Discussion

Bakker and Wilson (2001) found that, in a mixed prairie of Saskatchewan, Canada, the invasive species crested wheatgrass [Agropyron cristatum (L.) Gaertn.]

164 Стр плюс рисунок??????????? Доделать.

had a greater competitive ability than the native blue grama [Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths] because the former had a higher biomass allocation to roots than the latter. This same trend in root biomass allocation between the invasive and native grasses was found in our study, suggesting that the invasive grasses are physiologically primed for exploiting soil resources through a heavy investment of biomass to roots. In Kentucky bluegrass, with a shallow rooting habit (Gist and Smith, 1948; Peterson et al., 1979; Jiang and Huang, 2001) and sensitivity of stomatal conductance to developing water stress (Dong et al., 2011), the large biomass investment to roots means a high root density in the shallow soil layer. It has been observed that one salient feature of Kentucky bluegrass dominance in rangeland is the retardation of water infiltration to the deep soil profile due to the dense Kentucky bluegrass root mat (Dekey- ser et al., 2009). In our study, after 3 months of estab- lishment, the largest root system (both in terms of absolute root mass per pot and the fractional alloca- tion) occurred in smooth brome. This may render smooth brome the high capacity of drought resistance (Gist and Smith, 1948). On the other hand, both native grass species had lower biomass allocation to roots compared with two invasive species. The above discussion suggests that the two invasive species employ a higher competitive ability in terms of root growth and potential water uptake capacity, as compared to their native counterparts. Contrary to our original expectation, neither allocation to roots nor allocation to shoots was affected significantly by drought stress for all species (P > 0 05) except in root

allocation of green needlegrass, where severe drought marginally increased the biomass allocation to roots (P = 0 053; see Figure 1). While the highest rhizome allocation observed in Kentucky bluegrass may contribute to its ability of aggressive lateral expansion, the fact that the ‘strongly rhizomatous’ smooth brome (Sedivec et al., 2001) did not grow rhizomes (as observed in both the 2010 and 2011 experiments) in the first season’s growth does not explain the aggressive invading characteristic of this species in the mixed-grass prairie (Murphy and Grant, 2005; Dekeyser et al., 2010). Further study is needed to ascertain whether the competitive ability of the invasive smooth brome may not manifest itself fully in one season (perhaps rhizomes develop after seed set), or the smooth brome as used in our study happened to be a ‘bunch’ type, as with some varieties of the species (Cook, 1943). Our result that the effect of drought basically did not have a measurable impact to the biomass alloca- tion in roots or shoots in the studied grasses (except in green needlegrass where there was a marginal increase in allocation to roots with drought stress) agrees with the finding of Bakker and Wilson (2001). Similarly, Perkins and Owens (2003) found that the shoot/root biomass allocation in shrub and grass seedlings had limited response to the change in precipitation season- ality. Our data indicate that during the first few months of establishment, the effect of drought stress on seedling biomass allocation manifested itself more on rhizomes or crowns than on roots or shoots of the four grass species. Related to this, a study of five rhi- zomstronger response in rhizome than in the total biomass growth in response to clipping. It appears in our study that drought stress can weaken the vegetative growth potential of both the invasive Kentucky bluegrass and the native western wheatgrass. However, the mixed responses to drought among the four species suggest species-specific nature of responses. Similar to the relatively shallow-rooted Kentucky bluegrass that can alleviate the adverse effect of local- ized surface soil drying by setting roots to deeper wet soil profile (Huang and Fu, 2000; Dacosta et al., 2004), deep-rooted grasses, such as western wheatgrass and green needlegrass (Coupland and Johnson, 1965), may employ more profound capability to cope with drought stress under field conditions. These varied degrees of water stress compensation, however, render difficulty in comparing drought stress responses across species. The greenhouse pot experiment as used in our study, though impeding full rooting depth growth due to the confinement of pots, provided a simple yet con- sistent experimental system for comparing biomass allocation across different species from the whole plant perspective. In conclusion, our work suggests a clear distinction between the two invasive and two native species in water use, as seen in the root/shoot allocation and allocation to crowns in seedling stage. However, we did not find a consistent trend in the response of bio- mass allocation to drought that differentiates the inva- sive species from the native species. Instead, similar to the findings of Smith and Knapp (2001), the responses from our study either show similar response between the two groups of grasses or with some mixed responses that were species specific. Further studies spanning multiple years and using developed tillers are needed to compare these perennial grasses in their biomass allocation.atous grasses by Esmaeili et al. (2009) suggested a

