Effects of changes in the supplies of nitrogen and carbon on the phenology and growth of Nothofagus Fusca (Hook.F.) Oerst.

Abstract

The tree genus Nothofagus is widespread in the Southern Hemisphere as a characteristic component of the Southern flora Nothofagus fusca (Hook.f.) Oerst. is an important species in many natural forest systems of New Zealand. It is unusual in having an evergreen habit, with a marked deciduous loss of old leaves in spring as new leaves emerge and expand, so that the tree is never devoid of leaves. This dual strategy has implications for processes of uptake and allocation of nitrogen (N) and carbon (C) to new growth in spring, which have not been investigated before. Evergreen trees store N in leaves during winter, and remobilisation to new growth in spring is independent of leaf senescence. Winter-deciduous trees remobilise N from storage in stems or roots. Responses to manipulations of N and C supplies to young trees of this species were studied in order to characterise, and understand, this unusual phenology. Measurements of stem, bud and leaf growth, leaf loss, and photosynthetic characteristics of leaves were made on young trees, grown in sand while irrigated with high (HN, 6 mM), medium (MN, 3 mM) and low (LN, 0.5 mM) concentrations of nitrogen, during two successive annual growth cycles. There were differences between treatments in the nitrogen content per leaf on a mass and an area basis, and specific leaf area, in the first cycle, but not during the second cycle. During the first cycle, an initial adjustment to increased N supply resulted in larger average leaf size as well as more leaves. At the start of the second cycle the number and mass of buds had increased approximately 4 times in MN trees, and approximately 8 times in HN trees, relative to LN trees. Comparative increases in leaf number measured at the end of spring, and at autumn, of the second cycle were 6-fold, and 19- and 22-fold. However, there was no change in average leaf size, maximum rates of photosynthesis or values for photosynthetic parameters. Leaf loss from the canopy during late summer and mid-autumn increased with nitrogen supply. This was explained as a mechanism to avoid summer water deficit, and support for this was provided from measurements of the carbon isotope ratios of the leaves. The effect of nitrogen (N) supply on biomass, and N storage and remobilisation for spring growth was also investigated in the same young trees, during the same two annual growth cycles. N acquired during the first cycle was labelled with 15N enriched to 5.5 atom % excess. By mid-autumn of the second cycle, dry weights of whole tree, stem, total leaves and roots were over 10-fold greater in HN and MN trees. N was stored in roots and the quantity stored was 20- fold greater in MN trees (P < 0.001). Stored N was remobilised into new leaves and stem extension during spring, comprising approximately 40% of all N in those tissues. HN and MN trees continued N uptake during winter dormancy. The amounts approximated half of all labelled N acquired during the first cycle, showing the importance of winter uptake in this temperate species. N remobilisation from roots may have been accompanied by fine root turnover. The number and mass of overwintering buds set during the first cycle was significantly greater in HN trees. It is possible that the absence of significant growth differences between HN and MN trees was due to the HN supply stimulating foliage growth beyond an optimum. Root storage has implications for understorey competition because of the effect on sub-soil spatial interactions of N and water availabilities. Ammonium nitrate fertiliser enriched to 10 atom % was applied to juvenile trees of Nothofagus fusca grown for 5 years at ambient and elevated C02 concentrations, about 2.5 weeks before budburst. The aim was to determine the timing of root N-uptake relative to budburst. Bud samples and, (following budburst), leaf samples, were harvested three times per week until all leaves on each tree were fully expanded. There were no significant differences between treatments in the timing of onset of budburst, or its duration, or in the onset of loss of leaves from the previous season. This was notwithstanding the greater mass of buds on trees growing at ambient C02 concentration. There was a trend for trees growing at elevated CO2 concentration to retain 30% of old season's leaves beyond 70 days from the start of spring. There was no difference in the absolute rate of expansion of new leaves but the seasonal increase in the mass of individual leaves was delayed in trees growing at elevated C02 concentration. Individual leaves of trees growing at elevated C02 concentration were greater in area and mass. Enhancement of leaf area was probably not due to any interactive effect of C supply on tree N status. Nitrogen concentration of leaves, on a mass basis, Nm, was lower in trees growing at elevated CO2 concentration. Growth of these trees at elevated CO2 concentration for five years did not produce changes in the onset of budburst or leaf loss during spring-early summer. Nor were there changes in the pattern of simultaneous root N-uptake, seen in younger trees. Conclusions are drawn as to why N. fusca is unusual in obtaining benefits from both evergreen and deciduous habits. The phenology of leaf growth and leaf loss in this species is unique. It not only provides deciduous advantages for storage and remobilisation of N in roots, but also evergreen capabilities of N-remobilisation independently of senescence, and root N-uptake during winter. The response to increased N availability is to expand the canopy by an increase in leaf numbers and associated stems. This renders the tree vulnerable to occasional summer drought. The presence of a strategy that enables leaves to be shed to avoid this does not compromise the need to store N during winter, since storage is in roots.