Internal nutrient cycling in evergreen and deciduous tree species
This thesis compares the cycling of nutrients (N, P, K, Mg and Ca) within twigs and foliage, between the evergreen species Pinus sylvestris L. (Scots pine) and Picea abies (L.) Karst. (Norway spruce), and the deciduous species Quercus petraea (Matt.) Liebl. (oak) and Betula pubescens Ehrh. (birch). In addition, the role of foliage produced in previous seasons as a source of nutrients for current growth, in young stands of Scots pine and lodgepole pine (Pinus contorta Loud.), was investigated by means of defoliation experiments, where various age-classes of foliage were removed. In both evergreen and deciduous species, the main areas of overwinter storage and sources of nutrients to support current growth appear to lie outside the twig/foliage system. Patterns of leaf litter production were similar in both Scots pine, oak and birch, however the deciduous species lost a greater amount of nutrients in leaf litter than did the evergreen species. In comparisons between Scots pine and oak, it was observed that foliage production in spring and summer is much slower, and takes place over a longer period, in the Scots pine, which also maintains a greater amount of nutrients within its foliar biomass. Results from the artificial defoliation experiments, suggested that older needle fractions do not play a major role in supplying nutrients to sites of current growth, however they do appear to provide an essential source of photosynthate to support new biomass production. A mechanism is proposed, whereby evergreen species are able to reduce the level of current nutrient demands in two main ways, firstly by means of their slower more extended growing period, and secondly by maintaining lower foliar nutrient concentrations (thereby reducing nutrient losses via leaf litterfall), at the expense of maintaining a greater amount of nutrients within their biomass.