Abstract
Pinyon juniper woodlands in the American southwest face an uncertain ecological future with regard to climate altered precipitation. Although satellite remote sensing will be relied upon to assess the overall health of these plant communities more fine scaled information is needed to elucidate the mechanisms shaping the broader scaled regional assessments. We conducted a study to assess the NDVI response at the plant canopy level (insitu sensors placed over the canopies) of three tree and one shrub species to changes in precipitation, reference evapotranspiration and soil volumetric water content. Landsat data was used to compare stand integrated and satellite NDVI values. We also provided supplemental water in the amount of 10.85 cm over the study period to additional trees and shrubs which also had insitu NDVI sensors placed over their canopies. NDVI at the canopy level separated statistically by species and when contrasted with bare soil (p<0.001). Spring early summer dry down events were inversely related to increasing ETref-precipitation with a steeper dry down slope in the first year associated with no rainfall occurring in May and June. All three-tree species did not show any significant difference in canopy NDVI based on supplemental water, however the shrub species did reveal a significant response to water (p<0.001). Although all of the three-tree species revealed a one-month period in which they responded to precipitation in July of the first year after 11.2 cm of precipitation, no immediate (day of or next day) response was observed to precipitation or supplemental water events. Snowberry was unique in its NDVI response during the spring green up period in the second year revealing a highly linear shift over a 40-day period with a clear separation between treatments (p<0.001) with those plants receiving supplemental water having a higher more positive slope. Landsat NDVI values revealed an inverse sinusoidal relationship with ETref-precipitation (R2=0.59 p=0.012). Landsat values (0.19+/- 0.01) were found to have no significant difference with bare soil NDVI (0.17+/- 0.01) but were significantly different from all four tree and shrub species. Integrated NDVI based on sensor weighted % cover estimates (0.37+/-0.03) were nearly double Landsat values (0.19+/-0.01). Both NDVI values of pinyon pine and Utah juniper were found to be linear correlated with Landsat NDVI in the second Year (R2>0.75, p<0.001). Multiple regression analysis revealed that 95% of the variation in Landsat NDVI in the second year could be accounted for based on bare soil NDVI and pinyon pine NDVI (p<0.001). et al., NDVI interspace (bare soil) of pinyon juniper woodlands dominated the nature of the Landsat curve. Our results demonstrate the value of ground sensors to help fill the gap between what can be inferred at the forest canopy level and what is occurring at the plant level.
Highlights
Precipitation-derived soil moisture is one of the most important drivers of vegetation growth and productivity in semiarid ecosystems (Rodriguez-Iturbe, 2000), and temporal variation in soil moisture on the order of multiple days to multiple years exerts a strong control on vegetation dynamics at the plant and community stand level (Rasmussen et al, 2014; Bell et al, 2010)
NDVI Sensor and plant physiological measurements were taken over a two-year period in 2012-2013, during which the environmental demand was very similar on a yearly basis (ETref 193 cm in 2012, 189 cm in 2013), with highest monthly et al, ETref in June (29.0 cm in 2012, 28.8 cm in 2013)
No rainfall occurred in June of either 2012 or 2013, and in 2012 no rainfall occurred in May. et al, Volumetric water content (VWC) at 10 cm depth responded to rainfall events and subsequent water loss to evapotranspiration, as well as seasonal and shorter periods of rising ETref associated with high temperatures and low precipitation (Figure 1A)
Summary
Precipitation-derived soil moisture is one of the most important drivers of vegetation growth and productivity in semiarid ecosystems (Rodriguez-Iturbe, 2000), and temporal variation in soil moisture on the order of multiple days to multiple years exerts a strong control on vegetation dynamics at the plant and community stand level (Rasmussen et al, 2014; Bell et al, 2010). Whole-ecosystem functioning and responses to climate variation in pinyon-juniper woodlands are strongly shaped by the individual responses of et al, ecosystem components (tree and shrub composition, understory herbaceous vegetation, and bare soil) to seasonal variation in soil moisture (Flanagan et al, 1992; West et al, 2007; Petrie et al, 2015) It follows that the composition and physical structure of pinyon-juniper ecosystems underlie larger-scale evaluation using remote sensing indices (Krofcheck et al., 2015), yet these relationships have not been explored for the wide diversity in composition and physical structure exhibited by pinyon-juniper woodlands in the American southwest. Our study provides insight on how biotic and abiotic ecosystem components and their differing responses to seasonal water availability underlie measures usually attributed to the ecosystem as a whole
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