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Maintaining white spruce (Picea glauca (Moench) Voss) in mixture with trembling aspen (Populus tremuloides Michx.) and other broadleaf competitors following timber harvest in boreal and sub-boreal mixed-wood stands is challenging. Under-planting spruce within harvest gaps is one potential strategy, but it’s unclear which gap environments favor spruce in relation to aspen. Here we test the effect of gap position and woody release treatment on leaf area development and growth physiology of spruce and aspen. Five positions were measured from the southern understory through the gap center to the northern understory in replicated strip cuts. The woody release treatment cut all stems around spruce including aspen in subplots within each experimental gap; other subplots were left un-cut for comparison. We measured leaf area per wet weight, leaf photosynthesis, stomatal conductance, and transpiration in low- and full-light conditions. We also assessed photosynthetic plasticity in relation to light. Overall, aspen maintained higher leaf area per unit wet weight than spruce, but spruce leaf area was more sensitive to changes in gap position. Aspen also maintained higher photosynthesis and transpiration rates than spruce under both light conditions, and woody release treatment. The magnitude of difference in gas exchange rates between the two species, however, depended significantly on woody release treatment and gap position for photosynthesis, and on gap position for transpiration. Photosynthetic plasticity was not significantly different across species, gap position, or woody release treatment. Our results indicate that in order to ensure the competitive advantage of spruce in mixed-wood boreal stands, forest managers should concentrate spruce planting and competition control efforts in the centers of canopy openings.

Cloud forests are amongst the world’s most impacted and endangered forest types, with Mexican cloud forests amongst the most degraded. These species rich forests are characterized by a diversity of congeneric oak species which dominate the canopy of mature forests. An improved understanding of the establishment requirements of oak seedlings in cloud forests is needed for conservation and restoration purposes. The aim of this study was to assess the influence of light conditions during early establishment of three endangered Quercus species. Seedling growth and biomass allocation in Quercus insignis M. Martens & Galeotti, Q. sartorii Liebm. and Q. xalapensis Bonpl. was determined under two light levels: light gap (1338 µmol m-2 s-1) and closed canopy (118 µmol m-2 s-1) in a cloud forest in Veracruz, Mexico. Growth and development were evaluated over the first 13 months. Results suggest there was a significant effect of light conditions on growth rate and biomass allocation. Although survival rate was similar among both environments, the three species showed lower growth rates under the closed canopy during the first nine months, while elongation rate was higher during the last three months under this environment compared to the light gap. Across all species, fresh biomass and dry biomass of roots, stem and leaves were almost 50% higher in light gap than under closed canopy. Q. insignis produced more biomass in shoots and roots than Q. sartorii and Q. xalapensis, which may increase its establishment success in shaded conditions. Results suggest that these three oak species are suited to planting in small gaps, but also in shaded understory conditions, as high early survival (>90%) may allow enrichment planting in advance of gap creation.

The American quaking aspen (Populus tremuloides Michx.) and its close relative, the Eurasian quaking aspen (Populus tremula L.), cover a realm that is perhaps the most expansive of all tree species in the world. In North America, sudden aspen decline (SAD) is a growing concern that marks the rapid decline of quaking aspen trees leading to mortality at the stand and landscape scale. Research suggests that drought and water stress are the primary causes of SAD. Predisposing factors (age, structure, and landscape position), as well as associated stressors (i.e., pests and pathogens), have been linked to mortality in affected stands. The conflation of multiple interacting factors across the aspen&rsquo, s broad geographic range in North America has produced significant debate over the classification of SAD as a disease and the proper management of affected stands. Interestingly, no such effects have been reported for the Eurasian aspen. We here review and synthesize the growing body of literature for North America and suggest that SAD is a novel decline disease resulting from multiple inciting and interacting factors related to climate, land-use history, and successional dynamics. We suggest that the range of aspen observed at the onset of the 21st Century was bolstered by a wet period in western North America that coincided with widespread regional cutting and clearing of late-successional forests for timber and grazing. No comparable land-use history, successional status, or age-class structure is apparent or linked for Eurasian forests. Eurasian aspen is either absent or young in managed forests, or old and decadent in parks in Fenno-Scandinavia, or it grows more intimately with a more diverse mixture of tree species that have arisen from a longer period of frequent timber cutting in Russia. Based on these insights we provide recommendations for practical management techniques that can promote stand resilience and recovery across a range of stand conditions in North America. Managers should attempt to identify SAD-prone stands using the presence of predisposing conditions and focus treatments such as coppice or prescribed fire on stands with suitable topographies, elevations, and climates. We conclude that SAD will persist throughout the coming decades, given the enormity of past cutting history, fire exclusion, and current changes in climate until a more active restoration agenda is implemented.

