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Reports of forest damage have increased with the frequency of climatic extremes, but longer term impacts of such events on population dynamics of forest trees are generally unknown. Incited by the turn-of-the-century drought, sudden aspen decline (SAD) damaged 535 000 ha of Populus tremuloides Michx. in the Southern Rockies ecoregion of western North America. Although spread of the disease stopped in about 2009, most of the affected stands continued to deteriorate. Remeasurement of plots in southwestern Colorado showed that, since the peak of the epidemic, live basal area in sick plots decreased by an additional 28% to only 38% of that in healthy plots. Sick plots had much more recent damage than healthy plots, with almost three times as much recently dead basal area, over twice the density of recently dead trees, and almost four times as much recent crown loss. The important contributing agents in SAD were still active in sick stands in 2013. Density of small regeneration showed opposite trends, increasing in healthy plots and decreasing in sick plots. Timely regeneration treatments may be needed in some such stands to facilitate recovery. In addition to acute damage from climatic extremes, long-term decline diseases like SAD will likely be a common signature of forest damage from climate change. Key words: decline disease, climate change, regeneration, recent crown loss. Les rapports concernant les dommages aux forets ont augmente avec la frequence des evenements climatiques extremes mais les impacts a long terme de tels evenements sur la dynamique de population des arbres forestiers sont generalement inconnus. Declenche par la secheresse survenue au tournant du siecle, le deperissement soudain du peuplier (DSP) a endommage 535 000 ha de Populus tremuloides Michx. dans l'ecoregion des Rocheuses du Sud, dans l'ouest de l'Amerique du Nord. Bien que la maladie ait cesse de progresser en 2009, les peuplements les plus touches ont continue a deperir. Un nouveau mesurage des placettes dans le sud-ouest du Colorado a montre que depuis le point culminant de l'epidemie, la surface terriere vivante a connu une diminution additionnelle de 28 % dans les placettes malades, pour atteindre aussi peu que 38 % de la surface terriere vivante dans les placettes en bonne sante. Il y avait beaucoup plus de dommages recents dans les placettes malades que dans les placettes en bonne sante: presque trois fois plus de surface terriere morte recemment, plus de deux fois la densite des arbres morts recemment et presque quatre fois plus de perte de cime recente. Les agents qui ont contribue de facon importante au DSP etaient toujours actifs dans les peuplements malades en 2013. La densite de la petite regeneration suivait des tendances opposees: allant en augmentant dans les placettes en bonne sante et en diminuant dans les placettes malades. Des traitements de regeneration au moment opportun pourraient etre necessaires dans certains de ces peuplements pour faciliter la recuperation. En plus des dommages severes causes par les evenements climatiques extremes, les deperissements a long terme tels que le DSP deviendront probablement un type commun de dommages aux forets dus au changement climatique. [Traduit par la Redaction] Mots-cles: deperissement, changement climatique, regeneration, perte de cime recente., Introduction Partly as a result of recent climatic extremes, reports of tree mortality are increasing around the world (Allen et al. 2010). These include subtle increases in rates of 'background' [...]

: Reports of drought-associated forest mortality have increased around the world, but the mechanisms of mortality are rarely direct in nature. Biotic agents may kill trees that could otherwise recover and can perpetuate and expand mortality after the stress is relieved. Sudden aspen decline (SAD) has caused rapid, widespread branch dieback and mortality of quaking aspen (Populous tremuloides Michx.). We compared insects and diseases in 162 damaged and neighboring healthy plots to determine contributing factors and their ecological roles. Cytospora canker, bronze poplar borer, and aspen bark beetles were the most common agents in damaged plots and correlated with crown loss and other factors related to SAD. This was the first documented outbreak of Trypophloeus populi, an aspen bark beetle. As bark beetles and bronze poplar borer increased in damaged stands, they tended to attack trees with healthier crowns. Environmental stress may have directly affected the success of these agents by increasing host susceptibility followed by a density-dependent increase in the insects' invasive ability. In contrast, Cytospora canker had an identical relationship to crown loss in healthy versus damaged plots, suggesting that it was not limited by inoculum but responded to host susceptibility. Most other pathogens and insects contributed little to SAD and appear to be primary or weakening agents. The biotic agents of mortality in a decline differ greatly from primary agents and play complex and varied roles in healthy versus declining stands. Resume: Les rapports de mortalite des forets associee a la secheresse ont augmente partout dans le monde mais les mecanismes responsible de la mortalite sent garment de nature directed. Des agents biotiques peuvent tuer des arbres qui pourraient par ailleurs recuperer et peuvent perpetuer et augmenter la mortalite apres que le stress soit disparu. Le deperissement soudain du tremble (DST) est responsable de la mort en cime et de la mortalite rapide et generalisee du peuplier fauxtremble (Populus tremuloides Michx.). Nous avons compare les maladies et les insectes presents dans 162 placettes touchees par le deperissement appariees a des placettes saines avoisinantes pour determiner les facteurs contribuants et leurs roles ecologiques. Le chancre cytosporeen, l'agrile du peuplier et les scolytes du tremble etaient les agents les plus frequents dans les placettes touchees par le deperissement et ils etaient correles avec la mort en cime et d'autres facteurs relies au DST. Il s'agissait de la premiere epidemie documentee de Trypophloeus populi, un scolyte du tremble. A measure que les populations de scolytes du tremble et d'agrile du peuplier augmentaient dans les peuplements touches par le deperissement, ces insectes avaient tendance a s'attaquer a des arbres dont la cime etait plus saine. Un stress environnemental pourrait avoir directement influence le succes de ces agents en provoquant une augmentation de la sensibilite de l'hote, suivie d'une augmentation de la capacite invasive des insectes qui serait dependante de la densite. Au contraire, le chancre cytosporeen avait la meme relation avec la mort en cime que les placettes soient touchees ou non par le deperissement, ce qui indique que son incidence ne depend pas de la quantite d'inoculum mais plutot de la sensibilite de l'hote. La plupart des autres pathogenes et insectes contribuaient peu au DST et semblent etre des agents primaires ou qui affaiblissent leur hote. Les causes biotiques de mortalite dans le cas du deperissement different grandement des agents primaires et jouent des roles complexes et varies dans les peuplements sains comparativement aux peuplements qui deperissent. [Traduit par la Redaction], Introduction Increasing temperatures worldwide and more frequent and severe droughts projected in many regions in the coming decades (Core Writing Team 2008) will increase moisture stress in forests, especially during [...]

