Increased available soil nitrogen can increase biomass, lower species richness, alter soil chemistry and modify community structure in herbaceous ecosystems worldwide. Although increased nitrogen availability typically increases aboveground production and decreases species richness in mesic systems, the impacts of nitrogen additions on semiarid ecosystems remain unclear. To determine how a semiarid grassland responds to increased nitrogen availability, we examined plant community structure and above- and belowground net primary production in response to long-term nitrogen addition in a desert grassland in central New Mexico, USA. Plots were fertilized annually (10 g N m(-2)) since 1995 and NPP measured from 2004 to 2009. Differences in aboveground NPP between fertilized and control treatments occurred in 2004 following a prescribed fire and in 2006 when precipitation was double the long-term average during the summer monsoon. Presumably, nitrogen only became limiting once drought stress was alleviated. Belowground NPP was also related to precipitation, and greatest root growth occurred the year following the wettest summer, decreasing gradually thereafter. Belowground production was unrelated to aboveground production within years and unrelated to nitrogen enrichment. Species richness changed between years in response to seasonal precipitation variability, but was not altered by nitrogen addition. Community structure did respond to nitrogen fertilization primarily through increased abundance of two dominant perennial grasses. These results were contrary to most nitrogen addition studies that find increased biomass and decreased species richness with nitrogen fertilization. Therefore, factors other than nitrogen deposition, such as fire or drought, may play a stronger role in shaping semiarid grassland communities than soil fertility.

The replacement of native C4 -dominated grassland by C3 -dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m(-2) yr(-1) on average, while nearby native C4 grassland was a net source of 31 g C m(-2) yr(-1) over this same period. Differences in C exchange between these ecosystems were pronounced--grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.

Explosive population growth and increasing demand for rural homes and lifestyles fueled exurbanization and urbanization in the western USA over the past decades. Using National Land Cover Data we analyzed land fragmentation trends from 1992 to 2001 in five southwestern cities associated with Long Term Ecological Research (LTER) sites. We observed two general fragmentation trends: expansion of the urbanized area leading to Urban Ecosyst (2011) fragmentation in the exurban and peri-urban regions and decreased fragmentation associated with infill in the previously developed urban areas. We identified three fragmentation patterns, riparian, polycentric, and monocentric, that reflect the recent western experience with growth and urbanization. From the literature and local expert opinion, we identified five relevant drivers -water provisioning, population dynamics, transportation, topography, and institutions -that shape land use decision-making and fragmentation in the southwest. In order to assess the relative importance of each driver on urbanization, we linked historical site-specific driver information obtained through literature reviews and archival analyses to the observed fragmentation patterns. Our work highlights the importance of understanding land use decision-making drivers in concert and throughout time, as historic decisions leave legacies on landscapes that continue to affect land form and function, a process often forgotten in a region and era of blinding change.

Abstract. Grasslands occupy large areas in the northern Chihuahaun Desert. These grasslands, dominated by Bouteloua eriopoda, are subjected to periodic drought, infrequent fire and grazing by herbivores. Previous work shows that B. eriopoda is sensitive to disturbance but much work has been based on aboveground responses. We evaluated seasonal and annual recovery of belowground production and biomass following fire at two sites in ungrazed B. eriopoda-dominated grassland in Central New Mexico, USA. At one site, we quantified belowground standing crop and net primary production in burned and unburned areas during the first full growing season following wildfire the previous summer. At a second site, we measured annual below-and aboveground net primary production in burned and unburned grassland from 2005 through 2010 following a fire in 2003. At the first site, belowground standing crop did not change seasonally nor differ between burned/unburned areas. Patch types were different in that belowground standing crop was higher in soils under clumps of B. eriopoda than patches of unvegetated soil. Patterns of belowground biomass and daily production differed between patch types and over time in burned/unburned areas. Biomass was higher in soils below clumps of B. eriopoda than beneath unvegetated soil patches throughout the monsoon season. Patterns of belowground biomass and daily production differed in burned and unburned areas. Earlier in the growing season, biomass in the burned area was greater than in the unburned area. By early August, biomass increased rapidly in the unburned area and was higher than in the burned area. Daily rates of belowground production generally declined throughout the growing season with a large increase in rate of production in the unburned site in early August. At the second site's measured inter-annual responses, annual belowground production did not differ consistently between burned/unburned grasslands nor over time, nor was belowground production correlated with aboveground production. Our results demonstrate that despite the years required for aboveground production to recover following fire in B. eriopoda-dominated grassland, belowground standing crop and production was unchanged the year following fire. These results emphasize that aboveground production is not a reliable proxy for belowground production in this grassland.