Appendix A. Survival Strategies of the Ecotonal Species

Appendix A. Survival Strategies of the Ecotonal Species With greater numbers of species found in relatively high abundance in communities of both the boreal zone and the arctic regions, with fewer in the forest-tundra ecotonal communities, the question arises as to the strategies involved that permit adaptations to disparate environments yet prohibit all but a few from surviving in the ecotone, at least in numbers sufficient so they are recorded in quadrats of the size employed in transects of this study. There is a depauperate zone in the forest-tundra ecotone marked by a paucity of species, a zone in which only a relatively few wide-ranging species are persistently dominant (see Tables 5.1-5.9). Plant communities in both the forests to the south and the tundra to the north possess more species of sufficient abundance to show up in the transects. In the depauperate zone, there is no lack of vegetational cover over the landscape, but fewer species in total dominate the communities. There are fewer species demonstrating intermediate abundance, fewer rare species. The obvious question is, "Why should this be so?" As yet, there is perhaps no fully satisfactory answer, but there have been studies that at least seem to point the way for an approach to the question. Studies of the survival and reproductive strategies oftundra plants, arctic and alpine, have a long history, and these are thoroughly discussed in classic papers and reviews by Britton (1957), Jeffree (1960), Warren Wilson (1957a,b, 1966a,b, 1967), Johnson et al. (1966), Johnson and Packer (1967), Billings and Mooney (1968), Bliss (1956, 1958, 1962, 1971), Billings (1974), and Wiclgolaski et al. (1975). Concepts derived from more recent research programs are discussed by many workers, including Bliss (1977), Solbrig (1980), Brown ct al. (1980), Bell and Bliss (1977, 1978, 1980a,b), Survival Strategies of the Ecotonal Species 197 Addison and Bliss (1977, 1980, 1984), Sohlberg and Bliss (1984), Jackson and Bliss (1984), Tieszen (1972, 1973), Tieszen and Wieland (1975), and others. Early discus sions of the causes of timberline and the physiology of timberline trees are to be found in Daubenmire (1954), Lindsay (1971), Pisek and Winkler (1958), Larcher (1957), Tranquillini (1963, 1964, 1969, 1979), Vincent (1965), Viereck (1973), and Mosquin (1966). There also has been much recent work on the physiology and growth habit of northern evergreen trees of the genera Abies, Pinus, and Picea that are dominant species in the alpine and northern tree-lines of North America (Hom and Oechel, 1983; Strang and Johnson, 1981; Lawrence and Oechel, 1983a,b; Strong and La Roi, 1983; Van Cleve et al., 1981; Tryon and Chapin, 1983; Viereck et al., 1983; Vowinckel et al. , 1975; Yarie, 1983; Johnson, 1981; Kershaw, 1977, 1978; Lambert and Maycock, 1968; Arno, 1984; Ritchie, 1984; Rowe, 1966, 1984). Greater variability in environmental conditions in the ecotonal zone are likely to be a significant factor in the survival of these species. For example, there is only a few degrees difference in average summer temperature between the northern and southern edges of the boreal forest, with also a correspondingly great uniformity in the airmass characteristics of low latitude arctic tundra. Yet, at the ecotone these two climatic regimes are separated by a steep gradient clearly apparent in climatological data. This gradient is, however, deceptive in the sense that there is, in fact, not so much a persis tent gradient as a relatively rapid fluctuation from one regime to the other, not apparent in the averaged data: boreal one day, arctic the next, frost a possibility at any time dur ing summer, yet short periods of calm with high temperatures and clear skies, all of which follow one another with rapidity alien to other regions, one day hot, dry, and calm, the next windy, cold, wet, intermixed daily with a few frosty hours around mid night. Plant species adapted to the boreal regime do poorly in an arctic environment, arctic plants do poorly under boreal conditions, and only a few can survive persistent rapid oscillations between the two. For obvious reasons, most studies of ecological strategy have been conducted with temperate zone species, particularly with annuals and with some perennials possessing small well-defined bulbs. One early study was conducted on Equisetum sylvaticum, however, and revealed that this species, common in the northern spruce forest and a long-lived perennial, possesses an extensive rhizome which may outweigh the aerial shoots by a ratio of 100 to l. Moreover, marked variations in growth from year to year indicate that the species is capable of a markedly plastic response to variable condi tions. The conclusion drawn from the study is that phenotypic plasticity enables the species to survive marked environmental variability. This also provides assurance against major climatic changes and vegetational migrations that occur every few thou sand years. Other species with similar ecological strategy are MlCcinium spp., Ledum groenlandicum, Kalmia spp., Rubus spp., and others, according