Introduction
Climate change
Climate change is a long-term weather variation with significant and uncertain impacts on the natural environment. In the western Canada, the predicted climate is warmer and dryer in some provinces such as Alberta (Figure 1and Figure 2). Schneider et al. (2009) address the climate change impact on ecosystem distribution in Alberta, which may start a conversion of 12%-21% of boreal region into parkland in Alberta where aspen are widely distributed. Figure 3 shows the predicted ecosystem boundary change with the climate envelope modeling (Mbogga et al., 2009), where the expansion of the Prairie grassland ecosystem is significant. This may lead to mortality increases and productivity loss of aspen and spruce over large area (Hogg et al., 2008).
Due to the current global warming, the population responses of tree species are observed in different species. Sturm et al. (1997) found a northern shift of tree lines of white spruce near Kugururok River in the Arctic Circle; as well, in Alberta, Beaubien & Freeland (2000) reported earlier plant flowering due to climate change for a number of species including aspen. Although the global warming will change the distribution of plant species in the future (Chuine & Beaubien, 2001), it is not clear whether trees species such as aspen can still adapt to the local environment.
Due to the current global warming, the population responses of tree species are observed in different species. Sturm et al. (1997) found a northern shift of tree lines of white spruce near Kugururok River in the Arctic Circle; as well, in Alberta, Beaubien & Freeland (2000) reported earlier plant flowering due to climate change for a number of species including aspen. Although the global warming will change the distribution of plant species in the future (Chuine & Beaubien, 2001), it is not clear whether trees species such as aspen can still adapt to the local environment.
Figure 1 Comparison of climate deviations from the 1961–1990 normal of a recent 10-year average (1997–2006) and predicted climate for the 2020s for mean annual temperature (MAT), mean warmest month temperature (MWMT) and mean coldest month temperature (MCMT) (Mbogga et al., 2009). The warm color means higher deviation and blue color means minus deviation, which is getting cooler than the average.
Figure 2 Comparison of climate deviations from the 1961–1990 normal of a recent 10-year average (1997–2006) and predicted climate for the 2020s for mean annual precipitation (MAP), mean summer precipitation (MSP) (Mbogga et al., 2009). The warm color means decreasing of moisture and blue color means increasing deviation of moisture, which is getting wetter than the average.
Figure 3 Comparison of current ecosystems based on climate envelope with the baseline climate normals (1961–1990), and projected climate for the 2020s based on the Canadian Global Circulation Model (Mbogga et al., 2009).
Adaptation and seed transfer
Because forest trees may not be able to adapt to their new climate, productivity decrease will occur in the forestry sector. It is necessary to maintain the growth and adaptation of aspen in Alberta. Thus tree improvement programs and assisted migration of commercial tree species are needed to manage forest genetic resources sustainably. For the growth trait of aspen (e.g., tree height and Diameter of breast height ), Gylander et al. (2009) evaluated the provenance trials, clone trials and hybrid trials of aspen in Alberta with the quantitative analysis of the heritability. They found that transferring aspens from Minnesota to central Alberta is promising strategy to improve the aspen growth performance in Alberta in case of climate change.
Besides growth trait, adaptive traits such as phenological traits are useful to analyze maladaptation risks (Chuine and Beaubien 2001). The variation of adaptive traits indicates the distinctiveness of ecotypes, which adapt to local environment for long-term (White et al., 2007). The phenological traits of tree species, which are the timing of recurring phases of their reproduction and development, are indicators of the adaptation to the local environmental conditions (Li et al. 2010). Li et al.(2010) evaluated the spring phenology of aspens with a provenance trial and also suggested similar seed transfer from Minnesota to Alberta. However, the Minnesota aspen may suffer frost damages in late fall or early spring, though drought may not be a significant factor. As well, the fall phenology of aspen which relates to the risk of early frost in fall is not well studied for Minnesota aspen. The autumn phenology phases of the aspen, including the bud set and leaf abscission, are able to be observed with the remote sensing method. The autumn phonological traits of aspen differ among populations from different geographic origins (Luquez et al., 2008). The leaf abscission and bud set are affected by certain genes (Lechowicz, 1984), which shows the tree’s adaptive significance of long-term local environmental conditions.
