Stefan G. Schreiber

Genetic variation of hydraulic and wood anatomical traits in hybrid poplar and trembling aspen

Intensive forestry systems and breeding programs often include either native aspen or hybrid poplar clones, and performance and trait evaluations are mostly made within these two groups. Here, we assessed how traits with potential adaptive value varied within and across these two plant groups. Variation in nine hydraulic and wood anatomical traits as well as growth were measured in selected aspen and hybrid poplar genotypes grown at a boreal planting site in Alberta, Canada. Variability in these traits was statistically evaluated based on a blocked experimental design. We found that genotypes of trembling aspen were more resistant to cavitation, exhibited more negative water potentials, and were more water-use-efficient than hybrid poplars. Under the boreal field test conditions, which included major regional droughts, height growth was negatively correlated with branch vessel diameter (Dv ) in both aspen and hybrid poplars and differences in Dv were highly conserved in aspen trees from different provenances. Differences between the hybrid poplars and aspen provenances suggest that these two groups employ different water-use strategies. The data also suggest that vessel diameter may be a key trait in evaluating growth performance in a boreal environment.

Frost hardiness vs. growth performance in trembling aspen: an experimental test of assisted migration

Summary 1. According to the range limit hypothesis, the distribution of many temperate species is restricted by a trade-off between their capacity to survive winter extremes in the north (or high elevation) and their ability to compete with better-adapted species in the south (or low elevation range limits). This trade-off has important implications in forestry, particularly in the context of managed seed movement under climate change. 2. In this study, we aim to quantify trade-offs among growth, frost hardiness and timing of leaf senescence and bud break in populations of trembling aspen, Populus tremuloides Michx., which were observed in a large reciprocal transplant experiment across five planting sites in western Canada, including additional provenances from Minnesota. 3. After 10 years, we found pronounced increases in productivity as a result of long-distance transfers in a north-west direction. For example, provenances moved 1600 km north-west from Minnesota to central Alberta (a shift of 7° latitude to the north) produced almost twice the biomass of local sources. Similarly, provenances moved 800 km from central Alberta to north-east British Columbia (4° latitude north) also produced twice the biomass of local sources. 4. We further found that increased growth was not associated with lower survival rates. Bud break in provenances transferred north-west generally occurred slightly later than in local sources, suggesting decreased risk of spring frost injury. Leaf abscission was later in provenances transferred in a north-west direction, but they appeared to be very frost hardy, well ahead of very rare early fall frost events. 5. Synthesis and applications. This study demonstrated that assisted migration prescriptions have considerable potential to enhance forest productivity. In the case of aspen, even longdistance seed transfers in a north-west direction were successful. We conclude that benefits in productivity outweigh potential risks associated with northward transfer of aspen planting stock under both current and projected future climate conditions.

Sixteen years of winter stress: an assessment of cold hardiness, growth performance and survival of hybrid poplar clones at a boreal planting site

In recent years, thousands of hectares of hybrid poplar plantations have been established in Canada for the purpose of carbon sequestration and wood production. However, boreal planting environments pose special challenges that may compromise the long-term survival and productivity of such plantations. In this study, we evaluated the effect of winter stress, that is, frequent freeze-thaw and extreme cold events, on growth and survival of 47 hybrid poplar clones in a long-term field experiment. We further assessed physiological and structural traits that are potentially important for cold tolerance for a selected set of seven clones. We found that trees with narrow xylem vessels showed reduced freezing-induced embolism and showed superior productivity after 16 growing seasons. With respect to cold hardiness of living tissues, we only observed small differences among clones in early autumn, which were nonetheless significantly correlated to growth. Maximum winter cold hardiness and the timing of leaf senescence and budbreak were not related to growth or survival. In conclusion, our data suggest that reduction of freezing-induced embolism due to small vessel diameters is an essential adaptive trait to ensure long-term productivity of hybrid poplar plantations in boreal planting environments.

