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Trembling aspen (Populus tremuloides Michx.) and other poplars (e.g. Populus balsamifera L.; Populus deltoides Bartr. ex Marsh.; Populus trichocarpa Torr. & A. Gray) play an important role in North American ecosystems, particularly in the boreal forest and the aspen parklands of the prairie provinces (Alberta, Saskatchewan, Manitoba) in western Canada (Richardson et al., 2007). Poplars (Populus ssp.) are among the fastest growing temperate trees and are considered to be vegetational pioneers (Eckenwalder, 1996; Bradshaw et al., 2000). Poplars also represent an attractive and valuable forest resource as they grow quickly and are easy to propagate from both seed and vegetative propagation (Peterson & Peterson, 1992; Cooke & Rood, 2007). For instance, tree breeders in western Canada carry out intensive selection and breeding programs for poplars, searching for trees that produce high-quality wood for pulp and for oriented strand board production, but are also able to withstand the dry cold climate of the Canadian prairies. Tree improvement programs often include either native aspen or nonnative hybrid poplar clones in their breeding programs, and performance and trait evaluations are mostly made within these two groups, as reflected by a large number of studies conducted on either aspen or hybrid poplars. However, a comprehensive comparison between these two groups is still lacking (Lieffers et al., 2001), even though it may become very valuable information for species selection in the context of climate change.
When selecting suitable genotypes for a particular location, the ‘local is best’ concept is normally applied, where nearby seed sources are selected for reforestation. Using locally adapted planting material reflects physiological adaptations of numerous tree generations to the local climate and site conditions. However, an accelerated trend in global warming (Houghton, 2005) may require a human-based relocation of certain genotypes from their southern distribution limits up to places where natural migration through seed dispersal would not be sufficient, given the magnitude of current and predicted climate change (Aitken et al., 2008). In addition, hybrids among North American and Eurasian species of poplar are widely used for their superior growth characteristics. In both cases, physiological and field testing are required before large-scale deployment of this often nonlocal or novel plant material. These tests are typically common garden experiments that can differentiate environmental and genetic differences among genotypes in a shared environment (Gornall & Guy, 2007).
In central Alberta, it may be particularly beneficial to facilitate the introduction of aspen genotypes from more southern latitudes, as climate warming and decreases in precipitation for this region over the last 25 yr have been very pronounced. The province of Alberta, for instance, has experienced warming of c. 0.7°C and a reduction of mean annual precipitation of 20% over this period (Mbogga et al., 2009). In 2002, a severe regional drought led to massive aspen dieback and mortality in the aspen parklands of southern Alberta (Hogg et al., 2008). Historically, droughts have always been part of the climate in the Canadian prairies (Roberts et al., 2006; Bonsal & Regier, 2007). However, more frequent and more severe droughts have been recorded in the recent past (including another exceptional drought in 2009), and this poses a serious threat for local vegetation.
Since most poplar species are known to be sensitive to water deprivation (Blake et al., 1996; Shock et al., 2002), the question of how aspen and hybrid poplars will respond to drier conditions is becoming an important issue. Although poplar species are among the most susceptible trees to drought, considerable genotypic variability exists in water-use efficiency, growth performance, hydraulic traits, and tolerance to moderate water deficits, particularly in hybrid poplar clones (Morrison et al., 2000; Monclus et al., 2006; DesRochers et al., 2007; Silim et al., 2009; Fichot et al., 2010). Even greater differences are likely to exist between hybrid poplars and aspen as a group, but a comprehensive comparison of hydraulic traits between these two groups has, to our knowledge, not been conducted.
Xylem traits, along with root and soil properties, can play an important role in limiting canopy water supply (Sperry et al., 2002; McDowell et al., 2008). Xylem properties may be especially important in riparian cottonwoods (Rood et al., 2000) and hybrid poplars, which are known to be highly vulnerable to cavitation (Fichot et al., 2010). As a result of cavitation and subsequent embolism, hydraulic conductivity in the xylem (Kh) declines as the xylem pressure becomes more negative. This dependence of Kh on xylem pressure is often referred to as a vulnerability curve (Sperry et al., 2002). Comparisons of more or less distantly related taxa have shown that, at the interspecific level, cavitation resistance is often correlated with the water potential range that plants experience in their natural habitat (Hacke et al., 2000; Pockman & Sperry, 2000). Interspecific comparisons have also linked differences in cavitation resistance with trends in xylem structure and transport efficiency (Maherali et al., 2004; Hacke et al., 2006; Jacobsen et al., 2007; Jansen et al., 2009). However, such correlations may not be found when comparing closely related genotypes (Cochard et al., 2007) or populations of a single species (Martinez-Vilalta et al., 2009). For instance, a tradeoff between xylem safety and xylem transport efficiency was absent across eight hybrid poplar genotypes (Fichot et al., 2010), although it was found in a survey of 29 angiosperm species of diverse growth form and family affinity (Hacke et al., 2006).
In the present study, we measured genetic differences in hydraulic and wood anatomical traits of six aspen genotypes and seven hybrid poplar clones growing at a boreal planting site in Alberta, Canada. Aspen genotypes represented three provenances (Alberta, British Columbia, and Minnesota; Table 1). We assessed how traits varied within and across these two plant groups. We asked whether relationships between hydraulic traits seen in broad interspecific surveys would also be resolvable at a finer phylogenetic scale, that is, across the studied genotypes of the genus Populus. We also evaluated the potential of linking differences in xylem traits with growth performance. Growth was measured as height and diameter at breast height (DBH), integrated over 16 and 11 yr in hybrid poplar and aspen trial data, respectively. A long-term goal is to identify easily accessible traits that can serve as predictors of growth performance under field conditions in this boreal environment. Finally, we assessed which of the measured traits in aspen were conserved by geographic source (provenance) and which varied independently. The plantations were designed as long-term field experiments and represent a good opportunity to investigate the previously outlined issues in a common garden setting.
Table 1. Geographic origin of aspen seed sources and height and diameter at breast height (DBH) measured after 11 growing seasons in the field in a provenance field trial in central Alberta, Canada
|Region||Provenance #||Latitude||Longitude||Elevation (m)||Height11 (m)||DBH11 (cm)|
|British Columbia||9||58°12′N||123°20′W||1177||5.6 (0.2)||7.0 (0.5)|
|British Columbia||10||58°36′N||122°20′W||335||6.0 (0.5)||8.0 (0.5)|
|Alberta||25||55°36′N||113°25′W||762||8.8 (0.3)||9.5 (0.6)|
|Alberta||26||54°56′N||112°44′W||545||7.7 (0.3)||8.8 (0.5)|
|Minnesota||39||47°12′N||93°48′W||405||11.3 (0.2)||13.5 (0.6)|
|Minnesota||41||47°30′N||93°36′W||433||11.0 (0.2)||13.9 (0.5)|
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Fig. S1 Characterization of local climate conditions for the planting site and for the three aspen provenances locations Minnesota, Alberta and British Columbia.
Table S1 Analysis of Variance for physiological parameters and growth traits measured in hybrid poplar clones
Table S2 Analysis of Variance for physiological parameters and growth traits measured in aspen provenances
Table S3 Mean annual temperature (MAT), mean growing season precipitation (MGP), and mean annual precipitation (MAP) for the planting site as well as the aspen provenance locations
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