1 Data from elevations ranging from mixed hardwood-conifer forest at 600 m to subalpine Abies balsamea forest at 1120 m indicate that canopy gaps are not static but expand over time due to mortality of trees at the gap margin and coalescence of gaps. Gap expansion is more frequent than gap initiation. Ultimately such disturbance patches may become more extensive than is normally considered as typical of gap-phase disturbance, but the processes of development are the same. 2 Disturbance agents involved in gap initiation tend to differ from those involved in gap expansion. Spruce beetle, dwarf mistletoe and most root diseases predominate as agents of gap initiation, while windthrow/windsnap, chronic wind stress and Armillaria root disease are important agents of gap expansion. 3 Concepts of equilibrium gap-phase dynamics and a shifting-mosaic steady state do not fully account for the dynamics of these spruce-fir forests. A spruce beetle outbreak in the late 1970s/early 1980s killed most of a cohort of dominant, emergent Picea rubens. This epidemic initiated a long-term cycle of disturbance and release that is likely to be repeated in many years when a new cohort of P. rubens becomes sufficiently large to sustain another major bark beetle outbreak. 4 Episodic disturbance agents affect long-lived, dominant species at infrequent but regular intervals (up to hundreds of years) and operate at the landscape level. Gap-phase cycles appear to be nested within the long-term cycle. Over the long term, episodic disturbance drives such 'nested bicycle' dynamics. 5 Spatial and temporal distribution of disturbance results not only from stochastic events such as storms, but also from host specificity of agents of disturbance, their tendency to attack certain age classes of trees, local and regional contagion, and susceptibility of trees at the edge of disturbance patches.

1 Data from elevations ranging from mixed hardwood-conifer forest at 600 m to subalpineAbies balsameaforest at 1120 m indicate that canopy gaps are not static but expand over time due to mortality of trees at the gap margin and coalescence of gaps. Gap expansion is more frequent than gap initiation. Ultimately such disturbance patches may become more extensive than is normally considered as typical of gap-phase disturbance, but the processes of development are the same.2 Disturbance agents involved in gap initiation tend to differ from those involved in gap expansion. Spruce beetle, dwarf mistletoe and most root diseases predominate as agents of gap initiation, while windthrow/windsnap, chronic wind stress and Armillaria root disease are important agents of gap expansion.3 Concepts of equilibrium gap-phase dynamics and a shifting-mosaic steady state do not fully account for the dynamics of these spruce-fir forests. A spruce beetle outbreak in the late 1970s/early 1980s killed most of a cohort of dominant, emergentPicea rubens. This epidemic initiated a long-term cycle of disturbance and release that is likely to be repeated in many years when a new cohort ofP. rubensbecomes sufficiently large to sustain another major bark beetle outbreak.4 Episodic disturbance agents affect long-lived, dominant species at infrequent but regular intervals (up to hundreds of years) and operate at the landscape level. Gap-phase cycles appear to be nested within the long-term cycle. Over the long term, episodic disturbance drives such‘nested bicycle’ dynamics.5 Spatial and temporal distribution of disturbance results not only from stochastic events such as storms, but also from host specificity of agents of disturbance, their tendency to attack certain age classes of trees, local and regional contagion, and susceptibility of trees at the edge of disturbance patches.Journal of Ecology(2004) 10.1111/j.1365-2745.2004.00937.x [ABSTRACT FROM AUTHOR]

Collections of Heterobasidion spp. from Nebraska, Colorado, Arizona, and New Mexico were identified based on the sequence of the internal transcribed spacer region of the ribosomal DNA. The North American variant of Heterobasidion annosum sensu stricto was found on Pinus ponderosa and Juniperus virginiana in central Nebraska, southern Colorado, central Arizona, and southern New Mexico. The North American variant of H. parviporum was found on Abies concolor and Picea engelmannii in southern Colorado and central New Mexico. The pathogens were not found in a survey of conifer forests in Wyoming and the Black Hills of South Dakota. Historical records of annosus root disease are reviewed by host group to gain more insight into the potential distributions of the respective pathogens. An apparent lack of overlap in host range suggests that substitution of tree species may be a useful management approach in some cases.