By Jennifer Hushaw & Si Balch
Even if we could perfectly predict the details of future climate change, there would still be uncertainty about how ecological systems will respond. Most of the information we have about how forests respond to long-term climatic changes is related to the northward migration of species following the last deglaciation, but this is not necessarily a good proxy for the change we are experiencing today because the current rate of warming is much faster and the landscape is not the barren frontier it was after the glacial retreat – now the movement of species is complicated by other factors, including inter-species competition.
A great deal of work has been done to model how habitat suitability is likely to change in the future for individual tree species and, despite the inherent uncertainty in these models, the results provide some sense for how the species mix may change in various regions – a topic that will be explored in a future bulletin.
The following list includes some key topics in forest response to climate change and describes the aspects that are well-understood, as well as the major areas of uncertainty in each arena:
Response of Insects & Disease
The interaction of changing climate with both native and invasive insects and diseases is likely to be one of the most immediate and drastic climate-related impacts. While these pests are better understood than many of the following topics, there is still considerable uncertainty about life-cycles and population dynamics in new conditions – some pests may benefit from these changing conditions, while others may be held in check.
Increasing average temperatures and a longer growing season will provide an opportunity for increased forest productivity, especially in northern latitudes. Although, regions with increasing seasonal water stress may see fewer productivity gains than wetter areas.
Higher ambient CO2 levels can increase plant growth rates. The magnitude of additional carbon storage and whether the growth response will be offset by losses in productivity due to heat stress or other climate-related factors remains uncertain. There is likely to be little if any increase on poor sites where other nutrients are limiting.
CO2 & Water-use Efficiency
Higher ambient CO2 levels can also increase plant water-use efficiency (the ratio of water loss to carbon gain). This effect varies from one species to the next because there is variability in how plants regulate water loss during dry conditions, so the net gains (globally) are still uncertain.
The latest genetics research has revealed that some of an organisms genes can be turned “on” or “off” by external environmental factors and those changes can sometimes be inherited by offspring (a.k.a. transgenerational epigenetics). This is how plants with the same genome can flower at different times in response to different local temperatures. It is also the mechanism whereby environmental stressors experienced by the parent plant affect the next generation.
This has important implications for the adaptability and potential plasticity of different tree species in a novel climate. It suggests that some species may be more resilient to change than we might expect based on their current response to stress, since new environmental conditions can stimulate changes in gene expression that create a different response. For example, some research has shown that a parent plant exposed to repeated drought conditions can produce seedlings that express more drought tolerance. The role of epigenetics in shaping plant population response to climate change is very uncertain and is just beginning to be researched.
Fundamental, Realized, and Tolerance Niche
Each species has a fundamental niche – the full range of conditions and resources under which it can persist indefinitely – but species don’t live there, they live within a smaller realized niche – the places where competition doesn’t preclude them. Some researchers have also introduced the idea of a tolerance niche – this is a marginal zone beyond the fundamental niche in which species can survive, even if they can’t maintain self-sustaining populations (Figure 1, below). However, many of our assumptions about future changes in climate suitability are based only on a species’ current range (its realized niche) rather than the complete range of conditions (its fundamental or tolerance niches), lending uncertainty to habitat shift projections. Considering these broader niches suggests that species may be capable of persisting under a wider set of climate conditions than where they currently grow.
Limits to Knowledge about Plant Physiology
How an individual tree species responds to climate change is likely to be linked to its underlying physiological characteristics, such isohydric vs. anisohydric behavior (plant strategies for stomatal regulation), seedling vs. mature tree response to environmental stressors, heat tolerance, etc., but we only have detailed knowledge about these characteristics in a limited number of model species used in research and some of the more valuable timber species.
These unknowns all contribute to the ‘cascade of uncertainty’ that accumulates as researchers take the outputs of future emissions scenarios, use those in simulations of global climate change, downscale those outputs in a particular region, and then use those projections in forest models. At each step in this chain, there may be:
- Statistical uncertainty
- Sampling error, inaccuracy, etc.
- Scenario uncertainty
- Plausible changes, which are based on assumptions
- Uncertainty due to ignorance
- Known unknowns (uncertainty due to recognized ignorance)
- Unknown unknowns (uncertainty due total ignorance)
Coping with Uncertainty
Forestry has always required the consideration of future conditions – whether that is anticipating future market demand, timber prices, insect threats, or new invasive species of concern. In the past, however, we could assume that, aside from a few wet or dry years, the average climate conditions would be the same throughout the life of an individual tree from seedling establishment to harvest. We now understand that that assumption may not hold, particularly in regions with a long rotation age. The exact degree and rate of climate change in any given location is uncertain and this may change our ability to achieve desired stand conditions in the future, by hindering our ability to choose the species best-suited for a particular site, for example.
Forest management decisions are influenced by our assumptions about how well we can predict future conditions and our ability to “design” forests well-suited for those conditions. In the context of a changing climate, these assumptions are more relevant than ever, as managers try to determine the most appropriate strategy for developing “resilient” forests that will continue to provide desired products and services. On one end of the spectrum, we have completely deterministic strategies that aim to develop “forests of the future” (where the species mix and stand structure are well-adapted to future conditions) and, at the other end, we have purely indeterministic approaches that aim for more diverse forests (in terms of species mix, structure, age-class, etc) based on the idea that they have more natural capacity to buffer change (Figure 2, below). While the deterministic approach has proven to be successful in the 10- to 15-year planning horizon that is often of most concern for foresters, an indeterministic approach often has the advantage if reality doesn’t meet expectations (in this case, climate predictions). All forest management decisions fall somewhere along this continuum between two valid strategies that are not mutually exclusive, and forest managers will likely use some of each approach.
While these strategies can be employed at any scale, stands are likely best managed from a deterministic basis (the shorter the rotation the more appropriate the deterministic approach), while forests (made up of multiple stands and biologically diverse micro sites) are likely best managed on an indeterministic basis. The latter approach is aimed at developing a maximum flexible forest that has the ability to adapt to any change in the ecological or economic environments, by identifying the most important ecosystem features for allowing high resilience and self-regulating capacity, such as: diverse structural elements, increased species diversity and natural regeneration, secured vitality and fitness of highest possible number of species (including genetic families across all organisms). Ultimately, the ability and desire to implement one or the other of these strategies will depend on management objectives, risk tolerance, and planning horizon.
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