Zealand Radiata Pine Research Articles

Compression Strength Perpendicular to the Grain of New Zealand Radiata Pine Lumber

The compression strength perpendicular to the grain is one of the important timber properties for structural design. Exceeding the strength value will not only lead to large deformations and thus a serviceability issue, but it can also lead to a failure and thus a safety issue. Only some compression strength values and little information about the complete behaviour for Radiata Pine from New Zealand can be found in the literature. To correct this lack of information, tests have been conducted to investigate the compression behaviour, whereby compression strength values perpendicular to the grain much lower than the ones published in the current standard were noted. To make the designers become aware of this issue, the paper presents the experimental results of compression tests perpendicular to the grain with New Zealand Radiata Pine lumber. The test results are compared with strength values of the national and different international standards, as well as experimental research results of different species. It also gives an overview of the testing standards in use in different countries, showing the difficulty to determine a consistent strength value. According to the experimental results, the use of about half of the strength value published in the current design standard for compression perpendicular to the grain is recommended for structural sawn timber from New Zealand Radiata Pine.

Bolted and Dowelled Connections in Radiata Pine and Laminated Veneer Lumber Using the European Yield Model

Connections with mechanical fasteners are important for all cases of timber structures. The failure of these connections may occur in either ductile or brittle manner. For the calculation of the ductile failure strength or the load carrying capacity, the European yield model (EYM) is used in many standards and accepted as a very accurate model. In the current New Zealand timber standard NZS 3603:1993 (Standards New Zealand, 1993) and the Australian one AS1720.1–1997 (Standards Australia, 1997), the design concept for bolted or dowelled connections is not based on the EYM, and depends only on the diameter, the timber thickness and the species group. The most important parameters for the EYM are the fastener yield moment and the timber embedment strength, but embedment strength values are not available for New Zealand Radiata pine or laminated veneer lumber (LVL). To obtain the missing information and to implement the EYM into the New Zealand and Australian standards, embedment tests parallel, perpendicular and under various load-to-grain angles with different dowel diameters in Radiata pine lumber and LVL were conducted and compiled to build a database of embedment strength values. This paper includes the latest results of the investigations with dowel diameters extended up to 30 mm. Furthermore, different international testing standards are compared and their evaluation methods are used. The test results are also compared with the corresponding results using the Eurocode 5 formulas, and show that adjusted formulas of the Eurocode 5 can be used to predict the load carrying capacity of bolted and dowelled connections in Radiata pine lumber and LVL. Design examples comparing the current methods from the New Zealand/Australian design standards and the proposed method adopted from the EYM of the European design standard are given as well.

Predicting the vulnerability of New Zealand radiata pine plantations to the European pine weevil

Summary The European pine weevil, Hylobius abietis, is an important pest of temperate conifers and a significant biosecurity threat to countries such as New Zealand (NZ) where commercial forestry is largely dependent on pine monoculture. The economic impact of this insect is determined by its abundance and the degree of resistance of young pines to adult feeding. Assessing the likely impact of pine weevil in the event of accidental introduction is an important part of contingency planning. Resistance of 1- to 2-year-old trees was assessed by measuring the size of resin ducts within bark of the main stem and the mass of resin flowing from simulated feeding damage. Resin flow relative to duct area was much lower in radiata pine than in Corsican pines that are resistant to weevil feeding in the uK, indicating that NZ radiata pine transplants are likely to be highly susceptible. Local population size is influenced by the availability of pine root stumps after clearfelling. The short rotation of radiata pine and a likely shorter weevil generation time could increase weevil abundance and damage relative to the uK. however, larvae are likely to face competition from previously introduced bark beetles, and possibly native fungi, that rapidly exploit the bark of root stumps after felling. In particular, competition with Hylurgus ligniperda, which has an early spring flight and potential for two generations per year, is likely to reduce the number of weevils emerging from root stumps. Overall, pine weevil is likely to have less economic impact in NZ than in northern Europe.

Isolation and characterization of endophytic Beauveria spp. (Ascomycota: Hypocreales) from Pinusradiata in New Zealand forests

New Zealand Pinus radiata and associated exotic and native trees were surveyed for presence of Beauveria bassiana. Samples were collected from trees located in 33 sites in four geographically distinct areas of New Zealand. Needle samples were taken at all sites, while seed samples were taken from a single tree at one site and four root samples were collected from two sites where P. radiata seedlings were present. The bulk of the needle samples were from P. radiata, but exotic and indigenous trees growing in the same vicinity were also periodically sampled. After processing plant material from 167 trees, 21 fungal isolates resembling Beauveria spp. were recovered; DNA sequence analysis of a terminal region of the EF1-α locus showed that these isolates all classified as B. bassiana Clade A. The pathogenicity of these isolates against Hylurgus ligniperda adults and Tenebrio molitor larvae was confirmed in laboratory bioassays. This study demonstrated widespread occurrence of B. bassiana as endophytes of pine in New Zealand. This association may provide new opportunities to regulate insect pests of pine using a biological control agent.

