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ISSN: 2157-7625
Journal of Ecosystem & Ecography
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Leaf Photosynthesis and Plant Competitive Success in a Mixed-grass Prairie: With Reference to Exotic Grasses Invasion

Dong X1,2*, Patton J2, Gu L3, Wang J4 and Patton B2

1Texas A&M AgriLife Research and Extension Center, Uvalde, Texas-78801, USA

2North Dakota State University, Central Grasslands Research Extension Center, Streeter, N.D. 58483, USA

3Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee-37831, USA

4Chinese Research Academy of Environmental Sciences, No. 8 Dayangfang, Beiyuan, Chaoyang District, Beijing- 100012, China

*Corresponding Author:
Xuejun Dong
Assistant Professor of Crop Physiology
Texas A&M AgriLifer Research and Extension Center, Uvalde, Texas-78801, USA
Tel: 830-278-9151
E-mail: [email protected]

Received date: October 19, 2014; Accepted date: November 10, 2014; Published date: November 26, 2014

Citation: Dong X, Patton J, Gu L, Wang J, Patton B (2014) Leaf Photosynthesis and Plant Competitive Success in a Mixed-grass Prairie: With Reference to Exotic Grasses Invasion. J Ecosys Ecograph 4:152. doi:10.4172/2157-7625.1000152

Copyright: 2014 Dong X, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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The widespread invasion of exotic cool-season grasses in mixed-grass rangeland is diminishing the hope of bringing back the natural native plant communities. However, ecophysiological mechanisms explaining the relative competitiveness of these invasive grasses over the native species generally are lacking. We used experimental data collected in south-central North Dakota, USA to address this issue. Photosynthetic potential was obtained from the net assimilation (A) vs. internal CO2 (Ci) response curves from plants grown in a greenhouse. Plant success was defined as the average frequency measured over 25 years (1988 to 2012) on overflow range sites across five levels of grazing intensity. Also, estimated leaf area index of individual species under field conditions was used to indicate plant success. The correlation between photosynthetic potential based on A/Ci curves and plant frequency was negative. The correlation between leaf photosynthesis and plant success (defined as leaf area within a unit land area) was also negative, although statistically weak. These results suggest that the two cool-season grasses, Poa pratensis and Bromus inermis, do not rely on superior leaf-level photosynthesis for competitive success. Instead, some other traits, such as early and late-season growth, may be more important for them to gain dominance in the mixed-grass prairie. We propose that the negative photosynthesis-frequency relation as observed in this study results from a strong competition for limited soil nutrients in the mixed-grass prairie. It has implications for the stability and productivity of the grassland under various human disruptions influencing the soil nutrient status.


Bromus inermis; Invasive species; Leaf area index; Photosynthetic potential; Poa pratensis; Principal component analysis; Rangeland management


Worldwide, invasive plants pose profound impacts on plant communities and ecosystem function [1]. In the mixed-grass prairie of North America, the widespread invasion of exotic cool-season grasses, such as Poa pratensis L. and Bromus inermis Leyss., is diminishing the hope of bringing back the natural native plant communities [2-3]. A best management strategy for these rangelands, based on several plant community studies [4-6], is perhaps to utilize these cool-season exotics while reducing their cover and increasing diversity. However, knowledge of the ecophysiological mechanisms that explain the relative competitiveness of these cool-season invasive grasses over the native species generally is lacking [7].

Photosynthesis is the basis of plant growth and production. Although plant productivity is strongly associated with integrated seasonal photosynthetic activity [8], it is difficult to quantify seasonal photosynthesis simultaneously for many plant species. Leaf-level instantaneous photosynthesis is frequently measured [9-11], despite the difficulty of obtaining consistent results contrasting the native and invasive species [12,13]. Leaf-level photosynthesis was positively related to plant success in the C4 grass-dominated tallgrass prairie [14]. Whether this similar broad comparison exists in the highly invaded mixed-grass rangelands has important management implications because cool-season exotic grasses have already dominated these rangelands both in terms of herbage production [15] and cover [4]. This paper compares leaf-level photosynthesis of 26 plant species cooccurring in the mixed-grass prairie near Streeter, N.D. USA. The highlight is on the correlations between photosynthetic potential and plant success, with special reference to exotic invasive grasses Poa pratensis and Bromus inermis.

