Forest damage started discussions in public and in forest science whether management of stands as monocultures, usually dominated by spruce, is sustainable, i.e. whether or not this management affects the productivity of a site with a turnover time of about 100 years over several generations, and whether the change from deciduous forests to coniferous forests, which was enforced by man, increased the damage observed, and whether reverting from coniferous forests to deciduous forests would avoid such damage in future.
Deciduous and coniferous forests differ with respect to several parameters, including:
• Differences in radiation and heat balance (Chap. 2.1; Schulze 1982);
• Differences in water relations (Chap. 2.2; Schulze 1982);
• Differences in growth (Chap. 2.4; Schulze 1982);
• Differences in the formation of humus (Mitscherlich 1975).
Seasonal shedding of leaves in deciduous forests results in the energy exchanging surface being near the ground in winter and spring, but high in the canopy in summer (as with spruce; see Chap. 2.1, Fig. 2.1.9). Prior to leaf emergence in spring, temperatures in the litter rise rapidly and reach values far above the freezing point. This leads to activation of the species-rich ground vegetation (spring geophytes), which are important not only regarding the diversity of forests, but also for the storage of nutrients in the ecosystem because these herbaceous plants utilise nitrate that is formed in spring when tree roots are still inactive. In coniferous forests the permanent needle cover prevents light penetrating to the ground. Lack of light prevents the development of the spring flora. The soil remains cold, thus slowing down decomposition of litter. During the summer months, the climate for both types of trees is similar.
The greater needle and branch area of conifers compared with deciduous trees increases interception of precipitation by coniferous forests (Fig. 3.5.7; Schulze 1982). Transpiration for beech and spruce is approximately the same. Obviously, the increased needle area balances decreased stomatal conductance of conifers. It is the lower interception which leads to ca. 20% higher flow of water to the groundwater in deciduous forests than in coniferous forests. This is particularly important for the ecosystem service to supply water for human consumption. Areas acting as sources for water are therefore presently re-forested with deciduous forests or with herbaceous shrubs.
Conifers grow faster than deciduous trees in temperate climates because they start to assimilate earlier in the year and continue longer. This difference, however, is counterbalanced by the higher assimilation rates of deciduous trees. Conifers have an important additional advantage because only a fraction of their assimilating organs is exchanged every year, whilst deciduous trees renew the complete leaf mass. Also, in spruce, the ratio of twigs to stem mass is lower than for beech or oak (see Chap. 126.96.36.199). Thus, more assimilates remain for building stem wood. The increase in spruce is on average 10 m3 a-1 compared with 5 m3 a-1 for deciduous trees (Kramer 1988).
Effect of Deciduous and Coniferous Forests on Processes in Ecosystems
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