1. Introduction
The Great Lakes Basin is one of the most important natural resources in North America, providing numerous environmental, economic, and societal benefits. Zalesny et al. [
1] elaborated on these benefits, in addition to the substantial role of the Basin in provisioning freshwater and related ecosystem services to millions of people each year [
2,
3]. This unique water resource, however, is becoming increasingly degraded by anthropogenic activities. Legacy pollution, urban runoff and stormwater, and agricultural inputs (i.e., herbicides, pesticides, nutrients) have all contributed to declining water quality of the Basin, leading to 99% of the surface water being impaired for one or more designated use(s) [
4].
Landfills, waste dumps, and similar sites have contributed to non-point source pollution, especially due to the continuous rise in waste generation and concomitant increases in landfill size [
5]. Landfill leachate is a potential pollution source from municipal landfill sites that is often characterized by low biodegradability, high nitrogen content, and presence of other pollutants [
6]. Leachate and associated surface runoff are often managed through proactive preventative measures or reactive remediation strategies to prevent water contamination. Phytoremediation is one potential long-term, sustainable solution for achieving runoff reduction and cleaning/filtering of water, in which plants and their associated microorganisms are used for environmental cleanup [
7,
8]. Pollutants are remediated by various mechanisms such as accumulation in plant tissues, plant and microbe metabolism, and volatilization [
9,
10,
11]. Plant water uptake can also reduce contaminant mobility at a site [
12].
Purpose-grown trees, particularly poplars (
Populus spp.) and other short rotation woody crops (SRWCs), are well-suited to phytotechnology applications due to their ideal physiological, morphological, and genetic traits [
13]. Poplars can help managers achieve remediation goals in a condensed timeframe (e.g., <20 years) based on specific silvicultural prescriptions that are matched to site and management objectives [
14]. Additionally, poplar-based phytotechnologies can provide other ecosystem services such as carbon sequestration and biomass feedstocks for biofuels, bioenergy, and bioproducts [
15,
16,
17]. In recent decades, poplar biomass production systems have become more important globally, given the large demand for wood combined with sustainable forest management goals. As a result, tree breeding and improvement strategies are needed now more than ever to maximize the performance of poplars for achieving specific remediation and ecosystem service objectives.
As with agronomic and horticultural crops, tree breeding and improvement began hundreds of years ago, and over time has expanded to include numerous coniferous and broadleaved species [
18]. Significant results have been obtained within the
Populus genus through spontaneous and controlled hybridization and breeding throughout the last century [
19,
20]. Broad genetic variation, both within and among
Populus species, coupled with their ability to undergo successful intra- and inter-specific hybridization, in addition to the ability of some species to propagate readily from cuttings, have driven the success of poplar tree improvement [
21,
22,
23]. To prove the superiority of new collections and crosses, poplar genotypes and cultivars undergo complex testing in multi-environmental trials (MET), in which phenotypic responses to different environments, defined as genotype by environment interactions (G × E), are evaluated. Similarly, METs are used to test the robustness in genotypic performance across varying site and climatic conditions [
24,
25]. These G × E interactions have been studied often, leading to the characterization of genotypes as generalists or specialists [
26,
27]. Over the years, traits of interest in poplar breeding programs have evolved from agronomic characteristics (e.g., yield, pest and disease resistance, rooting capabilities) to more contemporary traits relating to biomass production (e.g., physiological drivers of productivity and wood properties) [
22] and ecosystem services [
13,
16].
