Responses of grassland production to single and multiple global environmental changes

Abstract

In this century, increasing concentrations of carbon dioxide (CO2) and other greenhouse gases in the Earth's atmosphere are expected to cause warmer surface temperatures and changes in precipitation patterns. At the same time, reactive nitrogen is entering natural systems at unprecedented rates. These global environmental changes have consequences for the functioning of natural ecosystems, and responses of these systems may feed back to affect climate and atmospheric composition. Here, we report plant growth responses of an ecosystem exposed to factorial combinations of four expected global environmental changes. We exposed California grassland to elevated CO2, temperature, precipitation, and nitrogen deposition for five years. Root and shoot production did not respond to elevated CO2 or modest warming. Supplemental precipitation led to increases in shoot production and offsetting decreases in root production. Supplemental nitrate deposition increased total production by an average of 26%, primarily by stimulating shoot growth. Interactions among the main treatments were rare. Together, these results suggest that production in this grassland will respond minimally to changes in CO2 and winter precipitation, and to small amounts of warming. Increased nitrate deposition would have stronger effects on the grassland. Aside from this nitrate response, expectations that a changing atmosphere and climate would promote carbon storage by increasing plant growth appear unlikely to be realized in this system.

Figure 1. Biomass Production, Cumulative Precipitation, and Cumulative Temperature from 1998 (the Year before Treatments Began) to 2003

Bars for shoot biomass (grey) and root biomass (black) represent mean values (±SE, n = 8) for quadrants in the “infrastructure control” treatment, which experienced ambient conditions (see Materials and Methods). Cumulative precipitation includes the germinating rain event (defined here as the event that brought cumulative rainfall after October 1 above 12.5 mm) and subsequent precipitation until the last harvest of the year. Cumulative temperature is the sum of average temperatures (in °C) over all days from germination until the final harvest. Shoot biomass (g m⁻²) was not related to growing season precipitation (mm; linear regression: p = 0.87) or cumulative temperature (°C; p = 0.27), but there was a weak relationship between shoot growth and fall precipitation (linear regression: p = 0.06, slope = 0.646, r ² = 0.627).

Figure 2. Proportional Responses of NPP (Measured as Root + Shoot Biomass), Shoot Biomass, Root Biomass, and Root-to-Shoot Ratio to the Four Global Change Treatments

Each line represents the response over time to a single global change factor, and each data point represents the sum of eight elevated treatment averages divided by the sum of eight ambient treatment averages. Elevated CO₂, C (gray dashed line and filled diamonds); increased temperature, T (thin red solid line); increased rainfall, R (thick blue dashed line and triangles); nitrate deposition, N (thick solid green line).

Figure 3. NPP (Measured as Root + Shoot Biomass) in Individual and Combined Global Change Treatments

Note that in the similar Figure 2 of Shaw et al. [27], the reference treatments were not the plots with all global change factors at ambient levels, but the average of all of the plots with the global-change factors identified under each bar at ambient levels. Note also that while Shaw et al. eliminated two blocks heavily invaded by non-native perennials in 2000–2001, the current analysis uses data from all of the blocks from every year. Paired bars depict mean values under ambient (open) and elevated CO₂ (grey). A line representing mean biomass of the ambient treatment is drawn across each panel to facilitate comparisons. Shaded yellow areas similarly mark zones within one standard error of the ambient treatment. Panels are shaded blue behind treatments receiving N to highlight plant responses to this treatment. Letters in each panel identify treatment effects (α < 0.05; C denotes elevated CO₂) from mixed model analyses of NPP data (*, p < 0.05; **, p < 0.01; ***, p < 0.001; ****, p < 0.0001). Interactions are presented as multiple letters. Ambient, amb; increased temperature, T; increased rainfall, R; nitrate deposition, N. Error bars denote one standard error.

Figure 4. Mean Shoot Biomass in Individual and Combined Global Change Treatments

Error bars denote one standard error. Shading, labels, and statistics as in Figure 3.

Figure 5. Mean Root Biomass in Individual and Combined Global Change Treatments

Error bars denote one standard error. Shading, labels, and statistics as in Figure 3, with one addition (†, 0.10 > p > 0.05).

Figure 6. Mean Total Biomass, Shoot Biomass, and Root Biomass in Individual and Combined Global Change Treatments during the 1999–2003 Period

Shading and labels as in Figure 3. Letters identify treatment effects (α < 0.05) from a repeated measures mixed model analysis (*, p < 0.05; **, p < 0.01; ****, p < 0.0001). Error bars denote one standard error.