Biology ETDs

Publication Date

Fall 10-6-2022

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.

Language

English

Keywords

drylands, fire, global change, net primary production, nitrogen, precipitation

Document Type

Dissertation

Degree Name

Biology

Level of Degree

Doctoral

Department Name

UNM Biology Department

First Committee Member (Chair)

Scott L. Collins

Second Committee Member

Cristina D. Takacs-Vesbach

Third Committee Member

Osvaldo E. Sala

Fourth Committee Member

John E. Barrett

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