Amanda Schoonmaker

Hydraulic acclimation to shading in boreal conifers of varying shade tolerance

The purpose of this study was to determine how shading affects the hydraulic and wood-anatomical characteristics of four boreal conifers (Pinus banksiana, Pinus contorta, Picea glauca and Picea mariana) that differ in shade tolerance. Plants were grown in an open field and under a deciduous-dominated overstory for 6 years. Sapwood- and leaf-area specific conductivity, vulnerability curves, and anatomical measurements (light and scanning electron microscopy) were made on leading shoots from six to nine trees of each treatment combination. There was no difference in sapwood-area specific conductivity between open-grown and understory conifers, although two of four species had larger tracheid diameters in the open. Shaded conifers appeared to compensate for small diameter tracheids by changes in pit membrane structure. Scanning electron microscopy revealed that understory conifers had thinner margo strands, greater maximum pore size in the margo, and more torus extensions. All of these trends may contribute to inadequate sealing of the torus. This is supported by the fact that all species showed increased vulnerability to cavitation when grown in the understory. Although evaporative demand in an understory environment is low, a rapid change into fully exposed conditions could be detrimental for shaded conifers.

Pit membrane structure is highly variable and accounts for a major resistance to water flow through tracheid pits in stems and roots of two boreal conifer species

The flow of xylem sap in conifers is strongly dependent on the presence of a low resistance path through bordered pits, particularly through the pores present in the margo of the pit membrane. A computational fluid dynamics approach was taken, solving the Navier-Stokes equation for models based on the geometry of pits observed in tracheids from stems and roots of Picea mariana (black spruce) and Picea glauca (white spruce). Model solutions demonstrate a close, inverse relationship between the total resistance of bordered pits and the total area of margo pores. Flow through the margo was dominated by a small number of the widest pores. Particularly for pits where the margo component of flow resistance was low relative to that of the torus, pore location near the inner edge of the margo allowed for greater flow than that occurring through similar-sized pores near the outer edge of the margo. Results indicate a surprisingly large variation in pit structure and flow characteristics. Nonetheless, pits in roots have lower resistance to flow than those in stems because the pits were wider and consisted of a margo with a larger area in pores.

Uniform versus Asymmetric Shading Mediates Crown Recession in Conifers

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.

CLUSTER PLANTING: EARLY ENHANCEMENT OF STRUCTRUAL DIVERSITY IN A RECLAIMED BOREAL FOREST

Planting trees is an important step in re-establishing functioning forest ecosystems after industrial land disturbances. Conventional planting practices create forests with evenly spaced trees, at low density, which maximizes individual tree growing space but delays the time until crown closure, potentially for decades. In this study, the first operational cluster planting trial for reclaimed boreal forest, we examined first year tree growth and vegetation competition results of a cluster planting trial in which trembling aspen (Populus tremuloides) trees were planted in clusters of 4, 10, or 20 trees with an internal spacing of 0.25 m along with non-clustered controls. Clustering of aspen seedlings had a measurable impact on the relative proportions of tree and competing vegetation cover with increased tree cover and decreased forb cover in the 10 and 20 seedling clusters compared to the controls. Average seedling height and first year height growth were similar across all cluster treatments but tended to be higher in the clusters, likely due to the suppression of competing vegetation. Operationally, there are still many questions to be answered before this practice can be implemented in a large scale across the landscape. However, based on our initial results, we believe that cluster planting has the potential to become a valuable tool for reclamation practitioners.