Box 41 Seedcaching birds

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The Corvidae, a widespread group of rather aggressive birds including the ravens, crows, rooks, jackdaws, magpies, jays, choughs and nutcrackers, show considerable intelligence and the rooks in particular seem to communicate by their calls. The group as a whole has a rather mixed diet, some feeding on carrion, but a number of corvids also form caches of seeds as a winter food supply. In doing so, they are crucial to the establishment of a number of tree species from seed. Most prominent are the 20 or so northern hemisphere pines that are reliant on corvids to disperse their large wingless seeds (Lanner, 1996). These include the pinyon pines, of which there are six in the USA alone. Islands of arolla pine Pinus cembra in the kampfzone of the European Alps, for example, arise from the activities of the nutcracker Nucifraga caryocatactes, which buries heaps of 10-30 seeds as winter food and either doesn't need them all or fails to relocate them. The next generation of trees grows from these ready-planted and undamaged seeds. The numbers of seeds buried by individual birds may be very large. The Clark's nutcracker of the mountains of western North America (N. columbiana) makes small caches of 1-15 seeds of the whitebark pine Pinus albicaulis. Over a season it may bury as many as 98 000 seeds in over 30 000 caches; relocating these is a remarkable feat of memory and it is not surprising that some seeds go unneeded or forgotten. Vander Wall and Balda (1977) found that a population of around 150 Clark's nutcrackers could cache 3.3-5.0 million seeds of the pinyon pine P. monophylla in one autumn, weighing between 658 and 1028 kg!

Corvids are also important in the dispersal of other species. For example, successful colonization by the European oaks is strongly influenced by the size of jay Garrulusglandarius populations; when these were low in the English Midlands in the 1970s oak seedlings were far rarer than they are now, when these birds can often be seen burying acorns in grassland and lawns.

predators by distributing seeds widely. The second phase often moves seeds to safe sites, which usually means below ground, where they are relatively protected from seed predators, and have a good chance of successful germination. Several techniques are showing promise in helping to track seeds as they disperse. These include radioactive labelling of seeds, attachment of fluorescent microspheres (particularly good at following seed dispersal in faecal material from birds), stable isotope analysis (since these naturally vary geographically, it might allow the origin of long-distance seed dispersal to be worked out) and molecular genetic markers, which would allow the matching of seedlings with their parents giving novel information on spatial patterns (see Wang and Smith, 2002 for more details).

A variety of nut-producing trees also depend upon animals (rodents such as squirrels, and birds, especially those in the crow family - corvids) for scatter hoarding, including walnuts Juglans, hickories Carya, oaks Quercus, beeches Fagus, chestnuts Castanea, chinquapins Castanopsis, tanoaks Lithocarpus, hazels Corylus, horse chestnuts Aesculus, and almonds, plums and cherries Prunus. Vander Wall (2001) puts forward evidence that this mutualistic relationship originated as early as the Paleocene, about 60 million years ago. Most nuts appear to have evolved from ancestors with wind-dispersed seeds to become highly nutritious often with a calorific value ranging from 5.7 to 153.5 kJ per nut, 10-1000 times greater than most wind-dispersed seeds.

Despite these mechanisms, many woodland plants, especially herbs in the field layer, are poor dispersers; they tend instead to be clonal (V) plants, have little recruitment from seed and possess no obvious mechanism for longdistance seed dispersal. This has led ecologists to wonder how they managed to migrate towards the poles after the last ice age. It has been assumed that given long periods of time, even small dispersal distances would allow woodland herbs to keep up with the migration of the forests. Cain et al. (1998) examined this assumption using mathematical models based on the seed dispersal of wild ginger Asarum canadense in north-east North America. Field studies showed that ants are responsible for dispersing wild ginger seeds (attracted by the nutritious elaiosome) and can move the seeds up to 35 m, the largest distance ants are known to move the seeds of any woodland herb. However, by this mechanism, over the last 16 000 years, wild ginger should have travelled only 10-11 km from its glacial refugia. In reality, wild ginger has moved hundreds of kilometres during this time. Their work strongly suggests that for wild ginger and other woodland plants occasional long-distance dispersals were important in allowing their spread. This has important implications for forest fragmentation since seeds may occasionally disperse further than expected and bridge gaps. There are, of course, any number of assumptions behind whether such dispersal would be effective in today's landscapes and against the current speed of loss. Trees have, throughout their lives, many difficulties to overcome, initial establishment in a suitable environment being one and the repeated onslaught of herbivores another. Woodland herbivores vary immensely in size; contrast the larva of the moth Cydia fagiglandana (Fig. 4.11) which invades and kills the embryos of beech seeds with the African elephant which attacks adult trees. Herbivores are important with regard to both defoliation and the destruction of seeds. Some trees, especially oak, are very resistant to defoliation. The seedling of an English oak depicted in Fig. 4.5, for example, developed from an acorn cached in a lawn at the margin of Cantreyn Wood, Bridgnorth, Shropshire. Its shoots were cut repeatedly

acorn cached in a lawn, probably by a jay or other corvid (see Box 4.1), and repeatedly cut to the ground. (Drawn by John R. Packham.)

and it remained unnoticed until the lawn was left uncut for a month, at the end of which time the radicle had reached a length of 28 cm and the shoot a height of 16 cm. The large food reserves contained in acorns, which enables oak seedlings to survive such adverse circumstances, make them a valuable food source for a variety of herbivores including grey squirrels. Healthy young oaks readily arise in gardens and the trees produce an adequate rain of acorns, yet young oaks are a rarity even beneath the many mature oaks in Cantreyn Wood, which is changing its nature and has an abundance of young ash.

