Examining ecosystem function in space and time within the critical zone through the lenses of ecology and biogeography

Background/Question/Methods

Current projections of climate change in the southwestern U.S. suggest increasing temperatures and longer periods of inter-storm drought. Both, high temperature and water deficits have major influences on ecosystem functioning by restricting plant productivity, but their influence is highly heterogeneous in space and time. Within our Critical Zone Observatory (CZO), we monitor ecosystem scale carbon and water fluxes using eddy covariance. This whole-ecosystem metric is a powerful integrating measure of carbon and water dynamics over time. However, details on spatial heterogeneity resulting from topographic features of a landscape are not captured, nor are the ecological interactions among below- and aboveground processes. We supplement eddy covariance monitoring with distributed measures of carbon and water flux from soil and vegetation across different aspects to better quantify the causes and consequences of spatial heterogeneity through time. Data on how physical attributes of a landscape influence plant sensitivity to temperature and drought stress are limited, and, thus, these dynamics are not often captured in ecosystem models. More intentional inclusions of ecological thinking within CZO research will illuminate the importance of biology as a driver and a response to the physical systems at play in the CZ.

Results/Conclusions

Given that (i) soil parent material influences soil water holding capacity; (ii) aspect influences how incoming energy drives evaporative water loss; and (iii) seasonality drives temporal patterns of soil moisture recharge, we aimed to examine the influence of these processes due to variation in aspect. We found that aspect was a significant source of spatial heterogeneity in soil CO₂ efflux, but the influence varied across seasonal periods. Snow on South-facing aspects melted earlier and yielded higher efflux rates in the spring. However, during summer, North- and South-facing aspects had similar amounts of soil moisture, but soil temperatures were warmer on the North-facing aspect, yielding greater rates of CO₂ efflux. Interestingly, aspect did not influence photosynthetic rates. Taken together, we found that physical features of the landscape yielded predictable patterns of levels and phenologies of soil moisture and temperature, but these drivers differentially influenced below- and aboveground sources of carbon exchange. Conducting these spatially distributed measurements are time consuming.  Looking forward, we are examining the potential for low-altitude remote sensing using unmanned aerial vehicles outfitted with thermal and multi-spectral cameras to quantify patterns of water flux, NDVI, needle browning due to moisture stress, overall phenology in the Critical Zone.