Radial growth patterns of jack pine in relation to climate, moisture availability, and dwarf mistletoe infection in the Boreal Plains Ecozone of Saskatchewan, Canada

Abstract

Drought stress is increasing as a result of climate change, and is thought to be driving episodes of tree mortality that have been observed in every forested region on Earth. In the western boreal forest, potential interactions between drought stress and forest diseases could lead to forest fragmentation and range contractions. Lodgepole pine dwarf mistletoe (Arceuthobium americanum) (DM) is a host specific, parasitic plant of jack pine (Pinus banksiana) that affects resource allocation, water use, and growth. Despite being one of the most damaging pests to jack pine in the Canadian boreal forest, it has not been well studied in relation to the potential impacts of climate change. I used tree ring and climate data to investigate growth-climate responses of jack pine in infected and uninfected stands, across the regional moisture gradient associated with the boundary of the boreal forest in southern Saskatchewan. I sampled across three study regions extending from remnant island forests in the drier Aspen Parkland Ecoregion, to the wetter interior of the mid-boreal upland and boreal transition ecoregions within the Boreal Plains Ecozone (BPE). Radial growth was negatively impacted by DM infection across all study regions. Severely infected trees showed a divergence in radial growth from healthy trees, with few exceptions. Ring-width correlation with summer temperature was altered in infected trees. Infected trees at the southern boundary of the boreal forest showed the greatest change in climate response, as the positive relationship with moisture variables (spring precipitation and spring Climate Moisture Index), seen in healthy trees, collapsed to zero or became marginally negative when trees were infected. This result and the decline in radial growth suggest that DM has an impact on trees growing in the southern BPE of Saskatchewan. This could lead to a contraction of the boreal forest as extensive mortality of infected jack pine could occur at its southern range limits. Jack pine is a commercially and ecologically valuable species in the boreal forest and understanding its potential responses to climate change will be important for the future sustainable use of forest resources.