In order to assist the seed transfer of aspen, we need to understand and predict the complex interrelationships between forest tree species and the changing environment in spring and autumn. Three major questions are critical: 1. whether the current populations can adapt to the future environmental conditions; 2. how fast can they adapt to the new environment, and 3. how far can they be migrated. In order to answers these questions, the adaptation characteristics of tree species are necessary to be analyzed.
Besides growth trait, adaptive traits such as phenological traits are useful to analyze maladaptation risks (Chuine and Beaubien 2001). The variation of adaptive traits indicates the distinctiveness of ecotypes, which adapt to local environment for long-term (White et al., 2007). The phenological traits of tree species, which are the timing of recurring phases of their reproduction and development, are indicators of the adaptation to the local environmental conditions (Li et al. 2010). Li et al.(2010) evaluated the spring phenology of aspens with a provenance trial and also suggested similar seed transfer from Minnesota to Alberta. However, the Minnesota aspen may suffer frost damages in late fall or early spring, though drought may not be a significant factor. As well, the fall phenology of aspen which relates to the risk of early frost in fall is not well studied for Minnesota aspen. The autumn phenology phases of the aspen, including the bud set and leaf abscission, are able to be observed with the remote sensing method. The autumn phonological traits of aspen differ among populations from different geographic origins (Luquez et al., 2008). The leaf abscission and bud set are affected by certain genes (Lechowicz, 1984), which shows the tree’s adaptive significance of long-term local environmental conditions.
In order to assist the seed transfer of aspen, we need to understand and predict the complex interrelationships between forest tree species and the changing environment in spring and autumn. Three major questions are critical: 1. whether the current populations can adapt to the future environmental conditions; 2. how fast can they adapt to the new environment, and 3. how far can they be migrated. In order to answers these questions, the adaptation characteristics of tree species are necessary to be analyzed.
Aspen biology
Quaking aspen (Populus, tremuloides Michaux.), is widely distributed in Canada and north of US，which can quickly grow at sites with bare soil, for higher germination rate and other adaptive advantages(Burns & Honkala, 1990) . Apsen has a growing demand in wood productivity in Alberta (Li et al. ,2009). Bud set of aspen is part of the cessation of its meristem before winter dormancy (Allona et al., 2008). The bud set and leaf senescence or leaf color brown- down are different phenological processes from other temperature-sensitive processes such as budburst in spring time, because budburst is closely related to heat-sum (Li et al. , 2009). The bud set occurs at similar dates as leaf senescence which is treated as the approximately equal temporal stage in the study.
Environmental conditions
Because aspen has a wide range of distribution, different populations can adapt to various climatic conditions. For example, the winter minimum temperatures, annual precipitation, and frost free period (length of growing season) (Table 1). The known widest range of temperature in the aspen sites can be from -61°C in Interior Alaska to as hot as 41 °C in Fort Wayne Indiana. In Alaska and northwest Canada, quaking aspen grows only on the warmest sites free of permafrost. The precipitation of aspen range includes mild humid condition, humid and wet conditions. The annual temperature limits of aspen distribution in the northern Canada is roughly 5.6 °C, where in the Rocky Mountain area the lower limit is around 7 °C (Perala, 1990). The primary factor limiting aspen distribution range is moisture regime with water surplus and the secondary factor is the maximum and minimum temperature during growing season (Perala, 1990).
Table 1 Examples of climatic condition range of aspen distribution in North America (Perala, 1990).
The geographic range of aspen distribution has a different soil types including Alfisols, Spodosols, and Inceptisols) ranging from shallow and rocky to deep loamy sands and heavy clays (Perala, 1990). Aspen often prefers soils which are well drained, loamy, and high in organic matter, calcium, magnesium, potassium, and nitrogen (Perala, 1990). And aspen plays important role in the forest nutrient cycling with rapid growth and higher demand of nutrients (Perala, 1990).