Legacies of organic matter removal: decreased microbial biomass nitrogen and net N mineralization in New Zealand Pinus radiata plantations

The effect of variations in organic matter removal during harvesting on microbial biomass nitrogen was determined in four Pinus radiata plantations between 9 and 17 years after harvesting. Variation in microbial biomass nitrogen with season and the response of net nitrogen mineralization to organic matter removal after 9 and 17 years were also determined at two of the sites. The microbial biomass nitrogen in the fermentation-humus (FH) layer was correlated with litter fall characteristics and did not vary with organic matter removal, but the total mass of microbial biomass nitrogen in the FH layer was significantly reduced by increased organic matter removal. The microbial biomass nitrogen in the mineral soil was decreased by increased organic matter removal and was strongly correlated with moisture content and total nitrogen concentration. The FH layer microbial biomass nitrogen did not vary with season, but mineral soil microbial biomass nitrogen varied with season at one site, probably due to increased moisture availability. Net nitrogen mineralization in the mineral soil, determined by an anaerobic incubation, was decreased by increased organic matter removal and was strongly correlated with microbial biomass nitrogen. The persistence of the significant differences in microbial biomass nitrogen and net nitrogen mineralization indicated that variations in organic matter removal during harvesting have long-term effects on soil microbial properties and activity during the life of the subsequent rotation. This has implications for the selection of harvesting techniques to promote the maintenance of site productivity and to minimise disruption to the soil biota.

Impacts of forest harvesting related soil disturbance on end-of-rotation wood quality and quantity in a New Zealand radiata pine forest

The long-term effect of soil disturbance (litter removal, topsoil removal and compaction) from forest harvesting on wood quality and quantity of second-rotation Pinus radiata growing on a clay loam soil, was assessed at the end the rotation, 26 years after planting. Relative to Control plots, average tree and stand total volume at rotation end was not significantly affected by litter removal and nil or light compaction, but was significantly reduced by 28% by litter and topsoil removal and moderate subsoil compaction, and further reduced by 38% by heavy compaction. Wood density at breast height in the inner rings of trees in the most disturbed treatments was elevated by up to 30 kg m. This occurred because these treatments were more N deficient as reflected by foliar N levels during the first 11 years of growth relative to the Control. However, no treatment differences in wood density were evident in outer rings, and by rotation age overall mean density did not differ significantly between treatments. Neither acoustic velocity of standing trees, nor acoustic velocity of logs, was significantly affected by soil disturbance, indicating that stiffness of lumber cut from trees in the trial was likely to be similar for all treatments. Economic impacts of soil disturbance and compaction on this soil type will therefore result largely from the considerable negative impacts on final tree size, with little or no compensation from improved wood properties.–3

Longitudinal shrinkage behaviour of compression wood in radiata pine

The behaviour of longitudinal shrinkage was investigated in the corewood of a swept, 17-year-old New Zealand radiata pine stem. Wood categories in terms of normal wood, mild compression wood and severe compression wood were identified microscopically using autofluorescence of lignin. Average longitudinal shrinkage was collated according to corewood location and wood category within corewood in the leaning and the vertical parts of the stem, and then maximum radial difference of longitudinal shrinkage within growth ring was examined. The results show that the average longitudinal shrinkage is significant (2.4%) in the corewood of the leaning part of the stem. Among wood categories, severer compression wood displays the highest (2.9%) average longitudinal shrinkage. In the context of this study, growth rings may consist of one of three types of wood: (1) only normal wood; (2) a single compression wood type; and (3) mixed-type wood. Where multiple compression woods co-existed with normal wood, the maximum radial difference of longitudinal shrinkage within the growth ring was found to be 4.0%. A strong correlation (R 2 = 0.90) between average MFA and average longitudinal shrinkage suggests a significant influence of the average MFA on average longitudinal shrinkage across the three growth ring types.

