Evergreen and deciduous strategies aspects of competitive advantage

Almost all conifers - including the Wollemi pine Wollemia nobilis discovered in 1994 - are evergreen, the exceptions including the larches Larix, the Chinese swamp cypress Glyptostrobus lineatus, the three American swamp cypresses Taxodium (of which the Mexican swamp cypress T. mucronatum is evergreen in warm climates), and the dawn redwood Metasequoia glyptostroboides. This last species was described from fossil material in 1941 and then discovered as a living tree in 1945. As with most angiosperm (broadleaved) tree genera many oaks are deciduous; one of the numerous evergreen species is the European holm oak Quercus ilex that is much used for windbreaks. Nevertheless, this does not address the question of the advantages and disadvantages of these two strategies.

Net production is obviously a major criterion. If growing conditions are favourable all year round, as in tropical rain forests, then there is no advantage in being deciduous and so evergreen angiosperms dominate. In climates with a dry or cold season, it is cheaper to grow disposable leaves than to grow more robust leaves capable of surviving the off-season, so in most moist temperate areas deciduous trees dominate. However, if the environmental conditions get worse - such as a very short growing season - it may once again be more beneficial to grow evergreen leaves since none of the growing season is wasted. Evergreen leaves also occur on trees growing on nutrient-poor soils (since it is too expensive in nutrients to grow a new set every year), and in very dry Mediterranean climates where the thick, expensive leaves needed to survive the growing season are retained for several years to repay the energy invested in them. In areas with even worse growing seasons, such as near timberlines, deciduous leaves re-appear. Here, despite the costs, the winter is so severe that it is cheaper to build new leaves every year rather than attempt to keep leaves alive. Thus the northernmost trees in the Arctic are birches and there are often birches, willows and larches in the krummholz. Thomas (2000) gives a more detailed account of this subject.

A number of detailed studies have been made on the relative merits of these evergreen and deciduous strategies. The photosynthetic capacity of individual leaves of temperate deciduous trees such as beech is considerably higher than that of evergreen conifers, though whole trees of the latter frequently have high net production. This is partially due to the prolonged growth period; photo-synthetic gains are made in the spring, winter and parts of the autumn when deciduous trees are bare of leaves. Leaf turnover rate is also important; when leaves are replaced annually the amount of photosynthate that can be put into the formation of wood and bark is greatly reduced. Photosynthetic capacity and respiratory activity are both high in deciduous trees. When young leaves are developing they respire intensely, have a small surface area, and are usually low in chlorophyll, making them a cause of overall carbon loss until their photo-synthetic capacity is at its peak within a few days of full expansion. While the photosynthetic capacity of evergreen leaves slowly falls over the years, as has been shown for white spruce Picea glauca and balsam fir Abies balsamea, they continue to make a substantial contribution for a very long period.

A very thorough comparison of the growth of beech (Schulze, 1970) and Norway spruce (Schulze et al, 1977a,b) growing within a kilometre of each other at 500 m on the Solling Plateau, North Germany was made as part of the

Parts Deciduous Forest Activities

Figure 3.20 A comparison of photosynthetic activities of common beech Fagus sylvatica and Norway spruce Picea abies growing on the Solling Plateau, Germany. In (a) and (b), columns for sun leaves are shown white, those for shade leaves are hatched. 0, 1 and 2 correspond, respectively, to spruce leaves developed in the current year, and one and two years previously. Carbon fixation per unit of leaf dry weight (mass) is much greater in beech which, however, has a much smaller leaf biomass (c) and lower annual photosynthetic gain (d) than spruce. (Drawn from the data of Schulze et al, 1977b. From Packham et al, 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 2.15. With kind permission of Springer Science and Business Media.)

Figure 3.20 A comparison of photosynthetic activities of common beech Fagus sylvatica and Norway spruce Picea abies growing on the Solling Plateau, Germany. In (a) and (b), columns for sun leaves are shown white, those for shade leaves are hatched. 0, 1 and 2 correspond, respectively, to spruce leaves developed in the current year, and one and two years previously. Carbon fixation per unit of leaf dry weight (mass) is much greater in beech which, however, has a much smaller leaf biomass (c) and lower annual photosynthetic gain (d) than spruce. (Drawn from the data of Schulze et al, 1977b. From Packham et al, 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 2.15. With kind permission of Springer Science and Business Media.)

International Biological Programme. Beech dominates many natural forests in central Europe, old-growth forests being particularly prominent in Poland, while spruce is the conifer most frequently planted in German re-afforestation programmes. The production values shown in Fig. 3.20 are thus of great interest. They are based on measurements of photosynthetic capacity of a typical tree of each species. The beech was 100 years old and 27 m high when examined in 1968, while the spruce investigated in 1972 was 89 years old and 25.6 m high; they differed in four major respects with regard to production. Both the sun and the shade leaves of beech had a much higher photosynthetic capacity per unit dry weight (mass) than even one-year-old needles of spruce. Beech had a short growing season, having a positive CO2 uptake on 176 days in the year as opposed to 260 for spruce. Though beech had a higher annual production of leaves than spruce, the latter had a much greater photosynthe-sizing biomass, some of its needles surviving for 12 years. Care has to be taken if such results are applied to a consideration of evergreen and deciduous forests elsewhere; increased levels of atmospheric pollution have reduced the useful life of conifer needles in many places.

In the Solling study the deciduous beech invested more dry matter in its leaves every year, but the long-term return for the investment made by the evergreen spruce was greater because its leaves continued to fix carbon so much longer, though at a slower rate. Thus in these terms the spruce has, despite its ancient lineage, a performance superior to that of its angiosperm rival, its primary production being 14.9 tC ha-1 y-1, while that of beech was 8.6tCha-1 y-1. In terms of the value of the two strategies it would be very interesting to make similar detailed comparisons on an evergreen and a deciduous species within the same genus such as oak Quercus.

Despite the high productivity of evergreen Norway spruce in the region, common beech Fagus sylvatica is frequently the dominant of many natural forests in central Europe. It is much more shade-tolerant, its seeds being able to germinate and grow where those of spruce cannot. Moreover, the deciduous habit entails a reduction of the surfaces on which ice can accumulate, so it is less affected by winter storms and by snow and ice breakage. The surface rooting of Norway spruce means that gales can topple it relatively easily, and its life is often shorter than that of beech, which competes powerfully with other trees and when growing actively suffers markedly lower mortality from fungal diseases than spruce. Clearly, relative growth rate, though important, is only a single aspect of competitive advantage, whose balance is often swayed by climatic factors.

Norway spruce flourishes in northern Scandinavia and Russia, where common beech would not ripen seed and is unknown as a natural forest tree. However, the deciduous habit and the production of strongly constructed, even xeromorphic, needles are both adapted to the water stresses ofnorthern winters, as the presence of aspen Populus tremula and Norway spruce in arctic Norway demonstrates.

Stem photosynthesis must play a part in maintaining carbohydrate levels in sessile oak Quercus petraea, one of the deciduous trees in the stems of which rates of photosynthesis are still appreciable at low light intensities. Nevertheless, evergreen conifers dominate most boreal forests, clearly gaining from the ability of their needles to photosynthesize for several years. When they yield to larches, their deciduous relatives, it is frequently on well-drained and often gravelly soils. The leaves of evergreen trees in the very different climate of tropical rain forests have a low rate of CO2 uptake but, because the active vegetative period is so long, can attain a high annual biomass gain.

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