Hybrid poplar plantations are suitable habitat for reintroduced forest herbs with conservation status

General results of transplant experiment

Four forest herb species were transplanted into plantations of two hybrid poplar clones and into woodlots. The four herb species varied considerably in initial size. None of the Trillium grandiflorum or Maianthemum racemosum plants was at reproductive maturity and 72% of Trillium plants were seedlings with only one leaf. Maianthemum plants had between one and 7 leaves each (3.34 ± 1.23; mean ± s.d.). Sanguinaria canadensis plants were somewhat larger at the time of transplanting; many showed evidence of having flowered earlier that spring, although flowers were not counted for year 1. Sanguinaria was planted as clumps with an initial size of one to four leaves (1.40 ± 0.74). Asarum canadense plants were the largest of the four species and all clumps contained mature, flowering individuals (10.6 ± 5.0).

In the spring of year 3 (two years after transplanting), average survival rates by plot (all stand types combined) were 61% ± 33% (mean ± s.d.) for Asarum, 52% ± 32% for Maianthemum, 77% ± 29% for Sanguinaria, and 59% ± 29% for Trillium. Of the surviving plants, 67% of Asarum plants, 77% of Sanguinaria and 12% of Maianthemum plants produced flowers. Of the surviving Trillium plants, 94% had reached the three-leaf adult stage and 18% flowered. Damage from slugs and other invertebrates was evident in many plots for all of these species, although this damage was not quantified. Important differences in biomass were visible among the six study sites. In several plots at the HAM and CAT sites, only a few small plants remained in the spring of year 3 while at the BED site, the lowest elevation site with rich bottomland soil, all four species were flourishing.

The average number of leaves and flowers of Asarum (as a proxy for biomass) remained approximately constant in the plantations and tended to decline in the woodlots over the two years after transplantation (Figure 1). The number of leaves of Sanguinaria and the sum of the leaf lengths of Trillium tended to increase over the two years after transplantation in all three stand types, while the sum of leaf lengths of Maianthemum tended to increase in the MxB poplar plantations and the woodlots, but not in the DxN poplar plantations. For Maianthemum and Trillium, the increase tended to be greater in the woodlots while the reverse was true for Sanguinaria. For all species, the variability among plots increased over the two years, reflecting a differential response to the different sites (Figure 1).

Effects of site and stand type

Response of transplants to environmental variables

Environmental variables were quite heterogeneous among the six sites and among stand types. At the most productive plantation sites, the plantations were similar to the woodlots in terms of light availability and leaf litter biomass, while at the less productive sites, the plantations provided much less shading and litter than the woodlots. The woodlot soils were on average more humid and more acidic than those of the poplar plantations, with higher C contents, higher C:N ratios, and higher concentrations of N, K, and Mg. However, the degree of difference in soil properties between the woodlots and the plantation/field environments were not consistent across all sites and at some sites the relationships were opposite those of the general trends (for detailed results of these environmental variables see Boothroyd-Roberts et al. 2013).

The Sanguinaria response was predicted by elevation and soil C:N ratio in the best model, explaining an estimated 60% of the response variability, and these were also the two variables with the highest importance values. Sanguinaria tended to have more leaves and flowers at lower elevation sites with lower soil C:N ratios. In particular, Sanguinaria had few leaves and flowers at the highest elevation site, LAP. Because of missing data, this site (LAP) was excluded from the ANOVA and the remaining two lower-elevation sites did not differ significantly in terms of Sanguinaria response. However, the six hybrid poplar plantation plots from the LAP site, our highest elevation site, were included in the regression analyses.

For Maianthemum, the availability of diffuse light was the most important environmental variable predicting the sum of leaf lengths (a proxy for biomass), and was also important for predicting flowering, with a better response in shadier conditions. June soil moisture (negative effect), soil Ca (positive effect) and soil Mg (negative effect) were also important for predicting the sum of leaf lengths and were included in the best model. However, these three soil variables were relatively unimportant for predicting flowering. Flowering was instead influenced by elevation. The environmental predictors in the best models explained an estimated 36% of the variability in total leaf lengths and 41% of the variability in flowering.

