Which trees die during drought? The key role of insect host‐tree selection

Abstract

During drought, the tree subpopulations (such as size or vigour classes) that suffer disproportionate mortality can be conceptually arrayed along a continuum defined by the actions of biotic agents, particularly insects. At one extreme, stress dominates: insects are absent or simply kill the most physiologically stressed trees. At the opposite extreme, host selection dominates: outbreaking insects kill trees independently of their stress, instead selecting trees based on size or other traits. Intermediate responses are also possible. Yet for mixed‐species forests, we lack a broad understanding of the relative importance of insects in determining exactly which subpopulations of trees suffer disproportionate mortality during drought, and whether these subpopulations differ among co‐occurring tree species. During an extreme drought, we documented the roles of native bark beetles in the mortality of five tree species in California’s Sierra Nevada. We analysed the patterns and agents of tree mortality in 12 permanent plots and the patterns of mortality in 89 temporary plots. Most tree mortality was associated with bark beetles. However, the growth rates (an indicator of chronic stress) and sizes of trees that suffered greatest bark beetle‐related mortality differed sharply among tree taxa, variously conforming with domination by stress (Abies concolor), domination by host selection (Pinus lambertiana and P. ponderosa) or a mix of the two (Calocedrus decurrens). Quercus kelloggii mortality remained relatively low. Thus, even during extreme drought substantial proportions of stressed trees survived because they were of sizes that mostly avoided fatal insect attack. Conversely, substantial proportions of comparatively unstressed trees died because they were of sizes that were selectively killed by outbreaking insects. Synthesis. Native bark beetles were primarily responsible for determining which subpopulations of trees suffered greatest mortality during drought. However, idiosyncratic host‐tree selection by the different bark beetle taxa meant that the tree subpopulations suffering greatest mortality differed strikingly among tree taxa—for example, high mortality of small trees of one species, but of large trees of another. If idiosyncratic host‐tree selection by biotic mortality agents proves to be a generally common phenomenon, it could help explain weak broadscale correlations between tree traits and tree mortality during drought.

Wildfire and drought moderate the spatial elements of tree mortality

“…Optimal host selection provides an additional explanation for this pattern of strong CNDD (i.e., mortality risk increases with more conspecifics): Beetle populations proliferated 2–3 yr post‐fire (when drought intensity peaked), and this may have enabled beetles to selectively attack larger and more vigorous host trees (Fig. 10; Boone et al 2011, Stephenson et al 2019). As conspecific BA is tightly correlated with the abundance of large Pinus , neighborhoods with high conspecific BA would have been preferentially selected by dispersing beetles (Safranyik and Carroll 2006, Barbosa et al 2009) and this would increase post‐fire mortality risk.…”

“…While density dependence has been used to describe patterns of both distance‐ and density‐dependent mortality associated with competition and biotic mortality agents (e.g., insects, pathogens), we define these terms separately to decouple the distinct relationships between each mechanism and forest spatial structure. Forest spatial structure regulates density‐dependent mortality directly through resource competition (Kenkel 1988), morphological constrains (King 1986), accumulation of host‐specific plant enemies (Janzen 1970, Connell 1971), and by moderating trees' ability to invest in defense mechanisms (Lorio 1986, Herms and Mattson 1992, Kolb et al 1998, Fettig et al 2007, Hood et al 2016, Stephenson et al 2019). Conversely, forest spatial structure does not directly regulate distance‐dependent mortality.…”

Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?

“…Trees can die during drought through water stress alone or through a combination of water stress and infestation of drought-associated beetle pests (Stephenson et al 2019). The long-term study at Teakettle Experimental Forest and California's historic 2012-2016 drought provided an opportunity to test these two inter-related pathways by manipulating stand densities and compositions through mechanical thinning and prescribed burning.…”

Linking drought legacy effects across scales: From leaves to tree rings to ecosystems

“…MMSF is a fairly mesic forest and tree-ring increments are not typically highly sensitive to water availability (Maxwell, Harley, & Robeson, 2016). In more xeric forests, the responses of understory versus canopy dominant trees may diverge to a greater extent due to (a) larger trees being more affected by drought (Bennett et al, 2015;Nepstad et al, 2007; though see Colangelo et al, 2017;Stephenson, Das, Ampersee, Bulaon, & Yee, 2019 for recent counterexamples); or (b) smaller trees having less access to soil water and smaller NSC stores (Sala et al, 2012). The site and context specificity of tree-ring sampling biases would be a fruitful avenue of research and assist future efforts at collecting ecologically relevant tree-ring chronologies.…”