Vegetation, land surface brightness, and temperature dynamics after aspen forest die‐off

Abstract

Forest dynamics following drought-induced tree mortality can affect regional climate through biophysical surface properties. These dynamics have not been well quantified, particularly at the regional scale, and are a large uncertainty in ecosystem-climate feedback. We investigated regional biophysical characteristics through time (1995-2011) in drought-impacted (2001-2003), trembling aspen (Populus tremuloides Michx.) forests by utilizing Landsat time series green and brown vegetation cover, surface brightness (total shortwave albedo), and daytime land surface temperature. We quantified the temporal dynamics and postdrought recovery of these characteristics for aspen forests experiencing severe drought-induced mortality in the San Juan National Forest in southwestern Colorado, USA. We partitioned forests into three categories from healthy to severe mortality (Healthy, Intermediate, and Die-off ) by referring to field observations of aspen canopy mortality and live aboveground biomass losses. The vegetation cover of die-off areas in 2011 (26.9% of the aspen forest) was significantly different compared to predrought conditions (decrease of 7.4% of the green vegetation cover and increase of 12.1% of the brown vegetation cover compared to 1999). The surface brightness of the study region 9 years after drought however was comparable to predrought estimates (12.7-13.7%). Postdrought brightness was potentially influenced by understory shrubs, since they became the top layer green canopies in disturbed sites from a satellite's point of view. Satellite evidence also showed that the differences of land surface temperature among the three groups increased substantially (≥45%) after drought, possibly due to the reduction of plant evapotranspiration in the Intermediate and Die-off sites. Our results suggest that the mortality-affected systems have not recovered in terms of the surface biophysical properties. We also find that the temporal dynamics of vegetation cover holds great potential for assessing propensity of subsequent mortality during drought itself, which could provide effective monitoring and potentially a much needed "early warning" of drought-induced tree mortality.

Detecting early warning signals of tree mortality in boreal North America using multiscale satellite data

“…Here we provide evidence that long‐term NDVI time series have the potential to detect early warning signals of tree mortality using a variety of sensors, time scales, forest systems, and tree species in western boreal North America. The utility of NDVI for mortality forecasting has been suggested (Huang & Anderegg, ; Vicente‐Serrano et al., ) and is consistent with known relationships between satellite‐based NDVI (or similar indices) and drought, infestations, productivity, leaf senescence and partial dieback, and mortality (Beck, Juday et al., ; Berner et al., ; Breshears et al., ; Bunn et al., ; Foster, Walter, Shugart, Sibold, & Negron, ; Kharuk, Im et al., ; Lloyd et al., ; McDowell et al., ; Meddens & Hicke, ; Spruce et al., ; Vogelmann, Tolk, & Zhu, ). However, to our knowledge these relationships have not yet been documented beyond short‐term (<5 years) associations during severe drought (Byer & Jin, ; Potter, ).…”

“… Relationships between mortality and NDVI‐based EWS metrics are strongest with finer spatial‐scale and longer temporal‐scale imagery. Relationships with NDVI trends are detectable over longer time windows compared to jumps, as the former captures gradually declining vigor and the latter inciting events. Relationships between mortality and EWS metrics are strongest in annually measured sites because longer remeasurement intervals add uncertainty as to the timing of mortality. Relationships are strongest in aspen‐dominated sites, and particularly those that are pure aspen (such as CIPHA). This is because (i) aspen's deciduous leaf habit results in more interannual variability in productivity (Welp et al., ) and leaf condition, as observed by NDVI, that responds more quickly to environmental stress compared to conifers (Gamon et al., ; Norman, Koch, & Hargrove, ); (ii) aspen are pioneer species and have comparatively high mean mortality rates, especially in later succession (Figure b) (Stephenson et al., ; Vanderwel, Zeng, Caspersen, Kunstler, & Lichstein, ); (iii) aspen die‐off begins in the upper canopy (Anderegg & Callaway, ; Frey, Lieffers, Hogg, & Landhausser, ), which can be detected with multispectral imagery (Huang & Anderegg, ); (iv) aspen are clonal, meaning patches of genetically identical trees die together, and relatively quickly as a strategy for effective resprouting (Frey et al., ); (v) and finally, aspen have documented sensitivity to defoliation and drought, including mortality (Bell, Bradford, & Lauenroth, ; Chen et al., ; Hogg, Brandt, & Kochtubajda, ; Worrall et al., ). …”

“…Tree mortality from drought, heat, and pests and pathogens is increasing in many locations globally in response to global change drivers, primarily climate change (Allen et al., ; Brienen et al., ; Carnicer et al., ; Kautz, Meddens, Hall, & Arneth, ; Kharuk, Im, Oskorbin, Petrov, & Ranson, ; van Mantgem et al., ; McDowell et al., ). Tree mortality precipitates a cascade of ecosystem impacts relevant for carbon cycling, energy budgets, nutrient cycling, hydrology, habitat and food webs, and ecosystem services (Adams et al., ; Anderegg, Kane, & Anderegg, ; Anderegg et al., ; Berner, Law, Meddens, & Hicke, ; Breshears, Lopez‐Hoffman, & Graumlich, ; Edburg et al., ; Huang & Anderegg, ). If mortality occurs over large enough areas, it can accelerate shifts in biome distributions (Allen & Breshears, ; Clifford & Booth, ).…”

“…services (Adams et al, 2012;Anderegg, Kane, & Anderegg, 2013;Anderegg et al, 2016;Berner, Law, Meddens, & Hicke, 2017;Breshears, Lopez-Hoffman, & Graumlich, 2011;Edburg et al, 2012;Huang & Anderegg, 2014). If mortality occurs over large enough areas, it can accelerate shifts in biome distributions (Allen & Breshears, 1998;Clifford & Booth, 2015).…”

Testing early warning metrics for drought‐induced tree physiological stress and mortality

“…Extensive research has shown that drought is the dominant cause of regional-scale aspen mortality in this area and time period (Anderegg, Berry, Smith, et al, 2012, Anderegg, Hicke, et al, 2015, Anderegg, Flint, et al, 2015Worrall et al, 2008Worrall et al, , 2010. Our focal study area was the San Juan National Forest (915 km 2 ; 37.5N, 108.3W, mean annual temperature = 3.9°C, mean annual precipitation = 699.7 mm) in western Colorado, USA (Huang & Anderegg, 2014), which experienced severe aspen mortality (Worrall et al, 2010).…”