The response of Trillium was less easily predicted by the environmental variables measured in this study, with the best models explaining only approximately 19% of the variability in total leaf lengths and 16% of the variability in flowering. Soil Ca (negative effect) had the highest importance value in predicting total leaf length and was the only variable included in the best model, but its importance value (0.78) was much lower than the highest values calculated for the other three species (1.0 for the most important variables), indicating a higher degree of uncertainty. Flowering was predicted by leaf litter biomass (negative effect) in the best model. This variable also had the highest importance value; however, this importance value (0.67) was lower than those calculated for the other response variables.

Reintroduction success

Two years after transplantation, all four species showed a capacity for survival and growth in hybrid poplar plantations. Sanguinaria plants had grown better in the hybrid poplar plantations than in the woodlots, while the opposite was true for Trillium plants. Asarum and Maianthemum plants showed no significant difference between stand types, although Asarum tended to respond with better growth in the plantations and Maianthemum tended to respond with better growth in the woodlots. The experimental introductions showed that site conditions are important for transplantation success, since all four species showed high mortality in certain plots. In the most suitable hybrid poplar plantations, Sanguinaria and Asarum showed a capacity for rapid clonal growth.

The four species each responded somewhat differently to the environments, suggesting different niches. Sanguinaria grew better at lower elevations, which is to be expected since this species is near the northern edge of its range in the study area (Kiger 1997). However, neither Asarum nor Trillium showed an obvious preference along the elevation gradient, which is unexpected since these species are also near the northern edge of their ranges in the study area (Case 2002; Whittemore et al. 1997), and the LAP site is actually outside of the natural range of Trillium grandiflorum (Québec, Ministère du Développement durable, de l’Environnement et des Parcs 2005). Another unexpected result is that Maianthemum was more likely to flower at lower elevations, despite the fact that it is a more northerly distributed species than the other three (LaFrankie 2002).

Shading was an important factor for Maianthemum, which showed poorer responses in less shaded environments. This may be explained by the increased pressure of competition from other understory species that were reinvading the transplantation plots in the less-shaded stands. At our sites, differences in canopy openness among the plantations were highly correlated with understory vegetation cover, which consisted mostly of shade-intolerant species (Boothroyd-Roberts et al. 2013). In agreement with our results, competition was an important factor in a transplant experiment in hybrid poplar plantations in Belgium (Baeten et al. 2009). We speculate that Asarum and Sanguinaria were less vulnerable to competition due to their larger initial size. In the case of Trillium, although it may have been vulnerable due to its small initial size, it was only planted in a subset of three relatively well-shaded sites, whereas Maianthemum was planted in all six sites, including those with less developed plantation canopies and understories with higher herbaceous total cover.

Soil properties were important for all species. Asarum and Maianthemum grew better in well-drained soils with high concentrations of Ca (and high pH). Maianthemum also grew better in soils with less Mg. Sanguinaria, on the other hand, responded more strongly to the C:N ratio, growing better in low C:N ratio soils. These results are unsurprising, given that all three of these species typically grow in rich sugar maple forests (Gagnon and Bouchard 1981; St-Jacques and Gagnon 1988; Gauthier and Gagnon 1990; Frère Marie-Victorin 1995; Lamoureux 2002).

These habitat associations help to explain the better performance of Asarum and Sanguinaria in the plantations compared to the woodlots. Three of the hybrid poplar plantations (at the BED, BRO and LAP sites) were located on rich, well-drained post-agricultural soils. Asarum grew particularly well in these three plantations, while Sanguinaria grew well at the BED and BRO plantations, located at lower elevations. In contrast, the woodlots in this study tended to have more poorly-drained, acidic soils with higher C:N ratios (Boothroyd-Roberts et al. 2013). These characteristics are probably typical of forested areas in many post-agricultural landscapes, in which lands considered marginal for agriculture were more likely to remain forested or to be abandoned earlier (Foster 1992; Flinn et al. 2005, Domon and Bouchard 2007).

