Drought Impacts and Compounding Mortality on Forest Trees in the Southern Sierra Nevada

Abstract

The increase in compounding disturbances, such as “hotter droughts” coupled with insect outbreaks, has significant impacts on the integrity of forested ecosystems and their subsequent management for important ecosystem services and multiple-use objectives. In the Southern Sierra Nevada, years of severe drought have resulted in unprecedented tree mortality across this mountainous landscape. Additionally, past land management practices, including fire suppression, have led to overly stocked, homogenous forest stand structures, dominated by small diameter, shade-tolerant and fire-intolerant tree species. Thus, the current condition of the landscape has further increased the susceptibility of forest trees to multiple stressors. We sought to determine the effects of extreme drought and insect outbreaks on tree mortality and their influence on forest stand structure and composition. To characterize mortality patterns, we monitored the condition of mature forest trees (>25.4 cm diameter at breast height) across 255 monitoring plots with four repeated measurements from 2015 through 2017. Tree mortality varied by species and through time. Reductions in pine species (Pinus lambertiana Douglas and P. ponderosa Lawson & C. Lawson) occurred earlier in the study period than Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr. or Calocedrus decurrens (Torr.) Florin. Across species, larger tree size, most often associated with tree height, was consistently related to increased survival in mature, overstory trees. As expected, sites with greater pine stocking and subsequently more bark beetle (Curculionidae: Scolytinae) host availability had increased pine mortality, especially for P. ponderosa. For Abies concolor, lower overstory basal area increased tree survival for this species. This study highlights the importance of effective forest monitoring, especially during a period of unprecedented ecological change as the compounding disturbance had a disproportional effect on pine species in smaller diameter classes. Proactive forest management may be necessary to maintain and promote these ecologically important species in heterogeneous mixtures across the landscape.

Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality

“…Notably, a similar pattern was shown by Stovall et al 65 in a study confined to the southern Sierra Nevada (i.e., the hottest, driest portion of the more spatially extensive results we present here) with a strong positive tree height/mortality relationship in areas with the greatest vapor pressure deficit and no tree height/mortality relationship in areas with the lowest vapor pressure deficit. Our work suggests that the WPB was cueing into different aspects of forest structure across an environmental gradient in a spatial context in a parallel manner to the temporal context noted by Stovall et al 65 and Pile et al 70 , who observed that mortality was increasingly driven by larger trees as the hot drought proceeded and became more severe. A temporal signal of bark beetles attacking larger and larger host trees reflects the positive feedback between forest structure and bark beetle population dynamics as the population phase cycles from endemic to epidemic 13 .…”

“…The positive main effect of host tree mean size on ponderosa mortality rates tracks the conventional wisdom on the dynamics of WPB in the Sierra Nevada, as well as other primary bark beetles 18 . WPB exhibits a preference for trees 50.8-76.2 cm DBH 68,69 , and a positive relationship between host tree size and levels of tree mortality attributed to WPB was reported by Fettig et al 14 in the coincident field plots as well as in other recent studies 9,15,70 . Larger trees are more nutritious and are therefore ideal targets if local bark beetle density is high enough to successfully initiate mass attack and overwhelm tree defenses, as can occur when many trees are under water stress 7,13,24 .…”

Time‐Varying Sensitivity Analysis Reveals Relationships Between Watershed Climate and Variations in Annual Parameter Importance in Regions With Strong Interannual Variability

“…(2018) found that increased evaporation rates due to warmer temperatures (1°C warmer than the previous decade) intensified the drought impact on mountain hydrology, reducing the fractional allocation of precipitation to runoff by 30%. These rising temperatures also amplified drought‐induced stress on watershed ecology as tree mortality across drought‐stricken landscapes was recorded at multiple studies (Goulden et al., 2019; Guardiola‐Claramonte et al., 2011; Pile et al., 2019; Young et al., 2017). This is noteworthy, since increasing temperatures also increase plant water demand while soil moisture decreases due to reduced precipitation levels.…”

Tree mortality in western U.S. forests forecasted using forest inventory and Random Forest classification

“…We assessed mortality rates for some wellstudied tree species and many other tree species that, to our knowledge, do not have any published mortality rates. The most common focal conifer species in recent mortality work have been conifers in the genera Abies, Pinus, and Juniperus (Gaylord et al 2013, Krofcheck et al 2014, Mortenson et al 2015, Fettig et al 2019, Flake and Weisberg 2019, Pile et al 2019, McDowell et al 2019. We improve on previous (Table 3 v www.esajournals.org mortality research for these species by including populations over wide areas, as well as including species closely related to common focal species that are less well studied.…”