Abstract
Long-term plot-scale studies have found water limitation to be a key factor driving ecosystem productivity in the Rocky Mountains. Specifically, the intensity of early summer (the ‘foresummer’ period from May to June) drought conditions appears to impose critical controls on peak ecosystem productivity. This study aims to (1) assess the importance of early snowmelt and foresummer drought in controlling peak plant productivity, based on the historical Landsat normalized-difference vegetation index (NDVI) and climate data; (2) map the spatial heterogeneity of foresummer drought sensitivity; and (3) identify the environmental controls (e.g. geomorphology, elevation, geology, plant types) on drought sensitivity. Our domain (15 × 15 km) includes four drainages within the East Water watershed near Gothic, Colorado, USA. We define foresummer drought sensitivity based on the regression slopes of the annual peak NDVI against the June Palmer Drought Severity Index between 1992 and 2010. Results show that foresummer drought sensitivity is spatially heterogeneous, and primarily dependent on the plant type and elevation. In support of the plot-based studies, we find that years with earlier snowmelt and drier foresummer conditions lead to lower peak NDVI; particularly in the low-elevation regions. Using random forest analysis, we identify additional key controls related to surface energy exchanges (i.e. potential net radiation), hydrological processes (i.e. microtopography and slope), and underlying geology. This remote-sensing-based approach for quantifying foresummer drought sensitivity can be used to identify the regions that are vulnerable or resilient to climate perturbations, as well as to inform future sampling, characterization, and modeling studies.
Highlights
IntroductionEcosystems in headwater catchments are important for water resources, because they influence hydrology through evapotranspiration (ET) and nutrient cycling (e.g., Lukas et al, 2015; Maxwell and Condon, 2016)
At the plot locations in Sloat et al (2015), Landsat-derived peak normalized difference vegetation index (NDVI) is positively correlated with June PDSI and snowmelt timing (Figure 2a and b, Table S3), which is consistent with their findings for peak NEP
Increased drought conditions and earlier snowmelt are associated with decreased peak NDVI
Summary
Ecosystems in headwater catchments are important for water resources, because they influence hydrology through evapotranspiration (ET) and nutrient cycling (e.g., Lukas et al, 2015; Maxwell and Condon, 2016). Recent global-climate-model ensembles predict increased temperature and earlier snowmelt in western North America (Higgins and Shi, 2001; Diffenbaugh et al, 2013). Some studies predict reduced spring precipitation and increased late-summer monsoon precipitation in the future (Seth et al, 2011). Together, these changes would increase the length of time between snowmelt and summer monsoon, or the “foresummer” part of growing seasons (Rauscher et al, 2008; Swain and Hayhoe, 2015). Low snowpack years with earlier snowmelt would expose plants to potentially longer and drier periods before the onset of monsoonal precipitation. Combined with predicted increasingly warmer temperatures, this foresummer period could become more drought-like
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