Materials and Methods

Field site and plant frequency data collection

The field site is located at the Central Grasslands Research Extension Center – NDSU near Streeter, N.D. USA. The climate is of the continental type with January mean temperature of -17ºC and August mean temperature of 20ºC. Mean annual precipitation is 458 mm, 70 percent of which occurs from May to September. The vegetation is dominated by grasses, followed by forbs (20 percent aboveground production) and shrubs (two percent) [16]. The rangeland had not been plowed for at least 50 to 60 years. Starting in 1988, a long-term cattle grazing intensity study was initiated on a land area of 156 ha, which was subdivided into 12 equal-size pastures stocked at four levels of season-long grazing (light, moderate, heavy and extremely heavy grazing) with yearling beef cattle. The light treatment left 65 percent of forage produced in an ‘‘average year’’ remaining at the end of the grazing season, moderate treatment left 50 percent, heavy treatment left 35 percent, and extreme treatment left 20 percent [15]. Frequency of occurrence of all plant species was monitored each year since 1988 in fifty 25 cm×25 cm frames along a 50 m transect on each site. Species frequency was defined as the number of frames in which the species occurred divided by the total number of frames used [17]. Data collected from the overflow range site and all grazing intensities combined [15] were used in this study because plants growing on this range site presumably experienced less resource stress and thus the measured frequency would reflect the species’ intrinsic characteristics.

Photosynthesis/intercellular CO2 curves measured in greenhouse

Seeds of 12 species, both native and introduced, were collected during the summer and fall of 2011 on native prairies of the Central Grasslands Research Extension Center near Streeter, N.D. On 20 Apr 2012, seeds were sown in plastic pots (7.6 cm×7.6 cm×7.6 cm) in replicates of four, using a local prairie soil of the Towner-Barnes complex (black loamy fine sand) that was sifted before use. Root cuttings of two shrubs and transplants of 12 more species were planted in pots in May and June, 2012. Pots were placed in a greenhouse and watered as needed. The plant species measured were all C3 species and included five grasses, one sedge, 18 forbs, and two shrubs (Table 1). The photosynthesis/intercellular CO2 (A/Ci) curve measurements were made using a LI-COR 6400 Portable Photosynthesis System (LICOR Bioscience, Lincoln, N.E. USA) from 7 Jul through 13 Aug 2012, with four replicates of each species. The chamber condition was set as: photosynthetic photon flux density (PPFD) 900 μmol m-2s-1; leaf temperature 28ºC; relative humidity 26 to 29 mmol H2O mol-1; and flow rate 300 mol s-1. The reference CO2 was varied in 15 or 16 steps from 0 to 1500 ppm. The A/Ci curves were fitted to the FvCB model according to [18] and implemented through The A/Ci parameters were scaled to the standard values at 25ºC and reported together with a global scale of datasets of A/Ci curves [19]. This paper provides additional analysis of this data set from North Dakota, focusing on the linkage between photosynthesis and plant competitive success.

