Assessing vegetation structure and ANPP dynamics in a grassland–shrubland Chihuahuan ecotone using NDVI–rainfall relationships

[摘要] Climate change and the widespread alteration of natural habitats are majordrivers of vegetation change in drylands. In the Chihuahuan Desert, largeareas of grasslands dominated by perennial grass species have transitionedover the last 150 years to shrublands dominated by woody species, accompaniedby accelerated water and wind erosion. Multiple mechanisms drive theshrub-encroachment process, including precipitation variations, land-usechange, and soil erosion–vegetation feedbacks. In this study, using a simpleecohydrological modelling framework, we show that herbaceous (grasses andforbs) and shrub vegetation in drylands have different responses toantecedent precipitation due to functional differences in plant-growth andwater-use patterns. Therefore, shrub encroachment may be reflected in theanalysis of landscape-scale vegetation–rainfall relationships. We analysethe structure and dynamics of vegetation at an 18 km²grassland–shrubland ecotone in the northern edge of the Chihuahuan Desert(McKenzie Flats, Sevilleta National Wildlife Refuge, NM, USA) byinvestigating the relationship between decade-scale (2000–2013) records ofremotely sensed vegetation greenness (MODIS NDVI) and antecedent rainfall.NDVI–rainfall relationships show a high sensitivity to spatial variations ondominant vegetation types across the grassland–shrubland ecotone, andprovide biophysical criteria to (a) classify landscape types as a function ofthe spatial distribution of dominant vegetation and to (b) decompose the NDVIsignal into partial components of annual net primary production (ANPP) forherbaceous vegetation and shrubs. Analysis of remotely sensed ANPP dynamicsacross the study site indicates that plant growth for herbaceous vegetationis particularly synchronized with monsoonal summer rainfall. For shrubs, ANPPis better explained by winter plus summer precipitation, overlapping themonsoonal period (June–September) of rain concentration. Our results suggestthat shrub encroachment was not particularly active in this Chihuahuanecotone for the period 2000–2013. However, future changes in the amount andtemporal pattern of precipitation (i.e. reductions in monsoonal summerrainfall and/or increases in winter precipitation) may enhance theshrub-encroachment process, particularly in the face of expected upcomingincreases in aridity for desert grasslands of the southwestern USA.

[学科分类] 地球化学与岩石