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Climate models suggest that more frequent drought events of greater severity and length, associated with climate change, can be expected in the coming decades. Although drought-induced tree mortality has been recognized as an important factor modulating forest demography at the global scale, the mechanisms underlying drought-induced tree mortality remain contentious. Above- and below-ground growth, gas exchange, water relations and carbon reserve accumulation dynamics at the organ and whole-plant scale were quantified in Populus tremuloides and P. balsamifera seedlings in response to severe drought. Seedlings were maintained in drought conditions over one growing and one dormant winter season. Our experiment presents a detailed description of the effect of severe drought on growth and physiological variables, leading to seedling mortality after an extended period of drought and dormancy. After re-watering following the dormant period, drought-exposed seedlings did not re-flush, showing that the root system had died off. The results of this study suggest a complex series of physiological feedbacks between the measured variables in both Populus species. Further, they reveal that reduced reserve accumulation in the root system during drought decreases the conversion of starch to soluble sugars in roots, which may contribute to the root death of drought-exposed seedlings during the dormant season by compromising the frost tolerance of the root system.

In the continued quest to explain the decline in productivity and vigor with aging forest stands, the most poorly studied area relates to root system change in time. This paper measures the wood production, root and leaf area (and mass) in a chronosequence of fire-origin lodgepole pine (Pinus contorta Loudon) stands consisting of four age classes (12, 21, 53, and ≥100 years), each replicated 

When fertilizer is broadcast in boreal forest stands, the applied nutrients must pass through a thick layer of either feather moss or leaf litter which covers the forest floor. In a growth chamber experiment we tested the transfer of N through living feather moss or aspen litter when fertilized with urea ((NH₂)₂CO) or NH₄NO₃ at a rate of 100 kg ha⁻¹ and under different watering regimes. When these organic substrates were frequently watered to excess they allowed the highest transfer of nutrients through, although 72% of the applied fertilizer was captured in the substrates. In a field experiment we also fertilized moss and aspen litter with urea ((NH₂)₂CO) or NH₄NO₃ at a more operationally relevant rate of 330 kg ha⁻¹. We captured the NO₃ ⁻ or NH₄ ⁺ by ion exchange resin at the substrate-mineral soil interface. In contrast to the growth chamber experiment, this fertilizer rate killed the moss and there was no detectable increase in nutrient levels in the aspen litter or feather moss layers. Instead, the urea was more likely transferred into the mineral soil; mineral soil of the urea treatment had 1.6 times as much extractable N compared to the NH₄NO₃ treatment. This difference between the growth chamber and field studies was attributed to observed fertilizer-damage to the living moss and possibly damage to the litter microflora due to the higher rate of fertilization in the field. In addition, the early and substantial rainfall after fertilization in the field experiment produced conditions for rapid leaching of N through the organic layers into the mineral soil. In the field, only 8% of the urea-N that was applied was captured by the ion exchange resin, while 34% was captured in for the NH₄NO₃ fertilization. Thus, the conditions for rapid leaching in the field moved much of the N in the form of urea through the organic layers and into the mineral soil before it was hydrolyzed.

The available evidence suggests that for trembling aspen ( Populus tremuloides ) seedlings, a high root-to-shoot ratio (R:S) and high root total non-structural carbohydrate (NSC) concentration are the best predictors of future growth and outplanting success in harsh reclamation environments. To facilitate more consistent production of aspen seedlings with these characteristics requires the identification of the environmental conditions most responsible for directing resources to growth or to reserve storage. During nursery production, aspen seedlings are often sheltered from environmental stressors in greenhouses and exposed to variation in light quality and quantity, compared to outside conditions; the common result is that greenhouse grown seedlings exhibit greater allocation to shoot growth rather than to root structures and reserves. It is currently unclear whether this imbalance in R:S is the result of stronger environmental controls within a greenhouse on factors such as relative humidity, wind and temperature or rather on differences in the quantity and quality of light. In this study, we sought to determine what influence differences in both light quality and quantity, along with sheltering, has on the development of desired characteristics in aspen seedlings. To do so, we grew aspen seedlings in the field under one of six light treatments [unsheltered, full-sun (100 % full-sun); sheltered, full-sun (~90 %): sheltered, shaded (~40 %): sheltered, shaded blue (~40 %); sheltered, shaded red (~40 %); and sheltered, shaded low red/far-red ratio (~40 %)] to identify the combination of sheltering, light quality and quantity with the greatest influence on R:S and root NSC. The largest increase in desired characteristics was a peak in R:S and in root NSC for seedlings grown under unsheltered, full-sun conditions. These results suggest that growing seedlings outside, with pre-exposure to more stressful conditions prior to bud hardening, is the driver for the desired characteristics in species with indeterminate growth strategies such as aspen seedling stock.