Trillium grandiflorum is also typically described as growing in “rich” environments (Case 2002; Québec, Ministère du Développement durable, de l’Environnement et des Parcs Québec, Ministère du Développement durable, de l’Environnement et des Parcs ; Québec, Ministère du Développement durable, de l’Environnement et des Parcs 2005; Lamoureux 2002); however, in the present study, Ca was weakly negatively correlated with Trillium growth. This result may be due to confounding effects from other, unmeasured variables. The better performance of Trillium in the woodlots, compared to the plantations, cannot therefore be explained by the present dataset.

Ecological implications

The success after two years of the transplants in the hybrid poplar plantations in this study is the first evidence for the suitability of certain hybrid poplar plantation environments for these four species of forest understory plants. The differential response of the four species to the different environmental variables underlines the importance of selecting understory species for reintroduction that are adapted to the conditions at the site of interest. These results support other findings (Boothroyd-Roberts et al. 2013; Truax et al. 2012; Gardiner et al. 2004) that hybrid poplar stands could function as transition stands in the restoration of deciduous forest environments on old fields.

Understory herbaceous plants could be transplanted into plantations as was done in this study and given a chance to establish while benefiting from protection from large herbivores and an initial reduction of understory competitors. Protection from herbivory is critical, as an over-abundant population of whitetail deer (Odocoileus virginianus) can seriously impair restoration efforts of understory herb populations (Lubbers and Lechowicz 1989; Ruhren and Handel 2003; Whigham 2004). This model of forest restoration would be especially useful for the rapid creation of habitat corridors to link existing woodlots across agro-forested landscapes. Sites should be chosen to correspond as closely as possible to the natural habitats of the understory species, in terms of elevation, drainage and soil type. Success is more likely in plantations that have attained a high level of canopy closure.

This study also provides evidence that the major limitation to the natural colonization of plantations and secondary forests by understory plants is in the dispersal and/or seedling emergence or establishment stages, rather than in the suitability of the habitats for mature plants. Indeed, many Eastern North American forest herb species are known to be very slow dispersers (Whigham 2004), such as Trillium, Sanguinaria and Asarum, whose seeds are likely dispersed by ants. Post-dispersal seed predation can also limit recruitment (Bruun et al. 2010), and environmental stresses can affect seedling emergence and establishment stages (e.g., Albrecht and McCarthy 2009; Bierzychudek 1982; Nault and Gagnon 1993; Vasseur and Gagnon 1994). Litter could also affect seedling recruitment, either positively by increasing soil moisture or negatively by acting as a physical barrier (Albrecht and McCarthy 2009). However, once juveniles or mature individuals are transplanted successfully into these sites, most deciduous forest understory species in north-eastern North America can propagate clonally and can eventually spread within a site without needing to produce seed. Long-term studies with more species are needed to validate these conclusions. The creation of a central database of reintroduction projects would also help to validate these conclusions since many reintroduction experiments are carried out without the results ever being published (Godefroid and Vanderborght 2011).

One factor that may be more important in a longer-term study is the effects of competition on clonal growth. In particular, below-ground competition from the hybrid poplar trees may have an increasing effect once the tree roots re-expand after being partly removed from the planting area prior to transplanting. The M×B hybrid poplar clone had a much denser root system than the D×N clone (K. Boothroyd-Roberts, field observations), the effects of which could create a difference in survival and growth of understory plants between the two plantation types in later years. It has also been suggested that secondary chemicals in Populus balsamifera litter can reduce nitrogen availability for other plant species (Schimel et al. 1998), and the M×B poplar clone, with this species as a parent, may also have such an effect. Different meteorological conditions in later years may also expose vulnerabilities of the transplanted plants in one stand type or another. For example, the difference in measured soil moisture between the plantations and the woodlots may become more critical during a severe summer drought or a more pronounced wet season.