The tropics are renowned for the numerous tree species that occur at low densities with individuals more evenly spread than expected through the forest. A number of explanations have been suggested but the Janzen-Connell hypothesis has caught the imagination of many ecologists. The hypothesis, named after two ecologists, Daniel H. Janzen and Joseph H. Connell, who published on the subject in 1970 and 1971, respectively, states that tree seedlings are more likely to survive a certain distance away fom the parent tree. Figure 4.6 shows how this works. The number of seeds on the ground falls with distance from the parent. Seeds and seedlings nearest the parent are more likely to be found by species-specific predators or diseases so the probability of survival increases with distance from the parent. Thus the number of successful seedlings will peak at a certain distance away from the tree. Connell suggested that since seed mortality is always so high, this effect would be most easily seen in seedling numbers; it is with these that species-specific predators would have most effect. The argument goes that this does not happen in temperate forests because most predators and diseases are not species-specific so the chance of being killed is roughly equal with distance from the parent. This hypothesis has important ramifications in that it helps explain how so many rare species continue to exist for long periods of time. No one species, or small group of species, is so common as to gain dominance, so many rare species persist together. Certainly, Rees et al. (2001) suggest that this is one of the few ways that the co-existence of 1000+ species growing together in a tropical forest can be explained (see Section 2.2.2 for the role of soils in this). Much research has been invested in testing this hypothesis, particularly whether distance enhances survival, in plants and animals, but with mixed results. Hyatt et al. (2003) took an overview of these studies using meta-analysis, a statistical tool that helps synthesize the results of a number of studies. While greater seedling survival with distance from the parent was found in some species, this was balanced by cases where the hypothesis is not supported. Thus, while this mechanism is undoubtedly important in some species, it does not look to be a general underlying principle in all species.

Janzen Connell Hypothesis

Figure 4.6 The Janzen-Connell hypothesis. This states that in tropical forests, tree seedlings are more likely to survive a certain distance away from the parent tree. With distance from the parent (left to right) the number of seeds on the ground declines (line I) but the probability of a seed or seedling being missed by species-specific predators or pathogens (which will be more numerous near the parent) increase with distance from the parent (line P). Most surviving seedlings will therefore be found in the dotted triangle, labelled PRC - the population recruitment curve. The distance from the parent tree at which this peaks will obviously vary with tree species and conditions. (Redrawn from Janzen, 1970. American Naturalist 104, University of Chicago Press.)

Figure 4.6 The Janzen-Connell hypothesis. This states that in tropical forests, tree seedlings are more likely to survive a certain distance away from the parent tree. With distance from the parent (left to right) the number of seeds on the ground declines (line I) but the probability of a seed or seedling being missed by species-specific predators or pathogens (which will be more numerous near the parent) increase with distance from the parent (line P). Most surviving seedlings will therefore be found in the dotted triangle, labelled PRC - the population recruitment curve. The distance from the parent tree at which this peaks will obviously vary with tree species and conditions. (Redrawn from Janzen, 1970. American Naturalist 104, University of Chicago Press.)

4.2.3 Early life of mangroves in relation to their adult distributions

Mangroves are a group of phylogenetically unrelated trees (i.e. not from the same immediate ancestors) that live in very adverse conditions along the margins of tropical river estuaries and sea coasts, usually supported over a muddy substrate by stilt roots, most of which are frequently out of the water, or roughly horizontal roots bearing pegs or knees which project above the waterline. As mentioned in Section 2.3.3, in all these three types of pneuma-tophore, lenticels and aerenchyma (wide air passages) in the above-water roots and trunk allow gas exchange in submerged roots with oxygen diffusing in and waste gases passing out. Mangrove roots and lower stems are regularly bathed in salt water and some have root filters that effectively prevent the entry of salts. Other plants in similar environments, like tamarisks (or saltcedars Tamarix spp.), excrete briny water containing excess salt through special salt glands, while some species allow salt to accumulate in leaves which are then dropped. The adult distribution of various mangrove species shows marked

Mangrove Zonation

Figure 4.7 Zonation of mangroves on the coast of East Africa. S, barren sand; Am, Avicennia marítima; Cr, Ceriops tagal; Rm, Rhizophora mucronata; Sa, Sonneratia alba; A, algal zone. Horizontal lines indicate extreme high and low tide levels. (Redrawn from Jenik, 1979. Pictorial Encyclopedia of Forests. Hamlyn.)