Scientific name Abbreviation Longevity/Functional group Native status Common name
AchilleamillefoliumL. Achmil perennial forb native common yarrow
Ambrosia psilostachyaDC. Ambpsi perennial forb native western ragweed
AntennarianeglectaGreene Antneg perennial forb native field pussy-toes
Artemisia absinthiumL. Artabs perennial forb/ subshrub introduced wormwood, absinthium
Artemisia frigidaWilld. Artfri perennial subshrub native fringed sagewort, prairie sagewort
Artemisia ludovicianaNutt. Artlud perennial forb native white sage, cudweed sagewort
BromusinermisLeyss. Broine perennial grass introduced smooth brome
CarexinopsL.H. Bailey ssp. heliophila(Mack.) Crins [CarexheliophilaMack.] Carhel perennial sedge native sun sedge
Cirsiumarvense(L.) Scop. Cirarv perennial forb introduced Canada thistle
Cirsiumflodmanii(Rydb.) Arthur Cirflo perennial forb native Flodman’s thistle
Elymusrepens(L.) Gould [AgropyronrepensL.] Agrrep perennial grass introduced quackgrass
GeumtriflorumPursh Geutri perennial forb native prairie smoke, torch flower
Grindelia squarrosa(Pursh) Dunal Grisqu biennial/ perennial forb native curly-cup gumweed
Helianthus pauciflorusNutt. ssp. pauciflorus[Helianthus rigidus(Cass.) Desf.] Helrig perennial forb native stiff sunflower
Melilotusofficinalis(L.) Lam. Meloff annual/biennial forb introduced yellow sweetclover
Nassellaviridula(Trin.) Barkworth [StipaviridulaTrin.] Stivir perennial grass native green needlegrass
Oligoneuronrigidum(L.) var. rigidum[SolidagorigidaL.] Solrig perennial forb native stiff goldenrod
Oxalis strictaL. Oxastr annual/ perennial forb native yellow wood sorrel,common yellow oxalis
Pascopyrumsmithii(Rydb.) Á. Löve [AgropyronsmithiiRydb.] Agrsmi perennial grass native western wheatgrass
PoapratensisL. Poapra perennial grass native/ introduced Kentucky bluegrass
Rosa arkansanaPorter Rosark shrub native prairie rose
SolidagocanadensisL. Solcan perennial forb native Canada goldenrod
SolidagomissouriensisNutt. Solmis perennial forb native Missouri goldenrod
SymphoricarposoccidentalisHook. Symocc shrub native western snowberry, buckbrush
Symphyotrichumericoides(L.) G.L. Nesom var. ericoides [Aster ericoidesL.] Asteri perennial forb native white aster, heath aster
TaraxacumofficinaleF.H. Wigg Taroff perennial forb native/ introduced common dandelion

Table 1: List of C3 species used in the photosynthesis-CO2 response curve measurement.

The pots were overwintered by placing them in a shallow trench located on a sloping field on 8 Nov 2012. The tops of the pots were level with the soil surface, with soil around each tray of pots. They then were covered with an eight-inch thick layer of straw fastened down by a wire mesh. The pots were lifted on 5 May 2013, returned to the greenhouse and watered when needed. The photosynthesis survey of the major species was conducted from 11 Jun to 19 Jun 2013 under conditions similar to that for the A/Ci curve measurement survey in 2012. Leaf samples were collected on 25 June 2013 for total nitrogen content analysis. Also, specific leaf area was measured in order to convert leaf nitrogen content from the dry matter to leaf area basis.

Leaf photosynthetic rates measured under field conditions

Leaf photosynthetic rates for a wide range of plant species were measured on 13 days in 2008 (from 18 Jun to 18 Sep), 12 days in 2009 (1 Jun to 24 Sep) and eight days in 2010 (19 May to 21 Sep). The measurements were made within two 10 m×10 m exclosures located in one moderately grazed pasture and one extremely heavy grazing pasture. The vegetation within the exclosure was clipped to mimic the removal of herbage by cattle grazing. Photosynthesis measurements were made on clear or mostly clear days from 9 AM to 12 PM at a leaf temperature of 16.4 to 32.9ºC, PPFD of 500 to 1330 μmol m-2 s-1, and relative humidity of 28.3 to 65.5 percent. Measurements made on days when the volumetric soil water content at 23 cm depth of soil was lower than 0.2 (permanent wilting point in this prairie soil) are not included as we are interested in photosynthesis under less stressful conditions. During each of the three years, 16 to 20 plant species were measured in the exclosures (with each species measured four to 20 times). Grand averages of photosynthetic rates for each species under different grazing pressures were used in further analysis linking photosynthesis with leaf area index.

Leaf area index for individual plant species

A leaf area survey was conducted monthly four times during the growing season, May to September, for three years (2009 through 2011). A 1-m2 frame was placed on two designated plots in the vicinity of the photosynthesis measurements. Four subplots within each frame were delineated and the percent contribution of total leaf area to the whole frame was noted for each. Separately, each subplot was visually surveyed to estimate leaf area by species. For each subplot, the value of each species’ contribution was adjusted to total 100 percent. The percent value of each species was then multiplied by the contribution of the subplot to the total frame. This information was combined with the total leaf area index within the 1-m2 frame measured using a LP- 80 Ceptometer (Decagon Devices, Pullman, Wash. USA) to obtain the total leaf area of the frame by species.

