- Top of page
- Materials and methods
- Supporting Information
Populations of widespread forest trees are typically adapted to a variety of local climate conditions in which they occur. As climate changes, locally adapted genotypes may become increasingly mismatched with their local environments, potentially leading to reduced productivity and forest health (Gray et al. 2011). This problem can be reduced with human-aided movement of reforestation stock to appropriate climate environments, and large-scale reforestation programes offer an opportunity to implement such a climate change adaptation strategy cost-effectively (Gray et al. 2011; Pedlar et al. 2012). However, such prescriptions, especially those that aim to pro-actively match planting stock to anticipated future climates, run the risk of mal-adaptation under current conditions, with frost damage the most obvious risk (Howe et al. 2003; Savolainen, Pyhäjärvi & Knürr 2007).
In North America, trembling aspen, Populus tremuloides Michx., is one of the most widespread and genetically diverse tree species. It is most abundant in the boreal forests of north-east British Columbia, northern and central Alberta and central Saskatchewan. Here, aspen covers an area of approximately 60 million hectares of boreal mixed wood forest, providing habitat for a variety of mammals, birds and insects, as well as other plant species (Stelfox 1995; Canadian Forest Service 2011). Aspen is also an important commercial forest resource in this region, accounting for approximately half of the annual forest harvest, and is primarily processed into oriented strand board (OSB) for construction purposes and pulp for paper products. More recently, aspen has also been used for conversion to biofuels and other biomaterials (Balatinecz, Kretschmann & Leclercq 2001; Sannigrahi, Ragauskas & Tuskan 2010).
Given current and predicted climate change for western Canada (Christensen et al. 2007; Mbogga, Hamann & Wang 2009), this important renewable forest resource appears to be under considerable threat. Over the last two decades, loss of forest productivity and dieback of aspen and other tree species has been well documented along the southern fringe of the boreal forest and further south in the United States (Hogg, Brandt & Michaelian 2008; Allen et al. 2010; Peng et al. 2011; Anderegg et al. 2012). Michaelian et al. (2011) conducted a detailed survey covering an area of 11·5 million hectares in western Canada to assess the impact of drought-induced aspen dieback. They report 45 megatonnes of dead above-ground biomass, which represented 20% of the total above-ground biomass (226 megatonnes) in the surveyed area. Climate change towards warmer and drier conditions appears to be the primary driver of the observed aspen decline, amplified by other factors such as forest pests, fire suppression policies and other management practices (Marchetti, Worrall & Eager 2011; Anderegg et al. 2012; Worrall et al. 2013).
One way to address these issues is to afforest the affected areas with different species, or differently adapted planting stock of the same species to better match current and anticipated climate conditions. In a study with lodgepole pine, Rehfeldt, Wykoff & Ying (2001) suggest that adapting to global climate change requires a major redistribution of forest tree species and genotypes across the landscape. They report, for example, that genotypes which are best suited to future climates in north-east British Columbia (latitude 60°) are currently located as much as 9° latitude to the south. Similar work for aspen indicates that relocating aspen planting material northwards by 1–2° latitude is required just to account for climate change observed over the last two decades (Gray et al. 2011).
Any movement of planting stock, however, could increase the risk of freezing injury if migrated genotypes are not synchronized with the local growing season (Aitken & Hannerz 2001). Frost hardening and de-hardening coincides with leaf senescence in autumn and bud break in spring. Early spring growth is particularly susceptible to late spring frosts, since tissues are actively growing and not lignified. Bud break is primarily a response to temperature and is initiated after a certain heat sum is acquired (Hunter & Lechowicz 1992; Li, Wang & Hamann 2010). In contrast, autumn leaf senescence in most species, including aspen, is triggered by photoperiod (Horvath et al. 2003; Keskitalo et al. 2005; Fracheboud et al. 2009). Notably, the trigger for the onset of cold hardiness (day length) is decoupled from the actual selective environmental factor (frost events). This poses a special concern when moving seed because a given day length may not correspond to the same frost risk in different geographic locations.
The distribution of many temperate tree species is thought to be determined by their adaptive capacity to survive winter extremes in the north or at high elevation, and their ability to compete with better-adapted species in the south or at low elevation range limits (MacArthur 1984; Woodward 1987). This is a consequence of trade-offs between maximizing growth by fully utilizing the available growing season, and avoiding injury or mortality due to late spring or early autumn frosts (Loehle 1998; Aitken & Hannerz 2001; Leinonen & Hänninen 2002; Koehler, Center & Cavender-Bares 2012). According to Loehle (1998), frost protection requires significant plant resources, being achieved by structural investments (e.g. increased lignification, thicker leaves and cell walls), physiological responses (e.g. accumulation of lipids, sugars or membrane proteins) and conservative growth strategies (e.g. early autumn leaf senescence and late bud break).
