Summary 1. Although there is ample support for positive species richness–productivity relationships in planted grassland experiments, a recent 48-site study found no diversity–productivity relationship (DPR) in herbaceous communities. Thus, debate persists about diversity effects in natural versus planted systems. Additionally, current knowledge is weak regarding the influence of evenness on the DPRs, how DPRs are affected by the variation in life-history traits among constituent species in polycultures and how DPRs differ among biomes. The impacts of these factors on DPRs in forest ecosystems are even more poorly understood. 2. We performed a meta-analysis of 54 studies to reconcile DPRs in forest ecosystems. We quantified the net diversity effect as log effect size [ln(ES)], the log ratio of the productivity in polycultures to the average of those in monocultures within the same type of mixture, site condition and stand age of each study. The first use of a boosted regression tree model in meta-analysis, a useful method to partition the effects of multiple predictors rather than relying on vote-counting of individual studies, unveiled the relative influences of individual predictors. 3. Global average ln(ES) was 0.2128, indicating 23.7% higher productivity in polycultures than monocultures. The final model explained 21% of the variation in ln(ES). The predictors that substantially accounted for the explained variation included evenness (34%), heterogeneity of shade tolerance (29%), richness (13%) and stand age (15%). In contrast, heterogeneity of nitrogen fixation and growth habits, biome and stand origin (naturally established versus planted) contributed negligibly (each £ 4%). Log effect size strongly increased with evenness from 0.6 to 1 and with richness from 2 to 6. Furthermore, it was higher with heterogeneity of shade tolerance and generally increased with stand age. 4. Synthesis. Our analysis is, to our knowledge, the first to demonstrate the critical role of species evenness, richness and the importance of contrasting traits in defining net diversity effects in forest polycultures. While testing the specific mechanisms is beyond the scope of our analysis, our results should motivate future studies to link richness, evenness, contrasting traits and life-history stage to the mechanisms that are expected to produce positive net biodiversity effects such as niche differentiation, facilitation and reduced Janzen–Connell effects.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2745.2011.01944.x

Forest productivity increases with evenness, species richness and trait variation: a global meta‐analysis

http://www.cof.orst.edu/cof/fs/kpuettmann/FEM%2009.pdf

Silviculture for old-growth attributes

Small-scale gap disturbance in forests is reviewed. Caused by the death of individual or multiple trees with subsequent fall from the canopy, gaps have been extensively studied in temperate deciduo...

Gap dynamics of forest trees: A review with particular attention to boreal forests

Understory vegetation is the most diverse and least understood component of North American boreal forests. Understory communities are important as they act as drivers of overstory succession and nutrient cycling. The objective of this review was to examine how understory vegetation abundance, composition, and diversity change with stand development after a major stand replacing disturbance. Understory vegetation abundance and diversity increase rapidly after fire, in response to abundant resources and an influx of disturbance adapted species. The highest diversity occurs within the first 40 years following fire, and declines indefinitely thereafter as a result of decreasing productivity and increased dominance of a small number of late successional feather mosses and woody plant species. Vascular plant and bryophyte/lichen communities undergo very different successional changes. Vascular plant communities are dynamic and change more dramatically with time after fire, whereas bryophyte and lichen communiti...

Understory Vegetation Dynamics of North American Boreal Forests

Boreal mixedwoods (BMWs) are the most productive and diverse forest ecosystems in North American boreal forests. A good understanding of BMW stand dynamics is a prerequisite for sustainable management of these vital resources. In this review, we describe the patterns and processes of BMWs created by natural disturbances, examine the biotic and abiotic factors that influence these patterns and processes, and discuss forest management implications related to stand development. Based on distinct structural and developmental features, BMW stand development is characterized by four stages: stand initiation, stem exclusion, canopy transition, and gap dynamics. These four stages of stand development provide a conceptual model of complex developmental processes. However, multiple pathways are possible during BMW stand development depending on disturbances, neighbour effects, and stand condition. Boreal mixedwood management at the stand level needs to emulate the natural development process and target a specific stand structure and species composition. Alternative silvicultural techniques are available to achieve the multiple objectives of BMWs. Further considerations at various temporal and spatial scales and at the operational level are required to ensure sustainable BMW management. Resume : Les forets mixtes boreales (BMWs) sont les ecosystemes forestiers les plus productifs et les plus diversifies des forets boreales nord-americaines. Une bonne comprehension de la dynamique des peuplements des BMW est un prealable a l'amenagement durable de ces ressources vitales. Dans cette revue, les auteurs decrivent les patrons et les processus des BMWs crees par des perturbations naturelles, examinent les facteurs biotiques et abiotiques qui influencent ces patrons et processus, et discutent leurs implications en amenagement forestier reliees au developpement du peuplement. Sur la base de caracteristiques distinctes de structure et de developpement, les peuplement des BMWs se caracterisent par quatre stades : initiation du peuplement, exclusion des tiges, transition vers la canopee et dynamique des ouvertures. Ces quatre stades du developpement du peuplement fournissent un modele conceptuel de processus de developpement complexe. Cependant, plusieurs avenues sont possibles au cours du developpement d'un peuplement de BMW, selon les perturbations, l'effet des voisins, et les conditions du peuplement. L'amenagement des forets boreales mixtes au niveau du peuplement doit se comparer au

