Introduction
1. Background and Rationale
Correlations between annual tree ring widths and climate can be traced back to the early 20th century (Douglass, 1919). These correlations led to the study of dendroclimatology, where tree rings are studied to estimate climatic conditions of the past (Sheppard, 2010). This assessment of climatic influences on past tree growth can be an important tool for forest managers as climate change is expected to result in increased water stress through rising temperatures and lower annual precipitation over much of North America (IPCC, 2014). In regions where precipitation is already a limiting climatic variable on forest productivity, this could lead to increased tree mortality and loss of suitable habitat.
The way in which dendrochronological records are used to estimate past climatic conditions is through the varying measurements of radial growth. We know that tree-ring formation occurs with four distinct phases: cell division and expansion, the formation of multilayered cell walls, lignification, and cell death (Rossi et al., 2006; Piermattei et al., 2015; Savidge, 1996). At the beginning of the growing season, the division of the cambial cells produces larger, thin-walled earlywood xylem cells while the thinner, thick-walled xylem cells produce latewood near the end of the growing season (Fritts, 1966). This delineation between the earlywood and latewood is what is then used to analyze annual growth increments where many concentric rings are stacked together (Fritts, 1996). The patterns observed in the varying ring-widths of a stand indicate varying historical levels of temperature and water availability, allowing us to then cross reference these growth patterns with historical climate data to make inferences on how productivity in that area is limited by climate.
Previous research studying tree-ring collections in northeastern North America concluded that high precipitation levels could outweigh the negative impacts of a warming climate under future climate change projections (D’Orangeville et al., 2016). The mean annual precipitation (MAP) received in the northeastern region of the continent is more than double the amount that is normally received in central and western areas of North America, creating a strong east-west gradient for water availability (D’Orangeville et al., 2016). Contrarily, various climate change studies have concluded that much of the western regions of North America are projected to see up to 5% in tree mortality per year as a result of regional warming and water stress (van Mantgem et al., 2009; Birdsey & Pan, 2011; Peng et al., 2011). Based on these findings, tree populations in areas that already experience more frequent periods of drought could be more at risk than populations where annual precipitation levels remain high (Figure 1).
Correlations between annual tree ring widths and climate can be traced back to the early 20th century (Douglass, 1919). These correlations led to the study of dendroclimatology, where tree rings are studied to estimate climatic conditions of the past (Sheppard, 2010). This assessment of climatic influences on past tree growth can be an important tool for forest managers as climate change is expected to result in increased water stress through rising temperatures and lower annual precipitation over much of North America (IPCC, 2014). In regions where precipitation is already a limiting climatic variable on forest productivity, this could lead to increased tree mortality and loss of suitable habitat.
The way in which dendrochronological records are used to estimate past climatic conditions is through the varying measurements of radial growth. We know that tree-ring formation occurs with four distinct phases: cell division and expansion, the formation of multilayered cell walls, lignification, and cell death (Rossi et al., 2006; Piermattei et al., 2015; Savidge, 1996). At the beginning of the growing season, the division of the cambial cells produces larger, thin-walled earlywood xylem cells while the thinner, thick-walled xylem cells produce latewood near the end of the growing season (Fritts, 1966). This delineation between the earlywood and latewood is what is then used to analyze annual growth increments where many concentric rings are stacked together (Fritts, 1996). The patterns observed in the varying ring-widths of a stand indicate varying historical levels of temperature and water availability, allowing us to then cross reference these growth patterns with historical climate data to make inferences on how productivity in that area is limited by climate.
Previous research studying tree-ring collections in northeastern North America concluded that high precipitation levels could outweigh the negative impacts of a warming climate under future climate change projections (D’Orangeville et al., 2016). The mean annual precipitation (MAP) received in the northeastern region of the continent is more than double the amount that is normally received in central and western areas of North America, creating a strong east-west gradient for water availability (D’Orangeville et al., 2016). Contrarily, various climate change studies have concluded that much of the western regions of North America are projected to see up to 5% in tree mortality per year as a result of regional warming and water stress (van Mantgem et al., 2009; Birdsey & Pan, 2011; Peng et al., 2011). Based on these findings, tree populations in areas that already experience more frequent periods of drought could be more at risk than populations where annual precipitation levels remain high (Figure 1).
Figure 1. A figure taken from Charney et al. (2014) showing the projected forest growth changes across North America over the next 50 years. Coastal areas are anticipated to experience net positive forest growth while much of the west-central interior is expected to experience a significant decrease in forest growth.
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2. Objectives
I will examine which historical climate variables have influenced and limited the growth of white spruce (Picea glauca) and lodgepole pine (Pinus contorta) by examining tree-ring data for these species across their North American range. These results could be informative to forest managers by creating a vulnerability map as a means of understanding which climate variables are primarily controlling inter-annual variations in radial growth within specified groups of white spruce and lodgepole pine. Because climate change projections predict an increase in evaporative demand driven by rising mean annual temperatures (MAT), this method could help predict which populations could expect to see net positive or negative impacts on growth.
3. Hypothesis
Following the work of D’Orangeville et al. (2016), I hypothesize that increases in MAT will negatively affect the growth of white spruce in areas where low MAP is the dominant climatic limitation. Conversely, I expect a positive growth response for white spruce in areas where low MAT is the dominant influence over MAP by lengthening the growing season. To this end, I will be testing the hypothesis that white spruce populations in eastern and coastal North America stand to benefit under future climate change projections while the western range will experience most of the negative impacts on forest productivity. Additionally, I will be observing whether a similar gradient exists for lodgepole pine from north to southern portions of its North American range.