An ecophysiological and developmental perspective on variation in vessel diameter

Variation in xylem vessel diameter is one of the most important parameters when evaluating plant water relations. This review provides a synthesis of the ecophysiological implications of variation in lumen diameter together with a summary of our current understanding of vessel development and its endogenous regulation. We analyzed inter-specific variation of the mean hydraulic vessel diameter (Dv ) across biomes, intra-specific variation of Dv under natural and controlled conditions, and intra-plant variation. We found that the Dv measured in young branches tends to stay below 30 µm in regions experiencing winter frost, whereas it is highly variable in the tropical rainforest. Within a plant, the widest vessels are often found in the trunk and in large roots; smaller diameters have been reported for leaves and small lateral roots. Dv varies in response to environmental factors and is not only a function of plant size. Despite the wealth of data on vessel diameter variation, the regulation of diameter is poorly understood. Polar auxin transport through the vascular cambium is a key regulator linking foliar and xylem development. Limited evidence suggests that auxin transport is also a determinant of vessel diameter. The role of auxin in cell expansion and in establishing longitudinal continuity during secondary growth deserve further study.

Variation of xylem vessel diameters across a climate gradient: insight from a reciprocal transplant experiment with a widespread boreal tree

Summary 1. Xylem vessel diameters represent an important plant hydraulic trait to ensure sufficient water supply from the roots to the leaves. The ability to adjust the hydraulic pathway to environmental cues is key in order to satisfy transpirational demands and maximize growth and survival. 2. We evaluated the variability of vessel diameters in trembling aspen in a reciprocal transplant experiment. We tested six provenances from three ecological regions of North America planted at four test sites in western Canada. All test sites were established at the same time with the same provenances arranged in a randomized complete block design. 3. Vessel diameter showed a strong interaction of population and test site suggesting a high degree of phenotypic plasticity in this trait. Gaussian kernel density estimates support plastic as well as genetic contributions in vessel diameter control trending from bimodal distributions at the most northern test site towards unimodal distributions at the warmest and mildest test site. 4. Furthermore, we used test site-specific climate data in form of a 2-year, 5-year and 10-year average of 21 directly and derived climatic variables and found that average site-specific vessel diameters correlated strongly with summer moisture availability. A previously found negative relationship with vessel diameter and tree height in central Alberta was also found at two other boreal test sites but reversed at a wetter and milder sub-boreal test site. 5. In summary, vessel diameters were highly plastic in response to different environments and varied with summer moisture availability. The variability of vessel diameter and tree height correlations suggests that vessel diameter alone cannot serve as a reliable proxy for long-term growth performance beyond boreal environments. Instead, selecting aspen populations with a high degree of plasticity in this trait appears to be the safest option for assisted migration and seed transfer programmes under climate change.

Leaf size serves as a proxy for xylem vulnerability to cavitation in plantation trees

Hybrid poplars are an important renewable forest resource known for their high productivity. At the same time, they are highly vulnerable to water stress. Identifying traits that can serve as indicators for growth performance remains an important task, particularly under field conditions. Understanding which trait combinations translate to improved productivity is key in order to satisfy the demand for poplar wood in an uncertain future climate. In this study, we compared hydraulic and leaf traits among five hybrid poplar clones at 10 plantations in central Alberta. We also assessed the variation of these traits between 2- to 3-year-old branches from the lower to mid-crown and current-year long shoots from the mid to upper crown. Our results showed that (1) hybrid poplars differed in key hydraulic parameters between branch type, (2) variation of hydraulic traits among clones was relatively large for some clones and less for others, and (3) strong relationships between measured hydraulic traits, such as vessel diameter, cavitation resistance, xylem-specific and leaf-specific conductivity and leaf area, were observed. Our results suggest that leaf size could serve as an additional screening tool when selecting for drought-tolerant genotypes in forest management and tree improvement programmes.