1 A fire of unusually great severity (deep burning) burned across the forest-tundra ecotone near Inuvik, Northwest Territories from August 8 to 18, 1968. 2 Burned-unburned paired study sites around the fire perimeter, which had been established in both tundra and forest-tundra in 1973 were relocated in 1990. These showed that total vascular plant cover had reached prefire levels after 22 years, that tall shrubs had become dominant in the tundra and that biomass was now sufficient to support another fire. Cryptogams showed minimum recovery between the two studies. 3 In previously treed areas postfire densities of Picea mariana and Picea glauca were much lower than before. Betula papyrifera and Populus balsamifera, however, showed an increase in density and had extended their range into previously treeless areas. 4 The results obtained have implications for vegetation changes in the Circumpolar North related to global warming. It is predicted that deciduous tree species with long distance seed dispersal mechanisms will increase in abundance and will invade the tundra in a stepwise fashion after each fire. This will be most noticeable near northward flowing rivers because these valleys provide the habitat for outlier tree populations and are therefore a major source of propagules.

Seasonal differences in photosynthesis and stomatal conductance of four herbaceous perennials from beneath a deciduous canopy was assessed at two light levels (60 and 400 μmol m-2s-1photosynthetic photon flux density, PPFD) and two leaf temperatures (7 and 20°C). Leaves of an evergreen, Pyrola asarifolia Michx., a wintergreen, Cornus canadensis L., and two summergreen species, Rubus pubescens Raf. and Aralia nudicaulis L., were collected at four times during the growing season. In addition, midsummer light response curves were obtained for one summergreen (A. nudicaulis) and one evergreen species (P. asarifolia) at both 7 and 20°C. Gas exchange measurements were made in the laboratory under controlled environmental conditions. For leaves collected in April, when insolation was high due to the leafless overstory, only P. asarifolia had green leaves, and there was no effect of temperature or light on this species' photosynthesis. P. asarifolia's net assimilation rate (NA) in April was about 30% of it's maximum in late summer. In early summer (June), A. nudicaulis and R. pubescens had higher NA at the higher temperature; at this time, these summergreen species also reached their maximum NA. Midsummer photosynthetic light response curves showed that the light-saturation point was higher and more responsive to leaf temperature in the summergreen A. nudicaulis than in the evergreen P. asarifolia. The summergreen species appear to have a photosystem which performs at high rates during early- and mid-summer, as well as a taller stature which allows them to intercept more light. The photosynthetic system of the ever/wintergreen species is adapted to the low ground-level light conditions in the summer and there does not appear to be an adjustment to take further advantage of the higher light in the spring and fall period. The adaptation of the evergreen and wintergreen understory species is tolerance to low temperatures, enabling them to photosynthesize into the fall till the first continuous frosts occur in the understory and also permitting the evergreen species to begin photosynthesis early in the spring.

Trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) seedlings were grown at uniform air temperatures but different soil temperatures (5, 15, and 25 degrees C), and gas-exchange and growth characteristics were examined during active growth and early dormancy. At 5 degrees C, Populus tremuloides had no root growth and limited growth in leaf area and shoot mass compared with the large increases in leaf and shoot mass at 25 degrees C. In contrast, Picea glauca had some root growth at 5 degrees C and moderate growth of roots at 15 and 25 degrees C; however, there were no differences in aboveground mass at the different soil temperatures. Net assimilation and stomatal conductance in Populus tremuloides were reduced with decreasing soil temperatures, while in Picea glauca soil temperatures did not affect these gas-exchange variables. In both species, root mass was higher in the dormant period than during the growing period, while root volume remained constant. Generally, the growth variables of Populus tremuloides were more suppressed by cold soils than in Picea glauca. Root total nonstructural carbohydrates (TNC) decreased between the active growth and dormancy period by nearly 50% in Populus tremuloides, while there was no change in TNC in Picea glauca. Results suggest a more conservative use of TNC reserves in Picea glauca combined with a tolerance to cold soil temperatures compared with Populus tremuloides.

Aspen (Populus tremuloides Michx.) is a clonal tree species that commonly regenerates via root suckering after disturbance. This paper reviews the literature and identifies critical gaps in our understanding of the dynamics of aspen root suckering. The role of plant growth regulators (e.g., hormones, carbohydrates), environmental conditions (e.g., soil moisture, temperature, nutrient availability), overstory disturbance (e.g., harvesting, wildfire), ground disturbance (e.g., soil compaction, wounding or severing of roots), vegetation competition, predisturbance stand condition, and clonal (genetic) differences are discussed as they relate to sucker initiation, sucker growth, and (or) patterns of site establishment. The paper presents a series of conceptual figures summarizing our knowledge of the factors controlling suckering dynamics and identifies areas of future research.

Sudden dieback and deterioration of mature aspen stands is commonly observed throughout North America. This dieback process has tremendous ecological and economic importance, yet remains poorly understood. This paper summarizes our understanding of aspen dieback in North America, identifies potential processes that contribute to reduced vigour and dieback of aspen stands, and examines the scales (stand, ecosite, regional) at which these processes operate. Many factors including pathogens, nutrition, or successional changes may be involved in the decline of aspen vigour and thereby contribute to the dieback process. However, insect defoliation, drought, and thaw-freeze events appear to be the most likely factors initiating dieback in mature aspen stands. Further study is clearly needed to elucidate the mechanisms and landscape patterns of dieback. Information needs related to identifying processes and modeling landscape patterns of dieback are indicated.

1 Removal of the above-ground portion of clonally regenerating trees results in a massive imbalance in the ratio of root to leaf area. We investigated the resilience of clones following above-ground disturbance in terms of root carbohydrates, leaf area renewal and root retention. 2 In a 2 x 2 factorial experiment, 40 Populus tremuloides saplings were cut at times of high (late fall) or low (spring after leaf flush) root carbohydrate reserves. Leaf area renewal was manipulated by allowing either only one or all root suckers to re-grow. 3 Root starch concentrations were 10 times higher at the time of cutting in the fall compared with the spring, whereas sugar concentrations were only 10% higher. When saplings were cut in fall and all suckers were allowed to develop (FA treatment), suckers were taller, and had more biomass and leaf area and higher leaf area ratio than all other treatments. In particular, leaf area and leaf area ratio recovered within a year to near pre-treatment levels compared with at least a 50% reduction in the other treatments. 4 At the end of the first season after cutting FA saplings also had the greatest root mass and lowest amount of dead root mass, and root starch concentrations of these saplings had returned to pre-treatment values, compared with a 20% recovery for spring-cut saplings with a single sucker. Root mass and root starch concentrations were correlated with leaf mass in all treatments. 5 Adequate carbohydrate reserves at the time of disturbance and rapid redevelopment of leaf area by extensive sprouting seem critical for resilience of the clone and allow for the retention of the clonal root system and a rapid rebuilding of carbohydrate reserves. Clones with poor sucker and leaf area development showed extensive root mortality and reduced carbohydrate reserves and their prospects for growth in the following year were comparatively poor.