Summary: Dothistroma needle blight (DNB), caused by the two fungiDothistroma septosporumandD. pini,is a major disease of pines with a worldwide distribution. Increases in the incidence and severity of disease in areas where the disease has long been established and notable range expansions have both recently been observed. The aim of this review was to assess the relationship betweenDNB, weather factors and climate to better understand possible underlying causes of this recent intensification in disease. A substantial body of literature shows that the life cycles of the fungi are closely related to weather factors such as precipitation and temperature. Given the rapid response ofDNBto favourable weather conditions, it seems plausible that changes in disease behaviour could be due to changes in climate. The recurrent El Niño‐Southern oscillation (ENSO) phenomenon influences patterns of temperature and precipitation in many regions of the world, often resulting in warmer and wetter conditions than normal. We found that since the 1950s, four of the past five strong El Niño events appear to have coincided with reports of increasedDNBactivity on an intercontinental scale. The lack of long‐term standardized data records limits our ability to fully interpret this relationship, but the projected future climatic conditions in the Northern Hemisphere appear to be increasingly favourable for the disease. Still, other areas of the world may become less favourable, and further research is required to be able to accurately predictDNBoutbreaks and their impact on pine forests in the future.

Over 14 million hectares of lodgepole pine-dominated forests in British Columbia (BC), Canada, have been severely impacted by the current mountain pine beetle epidemic. Simultaneously, a Dothistroma needle blight epidemic in northwest BC has been responsible for killing thousands of hectares of pine plantations and has even resulted in the death of mature trees, which is unprecedented. Both of these globally significant forest pest epidemics have been linked to climate change. The beetle epidemic has grossly exceeded the scale of all previously recorded outbreaks, in large part due to a lack of cold winters. In this sense, the link to global warming is relatively straightforward and foreseeable. Of all climate change projections, the prediction of an increase in winter temperatures has been associated with as high a degree of confidence as any. The Dothistroma needle blight epidemic and its link to climate change were not so predictable. Based on weather records over the past four decades, short-term increases in mean summer precipitation correlate closely with historical records of Dothistroma outbreaks in the northwest. The current most severe outbreak has occurred during a prolonged period of above-average summer precipitation. An increase in summer precipitation would more typically be thought of as beneficial for forests, but that increase in moisture has improved the conditions for a pathogen that has outweighed any benefits. Similar trends of increasing incidence and severity for other forest pathogens in BC appear in areas that have been receiving increased summer precipitation. Conversely, in the southern interior of the province decreased summer precipitation and increased drought conditions are resulting in stressed trees which can favour root diseases. Early indications are that climate change will have profound effects on forest health. Some effects will be more predictable than others.

We examined the performance of older even-aged plantations to check the validity of three of the most fundamental tenets of forest site productivity: the height–age site index, Eichhorn's rule and the thinning response hypothesis. We assessed the condition of >14 000 trees in 60 randomly selected plantations to determine whether the stands were following site productivity expectations and growth and yield projections. We evaluated the health status of all the trees by height class (4 m tall). We found strong evidence that older, managed plantations are subject to damage agents that are targeting dominant trees. We found natural ingress was not filling voids created by loss of planted trees. Our findings were clearly in conflict with the assumptions of low and stable levels of loss of dominant trees in aging plantations. The tendency of forest growth models to emphasize stability and predictability needs to be reconsidered. The assumptions driving the traditional forest growth models were developed largely in the absence of biotic and abiotic damage agents and certainly prior to the knowledge of climate change. The combined influence of these two drivers must be better accounted for in growth models through more intensive stand and forest level monitoring.

Many current tree improvement programs are incorporating assisted gene flow strategies to match reforestation efforts with future climates. This is the case for the lodgepole pine (Pinus contorta var. latifolia), the most extensively planted tree in western Canada. Knowledge of the structure and origin of pathogen populations associated with this tree would help improve the breeding effort. Recent outbreaks of the Dothistroma needle blight (DNB) pathogen Dothistroma septosporum on lodgepole pine in British Columbia and its discovery in Alberta plantations raised questions about the diversity and population structure of this pathogen in western Canada. Using genotyping-by-sequencing on 119 D. septosporum isolates from 16 natural pine populations and plantations from this area, we identified four genetic lineages, all distinct from the other DNB lineages from outside of North America. Modeling of the population history indicated that these lineages diverged between 31.4 and 7.2 thousand years ago, coinciding with the last glacial maximum and the postglacial recolonization of lodgepole pine in western North America. The lineage found in the Kispiox Valley from British Columbia, where an unprecedented DNB epidemic occurred in the 1990s, was close to demographic equilibrium and displayed a high level of haplotypic diversity. Two lineages found in Alberta and Prince George (British Columbia) showed departure from random mating and contemporary gene flow, likely resulting from pine breeding activities and material exchanges in these areas. The increased movement of planting material could have some major consequences by facilitating secondary contact between genetically isolated DNB lineages, possibly resulting in new epidemics.

We examine the direct effects of multiple disturbance agents on individual tree development and stand productivity in 15–40-year-old managed forests in British Columbia, Canada. Our primary interest was to establish a baseline assessment of damage in these forests and, especially, to focus on the degree to which biotic and abiotic stressors cause physical damage and diffuse mortality. Based on extensive climate data for the study area and the ecology of the disturbance agents we explore possible interactions between individual stressors and climate. Mean annual temperature increased by over 1 °C in the last century and annual precipitation increased by 8%, with that in the summer increasing by 18%. Disturbance agents were a central driver of mortality, growth and physical damage and their combined impact in lodgepole pine stands was as much as four times greater than expected particularly in the dominant trees most counted upon for stand productivity and timber supply. Climate-mediated disturbances accounted for five of the top six damage agent categories in terms of percent of basal area impacted but the lack of long-term disturbance monitoring data, a global information gap, limits our ability to conclusively link high damage rates to climatic changes.