On research priorities to advance understanding of the safety–efficiency tradeoff in xylem

We appreciate Bittencourt et al.’s (2016) constructive contributions following our paper, Gleason et al. (2016), on the proposed tradeoff between hydraulic safety and efficiency. To continue this dialog we would like to comment on which of the research directions proposed by Bittencourt et al. seem most promising to us. We agree that various xylem tissue fractions could potentially modify the safety–efficiency relationship. In principle, any tissue fraction could trade off with any other tissue fraction. However, as a matter of observation, parenchyma fraction is negatively correlated with fiber fraction, whereas parenchyma and fiber fractions are not strongly correlated with vessel lumen fraction (Ziemi! nska et al., 2015; Morris et al., 2016). As such, vessel lumen fraction, vessel diameter, and vessel frequency are largely uncoupled from nonvessel tissue fractions across self-supporting angiosperm species (Zanne et al., 2010), and are therefore unlikely to trade off with mechanical safety and hydraulic efficiency (or safety). Furthermore, vessel lumen fraction itself does not vary markedly across angiosperms, ranging from c. 5% to 20% (mean! 15%) (Zanne et al., 2010; Morris et al., 2016), although larger fractions are not uncommon in ring-porous and climbing species. Contrasts between climbing (e.g. lianas) and freestanding growth forms are more likely to show differences in allocation to vessel vs nonvessel space, and the climbing habit therefore may offer a more appropriate system for evaluating this idea (Gartner, 1991). Gymnosperm xylem differs from angiosperm xylem in that it generally lacks axial parenchyma, and conduits are both conductive and load-bearing. Greater mechanical safety may be negatively correlated with both hydraulic safety and efficiency across gymnosperm species (Mayr & Cochard, 2003; Mayr et al., 2003). Considering angiosperms, it is likely that nonvessel tissue fractions influence the safety–efficiency tradeoff indirectly (e.g. via their contribution to xylem capacitance, or whole-plant growth), rather than being forced by limited xylem space. Bittencourt et al. suggest that expressing conductivity as a ratio with mass, rather than cross-sectional area, might better characterize the energetic costs associated with xylem. We agree with this suggestion and did consider the influence of specific gravity on the safety–efficiency tradeoff in our paper (Table 2 and Figs 3d, 4d in Gleason et al., 2016). Here, we formulate these results by expressing the y axis explicitly as xylem-specific conductivity/ specific gravity (Fig. 1), as suggested by Bittencourt et al. Specific gravity, safety and efficiency values were generally obtained from the same published reports. Similar to the results we report in Gleason et al. (2016), including specific gravity in the analyses increases the tradeoff r 2 in both angiosperms (0.11–0.14) and gymnosperms (0.10–0.15) when safety is defined as P50. A similar increase in r 2 is achieved when defining safety as P88 and there is no change when defining safety as P12. Although including xylem density does increase the amount of variation explained by the models, they still fall far short of explaining why many species exhibit both low safety and low efficiency. Despite the analysis provided in Fig. 1, we feel that the clearest approach to analyzing these inter-correlated variables will be to consider all known sources of variation (e.g. structural equation models) rather than expressing them as a ratio with conductivity (e.g. conductivity/parenchyma fraction). Such ratios build an assumption of proportionality between the two elements of the ratio, which are not necessarily what we should expect. It remains a possibility that xylem safety, expressed as the xylem water potential at which a fraction of maximal conductance is lost, may not be an accurate approximation for all species in all situations. Although we agree in principle that safety (e.g. P50) may not correlate with mortality similarly across species, there is good evidence to suggest that it does for many angiosperm and gymnosperm species (Pratt et al., 2008; Brodribb & Cochard, 2009; Brodribb et al., 2010). This suggests that there may be an intrinsic property of xylem to resist desiccation (angiosperm Ψleaf > P88; gymnosperm Ψleaf > P50), beyond which the probability of mortality increases precipitously. Measurements of hydraulic safety, as well as conductivity during drought, should serve as more appropriate predictors of mortality than other measurements of water status (e.g. turgor loss point in leaves or stomatal response) because percentage loss of conductance is a meaningful representation of xylem desiccation. However, it is also clear that there are mechanisms that delay the time to reach a desiccation–mortality threshold. As suggested by Bittencourt et al. and Brodersen (2016), these would include deciduousness, deep rooting, reduced stomatal ‘leakiness’, reduced cuticular conductance, CAM and C4 metabolism, and capacitance. However, considering tradeoffs with either safety or efficiency, in isolation of one another (e.g. safety–capacitance), does not inform our efforts to understand the proposed link between safety and efficiency. For example, if greater capacitance reduces the requirement for safety, natural selection should still be free to improve efficiency, which would provide benefit via greater

