Nitrogen Allocation

The proportion of total tree nitrogen in each organ was assayed for every tree at each harvest point and is presented in (Figure 3-4 A). As previously stated,

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there was little root formation by the first harvest in June suggesting that most of the nitrogen found in the first harvest derived from that stored in the cutting (see section 3.4.4). This was confirmed by the N15 assessment. The initial mobilisation of nitrogen from the cutting was largely to the leaves, with around 53% of total

nitrogen reserves being allocated to this organ. 10 % of total nitrogen reserves were allocated to the stem at this time. In the following two harvests the overall amount of nitrogen vastly increased both in absolute amounts (with increased whole tree growth) and as proportions of leaf, stem and root DM (Figure 3-4 B and C). However, the initial allocation profile of total nitrogen between the different organs was not maintained through to August and October (Figure 3-4 A). The initial investment of 53 % of total nitrogen (cutting derived) to the leaves was maintained in August, even after the shift from cutting reserves towards recent assimilation as a nitrogen source, but by October the proportion of total nitrogen allocated to the leaves dropped to 35 %. The proportion of total nitrogen allocated to both the stem and roots of the trees increased in August to 19% and 8% respectively and in October to 27% and 19% respectively.

The lowest nitrogen concentration in any organ at any harvest point (other than the cutting as would be expected at establishment) was in the stem during August, with nitrogen comprising 0.7 % of stem DM (Figure 3-4 C). Even though October was the month where the smallest proportion of total nitrogen was dedicated to the leaves it was conversely also the point were the highest

A B

C D

Figure 3-4 Nitrogen Allocation of all Genotypes Pooled

A Mean percentage of total nitrogen allocated to each organ. B Mean nitrogen in each organ as a percentage of the whole trees DM (g/g). C The nitrogen in each organ as a percentage of the organ DM (g/g). D Mean Biomass yields (g DM) of all the trees at each harvest for each organ, total and growth (all organs with the exception of the cutting). The first, second and third column represent values for the June, August and October harvests respectively with the exception of the cutting where an extra column has been added indicating nitrogen allocation at the time of planting, April, for A-C. All error bars represent standard error (n > 60).

To specifically interpret these changes in nitrogen “strategy” the data is also presented as the shift between harvest points (Figure 3-5 C-D). Between April and June over 64 % of the nitrogen reserves in the cuttings were remobilised with the majority going to leaves and only a small amount going to the stem. Little to no nitrogen reserves were allocated to the roots between these times. Between June and August this large strategic investment in the leaves was maintained (but quantitatively increased with the large increase in total nitrogen assimilated from the soil). The investment in stem and roots at this time increased by 9% and 8% of the trees’ total nitrogen respectively. This trend of increased importance of stem and root nitrogen is maintained with further proportional increases between August and

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roots comes at the expense of investment of assimilated nitrogen allocation to leaves, resulting in a reduction of over 17% of total nitrogen to the leaves.

A B

C D

Figure 3-5 Nitrogen Allocation Shifts Between Harvests of all Genotypes Pooled A Mean total nitrogen allocation to each organ between all three harvest points: June, August and October and planting in April. Error bars represent the standard error (n > 60). B-D Mean shift of total nitrogen allocation to each organ between April and June, June and August and August and October. Error bars represent the standard error of the difference between the two means (n > 60).

When these shifts in allocation of total nitrogen are examined in terms of genotypes, significant variation can be observed (Figure 3-6 A + B). However, the large range of variation found mean that interpretation of any trends needs to be made with caution. If the data is considered with regard to biomass yield, by categorising into the originally defined biomass yield groups, then it becomes possible for distinct differences in strategy to be observed (Figure 3-6 C). The major difference between the biomass yield groups is between June and August, where the parents have a marked reduction in allocation away from the cutting and an increase in allocation away from the leaves. This distinct difference in the shift is also

which by June have only 29 % of their total nitrogen in the cutting, in contrast to 35% for the higher yielding progeny and 40% for the low yielding progeny (Figure 3-6 E).

A B

C D

E

Figure 3-6 Nitrogen Allocation Shifts Between Harvests for each Genotype and for Biomass Yield Groups

A + B Mean shift of total nitrogen allocation to each organ between June and August and August and October for each genotype. Error bars represent the standard error of the difference between the two means (n = 3-6). C + D Mean shift of total nitrogen allocation to each organ between June and August and August and October for each biomass yield group: Low, High and parents. Error bars represent the standard error of the difference between the two means (n = 12-30).E Mean total nitrogen

allocation to each organ in June for each biomass yield group: Low, High and parents. Error bars represent the standard error (n = 12-30).

3.4.4 Nitrogen Remobilisation/Secondary Allocation