Implications for non-timber forest product cultivation

The results of this study also suggest that it would be ecologically feasible to cultivate forest herb species in plantations such as the hybrid poplar plantations in this study, as an alternative to their cultivation in naturally regenerated woodlots, but especially to their destructive harvest from natural populations. Three of the transplanted species (Asarum, Sanguinaria, and Trillium) are officially designated as vulnerable in Quebec because of the threat that harvesting whole plants from natural populations represents.

More research is needed to evaluate the economic feasibility of this type of system; however, a study by Burkhart and Jacobson (2009) found that growing herbaceous NTFPs under natural forest canopies was not economically viable, except for ginseng (Panax quinquefolius) and goldenseal (Hydrastis canadensis). Economic feasibility would depend on an assessment of the long-term productivity of these plants on the transplantation sites, as well as the initial costs of the plant material. Several authors have suggested that transplanting adult plants is more likely to be successful than planting seeds, since seedlings and juveniles are generally more vulnerable to adverse environmental factors than are mature plants (Mottl et al. 2006; Ruhren and Handel 2003; Vasseur and Gagnon 1994). However, planting larger plants also carries a greater initial cost than planting from seedlings or seed. Economic feasibility may be increased by combining NTFP production with wood-production in plantations. Partial tree harvests can be carried out in winter when a snow cover protects understory plants; however, a more detailed assessment is necessary to evaluate the compatibility of these two types of production.

Study area and plantation sites

This study took place in the Eastern Townships region of Quebec, Canada (Figure 2), across a regional climatic gradient corresponding to an elevation gradient from the St. Lawrence Lowlands to the foothills of the Appalachian Mountains. This gradient spans three ecological regions: region1a-T, at the lowest elevations and with the mildest climate, with sugar maple and hickory (Acer saccharum - Carya cordiformis) mature forest stands typical of mesic sites; region 2c-T, at mid-elevations, characterized by sugar maple - basswood (A. saccharum - Tilia americana) stands; and finally region 3d-M, at the highest elevations, characterized by sugar maple - yellow birch (A. saccharum - Betula alleghaniensis) stands. The landscapes of southern Quebec represent an excellent opportunity to study the multi-functional potential of tree plantations. Many regions have been largely deforested in the past due to agriculture; however, agricultural activities have since been largely concentrated into the St. Lawrence Lowlands, resulting in the abandonment of many former fields outside this region (Domon and Bouchard, 2007).

The present study uses a series of similar experimental hybrid poplar plantations, planted in May 2000 for a study on hybrid poplar establishment and growth at eight sites across the Eastern Townships (Truax et al., 2012). A subset of six sites with the best poplar canopy development was used for the present study. The site characteristics are summarized in Table 4. The plantations were all established on privately-owned fields that had been used for either grazing or crops, and subsequently abandoned. The vegetation before planting ranged from herbaceous to mixed herbaceous - shrub communities. Sites were prepared for planting in the fall of 1999 by ploughing and disking, and re-sprouting or germinating vegetation was eliminated in June 2000 by an application of glyphosate herbicide over the entire plantation area. Following this treatment, 2 m long rooted cuttings of nine different hybrid poplar clones were planted by hand at each site at a stem density of 833 ha^-1 (4 m between rows and 3 m between stems along the row). Glyphosate was again applied in June 2001, but only between the rows.

After 8 seasons of growth, a wood volume production gradient had emerged across the sites (Truax et al. 2012). At certain sites, the plantations showed exceptional growth, while at others they showed average or poor growth. The majority of these plantations had, however, developed a closed canopy. In this study, understory native herbaceous plants were planted in the six most successful of these plantations.