Regional clonal development in the Midwestern United States has proliferated since the 1930s due to extensive open-pollination collections, intra- and inter-sectional hybridization, and increased interest in wood biomass production [
19,
20,
28]. Over 100,000 poplar offspring have been created since the 1950s [
14], with the majority produced by regional breeding programs at the University of Illinois (J. Jokela; B. McMahon), Iowa State University (R. Hall; B. McMahon), University of Minnesota (C. Mohn; D. Riemenschneider), and University of Minnesota Duluth (B. McMahon; W. Berguson). Clonal testing has been highly active since the 1990s [
20], with multiple MET networks being established around the Midwest to monitor biomass production [
29,
30,
31]. From these METs, Netzer et al. [
32] showed the greatest potential of clones was for
P. deltoides Bartr. ex Marsh ×
P. nigra L. ‘DN’ hybrids (a.k.a.,
P. ×
euramericana (Dode) Guinier;
P. ×
canadensis Moench) ‘DN21’, ‘DN154’, ‘DN164’, ‘DN170’, ‘DN177’, and ‘NE264’, in addition to
P. nigra ×
P. maximowiczii A. Henry ‘NM’ hybrid ‘NM2’. With the exception of ‘NM2’, all clones were ‘DN’ hybrids exhibiting generalist growth performance. Another poplar clonal regional testing network was established in 1995, 1997, and 2000 across Iowa, Michigan, Minnesota, and Wisconsin [
28]. This MET network initially contained 42 clones but was expanded to a total of 187 clones, most of which were from the aforementioned Midwestern breeding programs [
33]. Results from these METs showed greater biomass productivity rates than any previously recorded in the region, leading Riemenschneider et al. [
28] to conclude the need for continued tree improvement activities. Significant G × E interactions defined generalist (‘NC14105’, ‘Crandon’, ‘NM2’) and specialist (‘7300501’, ‘80 × 01015’, ‘NC14103’) clones [
33], which have since been tested for ecosystem services and environmental technologies [
16,
34].
The most recent poplar breeding and testing has been conducted at the University of Minnesota Duluth, Natural Resources Research Institute (NRRI) [
35,
36]. In parallel with traditional clonal testing of poplar productivity through evaluation of genotypic growth and stability [
36], NRRI researchers have tested the application of different silvicultural measures [
37] and defined geo-robust clones (i.e., extreme generalists) for establishment across broader latitudinal and longitudinal ranges [
38]. A contemporary goal of this and other poplar breeding efforts is to test clones for a wide range of ecosystem services such as carbon sequestration and phytoremediation [
17,
39].
Poplars have been tested and deployed extensively in phytoremediation systems to remediate organic [
9,
40,
41,
42,
43] and inorganic contaminants [
44,
45,
46,
47], in addition to newer classes of pollutants such as contaminants of emerging concern (CECs) [
48,
49,
50]. Testing poplar clones for phytoremediation is a complex process including breeding and selection for: (1) traditional traits related to growth and productivity [
51,
52,
53]; (2) tolerance of contaminants, determined by investigating physiological and metabolic processes [
42,
43,
47,
54]; and (3) phytoremediation potential exhibited by contaminant accumulation/degradation [
41,
55,
56]. Simultaneous selection for such a broad range of breeding traits can be achieved with phyto-recurrent selection, a stepwise testing process. In this method, crop and tree improvement strategies are implemented over multiple testing cycles to identify and select clones with superior performance [
14,
57]. Throughout the selection process, the number of clones decreases while the number of tested parameters and cycle length increase. Selection using basic traits such as growth and root:shoot ratio is enhanced with data on additional parameters such as tree health and growth performance index [
58]. Further investigation often includes greenhouse and field-testing clonal performance related to contaminant effects and accumulation, ecophysiology, and morpho-anatomical changes [
45,
46]. Following multiple selection cycles in the greenhouse, field validation of selected clones is a necessary step in phyto-recurrent selection. For example, testing clones used in the current study, Zalesny and Bauer [
59] reported broad clonal variation across eleven-year-old trees grown for nitrate phytoremediation in the Midwestern US. These phyto-recurrent selection results further emphasize the importance of long-term phytoremediation studies in evaluating clonal performance throughout stand development [
17].
