A synthesis of radial growth patterns preceding tree mortality

Abstract

Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1-100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks.

Growth response of different tree species (oaks, beech and pine) from SE Europe to precipitation over time

“…In our case, the previous year's precipitation deficit (which has been frequent in recent decades and is assumed to be the leading cause of mass mortality of oak forest in the region of SE Europe (Cailleret et al, 2017;Stojanović et al, 2015b;Stojanović et al, 2015c), negatively impacted on the growth of all four species at all sites (SPI values that reach the previous year, Fig. 6), despite the amplitude difference between the low and high elevation sites, which were 400 mm of precipitation, 4°C in temperature and 1000 m in elevation (Table 1).…”

Forest Growth Responses to Drought at Short- and Long-Term Scales in Spain: Squeezing the Stress Memory from Tree Rings

“…When the BAI data was modeled accounting for the influence of tree age on growth, we found that declining individuals show important legacy effects of drought on growth represented by a negative growth trend after the drought period ( Figure 5C). In this case, such negative growth trends could be used to indicate impending tree death (Camarero et al, 2015b;Cailleret et al, 2017). …”

“…For example, this is the case of some silver-fir (Abies alba) populations showing dieback in the Spanish Pyrenees due to a pronounced and long-term growth decline which started after the 1986 drought Gazol and Camarero, 2016). Ultimately, intense droughts can induce irreversible growth and vigor loss resulting in tree death (Sánchez-Salguero et al, 2012;Camarero et al, 2015b;Cailleret et al, 2017). Here we define the long-term legacy effects by selecting severe droughts representing "tipping-points" in tree vigor and triggering forest dieback (Camarero et al, 2015b).…”

Differences in functional and xylem anatomical features allow Cistus species to co-occur and cope differently with drought in the Mediterranean region

“…On a global scale, the frequency of these events has already increased during recent decades, thereby affecting forest productivity and plant mortality rates (Boisvenue and Running 2006, van Mantgem et al 2009, Allen et al 2015. Different studies have already reported substantial growth reductions and mortality events induced by drought stress not only at global scale (Cailleret et al 2016), but also specifically in the Mediterranean basin for species such as Quercus ilex, Pinus brutia and Pinus sylvestris (Peñuelas et al 2001, Corcuera et al 2004, Lloret et al 2004, Sarris et al 2007, generating growing concern about the effects of climate change on the distribution and composition of the Mediterranean forests (Martínez-Vilalta et al 2011).…”