The elevation of aspen site ranges from the creeks level (above sea level) to 3505m (Perala, 1990). But the trees prefer a elevation band in different regions: it is abundant between 1980 and 3050 m in Arizona and New Mexico; it is about higher than 300m in Colorado and Utah (Perala, 1990). Aspen likes warm south and southwest aspects, but it is still common on all aspects in the western mountains of the United States and grows well wherever soil moisture is not a limitation (Perala, 1990). However, the best stands in the Southwest are more frequently found on the northerly slopes where more favorable moisture conditions exist (Perala, 1990). Excessive drought condition will affect the tree growth (Perala, 1990). In the prairie provinces of Canada, particularly near the border between prairie and woodland, aspen often distribute on the cooler and wetter north and east slopes and to the depressions (Perala, 1990).
The elevation of aspen site ranges from the creeks level (above sea level) to 3505m (Perala, 1990). But the trees prefer a elevation band in different regions: it is abundant between 1980 and 3050 m in Arizona and New Mexico; it is about higher than 300m in Colorado and Utah (Perala, 1990). Aspen likes warm south and southwest aspects, but it is still common on all aspects in the western mountains of the United States and grows well wherever soil moisture is not a limitation (Perala, 1990). However, the best stands in the Southwest are more frequently found on the northerly slopes where more favorable moisture conditions exist (Perala, 1990). Excessive drought condition will affect the tree growth (Perala, 1990). In the prairie provinces of Canada, particularly near the border between prairie and woodland, aspen often distribute on the cooler and wetter north and east slopes and to the depressions (Perala, 1990).
Figure 3 Geographical range of aspen in North America (http://www.na.fs.fed.us/spfo/pubs/silvics_manual/volume_2/populus/tremuloides.htm).
Autumn phenology
Phenological traits of plants are the result of the interplay between genotypes and local environmental conditions (Li et al. ,2010). In spring, the budbreak is triggered by temperature which is measured as thermal time(Li et al. ,2010). The autumn phenotypic variations of aspen represent the genotypic differences in adaptation to the planting sites (Ingvarsson et al., 2006). There are two categories of phenotypic traits observed in the provenance trials: the survival traits and the adaptive traits. In the case of aspen, the autumn phenology include the timing of bud set, the timing of leaf abscission and leaf senescence etc., which are indicators of growth cessation in the observed year (Ingvarsson et al., 2006). The bud set trait is critical for the survival of buds at the end of a growing season. And the leaf senescence is important for aspen's endodormency and nutrient relocation before winter.
The timing of bud set is the initiation of dormancy before winter to avoid frost damage and survive during the winter. Ingvarsson et al. (2006) found that the bud set variation is triggered mainly by the shortening photoperiod, of which the trigger is independent of autumn senescence. The critical daylight or night length will initiate the bud set which is associated with phytochrome genes such as the putative candidate gene phyB2 (Ingvarsson et al., 2006). Such phenological traits are a result of a balance of a longer growing season with more yields and less mortality during the bitter cold winter.
The leaf senescence and abscission are affected by genotypes and environment with more complex relationships between phenological phase, shoot growth and leaf nutrient relocation. Weih (2009) found that in Willow (Salix spp.), the timing of bud-burst (onset of growing season) and delayed leaf abscission is important for willow biomass production than growth cessation.
The timing of bud set is the initiation of dormancy before winter to avoid frost damage and survive during the winter. Ingvarsson et al. (2006) found that the bud set variation is triggered mainly by the shortening photoperiod, of which the trigger is independent of autumn senescence. The critical daylight or night length will initiate the bud set which is associated with phytochrome genes such as the putative candidate gene phyB2 (Ingvarsson et al., 2006). Such phenological traits are a result of a balance of a longer growing season with more yields and less mortality during the bitter cold winter.
The leaf senescence and abscission are affected by genotypes and environment with more complex relationships between phenological phase, shoot growth and leaf nutrient relocation. Weih (2009) found that in Willow (Salix spp.), the timing of bud-burst (onset of growing season) and delayed leaf abscission is important for willow biomass production than growth cessation.
Objectives
This research project aims to (1) assess the variation of aspen autumn phenology in provenance trials; (2) validate the adaptive variation of aspen populations inferred from remote sensing images with phenological results of the provenance trials; and (3) evaluate whether the transferred aspen population can adapt to present and future changing climates in Alberta. Then two questions could be discussed: 1. How the bud set is controlled by environmental variables; 2. How far could we migrate the seed source.
