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.
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.
Relationships among stand composition, stemwood productivity, and canopy structure were investigated for 55 study areas in northeastern Minnesota. Tree species composition among study areas was deliberately allowed to vary. Aspen, primarily quaking (Populus tremuloides Michx.) and to a lesser degree bigtooth (Populus grandidentata Michx.), was a significant component of every study area. The two most common associates were paper birch (Betula papyrifera Marsh.) and balsam fir (Abies balsamea (L.) Mill.). Productivity was defined as mean annual increment of tree stemwood volume and ranged from 3.3 to 12.6 m3·ha1·year1. A multiple-regression approach was used to investigate the relationships between productivity, stand composition, and canopy structure. Base models relating productivity to aspen site index, aspen cohort age, and total basal area were developed. Measures of stand composition and canopy vertical structure were added to the base models, and their significance in explaining residual variation in productivity was tested. Productivity was found to be negatively correlated with stand composition and canopy vertical structure, with all other factors held constant. Pure aspen and single-canopied stands were expected to be the most productive. Exceptions were present in the data: the two most productive stands were vertically stratified, aspen balsam fir paper birch mixtures.
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