Spatiotemporal dynamics of above- and belowground production in response to global change drivers in the northern Chihuahuan Desert

Abstract:

A better understanding of temporal and spatial variability in net primary production (NPP) is needed to quantify impacts of global environmental change on ecosystem processes and improve carbon cycle models. While estimates of aboveground NPP (ANPP) are commonplace, long-term measurements of belowground production (BNPP) are rare. Moreover, recent work suggests ANPP and BNPP may respond independently to global change drivers, such as increases in precipitation variability, fire frequency, and nitrogen deposition. To investigate the spatiotemporal dynamics of NPP and quantify their impacts, we analyzed fifteen years (2005-2019) of ANPP and BNPP data collected from three sites spanning a semi-arid grassland (Bouteloua gracilis-B. eriopoda) to shrubland (Larrea tridentata) ecotone in the northern Chihuahuan Desert, USA. One grassland site was subjected to a management burn in 2003, while the other grassland site received 10 g m^-2 yr^-1 nitrogen prior to the summer monsoon. Both grassland sites contained treatment and control plots. ANPP was measured allometrically by species twice per year and BNPP was measured annually using permanent root ingrowth cores (0-30 cm) following the growing season.

Mean ANPP ranged from 73.6 ± 9.3 (mean ± se) m^-2 y^-1 in burned grassland to 113.0 ± 16.8 m^-2 y^-1 in fertilized grassland. Mean BNPP ranged from 74.6 ± 11.5 g m^-2 y^-1 in burned grassland to 93.2 ± 18.4 g m^-2 y^-1 in unfertilized grassland. Temporal variability of ANPP was generally low (31.3% < CV < 57.7%) and significantly correlated among sites, whereas BNPP was more highly variable (59.6% < CV < 80.7%) and weakly correlated among sites. Thus, while ANPP generally followed similar trends across sites in a given year, BNPP was less temporally and spatially consistent across this semi-arid grassland-shrubland ecotone. Within sites, temporal correlations of above- and belowground NPP ranged from R²=0.01 in unfertilized grassland to R²=0.19 in burned grassland. However, none of these correlations were significant, meaning above- and belowground production fluctuated independently across sites and years. Our results suggest long-term measurements of both above- and belowground production are needed to quantify the impacts of global change drivers on dryland ecosystem processes and improve carbon cycle models in these systems.