Tree rings provide an invaluable long‐term record for understanding how climate and other drivers shape tree growth and forest productivity. However, conventional tree‐ring analysis methods were not designed to simultaneously test effects of climate, tree size, and other drivers on individual growth. This has limited the potential to test ecologically relevant hypotheses on tree growth sensitivity to environmental drivers and their interactions with tree size. Here, we develop and apply a new method to simultaneously model nonlinear effects of primary climate drivers, reconstructed tree diameter at breast height (DBH), and calendar year in generalized least squares models that account for the temporal autocorrelation inherent to each individual tree's growth. We analyze data from 3811 trees representing 40 species at 10 globally distributed sites, showing that precipitation, temperature, DBH, and calendar year have additively, and often interactively, influenced annual growth over the past 120 years. Growth responses were predominantly positive to precipitation (usually over ≥3‐month seasonal windows) and negative to temperature (usually maximum temperature, over ≤3‐month seasonal windows), with concave‐down responses in 63% of relationships. Climate sensitivity commonly varied with DBH (45% of cases tested), with larger trees usually more sensitive. Trends in ring width at small DBH were linked to the light environment under which trees established, but basal area or biomass increments consistently reached maxima at intermediate DBH. Accounting for climate and DBH, growth rate declined over time for 92% of species in secondary or disturbed stands, whereas growth trends were mixed in older forests. These trends were largely attributable to stand dynamics as cohorts and stands age, which remain challenging to disentangle from global change drivers. By providing a parsimonious approach for characterizing multiple interacting drivers of tree growth, our method reveals a more complete picture of the factors influencing growth than has previously been possible.

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Background The epidemic Dendroctonus rufipennis (spruce beetle) outbreak in the subalpine forests of the Colorado Plateau in the 1990s killed most larger Picea engelmannii (Engelmann spruce) trees. One quarter century later, the larger snags are beginning to fall, transitioning to deadwood (down woody debris) where they may influence fire behavior, regeneration, and habitat structure. Methods We tracked all fallen trees ≥ 1 cm in diameter at breast height (1.37-m high) and mapped all pieces of deadwood ≥ 10-cm diameter and ≥ 1 m in length within 13.64 ha of a high-elevation mixed-species forest in the Picea–Abies zone annually for 5 years from 2015 through 2019. We examined the relative contribution of Picea engelmannii to snag and deadwood pools relative to other species and the relative contributions of large-diameter trees (≥ 33.2 cm at this subalpine site). We compared spatially explicit mapping of deadwood to traditional measures of surface fuels and introduce a new method for approximating vertical distribution of deadwood. Results In this mixed-species forest, there was relatively high density and basal area of live Picea engelmannii 20 years after the beetle outbreak (36 trees ha−1 and 1.94 m2 ha−1 ≥ 10-cm diameter) contrasting with the near total mortality of mature Picea in forests nearby. Wood from tree boles ≥ 10-cm diameter on the ground had biomass of 42 Mg ha−1, 7 Mg ha−1 of Picea engelmannii, and 35 Mg ha−1 of other species. Total live aboveground biomass was 119 Mg ha−1, while snag biomass was 36 Mg ha−1. Mean total fuel loading measured with planar transects was 63 Mg ha−1 but varied more than three orders of magnitude (0.1 to 257 Mg ha−1). Planar transects recorded 32 Mg ha−1 of wood ≥ 7.62-cm diameter compared to the 42 Mg ha−1 of wood ≥ 10-cm diameter recorded by explicit mapping. Multiple pieces of deadwood were often stacked, forming a vertical structure likely to contribute to active fire behavior. Conclusion Bark beetle mortality in the 1990s has made Picea an important local constituent of deadwood at 20-m scales, but other species dominate total deadwood due to slow decomposition rates and the multi-centennial intervals between fires. Explicit measurements of deadwood and surface fuels improve ecological insights into biomass heterogeneity and potential fire behavior.

Variation in tree recruitment, mortality, and growth can alter forest community composition and structure. Because tree recruitment and mortality events are generally infrequent, long‐time scales are needed to confirm trends in forests. We performed a 50‐yr demographic census of a forest plot located on the southern edge of the Canadian boreal forest, a region currently experiencing forest die‐back in response to direct and indirect effects of recent severe droughts. Here, we show that over the last 30 yr biomass, basal area, growth, and recruitment have decreased along with a precipitous rise in mortality across the dominant tree species. The stand experienced periods of drought in combination with multiple outbreaks of forest tent caterpillar (Malacosoma disstria) and bark beetles. These insect disturbances interacted to increase mortality rates within the stand and decrease stand density. The interaction of endogenous and exogenous factors may shift forests in this region onto novel successional trajectories with the possibility of changes in regional vegetation type.