The impacts of climatic changes on forests may appear gradually on time scales of years to centuries due to the long generation times of trees. Consequently, current forest extent may not reflect current climatic patterns. In contrast with these lagged responses, abrupt transitions in forests under climate change may occur in environments where alternative vegetation states are influenced by disturbances, such as fire. The Klamath forest landscape (northern California and southwest Oregon, USA) is currently dominated by high biomass, biodiverse temperate coniferous forests, but climate change could disrupt the mechanisms promoting forest stability (e.g. growth, regeneration and fire tolerance). Using a landscape simulation model, we estimate that about one-third of the Klamath forest landscape (500,000 ha) could transition from conifer-dominated forest to shrub/hardwood chaparral, triggered by increased fire activity coupled with lower post-fire conifer establishment. Such shifts were widespread under the warmer climate change scenarios (RCP 8.5) but were surprisingly prevalent under the climate of 1949–2010, reflecting the joint influences of recent warming trends and the legacy of fire suppression that may have enhanced conifer dominance. Our results demonstrate that major forest ecosystem shifts should be expected when climate change disrupts key stabilizing feedbacks that maintain the dominance of long-lived, slowly regenerating trees.

As trees are long-lived organisms, the impacts of climate change on forest communities may not be apparent on the time scale of years to decades. While lagged responses to environmental change are common in forested systems, potential for abrupt transitions under climate change may occur in environments where alternative vegetation states are influenced by disturbances, such as fire. The Klamath mountains (northern California and southwest Oregon, USA) are currently dominated by carbon rich and hyper-diverse temperate conifer forests, but climate change could disrupt the mechanisms promoting forest stability– regeneration and fire tolerance— via shifts in the fire regime in conjunction with lower fitness of conifers under a hotter climate. Understanding how this landscape will respond to near-term climate change (before 2100) is critical for predicting potential climate change feedbacks and to developing sound forest conservation and management plans. Using a landscape simulation model, we estimate that 1/3 of the Klamath could transition from conifer forest to shrub/hardwood chaparral, triggered by an enhanced fire activity coupled with lower post-fire conifer establishment. Such shifts were more prevalent under higher climate change forcing (RCP 8.5) but were also simulated under the climate of 1950-2000, reflecting the joint influences of early warming trends and historical forest legacies. Our results demonstrate that there is a large potential for loss of conifer forest dominance—and associated carbon stocks and biodiversity- in the Klamath before the end of the century, and that some losses would likely occur even without the influence of climate change. Thus, in the Klamath and other forested landscapes subject to similar feedback dynamics, major ecosystem shifts should be expected when climate change disrupts key stabilizing feedbacks that maintain the dominance of long-lived, slowly regenerating trees.

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Climate change will accelerate forest mortality due to insects, disease, and wildfire. As a result, substantial resources will be necessary where and when forest managers seek to maintain multiple management objectives. Because of the increasing managerial requirements to offset climate change and related disturbances, the uncertainty about future forest conditions is magnified relative to climate change alone. We provide an analytical approach that quantifies the key drivers of forest change-climate, disturbance, and forest management-using scenarios paired with simulation modeling to forecast and quantify uncertainties in the Lake Tahoe Basin of California and Nevada (USA), a montane seasonally dry conifer forest. We partitioned uncertainty among climate change (including associated changes to wildfire and insect outbreaks), forest management (including thinning, prescribed fire, and fire suppression), and other sources using a fully factorial experimental design and analysis of variance. We focused on three metrics that are important for forest management objectives for the area: forest carbon storage, area burned at high severity, and total area burned by wildfire. Management explained a substantial amount of variance in the short term for area burned at high severity and longer term carbon storage, while climate explained the most variance in total area burned. Our results suggest that simulated extensive management activities will not meet all the desired management objectives. Both the extent and intensity of forest management will need to increase significantly to keep pace with predicted climate and wildfire conditions.

Because of past land use changes and changing climate, forests are moving outside of their historical range of variation. As fires become more severe, forest managers are searching for strategies that can restore forest health and reduce fire risk. However, management activities are only one part of a suite of disturbance vectors that shape forest conditions. To account for the range of disturbance intensities and disturbance types (wildfire, bark beetles, and management), we developed a disturbance return interval (DRI) that represents the average return period for any disturbance, human or natural. We applied the DRI to examine forest change in the Lake Tahoe Basin of California and Nevada. We specifically investigated the consequences of DRI on the proportion of highseverity fire and the net sequestration of carbon. In order to test the management component of the DRI, we developed management scenarios with forest managers and stakeholders in the region; these scenarios were integrated into a mechanistic forest landscape model that also accounted for climate change, as well as natural disturbances of wildfire and insect outbreaks. Our results suggest increasing the frequency of disturbances (a lower DRI) would reduce the percentage of high-severity fire on landscape but not the total amount of wildfire in general. However, a higher DRI reduced carbon storage and sequestration, particularly in management strategies that emphasized prescribed fire over hand or mechanical fuel treatments.