Quaternary Variations Of Vegetation

Palynologically-based paleoecology has given strong evidence in recent years about altitudinal variations in the period c. 25,000-15,000 years ago, the coldest period of

Tropical Jungle
Figure 8.3. The daily weather regime in the New Guinea Highlands (modified after Brookfield, 1964). Note especially the cloud below the peaks at 6:30 a.m. followed by heavy clouding at 12: 30 p.m. and later.

the last glaciation. The evidence from the New Guinea Highlands consists of pollen diagrams from a variety of altitudes between 1,900 and >4,000 m (Flenley, 1979). These are consistent with a lowering of the altitudinal forest limit to c. 2,000 m in the Late Pleistocene (Figure 4.13). This could be explained by a lowering of mean annual temperatures.

There is, however, an anomaly which cannot be explained by mean annual temperature change alone: the fact that in the Late Pleistocene the Dwarf Forest apparently disappeared almost completely (Walker and Flenley, 1979). Its constituent taxa must have survived somewhere, presumably as rare individuals near the altitu-dinal forest limit. In its place, the alpine grassland was greatly expanded, and was apparently rich in tree ferns. This phenomenon has now been reported to be repeated during each glacial phase (see Chapter 4).

If this absence of the Dwarf Forest in New Guinea in the Late Pleistocene had been an isolated instance, one could perhaps have ignored it. A similar phenomenon has, however, been reported from the Colombian Andes (Salomons, 1986). In this case it was the subparamo, the subalpine shrubbery of the Andes, which was discontinuous in the Pleistocene. Both cases amount, however, to a great reduction in the Late Pleistocene of the upper woody formations which are characterized by stunted growth, small thick leaves, and a hypodermis.

The usual explanation advanced for this is that the climate of the Last Glacial Maximum was somewhat drier than the present one, as well as cooler. Thus, the Dwarf Forest disappears. However, since desiccation is more likely to favor stunting than to discourage it, this explanation does not appear completely satisfactory. If, however, the UV-B hypothesis advanced in the last section has any credibility, the occurrence of Dwarf Forest is related—at least, partially—to high UV-B. The disappearance of the Dwarf Forest during the glaciation can then be explained in the following way (Figure 8.4). In the Holocene, at lower altitudes—below c. 3,000 m— the genetically stunted trees of the Dwarf Forest would be at a selective disadvantage compared with the larger trees of the Lower Altitude Forest. Above c. 3,000 m they would be at an advantage, because of the greater UV-B insolation there. In the Late Pleistocene, lower temperatures brought the forest limit below 3,000 m, therefore the stunted species became rare. In the Holocene, warmer temperatures allowed forests to expand uphill, but only those species genetically adapted to high UV-B insolation could take advantage of this. The present forest limit thus could be controlled by temperature or UV-B, or a combination of both.

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