Adaptation of stomatal conductance, photosynthesis and water-use efficiency at shoot and canopy scales in adjacent stands of Dacrycarpus dacrydioides and Podocarpus totara.

We tested an approach to estimate daily canopy net photosynthesis, A, based on estimates of transpiration, E, using measurements of sap flow and water-use efficiency, ω, by measuring δ13C in CO2 respired from shoots in the canopies of two conifers (Podocarpaceae) native to New Zealand. The trees were planted in adjacent 20-year-old stands with the same soil and environmental conditions. Leaf area index was lower for Dacrycarpus dacrydioides (1.34 m2 m-2) than for Podocarpus totara (2.01 m2 m-2) but mean (± standard error) stem diameters were the same at 152 ± 21 mm for D. dacrydioides and 154 ± 25 mm for P. totara. Over a 28-day period, daily A (per unit ground area) ranged almost five-fold but there were no significant differences between species (mean 2.73 ± 1.02 gC m-2 d-1). This was attributable to higher daily values of E (2.63 ± 0.83 mm d-1) and lower ω (1.35 ± 0.53 gC kg H2O-1) for D. dacrydioides compared with lower E (1.82 ± 0.72 mm d-1) and higher ω (1.90 ± 0.77 gC kg H2O-1) for P. totara. We attributed this to higher nitrogen availability and nitrogen concentration per unit foliage area, Na, and greater exposure to irradiance in the D. dacrydioides canopy compared with P. totara. Our findings support earlier observations that D. dacrydioides is more adapted to sites with poor drainage. In contrast, the high retention of leaf area and maintaining low rates of transpiration by P. totara, resulting in higher water-use efficiency, is an adaptive response to survival in dry conditions. Our findings show that physiological adjustments for two species adapted to different environments led to similar canopy photosynthesis rates when the trees were grown in the same conditions. We demonstrated consistency between whole-tree and more intensive shoot-scale measurements, confirming that integrated approaches are appropriate for comparative estimates of carbon uptake in stands with different species.

Relating nutritional and physiological characteristics to growth of Pinus radiata clones planted on a range of sites in New Zealand.

Six clones of radiata pine with known differences in growth rate were examined for clonal nutritional characteristics and for physiological determinants of clonal growth rate. We compared growth, foliar characteristics and nutrient, ¹³C and ¹5N concentration data for the six clones in 4- to 6-year-old field trials planted over a range of nutritionally contrasting sites. These data were also compared with growth, nutrient uptake and remobilization, foliar characteristic and gas exchange data from intensive physiological glasshouse experiments using 1- and 2-year-old plants of the same clones. Significant genotype x environment interactions in our field experiments conducted over strong nutritional gradients allowed us to identify radiata pine clones with consistent, superior growth and nutritional characteristics and clones that may be suited to particular site conditions. Our results suggest that the opportunity exists to exploit clone x site variation for site-specific clonal deployment and planting of fast-growing clones could be accompanied by planting of clones able to take relative advantage of site nutritional characteristics. Faster tree growth was not strongly related to any physiological characteristic, and the factors influencing growth rate differed among clones. The fastest-growing clone had consistent, high uptake of all nutrients, high fascicle weights and high water-use efficiency.

The influence of nitrogen and phosphorus supply and genotype on mesophyll conductance limitations to photosynthesis in Pinus radiata.

Mesophyll conductance, g(m), may pose significant limitations to photosynthesis and may be differentially affected by nutrition and genotype in Pinus radiata D. Don. Simultaneous measurements of gas exchange and chlorophyll fluorescence were made to determine g(m), using the constant J method (Harley, P.C., F. Loreto, G. Di Marco and T.D. Sharkey. 1992. Theoretical considerations when estimating the mesophyll conductance to CO(2) flux by analysis of the response of photosynthesis to CO(2). Plant Physiol. 98:1429-1436), in a fast- and a slow-growing clone of P. radiata grown in a greenhouse with a factorial combination of nitrogen (N) and phosphorus (P) supply. Values of g(m) increased linearly with the rate of photosynthesis at saturating irradiance and ambient CO(2) concentration, A(sat) (g(m) = 0.020A(sat), r(2) = 0.25, P < 0.001) and with stomatal conductance to CO(2) transfer, g(s) (g(m) = 1.16g(s), r(2) = 0.14, P < 0.001). Values of g(m) were greater than those of stomatal conductance, g(s), and the ratio (g(m)/g(s)) was not influenced by single or combined N and P additions or clone with a mean (+/-SE) value of 1.22 +/- 0.06. Relative limitations to mesophyll conductance, L(m) (16%) to photosynthesis, were generally greater than those imposed by stomata, L(s) (13%). The mean (+/-SE) CO(2) concentration in the intercellular air spaces (C(i)) was 53 +/- 3 mumol mol(-1) lower than that in the atmosphere (C(a)). Mean (+/-SE) CO(2) concentration in the chloroplasts (C(c)) was 48 +/- 2 mumol mol(-1) lower than C(i). Values of L(s), L(m) and CO(2) diffusion gradients posed by g(s) (C(a) - C(i)) and g(m) (C(i) - C(c)) did not significantly differ with nutrient supply or clone. Mean values of V(cmax) and J(max) calculated on a C(c) basis were 15.4% and 3.1% greater than those calculated on a C(i) basis, which translated into different slopes of the J(max)/V(cmax) relationship (C(c) basis: J(max) = 2.11V(cmax), r(2) = 0.88, P < 0.001; C(i) basis: J(max) = 2.43V(cmax), r(2) = 0.86, P < 0.001). These results will be useful for correcting estimates of V(cmax) and J(max) used to characterize the biochemical properties of photosynthesis for P. radiata.

