Oil Sands Terrestrial Habitat and Risk Modelling for Disturbance and Reclamation: The Impact of Climate Change on Tree Regeneration and Productivity – Phase III Report

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  • The overall objective of this project is to develop a framework that integrates risk management and strategic decision-making to evaluate the impact of disturbance (natural and industrial) on ecosystem products and services, and on habitat availability for terrestrial species in Alberta’s Lower Athabasca planning region. This also includes an evaluation of conservation, and reclamation activities associated with oil sands development both at the lease and regional levels. The project has been conducted in phases. Each phase is sequential such that its results and conclusions represented the foundation for subsequent work. This report summarizes activities conducted as part of Phase III, consisting of the following: (1) Model projections of tree regeneration under climate change on actual oil sands reclamation materials, and (2) A comprehensive model analysis of the risks to ecosystem productivity from climate change as a consequence of the impact of moisture stress on tree mortality. Model projections of plant regeneration under climate change on actual oil sands reclamation materials Six climate change scenarios for Alberta were selected that encompassed a range of predictions in future temperature and precipitation change. The tree and climate assessment (TACA) model was calibrated for reclaimed sites that varied in their soil moisture regimes (from xeric to subhygric) and three natural sites, High Level (subxeric), Calling Lake (mesic), and Fort Chipewyan (subhygric). TACA was used to predict regeneration probabilities on these sites for jack pine, aspen, and white spruce, in conjunction with the climate change scenarios. A comparison between the natural sites and their corresponding moisture regimes on reclaimed sites showed little quantitative difference in predicted regeneration for High Level. Regeneration probabilities for Calling Lake and Fort Chipewyan, however, were lower than the corresponding moisture regimes on reclaimed sites (mesic and subhygric, respectively). The differences in the Calling Lake and Fort Chipewyan sites are largely a consequence of the fact that percolation rates were higher on natural versus the reclaimed sites. These results highlight the importance of assessing soil moisture regime using a variety of metrics. Across climate periods, regeneration in this northern region was generally improved in jack pine and aspen because of the warming temperatures and in some scenarios, increases in annual precipitation, predicted under climate change. This was particularly the case in the wetter moisture regimes (submesic to subhygric) than the subxeric and xeric regimes, probably due to increases in growing season moisture deficits in the latter. Aspen regeneration from suckering had substantially greater predicted success than aspen regenerated from seed. Predicted trends in white spruce regeneration were in sharp contrast to the other species. Spruce regeneration was reduced substantially in future periods to the point where it was predicted to be less than 20% in subxeric and xeric moisture regimes. These results indicate that from a reclamation perspective, the impact of climate change on regeneration requires careful consideration of the tree species and its associated moisture regime. Soil moisture regime generated pronounced differences in regeneration probabilities both within a given future time period, and across periods. As might be expected, regeneration was highest in the wettest moisture regime and declined as the moisture regime became drier. However, the difference between moisture regimes within a given time period also increased over time for all species. From the perspective of reclamation outcomes, these results suggest soil prescriptions should be developed and/or applied which generate moisture regimes that are submesic and wetter. Drier regimes (subxeric and xeric) appear to introduce a substantially greater average risk that revegetation success in a future climate may be compromised through regeneration failure. How well might current reclamation prescriptions be expected to perform under climate change with respect to regeneration success? Overall, results suggest that no single set of prescriptions will be adequate to maintain the current suite of tree species common to the region. Nevertheless, current one-layer prescriptions seem adequate for maintaining pine and aspen regeneration, at least on average. Practices governing spruce, in contrast, should transition over the next several decades towards an emphasis on constructing two-layer prescriptions only, in an effort to minimize the risk of inadequate regeneration. This has important implications for mass balance calculations associated with soil amendment materials. In short, drier sites should focus on pine and possible aspen regeneration, and spruce on wetter sites. For a risk management perspective, reclamation practices that generate the two wettest moisture regimes (mesic and subhygric) are most likely to result in successful outcomes, at least through the 2050s. Drier moisture regimes can have lower regeneration probabilities but results were often highly inconsistent across the climate scenarios; constructing covers that generate drier moisture regimes thus entails considerably more risk of inadequate regeneration. Although regeneration was high in the 2080s, in many moisture regimes uncertainty in model predictions was also high. However, because of this extended time frame, modifying current reclamation practices or planting prescriptions to mitigate this risk is not warranted. Taken together, results emphasize the point that the climate will continue to change and highlight the necessity for ongoing investment in this type of analysis to facilitate the process of continuous learning that can form the basis for adaptive management. Analysis of risks to ecosystem productivity from climate change using FORECAST Climate Drought is anticipated to be an increasingly limiting factor for plant productivity and survival in the Fort McMurray region. Regional climate data indicate that this trend has already begun with patterns of growing season moisture deficits increasing since the 1960s. A new drought mortality function was developed and implemented within FORECAST Climate. In contrast to the threshold mortality approach employed in previous analyses, the