We tested the hypothesis that leaf/sapwood area ratios (S) in Scots pine (Pinus sylvestris L.) differ across Europe. Data from published records were collected and critically reviewed, and five new sites were studied to increase the overall sample size available for analyses. For seven studies for which data were available, we also determined the magnitude of the errors resulting from the use of ratio and regression-type estimators during subsampling for leaf areas of individual trees. These subsampling errors were then compared with those resulting from least square regressions of leaf area against sapwood area, and the total error was determined. Finally, correlation analysis was used to test for significant relationships between S and site-specific and stand-specific variables. Subsampling for leaf area resulted in errors, the magnitude of which was site specific and depended on sampling sizes and protocols. In general, despite larger total errors resulting from accounting for subsampling, significant differences were found among S, both at breast height and at the base of the living crown, at least for the most extreme cases. Significant negative relationships were found between S and summer vapour pressure deficit and maximum summer temperature. Although preliminary, our results confirm previous suggestions about climatic effects on Scots pine leaf/sapwood area ratios by enlarging the analysis to a wider range of European sites and by including genetic variability across stands.

Background and AimsThe major objective was to identify plant traits functionally important for optimization of shoot growth and nitrogen (N) economy under drought. Although increased leaf N content (area basis) has been observed in dry environments and theory predicts increased leaf N to be an acclimation to drought, experimental evidence for the prediction is rare.MethodsA pedigree of 200 full-sibling hybrid willows was pot-grown in a glasshouse in three replicate blocks and exposed to two water regimes for 3 weeks. Drought conditions were simulated as repeated periods of water shortage. The total leaf mass and area, leaf area efficiency (shoot growth per unit leaf area, EA), area-based leaf N content (NA), total leaf N pool (NL) and leaf N efficiency (shoot growth per unit leaf N, EN) were assessed.Key ResultsIn the water-stress treatment, shoot biomass growth was N limited in the genotypes with low NL, but increasingly limited by other factors in the genotypes with greatest NL. The NA was increased by drought, and drought-induced shift in NA varied between genotypes (significant G × E). Judged from the EA–NA relationship, optimal NA was 16 % higher in the water-stress compared with the well-watered treatment. Biomass allocation to leaves and shoots varied between treatments, but the treatment response of the leaf : shoot ratio was similar across all genotypes.ConclusionsIt is concluded that N-uptake efficiency and leaf N efficiency are important traits to improve growth under drought. Increased leaf N content (area basis) is an acclimation to optimize N economy under drought. The leaf N content is an interesting trait for breeding of willow bioenergy crops in a climate change future. In contrast, leaf biomass allocation is a less interesting breeding target to improve yield under drought.

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Hybrids of willow (Salix spp.) are today grown as biomass crops for energy in several cool-temperate regions including Sweden. These trees have great growth potential also under warmer climates, but may suffer from drought and heat. Research on tree growth under drought has often focused on the responses to permanent water stress. Less attention has been paid to the effects of temporary water stress on tree growth, although periods of strong water shortage are recurrent in many regions of the world. We performed a greenhouse experiment to assess the impact of five treatments including permanent (PWS; 30 days of water shortage) and temporary (TWS; 4-12 days of water shortage followed by normal watering) water stress on growth and leaf traits of 15 Salix genotypes. Plant biomass and height increment decreased across treatments in the order control (well-watered) [ TWS [ PWS, but there was also a significant genotype 9 treatment interaction. The pattern in growth traits was not directly reflected by the leaf traits, for which there was no significant genotype 9 treatment interaction (except for leaf angle). The significant genotype 9 treatment interactions for growth traits indicate a potential for breeding. Permanent water shortage does not necessarily represent an appropriate environment for the selection of genotypes for environments characterised by temporary water stress.

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