Post-glacial biogeography of trembling aspen inferred from habitat models and genetic variance in quantitative traits

Using species distribution models and information on genetic structure and within-population variance observed in a series of common garden trials, we reconstructed a historical biogeography of trembling aspen in North America. We used an ensemble classifier modelling approach (RandomForest) to reconstruct palaeoclimatic habitat for the periods 21,000, 14,000, 11,000 and 6,000 years before present. Genetic structure and diversity in quantitative traits was evaluated in common garden trials with 43 aspen collections ranging from Minnesota to northern British Columbia. Our main goals were to examine potential recolonisation routes for aspen from southwestern, eastern and Beringian glacial refugia. We further examined if any refugium had stable habitat conditions where aspen clones may have survived multiple glaciations. Our palaeoclimatic habitat reconstructions indicate that aspen may have recolonised boreal Canada and Alaska from refugia in the eastern United States, with separate southwestern refugia for the Rocky Mountain regions. This is further supported by a southeast to northwest gradient of decreasing genetic variance in quantitative traits, a likely result of repeated founder effects. Stable habitat where aspen clones may have survived multiple glaciations was predicted in Mexico and the eastern United States, but not in the west where some of the largest aspen clones have been documented.

On research priorities to advance understanding of the safety-efficiency tradeoff in xylem: A response to Bittencourt et al.'s (2016) comment 'On xylem hydraulic efficiencies, wood space-use and the safety-efficiency tradeoff': in this issue of New Phytologist, pp. 1152-1155

