Why do trees die? Characterizing the drivers of background tree mortality

Abstract

The drivers of background tree mortality rates-the typical low rates of tree mortality found in forests in the absence of acute stresses like drought-are central to our understanding of forest dynamics, the effects of ongoing environmental changes on forests, and the causes and consequences of geographical gradients in the nature and strength of biotic interactions. To shed light on factors contributing to background tree mortality, we analyzed detailed pathological data from 200,668 tree-years of observation and 3,729 individual tree deaths, recorded over a 13-yr period in a network of old-growth forest plots in California's Sierra Nevada mountain range. We found that: (1) Biotic mortality factors (mostly insects and pathogens) dominated (58%), particularly in larger trees (86%). Bark beetles were the most prevalent (40%), even though there were no outbreaks during the study period; in contrast, the contribution of defoliators was negligible. (2) Relative occurrences of broad classes of mortality factors (biotic, 58%; suppression, 51%; and mechanical, 25%) are similar among tree taxa, but may vary with tree size and growth rate. (3) We found little evidence of distinct groups of mortality factors that predictably occur together on trees. Our results have at least three sets of implications. First, rather than being driven by abiotic factors such as lightning or windstorms, the "ambient" or "random" background mortality that many forest models presume to be independent of tree growth rate is instead dominated by biotic agents of tree mortality, with potentially critical implications for forecasting future mortality. Mechanistic models of background mortality, even for healthy, rapidly growing trees, must therefore include the insects and pathogens that kill trees. Second, the biotic agents of tree mortality, instead of occurring in a few predictable combinations, may generally act opportunistically and with a relatively large degree of independence from one another. Finally, beyond the current emphasis on folivory and leaf defenses, studies of broad-scale gradients in the nature and strength of biotic interactions should also include biotic attacks on, and defenses of, tree stems and roots.

A synthesis of radial growth patterns preceding tree mortality

“…This group especially included trees that died because of high competition intensity, confirming that shading can suppress trees for a long period before they actually die (Abrams & Orwig, ). However, the effects of shading (and competition in general) and other stress factors frequently interact (Myers & Kitajima, ; Das et al ., ) and are difficult to disentangle in field settings.…”

Near‐future forest vulnerability to drought and fire varies across the western United States

“…According to this algorithm, a tree will experience drought stress when the soil moisture drops below the water potential that would cause a 50% decline in hydraulic conductivity (Supporting information Table S2) for a period of 30 consecutive days. Leaf shed rate and drought stress duration parameters (Supporting information Figure S1) were selected, based on observations (Kelly, 2016) and comparisons of modeled and plot-level measurements of aboveground carbon in the Sierra Nevada Mountains, CA (Das, Stephenson, & Davis, 2016). Leaf shed rate and drought stress duration parameters (Supporting information Figure S1) were selected, based on observations (Kelly, 2016) and comparisons of modeled and plot-level measurements of aboveground carbon in the Sierra Nevada Mountains, CA (Das, Stephenson, & Davis, 2016).…”

Global importance of large‐diameter trees

“…Previous studies have showed that large‐diameter trees comprise a large fraction of the biomass of many forests (Bastin et al, ; Brown et al, ; Clark & Clark, ; Lutz, Larson, Swanson, & Freund, ) and that they modulate stand‐level leaf area, microclimate and water use (Martin et al, ; Rambo & North, ). Large‐diameter trees contribute disproportionately to reproduction (van Wagtendonk & Moore, ), influence the rates and patterns of regeneration and succession (Keeton & Franklin, ), limit light and water available to smaller trees (Binkley, Stape, Bauerle, & Ryan, ), and contribute to rates and causes of mortality of smaller individuals by crushing or injuring sub‐canopy trees when their bole or branches fall to the ground (Chao, Phillips, Monteagudo, Torres‐Lezama, & Vásquez Martínez, ; Das, Stephenson, & Davis, ). Large‐diameter trees (and large‐diameter snags and large‐diameter fallen woody debris) make the structure of primary forests and mature secondary forests unique (Spies & Franklin, ).…”