Lagged mortality among tree species four years after an exceptional drought in east Texas

Abstract

In 2011, east Texas experienced the worst drought on record causing extensive tree mortality.Initial mortality estimates for 2012 varied among tree genera. A rapid damage assessment (RDA) estimated that 65.5 (AE 7.3) million trees died as a result of the drought in this region one year post-drought. However, this estimate was untested against established monitoring networks. Moreover, pests and physiological damage can elevate tree mortality multiple years beyond a drought event. Since the RDA was unable to quantify multi-year trends, it remained unclear whether these drivers caused increased tree mortality in east Texas beyond one year post-drought and how different species responded over time. To address these questions, we compared total 2012 standing dead tree (SDT) estimates (i.e., drought-killed plus all other SDT excluding harvested or salvaged trees) derived from the RDA and U.S. Forest Service Forest Inventory and Analysis (FIA) data for east Texas. Total SDT estimates did not significantly differ between the RDA (120.5 AE 8.5 million) and FIA (108.4 AE 8.7 million). Furthermore, total SDT estimates for the four most common genera (Pinus, Quercus, Liquidambar, Ulmus), which comprised over 80% of all species, did not significantly differ between the RDA and FIA. Additionally, we used logistic regression and FIA data from east Texas for 2011 through four years post-drought (2012)(2013)(2014)(2015) to examine temporal trends in plot-level drought-and pest-driven tree mortality of seven key species (Pinus taeda, Pinus echinata, Quercus nigra, Quercus stellata, Quercus falcata, Liquidambar styraciflua, Ulmus alata) from the four most common genera. At the plot-level, drought-driven mortality was immediate for the three Quercus species (notably Q. falcata) and L. styraciflua which significantly increased in 2012 while P. taeda mortality was delayed, not increasing significantly until 2013. Pest-driven mortality increased from 2013 to 2015 for all species, with the highest mortality observed in Q. falcata and lowest in P. taeda and U. alata. This study affirms the validity and value of independent sampling efforts to quantify mortality immediately following major disturbance and also demonstrates the need for longer-term species-level assessments beyond the initial year post-drought to account for differential impacts from drought and pests.

Tree Mortality After a Hot Drought: Distinguishing Density-Dependent and -Independent Drivers and Why It Matters

“…Here, we present a case study that takes the framework of density-dependence as the organizing principle to characterize drivers of crown death following a hot drought that took place in Texas, USA in 2011. This drought reportedly killed 110-120 million trees statewide over 3 years, at a 9x higher annual rate than previously observed (Moore et al, 2016;Klockow et al, 2018). Although this event was not as destructive as some drought events reported in southwestern US forests [e.g., Breshears et al, 2005], it was extreme in terms of its spatial, biogeographic and taxonomic extent.…”

“…Thus, although the survey method may have included some trees that died before 2011, their proportion in the total sample volume would have been small. In addition, we considered trees that died since 2011 as late victims of the drought, since drought conditions do not always kill trees outright but may start them on a downward spiral from which they cannot recover (Klockow et al, 2018;Trugman et al, 2018).…”

Influence of Drought on Foliar Water Uptake Capacity of Temperate Tree Species

“…The presence or absence of a relationship between Ψ leaf and FWU can partly be explained by iso-and anisohydric behavior of the species. Isohydric species maintain a constant midday Ψ leaf to reduce the risk of embolism formation during drought [23,24], which partly decouples Ψ leaf from Ψ soil [24,25]. Anisohydric species exhibit a progressive decrease in midday Ψ leaf as Ψ soil decreases [24], allowing them to function with a small hydraulic safety margin, which may result in embolism formation [23].…”

Assessing the vulnerability of Australia’s urban forests to climate extremes

“…Future changes in temperature and precipitation patterns will undoubtedly affect the performance and survival of urban forests globally. Rainfall is well-known to play an important role in determining species distributions (Moles et al, 2014;O'Donnell & Ignizio, 2012), and both natural and urban forests are increasingly threatened by drought-induced tree mortality (Anderegg et al, 2015;Klockow, Vogel, Edgar, & Moore, 2018;Vogt et al, 2017). However, the adverse effects of limited rainfall can be reduced in areas with low potential evapotranspiration and also mitigated by irrigation (Jenerette et al, 2016;Vogt et al, 2017).…”