We appreciate Bittencourt et al.’s (2016) constructive contributions following our paper, Gleason et al. (2016), on the proposed tradeoff between hydraulic safety and efficiency. To continue this dialog we would like to comment on which of the research directions proposed by Bittencourt et al. seem most promising to us. We agree that various xylem tissue fractions could potentially modify the safety–efficiency relationship. In principle, any tissue fraction could trade off with any other tissue fraction. However, as a matter of observation, parenchyma fraction is negatively correlated with fiber fraction, whereas parenchyma and fiber fractions are not strongly correlated with vessel lumen fraction (Ziemi! nska et al., 2015; Morris et al., 2016). As such, vessel lumen fraction, vessel diameter, and vessel frequency are largely uncoupled from nonvessel tissue fractions across self-supporting angiosperm species (Zanne et al., 2010), and are therefore unlikely to trade off with mechanical safety and hydraulic efficiency (or safety). Furthermore, vessel lumen fraction itself does not vary markedly across angiosperms, ranging from c. 5% to 20% (mean! 15%) (Zanne et al., 2010; Morris et al., 2016), although larger fractions are not uncommon in ring-porous and climbing species. Contrasts between climbing (e.g. lianas) and freestanding growth forms are more likely to show differences in allocation to vessel vs nonvessel space, and the climbing habit therefore may offer a more appropriate system for evaluating this idea (Gartner, 1991). Gymnosperm xylem differs from angiosperm xylem in that it generally lacks axial parenchyma, and conduits are both conductive and load-bearing. Greater mechanical safety may be negatively correlated with both hydraulic safety and efficiency across gymnosperm species (Mayr & Cochard, 2003; Mayr et al., 2003). Considering angiosperms, it is likely that nonvessel tissue fractions influence the safety–efficiency tradeoff indirectly (e.g. via their contribution to xylem capacitance, or whole-plant growth), rather than being forced by limited xylem space. Bittencourt et al. suggest that expressing conductivity as a ratio with mass, rather than cross-sectional area, might better characterize the energetic costs associated with xylem. We agree with this suggestion and did consider the influence of specific gravity on the safety–efficiency tradeoff in our paper (Table 2 and Figs 3d, 4d in Gleason et al., 2016). Here, we formulate these results by expressing the y axis explicitly as xylem-specific conductivity/ specific gravity (Fig. 1), as suggested by Bittencourt et al. Specific gravity, safety and efficiency values were generally obtained from the same published reports. Similar to the results we report in Gleason et al. (2016), including specific gravity in the analyses increases the tradeoff r 2 in both angiosperms (0.11–0.14) and gymnosperms (0.10–0.15) when safety is defined as P50. A similar increase in r 2 is achieved when defining safety as P88 and there is no change when defining safety as P12. Although including xylem density does increase the amount of variation explained by the models, they still fall far short of explaining why many species exhibit both low safety and low efficiency. Despite the analysis provided in Fig. 1, we feel that the clearest approach to analyzing these inter-correlated variables will be to consider all known sources of variation (e.g. structural equation models) rather than expressing them as a ratio with conductivity (e.g. conductivity/parenchyma fraction). Such ratios build an assumption of proportionality between the two elements of the ratio, which are not necessarily what we should expect. It remains a possibility that xylem safety, expressed as the xylem water potential at which a fraction of maximal conductance is lost, may not be an accurate approximation for all species in all situations. Although we agree in principle that safety (e.g. P50) may not correlate with mortality similarly across species, there is good evidence to suggest that it does for many angiosperm and gymnosperm species (Pratt et al., 2008; Brodribb & Cochard, 2009; Brodribb et al., 2010). This suggests that there may be an intrinsic property of xylem to resist desiccation (angiosperm Ψleaf > P88; gymnosperm Ψleaf > P50), beyond which the probability of mortality increases precipitously. Measurements of hydraulic safety, as well as conductivity during drought, should serve as more appropriate predictors of mortality than other measurements of water status (e.g. turgor loss point in leaves or stomatal response) because percentage loss of conductance is a meaningful representation of xylem desiccation. However, it is also clear that there are mechanisms that delay the time to reach a desiccation–mortality threshold. As suggested by Bittencourt et al. and Brodersen (2016), these would include deciduousness, deep rooting, reduced stomatal ‘leakiness’, reduced cuticular conductance, CAM and C4 metabolism, and capacitance. However, considering tradeoffs with either safety or efficiency, in isolation of one another (e.g. safety–capacitance), does not inform our efforts to understand the proposed link between safety and efficiency. For example, if greater capacitance reduces the requirement for safety, natural selection should still be free to improve efficiency, which would provide benefit via greater

On research priorities to advance understanding of the safety-efficiency tradeoff in xylem: A response to Bittencourtet al.'s (2016) comment ‘On xylem hydraulic efficiencies, wood space-use and the safety-efficiency tradeoff’