The influence of N and P supply and genotype on carbon flux and partitioning in potted Pinus radiata plants.

Carbon (C) flux and partitioning responses of Pinus radiata (D. Don) clones to a factorial combination of nitrogen (N) and phosphorus (P) supply were estimated in small trees growing in a greenhouse over 44 weeks. Our objective was to use a C budget approach at the plant level to examine how a factorial combination of N and P additions and genotype modify gross primary production (GPP), net primary production (NPP), absolute C fluxes apportioned to aboveground net primary production (ANPP), aboveground plant respiration (APR), total belowground carbon flux (TBCF) and the partitioning of GPP to ANPP, APR and TBCF. Single N or P additions increased plant NPP and GPP similarly, but their combined effects exceeded those of their individual contributions. Nitrogen and to a lesser extent P additions enhanced carbon-use efficiency (CUE, NPP:GPP) and C partitioning to ANPP at the expense of TBCF. The fraction of GPP partitioned to APR was invariant to N or P additions. The ratio of soil respiration (FS) to TBCF was significantly greater in the low-N low-P addition treatment (61%) than in those treatments with single or combined N and P additions (49%). The slowest growing clone partitioned a significantly smaller fraction of GPP to ANPP (29%) than one of the faster-growing genotypes (33%). This research provides insight into how N and P regulate the C fluxes and partitioning in individual plants. Our results contribute to explaining clonal variation in aboveground growth rates and suggest that greater gains in CUE and partitioning to ANPP occur with addition of N rather than P supply.

Partititioning concurrent influences of nitrogen and phosphorus supply on photosynthetic model parameters of Pinus radiata.

Responses of photosynthesis (A) to intercellular CO(2) concentration (C(i)) were measured in a fast- and a slow-growing clone of Pinus radiata D. Don cultivated in a greenhouse with a factorial combination of nitrogen and phosphorus supply. Stomatal limitations scaled with nitrogen and phosphorus supply as a fixed proportion of the light-saturated photosynthetic rate (18.5%) independent of clone. Photosynthetic rates at ambient CO(2) concentration were mainly in the V(cmax)-limited portion of the CO(2) response curve at low-nitrogen supply and at the transition between V(cmax) and J(max) at high-nitrogen supply. Nutrient limitations to photosynthesis were partitioned based on the ratio of foliage nitrogen to phosphorus expressed on a leaf area basis (N(a)/P(a)), by minimizing the mean square error of segmented linear models relating photosynthetic parameters (V(cmax), J(max), T(p)) to foliar nitrogen and phosphorus concentrations. A value of N(a)/P(a) equal to 23 (mole basis) was identified as the threshold separating nitrogen (N(a)/P(a) < or = 23) from phosphorus (N(a)/P(a) > 23) limitations independent of clones. On an area basis, there were significant positive linear relationships between the parameters, V(cmax), J(max), T(p) and N(a) and P(a), but only the relationships between T(p) and N(a) and P(a) differed significantly between clones. These findings suggest that, in genotypes with contrasting growth, the responses of V(cmax) and J(max) to nutrient limitation are equivalent. The relationships between the parameters V(cmax), J(max), T(p) and foliage nutrient concentration on a mass basis were unaffected by clone, because the slow-growing clone had a significantly greater leaf area to mass ratio than the fast-growing clone. These results may be useful in discriminating nitrogen-limited photosynthesis from phosphorus-limited photosynthesis.

Modelling environmental variation in Young's modulus for Pinus radiata and implications for determination of critical buckling height.

BACKGROUND AND AIMS: Although density-specific stiffness, E/rho, (where E is Young's modulus and rho is wood density) is often assumed constant by the elastic similarity model, and in determination of critical buckling height (H(crit)), few studies have tested this assumption within species. Here this assumption is tested for Pinus radiata growing across an environmental gradient, and theory is combined with data to develop a model of Young's modulus. METHODS: Analyses use an extensive series of environmental plots covering the range of climatic and edaphic conditions over which P. radiata is grown in New Zealand. Reduced major axis regression was used to determine scaling exponents between log-log plots of H(crit) vs. groundline diameter (D), and E/rho vs. D. Path analysis was used to identify significant direct and indirect (through stem slenderness) edaphic and climatic influences on E. KEY RESULTS: Density-specific stiffness exhibited 3-fold variation. As E/rho scaled positively with D, the exponent of 0.95 between H(crit) and D exceeded the assumed value of 0.67 under constant E/rho. The final path analysis model included mean air temperature in early autumn (T(aut)) and slenderness as significant (P < 0.05) positive direct influences on E. Tree leaf area index and T(aut) were indirectly associated with E through their significant (P < 0.05) positive direct relationship with stem slenderness. Young's modulus was most sensitive to T(aut), followed by stem slenderness then leaf area index, and the final model explained 76 % of the variance in E. CONCLUSIONS: The findings suggest that within species E/rho variation may influence H(crit) and the scaling exponent between D and H(crit) so important in assumptions regarding allometric relationships. The model presented may provide a useful means of determining variation in E, E/rho and H(crit) across environmental gradients.