Data analysis

The estimated eight parameters (Table 1) were subjected to principal component analysis (PCA) to identify the general trend of the multi-dimensional data sets. Differences in photosynthetic parameters among plant species were tested using one-way analysis of variance (ANOVA). As ‘photosynthetic capacity’ is often used to refer to maximum photosynthetic rates under no resource limitations, we denote the PCA-summarized variable as ‘photosynthetic potential.’ This is correlated with the long-term averaged frequency data for the 26 species. Also, grand averages of photosynthetic rates of dominant species were correlated with the corresponding leaf area indices estimated from field surveys from the same location. Data of leaf area index were square root transformed in order to meet the assumptions of the correlation analysis.

Results and Discussion

The first axis of PCA (Figure 1 - horizontal) is correlated with photosynthetic carboxylation (Vcmax) and electron transport (Jmax) and the second (vertical axis) with the mesophyll conductance (Gmeso) (Table 2). Six forbs (five from the Asteraceae family and one legume) had a greater intrinsic photosynthetic capacity than the remaining species, which occupied the middle or lower range of variation. Located at the lowest end of the horizontal axis were Poa pratensis, Bromus inermis and Taraxacum officinale F.H. Wigg. It appears that the maximum carboxylation rate (reflecting mainly the Vcmax in the first PCA axis) and the mesophyll conductance (the second PCA axis) may compensate to give similar maximum photosynthesis. In this respect, Achillea millefolium L. and Rosa arkansana Porter exhibited different patterns from other species (Figure 2). As shown in Figure 3, frequency was negatively correlated with photosynthetic potential of prairie plants, though the correlation was stronger when the information from the first two PCA axes were combined (Figure 3b, r=-0.435, p=0.026). Also, there was a positive correlation between photosynthetic rates and leaf nitrogen in these species (Figure 4). Furthermore, using our leaf photosynthesis data collected from 2008 through 2010 and leaf area index data for 25 species collected from 2009 through 2011, we observed also a negative correlation between leaf photosynthesis and plant success (here defined as leaf area within a unit land area) in both moderately and heavily grazed pastures, although the correlation was statistically weak (p=0.311 for moderate grazing and p=0.365 for heavy grazing), (Figures 5 and 6).


Figure 1: Principal component analysis (PCA) of seven parameters (Vcmax, Jmax, Rdlight, Gmeso, Γ*, Γ, and Anet) of the FvCB model based on the automated analysis scheme available at Abbreviations of plant species are defined in Table 1 and the abbreviations of parameters are defined in Table 2. The horizontal and vertical axes stand for the first and second axes of PCA, respectively, which account for 70 percent of the variance of the seven original parameters.


Figure 2: Relationships between maximum carboxylation limited photosynthesis (Vcmax) and mesophyll conductance (Gmeso) (top panel) and electron transport limited photosynthesis (Jmax) and mesophyll conductance (Gmeso) (lower panel).


Figure 3: Correlation of plant frequency and photosynthetic potential defined as the first axis of the principal component analysis (PCA) of the seven photosynthetic parameters (a) and the combination of the first and second axes of the PCA analysis (b) for 26 C3 plant species found in the mixed-grass prairie.


Figure 4: Relationship between photosynthetic rates and leaf nitrogen contents for prairie plants measured under common greenhouse conditions.


Figure 5: Correlation between photosynthetic rates and the square root of leaf area index of 16 plant species growing in a moderately grazed pasture. Photosynthetic rates were averaged based on measured values on 33 clear to mostly clear days during the growing seasons from 2008 to 2010, and leaf area indices were averaged values based on field surveys conducted in the same location from 2009 to 2011. Abbreviations of plant species are defined in Table 1, as well as: Psoarg=Pediomelum argophyllum (Pursh) J. Grimes [Psoralea argophylla Pursh] and Solmol=Solidago mollis Bartl.


Figure 6: The same as in Figure 5 except that the measurements were made on 20 species in a heavily grazed pasture. Abbreviations of plant species are defined in Table 1, as well as: Astagr = Astragalus agrestis Douglas ex G. Don., Lacobl =Lactuca tatarica (L.) C.A. Mey. var. pulchella (Pursh) Breitung [Lactuca oblongifolia Nutt.], Ratcol=Ratibida columnifera (Nutt.) Woot. & Standl., and Vicame=Vicia americana Muhl. ex Willd. et al.