Nitrogen fertiliser effects on litter fall, FH layer and mineral soil characteristics in New Zealand Pinus radiata plantations

The effects of nitrogenous fertilisation on litter fall, FH layer and soil characteristics were investigated in replicated trials in six second rotation New Zealand Pinus radiata plantation forests. Four trial sites also incorporated three different post-harvest organic matter removal treatments. All sites were sampled in early 2002 and 2003. Fertilisation significantly increased the nitrogen content and decreased the carbon:nitrogen ratio of the litter fall. Fertilisation significantly increased the mass of the FH layer in the treatment plots, moisture content in the FH layer, the concentration of nitrogen in the FH layer and the pool of carbon and nitrogen stored in the FH layer. Fertilisation significantly increased the nitrogen concentration of the mineral soil, and decreased the mineral soil carbon:nitrogen ratio and pH. Several significant site × fertilisation interaction terms indicated that variations in the fertilisation regimes and site characteristics substantially influenced the effects of fertilisation. Fertilisation did not significantly decrease the relative differences between the organic matter removal treatments. The significant differences in the litter fall, FH layer and mineral soil characteristics strongly suggest that nitrogenous fertilisation has the capacity to significantly alter the forest floor environment, and may be able to increase carbon storage over the life of the rotation.

Postharvest organic matter removal effects on FH layer and mineral soil characteristics in four New Zealand Pinus radiata plantations

The effects of three postharvest organic matter removal treatments on fermentation-humus (FH) layer and soil characteristics were compared in replicated trials in four second rotation New Zealand Pinus radiata plantation forests, planted 8–16 years prior to sampling. All sites were sampled in early 2002 and 2003. Increasing organic matter removal significantly decreased the mass of the FH layer in the treatment plots, the moisture content in the FH layer and mineral soil, the concentration of carbon in the FH layer and mineral soil, the pool of carbon stored in the FH layer, the concentration of nitrogen in the mineral soil and the pool of nitrogen stored in the FH layer. Mineral soil pH was significantly increased with increasing organic matter removal. The persistence of the significant differences in the FH layer and mineral soil characteristics strongly suggested that variations in organic matter removal have long-term effects on forest floor properties, and significantly influences carbon storage over the life of the rotation.

Isolation and characterization of Beauveria spp. associated with exotic bark beetles in New Zealand Pinusradiata plantation forests

A survey of entomopathogenic fungi associated with Hylastes ater and Hylurgus ligniperda was undertaken in six Pinus radiata forest sites in New Zealand. In each site soil, bark and frass associated with beetle galleries were sampled, as well as a selection of live beetles, larvae and mycosed cadavers. The density of Beauveria spp. (as measured by colony forming units) varied both within and between the sites and substrates sampled. The highest CFU density detected was 4 × 10 6 CFU/g dry frass, which was collected from a stump at the Mahurangi site in the North Island. A number of isolates were selected and characterized using PCR amplification of a terminal region of the EF1-α fragment. This analysis revealed that two Beauveria species, Beauveria caledonica and Beauveria bassiana, were commonly associated with bark beetle habitat. B. caledonica was the only species isolated from mycosed beetles, and was also recovered from soils, bark and frass, though not live insects. B. bassiana was also found in soils, bark, frass and live insects; however, it was not isolated from mycosed cadavers in this study. It is speculated that B. caledonica is the common ‘natural’ pathogen of bark beetles in their breeding habitat. A third species, Beauveria malawiensis, was isolated once from soil. This is the first report of this species from New Zealand. Thirteen isolates (selected from all substrates sampled and live beetles) were selected for laboratory bioassays. Under laboratory conditions all isolates were pathogenic to H. ater and H. ligniperda adults; there was little difference among the isolates in terms of efficacy despite the paucity of B. bassiana infection observed in the field. The mechanisms behind the lack of field infection observed for B. bassiana are not known, but are likely to be linked to limiting environmental factors in the breeding sites. These results indicate potential risks associated with the selection of strains for use in biological control programmes solely on the basis of laboratory bioassays, especially if the mode of action and biology of the species used are not fully understood. The importance of selecting strains which are environmentally competent is evident. Overall, however, this study demonstrates that entomopathogenic fungi can play a significant role in the regulation of H. ater and H. ligniperda populations in pine plantations.

