Changing patterns, changing forests

I’ve spent the last week enmeshed in the intricacies of climate change research and trying to understand how temperate forests will be impacted- and there were quite a few more variables than I had been anticipating. I guess I had assumed that warmer temperatures would mean shorter winters, earlier springs, and probably more bark beetles, and that was about it. Instead, I learned that trees would be impacted by more than just warmer days, and the results will really depend on more than just temperatures. What I’ve put together below is meant to provide a little background on the situation.

Each species will respond differently to climate change- which will be the most successful?

How will rising temperatures alter forests? Well, there are still a few unknowns with this, but the general trend is longer growing seasons and increased plant growth (Saxe et al. 2001). Warmer days will also mean warmer soils. So that’s a good thing, right? Well, it’s a bit of a tricky situation, because some plants and some natural processes operate better at lower temperatures (this is why, for example, lilacs won’t make it in Georgia, much to the disappointment of a woman I know). In addition, when scientists forecast overall warmer temperatures, they are usually working with averages, but, as anyone who was on the East Coast over the last 2 weeks can tell you, extreme weather events seem to be a bit too common for comfort and extreme temperatures go along with that. It’s true that longer growing seasons can mean bigger plants, but there are concerns that earlier springs and later autumn frosts will cause communication errors for trees responding to seasonal changes (Saxe et al. 2001). Trees develop cold-hardiness for winter based upon temperature and daylight cues, and then use those same cues to start the growing season in the spring. If the timing of those cues is out of sync, trees may not prepare for winter adequately, or they may come out of dormancy too soon and risk frost damage (Aitken et al. 2008). If they then experience an extreme weather event, such as the recent nor’easter, they could face damage or even death. Another concern is that trees which depend more on daylight than temperature cues could function at a disadvantage and be out-competed by other species taking better advantage of the longer growing season. What about warmer winter temperatures? One worry here is how precipitation will be affected (see below), but another issue is that warmer days during winter can cause trees to dry out. When conifers were subjected to higher winter temperatures without an increase in light, some lost their needles (Saxe et al. 2001).

What about warmer soils? As it turns out, soil temperature, soil moisture, and soil nutrients will all play a big role in how forests react to climate change. Typically, the warmer the habitat, the more equal the distribution of carbon between living biomass and soil (Malhi et al. 1999), so, as temperate and boreal areas warm up, you may find increased soil respiration and nutrient cycling; soil carbon stocks will go down and more carbon will be stored in living tissue. At the same time, soils in some areas may become drier and some studies have predicted increased water stress for plants under those conditions (Aber et al. 1995). This could be caused by changes in where precipitation falls, but also changes in what form it takes- if warmer winter temperatures mean that less snow and more rain falls or if snow melts faster in the spring, trees may not have access to enough moisture later in the year (Weltzin et al. 2003). Given their stationary nature, they can’t go looking for better conditions, but will simply have to make do.

These trees are prepared for and, in some ways, depend on cold winters with lots of snow- but they may not be ready for greater extremes in the future

Should we expect changes in the forests around us? Once again, this is a difficult question to answer- so many reports commented that each species and each location was an individual case because of differences in tolerance, nutrients, water-cycling, etc. Pastor & Post (1988) felt that spruce forests in northern Quebec would continue as such despite the impact of climate change, while forests from the west of the Great Lakes to the Atlantic would become mostly northern hardwood at the expense of spruce and fir. One of the big questions is which species will adapt best to new conditions and which will be unable to compete. And there are concerns over how pests and diseases will be influenced by climate change- warmer winters could mean more bark beetles, warmer and wetter summers could cause fungal and disease outbreaks, but higher carbon dioxide levels might cause leaves to become less nutritious for insects and limit their numbers (Saxe et al. 1998).

I feel that the future composition and health of forests as climates around the globe change is a big topic that needs more investigation and discussion, so that will be my focus for next week, as well as a peek into changes in animal populations as a result of shifts in vegetation. There are ways to research future changes, but the complexity of the systems involved and the sheer number of variables mean that some consequences of a warmer climate are not yet known. Hopefully, however, we can get of sense of the general trend and then refine our understandings with more study.

Works Cited:

Aber, J.D., Ollinger, S.V., Federer, C.A., Reich, P.B., Goulden, M.L., Kicklighter, D.W., Melillo, J.M., and R.G. Lathrop, Jr. 1995. Climate Research 5: 207-222

Aitkin, 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.

Malhi, Y., Baldocchi, D.D., and P.G. Jarvis. 1999. The carbon balance of tropical, temperate and boreal forests. Plant, Cell and Environment 22: 715-740.

Pastor, J. and W.M. Post. 1988. Response of northern forests to CO₂-induced climate change. Nature 334: 55-58.

Saxe, H., Cannell, M.G.R., Johnsen, O., Ryan, M.G., and G. Voulitis. 2001. Tansley review no. 123: Tree and forest functioning in response to global warming. New Phytologist 149: 369-400.

Saxe, H., Ellsworth, D.S., and J. Heath. 1998. Tansley review 98: Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436.

Weltzin, J.F., Loik, M.E., Schwinning, S., Williams, D.G., Fay, P.A., Haddad, B.M., Harte, J., Huxman, T.E., Knapp, A.K., Lin, G., Pockman, W.T., Shaw, M.R., Small, E.E., Smith, M.D., Smith, S.D., Tissue, D.T., and J.C. Zak. Assessing the reponse of terrestrial ecosystems to potential changes in precipitation. BioScience 53: 941-952.