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Title: Production and nutrient cycling in two birchwood ecosystems
Author: Forbes, James C.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1973
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The annual organic matter production and cycling of mineral nutrients are compared in two birchwood ecosystems in north-east Scotland. One is a pure stand of Betula pendula. at an altitude of 300 feet; the other an almost pure stand of B. pubescens subsp. odorata. at an altitude of 950 feet. Both stands are about fifty years old. The total above-ground tree biomass of the B. pendula stand (123 x 10³ kg ha⁻¹) is much greater than that of the B. odorata stand (78 x 10³ kg ha⁻¹), hut the biomass of twigs is similar in the two stands, B. odorata carries a greater biomass of leaves, having a much denser canopy than B. pendula. In 1971 about 20% of the B. pendula leaves were apparently removed from the site by high winds: this is a significant drain of nutrients from the site. That this does not occur in the B. odorata stand is probably due to the more bushy growth form of the trees, which is not prone to losses of leaves by wind, and acts as an efficient trap for wind-blown leaves. Annual death of perennial parts is greater in B. pendula than in B. odorata, but annual non-perennial part litter production is greater in B. odorata. Total annual above-ground production of the trees is similar in the two stands (just under 11 x 10³ kg ha⁻¹), but the production per gram of leaves is 6.0 g in the B. pendula and only 4.4 g in the B. odorata. The annual above-ground production of the ground vegetation accounts for 16% of the total above-ground production in the B. pendula stand, but only 10% in the B. odorata stand. Both stands utilise about 3% of the photosynthetically active radiation energy received during the growing season in their organic matter production. A new mathematical model of twig growth and death was used in the calculation of the seasonal biomass of twigs, buds, leaves and catkins. It gave results which were consistent with other observations, including measurement of litter fail. Annual uptake, and return in litter and rainwater, of N, P, K, Ga and Mg are greater in the B. odorata than in the B. pendula stand, not only as a result of the greater biomass of nutrient-rich non-perennial parts, but also because of higher nutrient concentrations in the B. odorata. Rainfall percolating through the canopy contains a significant proportion of the annual return of nutrients (particularly K) to the forest floor. Minor components of the litter are also of considerable importance in this respect, especially in the return of P. A very large proportion of the annual uptake of nutrients is recycled: over 95% of the K uptake (hut only 84% of the Ca uptake) returning annually to the floor of the B. odorata stand. One year after litter fall, virtually all the P, K and Mg in the leaves has been incorporated into the soil by decomposition. Almost all the differences between the stands outlined above result from the contrasting growth forms of the two species. Water and light penetration, temperature and relative humidity, and production and floristics of the ground vegetation are among the other features of birchwood ecology influenced by growth form of the trees. Branching pattern (which generates growth form) was analysed using a simple mathematical model, and the more open canopy of B. pendula was shown to result from greater twig and branch death rather than less profuse branch production. The results of the project are discussed with reference to the distinct geographical and ecological ranges of the two species. The theory is advanced that B. odorata is adapted to the cooler climate and shorter growing season of higher altitude by a greater leaf production (permitted by greater longevity of its branches) and to nutrient-poor soils by a capacity to take up a greater proportion of the available nutrients than B. pendula.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available