This study examined the relative importance of soil, stand development and climate hypotheses in driving productivity for a species that is widely distributed in North America. Inventory plots, 3548 of such, either dominated by aspen or made up of species mixture of which aspen occurs in dominant canopy position were sampled along a longitudinal gradient from Quebec to British Columbia. Site index (SI), was used as a measure of productivity, and soil, climate and stand attributes were correlated with site index in order to determine their effects on productivity. Results show a decline in productivity with high moisture deficit. Soil humus correlates significantly with SI but does not sufficiently capture differential rates of litter deposition and decomposition effects over the long-term. Consequently, aspen composition, stand ageing, and stand structural changes dominate variability in productivity. Within the context where deciduous cover has being increasing, there are implications for forest productivity.

Abstract

Questions

High moisture levels and low occurrences of wildfires have contributed during recent millennia to the accumulation of thick layers of organic soil and to a succession into open black spruce (Picea mariana)–Sphagnum-dominated forests in the Clay Belt boreal landscapes of eastern North America. In these forests, the anticipated increase in drought frequency with climate change could lead to a shift in forest structure and composition via increased fire disturbance. Here, we quantify the expected changes in fire behaviour, biomass burning and vegetation composition in the Clay Belt forest of North America that could arise under climate change over the next century.

Location

A managed forest unit in the Clay Belt boreal forest of eastern North America.

Methods

The impact of a changing climate from 1971 to 2100 on fire regime characteristics (i.e. rate of spread, fuel consumption, fire intensity, type of fire and depth of burn) and vegetation dynamics (mortality and recruitment) was investigated using the Canadian Fire Effects Model (CanFIRE). Vegetation dynamics were governed by the fire danger and behaviour that affect tree mortality and post-fire recruitment of species, and by long-term successional pathways that are driven by post-fire recruitment and forest age. An ensemble of two climate models forced by three scenarios of greenhouse gas emissions was used to drive CanFIRE simulations.

Results

Results from multiple scenarios suggested that fire danger will increase significantly during the 21st century in the Clay Belt forest. The burn rate was projected to change from 4.2% decade−1 during 1971–2000 to 18.6% decade−1 during 2071–2100. Stand mortality, fire intensity and areas affected by crown fires were also projected to increase. A shift in forest composition did not occur over the simulation period across most of our fire regime scenarios. Dominance of open black spruce–Sphagnum forests was projected to remain in future landscapes.

Conclusions

Moist and cool conditions in these forests prevent high depth of burn and contribute to the ecological resistance of these forests to increasing fire danger.

Forest productivity is driven by a suite of direct climatic and non-climatic factors that are transient or permanent. The kind of productivity driver and the nature of their effects vary by species, and scale dependencies potentially complicate these relationships. This study explored productivity-driver relations in eastern Boreal Canada and determined spatial effects in productivity control when expressed with stand dominant height at a reference age (site index). Data from 4,217 temporary sample plots obtained from boreal mixedwood and conifer bioclimatic domains, and with varied species composition, were used in this study. A single-level global model that assumes equal sensitivities across spatial scales was calibrated and compared with three alternative models reflecting different hypotheses on possible spatial heterogeneities. Alternative models were calibrated by plot-level soil deposit types (microscale), landscape dominant deposits (mesoscale) and bioclimatic domains (macroscale). A marked difference between the global and alternative models was observed, suggesting that a single global model does not sufficiently reflect existing heterogeneity in productivity-driver relationships. A combination of macro- and microscale models provided the best explanation of site index. Results further showed that site index is mainly driven by species composition (complementarity effects of aspen and jack pine compositions) and stand diameter structural diversity effects. It is concluded that successional changes, more than direct climatic effects, drive productivity.

Existing models that use the site-index concept (dominant canopy height of a tree species at a reference stand age) are fundamentally stand-level models that do not account for stand dynamics, limiting their use to only a part of successional trajectories. Given that stand dynamics is influenced by both large and fine scale processes, we took a multi-level look at aspen (Populus tremuloides Michaux) productivity by determining landscape- and plot-level factors related to productivity as rated with site index. The study area extends from latitude 45° to 50°N in western Québec, from which were sampled 62 landscapes made up of 4948 plots, 25% of which had aspen as dominant and/co-dominants in the canopy. There, aspen is most often found in mixed stands. A stepwise procedure with forward selection was used in building landscape- and plot-level models and models were then arranged hierarchically such that (a) predicted estimates of the landscape model were inputs to the plot-level model (top-down) and (b) significant landscape variables were added to selected plot level variables (bottom-up). For the plot-level model, none of the climate variables considered were selected but at the landscape level, annual sum of degree–days was only the third to enter. In both cases, aspen site index was more related to the proportion of spruce (Picea mariana (Mill.) B.S.P. and Picea glauca (Moench.) Voss). At the level of landscapes, this observation might be due to the existence of particular vegetation mosaics, of which spruce proportion could be a surrogate. At the level of plots, influence of spruce on aspen site index is probably indicative of niche sharing with aspen. A high random variability was associated with the plot-level model but not with the landscape-level model. The similarity in drivers of aspen site index at both levels and the fact that both top-down and bottom-up approaches provided the same information, suggest that the use of landscape variables when modelling site index in mixed stands may help distinguish peculiarities shared by plots located in a landscape and improve the signal between site index and explanatory variables by reducing the random noise observed at the level of plots.

Studies relating site index to climatic variables basically assume that the sensitivity of a species to climate remains stable across the geographic range of their study area. Yet, provenance trials speak to the contrary and show that populations are adapted to their local climatic conditions and tend to respond differently to climate. Spatial and temporal complexity of forest productivity and climate-relationships has been globally reported and recent studies have emphasized the necessity for regional studies on forest growth dynamics of current and future populations. The objective of this study was to determine whether the main climatic and non-climatic drivers of trembling aspen (Populus tremuloides Michx.) growth in Québec should be treated as regional (the study area reacts as a unique population) or local factors (the area is composed of different populations) when modeling the spatio-temporal variability of aspen productivity as measured with site index. Stem analysis data was collected from 124 trees (32 stands) that span a north-south (latitude 46–51°N) transect in western boreal Québec. Most stands were dense with cover density above 60%, even-aged, 50–90 years old, and very often mixed. The northernmost regions (latitude 48–51°N) are characterized by either organic or clay deposits, while in the south (latitude 46–48°N) till or clay deposits predominate. Climate variables that met selection criteria as major regional or local factors that influence aspen productivity were selected. A mixed modeling approach was subsequently employed to identify the categorization unit that could be defined as a population. We then predicted variation in the random error with prior information obtained at stand level. Our results show that aspen height growth is mainly driven by annual sums of degree days and stand age. Surface deposit type, which is an indicator of soil nutritive status and moisture potential, was found to have modulated climate influence. Finally, aspen productivity is better explained with a model that assumes that specific populations have a different response function to climate and are adapted to their local climatic conditions. This has implications when predicting the response to climatic change for forest growth models that assume that conspecifics respond to climate similarly.