Dynamics of North American boreal mixedwoods

First-summer establishment and overwinter survival of planted and seeded white spruce (Picea glauca (Moench) Voss) germinants were assessed in five microsites in the understory of trembling aspen (Populus tremuloidesMichx.) dominated boreal mixedwood stands. Mineral soil mounds, rotten logs, large exposed patches of mineral soil, normal LFH forest floor, and small screefed patches of mineral soil were examined. The mounds and rotten logs had the highest soil temperature, lowest soil strength, and accumulated the least amount of deciduous leaf litter whereas the small screefs were at the other extreme. First-summer establishment of planted seedlings was greatest on the rotten logs, mounds, and large exposed patches; the large exposed patches had the highest establishment of seeded trees. Over 85% of the planted seedlings that were in good condition in September survived the winter. Overwinter survival of seeded trees was least on small screefed patches, but survival of these trees was increased by covering them with small plastic cups. Low soil temperature, high seedbed strength, and high vegetation cover were associated with reduced seedling establishment whereas high litter accumulation was associated with reduced overwinter survival. Resume : L'etablissement au cours de la premiere saison estivale et la survie apres une saison hivernale de semis d'epinette blanche (Picea glauca (Moench) Voss), plantes ou ensemences, ont ete evalues dans cinq microsites en sous-etage de peuplements melanges domines par le peuplier faux-tremble (Populus tremuloidesMichx.) et situes en foret boreale. Des buttes de sol mineral, des billes en decomposition, de larges plaques de sol mineral expose, le parterre forestier normalement recouvert de litiere et de petites plaques de sol mineral denude ont ete examines. Les buttes et les billes en decomposition etaient les microsites ou la temperature du sol etait la plus elevee et la cohesion du sol la moins elevee. Ces microsites accumulaient la plus faible quantite de litiere de feuilles de feuillus tandis que les petites plaques denudees se situaient a l'autre extreme. L'etablissement des semis plantes au cours de la premiere saison estivale etait le plus eleve sur les billes en decomposition, sur les buttes et les grandes plaques de sol mineral expose. Dans le cas des semis ensemences, l'etablissement etait le plus eleve sur les grandes plaques de sol mineral expose. Plus de 85% des semis plantes qui etaient en bon etat en septembre ont survecu pendant l'hiver. La survie des semis ensemences apres une saison hivernale etait la plus faible sur les petites plaques denudees mais la survie de ces abres a ete amelioree en les recouvrant de petits verres en plastique. Une temperature du sol basse, la cohesion elevee du lit de germination et une couverture de vegetation haute etaient les facteurs associes a un plus faible etablissement des semis tandis qu'une forte accumulation de litiere etait associee a une survie hivernale reduite. (Traduit par la Redaction)

Microsite effects on first-year establishment and overwinter survival of white spruce in aspen-dominated boreal mixedwoods

In northeastern Alberta, the current biomass and periodic annual biomass increment (PAI) was measured in 29 stands of maturing aspen (Populus tremuloides)-white spruce (Picea glauca), aged 48 to 105 years. Plots in pure aspen were paired with nearby plots of aspen growing on a similar landform but with a spruce understory. Biomass was estimated by diameter at breast height and allometric equations. Totalled over both species, there was 10.5 % greater PAI and 10.0 % greater biomass in the mixed plots than in the pure aspen plots. Pure aspen plots, however, had 12.9% greater aspen biomass and 25.2% greater aspen PAI than the aspen component of mixed plots. The apparent decline in productivity of aspen in the mixed stands, however, could not be related to the variation in spruce abundance in these mixed stands. Key words: mixed wood management, understory, spruce boreal mixed wood

Productivity of aspen stands with and without a spruce understory in Alberta's boreal mixedwood forests

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 Alb...

The influence of partial harvesting and forest floor disturbance on nutrient availability and understory vegetation in boreal mixedwoods

In recent years boreal forests have been harvested to retain a portion of the original canopy, thereby providing forest structure, mostly for biodiversity reasons. Boreal mixedwood cutovers were surveyed at one and five years after harvesting with approximately 10% structural retention, to quantify the mean annual mortality rates of the residual trembling aspen, balsam poplar, paper birch and white spruce trees. For comparison, “natural” mortality rates by species were estimated from permanent sample plots in stands of similar composition. Species ranking of the annual mortality rates of residuals in areas harvested with structural retention were: poplar (10.2%) > birch (8.7%) > aspen (6.1%) > spruce (2.9%). Annual mortality rates were 2.5 to 4 times greater than in the reference stands. The majority of broadleaved species died as snags (~70%‐90%), while most spruce died due to windthrow (80%). Mortality rates increased with slenderness coefficient for codominant and understory poplar and for understory birch. For aspen, codominants were most likely to die, while in spruce, dominant trees and trees with the greatest damage to the bole from harvesting operations had the highest mortality.

/pdf/elevated-mortality-of-residual-trees-following-structural-58iric364l.pdf

Peter V. Blenis

Papers

Microsite effects on first-year establishment and overwinter survival of white spruce in aspen-dominated boreal mixedwoods

Productivity of aspen stands with and without a spruce understory in Alberta's boreal mixedwood forests

The influence of partial harvesting and forest floor disturbance on nutrient availability and understory vegetation in boreal mixedwoods

Elevated mortality of residual trees following structural retention harvesting in boreal mixedwoods

Cattle habitat selection and associated habitat characteristics under free-range grazing within heterogeneous Montane rangelands of Alberta