Deterministic Simulation of Gains for Seedling and Cloned Main and Elite Breeding Populations of Pinus radiata and Implications for Strategy

Abstract Genetic gains in breast-height diameter were estimated using deterministic simulation. Simulations of gain from one generation of selection were undertaken in large Main and small Elite breeding populations for a range of heritabilities, with varying numbers of parents, families, seedlings/family, clones/family and ramets/ clone to aid revision of the New Zealand Pinus radiata breeding strategy. Cloned versus seedling populations of equal numbers of plants were simulated, derived from open pollination, polycrossing, and pair crossing. Balanced within-family selection was used for 200, 400 and 800-parent Main breeding populations and among- and within-family selection for 25-parent Elite populations of 25 up to 100 full-sib families. Predicted gains from within-family selection in the Main population were highest from cloned polycross families at all heritabilities and lowest for seedling fullsib families. Gains from cloned populations were higher than seedling equivalents at heritabilities ≤0.5, and their advantage in gain was greatest at lower heritabilities. Elite populations of 25 parents showed similar trends but intensive among- and within-family selection resulted in much higher gains than from the Main, highest from the cloned options. The increase in gain with increased number of families diminished with more than 2-3 times as many families as parents. A new strategy was proposed for P. radiata, based on the simulation results, involving an expanded Main breeding population of open-pollinated (OP) seedling families, together with pair-cross family seedlots already available, supported by parentage reconstruction using DNA markers. Forwards selection in small cloned Elite populations was proposed as the main source of seed orchard clones.

The Effect of Poly[methylene(polyphenyl isocyanate)] and Maleated Polypropylene Coupling Agents on New Zealand Radiata Pine Fiber–Polypropylene Composites

The effects of poly[methylene (polyphenyl isocyanate)] (PMPPIC) and maleated polypropylene (MAPP) coupling agents, separately and combined, on the strength and Young’s modulus of New Zealand radiata pine-reinforced polypropylene (PP) composites were assessed. A modest improvement compared to the unreinforced matrix strength of 4% and much greater improvement of 123%, substantially greater than reported elsewhere in the literature, were obtained using PMPPIC and MAPP respectively. Young’s modulus improvements compared to the unreinforced matrix of 77 and 177% were obtained using PMPPIC and MAPP respectively. The MAPP coupling agents with the longest chain lengths and lower functionality gave the greatest benefit, supporting chain entanglement as the major mechanism for enhanced bonding with PP. Relatively high moulding temperatures and low injection pressures, bringing about enhancement of fiber orientation in the test direction, also appear to have contributed to such large improvements in composite properties in the current work. Combination of PMPPIC and MAPP coupling agents produced mixed results: benefits were obtained by combining PMPPIC and MAPP with the longest chain lengths which can be explained by PMPPIC bonding with hydroxyl groups not accessible to further coupling with MAPP once saturation is achieved.

Optimum spot weed control treatment for a New Zealand radiata pine (<i>Pinus radiata</i>) plantation

A trial was established in the South Island of New Zealand to investigate the response of Pinus radiata to varying the area and duration of weed control around individual trees Results after six years demonstrated that weed control was essential to ensure maximum survival and growth of Pinus radiata seedlings Maintaining weed control for two seasons following planting resulted in marginally higher stand productivity than where weed control was applied in the year of planting only A modelling approach was used to determine treatment effects in terms of a time difference and to make inferences on longterm treatment effects Without weed control tree volume six years after planting was more than 15 years behind trees given complete weed control Spot vegetation control resulted in similar tree growth to total control as long as the area and duration of weed control was greater than a 1 m spot maintained for 1 year

Testing for linkage disequilibrium in the New Zealand radiata pine breeding population.

Linkage analysis is commonly used to find marker-trait associations within the full-sib families of forest tree and other species. Study of marker-trait associations at the population level is termed linkage-disequilibrium (LD) mapping. A female-tester design comprising 200 full-sib families generated by crossing 40 pollen parents with five female parents was used to assess the relationship between the marker-allele frequency classes obtained from parental genotypes at SSR marker loci and the full-sib family performance (average predicted breeding value of two parents) in radiata pine ( Pinus radiata D. Don). For alleles (at a marker locus) that showed significant association, the copy number of that allele in the parents was significantly correlated, either positively or negatively, with the full-sib family performance for various economic traits. Regression of parental breeding value on its genotype at marker loci revealed that most of the markers that showed significant association with full-sib family performance were not significantly associated with the parental breeding values. This suggests that over-representation of the female parents in our sample of 200 full-sib families could have biased the process of detecting marker-trait associations. The evidence for the existence of marker-trait LD in the population studied is rather weak and would require further testing. The exact test for genotypic disequilibrium between pairs of linked or unlinked marker loci revealed non-significant LD. Observed genotypic frequencies at several marker loci were significantly different from the expected Hardy-Weinberg equilibrium. The possibilities of utilising marker-trait associations for early selection, among-family selection and selecting parents for the next generation of breeding are also discussed.

