Abstract
In this study we explore the impact of asymmetrical vs. uniform crown shading on the mortality and growth of upper and lower branches within tree crowns, for two conifer species: shade intolerant lodgepole pine (Pinus contorta) and shade tolerant white spruce (Picea glauca). We also explore xylem hydraulics, foliar nutrition, and carbohydrate status as drivers for growth and expansion of the lower and upper branches in various types of shading. This study was conducted over a two-year period across 10 regenerating forest sites dominated by lodgepole pine and white spruce, in the lower foothills of Alberta, Canada. Trees were assigned to one of four shading treatments: (1), complete uniform shading of the entire tree, (2) light asymmetric shading where the lower 1/4–1/3 of the tree crown was shaded, (3) heavy asymmetric shading as in (2) except with greater light reduction and (4) control in which no artificial shading occurred and most of the entire crown was exposed to full light. Asymmetrical shading of only the lower crown had a larger negative impact on the bud expansion and growth than did uniform shading, and the effect was stronger in pine relative to spruce. In addition, lower branches in pine also had lower carbon reserves, and reduced xylem-area specific conductivity compared to spruce. For both species, but particularly the pine, the needles of lower branches tended to store less C than upper branches in the asymmetric shade, which could suggest a movement of reserves away from the lower branches. The implications of these findings correspond with the inherent shade tolerance and self-pruning behavior of these conifers and supports a carbon based mechanism for branch mortality – mediated by an asymmetry in light exposure of the crown.
Highlights
Light availability is an important driver of plant growth and crown development, in multilayered forests [1,2,3]
Terminal shoot growth of the upper branches of US pine trees was reduced by 6 cm compared with NS trees while asymmetrical shading (AS-H and AS-L) had little effect on the terminal shoot growth of the upper branches (Figure 2a,b)
The growth of terminal shoots of the lower branches was 2.5 cm less in the US treatment compared to the control (NS) trees (Figure 2e,f); the terminal shoots of lower branches in both the ASL and asymmetric shading (AS-H) was 5 cm less compared to the NS control (.50% reduction in shoot growth) (Figure 2e,f)
Summary
Light availability is an important driver of plant growth and crown development, in multilayered forests [1,2,3]. In closed-canopy forests, light limitation is a significant driver of lower branch mortality and crown recession for trees [7,8]. As it relates to carbon (C), it is thought that branches in a crown behave as autonomous units [9], as there is little evidence for longdistance C movement between branches within a crown [10,11,12,13,14,15,16,17]. There is growing evidence, that C limitation due to reduced light is not the only driver of branch mortality, especially in large trees. Other factors such as nutrient limitation [18,19]; hydrological constraints [20,21], and heterogeneity in light within crowns [22,23,24] have been linked to the mortality of lower branches, which makes branch recession a more complex issue than previously thought
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