Above- and Belowground Responses to Environmental Change in the Northern Chihuahuan Desert
Brown, R. F. 2022.
Abstract
Drylands cover 45% of the terrestrial surface and are expanding rapidly due to anthropogenic
drivers. Altered precipitation regimes, atmospheric nitrogen deposition, and wildfire will likely
have significant consequences in these regions where ecological processes are limited by water
and nitrogen. In this dissertation, I explored temporal dynamics of net primary production
(NPP) and related above- and belowground processes under several environmental change
drivers in the Sevilleta National Wildlife Refuge, central New Mexico, USA. Located in the
northern Chihuahuan Desert, this region experiences strong seasonal precipitation patterns
driven by the North American Monsoon, historically characterized by frequent small rain events
hypothesized to benefit microbial processes, such as nitrogen mineralization. Climate models
predict a shift to a more extreme precipitation regime characterized by fewer, but larger rain
events hypothesized to benefit plant production. Yet, it remains unclear how drylands will
respond to increased precipitation variability and other drivers of environmental change. To
address this knowledge gap, I explored daily and seasonal responses of plant available nitrogen
and related belowground parameters to altered rainfall size and frequency using a long-term
rainfall manipulation experiment. Next, I explored long-term responses of aboveground NPP (ANPP) and plant community composition to these altered rainfall patterns, along with chronic
nitrogen enrichment. In contrast to long-standing ecological theories, small frequent rain events
stimulated the greatest amounts of plant available nitrogen and ANPP, and when combined with
nitrogen enrichment, led to the greatest shift in plant community composition following a
wildfire. A more extreme growing season rainfall regime decoupled microbial and plant
processes, and only when nitrogen limitation was alleviated did ANPP respond strongly to large
infrequent rain events. Finally, I explored spatiotemporal impacts of precipitation, nitrogen
enrichment, and a prescribed fire on belowground NPP (BNPP) and ANPP using sixteen years
of annual measurements across a grassland-shrubland ecotone. Surprisingly, BNPP was largely
unaffected by these drivers of environmental change and was not correlated with ANPP.
Drylands are vastly understudied compared to other terrestrial ecosystems despite comprising
the largest terrestrial biome. This research contributes important advances to understanding how
dryland ecosystem structure and functioning may respond to future environmental change.