As described by Zalesny et al. [
1], phyto-recurrent selection was used to establish an ongoing MET testing network consisting of sixteen phytoremediation buffer systems (i.e., phyto buffers) at sites located in the Lake Superior (i.e., Michigan’s Upper Peninsula) and Lake Michigan (i.e., eastern Wisconsin) watersheds. Given the potential of new genotypes in the biomass productivity networks illustrated above, our overarching objective in the current study was to test for ecological restoration potential of new clones developed at NRRI. To do so, we divided clones into Experimental (i.e., genotypes with a rich history of testing but are still at the experimental stage) and Common (i.e., genotypes commonly used for commercial and/or research purposes in the region) clone groups that we then compared with each other and each NRRI clone planted at the phyto buffers. Although Zalesny et al. [
1] compared individual clones, these current comparisons are warranted because poplar clones in the Midwestern United States are often selected in groups based on stage of testing (i.e., Experimental versus Common) rather than individually, due to uncertainties with nursery production and availability of clonal material. Specifically, we tested for differences in the three clone groups (i.e., NRRI, Experimental, Common), phyto buffers (i.e., environments), and their interactions for health, height, diameter, and volume during early field establishment (i.e., from one to four years after planting). These data are useful to advance poplar tree improvement efforts throughout the region, continent, and world, informing clonal selection for multiple end-uses, including phytotechnologies.
4. Discussion and Conclusions
Selection of
Populus and other short rotation woody crop (SRWC) species to match specific site and growing conditions is imperative for maximizing productivity [
64]. The availability of appropriate genotypes can be necessary for plantation or site managers in the absence of precise site information [
65]. Species of
Populus, a genus utilized ubiquitously for environmental applications, have been bred and tested extensively for biomass production [
66], especially beginning in the early 1990s with international germplasm exchanges and other cooperative tree improvement efforts between the United States and Europe [
67]. Results of these testing efforts have shown great potential of new genotypes for biomass production. Building on these successful partnerships, the poplar breeding and testing program at the University of Minnesota Duluth, Natural Resources Research Institute (NRRI) has produced thousands of genotypes since the mid-1990s [
35,
36]. Some of these clones have been defined as geo-robust, meaning they are extreme generalists with the capability for establishment across broader latitudinal and longitudinal ranges [
38]. The ecological restoration potential of a subset of these clones was tested in the current study, and clones ‘99038022’ and ‘9732-31’ exhibited exceptional survival and growth across eleven and ten phytoremediation buffer systems (i.e., phyto buffers), respectively, in the Lake Superior watershed of the Upper Peninsula of Michigan, USA and the Lake Michigan watershed of eastern Wisconsin, USA. Other NRRI clones showed exceptional promise at individual phyto buffers, demonstrating the value of matching individual genotypes to specific site conditions. This combination of generalist and specialist genotypes corroborated the importance of such multi-environmental trials (MET) throughout plantation development, making the current data useful for advancing poplar tree improvement efforts throughout the region, continent, and world, informing clonal selection for multiple end-uses, including phytotechnologies.
Across the United States, average annual poplar productivity of approximately 9 Mg ha
−1 yr
−1 is common, with advanced genotypes exhibiting nearly 2.5 times as much growth [
68]. In the Midwestern United States, the location of the current study, a wide range of poplar biomass productivity potential has been reported. Most common stand densities of 1075 and 1736 trees ha
−1 (i.e., 3 × 3 and 2 × 2 m spacing, respectively) have resulted in mean annual increment (MAI) ranges similar to those of our study for the same age. Poplar biomass plantations with 1736 trees ha
−1 had MAI values ranging from 2.8 to 6.1 Mg ha
−1 yr
−1 at four years after planting [
29,
32] and 6.7 to 9.0 Mg ha
−1 yr
−1 for five-year-old trees [
30]. Maximum productivity resulting from 3-PG modeling resulted in 13.0 Mg ha
−1 yr
−1 at the end of ten-year rotations [
69]. Plantations of the same stand density as the current study (i.e., 1075 trees ha
−1) exhibited productivity ranging from 4.3 to 5.3 Mg ha
−1 yr
−1 at age four years [
43] and 5.1 to 16.8 Mg ha
−1 yr
−1 after six years of growth [
