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Several models suggest that tree populations are capable of rapid migration in the face of climatic variation (Iverson et al., 2008; Morin & Thuiller, 2009). Accordingly, the reconstruction of post-glacial vegetation in North America shows several forest tree species reaching their maximum extent in the middle Holocene, followed by contractions that could have been caused by a cooling climate (McLachlan et al., 2005). Given ongoing climate changes, many species are expected to expand their ranges, which could be particularly marked for populations located at the limits of their distribution ranges (Iverson et al., 2004). For example, the northern limits of many tree species are modulated primarily by climatic factors, mainly temperature and light availability, which constrain the chances of survival at various stages of the life cycle (Woodward, 1987). A progressive decline in reproductive success results in a reduced capacity of these species to sustain themselves and to colonize new sites (Gaston, 2009). Among various natural processes, disturbances (e.g. fire, insect outbreaks and disease) play a major role in regulating species distributions at their latitudinal limits by altering population densities and limiting reproduction capacity (Ali et al., 2008). Human activities, such as agriculture and forest exploitation, are superimposed upon these factors and can contribute to habitat fragmentation, thereby accentuating the isolation of populations at a landscape scale (Vranckx et al., 2012).
To date, only a few studies have specifically addressed climate effects on species that are present within transition zones between forested areas. Most of these studies have focused on species that are present in northern tree-line ecosystems (i.e. the transition zone between boreal forest and the tundra) that are very sensitive to changes in climatic conditions, and which have shown recent species expansions (Lloyd & Fastie, 2003; Caccianiga & Payette, 2006). However, recent empirical observations in North America have shown that range contraction can also be observed (Zhu et al., 2012).
Sugar maple (Acer saccharum Marshall) is a widespread and abundant tree in north-eastern North America that reaches its northern continuous distribution range at the transition between boreal mixed-wood and temperate deciduous forests (Saucier et al., 2003). It is a deciduous, shade-tolerant species (Logan, 1965) that forms uneven-aged stands (Majcen et al., 1984) and has major ecological and economic value in eastern North America (Godman et al., 1990). Like other tree species in the Northern Hemisphere, sugar maple is predicted to migrate northwards from its current range. In the United States, models predict decreases in abundance at the southern edge of this species’ range (Iverson et al., 2008) and a northward expansion that will eventually lead to an increase in sugar maple abundance towards its northern limits in Canada (Goldblum & Rigg, 2005).
Climate controls species distribution in part by affecting recruitment at different phases of sexual reproduction (Walck et al., 2011). Sugar maple seeds require high soil moisture levels during germination (Janerette, 1979) and a period of stratification at low temperatures, between 1 and 5 °C, to break embryo dormancy and stimulate germination (Shih et al., 1985; Godman et al., 1990). Germination of northern seed sources begins one week earlier at 1 °C than at 7 °C, but the cumulative proportion of germination after 90 days is 20% higher at 7 °C than at 1 °C (McCarragher et al., 2011). Sugar maple seedlings and mature trees may be affected by early leaf senescence and eventually die due to frost damage (Pilon et al., 1994). Sufficient understorey light is limiting during the growing season for seedlings at northern latitudes and may lead to decreased seedling growth because the time period between seedling leaf emergence and canopy closure by mature trees is shorter in northern stands (Kwit et al., 2010).
In the present study, we analysed the reproductive capacity of sugar maple populations along a climatic gradient. Our objective was to determine whether or not sugar maple recruitment differs between the discontinuous and continuous zones within its northern range. We hypothesized that sugar maple had lower recruitment in the discontinuous than in the continuous part of its range, because seed production and seedling survival are reduced by low temperatures. To test this hypothesis, we examined stand structure, seed abundance and germination, seedling density and seedling age structure in populations along latitudinal transects ranging from the southern limit of the continuous sugar maple distribution in the sugar maple–yellow birch bioclimatic domain to its northern limit in the balsam fir–white birch bioclimatic domain.
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Our results indicated that sugar maple regeneration was globally similar across the discontinuous and continuous parts of its distributional range. Seedling age structures was also similar across its range with high recruitment, higher mortality at early stages, and decreasing mortality rates over time in most sampled sites. However, seedling density varied significantly among year (seedling emergence year), transects (west–east), and bioclimatic domains (zones). Variation in sugar maple seedling density was best explained by stand characteristics (mature tree species, mature sugar maple, tree species saplings, and sugar maple sapling basal areas) and climate conditions (July average temperature and precipitation), rather than by zonation.
Sugar maple seedling density was estimated to be 25.5 stems m−2, when averaged over the 24 stands, and 32.6 stems m−2, when we considered only the three northernmost sites (1-D-A′, Lac Labelle; 2-D-A′, Lac Pénobscot; 3-D-A′, Lac Patrick). These values were higher than those reported by Caspersen & Saprunoff (2005) for central Ontario (i.e. 11.1 stems m−2; Haliburton Forest and Wildlife Reserve, located south of our study area), and the range of values (14.7–29.9 stems m−2) that are reported by Goldblum & Rigg (2002, 2009) on the north-eastern shore of Lake Superior (Lake Superior Provincial Park; 47°45′ N, 84°42′ W). Variation in recruitment was low in our study area, in that sugar maple seedlings were absent from only 9% of circular plots in the continuous zone, which was consistent with a value of 20% for all quadrats measured by Caspersen & Saprunoff (2005). Multi-model inference indicated that five of our six first models included mature sugar maple basal area and that this variable was positively related to sugar maple seedling density (Table 1, Fig. 7, Table S4). This result was also consistent with Caspersen & Saprunoff (2005), who showed that sugar maple seedling density is correlated with higher sugar maple basal area, and with Garrett & Graber (1995), who showed that larger sugar maple trees generally producing more seeds than did smaller individuals. In contrast, Houle (1992) reported a negative relationship between sugar maple basal area versus sugar maple seed and seedling abundance, within a stand located south-east of our study area. This difference could be related to a different bioclimatic domain (sugar maple–basswood; Houle, 1992) or to stand characteristics that are not representative of a general pattern for sugar maple (Houle, 1992).