Parameter1 PC1 (48.4%) PC2 (21%) PC3 (16%)
Vcmax 0.76*2 -0.05 0.48*
Jmax 0.88* -0.36 -0.07
Rdlight -0.22 -0.89* 0.09
Gmeso 0.51* 0.53* 0.56*
Γ* -0.51* -0.26 0.72*
Γ -0.87* -0.14 0.22
Anet 0.84* -0.43* -0.01

Table 2: Correlation coefficients between the first three axes of the principal component analysis (PCA) and the seven photosynthetic parameters used in the PCA analysis. The analysis was based on the averaged data from photosynthesisintercellular CO2 concentration curves for 26 C3 rangeland plant species. Measurements were made at 28ºC, but the parameters were scaled to 25ºC. The percentages of variance of the original seven parameters explained by the PCA axes are included in the parentheses.

Averaged over the growing season from May to September, the leaf area indices of Poa pratensis and Bromus inermis combined accounted for 46 percent and 24 percent of the total LAI in plots of moderately and heavily grazed pastures, respectively (Tables 3 and 4). In terms of leaf nitrogen on ground area basis, the two exotic species combined represented 30 percent and 15 percent of the total leaf nitrogen of the plant communities under the two grazing treatments.

Species ǂLAI-2009 LAI-2010 LAI-2011 LAI-mean N (g/m2 ground area)§
Bromusinermis 0.3233 0.5416 0.6826 0.5158 0.2182
Poapratensis 0.3081 0.4069 0.5165 0.4105 0.1908
Helianthus pauciflorus 0.1174 0.1496 0.1976 0.1548 0.1626
Symphoricarposoccidentalis 0.1746 0.196 0.2518 0.2075 0.1106
Aster ericoides 0.115 0.1257 0.1494 0.13 0.1029
Rosa arkansana 0.0882 0.1168 0.1618 0.1223 0.0834
Oligoneuronrigidum 0.0941 0.1178 0.1654 0.1258 0.0782
Melilotusofficinalis 0.0316 0.0933 0.1355 0.0868 0.0672
Artemisia ludoviciana 0.0907 0.0941 0.1242 0.103 0.0537
Cirsiumflodmanii 0.0301 0.042 0.0774 0.0499 0.0258
Solidagomissouriensis 0.0237 0.0053 0.0299 0.0196 0.0122
Solidagomollis* 0 0.0152 0.0259 0.0137 0.0090
Tragopogondubius* 0.0047 0.0187 0.0101 0.0111 0.0073
Lotus purshianus* 0 0 0.0305 0.0102 0.0067
Stipaviridula 0.0021 0 0.0137 0.0053 0.0045
Solidagocanadensis 0.0179 0 0.0023 0.0067 0.0043
Anemone cylindrica* 0.0005 0.0044 0.0127 0.0059 0.0039
Psoraleaargophylla* 0.0019 0 0.0119 0.0046 0.0030
Asclepiasovalifolia* 0 0 0.0117 0.0039 0.0026
Cirsiumarvense* 0.0045 0.003 0.0044 0.004 0.0025
Achilleamillefolium 0.0032 0.0038 0.0068 0.0046 0.0022
Carexspp. 0.0042 0.0023 0.001 0.0025 0.0020
Phleum pretense* 0 0 0.0066 0.0022 0.0014
Taraxacumofficinale 0.0025 0 0 0.0008 0.0003
Total 1.4383 1.9365 2.6297 2.0015 1.3156

Table 3: Leaf area indices (LAI) and leaf nitrogen per unit ground area for 24 plant species found in a moderately grazed pasture in the mixed-grass prairie near Streeter N.D., USA.

