Understanding the ecophysiological and biomechanical properties of juvenile Pinus radiata in response to water deficits
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
As the frequency and severity of drought events are expected to increase globally, drought induced reductions in plantation productivity are likely to become more important. This will concern forest managers who wish to improve forest productivity during the establishment and initial growth phases of plantation-grown Pinus radiata D. Don. Wood grown during the initial growth phases is in the corewood zone, which in Pinus radiata generally has poor wood properties. This restricts its usability, particular for solid timber applications. Therefore, understanding how water deficits impact on patterns of juvenile P. radiata productivity and wood variation is important. The objective of this research was to assess how key morphological and physiological processes, carbon fluxes and partitioning, and wood property formation in juvenile P. radiata responded to the timing and duration of water deficits. Using two-year-old P. radiata cultivated in a controlled environment, trees were subjected to a well watered control, plus early season cyclical drought, late season cyclical drought and summer drought treatments over the course of one growing season. Needle water potential, tree growth, crown characteristics, biomass partitioning, leaf characteristics, physiological processes and water use efficiency were measured. A carbon balance approach was used to examine how the timing and duration of water deficits modified gross primary production (GPP), net primary production (NPP), carbon fluxes to aboveground net primary production (ANPP) and total belowground carbon flux (TBCF), and the partitioning of GPP to ANPP and TBCF. Wood samples were analysed by X-ray absorption, X-ray diffraction and automated image analysis using Silviscan. Measures of wood density, microfibril angle (MFA), fibre dimensions and modulus of elasticity (E) were examined by matching seasonal variation in growth, measured temporally, with variation in wood properties, measured spatially. Further wood samples were assessed for longitudinal shrinkage and acoustic velocity. Cyclical drought treatments resulted in large fluctuations in needle water potential, while the summer drought treatment resulted in a sustained negative needle water potential over the summer months. Water stress integrals (Sψ) were 41.4, 66.8, 55.2 and 97.6 MPa-days for the well watered, early season cyclical drought, late season cyclical drought and summer drought treatments, respectively. In general, water deficits decreased tree growth, reduced crown size, reduced biomass accumulation and leaf area, reduced physiological activity and resulted in more enriched values of δ13C, all of which were significantly (P < 0.05) affected by treatment. Although the early season drought treatment experienced greater levels of water stress, growth and productivity were superior to those of the late season drought treatment. Summer drought reduced height, diameter and basal area by 24.7%, 33.1%, and 52.3%, respectively, while aboveground biomass was reduced by 68.3% and total leaf area by 40.0%. Water deficits substantially decreased fluxes to GPP, NPP, ANPP and TBCF following gradients of tree productivity. Treatment values of GPP were between 1470 and 4142 g C per tree per year. Partitioning of GPP to ANPP and TBCF was not affected by treatment, nor were the FS /ΔCR, TBCF/ANPP and ΔCR/ANPP ratios. Partitioning of GPP was predominantly to TBCF (45 - 56%) for all treatments. Partitioning of soil respiration (FS) did not significantly differ with treatment but FS was the dominant component of TBCF (77 - 88%) for all treatments. Wood properties of juvenile P. radiata were sensitive to temporal changes in water availability and associated growth rates. Imposition of seasonal water deficits resulted in higher values of air-dry density and modulus of elasticity (E) and decreases in microfibril angle (MFA). These differences were more evident for the late season drought treatment than for the early season drought treatment when compared to the well watered treatment. Late season drought increased density by 10%, E by 18% and decreased MFA by 5%, compared with the well watered trees. Seasonal water deficits had no impact on annual average values of density, E and MFA in this experiment but significant annual differences existed in cell wall thickness, cell radial diameter and cell populations between the faster and slower growing trees. Well watered trees had higher velocity and also greater longitudinal shrinkage which were significantly different (P < 0.05) from the water deficit treatments. Within treatment variation for acoustic velocity and longitudinal shrinkage was greatest for the well watered treatment. Weak to non-existent relationships were observed between longitudinal shrinkage and other wood properties. The results provide insight for forest managers of P. radiata into the importance of managing water deficits to maximise forest production and improve wood quality of juvenile trees. This study demonstrated that late season drought has a more marked impact on absolute growth and wood properties than early season drought, and that water deficits have a greater impact on growth than on partitioning of carbon or ring level wood properties.