Predictions of forest succession have a tremendous influence on forest management decisions, strategies, and policies. Although generalized forest succession models are intensively studied, there is a distinct lack of long-term studies confirming these models due to the logistical constraints of tracking a forest as it develops over decades or centuries. Here we use a long-term dataset spanning >50 years of third-growth forest development in experimental forest gaps within a second-growth northern temperate hardwood forest to examine the influence of gap position on the structure and composition of regeneration and confirm past models of forest succession. We document key stand development patterns in even-aged mixed-species stands, including vertical stratification and shifts in diameter distributions. While the stands have shifted into a classic inverse-J diameter distribution, these shifts are due to a combination of unimodal distributions of the shade-intolerant and intermediate guilds that grow in mean diameter over time with consistently high densities of small individuals from the shade-tolerant guild. The greatest vertical stratification between the shade-intolerant and intermediate guilds occurred 30 years after clearing but then disappeared as the intermediate guild increased in height. In contrast, the separation in strata between the shade-tolerant species and the more intolerant guilds only increased over time. Finally, we found that gap position had a significant effect on productivity. The basal area, height, and species richness (in this case, an indicator of canopy stratification) were best modeled with negative quadratic regressions with highest values in gap centers and lower values towards the edges of gaps. This pattern aligns more with expected moisture and nutrient availability within gaps than expected solar radiation in the northern hemisphere, suggesting that moisture and nutrient availability may play a greater role than is often acknowledged in temperate forest types. Furthermore, these relationships were readily apparent only after 30 years yet became more accentuated with time, highlighting the importance of long-term research on successional dynamics for management. Ultimately our results document a clear successional trajectory in these gaps that follow expectations for even-aged mixed-species stand development where a full suite of species leads to rolling periods of dominance based on species functional guilds, with early successional species replaced by late-successional species in the progression of succession.

Across the central and eastern U.S., decades of fire exclusion have coincided with upland oak (Quercus spp.) regeneration problems and a compositional shift toward shade-tolerant, fire-sensitive species like red maple (Acer rubrum L.) and American beech (Fagus grandifolia Ehrh.). Because oaks are fire-adapted and moderately shade-intolerant, prescribed fire is commonly used as a management tool to decrease competition, increase light, and promote oak regeneration. However, prescribed fire alone often fails to sufficiently open the canopy and improve oak competitive status, suggesting the combination of fire and canopy gaps may be necessary for oak success. To better understand the effects of single and multiple prescribed fires alone and combined with naturally-formed canopy gaps on tree regeneration, we measured tree densities (both seed and resprout origin) in five height size classes (small seedlings: ≤0.5 m, large seedlings (>0.5–1.0 m), saplings (1.1–4.0 m), midstory (4.1–7.0 m), and poles (7.1–12 m)) within gaps and non-gap areas treated with no fire, single fire, or multiple fires (2–3) across six sites within the Knobs Region of Kentucky (U.S.A.) in 2017. Oaks were common as small and large seedlings without fire, especially within gaps, but they were largely absent from larger size classes. Instead, red maples and American beech dominated sapling and midstory size classes without fire, regardless of gap treatment. Single and multiple fires reduced both absolute and relative density of American beech saplings and red oaks (Q. velutina Lam., Q. coccinea Munchh. and Q. rubra L.) of all sizes, but single fires, both within and outside of gaps, increased red maple large seedling, sapling, and midstory dominance. Multiple fires, both within and outside of gaps, reduced red maple abundance, and this coincided with increased relative density of white (Q. alba L.) and chestnut oaks (Q. montana Willd.), but not of red oaks. Red oak small and large seedlings were the only oaks where absolute density increased in the small (~300 m2), relatively old (20–30 yr), naturally-formed canopy gaps in this study. Our findings suggest that management techniques that include multiple prescribed fires and large canopy gaps (>300 m2) created relatively soon after fire will likely be necessary to reduce competing species, increase oak density, and allow oaks to reach sapling and midstory size classes.

