Drought-Related Tree Mortality

Suzanne Shears wrote:

During fall of 2018, I observed significant tree mortality in young stands and increased mortality in some mature stands around Merritt, BC. This mortality appears to be a result of the 2017-2018 drought. A variety of species and ages appear to be affected, from newly planted to well-established plantations located predominately on hilltops where soils are shallow and rocky. Forest industry professionals have confirmed that my observation is not an isolated incident. Figures 1 through 3 show an eight-year old plantation of Lodgepole pine, Subalpine fir and interior Douglas fir between Merritt and Kamloops off of the MAB Forest Service Road where there is mortality due to drought.

Drought not only causes tree mortality, reduced growth and defoliation but also increases the susceptibility of the tree to pests and diseases (Joy & Maclauchlan, 2000). The ecosystem services or, “benefits human populations derive, directly or indirectly” (Costanza et al., 1997, p. 253), from a healthy forest would diminish. Contemplating this through a timber perspective, this increase in tree mortality will have an impact on the future supply thereby having cascading social and economic consequences for Merritt. Merritt’s link to a healthy forest inventory is the reason I choose to share this observation. Other ecosystem services reduced include carbon sequestration, habitat for wildlife, aesthetics, recreation, and increasing costs to reforest.

High elevation snowpack which acts as a temporary water storage system (Abbott & Chapman, 2018) was below normal for 2017 and just at normal for 2018 (MoFLNRORD, 2019). Both years have also been unseasonably warm in May causing the snowpack to melt faster leaving little stored moisture for late spring and summer. This in addition to low levels of precipitation, high temperatures and increased wind have left trees with insufficient moisture. Although this was observed across species, climatic changes such as those described above are affecting the range of Lodgepole pine and spruce (Liepe, Hamann, Smets, Fitzpatrick & Aitken, 2016). Other causal factors to be considered include aspect, timing of planting, and planting on rocky thin soils.

An expert consult suggested some reasons for this occurring and at seemingly increased amounts (drought-induced mortality) is in part due to the accelerated harvest of MPB-killed stands with the ensuing reforestation. There are far more young stands on our landscape than in past decades, often planted at lower densities and exposed to more variable and often severe climatic environments.

On a local scale, where communities rely on a continuous timber supply, any loss of production will have social and economic impacts including loss of jobs, increased unemployment, relocation to bigger centres, increase costs of protecting timber against pests/ disease, and maintenance of current infrastructure. Such degradation may trigger more of a pathological cycle of trying to control the ecological system for the production of a desired ecosystem service, as described by Holling (1986). On broader scales, such changes would create a volatile market, decrease carbon sequestration, exacerbating climate change which, and in turn increasing drought mortality. It would also likely cause “...locally generated surprises … in a web of global ecological interdependencies.” (Holling 1986; p. 313).

References

Abbott, G. & Chapman, M. (2018). Addressing the new normal: 21st century disaster management in British Columbia. Retrieved from https://www2.gov.bc.ca/assets/gov/public-safety-and-emergency-services/emergency-preparedness-response-recovery/embc/bc-flood-and-wildfire-review-addressing-the-new-normal-21st-century-disaster-management-in-bc-web.pdf

Costanza, R., d'Arge, R., de Groot, R., Farber, S., Grasso, M., Hannon, B., Limburg, K., Naeem, S., O'Neill, R. V, Paruelo, J., Raskin, R. G., Sutton, P., & van den Belt, M. (1997). The value of the world's ecosystem services and natural capital. Nature, 387, 253-260.

Holling, C. S. (1986). The resilience of terrestrial ecosystems: local surprise and global change. In Clark, W. C. & Mann, R. E. (eds.). Sustainable development of the biosphere. Cambridge UK: Cambridge University Press, 292-317.

Joy, F. & Maclauchlan, L. (2000). Kamloops forest region drought assessment project. Retrieved from http://www7.nau.edu/mpcer/direnet/publications/publications_j/files/joy-2000.pdf

Liepe, K.J., Hamann, A., Smets, P., Fitzpatrick, C.R., & Aitken, S.N. (2016). Adaptation of lodgepole pine and interior spruce to climate: Implications for reforestation in a warming world. Evolutionary Applications, 9(2), 409-419.

Ministry of Forests, Lands, Natural Resource Operations, and Rural Development [MoFLNRORD] (2019). Snow conditions and water supply bulletins for 2017 and 2018. Retrieved from https://www2.gov.bc.ca/gov/content/environment/air-land-water/water/drought-flooding-dikes-dams/river-forecast-centre/snow-survey-water-supply-bulletin