The mass of fine litterfall and nutrient circulation through litterfall were determined in four Melrosideros polymorpha/Cibotium spp.-dominated rainforests that differed in substrate age, parent material texture and annual precipitation on Kilauea and Mauna Loa volcanoes on the island of Hawaii. Three of the sites had rates of litterfall of 5.2 Mg ha−1 y−1; the fourth, which was on the most fertile soil, produced 7.0 Mg ha−1 y−1 of litterfall with higher concentrations of nitrogen and phosphorus. Tree ferns of the genus Cibotium cycled relatively large amounts of nitrogen, phosphorus and potassium through litterfall; their contribution to nutrient circulation was disproportionate to their mass in the forest, or in litterfall. The forest on the youngest substrate, which also had the lowest concentrations of nitrogen in litterfall, was fertilized with complete factorial combinations of nitrogen, phosphorus and a treatment consisting of all other plant nutrients. Additions of nitrogen increased the quantity and nitrogen concentration in litterfall during the second year following the initiation of fertilization, while no other treatment had a significant effect. Additions of nitrogen had no effect on litterfall mass or nutrient concentrations in the most nutrient-rich site.
Questions Do long‐term observations in permanent plots confirm the conceptual model of Metrosideros polymorpha cohort dynamics as postulated in 1987? Do regeneration patterns occur independently of substrate age, i.e. of direct volcanic disturbance impact? Location The windward mountain slopes of the younger Mauna Loa and the older Mauna Kea volcanoes (island of Hawaii, USA). Methods After widespread forest decline (dieback), permanent plots were established in 1976 in 13 dieback and 13 non‐dieback patches to monitor the population structure of M. polymorpha at ca. 5‐yr intervals. Within each plot of 20 × 20 m, all trees with DBH >2.5 cm were individually tagged, measured and tree vigour assessed; regeneration was quantified in 16 systematically placed subplots of 3 × 5 m. Data collected in the subplots included the total number of M. polymorpha seedlings and saplings (five stem height classes). Here we analyse monitoring data from six time steps from 1976 to 2003 using repeated measures ANOVA to test specific predictions derived from the 1987 conceptual model. Results Regeneration was significantly different between dieback and non‐dieback plots. In dieback plots, the collapse in the 1970s was followed by a ‘sapling wave’ that by 2003 led to new cohort stands of M. polymorpha. In non‐dieback stands, seedling emergence did not result in sapling waves over the same period. Instead, a ‘sapling gap’ (i.e. very few or no M. polymorpha saplings) prevailed as typical for mature stands. Canopy dieback in 1976, degree of recovery by 2003 and the number of living trees in 2003 were unrelated to substrate age. Conclusions Population development of M. polymorpha supports the cohort dynamics model, which predicts rebuilding of the forest with the same canopy species after dieback. The lack of association with substrate age suggests that the long‐term maintenance of cohort structure in M. polymorpha does not depend on volcanic disturbance but may be related to other environmental mechanisms, such as climate anomalies.
Approximately 50,000 ha of native '6hi'a (Metrosideros polyrnorpha) rain forest on the windward side of the island of Hawaii experienced a pronounced dieback of the tree canopy during the 1960s and early 1970's. The forests affected were located between 600 and 1,500 m elevation where the median annual rainfall was greater than 2500 mm. Dieback stands were found on a variety of different substrates ranging from 500-10,000 years in age. Most of the stands that experiencel dieback had canopy cover greater than 60% prior to 1960.In 1976 and 1977 we established 43 relevrs in dieback and non-dieback forest stands to assess canopy tree vigor, composition and structure of the vegetation, and to describe substrate characteristics. Metrosideros population structure and tree vigor were resampled for 25 of these relevrs in 1982 and 1985-1986. The results of air photo analysis and ground sampling showed that dieback has not expanded very much within the study area since 1977. However, 5 of the relevrs sampled in 1977 continued to decline in tree canopy vigor between 1977 and 1982 while 2 others showed a slight recovery in vigor over this same period.Seedling and sapling regeneration has been extremely vigorous in most of the sites that experienced a breakdown of the canopy while, in stands with an intact, dense tree canopy no such regeneration occurred. It appears that most of the stands which experienced canopy dieback have the potential to become closed forest communities again, dominated by Metrosideros. The natural recovery process may be disrupted in some areas due to additional competition for light and nutrients from invading populations of both native and alien plant species that have become established following canopy dieback.
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.
Large numbers of Metrosideros polymorpha trees have died in the montane rain forest on the Island of Hawai'i, but previous research has failed to identify a principal cause. This paper describes an experiment that tests the hypothesis that nutrient deficiency is the principal cause of tree death and stand—level dieback. Treatments were fertilizing, stand thinning, and a combination of the two. No significant change in the mortality rate was caused by the treatments; however, stem diameter growth of surviving trees was found to be nutrient limited at all sites. We conclude that nutrient deficiency is not the principal cause of most Metrosideros dieback, but it may be a contributing factor.
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