The forest understory contains the majority of vascular plant diversity in eastern temperate forests, and its diversity, composition, and dynamics contribute directly to ecosystem function. Forest managers have traditionally viewed the understory as primarily affecting forest regeneration or wildlife habitat, but the growing recognition of goods and services the understory provides (e.g., ecosystem function, ecological resiliency, non-timber forest products) has increased concerns about the impacts of forest management on understory diversity. We monitored response of understory diversity to microsite position and degree of ground-level disturbance within experimental gaps for 10 years. We did this at four sites with distinct soil types and topographic positions of a glacial geology in southern New England that were categorized as (i) mesic, (ii) mid-slope, (iii) outwash, and (iv) sandy-skeletal. We analyzed differences in patterns of species richness, Shannon diversity, and evenness across sites and through time. Understory species richness was generally enhanced by gap formation. Gap position was the primary factor influencing species richness across all sites, but the patterns of diversity and evenness within gaps was site specific. Ground-disturbance was influential on drier sandy sites, and more pronounced earlier in the experiment. Temporal differences were also evident across sites, with richness stabilizing at all sites 10 years after gap creation. The one exception was the sandy-skeletal site, which was still increasing in richness. Resource managers interested in protecting and enhancing understory species diversity need to consider underlying site, specifically soil type when planning silvicultural treatments, as the response of the understory community to disturbance can vary greatly with site.

In many forests, advance regeneration represents an important ‘seedling bank’ for replacing overstory trees after canopy disturbance. However, long-term spatial and temporal dynamics of understory tree seedlings are poorly understood, particularly in topographically complex southern New England mixed-deciduous forests. From 1996 to 2005 we tracked the recruitment, growth and survival of seedlings of masting tree species across different topographic positions. Species included the largely valley associated sugar maple ( Acer saccharum ) and white ash ( Fraxinus americana ), the primarily ridge associated white oaks ( Quercus alba and Q. prinus ), and the site generalist red oaks ( Q. rubra and Q. velutina ) and red maple ( A. rubrum ). Four 1 m 2 circular plots were randomly established in nine different sites, which included three replicates of each topographic position (ridge, midslope and valley). Seedling recruits were tagged and measured annually. Densities varied largely between species, both over time and by topographic position. Ridge and midslope positions had similar patterns, with high densities of red oak (>200,000 seedlings/ha) declining over time, and other species generally fluctuating at much lower densities (

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 re- duced 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 ap- pear 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. Resume : Le deperissement soudain et la degradation des peuplements matures de peuplier faux-tremble sont couram- ment observes un peu partout en Amerique du Nord. Ce deperissement a une enorme importance ecologique et econo- mique; pourtant il demeure mal compris. Cet article resume notre comprehension du deperissement du peuplier faux- tremble en Amerique du Nord, identifie les processus qui pourraient contribuer a la perte de vigueur et au deperisse- ment des peuplements de peuplier faux-tremble et examine l'echelle (peuplement, aire ecologique, region) a laquelle operent ces processus. Plusieurs facteurs incluant les agents pathogenes, la nutrition et les changements du stade de succession pourraient etre responsables de la perte de vigueur du peuplier faux-tremble et contribuer par consequent au processus de deperissement. Cependant, les facteurs plus probablement a l'origine du deperissement dans les peuple- ments matures de peuplier faux-tremble semblent etre la defoliation par les insectes, la secheresse et les episodes de gel et degel. D'autres etudes seront evidemment necessaires pour elucider les mecanismes du deperissement et son comportement a l'echelle du paysage. Il serait utile de combler les besoins d'information reliee aux processus d'identification et a la modelisation du comportement du deperissement a l'echelle du paysage.