We appreciate Bittencourt et al.’s (2016) constructive contributions following our paper, Gleason et al. (2016), on the proposed tradeoff between hydraulic safety and efficiency. To continue this dialog we would like to comment on which of the research directions proposed by Bittencourt et al. seem most promising to us. We agree that various xylem tissue fractions could potentially modify the safety–efficiency relationship. In principle, any tissue fraction could trade off with any other tissue fraction. However, as a matter of observation, parenchyma fraction is negatively correlated with fiber fraction, whereas parenchyma and fiber fractions are not strongly correlated with vessel lumen fraction (Ziemi! nska et al., 2015; Morris et al., 2016). As such, vessel lumen fraction, vessel diameter, and vessel frequency are largely uncoupled from nonvessel tissue fractions across self-supporting angiosperm species (Zanne et al., 2010), and are therefore unlikely to trade off with mechanical safety and hydraulic efficiency (or safety). Furthermore, vessel lumen fraction itself does not vary markedly across angiosperms, ranging from c. 5% to 20% (mean! 15%) (Zanne et al., 2010; Morris et al., 2016), although larger fractions are not uncommon in ring-porous and climbing species. Contrasts between climbing (e.g. lianas) and freestanding growth forms are more likely to show differences in allocation to vessel vs nonvessel space, and the climbing habit therefore may offer a more appropriate system for evaluating this idea (Gartner, 1991). Gymnosperm xylem differs from angiosperm xylem in that it generally lacks axial parenchyma, and conduits are both conductive and load-bearing. Greater mechanical safety may be negatively correlated with both hydraulic safety and efficiency across gymnosperm species (Mayr & Cochard, 2003; Mayr et al., 2003). Considering angiosperms, it is likely that nonvessel tissue fractions influence the safety–efficiency tradeoff indirectly (e.g. via their contribution to xylem capacitance, or whole-plant growth), rather than being forced by limited xylem space. Bittencourt et al. suggest that expressing conductivity as a ratio with mass, rather than cross-sectional area, might better characterize the energetic costs associated with xylem. We agree with this suggestion and did consider the influence of specific gravity on the safety–efficiency tradeoff in our paper (Table 2 and Figs 3d, 4d in Gleason et al., 2016). Here, we formulate these results by expressing the y axis explicitly as xylem-specific conductivity/ specific gravity (Fig. 1), as suggested by Bittencourt et al. Specific gravity, safety and efficiency values were generally obtained from the same published reports. Similar to the results we report in Gleason et al. (2016), including specific gravity in the analyses increases the tradeoff r 2 in both angiosperms (0.11–0.14) and gymnosperms (0.10–0.15) when safety is defined as P50. A similar increase in r 2 is achieved when defining safety as P88 and there is no change when defining safety as P12. Although including xylem density does increase the amount of variation explained by the models, they still fall far short of explaining why many species exhibit both low safety and low efficiency. Despite the analysis provided in Fig. 1, we feel that the clearest approach to analyzing these inter-correlated variables will be to consider all known sources of variation (e.g. structural equation models) rather than expressing them as a ratio with conductivity (e.g. conductivity/parenchyma fraction). Such ratios build an assumption of proportionality between the two elements of the ratio, which are not necessarily what we should expect. It remains a possibility that xylem safety, expressed as the xylem water potential at which a fraction of maximal conductance is lost, may not be an accurate approximation for all species in all situations. Although we agree in principle that safety (e.g. P50) may not correlate with mortality similarly across species, there is good evidence to suggest that it does for many angiosperm and gymnosperm species (Pratt et al., 2008; Brodribb & Cochard, 2009; Brodribb et al., 2010). This suggests that there may be an intrinsic property of xylem to resist desiccation (angiosperm Ψleaf > P88; gymnosperm Ψleaf > P50), beyond which the probability of mortality increases precipitously. Measurements of hydraulic safety, as well as conductivity during drought, should serve as more appropriate predictors of mortality than other measurements of water status (e.g. turgor loss point in leaves or stomatal response) because percentage loss of conductance is a meaningful representation of xylem desiccation. However, it is also clear that there are mechanisms that delay the time to reach a desiccation–mortality threshold. As suggested by Bittencourt et al. and Brodersen (2016), these would include deciduousness, deep rooting, reduced stomatal ‘leakiness’, reduced cuticular conductance, CAM and C4 metabolism, and capacitance. However, considering tradeoffs with either safety or efficiency, in isolation of one another (e.g. safety–capacitance), does not inform our efforts to understand the proposed link between safety and efficiency. For example, if greater capacitance reduces the requirement for safety, natural selection should still be free to improve efficiency, which would provide benefit via greater