Marginal areas present both advantages and disadvantages for seedling establishment as compared with more central situations. In many forests regeneration is often episodic as it is dependent on some form ofdisturbance or catastrophe to create space for the next generation. In the southern beech forests (Nothofagus dombeyii and N. pumilio) of south-central Chile the beeches at lower elevations are dependent on large-scale disturbance by landslides, blowdown, or even rooting by wild pigs. By contrast, in stable regions they become replaced by more shade-tolerant, late-successional species, e.g. Aetoxilon punctatum and Lauerelia phillipiana, which can be found with ages from 300 to over 635 years old. At the upper peripheral margins of the forest the more open nature of the canopy allows a more suitable habitat for Nothofagus regeneration (Pollmann & Veblen, 2004). At subalpine elevations the frost-tolerant N. pumilio is particularly successful and has no need in this relatively open forest for any disturbance as an aid to regeneration (Fig. 4.4).
The availability of suitable sites is not sufficient on its own to ensure reproduction. There has also to be an adequate supply of propagules. The various methods used by plants to disperse seeds are well known. However in marginal areas there can also be found highly specialized biological adaptations that have received less attention. Stone pines (five-needled Pinus spp. subsection Cembrae) are subalpine and timberline species both in North America and Eurasia. The European arve or stone pine (Pinus cembra) has long been known to have a mutualistic association with the European spotted nutcracker (Nucifraga caryocatactes). The pine seeds are large and wingless and remain within the cones and are therefore maladapted for wind dispersal. Their large size makes them rewarding fodder for the nutcracker birds which collect and distribute the pine seeds in hoards or caches. These hoards are buried, creating a bird-collected soil seed bank (Tomback et al., 2001).
Although this mutualistic relationship has long been noted in Europe it is only recently that detailed studies have revealed a similar connection between the North American whitebark pine (Pinus abicaulis) and Clark's nutcracker (Nucifraga columbiana; Figs. 4.5-4.6). Detailed studies have shown that this relationship goes
far beyond being merely a local aid to seed dispersal but has far-reaching long-term effects on the ecology, timing of regeneration, and population genetics, of the Cembrae pines (see reviews in Tomback, 2001, 2005).
The quantity of seed transported by the nutcrackers is impressive. In their specially adapted sublingual pouch they can hold 100 or more pine seeds. It has been estimated that one bird collecting seeds for about
80 days and working for 9 hours per day will transport as many as 98 000 seeds in a season into caches with 1-15 seeds (mean 3.2) which represents more than 30600 caches per bird. As has been pointed out (Tomback,
2005), where the whitebark pines will grow is determined first by the cache-site preferences of the nutcrackers and secondly on aspect, substrate, water availability, and climatic conditions of the site. The birds also have a homogenizing effect on genetic differences between populations. The tendency for the nutcrackers to cache seeds both above and below the timberline enables the pines to migrate rapidly with respect to climatic warming or cooling trends.
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