Experimental design

Each plantation contains three replicate 12 m × 12 m plots of each of nine hybrid poplar clones in a randomized block design. For this study we used only two of these clones, namely clone 915303, a hybrid of Populus maximowiczii and P. balsamifera (M × B), and clone 131, a hybrid of P. deltoides and P. nigra (D × N), both developed in Quebec. Clone 3333, another D × N hybrid, was used in place of clone 131 where the latter showed high damage. At each site, three 12 m × 12 m woodlot plots were selected within a secondary forest stand close to the plantation. At the BED and HAM sites, the woodlot plots were located in two different forest stands, both close to the plantations. These forested areas had regenerated naturally after either the harvest of a previous stand, or the abandonment of agriculture. The age of each stand was estimated based on cores taken from a sample of the largest trees and interviews with landowners. The vegetation and environmental characteristics of these woodlot plots are described in Boothroyd-Roberts et al. (2013) (see also Table 4 for dominant tree species). Thus, the present study consisted of three woodlot plots per site and three plots per site in the M × B poplar plantations and in the D × N poplar plantations, for a total of 54 plots (6 sites × 3 stand types × 3 replicates). Four woodlot plots (three from the LAP site and one from the HAM site) were subsequently eliminated from the study due to damage from fallen trees or crushed deer exclusion cages.

We established experimental plantations of understory native herbaceous plants within each of the plots described above. We chose four species typical of mature broadleaf forests in the region, with conservation value and economic value, which could potentially serve as non-timber forest products (NTFPs). These are: Asarum canadense L., Maianthemum racemosum (L.) Link (syn. Smilacina racemosa), Sanguinaria canadensis L. and Trillium grandiflorum (Michx.) Salisb. All of these species, apart from Maianthemum, are officially designated as vulnerable species in Quebec, due to the threat of overexploitation of wild populations. Asarum is sought because of the essential oils contained in its rhizomes, while Sanguinaria is sought for its rhizome containing sanguinarine, a powerful alkaloid with known medicinal uses. Trillium and Asarum are both valued for their ornamental value in shaded gardens. Maianthemum is also used for horticulture to a lesser extent. No natural populations of any of the transplanted species were present in any plot; however, a patch of Asarum was observed near one of the plots in the Bromptonville woodlot.

Transplantation of understory herbs

Small plants (well-established seedlings) of these four species were purchased from Horticulture Indigo, a specialized nursery that cultivates native plants from seed, thus avoiding an impact on wild populations. This nursery is located in Melbourne, at the centre of our study area, avoiding any unnecessary transportation time that could compromise plant quality. We chose to use plants because introductions of forest perennial herbs have been shown to be more successful from transplants than from seed (Francis and Morton 1995, Primack 1996). We planted Asarum and Maianthemum in all 54 plots over the six sites. Due to the limited availability of plants, we planted Sanguinaria and Trillium at only three of the sites (27 plots; BED, BRO, LAP). In each plot, we planted 10 individuals (Trillium and Maianthemum) or 10 clumps (Sanguinaria and Asarum) of each species. Occasionally, 11 or 12 individuals of Trillium or Maianthemum were planted in cases in which it was impossible to separate two that had grown together. In total, 1636 plants were planted. The plants were introduced into small rectangular understory plantations from which all understory vegetation, large surface roots (to 15 cm depth), and litter were removed by hand. These understory plantations consisted of two rows per species and five plants per row with a spacing of 20 cm between plants and 20 cm between the rows (total areas of 120 cm × 180 cm for four species, and 120 cm × 100 cm for two species). The experimental plants were transplanted between May 25, 2009 and June 4, 2009. No weeding has been done since transplantation, but we protected the plants from larger herbivores (mostly white-tailed deer) with exclusion cages.

Understory herb measurements

The size of all transplanted plants was recorded following transplantation, between June 10 and 18, 2009, and again in the two following springs (2010 and 2011). Sanguinaria flowers were counted on April 22, 2010 and May 5, 2011 at the Bedford site; April 29, 2010 and May 7, 2011 at the Bromptonville site; and May 30, 2010 and May 14, 2011 at the La Patrie site. All remaining counts and measurements were taken between May 21 and June 3, 2010 and June 6 and June 9, 2011. Sanguinaria flowers were not counted the first year because the flowering season had finished before the time of transplantation.

For each species, we used different measurements, appropriate for the morphology and growth patterns of each, as proxies for the total biomass of the species in the plot. For Asarum and Sanguinaria, we measured the sum of the number of leaves and the number of flowers in each plot, while for Maianthemum and Trillium, we measured the sum of the leaf length of all leaves in the plot. Maianthemum and Trillium flowers were also counted.

