Tree mortality from an exceptional drought spanning mesic to semiarid ecoregions

Abstract

Significant areas of the southern USA periodically experience intense drought that can lead to episodic tree mortality events. Because drought tolerance varies among species and size of trees, such events can alter the structure and function of terrestrial ecosystem in ways that are difficult to detect with local data sets or solely with remote-sensing platforms. We investigated a widespread tree mortality event that resulted from the worst 1-year drought on record for the state of Texas, USA. The drought affected ecoregions spanning mesic to semiarid climate zones and provided a unique opportunity to test hypotheses related to how trees of varying genus and size were affected. The study was based on an extensive set of 599 distributed plots, each 0.16 ha, surveyed in the summer following the drought. In each plot, dead trees larger than 12.7 cm in diameter were counted, sized, and identified to the genus level. Estimates of total mortality were obtained for each of 10 regions using a combination of design-based estimators and calibrated remote sensing using MODIS 1-yr change in normalized difference vegetation index products developed by the U.S. Forest Service. As compared with most of the publicized extreme die-off events, this study documents relatively low rates of mortality occurring over a very large area. However, statewide, regional tree mortality was massive, with an estimated 6.2% of the live trees perishing, nearly nine times greater than normal annual mortality. Dead tree diameters averaged larger than the live trees for most ecoregions, and this trend was most pronounced in the wetter climate zones, suggesting a potential re-ordering of species dominance and downward trend in tree size that was specific to climatic regions. The net effect on carbon storage was estimated to be a redistribution of 24-30 Tg C from the live tree to dead tree carbon pool. The dead tree survey documented drought mortality in more than 29 genera across all regions, and surprisingly, drought resistant and sensitive species fared similarly in some regions. Both angiosperms and gymnosperms were affected. These results highlight that drought-driven mortality alters forest structure differently across climatic regions and genera.

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

“…Recent severe droughts have led to increased tree mortality in forests worldwide and are predicted to be more common and impactful in the future (Allen et al 2015). In North America, many forest ecosystems have been affected by these events including southwestern U.S. pine forests (Breshears et al 2005), aspen forests in south-central Canada (Michaelian et al 2011), and diverse angiosperm and gymnosperm forests of Texas (Moore et al 2016) and California (Young et al 2017). It is difficult to generalize about regional vulnerability to extreme events in diverse forests with species varying in their strategy for surviving drought and sensitivity to insects and diseases (Anderegg et al 2016, Adams et al 2017.…”

Vulnerability of Southern Plains agriculture to climate change

“…For example, the 2011 drought killed more than 300 million rural trees across Texas (Moore et al 2016). While predictions of future precipitation vary among different climate models, most agree that climate change is likely to result in increased temperatures with associated increases in evapotranspiration and water demand (Sun et al 2008).…”

Review of broad-scale drought monitoring of forests: Toward an integrated data mining approach

“…Wu et al (2016), by combining elaborate ground and tower measurements with remotely sensed data across a rainfall gradient, found that coordinated leaf development and demography may explain large increases in ecosystem photosynthesis during dry conditions. An all-data approach such as this, where remote sensing methods are leveraged with other supplementary data streams, including ground-based measurements, may represent the most promising approach for cohesively and coherently addressing drought effects at broad scales (AghaKouchak et al, 2015;Cohen et al, 2016;Moore et al, 2016). A number of integrated wide-area monitoring systems have been developed that loosely follow this type of conceptual model, including the US Drought Monitor and the related North American Drought Monitor (Lawrimore et al, 2002).…”

“…This drought was remarkable because of its severity and duration, and because of the extensive area burned during the 2011 wildfire season (Nielsen-Gammon, 2012). An estimated 6.2% of all live trees died statewide (Moore et al, 2016). Where and when they co-occur, drought and wildfire may have additive or redundant effects in reducing NDVI.…”

“…A number of studies have employed this multivariate approach (e.g., Shaw et al, 2005;Klos et al, 2009;Dietze and Moorcroft, 2011;Williams et al, 2013), albeit on a limited scale. Through specially commissioned FIA re-measurement surveys, it may be possible to quantify areas experiencing major forest impacts in terms of trees lost and extent of the affected area, as was done by the Texas A&M Forest Service after the exceptional Texas drought in 2011, with use of MODIS-based ForWarn data products (Nielsen-Gammon, 2012; Moore et al, 2016).…”