28]. Optimizing genotype × environment interactions for the best performing clones resulted in MAI values of 3.0 to 11.0 Mg ha
−1 yr
−1 for four-year-old trees [
28]. Such a wide range in productivities can be attributed in part to site conditions and planting stock (i.e., rooted vs. unrooted cuttings). Effective clonal selection is integral to maximizing productivity, regardless of application (e.g., biomass for bioenergy, phytotechnologies, etc.). Productivity values in the lower part of this range have been shown for poplars grown for phytotechnologies. At phytoremediation plantations in the Midwest planted at stand densities from 434 to 4310 trees ha
−1, MAI values ranged from 4.4 to 15.5 Mg ha
−1 yr
−1 for some of the same clones as the current study (‘DN5’, ‘DN34’, ‘NM2’, ‘NM6’) [
17]. However, lower productivity (0.5 to 2.5 Mg ha
−1 yr
−1) also has been reported for poplar clones ‘DN5’, ‘NC14106’, ‘NM2’, and ‘NM6’ irrigated with landfill leachate grown for two years with a stand density of 3472 trees ha
−1 [
52]. These results corroborated the growth productivity of clones in the current study, for which MAI ranged from 1.6 to 3.9 Mg ha
−1 yr
−1 across all phyto buffers and clones. Considering that phyto buffers in our study were located adjacent to landfills and similar sites, clone productivity can be considered satisfactory because the presence of potential soil heterogeneity can significantly affect biomass production of poplar clones [
14,
42,
46,
47,
53,
57,
70].
Optimal site conditions for poplar growth include deep, fertile sandy-loam to clay-loam soils with pH ranging from 5.0 to 7.5 that are well drained, but not droughty [
71]. Thus, annual precipitation is another influential factor and, in the present study, all phyto buffers fit within the regional precipitation gradient range of 76.2 to 88.9 cm [
30]. Site conditions at the buffers significantly affected growth and productivity of the tested clones, specifically concerning soil water availability and pH, which serve as limiting factors for poplar growth. By comparison, there was a lack of phyto buffer × clone group interaction regarding MAI at four years after planting. Such an outcome can be explained by the origin of the hybrids; NRRI clones belong to the ‘DN’ genomic group, whereas Control and Experimental clone groups contain clones originating from different poplar species and inter- and intra-sectional hybrids [
17].
Trends in health were similar across phyto buffer groups; phyto buffer and clone group main effects governed health during the year of establishment, and in the following years, phyto buffer × clone group interactions were expressed. Such results can be explained by a stronger influence of site conditions and clone group characteristics (i.e., rooting ability) on vitality during the year of establishment, whereas the interaction of the factors evolved in subsequent years. Greenhouse experiments of Rogers et al. [
58] showed a similar health response of NRRI clones ‘99038022’ and ‘9732-36’ compared to Experimental (‘NC14106’) and Common (‘DN34’, ‘NM2’, ‘NM6’) clones grown in soils from six of the phyto buffers of the current study (BW: Bellevue (West); CE: Caledonia (East); ME: Menomonee Falls (East); MW: Menomonee Falls (West); SL: Slinger; WH: Whitelaw). Finally, despite significant effects of phyto buffer, clone group, and their interaction, all health assessment values were within the optimal health category, with values ranging from 1.11 to 1.36 across all phyto buffer × clone group × year combinations (
Figure 1,
Figure 2 and
Figure 3,
Figures S1 and S2; Table S1), indicating no substantial influence on clonal vitality across all sites.
As expected, the phyto buffer × clone group × year interaction for diameter and volume production of clones was significant, indicating different growth patterns of tested clones and, further, changes in annual growth increment of poplars throughout the production cycle [
72]. Such an explanation could also be applied for MAI, which was lower (though not always significantly) for NRRI clones than those of the Common clone group. These results were corroborated considering volume production of the clones in the 2017 Phyto Buffer Group. NRRI clones ‘99059016’ and ‘9732-36’ had significantly lower wood volume than Common clones after the first year, whereas these differences were negligible after two and three years of growth. In the current study, the lack of a significant phyto buffer × clone group × year interaction for height can be explained by the fact that although height and diameter are typically positively correlated for poplars (and trees in general), this correlation is influenced by variation due to the site and G × E interactions, leading to the need for matching clones to specific site conditions [
73]. In addition, different biomass allocation growth patterns (e.g., terminal vs. lateral shoot growth) among clones could have impacted the current results [
64].
