Press–pulse interactions: effects of warming,N deposition,altered winter precipitation,and fire on desert grassland community structure and dynamics

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long‐term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production (ANPP) in a northern Chihuahuan Desert grassland. In 2009, a lightning‐caused wildfire burned through the experiment. Here, we report on the interactive effects of these global change drivers on pre‐ and postfire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared with control plots. Average soil volumetric water content (VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared with control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced the cover of the dominant grasses by more than 75%. Following the fire, forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change.     

Revegetation of prairie pothole wetlands in the mid-continental US: twelve years post-reflooding

In the mid-1980's, thousands of wetlands in the mid-continental Unites States were restored by interrupting drainage lines; revegetation of these systems, often cropped for decades and positioned in a predominantly agricultural landscape, relied solely on natural recolonization. A study of 64 of these wetlands determined that by 1991, three years after initial reflooding, aquatic species had efficiently recolonized whereas sedge meadow and wet prairie species had not. In 2000, 41 of these restorations that had not been significantly altered or returned to cultivation were revisited and their floras characterized by cover within distinct zones. While species richness increased on every site, the rate of accumulation varied widely. Furthermore, species that had colonized since 1991, including a variety of native wet prairie and sedge meadow species, were detected only at very low abundance. In contrast, Phalaris arundinacea L., an invasive perennial, was now present on every site, often at covers approaching 75–100% in the zones in which it occurred. Other invasive perennials, including Cirsium arvense (L.) Scop. and Typha angustifolia L./glauca Godr., had expanded significantly on many sites. The overall dominance of invasive perennials has resulted in basins that are becoming more similar over time. However, present variations in species richness and composition can be attributed to flooding frequency, and, potentially, basin size and isolation from nearby natural wetlands, as shown by TWINSPAN and graphical analysis. Basins that have not been flooded at midsummer for at least seven of 12 years are among the most depauperate in the study. Yet even frequently flooded basins lack diversity if they are small (< 1.5 ha) or isolated from seed sources. Across the study, numerous species common to natural systems were notably absent or infrequently occurring, including Calamagrostis canadensis (Michx.) Beauv., Carex lacustris Willd. and Lysimachia thrysiflora L. Continuous areas of sedge-dominated meadow, a signature of prairie potholes, did not exist at any study site, nor did they appear to be forming. Given the dominance of invasive perennials and absence of many native wetland species, it appears that without significant seeding, planting and aftercare wetland restorations in fragmented landscapes have a low probability of resembling those that existed historically. This revised version was published online in August 2006 with corrections to the Cover Date.