to the authors (Beas leigh and Yarranton, 1974), who add that longevity in these species permits survival where microclimatic and edaphic conditions fluctuate wildly and the vegetation cover varies from zero to closed forests. Plasticity, thus, permits survival during changing conditions. Species that lack such plasticity, although they may inhabit colder climatic regimes northward or warmer and wetter conditions southward in the boreal forest, are eliminated as competitors in forest-tundra ecotonal regions. Moreover, this is seen to be the case especially in the 198 Appendix A. Survival Strategies of the Ecotonal Species zone where environmental fluctuations are most pronounced, in the forest-tundra ecotonal fringe at the northern edge of the forest-tundra transition zone. Only additional studies of the individual strategies of the species will reveal whether this hypothesis is a valid one, particularly with such species as Ledum groenlalldicum, L. decumbens, Vaccillium uliginosum, Rubus chamaemorus, Salix spp., Arctostaphylos alpina, Andromeda polifolia, Carex spp., Eriophorum spp., and perhaps a few other wide-ranging species that tend to dominate the relatively species-poor plant communi ties at the northern edge of the forest and the southern fringe of tundra, the extreme northern edge of the forest-tundra ecotone (Larsen, 1965, 197Ia,b, 1974, 1980). Somewhat like repeated fire, these environmental fluctuations tend to stimulate vegetative reproduction of the prevailing dominants, to the extent that they ultimately eliminate species of lower densities and dominance (Solberg, 1980; Strang and John son, 1981). It has also been noted that the long period of germination of Ledum groenlandicum and L. decumbens seed serves to minimize the influence of brief periods of unseasona bly warm temperatures that may occur in fall, presumably as well as brief periods of very severe cold in late spring. The seeds, moreover, have relatively high temperature requirements for germination. This tends to delay shoot emergence until at least early summer conditions are firmly established in both soil and atmosphere. Seed of L. decumbens germinates more rapidly than that of L. groenlandicum, however, which would account for the more northern distribution of the former, extending far into tun dra while the latter is more or less restricted to the northern forest and forest-tundra ecotone (Karlin and Bliss, 1983). It is undoubtedly significant that these are wide ranging species, among the group listed above, indicating by the breadth of their geographical range that they do, indeed, possess a degree of phenotypic plasticity and perhaps genetic diversity absent in species of more restricted range and adaptation. It has been observed that studies of the structure and dynamics of plant communities have at present no theoretical base (Austin, 1985), at least broadly so, although con cepts of continuum and niche are certainly valid steps along the way (Johnson, 1977a,b). There are, however, no predictive equations describing causal relationships that establish which plant species will be found under specific habitat conditions, in which combinations, in what numbers, and whether or not inter- or intraspecific com petition represent important factors establishing the structure observed. It is almost as though much of what is observed is the consequence of random events in the environ ment occurring concurrently with virtually random opportunities for species to gain a foothold at a given time and place, within the necessary limits imposed by the physio logical tolerance of the species, existence of a sufficiently abundant supply of seed, and favorable habitat conditions. On the other hand, it is possible to be rather sanguine over the fact that it is now pos sible to find descriptive, perhaps even quantitative, information on the composition and structure of hundreds of communities that have been sampled by ecologists, but the absence of a theoretical basis for this effort results in a corresponding absence of a uni form methodology for statistical description of community composition and structure, making comparison difficult, the pathway of successional change, and so on, largely conjectural. In the instance of forest-tundra ecotonal communities described above (and in Tables 5.1-5.9), in which a few wide-ranging species attain dominance over a Survival Strategies of the Ecotonal Species 199 broad longitudinal range, the observation that this is a community structure found most commonly in harsh environments is one of great interest and perhaps broad sig nificance (Giller, 1984, pp. 121-123). Additional detailed analyses of the structure of these communities was given in previous publications (see tables, figures, and refer ences in Larsen, 1974). While much remains to be done, there are, nevertheless, recent studies of northern plant communities that encourage the belief that a certain degree of underlying predic tability exists in the behavior of the northern species and the plant communities of northern regions, forest, ecotone, and tundra.

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