The impacts of partial cut systems on nutrient availability and understory vegetation are poorly understood. To examine these responses, white spruce dominated stands in the boreal mixedwood of Alberta were clear-cut or partial-cut and the forest floor treated by slash burning, mixing, mounding, or scalping in a split-plot design. Soil nutrient availability (ion exchange resin), net N mineralization (in situ incubations), and vegetation (density and cover) responses were assessed. With the exception of higher Mg availability in the clearcuts, differences in nutrient availability were driven by forest floor disturbance and not harvest method. Relative to controls, burning increased availability of NH4+, NO3–, and P, and scalping increased Ca and Mg but diminished K. Controls had low levels of NO3–. The mixing treatment substantially reduced net N mineralization. In terms of vegetation, partial cuts reduced root suckering by Populus spp. (Populus tremuloides Michx., Populus balsamifera L.) relative to clearcuts. Burning and mounding stimulated fireweed (Epilobium angustifolium L.) cover, while scalping increased Populus spp. sucker density. In contrast, mixing largely reduced vegetation establishment, likely because of the destruction of roots and rhizomes and reduced N supply. Nutrient availability and vegetation establishment were more strongly controlled by forest floor disturbance than by partial canopy retention.

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.

Although short-rotation woody crop biomass production technology has demonstrated a promising potential to supply feedstocks for bioenergy production, the water and nutrient processes in the woody crop planation ecosystem are poorly understood. In this study, a computer model was developed to estimate the dynamics of water and nitrogen (N) species (e.g., NH-N, NO-N, particulate organic N, and soluble organic N [SON]) in a woody crop plantation using STELLA (tructural hinking and xperiential earning aboratory with nimation) software. A scenario was performed to estimate diurnal and monthly water and N variations of a 1-ha mature cottonwood plantation over a 1-yr simulation period. A typical monthly variation pattern was found for soil water evaporation, leaf water transpiration, and root water uptake, with an increase from winter to summer and a decrease from summer to the following winter. Simulations further revealed that the rate of soil water evaporation was one order of magnitude lower than that of leaf water transpiration. In most cases, the relative monthly water loss rates could be expressed as evapotranspiration > root uptake > percolation > runoff. Leaching of NO-N and SON depended not only on soil N content but also on rainfall rate and duration. Leaching of NO-N from the cottonwood plantation was about two times higher than that of SON. The relative monthly rate of N leaching was NO-N > SON > NH-N. This study suggests that the STELLA model developed is a useful tool for estimating water and N dynamics from a woody crop plantation.

In two separate studies, the suckering of trembling aspen (Populus tremuloides Michx.) roots was assessed in response to different daily maximum soil temperatures and in relation to available nutrients (CaSO4 and NH4NO3). In the first study, aspen root sections were incubated under high:low temperature regimes of 12:8, 14:8, 16:8, 18:8, or 20:8°C until 124 degree-days above 8°C had been reached. Daily maximum temperature did not affect the number of suckers produced per square centimetre of root surface area or per root section. However, more time was required to initiate suckers on root sections grown under the 12:8°C temperature regime compared with those grown at 20:8°C. Furthermore, when calculated from a base temperature of 5°C, the number of degree-days needed to initiate aspen suckers was not different across the temperature regimes. In the second study, CaSO4, NH4NO3, or distilled water was added to aspen root sections. While nutrients did not affect the number of suckers produced, the addition of CaSO4 or NH4NO3 significantly increased sucker dry mass. Nutrient addition appeared to stimulate sucker growth and mobilization of stored reserves to support this growth.

As one of the largest and most intact biomes, the boreal forest occupies a prominent place in the global carbon budget. While it contains about 13% of global terrestrial biomass, its organic-rich soils hold 43% of the world’s soil carbon. A growing body of research has attempted to measure how climate influences the processes governing carbon uptake and release, and to predict further changes due to climate change. Current research on boreal forest carbon pools and the processes that affect them suggest that this biome acts as a weak sink for atmospheric carbon. However, evidence of rapid climate change at northern latitudes has raised concern that the boreal forest could readily shift to a net carbon source if the ecophysiological processes facilitating carbon uptake are sufficiently disrupted. Changes in soil temperatures, respiration rates, and disturbance dynamics (type, extent, and frequency) brought about by climate change or other factors could switch the biome to a net source of carbon. Based on current knowledge, it appears that a warming climate will likely create the conditions for increased carbon release from boreal forests.