Albino Strains of Ophiostoma Species for Biological Control of Sapstaining Fungi

SummaryAlbino strains ofOphiostoma floccosum,O. piceaeandO. pluriannulatumwere selected and screened for biological control of sapstaining fungi on New Zealand radiata pine (Pinus radiata). Albino strains were obtained through matings and single ascospore isolations from cultures of prevalent species in New Zealand. These strains do not synthesize the melanin-like hyphal pigments of common sapstaining fungi. Additional mating studies were also carried out to obtain isolates ofO. piceaeandO. floccosumthat lacked pigmented fruiting structures. Several albino isolates ofO. piceaewith colorless synnemata and isolates ofO. floccosumwith little to no synnemata were obtained. Biological control potential of the albino isolates was evaluated in the laboratory by challenging them on wood chips with fungi that cause extensive sapstain in pine,Leptographium procerum,Ophiostoma piliferumandSphaeropsis sapinea. Many albino isolates ofO. floccosum,O. piceaeandO. pluriannulatumwere effective in stopping the challenge fungi from staining wood chips and were fast growing and colorless when grown unchallenged on wood chips. Selected albino isolates ofO. pluriannulatumwere used in two field trials in New Zealand to control sapstain. Several strains were found to significantly reduce dark sapstain as compared to the untreated control logs.

Scaling up or scaling down: the use of foliage and soil information for optimising the phosphate nutrition of radiata pine

Fertilising with phosphate is important in New Zealand radiata pine plantations. Phosphate fertiliser trials have provided functions for determining potential growth gains from stands based on foliar P status. We used the national foliage database to determine the proportion of forest stands likely to be deficient in NZ. This gave a statistical representation but without a spatial reference. We used a nested design to test the spatial representation of foliar nutrient status and responsiveness at varied map scales. First we used the national soil map (1:10 6 scale) to show where P deficiency, and hence response, was most likely to occur. It was apparent that this representation was not acceptable for any operational scale planning. General trends in foliar status could be seen but there was no way of defining where foliage sampling should be concentrated within soil units, leading to inefficient sampling strategies. We then concentrated on the pumice, allophanic and podsol soil orders in the central North Island and used foliage data associated with 1:100,000 scale soil series map units. The spatial variation in likelihood of P deficiency and responsiveness was much clearer at this scale and more efficient foliar sampling programmes could be designed. At the most intensive scale (1:10,000) there were further gains to be made in foliage sampling efficiency and identification of responsive sites in some, but not all cases, and this depended on the soil pattern. We then tested whether spatial analysis of foliage data without reference to soil was better than using soil series as a defining class. Semivariograms constructed for foliar P concentration indicated that errors involved with predicting foliar P concentration were no less than if soil series was used as a predictor of foliar P status and responsiveness. We concluded that 1:100,000 soil series maps were adequate for general nutritional management planning and definition of areas likely to be responsive. However, for really precise work 1:10,000 maps would give additional definition for P fertilisation operations and allow us to refine fertiliser prescriptions at the within stand scale in some cases.

Adapting Finnish Multi-Source Forest Inventory Techniques to the New Zealand Preharvest Inventory

To measure many small areas in a forest, each to the required detail and precision, and sufficiently frequently, is too expensive using ground-based inventories alone. The Finnish satellite image-aided National Forest Inventory (NFI) method was modified and tested with a view to developing a new preharvesting forest inventory method for assessing the volumes of potential timber assortments (log products) in New Zealand radiata pine (Pinus radiata D. Don) plantations. Data from 188 ground plots were supplied by a forest company for a 1000 ha block of radiata pine. This material was combined with known stand boundaries from maps and data from a Landsat TM image in order to predict for each pixel and each stand the pruned and unpruned sawlog, pulp and total standing volumes. A k-nearest neighbour algorithm was applied for the estimation. Some stand characteristics, obtained from stand histories, were used as co-variates, in addition to spectral features. Cross-validation tests indicated that estimates of plot variables were unbiased, though with high RMSEs. Stand level estimates were also unbiased on average, but showed high RMSEs. Research into the effects of stratification and methods by which to segment stands into sub-areas uniform with respect to timber assortments is continuing.

Net ecosystem productivity, net primary productivity and ecosystem carbon sequestration in a Pinus radiata plantation subject to soil water deficit.