Species ǂLAI-2009 LAI-2010 LAI-2011 LAI-mean N (g/m2ground area)§
Artemisia ludoviciana 0.0373 0.0788 0.1524 0.0895 0.0467
Poapratensis 0.0321 0.0766 0.1665 0.0917 0.0426
Solidagomissouriensis 0.0452 0.0665 0.0749 0.0622 0.0387
Bromusinermis 0.0301 0.0707 0.1337 0.0782 0.0331
Stipaviridula 0.0128 0.0245 0.0502 0.0292 0.0250
Aster ericoides 0.0115 0.0253 0.0407 0.0259 0.0205
Grindelia squarrosa 0.0353 0.047 0.0456 0.0426 0.0184
Oligoneuronrigidum 0.0111 0.0322 0.0328 0.0253 0.0157
Carexspp. 0.0199 0.019 0.0155 0.0181 0.0145
Achilleamillefolium 0.008 0.0194 0.0584 0.0286 0.0138
Taraxacumofficinale 0.0188 0.0367 0.0394 0.0317 0.0124
Geumtriflorum 0.0038 0.0129 0.0209 0.0125 0.0118
Boutelouagracilis* 0.0081 0.0155 0.0286 0.0174 0.0114
Artemisia frigida 0.0134 0.0134 0.0158 0.0142 0.0111
Symphoricarposoccidentalis 0.0128 0.018 0.0255 0.0188 0.0100
Cerastiumarvense 0.0054 0.0186 0.0176 0.0139 0.0089
Antennarianeglecta 0.0088 0.0143 0.0265 0.0165 0.0077
Oxalis stricta* 0 0.0035 0.0309 0.0115 0.0076
Dichantheliumwilcoxianum* 0.0043 0.004 0.0244 0.0109 0.0072
Medicagolupulina* 0.0017 0.0104 0.0187 0.0103 0.0068
Ratibidacolumnifera* 0.0018 0.0075 0.0202 0.0098 0.0064
Stipacurtiseta* 0 0.0151 0.0112 0.0088 0.0058
Cirsiumflodmanii 0 0.0069 0.0252 0.0107 0.0055
Astragalusflexuosus* 0.0026 0 0.0198 0.0074 0.0049
Astragalusagrestis* 0.0026 0.0121 0.0071 0.0073 0.0048
Anemone cylindrica* 0.0028 0.0042 0.0143 0.0071 0.0047
Viola pedatifida* 0.001 0.0043 0.0137 0.0063 0.0041
Lactucatatarica* 0.0006 0.0014 0.006 0.0027 0.0018
Sphaeralceacoccinea* 0.0011 0.0031 0.0028 0.0023 0.0015
Viciaamericana* 0 0.0003 0.0027 0.001 0.0007
Potentillapensylvanica* 0.0003 0 0.0024 0.0009 0.0006
Pascopyrumsmithii 0 0 0.0017 0.0006 0.0005
Ambrosia psilostachya 0.0008 0.0003 0 0.0004 0.0003
Euphorbia serpyllifolia* 0 0.0013 0 0.0004 0.0003
Helianthus pauciflorus 0 0.0005 0 0.0002 0.0002
Agrostishyemalis* 0 0.0009 0 0.0003 0.0002
Onosmodium, molle* 0 0.0008 0 0.0003 0.0002
Polygonum convolvulus* 0 0.0001 0 0 0.0000
 Total 0.3340 0.6661 1.1461 0.7155 0.4703

Table 4: Leaf area indices (LAI) and leaf nitrogen per unit ground area for 38 plant species found in a heavily grazed pasture in the mixed-grass prairie near Streeter N.D., USA.

Past studies indicate that photosynthetic capacities of prairie forbs when compared to grasses are similar [20], higher [21] or lower [14,22]. Our results suggest that photosynthetic capacity of prairie forbs is widely variable, which is not surprising considering the number of plant families and life forms represented. The negative correlation between photosynthesis and plant frequency as observed in our study contrasts the findings of McAllister et al. [14] done in a highly productive tallgrass prairie. We speculate that the negative photosynthesis-plant success relationship as found in our study might indicate an outcome of severe interspecific competition for soil resources in the mixed-grass prairie.

The data from this study has several implications for understanding plant community and ecosystem functions in the mixed-grass prairie. First, the negative correlation between leaf photosynthesis and plant success suggests that the instantaneous photosynthetic characteristics measured at leaf level can only represent shortly-lived trends of leaf carbon gain, which cannot portray the overall photosynthetic capacity of the plant species during the entire growing season. Our measurements were conducted in the warm months, and did not capture photosynthetic activity in early spring and late fall when Poa pratensis and Bromus inermis maintain photosynthesis and growth while many other species in the prairie are dormant. Taking advantage of ample spring soil water for rapid growth is especially important for the competitive success of these two exotic species. Second, maintaining a low photosynthetic rate for the most abundant species also reduces excessive depletion of soil resources, and thus favors a more diverse plant community. This can also be seen from the peak-season value of the ground area-based nitrogen for each species co-occurring in the moderate and heavy grazing pastures (Tables 3 and 4). We can see that on a unit ground area basis on this prairie, these two exotic species are strong competitors for nitrogen, especially in the moderately grazed pasture. This occurred despite their low nitrogen content and low photosynthetic rate per unit leaf area.