Like seedling density, basal area for mature trees (total and sugar maple) did not vary among transects and between zones. Mature sugar maple trees in transect 1 (western region, Abitibi-Témiscamingue) were larger but occurred at lower densities than those in the central region of Québec (transect 2; Fig. 3). We had expected to find larger sugar maple trees in the continuous zone where climatic conditions are more favourable to the growth of this species. Our results showed a negative influence of sugar maple sapling basal area on seedling density. The negative effect of saplings may be attributed to the shade produced by sugar maple leaves. This closed shade-cover (compared to mature trees) may induce higher sugar maple seedling mortality, even though sugar maple is a shade-tolerant species (Logan, 1965). This hypothesis of light limitation is supported by Kellman (2004), who found that sugar maple seedling mortality is lower in boreal stands than in sugar maple stands at the same latitude.
In transect 1 (west), differences between the continuous and discontinuous zones were very clear with respect to seed abundance and seedling density. In 2008, seeds were produced only in the continuous zone (34.0 seeds m−2; Fig. 4). However, this trend was not observed in 2009 and 2010, where very few seeds were produced in either zone, or in 2011, where high production was observed in both zones. Only 2.5 seeds m−2 germinated in 2008, which was not consistent for a mast seed year, while 43.0 seeds m−2 germinated in 2011. On the basis of seedling density structure (Fig. 5) and seed collections, we identified four mast seed years in transect 1 (1996, 2002, 2006 and 2011). Overall, mast years were well synchronized in all sites, if we excluded 2002–2003. There was a mast year in 2002 in transect 1 (west), 2002 and 2003 in transect 2 (centre), and 2003 in transect 3 (east), which suggested a ‘west–east gradient’. Four to 6 years separated each mast, which is consistent with the 3–8 years reported for Canada (Wang, 1974). Cleavitt et al. (2011) identified 1998, 2002 and 2006 as mast years in north-central New Hampshire. This synchronized and intermittent reproduction across the range of sugar maple could be due to endogenous rhythms (Kelly, 1994), climate conditions (Houle, 1999; Kelly & Sork, 2002), or seed predators (Tachiki & Iwasa, 2010). With respect to climate conditions, our data suggested that a warm and dry July in the previous year can induce mast seeding (Fig. 8). If some seedlings survived for several years, the presence of a seedling bank on the forest floor could compensate for the disadvantage of intermittent reproduction.
Model selection identified July mean temperature and precipitation (see Table S4) as variables positively influencing sugar maple seedling density (Fig. 7). Sugar maple seeds have high soil moisture requirements during germination (Janerette, 1979). Our results showed that precipitation during April and May did not influence seedling density. This suggested that moisture during germination was not a limiting factor. We could hypothesize that warm July temperature, combined with high precipitation, might reduce seedling mortality. However, more studies are needed to identify the direct effect of climate conditions on seedling survival in sugar maple.
Climatic models for eastern North America have predicted a mean increase of +3.3 °C (minimum, 2.1 °C; maximum, 5.4 °C) and +1% in precipitation (min. −17%, max. +13%) in June, July and August for the decade 2080–2099 compared to 1980–1999 (Christensen et al., 2007). Applying these mean values with our model for the northernmost site of transect 1 (1-D-A′, Lac Labelle), our model predicted a mean increase of 5.3% (min. 2.1%, max. 18.4%) in sugar maple seedling densities for 2080–2099 compared to 1980–1999. However, the uncertainties around these estimates progressively increased as temperature and precipitation increased (Fig. 7). Therefore, our predictions are limited by these uncertainties, but an increase in sugar maple seedling density is predicted to occur with climate change.
Species distributional range-shift predictions are based on niche models, climatic envelopes, and process-based models (McKenney et al., 2007; Morin & Thuiller, 2009). All of these approaches have more or less emphasized the influence of climate in predicting future species distributions, in part because other predictors are frequently unavailable. Our sugar maple seedling density model and habitat distribution models showed that the sugar maple limit was not controlled exclusively by the effect of tested climatic variables on regeneration. Therefore, northward expansion could possibly occur under different climatic scenarios (Iverson et al., 2008; Morin & Thuiller, 2009). All of the sites from the discontinuous zone showed very good sugar maple regeneration. For example, the northernmost site in transect 2 (2-D-A′, Lac Pénobscot; Fig. 6) showed high sugar maple seedling densities and a typical J-shaped age structure, which indicates continuous recruitment over time. It is possible that soil characteristics (type and nutrients) could serve as explanatory factors. For example, a lack of rotten wood and leaf litter can represent a barrier to seedling establishment (Caspersen & Saprunoff, 2005), while an increase in calcium availability and a thinner litter layer are important for improving early sugar maple seedling survival (Cleavitt et al., 2011). The inclusion of soil characteristics may increase the predictive power of our sugar maple seedling density model. Sugar maple regeneration could also be influenced by factors such as herbivore grazing (Salk et al., 2011), seed predation (Hsia, 2009), disease (Cleavitt et al., 2011), and interspecific competition (Gravel et al., 2011). Accumulating evidence has shown that important non-climatic factors must be included to increase model predictive power for actual and future tree species range shifts (McMahon et al., 2011).