A moveable forest?

If the trees on the hillside succumb to water stress, the bushes and grasses down below may find their world contains more water than they can handle.

As I mentioned in my last post, predicting the form of future forests is complicated due to the wide variety of factors involved, but there are some common concerns that researchers share about the challenges trees will face with global climate change. Changes in temperature and precipitation patterns will alter the suitability of forest locations (and not always in the ways you might think- in the Netherlands, for example, summer precipitation is expected to decrease while winter precipitation is expected to increase (Brolsma 2010)- because water access is typically greater downslope than upslope, researchers predict that summer water stress will cause thinning of vegetation upslope, which you might expect. But, as a result of more winter precipitation and fewer plants upslope to absorb moisture, they expect wetter conditions downslope and a change in species composition in response. I find this situation amazing- too little water in summer will lead to too much water in winter- and those species that can handle the drier summer conditions may not be able to adapt to increased moisture).

Will trees be able to find the areas that work best for them? Scientists have been looking at this issue from several angles, not all of which have rosy scenarios. Tree populations contain high levels of genetic diversity which can help them adapt to changing conditions, but when a changing climate encourages genetic variability, it is possible that none of the new variations will be particularly well-adapted and you can see high juvenile mortality as a result (Kremer et al. 2012). With an increase in temperatures, we could expect that temperate forests would move away from the heat and into areas that had previously been cooler- Kremer et al. (2012) felt that trees growing along the line of expansion in the northern hemisphere would benefit from ‘pre-adapted’ genes coming from trees in the south already prepared for a certain degree of heat, but those trees along the southern edge of the range would be at a disadvantage when dealing with even hotter temperatures. How quickly can tree populations shift location? That is one of the big questions- based on previous changes in the earth’s climate, we know that forests can expand and contract in response to warmer and colder climates, but we’re not sure if they can keep up with the changes in store this time around. Models predict that climatic zones may shift as much as 150 km over the next century (see the European Commission’s Dept. of Agriculture for 2008 reports on how climate change is expected to impact Europe’s forests), but pollen analysis suggests that migration rates after the last ice age were 100-200 m per year (Aitken et al. 2008). At that rate, forests could shift up to 20 km in a century, but not nearly enough to keep pace with climate change. Aitken et al. (2008) suggest that, by 2050, if we experience a mid-level climate change scenario, as much as 38% of plant species could be extinct.

It’s likely that a picture of this location in Franconia Notch State Park will show a different plant community in 50 or 100 years.

So what will our forests look like? It’s a little difficult to predict this exactly, but here are the general trends:

  • Forest fire events will likely become more common and severe in areas along the Mediterranean, southeastern Australia, and western US, and fire suppression techniques may lose some of their effectiveness (Keenan 2012; Loehman et al. 2011).
  • Northern Europe will see increased forest growth, while southern Europe experiences decreased forest growth (Kolstrom et al. 2011).
  • In eastern North America, species composition of forests will change as some, such as paper birch, flee higher temperatures and others, like beech, have problems with water stress (Thompson et al. 2009- if you live in this area and would like to see what the future possibilities are, check out my Just Fascinating Stuff page for a link to prediction maps by species, state, and region).
  • Forests will experience an increase in disease and pest outbreaks, such as mountain bark beetles which are projected to spread 7° farther north with a 2.5° C increase in temperature (Cudmore et al. 2010).
  • Species composition in general may change as habitat conditions alter- there is concern, for example, that the northern flying squirrel (Glaucomys sabrinus) may become rare or disappear from the southern part of its range due to habitat loss from both human and climatic changes to the landscape (Weigl 2007).

So this generally sounds like there are big problems over a wide area, but we know that forests have endured and overcome past climatic shifts, and we know that there are people working to address these issues- there is hope for the future of the forests. Next week I’ll be looking at ways we can mitigate and adapt to climate change impacts, so that we can all pitch in and give trees a hand.


Works cited:

Aitken, S.N., Yeaman, S., Holliday, J.A., Wang, T., and S. Curtis-McLane. 2008. Adaptation, migration or extirpation: climate change outcomes for tree populations. Evolutionary Outcomes 1: 95-111.

Brolsma, R.J. 2010. Effect of climate change on temperate forest ecosystems. Netherlands Geographical Studies 396: 1-157.

Cudmore, T.J., Bjorklund, N., Carroll, A.L., and B.S. Lindgren. 2010. Climate change and range expansion of an aggressive bark beetle: evidence of higher beetle reproduction in naïve host tree populations. Journal of Applied Ecology 47: 1036-1043.

Keenan, R.J. 2012. Adaptation of forests and forest management to climate change: an editorial. Forests 3: 75-82.

Kolstrom, M., Lindner, M., Vlien, T., Maroschek, M., Seidl, R., Lexer, M.J., Netherer, S., Kremer, A., Delzon, S., Barbati, A., Marchetti, M., and P. Corona. 2011. Reviewing the science and implementation of climate change adaptation measures in European foresty. Forests 2: 961-982.

Kremer, A., Ronce, O., Robledo-Arnuncio, J.J., Guillaume, F., Bohrer, G., Nathan, R., Bridle, J.R., Gomulkiewicz, R., Klein, E.K., Ritland, K., Kuparinen, A., Gerber, S., and S. Schueler. 2012. Long-distance gene flow and adaptation of forest trees to climate change. Ecology Letters 15: 378-392.

Loehman, R.A., Clark, J.A., and R.E. Keane. 2011. Modeling effects of climate change and fire management on western white pine (Pinus monticola) in the northern Rocky Mountains, USA. Forests 2: 832-860.

Thompson, I., Mackey, B., McNulty, S., and A. Mosseler. 2009. Forest Resilience, Biodiversity, and Climate Change: A synthesis of the biodiversity/resilience/stability relationship in forest ecosystems. Secretariat of the Convention on Biological Diversity, Montreal. Technical Series no. 43, 67 pages.

Weigl, P.D. 2007. The northern flying squirrel (Glaucomys sabrinus): a conservation challenge. Journal of Mammalogy 88: 897-907.