A Study on the Response of Non-photosynthetic Vegetation (NPV) towards the Anomalies of Climate in the Southwestern U.S.

Non-photosynthetic vegetation (NPV), including the standing dead leaves and trunks of plants, is not only a crucial component of aboveground biomass in the dry ecosystems but also an effective indicator of drought, since the photosynthetic plants (PV) usually degrade to NPV after drought. With the multiple-endmember spectral mixture (MESMA) analysis, it is possible to extract the NPV coverage to a selected baseline date from MODIS MOD43 NBAR data. In this study, we used a baseline image derived based on JHU Spectral Library to obtain the NPV, PV and bare soil of southwest U.S. from 2000 to 2012. To investigate the response of NPV, we then calculated the lagged and non-lagged correlation between the anomalies of land cover and climate variables. The major land use categories are also employed to investigate the spatial pattern of the response. 

The more significant correlation between NPV or PV and precipitation than temperature indicates the importance of moisture in the study site. In addition, there is an asymmetric response between NPV to the drought and the increased precipitation. In drier inland, the most significant response of NPV after a deficit of precipitation occurs later than after an increase. The increase of temperature, especially under the deficits of moisture, facilitates the presence of NPV with lag, which is due to the response time of detectable withering of PV. The response of NPV also varies between the different land cover categories. In southwest U.S., the NPV from shrubs and grassland have more sensitive feedbacks on the dynamics of climate than wetter region. The nature of the ecosystems can partly explain the difference, but finer scale studies are necessary for further investigation of specific regions. 

Considering the increase of drought in southwest U.S., obtaining a better understanding on the response of vegetation is crucial to further evaluate its impacts on the dry ecosystems. This study provides a perspective by examining NPV, another direct indicator of drought. For further studies, temporal and spatial patterns of NPV response to the climate need more scrutiny, such as the spatial pattern of the lags, hotspots of change, and regional-specific feedbacks. Different indicators of extreme events, such as the U.S. Drought Monitor may also be employed to provide more direct evaluation.