Figure 4.7 Zonation of mangroves on the coast of East Africa. S, barren sand; Am, Avicennia marítima; Cr, Ceriops tagal; Rm, Rhizophora mucronata; Sa, Sonneratia alba; A, algal zone. Horizontal lines indicate extreme high and low tide levels. (Redrawn from Jenik, 1979. Pictorial Encyclopedia of Forests. Hamlyn.)

zonation patterns (Fig. 4.7); these are now widely regarded as being largely due to a sorting of species along a salinity gradient as a result of competitive interactions. The mangrove and freshwater swamps of the tropics are commonly inhabited by crabs (true crabs belonging to the infraorder Brachyura); these excavate large corridors in the soil and so improve drainage and root oxygenation. Crustaceans of all sizes live in forests and in drier situations, such as the oak woods of Europe, woodlice are common, consuming plant remnants.

Clarke et al. (2001) investigated the dispersal potential and early growth of 14 tropical mangroves on the northern coast of Australia in an attempt to discover whether these correlated with their patterns of adult distribution. The species investigated belonged to eight different families; the Avicenniaceae, Combretaceae, Fabaceae, Meliaceae, Myrsinaceae, Plumbaginaceae, Steruliaceae and Rhizophoraceae. Some were viviparous (having seeds which could develop while still attached to the parent tree) and others were not, but both groups had broadly similar seed weights, buoyancy and rates of root and shoot initiation. Diaspores (reproductive units), which were either seeds or one-seeded fruits, were immersed in solutions containing 100%, 50%, 10% and 0% sea water, and then stranded on surfaces with the same salinity (trays with 5 cm depth of damp sand). Some floated while others sank even in fresh water, and there were also differences in predominant orientation of the diaspores and the time before roots were initiated. In all the species investigated the diaspores were relatively widespread; dormancy was found in only one species. Though dormancy is an advantage in many widely dispersed seeds - because it allows more time for dispersal - this is not the case with mangroves as swift development is an advantage upon stranding in a favourable habitat before they can be washed away. There seems to be a clear trend against the production of small dormant diaspores that are not likely to develop into saplings in such an unstable environment.

When early life-history traits of 12 of the 14 species were compared with patterns of adult distribution, correlation was poor. Traits concerning establishment were, however, better predictors of occurrence than those associated with dispersal. The proportions of diaspores germinating after being soaked for 15 days in various salinities varied widely amongst the species tested, as did the heights reached after 15 weeks in early growth trays. Xylocarpus mekon-gensis and Avicennia mariuna were amongst the species which grew well and rapidly under all four treatments, while Aegiceras corniculatum was adversely affected by high salinity and achieved optimal growth in 5% sea water.

4.2.4 Angiosperms, conifers and ferns: tree regeneration and dominance in South Island, New Zealand

The angiosperms rapidly assumed dominance of most terrestrial ecosystems after their origination early in the Cretaceous period (Fig. 1.1). While they diversified and spread, coniferous trees and other gymnosperms were virtually eliminated from the tropics and gymnosperm abundance in many other terrestrial ecosystems was reduced. Coniferous trees now generally dominate only at high latitudes, in subalpine forests, in arid regions and on nutrient-poor or poorly drained soils. Although they grow less rapidly than angiosperms during the early regeneration phase, they are better able to resist severe cold and can persist in less productive habitats where their tough, long-lived needles are advantageous on nutrient-poor soils. Thirteen co-authors from many parts of the world co-operated in investigations of the forests of the Waitutu Ecological Region of the Fiordland National Park, New Zealand, in an attempt to throw light on the mechanisms influencing the relationships between angiosperm and coniferous trees (Coomes et al, 2005).

The forests investigated developed on a soil chrono-sequence in which soil nutrient availability became less and the drainage poorer on the older sites. Angiosperm trees were dominant on the 'recent' alluvial terraces which have been dated at less than 24 000 years old, while coniferous trees dominated the older marine terraces whose age varied between 79 000 and 121 000 years. The oldest marine terraces were 291 000 years old and were dominated by coniferous shrubs. The three main habitats investigated were the alluvial forest (most productive), terrace forest and shrubland (least productive). Soil phosphorus becomes increasingly limiting along this sequence and its scarcity is very marked in the shrubland. The ferns characteristic of the alluvial forest are Dicksonia squarrosa, Cyathea smithii and Blechnum discolor, while those of the terrace forest are Blechnumprocerum and B. discolor (again). The dominant ferns of the shrubland are more varied consisting of Hymenophyllum multifidum, Gleichenia dicarpa, Grammitis billardierei and Schizaea fistulosa (Coomes et al, 2005).

The title of their paper ('The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering') reflected the nature of the three groups; the very active angiosperms dominating the better sites, the more conservative conifers coping well with less nutrient-rich and poorer areas while the ferns, a very ancient group, influence the regeneration of the trees. Tall ferns (crocodiles) and deep shade restrict regeneration opportunities in the relatively productive forests of New Zealand, thus diminishing the opportunities for conifers (tortoises) to escape competition from fast-growing angiosperms (hares). Less than 1% of total light reaches the floor of the alluvial forest; transmission of PAR is largely prevented by dense groves of tree and ground ferns, and by large-leaved subcanopy trees. Few young tree seedlings of any species were found on the forest floor, though angiosperm trees were particularly successful in colonizing rotting logs and tree fern trunks.

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