Effect of P supply upon seasonal growth and internal cycling of P in Sitka spruce (Picea sitchensis(Bong.)Carr.) seedlings

The availability of phosphorus in many UK forest soils limits growth of Sitka spruce (Picea sitchensis (Bong.) Carr.). Efficient cycling of P within such systems is therefore necessary for sustained tree growth. Internal cycling of P is an important component of the overall P cycle in forests and the current work aims to quantify the impact of P nutrition on internal cycling and seasonal growth of Sitka spruce.Two-year old seedlings of Sitka spruce were grown in sand culture in the glasshouse for one year. Two treatments were imposed in which trees received either a complete nutrient solution from which P was excluded (-P) or one in which P was applied as labelled ³²P (+P). Internal cycling of P was measured directly in plants which had received no P and by difference in those which received ³²P.The contrasting P treatments produced an eight-fold difference in P content and a three-fold difference in tree growth between May and October. Root:shoot ratios increased during the growing season from 0.29 to 0.38 and from 0.29 to 0.52 in +P and-P treatments, respectively. In both treatments P was translocated from old shoots to support new shoot growth. P supply did not affect the amount of P remobilised but there was evidence that the rate of remobilisation may have been affected. The partition of remobilised P was affected by current P supply and differed from the partition of current P uptake.Results are compared to those from studies of growth and internal cycling of nitrogen in Sitka spruce.     

Modelling the Seasonal Nitrogen Partitioning in Young Sycamore (Acer pseudoplatanus) Trees in Relation to Nitrogen Supply

A model of nitrogen partitioning during the seasonal growthof sycamore (Acer pseudoplatanus) seedlings was developed andtested against data from trees grown with two contrasting levelsof nitrogen supply. The model considered each tissue type (roots,trunk, stems and leaves) as sources and sinks for nitrogen andused flow equations to simulate the dynamics of nitrogen partitioningduring a growing season, with increases in tissue dry matteras driving force variables. Withdrawal of nitrogen from leavesduring senescence was allocated back to other tissues assuminga linear decrease in leaf mass. The model was fitted to data from trees grown in sand culturewith 6·0 molN m^-3 (high N) supplied with the irrigation.Model parameters thus determined were used to predict nitrogenpartitioning in trees grown with 1·0 molN m^-3 (low N)in the same year, and for trees from both treatments given eitherhigh or low N during a second year. The model accurately predictedthe nitrogen content of roots and leaves and gave small errorsin the amount of nitrogen partitioned to stems. In contrast,the nitrogen content of the trunks were over-estimated due toa failure to simulate the decreased in nitrogen content foundat the start of the growing season. The ability of the modelto simulate nitrogen partitioning by changes in tissue dry matterin trees of varying size and nitrogen status is discussed andpossible modifications to model partitioning of trunk nitrogenmore accurately suggested.Copyright 1993, 1999 Academic Press Modelling, nitrogen partitioning, ¹⁵N supply, Acer pseudoplatanus (sycamore), young seedlings     

Leaf habit influences nitrogen remobilization in Vaccinium species.

The effect of N supply on plant growth and leaf demography of a deciduous and an evergreen Ericaceae was studied in relation to their internal cycling of N. Mature ramets of Vaccinium myrtillus (deciduous) and Vaccinium vitis-idaea (evergreen) were established in sand culture for 1 year with an adequate supply of a balanced nutrient solution. During one growing season, the plants were given two levels of N supply enriched with 15N and eight sequential destructive harvests were taken. Recovery of unlabelled N in the new shoot was used to determine the remobilization of N from storage. Initially, growth was unaffected by N supply. After May, High N enhanced growth for both species but the nature of their growth response differed. For both species, new shoot biomass and leaf number increased but root biomass production was affected for V. myrtillus only. Whole plant biomass production was similar for both species under High N, but was greater for V. vitis-idaea under Low N. The amount of N remobilized to support new shoot growth was similar for the two species and was independent of N current supply. N was remobilized predominantly from previous year leaves for V. vitis-idaea and from previous year stems and roots for V. myrtillus. The contribution of remobilization to new shoot N was similar for the two species, but depended on N supply. Remobilization was faster in V. myrtillus, but lasted longer in V. vitis-idaea. The results are discussed in relation to species growth in N-poor environments, focusing on the extent to which species-differences in the dynamic of N remobilization and growth may explain their adaptation to constant and/or changeable N supply.