Relating Carbon Allocation Patterns to Tree Senescence in Metrosideros Forests

Abstract

The ratios of carbohydrate sink to source in the shoot systems of mature and juvenile Metrosideros polymorpha trees were compared, testing a prediction of the pipe model theory that this ratio is proportional to height, thus explaining the synchrony of forest decline in even—height stands. Volumes of sapwood (S) and inner bark (IB), and leaf mass (L) were measured by destructive sampling in 5 mature and 12 juvenile trees. S:L and IB:L of mature tree shoot systems were higher than of juvenile trees, strongly supporting the hypothesis that mature trees have a lower surplus of carbohydrate production and are less resistant to many forms of stress. The specific leaf burden (S:L and IB:L jointly) within the mature tree sample varied by a factor of about 2, compared with a variation in height by a factor of about 1.35, suggesting that a relatively strong environmental stress is needed to trigger the synchronous death of all or most trees in a stand. The pipe model, with its prediction of constant Huber value (stem cross—sectional area per unit supported leaf mass), does not accurately represent the distribution of either the sapwood or the inner bark in M. polymorpha. The sapwood Huber values varied significantly among the organ systems of individual trees and between life states. Within each organ system the sapwood cross—sectional area in larger stems was strongly correlated with supported leaf mass, while the inner—bark cross—sectional areas was better correlated with the cross—sectional area of the stem. Sapwood Huber values were similar in twigs and small branches of juvenile and mature trees, but were much lower in the larger stems of mature trees than at any point in the juvenile trees. The pattern of variation of the inner—bark Huber values among organ systems was similar in juvenile and mature trees. A reduction of the sapwood Huber value in the large stems of mature trees associated with heartwood development decreased the rate of increase of S:L with size. IB:L increased very little in the juvenile state, but it increased rapidly in the mature state when leaf mass failed to increase in linear proportion with total tree volume. The volume of inner bark was found to be about one—half as great as the volume of sapwood in mature trees, but is likely to be responsible for a disproportionate part of total aboveground respiration.