Soil in this mixed-grass prairie typically is low in nitrogen (N) and phosphorus (P) and fertilization of N and P can increase forage production [23]. At the same time, high N application tends to favor exotic grasses such as Poa pratensis and reduce plant diversity, depending on stocking history [24]. Studies in the tallgrass prairie [25,26] also demonstrate that nitrogen addition may encourage native warm-season grasses to be displaced by exotic cool-season grasses such as Poa pratensis and Elymus repens (L.) Gould. It has been estimated that atmospheric nitrogen deposition due to burning of fossil fuels, crop fertilization and intensive livestock operations may increase plant N supply by 10-25 percent in the tallgrass prairie [27]. The relative contribution of nitrogen deposition may be higher in the mixed-grass prairie, perhaps as high as 50 percent, considering an estimated annual N mineralization rate of 1.2 g m-2 [28] and an annual total atmospheric N deposition of 0.6 g m-2 based on data of NADP for the year 2000. Bromus inermis and Poa pratensis, which are considered invasive [3] and investing larger proportion of its production to belowground biomass compared with co-occurring natives [29], have been shown to increase in both biomass production and cover on the silty range sites of the mixed-grass prairie over past 26 years, especially under rest and light grazing situations [15]. Wedin and Tilman [27] used a positive feedback model to describe the interaction between tallgrass prairie natives and soil nitrogen cycling. Perhaps the same framework can be used to speculate why in the mixed-grass prairie Poa pratensis and Bromus inermis have extensively displaced native grasses and forbs may partly be due to the low photosynthesis/low nitrogen, and low litter quality nature of these exotic species. Nevertheless, as the positive feedback system is inherently unstable, the future of the mixed-grass prairie will to a substantial extent be influenced by how the plant species, especially those representing a large fraction of LAI and per land area based nitrogen, such as Poa pratensis and Bromus inermis, allocate nitrogen among different organs and adjust their physiology strategies in response to various disturbances, including atmospheric N deposition at regional scale spanning tallgrass and mixed-grass prairies.

In addition to nitrogen deposition, other global change factors, such as increases in atmospheric CO2 concentration and temperature, may have differential impacts to exotic vs. native species. A synthesis of available data suggests that invasive, noxious weeds have a larger than expected growth increase to both recent and projected atmospheric CO2 increase as compared to native plant species [30]. Actually, it has been shown that, in Poa pratensis, increased atmospheric CO2 concentration not only mitigated high temperature stress on net carbon gain, but also significantly increased root growth [31]. It will be interesting to test experimentally if this CO2-related growth enhancement would increase the invasive potential of this and related species. Although our data showed a relative low photosynthetic potential in several invasive plants (Figure 1 and Table 2), the low mesophyll conductance measured in several invasive plants (Supplementary Table 1) suggests potentials for photosynthesis to increase under high atmospheric CO2. To what extent this will be integrated into whole plant growth and competitive success is yet to be seen using manipulative experiments contrasting the invasive against the native plants, for which comprehensive data is merger yet critically important for understanding the integrity of rangeland ecosystems under future climate and land use changes [30]. As the invasion and naturalization of exotic plant species is influenced by not only biophysical but also social economic factors, including global trade [32], it will be a challenging task to constantly contain the vigor of the exotic species using a combination of tools, such as early grazing and prescribed fire targeted at the most vulnerable growth stages while maintaining the integrity of native species [4-6,33,34]. This highlights the importance of integrated, transdisciplinary studies for the rehabilitation of degraded rangelands [35].


This paper was part of North Dakota Agricultural Experiment Station projects ND6146, ND6147 and ND6149. We appreciate editors and reviewers for valuable suggestions that helped to improve the paper.


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