Mycorrhizal networks (MNs) are fungal hyphae that connect roots of at least two plants. It has been suggested that these networks are ecologically relevant because they may facilitate interplant resource transfer and improve regeneration dynamics. This study investigated the effects of MNs on seedling survival, growth and physiological responses, interplant resource (carbon and nitrogen) transfer, and ectomycorrhizal (EM) fungal colonization of seedlings by trees in dry interior Douglas-fir (Pseudotsuga menziesii var. glauca) forests. On a large, recently harvested site that retained some older trees, we established 160 isolated plots containing pairs of older Douglas-fir "donor" trees and either manually sown seed or planted Douglas-fir "receiver" seedlings. Seed- and greenhouse-grown seedlings were sown and planted into four mesh treatments that served to restrict MN access (i.e., planted into mesh bags with 0.5-, 35-, 250-microm pores, or without mesh). Older trees were pulse labeled with carbon (13CO2) and nitrogen (15NH4(15)NO3) to quantify resource transfer. After two years, seedlings grown from seed in the field had the greatest survival and received the greatest amounts of transferred carbon (0.0063% of donor photo-assimilates) and nitrogen (0.0018%) where they were grown without mesh; however, planted seedlings were not affected by access to tree roots and hyphae. Size of "donor" trees was inversely related to the amount of carbon transferred to seedlings. The potential for MNs to form was high (based on high similarity of EM communities between hosts), and MN-mediated colonization appeared only to be important for seedlings grown from seed in the field. These results demonstrate that MNs and mycorrhizal roots of trees may be ecologically important for natural regeneration in dry forests, but it is still uncertain whether resource transfer is an important mechanism underlying seedling establishment.

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 opengrown 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.

SummaryThe 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.

Hydraulic redistribution (HR) is a process by which water moves through plant roots from moist to dry soils. An experiment was conducted to quantify the influence of common mycorrhizal networks (CMNs) and proximity to mature HR-source trees on the water relations of surrounding seedlings. Douglas-fir (Pseudotsuga menziesii var glauca (Mirb.) Franco) seedlings were planted at four distances (0.5, 1, 2.5, and 5 m) from six mature Douglas-fir trees, either directly into soil (soil plus CMN pathway) or inside 0.5 microm mesh bags (soil-only pathway). Deuterated water was used to irrigate soil beside mature trees in order to identify different HR water pathways to surrounding seedlings. This was followed by measurements of seedling deuterium enrichment, seedling water potential, soil water potential, gravimetric soil water content, and tree root density surrounding the seedlings. There was no significantly detectable difference in the quantity of HR water transferred to seedlings having access to soil and CMN pathways or soil-only pathways of water movement. Water from the irrigation plot contributed up to 1.4% of the water of Douglas-fir seedlings. Based on the assumption that the only pathway through which seedlings could access irrigation water was through the mature trees, we estimate that as much as 21.6% of the seedling water was supplied by the nearby tree. Seedling water potential was not significantly affected either by proximity to mature trees or pathway, suggesting HR may have compensated for increasing tree competitive effects with proximity. It is also possible that the lack of difference was due to a relatively moist summer. Our results suggest that residual mature trees are potentially important for hydraulic redistribution to regenerating seedlings in harvested dry interior Douglas-fir forests.

In an attempt to comprehensively study the dynamics of non-structural carbon compounds (NCC), we measured the seasonal changes of soluble sugars, starch, lipids, and sugar alcohols in the leaves, branches, stem, and roots of the fast-growing Pinus contorta (pine) and slow-growing Picea glauca (spruce) trees growing in a boreal climate. In addition to measuring seasonal concentrations of these compounds, the relative contribution of these compounds to the total NCC pool within organs of trees (~8 m tall) was estimated and compared across different phenological and growth stages. Both species showed large seasonal shifts from starch to sugars from spring to fall in nearly all organs and tissues; most likely an adaptation to the cold winters. For both species the total fluctuation of sugar + starch across the year (i.e., the difference between the minimum and maximum observed across collection times) was estimated to be between 1.6 kg and 1.8 kg for all NCCs. The fluctuation, however, was 1.40 times greater than the minimum reserves in pine while only 0.72 times the minimum reserves in spruce. By tissue type, NCC fluctuations were greatest in the roots of both species. Roots showed a large build-up of reserves in late spring, but these reserves were depleted over summer and fall. Storage reserves in needles and branches declined over the summer and this decline may be linked to the sink strength of the stem during diameter growth. Some notable highlights of this holistic study: a late winter build-up of sugars in the stem xylem of both species but especially spruce; an increase in sugar alcohols in the bark of spruce in very late winter which could indicate mobilization to support early growth in spring; high lipid reserves in the bark of pine, that appeared not to be impacted by seasonal changes between summer and winter. Collectively, these observations point towards a more conservative NCC reserve strategy in spruce compared to pine which is consistent with its stress tolerance and greater longevity.

Alnus viridis (Chaix) DC. subsp. crispa (Aiton) Turrill (Betulaceae), commonly known as green alder or mountain alder, is a boreal shrub used to revegetate disturbed lands because of its ability to persist and flourish in adverse conditions. Cold stratification, as a seed pretreatment, has been effective in breaking the embryo dormancy of green alder seed; however, the recommended duration of cold stratification varies from 0 to 60 d. To determine the optimum time for seed collection and the impact of duration of cold stratification, seed was collected from 3 locations in 2013. One location was chosen for collection of seed over 3 consecutive mo. Seed was subjected to 4 cold stratification treatments at +4 °C (39.2 °F), and we included a non-treated control. Mean germination time was significantly lower with cold-stratified seed than with the non-stratified seed. Green alder seed showed more rapid and uniform germination when cold stratified for 2 wk or 6 wk as compared to 12 wk of stratification or non-stratified. Germination rate of green alder seed was similar over a 3-mo collection period from late summer to fall.

No 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.