Approximately 27 species (c. 5%) of the New Zealand woody flora have a marked loss of leaves in winter, although only 10 species are consistently fully deciduous and no extensive vegetation type is dominated by them. There are no summer deciduous species. The deciduous habit appears in most cases to have evolved independently within New Zealand from either ancestors or immigrants with short leaf life spans. New Zealand deciduous or semideciduous species typically lose substantial numbers of leaves throughout the growing season, and leaf loss is often gradual throughout winter but accelerated by frost. The deciduous habit in New Zealand tends to be a leaf-or shoot-level feature reflecting short leaf life spans and winter stress on individual shoots, rather than a systematic feature of the canopy-level organisation of the plant. Degree of deciduousness varies with taxon, plant age, climate, and soil fertility. A physiological study of cooccurring fully deciduous (Fuchsia excorticata) and semideciduous (Aristotelia serrata) trees in the South Island established that higher growing-season productivity of Fuchsia compensated for carbon gain forgone over winter. However, Fuchsia was significantly favoured over Aristotelia only on the coldest sites. Aside from some inland basins of the South Island, New Zealand has milder winters than those that favour deciduous taxa globally, and this is the primary reason for the low incidence of deciduous species. Even so, a number of deciduous species are common throughout. New Zealand deciduous trees and shrubs are typically fast growing and are characteristic of seral or forest marginal habitat on nutrient-rich soils developed in recent alluvium or debris. The relatively low nutrient status of most New Zealand forest soils makes a deciduous phenology, with its necessarily high turnover of nutrients, less competitive than a nutrient-conserving evergreen phenology. Deciduous species are often also divaricating, or have close relatives that are, and share a preference for nutrient-rich soils with this growth form. This strong relationship between the two habits suggests both are, in part, adaptations to stressful climates by plants with high-productivity leaves.
Two process-based models were used to identify the environmental variables limiting productivity in a pristine, mature forest dominated by rimu (Dacrydium cupressinum Sol. ex Lamb.) trees in South Westland, New Zealand. A model of canopy net carbon uptake, incorporating routines for radiation interception, photosynthesis and water balance was used to determine a value for quantum efficiency when climate variables were not limiting. The annual net carbon uptake by the canopy was estimated to be 1.1 kg C m(-2) and the quantum efficiency 22.6 mmol mol quanta(-1). This value of quantum efficiency, combined with other parameters obtainable from the literature, was then used in a model of forest productivity (3-PG), to simulate changes in net productivity and the allocation of carbon to tree components. The model was adjusted to match a measured stem increment of 10.6 Mg ha(-1) over a period of 13 years. To achieve this while maintaining a low, but stable value for leaf area index, it was necessary to set the site fertility rating very low and select high values for the parameters describing the proportional allocation of total carbon to roots. This approach highlighted nutrient availability as the principal constraint on productivity for the ecosystem and identified critical measurements that will be necessary for using the model to predict the effects of climate change on carbon sequestration. The low rates of carbon uptake and productivity are consistent with the low nutrient supply available from the highly leached, acid soils, most likely attributable to frequent saturation and a very shallow aerobic zone.
A forest biome map for New Zealand is presented, based on the ecosystem process model LINKNZ. Climate surfaces, landforms, slope, and soil types defined 88 933 homogeneous landscape units covering the North and South islands (264000 km 2 ). Forest development (2000 years) was simulated on each unit with 78 individually parameterised species selected by ecological importance. Forest biomes for the units were assigned by the relative biomass predicted for 21 plant functional types, categorised from the available species. An assessment of model performance against a systematic sample of 559 measured forest plots was satisfactory (dissimilarity index = 0.25). Direct validation was not possible over most of the landscape where native forest has been removed, but performance was sensible against data from 136 pre-deforestation pollen sites (dissimilarity index = 0.20). The modelled biome map reproduced the main characteristics of the current forest distribution in New Zealand and departures from the observed forest distribution were generally explained by omission of whole-stand disturbance effects from simulations. The model reproduced lowland areas of the striking "beech gap" in the west-central zone of the Southern Alps, but not the distribution of Nothofagus species in montane areas of this zone. The most likely explanation supported previous conclusions that the absence was due to a slow reinvasion of Nothofagus after exclusion during the Last Glaciation. Predictions of Nothofagus across some regions of the south-eastern South Island where it was nearly absent before settlement suggested that the ecological knowledge of some competing conifer species was incomplete.
A generalized computer model of forest growth and nutrient dynamics (LINKAGES) was adapted for the temperate evergreen forests of New Zealand. Systematic differences in species characteristics between eastern North American species and their New Zealand counterparts prevented the initial version of the model from running acceptably with New Zealand species. Several equations were identified as responsible, and those modeling available light were extended to give more robust formulations. The resulting model (LINKNZ) was evaluated by comparing site simulations against independent field measurements of stand sequences and across temperature and moisture gradients. It successfully simulated gap dynamics and forest succession for a range of temperate forest ecosystems in New Zealand, while retaining its utility for the forests of eastern North America. These simulations provided insight into New Zealand conifer–hardwood and beech species forest succession. The adequacy of the ecological processes, such as soil moisture balance, decomposition rates, and nutrient cycling, embodied in a forest simulation model was tested by applying it to New Zealand forest ecosystems. This gave support to the model’s underlying hypothesis, derived from LINKAGES, that interactions among demographic, microbial, and geological processes can explain much of the observed variation in ecosystem carbon and nitrogen storage and cycling. The addition of a disturbance option to the model supported the hypothesis that large‐scale disturbance significantly affects New Zealand forest dynamics.
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