In general, NRRI clones showed potential for use in phytotechnologies, with high productivity exhibited for clones ‘99038022’ and ‘9732-31’. Previously, NRRI clones ‘99038022’, ‘99059016’, ‘9732-11’, ‘9732-24’, and ‘9732-31’ demonstrated high productivity for mean basal area and volume, often outperforming Common clones [
36,
37]. Although the productivity of NRRI clones have varied markedly across sites, the identification of geographically robust clones holds promise for efficiently meeting diverse environmental objectives [
38]. Breeding and selecting clonal forest reproductive material has many advantages, including utilization of both additive and non-additive variance, resulting in larger genetic gains [
35,
74,
75]. On the other hand, environmental factors can diminish genetic gains. According to Pliura et al. [
76], the presence of a significant G × E interaction implies that: (1) a genotype’s performance in a specific environment can be less accurately predicted by the overall genotypic mean, and (2) a genotype’s overall performance can be less accurately predicted by the genotypic mean in a specific environment. Both of these responses can result in biased estimates and, thus, decreases in genetic gains [
76].
The aforementioned results, including those of the present study, indicated that NRRI clones, which originated from a narrow range of latitudes, were well-suited to the latitudinal range of the phyto buffers. For example, ‘D125’ (selected from Dr. Carl Mohn’s long-term
P. deltoides program at the University of Minnesota) is the female Minnesota
P. deltoides parent used for all F
1 full-sib progeny within family pedigree ‘9732’. In contrast, some genotypes of the Common and Experimental clone groups originated from other parts of North America and Europe, making them less adapted to certain phyto buffer site conditions. The intra-specific breeding strategy for NRRI clones uses
P. deltoides parents of a limited geographic range (Minnesota) combined with other
Aigeiros species (e.g.,
P. nigra) to produce progeny of increased performance [
35]. The
P. nigra component of ‘DN’ hybrids has produced a strong heterotic effect not exhibited in
P. trichocarpa Torr. et Gray ×
P. deltoides ‘TD’ hybrids due to greater genomic relatedness between
P. deltoides and
P. nigra relative to poplars from the
Tacamahaca section (e.g.,
P. trichocarpa,
P. maximowiczii A. Henry) [
77]. This genetic closeness was corroborated by mitochondrial DNA variation [
78] and simple sequence repeat (SSR) markers [
79]. In addition, species biology likely contributed substantially to the performance of NRRI hybrids. According to Sixto et al. [
65], the plasticity of certain
Aigeiros species enabled them to grow on a vast range of habitats (e.g., from poor, dry and stony to optimal silty or sandy loamy soils) versus
Tacamahaca balsam poplars that preferred alluvial, fertile soils in wetter climates and higher elevations. Their results were verified by findings of positive sensitivity to increases in median temperature and negative sensitivity to increased sand content by
P. nigra clones, with the opposite occurring for
P. trichocarpa ×
P. deltoides hybrids [
65]. Nelson et al. [
37] hypothesized that the
P. nigra male component of
P. deltoides ×
P. nigra hybrids imparts broad adaptability to these genotypes.
Overall, in the current study, NRRI clones exhibited positive growth performance at all sixteen phyto buffers during the first four years of establishment. Their height, diameter, and volume, like those of the Common and Experimental clone groups, were influenced by site conditions, which was expected considering soil heterogeneity at the phyto buffers. NRRI clones, the progeny of Minnesota-selected
P. deltoides and
P. nigra, were robust and well-adapted to the varying climate and soils at the phyto buffers. Our results corroborated previous testing of NRRI clones in more traditional SRWC production plantations [
35,
36,
37,
38], indicating their potential for use in phytotechnologies.