling of growing larger is to become less at risk of damage by falling litter or animal activity. Clark & Clark (1989) used artificial seedlings made of plastic drinking straws and wire to assess seedling damage rates in the forest at La Selva. In one year, 82% of the seedlings were knocked over, flattened or uprooted. In the 49% of cases where the agent of destruction was identifiable, roughly half were due to falling debris and the other half to trampling and uprooting by vertebrates. Guariguata (1998) studied saplings 1-2.5 m tall of four species with an erect, unbranched architecture on Barro Colorado Island. About 3% per year of the saplings suffered damage. Over a four-year period mortality resulting from saplings being pinned to the forest floor was almost double that due to loss of the upper half of the sapling by breakage. Alseis blackiana, the most resilient of the species studied, was able to produce adventitious roots on the bent saplings. Interestingly, it was the most slow-growing of the four species and therefore would have the longest average residence time in the understorey.

Saplings may require to be more robustly designed than later stages to withstand falling debris; this idea fits with studies of safety factors calculated from allometric analyses of height-diameter relations (King 1996; Claussen &

Maycock 1995; Rich et al. 1986). However, this approach to design safety may be flawed because saplings tend to invest heavily in foliage, making crown mass a large proportion of total mass. Saplings may not therefore be as safe as they appear from trunk allometry; King (1987), employing a different approach, seems to have confirmed this. He used weights to load the crowns of treelets and saplings of canopy species at La Selva, Costa Rica. The results of the experiment led to the conclusion that the plants had stem diameters only 1.3-1.4 times the minimum diameter to prevent instability when growing in the shaded understorey. The absence of strong winds inside the forest probably allows such narrow safety limits.

Seedling predators

A range of different animals will attack seedlings, mostly for food, and cause their death. At the earliest stages seed-eating animals may eat the cotyledons and rob the seedling of its energy reserves. Such activity can also lead to the seedling being pulled up. Herbivorous mammals may eat the whole seedling before it becomes woody. Nest-making wild pigs are a major cause of mortality among large seedlings and saplings in Pasoh Forest, Malaysia. The pigs build a shelter of thin stems in which to raise their young and collect many saplings from the understorey for this purpose. Mammalian herbivory was observed to be a major cause of direct mortality in seedlings of Virola nobilis on Barro Colorado Island, and it also contributed to susceptibility to death due to drought as defoliated plants could not grow sufficiently large root systems to cope with water shortage (Howe 1990). Exclusion of mammals from seedlings in the forest reduces mortality rates (Osunkoya et al. 1992; Molofsky & Fisher 1993).

Pests and disease

Damping-off is a frequent cause of mortality in newly germinated seedlings. Damping-off is a disease caused by soil-borne fungi. It was studied, by means of field and shade-house experiments, by Augspurger & Kelly (1984). Newly germinated seeds of Platypodium elegans suffered density- and distance-dependent mortality from damping-off on Barro Colorado Island. Seeds placed out in the forest at four-times higher density had a greater incidence of damping-off, as did seeds nearer adult trees. Seeds of 18 species of wind-dispersed tree from Barro Colorado Island, were sown at two densities in shade houses imitating small gap (300 mol m" s" PAR) and understorey (17.5 mol m" s" ) conditions. Species varied considerably in their susceptibility to damping-off. Light was found to be more important than seed density in determining likelihood of damping-off, with most mortality in the deep shade. There are probably many other diseases present among populations of tropical trees, but there has been relatively little work conducted in this field. Potentially pathogenic nematodes have been reported from the roots of seedlings of Turraeanthus africana in Ivory Coast (Alexandre 1977) and Dicorynia guianensis in French Guiana (Queneherve et al. 1996).

Drought

The inability of young seedlings to grow very deep roots leaves them particularly susceptible to drought. High seedling mortality in dry spells has been widely reported (Turner 1990a). Mortality among seedlings of Virola nobilis was greatest in the understorey during the dry season on Barro Colorado Island (Fisher et al. 1991). Irrigation of seedlings through the drought resulted in higher survival. Seedlings in gaps survived the dry season best, despite having greater transpirational loads, because they could grow faster in the higher light of the gap and develop more extensive root systems that allowed them better access to water during the dry period.

Shading

The survival and growth of the seedlings of most species is reduced in the deep shade of the forest understorey. This is the topic of a later section of this chapter. For the present, suffice it to say that the low light of the forest understorey may impose such low photosynthetic rates that seedlings barely make a positive carbon gain. A few cloudy days may be sufficient to exhaust resources and kill the seedling.

Competition

The majority of seedlings grow in the forest understorey where they must establish despite being surrounded by many other individuals, most of which are far larger than they are. This is a highly competitive environment, and superior competitors may deprive weaker individuals of resources, reduce their growth rates and possibly even starve them altogether. Resource shortage caused by competition, and in other ways, will increase the susceptibility of seedlings to disease. Proximity to understorey palms negatively influenced survival in seedlings of two species of Inga planted into the forest understorey (Denslow et al. 1991). Coomes & Grubb (1998a) found that trenching had a positive effect on the height growth and leaf production in seedlings in the very infertile caatinga forests of Venezuela. The magnitude of response to trenching was similar in understorey and gap sites (Fig. 5.11).

Seed and seedling mortality are high for most tree species, and in combination with inadequacies in fecundity and dispersal, probably mean that many species are recruitment-limited in the forest.

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