Tree carbon (C) uptake (net primary productivity excluding fine root turnover, NPP') in a New Zealand Pinus radiata D. Don plantation (42 degrees 52' S, 172 degrees 45' E) growing in a region subject to summer soil water deficit was investigated jointly with canopy assimilation (A(c)) and ecosystem-atmosphere C exchange rate (net ecosystem productivity, NEP). Net primary productivity was derived from biweekly stem diameter growth measurements using allometric relations, established after selective tree harvesting, and a litterfall model. Estimates of A(c) and NEP were used to drive a biochemically based and environmentally constrained model validated by seasonal eddy covariance measurements. Over three years with variable rainfall, NPP' varied between 8.8 and 10.6 Mg C ha(-1) year(-1), whereas A(c) and NEP were 16.9 to 18.4 Mg C ha(-1) year(-1) and 5.0-7.2 Mg C ha(-1) year(-1), respectively. At the end of the growing season, C was mostly allocated to wood, with nearly half (47%) to stems and 27% to coarse roots. On an annual basis, the ratio of NEP to stand stem volume growth rate was 0.24 +/- 0.02 Mg C m(-3). The conservative nature of this ratio suggests that annual NEP can be estimated from forest yield tables. On a biweekly basis, NPP' repeatedly lagged A(c), suggesting the occurrence of intermediate C storage. Seasonal NPP'/A(c) thus varied between nearly zero and one. On an annual basis, however, NPP'/A(c) was 0.54 +/- 0.03, indicating a conservative allocation of C to autotrophic respiration. In the water-limited environment, variation in C sequestration rate was largely accounted for by a parameter integrative for changes in soil water content. The combination of mensurational data with canopy and ecosystem C fluxes yielded an estimate of heterotrophic respiration (NPP' - NEP) approximately 30% of NPP' and approximately 50% of NEP. The estimation of fine-root turnover rate is discussed.

Primer cure and adhesion after light organic solvent preservative treatment of Radiata pine

This study examined the effect of preservative treatment and wood drying history on paint primer cure and adhesion for New Zealand radiata pine (Pinus radiata D. Don). Double Vacuum LOSP treatments resulted in a significantly lower LOSP uptake than did Lowry treatments, for both sapwood and heartwood. Wood drying history had no effect on uptake by sapwood, but did affect uptake by heartwood; high-temperature-kiln-dried heartwood retained more solvent than air-dried or conventionally dried heartwood. For sapwood, LOSP treatment method and wood drying history did not significantly affect either primer cure or primer adhesion. For heartwood, primer adhesion in the early stage of cure was better with Double Vacuum treatments. Double Vacuum treatments also resulted in a higher cure of primer, two days after priming, than did Lowry treatment when either conventionally or high-temperature-kiln-dried heartwood was used, but no significant difference was found with air-dried heartwood. It was concluded that, for radiata pine heartwood, Double Vacuum treatment, combined with either air-drying or conventional drying, will reduce LOSP uptake and give better primer performance, so long as effective ventilated solvent removal is achieved before priming.

Fluxes of carbon and water in a Pinus radiata forest subject to soil water deficit

We measured, by eddy covariance, seasonal CO2 (FCO2) and water (E) fluxes in an 8-year-old New Zealand Pinus radiata D.Don plantation subject to growing season soil water deficit. Average rates of FCO2 and E were highest in spring (324 mmol m-2 d-1 and 207 mol m-2 d-1, respectively) when the abiotic environment was most favourable for surface conductance and photosynthesis. During summer, fluxes were impeded by soil water (θ) deficit and were equal to or smaller than during winter (FCO2 = 46 mmol m-2 d-1 in summer and 115 mmol m-2 d-1 in winter; E = 57 and 47 mol m-2 d-1, respectively). On particularly hot and dry days, respiration exceeded photosynthetic uptake and the ecosystem was a net carbon source. Portraying the underlying biochemistry of photosynthesis, daytime half-hourly FCO2 increased with quantum irradiance absorbed by the canopy (Qabs) following a non-saturating, rectangular hyperbola. Except for winter, this relation was variable, including hysteresis attributable to diurnal variation in air saturation deficit (D). Daily ecosystem FCO2, FCO2/Qabs and FCO2/E were inversely proportional to maximum daily D, but in the cases of FCO2 and FCO2/Qabs only after soil moisture deficit became established. Consequently, as the tree growing season progressed, ecosystem carbon sequestration was strongly limited by the co-occurrence of high D at low θ.