Community Level Offset of Rain Use- and Transpiration Efficiency for a Heavily Grazed Ecosystem in Inner Mongolia Grassland

<div><p>Water use efficiency (WUE) is a key indicator to assess ecosystem adaptation to water stress. Rain use efficiency (RUE) is usually used as a proxy for WUE due to lack of transpiration data. Furthermore, RUE based on aboveground primary productivity (RUE_ANPP) is used to evaluate whole plant water use because root production data is often missing as well. However, it is controversial as to whether RUE is a reliable parameter to elucidate transpiration efficiency (TE), and whether RUE_ANPP is a suitable proxy for RUE of the whole plant basis. The experiment was conducted at three differently managed sites in the Inner Mongolia steppe: a site fenced since 1979 (UG79), a winter grazing site (WG) and a heavily grazed site (HG). Site HG had consistent lowest RUE_ANPP and RUE based on total net primary productivity (RUE_NPP). RUE_ANPP is a relatively good proxy at sites UG79 and WG, but less reliable for site HG. Similarly, RUE_ANPP is good predictor of transpiration efficiency based on aboveground net primary productivity (TE_ANPP) at sites UG79 and WG but not for site HG. However, if total net primary productivity is considered, RUE_NPP is good predictor of transpiration efficiency based on total net primary productivity (TE_NPP) for all sites. Although our measurements indicate decreased plant transpiration and consequentially decreasing RUE under heavy grazing, productivity was relatively compensated for with a higher TE. This offset between RUE and TE was even enhanced under water limited conditions and more evident when belowground net primary productivity (BNNP) was included. These findings suggest that BNPP should be considered when studies fucus on WUE of more intensively used grasslands. The consideration of the whole plant perspective and “real” WUE would partially revise our picture of system performance and therefore might affect the discussion on the C-sequestration and resilience potential of ecosystems.</p></div

Transpiration efficiency (TE, g m⁻²mm⁻¹) based on aboveground-(ANPP) and net primary productivity (NPP) of the three grassland sites in 2004, 2005 and 2006.

<p>Significant differences in TE_ANPP and TE_NPP among years within a site are indicated by letters a-c and significant differences among sites within a year by letters x–z, respectively. Error: ± SE. Symbols for different sites as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-t001" target="_blank">Table 1</a>.</p

Relationship between RUE_ANPP (g m⁻²mm⁻¹) and TE_ANPP (g m⁻²mm⁻¹).

<p>Site UG79 (A), site WG (B), and site HG (C). The data for each site were from 2004, 2005 and 2006. The regression equations for UG79: y = 0.040+0.438 x, r² = 0.9446, n = 15, P<0.001; WG: y = 0.024+0.393 x, r² = 0.9387, n = 15, P<0.001; HG: y = 0.160+0.158 x, r² = 0.4750, n = 15, P = 0.004. TE_ANPP = Transpiration efficiency based on aboveground net primary productivity. Same symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-g002" target="_blank">Figure 2</a>.</p

Relationship between RUE_NPP (g m⁻²mm⁻¹) and TE_NPP (g m⁻²mm⁻¹).

<p>Site UG79 (A), site WG (B), and site HG (C). The data for each site were from 2004, 2005 and 2006. The regression equations for UG79: y = 0.177+0.406 x, r² = 0.9542, n = 15, P<0.001; WG: y = 0.084+0.370 x, r² = 0.9626, n = 15, P<0.001; HG: y = 0.2734+0.176 x, r² = 0.8055, n = 15, P<0.001. RUE_NPP = rain-use efficiency based on total net primary productivity; TE_NPP = transpiration efficiency based on total net primary productivity. Same symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-g002" target="_blank">Figure 2</a>.</p

Differences between grassland sites and years in rain-use efficiency (RUE, g m⁻²mm⁻¹) based on aboveground-(ANPP) and net primary productivity (NPP) in 2004, 2005 and 2006.

<p>Significant differences in RUE_ANPP and RUE_NPP among years within a site are indicated by letters a-c and significant differences among sites within a year by letters x–z, respectively. Error: ± SE. Symbols for different sites as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-t001" target="_blank">Table 1</a>.</p

Relationship between RUE_ANPP (g m⁻²mm⁻¹) and RUE_NPP (g m⁻²mm⁻¹).

<p>The data were from three differently managed sites in 2004, 2005 and 2006. The regression equations for UG79: y = 0.025+0.391 x, r² = 0.673, n = 15, P<0.001; for WG: y = 0.112+0.326 x, r² = 0.742, n = 15, P<0.001; for HG: y = 0.213+0.201 x, r² = 0.306, n = 15, P = 0.029. UG79 =  ungrazed site since 1979; WG = winter grazing site; HG = heavy grazing site; RUE_ANPP = rain-use efficiency based on aboveground net primary productivity; RUE_NPP = rain-use efficiency based on total net primary productivity.</p

Carbon isotope discrimination (δ¹³C ‰) (mean ± SE) of dominant species at three differently managed grassland sites in July 2005.

<p>– not found.</p><p>Lc = <i>Leymus chinensis</i>; Sg = <i>Stipa grandis</i>; Cs = <i>Cleistogenes squarrosa</i>; Pa = <i>Potentilla acauli</i>; Af = <i>Artemisia frigida</i>. Kruskal-Wallis test was performed to show the significant difference among sites. Different small letters (a–c) indicate significant differences among sites within one species. Symbols for different sites as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-t001" target="_blank">Table 1</a>.</p

Correlation between transpiration efficiency and carbon stable isotope discrimination.

<p>The data were from the vegetation growth period of 2005 at three differently managed sites. Same symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-g002" target="_blank">Figure 2</a>.</p

Relative distribution of root dry mass (%) in soil depths 0–100 cm.

<p>2004 (A), 2005 (B), and 2006 (C). The data of each year were from three differently managed sites. Error bars represent mean ± SE. Same symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074841#pone-0074841-g002" target="_blank">Figure 2</a>.</p