Environmental characteristics

The environment within each plot was characterized during the 2009 growing season by its soil chemical and physical properties, stand basal area, light availability in the understory, and leaf litter biomass. Detailed methods and results of these measurements are presented in a previous article (Boothroyd-Roberts et al. 2013). Briefly, we collected five soil samples distributed systematically within each plot for chemical analysis, which were then combined into one composite. Samples were taken from the mineral soil at a depth of between 5 cm and 10 cm, corresponding to the principal rooting zone of herbaceous plants. We measured pH in a soil-water suspension. The available potassium (K), calcium (Ca), and magnesium (Mg) contents were determined through extraction with BaCl₂ and detection by atomic absorption. The extractable phosphorus (P) content was measured using the Bray-2 method (Bray and Kurtz 1945) (modified by F. Lambert). The total nitrogen (N) and total carbon (C) were measured using dry combustion, high-temperature reduction of the combustion products, and thermo-conductometric detection. Soil moisture at a depth of 10 cm was sampled twice, in June and August, corresponding to dry periods with no significant rainfall events in the 48 hours prior, and measured gravimetrically after oven drying. Light availability at a height of 90 cm was measured using a digital hemispheric photograph, taken at the centre of the plot, to determine canopy openness and the average light received during the growing season. Leaf litter was collected after almost all the leaves had fallen from every tree from a 50 cm × 50 cm microplot and was subsequently dried and weighed.

Analyses

The responses of each understory herb species were analyzed using separate 2-way analysis of variance tests (ANOVAs) with stand type (M×B poplar plantation vs. D×N poplar plantation vs. woodlot), site and their interaction as fixed factors. The LAP site was excluded from ANOVAs because data was missing from all woodlot plots from this site. This left two remaining sites for Sanguinaria and Trillium, and five sites for Asarum and Maianthemum. The experimental unit was the plot (n = 3 per cell). All response measures were subjected to square-root transformations to improve the normality of ANOVA residuals and homoscedascicity; for presentation purposes the raw data are used in figures and tables. ANOVAs were done using the JMP software package (SAS Institute, Cary, NC).

To explore the relative influence of different biotic and abiotic environmental variables on the response of transplanted plants, we fit a series of linear mixed-effect regression models, which we then used for model selection and multi-model inference. Each understory herb response was fit separately and all response variables were square-root transformed prior to analysis. The replication level for all models was the plot, using all plots except for the four woodlot plots with missing data (n = 50 for Asarum and Maianthemum; n = 24 for Sanguinaria and Trillium). We began with an initial set of eight environmental variables as fixed factors and site as a random factor to account for the spatial clustering of plots. Site was not considered a fixed factor for this analysis since the question of interest was the relative effects of the different environmental variables rather than site effects. The initial set of environmental variables consisted of elevation, June soil moisture, soil C:N ratio, soil Ca, soil Mg, soil P, leaf litter biomass and availability of diffuse light. August soil moisture, soil C, soil N, soil K, soil pH and availability of direct light were excluded from the analyses because they were highly correlated with one of the selected variables (r > 0.6). For each response variable, we selected a single best model from all possible subsets of the initial set of environmental variables, using maximum likelihood and Aikaike’s Information Criterion for small samples (AICc: Burnham and Anderson 2002). We calculated a marginal R² for these best models as an estimate of the variance explained by the fixed factors in the model (Nakagawa and Schielzeth 2013). Because there is often uncertainty in choosing the best model, multi-model inference was also done, using all models within 4 AICc of the best model. A relative importance value was calculated for each environmental variable as the sum of the Aikaike weights of all models in which the variable was included (Aikaike weights quantify the probability that a given model is the best model). Model-averaged regression coefficients were also calculated. Initial mixed-effects models were fit using the lme4 package (Bates et al. 2013) in R software (R Development Core Team 2011) and the MuMIn package (Barton 2013) was used for model selection and multimodel inference.