This paper aims at developing recommendations for moving planting stock, considering such trade-offs in growth and adaptive traits. Hence, the objectives of the present study are (1) to assess the impact of moving aspen seed sources throughout western Canada on growth and survival in a large-scale reciprocal transplant experiment; (2) to investigate geographic patterns of genetic variation in adaptive traits, including the timing of bud break and leaf senescence, and the onset and degree of frost hardiness; (3) to quantify frost risk environments in early autumn and late spring to which local and transferred aspen populations need to be adapted; and (4) to assess risks and potential benefits of seed movement throughout the western boreal forest. This information could in principle be used to develop sophisticated climate change adaptation strategies that account for uncertainty in future climate projections. For example, we could potentially lower risks (e.g. due to variable future climate), by sacrificing some growth potential (e.g. through a more conservative growth strategy). However, to keep the scope of the study manageable, we do not model performance and trade-offs under uncertain future environments. Instead, this paper aims at management recommendations that enhance health and productivity of planted aspen forests in response to recent climate change trends.
- Top of page
- Materials and methods
- Supporting Information
Adaptations for growth optimization vs. survival optimization are normally expected to be important trade-offs for temperate tree species (Leinonen & Hänninen 2002). Fitness of trees from high-latitude ecosystems should be strongly influenced by their ability to withstand harsh frost, whereas trees from milder climates should be favoured by natural selection based on higher growth rates and competitive fitness (Loehle 1998). By moving trees north out of their local habitat, one would generally expect an increasing risk of frost damage in autumn due to delayed growth cessation (Howe et al. 1995). This is broadly what we found in the current study with Minnesota provenances or Alberta sources moved to more northern positions showing a delay in the timing of leaf senescence, and a lesser degree of cold hardiness than local sources. However, this did not compromise survival, and the timing of the onset of dormancy and frost hardiness suggests that there should be no severe risks involved with moderate northward transfers of planting material.
Spring phenology was quite similar across all provenances observed at the central Alberta test site, except for the northern Alberta provenances. It is not uncommon that provenances from very high latitudes or very high elevation are adapted to make the most out of a short period of favourable temperatures and extended photoperiods and tend to be less conservative in utilizing the available growing season (Beuker 1994; Aitken & Hannerz 2001). In our case, this means that a northward movement of more southern provenances would typically lead to similar or slightly delayed onset of growth of introduced genotypes relative to local provenances, and therefore, northward transfers would not pose additional risks.
Another interesting observation is that the inferred dates of bud break and leaf senescence (for the regions BC, nAB, ABf, SK, MN) were not drastically different from the common garden site at which they were all observed (cAB), and this may have two explanations: while the risk of severe frost increases from south-east to north-west, there are virtually no differences in the frost-free period from Minnesota to north-east British Columbia (Fig. 4). Secondly, the date of leaf senescence of aspen populations coincides exactly with the inflection point of the day length curve (see Fig. S2, Supporting information). This means that although the day length trigger is temperature-decoupled, it will nevertheless work more or less appropriately under latitudinal transfers, because the day length does not vary with latitude around the date of the southern equinox (22 September), which coincides with the observed leaf senescence in aspen. The true critical day length that initiates senescence must be somewhat earlier than the date where we observe leaf senescence, so there may be small shifts in the timing of senescence under long-distance transfers. However, we find these quite small in absolute terms. For example, Minnesota provenance senesced six days later than the local sources when moved over 7° of latitude to the central Alberta planting site (Table 3).
Perhaps the most striking result of this large-scale transplant experiment is that moving aspen as far as 2300 km north-west from Minnesota to north-east British Columbia did not result in higher mortality rates or inferior growth. In fact, trees from Minnesota outperformed all local sources at Saskatchewan, Alberta and north-east British Columbia test sites. It seems that moving aspen as far as 2300 km north-west was not enough to reach the cold tolerance limit of aspen from Minnesota. That said, we should acknowledge that there are clearly discernible differences in frost hardiness from south-east to north-west, suggesting a typical trade-off between investments in growth (Minnesota sources) vs. investments in cold resistance (north-east British Columbia sources). However, when looking at the corresponding risk environments, investments in cold resistance appear nonoptimal for current climate conditions, that is, too conservative (Fig. 3b). All provenances appear to be sufficiently hardy early enough to withstand extremely unlikely cold events, for example −30°C in mid-September.
