Potential effects of climate change on the growth of lodgepole pine across diameter size classes and ecological regions

Abstract

We examined 65 lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) sites in Alberta using a dendrochronological approach in order to examine the relationships between climate and growth of lodgepole pine across elevational ecoregions and diameter size classes. The 4 elevational ecoregions sampled included the Boreal Highlands (BH: 13 sites); the Foothills (FH: 36 sites); a grouping of the montane and subalpine zones of the southern Rocky Mountains (RM: 12 sites); and the montane zone of the Cypress Hills (CH: 4 sites). The first diameter size class was comprised of the three largest (top) diameter trees at each site. The tree list of each plot was ranked, then divided into three groups of equal basal area and the tree at the midpoint of each group (small, medium, and large) was selected for the other three size classes. Annual growth in basal area of lodgepole pine was generally sensitive to heat and moisture stress in late summer of the previous year, the degree of winter harshness, and the timing of the start of the growing season. Growth was inhibited by low temperature in all winter months at the most northern BH sites which had the coldest winters, but this effect was interrupted in some of the midwinter months in the more southerly sites in the RM, and we postulated this was due to the damaging influences of Chinook winds. Interannual growth patterns were strongly correlated between top diameter trees and the other classes, and trees of all diameter classes generally responded to climate in the same way, which indicated that it is sufficient to sample only the largest diameter trees in a stand to provide insight into growth–climate relationships. The forecasted growth estimates indicate that future climate warming will negatively impact the productivity of lodgepole pine in the FH, the heart of lodgepole distribution and productivity in Alberta.

Introduction

Climate models forecast a warming trend of 1.4–5.8 °C under an approximate doubling of atmospheric CO₂ concentrations by 2071–2100 (IPCC, 2001). Climate is expected to change most in northern regions and in mountainous areas including the Cordilleran forest of western Canada (Luckman, 1998). Examining past relationships between tree growth and climate in Cordilleran forests will help provide insight into how the productivity of these forests might be affected by future climatic change.

Most tree-ring studies (dendrochronology) involve examining the largest diameter trees to minimize the effect of competition and to simplify cross-dating due to fewer instances of missing rings (Fritts, 1976). Trees of smaller diameter classes are typically aged in demographic studies (i.e., dendroecology), but there have been few attempts to examine how interannual variations in radial growth–climate relationships may differ between trees of different diameter size classes (Piutti and Cescatti, 1997, Cescatti and Piutti, 1998, Meyer and Bräker, 2001). Previous studies have examined diameter growth patterns across elevational ecoregions in the Rocky Mountains but did not directly examine growth–climate relationships (Berg et al., 2007). Past studies examining the influences of climate on the radial growth of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) have been carried out at specific locations (1–4 sites) in the United States (Villalba et al., 1994, Sauchyn et al., 2003). Sauchyn et al. (2003) examined lodgepole pine in the Cypress Hills reporting on radial growth in relation to precipitation. In contrast, the current study describes the relationships between growth and climate for trees of different diameter size classes and for a broad network of 65 lodgepole pine sites that spans a range of elevational ecoregions.

Lodgepole pine forms even-aged, pure stands over broad areas of western North America due to its wide ecological amplitude (Lotan and Critchfield, 1990). In the western Canadian province of Alberta, lodgepole pine is found in every forested region (Natural Regions Committee, 2006) but is the predominant forest type along the eastern slopes of the Rocky Mountains (Huang et al., 2001) and in Cypress Hills of South-eastern Alberta (Henderson et al., 2002) (Fig. 1). Lodgepole pine accounts for approximately 40% of the annual harvest in Alberta (Huang et al., 2001). There is growing concern of the potential vulnerability of Alberta's forests to future climate change (Alberta Environment, 2007). To help address these concerns, the objective of this study was to examine radial growth–climate relationships of lodgepole pine in the Cordilleran forests of Alberta over an 80-year period (1924–2003). Other specific objectives included examining whether the growth response to climate in lodgepole pine varied between diameter size classes and between elevational ecoregions. The other specific objective was to make projections of growth under different climate change scenarios in the 21st century.