Introduction

Climate change will affect how Alberta’s forests grow in the future, especially through changing patterns of temperature and precipitation.1–6

Experimental warming of forests has shown that increased temperature will likely have a positive effect on tree growth. However, the associated growth increase is offset by a reduction in soil moisture. Crucially, growth in experimentally warmed forest decreased more during dry periods than growth in control forest during the same periods. This suggests that the negative effects of decreased soil moisture will offset the positive effects of increased soil temperature.7

We know that reduced tree density in forests can increase tree-level growth, especially through periods of drought.8–17

In Alberta, forest thinning (reduction in tree density) on large scales is done by the forest products industry to improve tree growth. Figure 1 shows a mixed white spruce and trembling aspen stand in the boreal forest near Slave Lake, Alberta. This stand was thinned in 2021 with the objective of reducing resource competition between trees and consequently producing larger trees faster.

Modelling the effect of silvicultural treatments, such as thinning, on tree growth is difficult and requires datasets that cover long time periods.18,19 No dataset exists for thinned forest grown in a warmed climate. While it would be fascinating to experimentally alter the world's climate and measure tree growth, the University of Alberta research ethics boards would probably rain on our parade. Additionally, the available dataset only describes conditions immediately after thinning, making it impossible to measure growth directly.

However, we know a few things about the conditions that drive tree growth:

1.       Temperature and moisture availability affect tree level growth in the forest.

2.       Thinning increases tree level growth in the forest, in part through altering temperature and moisture availability.

3.       Climate change is likely to decrease future tree level growth in the forest, in part through altering temperature and moisture availability.

Given these 3 pieces of information, this project will try to answer the following question:

Does forest thinning mitigate the expected effects of climate change by allowing trees to take advantage of increased soil temperature at the same time as increased soil moisture?

Citations:

1.        Kao, S.-C. & Ganguly, A. R. Intensity, duration, and frequency of precipitation extremes under 21st-century warming scenarios. J. Geophys. Res. 116, D16119 (2011).

2.        Novick, K. A. et al. The increasing importance of atmospheric demand for ecosystem water and carbon fluxes. Nat. Clim. Change 6, 1023–1027 (2016).

3.        Price, D. T. et al. Anticipating the consequences of climate change for Canada’s boreal forest ecosystems. Environ. Rev. 21, 322–365 (2013).

4.        Hogg, E. H., Michaelian, M., Hook, T. I. & Undershultz, M. E. Recent climatic drying leads to age‐independent growth reductions of white spruce stands in western Canada. Glob. Change Biol. 23, 5297–5308 (2017).

5.        Sherwood, S. & Fu, Q. A Drier Future? Science 343, 737–739 (2014).

6.        Wang, Y., Hogg, E. H., Price, D. T., Edwards, J. & Williamson, T. Past and projected future changes in moisture conditions in the Canadian boreal forest. For. Chron. 90, 678–691 (2014).

7.        Reich, P. B. et al. Effects of climate warming on photosynthesis in boreal tree species depend on soil moisture. Nature 562, 263–267 (2018).

8.        Steckel, M., Moser, W. K., del Río, M. & Pretzsch, H. Implications of Reduced Stand Density on Tree Growth and Drought Susceptibility: A Study of Three Species under Varying Climate. Forests 11, 627 (2020).

9.        Comeau, P. G. Effects of Thinning on Dynamics and Drought Resistance of Aspen-White Spruce Mixtures: Results From Two Study Sites in Saskatchewan. Front. For. Glob. Change 3, 621752 (2021).

10.     D’Amato, A. W., Bradford, J. B., Fraver, S. & Palik, B. J. Effects of thinning on drought vulnerability and climate response in north temperate forest ecosystems. Ecol. Appl. 23, 1735– 1742 (2013).

11.     Cortini, F., Comeau, P. G. & Bokalo, M. Trembling aspen competition and climate effects on white spruce growth in boreal mixtures of Western Canada. For. Ecol. Manag. 277, 67–73 (2012).

12.     Neufeld, B. A. et al. The influence of competition and species mixture on plantation-grown white spruce: Growth and foliar nutrient response after 20 years. For. Chron. 90, 70–79 (2014).

13.     Bjelanovic, I., Comeau, P., Meredith, S. & Roth, B. Precommercial Thinning Increases Spruce Yields in Boreal Mixedwoods in Alberta, Canada. Forests 12, 412 (2021).

14.     Kabzems, R., Bokalo, M., Comeau, P. & MacIsaac, D. Managed Mixtures of Aspen and White Spruce 21 to 25 Years after Establishment. Forests 7, 5 (2015).

15.     Comeau, P. G., Wang, J. R. & Letchford, T. Influences of paper birch competition on growth of understory white spruce and subalpine fir following spacing. Can. J. For. Res. 33, 1962–1973 (2003).

16.     Hawkins, Ch. D. B. & Dhar, A. Birch (Betula papyrifera) × white spruce (Picea glauca) interactions in mixedwood stands: implications for management. J. For. Sci. 59, 137–149 (2013).

17.     Filipescu, C. N. & Comeau, P. G. Aspen competition affects light and white spruce growth across several boreal sites in western Canada. Can. J. For. Res. 37, 1701–1713 (2007).

18.     Burkhart, H. E. & Tomé, M. Modeling Forest Trees and Stands. (Springer Netherlands, 2012). doi:10.1007/978-90-481-3170-9.

19.     Forest growth and yield modeling. (Wiley, 2011).