Such observations are normally interpreted as adaptational lag, caused by environmental change that exceeds the speed of evolutionary change (Matyas 1990; Matyas & Yeatman 1992). It is not uncommon that provenances transferred around two degrees north show increases in growth relative to local sources (Namkoong 1969; Mangold & Libby 1978; Morgenstern 1996). What is remarkable in the study is the magnitude of the adaptational lag in aspen, which may be due to the unique life history and regeneration biology of aspen. Aspen predominantly reproduces through vegetative reproduction from the distal portion of the root system, resulting in clones that are the oldest and largest known organisms (Mitton & Grant 1996). Aspen seed are very small and lack endosperm, resulting in a narrow window of viability. Suitable conditions with adequate moisture, bare ground and sufficient light are rarely met, limiting reproduction by seed (Peterson & Peterson 1992). Thus, adaptation through evolutionary processes is expected to be slow, which could explain the unusually large adaptational lag observed in this study.
A strong adaptational lag implies that a species should be more vulnerable to movement in one geographic or climatic direction than the opposite. We do, in fact, find indications that heat tolerances may be exceeded and compromise survival in provenances that were transferred southward. A transfer of the most northern provenances from north-east British Columbia to the warmest test site in the Alberta foothills (ABf being 3·6°C warmer than BC, Table 1) yielded by far the lowest survival of any transfer tested in this experiment (45%, Table 3). The data indicate that northward rather than southward movement of trees would be associated with less risk, which also suggests that inaction in the face of climate change may result in higher mortality.
To support decisions on regional seed transfers, we use means and standard errors for height (Table 2) to calculate the probability of a transferred provenance to match or exceed the productivity of local sources or to exceed predetermined reference values of 10%, 20% or 30% gain over the local sources (Table 5). It should be noted that regional representation from British Columbia is quite low with three provenances (or 90 genotypes per site). However, we only report probabilities for north or north-west transfers and exclude transfer recommendations for British Columbia provenances. Transfers in the opposite directions yield probabilities near zero (data not shown).
Table 5. Probability of a transferred provenance to match or exceed the productivity of local sources, based on the means and standard errors for height (Table 2). We only report probabilities for north or north-west transfers. Transfers in the opposite directions yield probabilities near zero
|Seed sources from||Transferred to||Probability of match or gain|
|Alberta Foothills (ABf)||cAB||0·50||<0·01||<0·01||<0·01|
|Central Alberta (cAB) ||nAB||0·05||<0·01||<0·01||<0·01|
|Northern Alberta (nAB)||BC||>0·99||>0·99||>0·99||0·98|
When interpreting these probabilities, it is important to keep in mind that these probabilities are based on the ranking of provenances at single test sites. Nevertheless, while different planting sites may strongly influence absolute productivity, the relative ranking of provenances should not change. Only if planting environments are so different from test sites that genotype by environment interactions become a major factor, the regional rankings could change. An example for a site that does not conform to the general pattern of this trial series may be the northern Alberta (nAB) test site in a dry ecoregion, where provenances transferred from more southern or eastern origins have a low probability of matching or exceeding local sources.
There may be other important trade-offs, where more northern sources sacrifice growth and instead invest in resistance mechanisms to biotic or abiotic risk factors that we have not considered. One possible risk factor, drought resistance, was excluded by a related study (Schreiber et al. 2011) that showed that the Minnesota provenances tested in this experiment also have small vessel diameters, which conferred adequate drought resistance across all genotypes tested in this experiment. Adaptations to biotic factors such as pests and diseases by northern provenances that are absent in southern sources also appear unlikely. Sources from warmer environments and milder winters would generally be expected to be more exposed and therefore better adapted to pest and disease factors.
This study evaluated potential trade-offs and risks associated with seed transfer of aspen seedlings for reforestation in western Canada. Gray et al. (2011) suggested that in order to adapt to observed and predicted climate warming for western Canada, planting stock should be moved 2–3° of latitude northward. Such a prescription could lead to increased frost damage and a mismatch in the timing of bud break and leaf senescence with the available growing season.
Experimental cold hardiness testing and phenology observations in a common garden experiment revealed that seed transfer to more northern locations results in delayed timing of leaf senescence, but the onset of dormancy and frost hardiness suggests that there should be no severe risks involved with northward transfers of planting material. Northward movement was also associated with a slightly delayed onset of growth of introduced genotypes relative to local provenances and therefore poses no additional risks. We conclude that benefits in growth outweigh potential risks to survival associated with a northward movement of aspen populations in forestry operations. Even extreme long-distance northward movements had positive or neutral effects on growth and survival, while southward movement had clear negative consequences, highlighting the risk of inaction in the face of climate change. We therefore recommend that seed transfer guidelines in western Canada allow a moderate movement of aspen planting stock to account for adaptational lag. As for true long-distance transfers, notably the use of Minnesota sources in western Canada, we encourage forest companies and government agencies to pursue this option first on a relatively small operational scale. General recommendations of long-distance transfers should await results from this test series at rotation age, and concurrent experience from small-scale operational plantations.