The deliberate approach of Natural Systems Agriculture of restoring ecological function in agroecosystems calls for some understanding of the underlying interactions and mechanisms. To explore the coupling between structural diversity and function in water-limited systems, two distinctly different ecosystems are considered: tropical savanna and a legume-cereal intercrop. Here we study resource acquisition at the plot scale, in a legume-perennial grain (Kernza) intercrop, and ecological stability at the sub-continental scale, in tropical savanna.
Savanna: We use a 15-year record of daily precipitation (ERA-Interim), and satellite data for Normalized Difference Vegetation Index (NDVI), matching the same time period (2001-2015), to determine ecosystem response across a climate gradient in Southern Africa. We confine our definition of stability to a trade-off metric, balancing the capacity to optimize resource use with stress avoidance. In addition, stability is defined as the capacity to resist and recover from perturbations.
Intercrop: To determine the degree of resource partitioning in the intercrop we compare stable isotopes of soil- and xylem-water (δD and δ18O) in alfalfa-Kernza intercrop plots, compared to Kernza monoculture.
Results/Conclusions
Savanna ecosystems show a varied temporal sensitivity to environmental fluctuation going from 200 mm to 700 mm of mean annual precipitation. Increasing grass cover results in higher sensitivity of NDVI to intra-annual dynamics, while an increased woody cover reflects the annual pattern.
Intercropping trials with Kernza and alfalfa show a 3-fold increase in aboveground biomass production when compared to Kernza monoculture, with no negative effect on either Kernza biomass-, nor grain-production. This result is inline with a clear water partitioning by depth, with alfalfa xylem water reflective of a much lighter signal than what we observed in the depths sampled (0-100 cm). In contrast, Kernza xylem water showed a clear match with the soil water sampled, supporting the importance of niche partitioning as an important mechanism for resource use optimization in a structurally diverse agroecosystem.
Both studies point to interactions between ecosystem structure and function whereby ecosystem composition can have effective feedback on resource use and stability. Finally, using the savanna example to better understand trade-offs, such as how resistance versus recovery, and similarly, stress avoidance versus resource use optimization plays out under a gradient of conditions, can inform the importance of structural diversity in the stability of agroecosystems.