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 CO₂ flux by analysis of the response of photosynthesis to CO₂. 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 CO₂ concentration, A_sat (g_m = 0.020A_sat, r² = 0.25, P < 0.001) and with stomatal conductance to CO₂ transfer, g_s (g_m = 1.16g_s, r² = 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) CO₂ concentration in the intercellular air spaces (C_i) was 53 ± 3 μmol mol⁻¹ lower than that in the atmosphere (C_a). Mean (±SE) CO₂ concentration in the chloroplasts (C_c) was 48 ± 2 μmol mol⁻¹ lower than C_i. Values of L_s, L_m and CO₂ 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, r² = 0.88, P < 0.001; C_i basis: J_max = 2.43V_cmax, r² = 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.

Forest plantations have a large potential for carbon sequestration, playing an important role in the global carbon cycle. However, despite the large amount of research carried out worldwide, the absolute contribution of forest plantations is still incomplete for some parts of the world. To help bridge this gap, we calculated the amount of C stock in three fast growing forest species in Chile. Carbon pools in above-ground and below-ground biomass, forest floor, and soil were considered for this analysis. Across the plantation forests of Chile, carbon accumulated in the above-ground biomass was 181–212 Mg · ha${}^{-1}$ for Pinus radiata, 147–180 Mg · ha${}^{-1}$ for Eucalyptus nitens, and 95–117 Mg · ha${}^{-1}$ for Eucalyptus globulus (age 20–24 years for P. radiata and 10–14 years for Eucalyptus). Total C stocks were for 343 Mg · ha${}^{-1}$ for P. radiata, 352 Mg · ha${}^{-1}$ for E. nitens, and 254 Mg · ha${}^{-1}$ for E. globulus, also at the end of a typical rotation. The carbon pool in the forest floor was found to be significantly lower (less than 4% of the total) when compared to the other pools and showed large spatial variability. Our results agree with other studies showing that 30–50% of the total C stock is stored in the soil. The baseline data will be valuable for modelling C storage changes under different management regimes (changes in species, rotation length and stocking) and for different future climates. Given the contribution of soils to total carbon stocks, special attention should be paid to forest management activities that affect the soil organic carbon pool.

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 (F_S) 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.

Although the understory vegetation abundance, diversity, and composition associated with stand development in natural forests have been well reported, only a few studies have examined the age-related changes of understory vegetation in fast-growing planted forests in reclaimed soils. This study measured the understory vegetation and soil variables in 8-, 12-, and 18-year-old poplar plantations in reclaimed coastal saline soil of Eastern China. This study examined how the abundance, diversity, and composition changed with stand development and the soil variables. Further, structural equation modeling (SEM) was used to evaluate the direct and indirect factors influencing the abundance and plant diversity throughout stand development. Herb abundance was significantly higher in the youngest and oldest stands, whereas shrub abundance was higher in the middle-aged stands. Shannon’s diversity index was significantly higher in the youngest stand for herbs, whereas it was highest in the middle-aged stands for shrubs. A multivariate analysis revealed that the herb and shrub composition were influenced by the stand age, total soil carbon and soil pH. The most parsimonious SEM model showed the negative direct effects of the stand age and the negative indirect effects of the stand age via the soil variables on shrub abundance, shrub diversity, and herb diversity, suggesting that the increase of overstory biomass with the stand age reduces resources available for the understory. Our results revealed that understory diversity and composition might change with stand development mediated by the changes in understory light and soil resources in fast-growing plantations.

Quillaja saponaria Mol., Cryptocarya alba Mol. Looser, and Lithraea caustica Molina Hook et Arn., are common sclerophyllous species in Mediterranean Central Chile. Mesophyll conductance, g m, may strongly limit photosynthesis in these semiarid environments.