Interannual and Seasonal CO2 Exchange and Rainfall Dynamics in a Northern Chihuahuan Desert Shrubland

Dryland ecosystems are an important global driver of inter-annual variation of terrestrial carbon storage. Variation in rainfall is considered the primary driver of dryland carbon dynamics. The relationships between net ecosystem exchange (NEE) of carbon and rainfall are however complex and it remains unclear how seasonal timing, and inter-annual rainfall variation influence plant productivity and NEE. Here, we examine twelve years of NEE measurements from a mixed Creosote-Mesquite shrubland in the northern Chihuahuan Desert (Jornada LTER) to understand variation in CO2 sink strength in relation to seasonal and interannual precipitation dynamics. At this Chihuahuan Desert site mean daily air temperatures range from -5C to 40C and precipitation occurs in both summer and winter, with summer monsoons considered to be the main source of ecosystem water availability. These seasonal temperature and rainfall extremes create large variation in evaporative demand and span a wide range in plant phenology. We report annual cumulative NEE that ranged from a sink of -75gCm -2 to -200gCm -2 . Cumulative rainfall ranged from 150mm to 350mm with markedly different rainfall distribution patterns. Surprisingly, the dry years did not have the smallest sinks and in a wet year following a series of low rainfall years, the net sink strength was not suppressed. Seasonal patterns of NEE were bimodal in all years, with two distinct uptake periods. The strongest uptake period was always prior to summer monsoon, consistent with mesquite leaf-out and a second, weaker uptake period occurred once monsoonal rain arrived. The cumulative NEE during the early uptake period was remarkably consistent between years, with divergences in annual cumulative NEE governed by differences in the second peak, most likely linked to the timing and magnitude of summer monsoon rain. The consistent pre-monsoon net CO2 uptake suggests strong reliance on stored water supplies, a certain amount of resiliency in plant photosynthetic capacity across years, and that differences in annual NEE may depend on vegetation stress later in the season. Results presented here provoke the need for further evaluation of water-limits on productivity, the role of winter precipitation, and soil water storage.