This study examines influences of climate variability on spruce beetle (Dendroctonus rufipennis) outbreak across northwestern Colorado during the period 1650 2011 CE. Periods of broad-scale outbreak reconstructed using documentary records and tree rings were dated to 1843-1860, 1882-1889, 1931-1957, and 2004-2010. Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index (PDSI), and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Classification trees showed that outbreaks can be predicted most successfully from above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Notably, we find that spruce beetle outbreaks appear to be predicted best by interannual to multidecadal variability in drought, not by temperature alone. This finding may imply that spruce beetle outbreaks are triggered by decreases in host tree defenses, which are hypothesized to occur with drought stress. Given the persistence of the AMO, the shift to a positive AMO phase in the late 1990s is likely to promote continued spruce beetle disturbance.

Question As the extent, magnitude and/or frequency of various forest disturbances are increasing due to climate change, it is becoming increasingly likely that forests may be affected by more than one type of disturbance in short succession. We studied how compounded disturbances and pre‐fire composition influence post‐fire tree regeneration. Specifically, do compounded disturbances reduce overall regeneration and favour initial dominance of species that regenerate vegetatively? Location Sub‐alpine forests in northwestern Colorado. Methods The study region was affected by a severe outbreak of Dendroctonus rufipennis in the 1940s, a severe wind storm in 1997 and severe fires in 2002. Permanent plots to monitor regeneration were established in 2003 and were re‐measured in 2004, 2005 and 2010. Plots were located in stands that varied in long‐term disturbance history (stands that originated following fires in 1879 or 1880 vs older stands), recent disturbance history (fire only; outbreak then fire; blowdown then fire) and pre‐fire forest dominance (Populus tremuloides, Pinus contorta, or Picea engelmannii–Abies lasiocarpa). Results Combined density of regeneration of all tree species was highest in stands dominated by P. tremuloides prior to the 2002 fires. In P. contorta stands that were affected only by the 2002 fires, regeneration density was higher in stands that were younger prior to the fire (those that originated in the 1880s), in which cone serotiny is more prevalent, than in older stands (those that originated >200 yr ago). However, the advantage of relatively young P. contorta stands to regenerate following fire was inhibited by compounded disturbances of wind and then fire. Similarly, following compounded disturbances the combined density of conifer seedlings of all species was lower than following only fire. In contrast, the density of P. tremuloides was not lower following compounded disturbances than following fire only, and was higher than that of other species. Conclusions Pre‐fire forest composition and disturbance history influence the abundance and composition of post‐fire regeneration. Compounded disturbances generally reduce the regeneration of conifers, which regenerate exclusively from seed, and appear to favour initial stand dominance by P. tremuloides, which regenerates both sexually and asexually. Such differential effects may thereby alter trajectories of post‐fire regeneration. As P. tremuloides are less susceptible than conifers to fires, bark beetle outbreaks and wind disturbances, increased dominance by P. tremuloides may contribute to a negative feedback that may diminish the probability and/or severity of future disturbances and thus increase overall forest ecosystem resiliency.

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Aim As climate change is increasing the frequency, severity and extent of wildfire and bark beetle outbreaks, it is important to understand how these disturbances interact to affect ecological patterns and processes, including susceptibility to subsequent disturbances. Stand-replacing fires and outbreaks of mountain pine beetle (MPB), Dendroctonus ponderosae, are both important disturbances in the lodgepole pine, Pinus contorta, forests of the Rocky Mountains. In the current study we investigated how time since the last standreplacing fire affects the susceptibility of the stand to MPB outbreaks in these forests. We hypothesized that at a stand-scale, young post-fire stands (< c. 100-150 years old) are less susceptible to past and current MPB outbreaks than are older stands.Location Colorado, USA.Methods We used dendroecological methods to reconstruct stand-origin dates and the history of outbreaks in 23 lodgepole pine stands. ResultsThe relatively narrow range of establishment dates among the oldest trees in most sampled stands suggested that these stands originated after stand-replacing or partially stand-replacing fires over the past three centuries. Stands were affected by MPB outbreaks in the 1940s/1950s, 1980s and 2000s/ 2010s. Susceptibility to outbreaks generally increased with stand age (i.e. time since the last stand-replacing fire). However, this reduced susceptibility of younger post-fire stands was most pronounced for the 1940s/1950s outbreak, less so for the 1980s outbreak, and did not hold true for the 2000s/2010s outbreak.Main conclusions Younger stands may not have been less susceptible to the most recent outbreak because: (1) after stands reach a threshold age of > 100-150 years, stand age does not affect susceptibility to outbreaks, or (2) the high intensity of the most recent outbreak reduces the importance of pre-disturbance conditions for susceptibility to disturbance. If the warm and dry conditions that contribute to MPB outbreaks concurrently increase the frequency and/or extent of severe fires, they may thereby mitigate the otherwise increased landscape-scale susceptibility to outbreaks. Potential increases in severe fires driven by warm and dry climatic trends may lead to a negative feedback by making lodgepole pine stands less susceptible to future MPB outbreaks.

Abstract. The ability to estimate and model future vegetation dynamics is a central focus of contemporary ecology and is essential for understanding future ecological trajectories. It is therefore critical to understand when the influence of initial post-disturbance regeneration versus stochastic processes dominates long-term post-disturbance ecological processes. Often, conclusions about post-disturbance dynamics are based upon initial regeneration in the years immediately after disturbances. However, the degree to which initial post-disturbance regeneration indicates longer-term trends is likely to be contingent on the types, intensities, and combinations of disturbances, as well as pre-disturbance ecosystem structure and composition. Our relatively limited understanding of why initial post-disturbance regeneration is sometimes a poor predictor of future ecosystem trajectories represents a critical gap in post-disturbance ecological forecasting. We studied the composition and density of regeneration of tree species following wind blowdown in 1997, wildfire in 2002, and compounded disturbances by blowdown and wildfire in subalpine forests of Colorado. We examined regeneration of Picea engelmannii, Abies lasiocarpa, Pinus contorta, and Populus tremuloides in 180 permanent plots across 12 sites (classified by pre-disturbance age and composition) in 2003, 2010, and 2015. At sites that were blown down but not burned, regeneration was dense and dominated by Picea and Abies. At these sites, regeneration observed from 2003 to 2005 (hereafter initial regeneration) was also highly predictive of regeneration 5-10 yr later. In contrast, at sites that were burned and sites that were blown down and burned, regeneration was less dense and dominated by a mix of species. At these sites, initial regeneration was a poor predictor of longer-term trends as species dominance and overall density fluctuated over the 13-yr period. These findings call into question our ability to confidently predict ecosystem trajectories based upon observations made in the years immediately after large, severe disturbances such as wildfires and compounded disturbances. As compounded disturbances become more common under climatically driven changes in disturbance regimes, post-disturbance ecosystem trajectories may become increasingly stochastic and unpredictable.