new continuous function simulates drought mortality using a two-year running average of a species-specific moisture stress as a predictor of annual mortality. The 2-year running average is designed to capture the compounding effect of consecutive dry years. The amplitude of the function curve was fitted to historical climate data for each species so that mortality rates were consistent with empirical observations of actual mortality events. Two different mortality curves (low and high) were simulated for each tree species to explore the sensitivity of the model to assumptions regarding tree susceptibility to drought stress. To simulate the effects of a changing climate, five climate-change and associated emissions scenarios were utilized, and one scenario representing the historical climate regime. Simulations were conducted for ecosites dominated by jack pine (ecosite a1), aspen (d1), and white spruce (d3). Jack pine showed very little mortality under the historical climate regime at either index of drought sensitivity. In the case of aspen (ecosite d1) and spruce (ecosite d3), historical drought-related mortality events were not uncommon in the simulations, consistent with empirical data. Projections of future climate conditions generated mixed results in terms of mortality, depending on the emission scenario. With the exception of A1FI, all other emission scenarios triggered mortality below historical conditions at various points in the simulation. Given that primary productivity at high latitudes is temperature limited, a warming climate thus has the potential to improve survival under some circumstances, though not necessarily on sites where drought is already problematic. Within a given species, the highest mortality almost always occurred under the A1FI emissions scenario. Though A1FI was considered a pessimistic outcome in terms of CO2 emissions, current evidence indicates that, in fact, it may be close to reality. Pine and spruce appear generally robust to drought conditions at least over the next several decades, regardless of the climate regime. Mortality tended to increase thereafter as the simulation years got longer (i.e., later in the century). In absolute terms, pine is projected to have the lowest overall drought-related mortality (the exception being mortality under the A1FI emission scenario) while spruce is projected to have the highest mortality, particularly late in the century. Aspen showed a small increase in mortality over time beginning in the first decade of the simulations. The Climate Response Index (CRI) is a metric calculated in FORECAST Climate that integrates the impact of temperature and precipitation. Similarly, the decomposition response index (DRI) links decomposition (i.e., nutrient availability) to temperature and moisture. Both indices thus serve as proxy measures of climate-related growth conditions. The A1FI scenario, by example, always generated higher CRI and DRI values than occur under historical climate conditions. Nevertheless, assumptions regarding tree sensitivity to drought stress had a significant impact on volume production and its relation to climate change. When the mortality rate was low (i.e., species were robust to moisture stress), volume production under climate change always exceeded that projected under the historical climate regime. If species are less tolerant of moisture stress (i.e., the mortality rate function was high) climate change will have a negative impact on stand-level productivity later in the century, though how much depends on the particular species and a given emissions scenario. Significant reductions in productive capacity from climate-driven mortality threaten to destabilize ecosystems beyond their resilient capacity. One feature that would serve to promote resilience by avoiding drought stress is to ensure the rooting zone possesses adequate available water holding capacity. This can be accomplished by ensuring capping materials have higher organic matter content, are not predominantly coarse textured, and of sufficient depth. Layering of capping materials to generate textural breaks also serves to increase moisture storage, at least temporarily. Another important feature in creating resilience is to properly match tree species to their edatopic position. Aspen, and particularly spruce, occupy wetter positions on the edatopic grid. For the most part, these species are more prone to drought than pine. It is important then to ensure they are not planted on sites that may become marginal in terms of available moisture. In that respect, another consideration is to actively modify planting prescriptions in anticipation of a drier climate. Conceptually, this approach is based on the assumption a given soil moisture regime will for all intents and purposes transition to a drier edatopic position with further climate warming. In Europe, mitigative activities against climate change at the stand level are focusing on the regeneration phase. This is because a well-established plant population will have better prospects for surviving the vagaries of future (and largely uncertain) climate conditions and the fact little can be done to affect survival in stands that are mature today. Hence, one approach is to increase the genetic or species diversity in seeded and planted stands. This can be accomplished with traditional tree-breeding programs (termed provenance trials) though molecular genetics techniques have been developed that significantly reduce the time and resources needed for the selection process. Other possible silvicultural measures to promote establishment and maintenance of desired communities include moving up the planting season to take advantage of earlier spring conditions, using containerized stock to reduce drought risk, enhancing drought tolerance by employing seedlings with higher root:shoot ratios, and reduced spacing to increase recovery after dry periods. Quantitative models, such as TACA and FORECAST Climate, can project forest responses and the goods and services those forests provide to a range of future climate change scenarios. Predictions made using these climate-based models need to inform best management practices and can be coupled to the continuous learning that forms the basis of an adaptive management process, thereby reducing the uncertainty associated with reclamation decisions. The report closes with conclusions and associated recommendations, and a final section describing potential next steps.

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    Attribution 3.0 International