Abstract
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. Such reductions will be of concern to forest managers looking to improve forest productivity during the establishment and initial growth phases of plantation-grown Pinus radiata D. Don. The objective of this research was to assess how growth, biomass, leaf area and water-use efficiency in juvenile Pinus radiata responded to the timing and duration of water deficits. Two-year-old Pinus radiata seedlings (cultivated in a polyhouse) were subjected to various water deficit treatments. Needle water potential, tree growth, biomass partitioning, leaf characteristics and water-use efficiency were measured to assess the impact that the timing and duration of water deficits had on productivity. Cyclical re-watering of the early- and late-season drought treatments led to large fluctuations in needle water potential. The summer drought treatment resulted in a sustained low needle water potential over the summer months. Total water stress integral (S ψ ) was 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, and resulted in more enriched values of δ13C, all of which were significantly (P < 0.05) affected by treatment. Summer drought reduced height, diameter and basal area by 24.7%, 33.1% and 52.3%, respectively. Total biomass was reduced by 64.7% and total leaf area by 40.0%, compared with the well-watered treatment. The reduction in stem diameter growth was larger for the late-season treatment than the early-season treatment when compared with the well-watered trees suggesting that late-season drought is more detrimental than early-season drought to tree growth. The results provide insight for forest managers of Pinus radiata into the importance of managing water deficits in order to maximise forest production of juvenile trees. Results suggest that late-season drought is more detrimental to absolute growth and biomass accumulation than early-season drought.
Highlights
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
The higher slope for the late-season drought treatment in Figure 4 means that similar monthly water stress integral had a more pronounced effect on monthly stem diameter growth compared with the early-season drought treatment, whilst demonstrating that the earlyseason drought trees experienced greater levels of water stress
The strong positive relationship between woody biomass and foliage biomass across all treatments and tree sizes in this experiment conforms to the pipe model theory advocated by Shinozaki et al (1964a, b). These results suggest that, in juvenile P. radiata, estimates of canopy leaf area can be made from the sapwood area at the base of the tree
Summary
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. Reductions in available soil water result in closure of stomata, which in turn constrains growth of P. radiata and other conifers. While this response to water deficits helps conserve water by Waghorn et al New Zealand Journal of Forestry Science (2015) 45:3 limiting evaporative losses, it reduces carbon dioxide uptake, thereby reducing photosynthetic rate, tree growth and wood production (Waring and Franklin 1979; Benecke 1980; Whitehead 1985; Thompson and Wheeler 1992; Yunusa et al 1995a). Earlier work by Rook et al (1977) showed that greater levels of water stress caused transpiration and photosynthesis to decline to near zero, which induced shedding of older foliage
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