stronger response in rhizome than in the total biomass growth in response to clipping. It appears in our study that drought stress can weaken the vegetative growth potential of both the invasive Kentucky bluegrass and the native western wheatgrass. However, the mixed responses to drought among the four species suggest species-specific nature of responses. Similar to the relatively shallow-rooted Kentucky bluegrass that can alleviate the adverse effect of local- ized surface soil drying by setting roots to deeper wet soil profile (Huang and Fu, 2000; Dacosta et al., 2004), deep-rooted grasses, such as western wheatgrass and green needlegrass (Coupland and Johnson, 1965), may employ more profound capability to cope with drought stress under field conditions. These varied degrees of water stress compensation, however, render difficulty in comparing drought stress responses across species. The greenhouse pot experiment as used in our study, though impeding full rooting depth growth due to the confinement of pots, provided a simple yet con- sistent experimental system for comparing biomass allocation across different species from the whole plant perspective. In conclusion, our work suggests a clear distinction between the two invasive and two native species in water use, as seen in the root/shoot allocation and allocation to crowns in seedling stage. However, we did not find a consistent trend in the response of bio- mass allocation to drought that differentiates the inva- sive species from the native species. Instead, similar to the findings of Smith and Knapp (2001), the responses from our study either show similar response between the two groups of grasses or with some mixed responses that were species specific. Further studies spanning multiple years and using developed tillers are needed to compare these perennial grasses in their biomass allocation.

Acknowledgments

This work was part of North Dakota Agricultural Experiment Station projects ND06146, ND6147 and ND6149. We appreciate faculty and staff members of Central Grasslands Research Extension Center for assistance. We especially thank Bob Patton for valu- able suggestions and Chunyun Zhang for help with the experiment.

References

BAKKER J. and WILSON S. (2001) Competitive abilities of invasive and native grasses. Plant Ecology, 157, 119–127. COOK C.W. (1943) A study of the roots of Bromus inermis in relation to drought resistance. Ecology, 24, 169–192.

COUPLAND R.T. and JOHNSON R.E. (1965) Rooting characteristics of native grassland species in Saskatchewan. Journal of Ecology, 53, 475–507. CULLEN B.R., CHAPMAN D.F. and QUIGLEY P.E. (2005) Carbon resource sharing and rhizome expansion of Phalaris aquatica in grazed pastures. Functional Plant Biology, 32, 79 –85. DACOSTA M., WANG Z. and HUANG B. (2004) Physiological adaptation of Kentucky bluegrass to localized soil drying. Crop Science, 44, 1307–1314. DEKEYSER S., CLAMBEY G., KRABBENHOFT K. and OSTENDORF J. (2009) Are changes in species composition on central North Dakota rangelands due to non-use management? Rangelands, 31, 16 –19. DEKEYSER S., MEEHAN M., SEDIVEC K. and LURA C. (2010) Potential management alternatives for invaded rangelands in the Northern Great Plains. Rangelands, 32, 26–31. DONG X., ZHANG X. and YANG B. (1997) A preliminary study on the water balance for some sandland shrubs based on the transpiration measurements in field conditions. Acta Phytoecologica Sinica, 21, 208–225. DONG X., PATTON B.D., NYREN A.C., NYREN P.E. and PRUNTY L.D. (2010) Quantifying root water extraction by rangeland plants through soil water modeling. Plant and Soil, 335, 181–198. DONG X., PATTON B., NYREN P., LIMB R., CIHACEK L., KIRBY D. and DECKARD E. (2011) Leaf-water relations of a native and an invasive grass species in the mixed- grass prairie under cattle grazing. Applied Ecology and Environmental Research, 9, 311–331. ESMAEILI M.M., BONIS A., BOUZILLE ´ J.B., MONY C. and BENOT M.L. (2009) Consequence of ramet defoliation on plant clonal propagation and biomass allocation: example of five rhizomatous grassland species. Flora, 204, 25 –33. GIST G.R. and SMITH R.M. (1948) Root development of several common forage grasses to a depth of eighteen inches. Agronomy Journal, 40, 1036–1042. HESTERBERG T., MOORE D.S., MONAGHAN S., CLIPSON A. and EPSTEIN R. (2006) Bootstrap methods and permutation tests. In: Moore D.S. and McCabe G.P. (eds) Introduction to the practice of statistics, Chap 14, pp. 14.1–14.70. New York, USA: W. H. Freeman. HUANG B. and FU J. (2000) Photosynthesis, respiration, and carbon allocation of two cool-season perennial grasses in response to surface soil drying. Plant and Soil, 227, 17 –26. JIANG Y. and HUANG B. (2001) Osmotic adjustment and root growth associated with drought preconditioning- enhanced heat tolerance in Kentucky bluegrass. Crop Science, 41, 1168–1173. KINMONTH-SCHULTZ H. and KIM S.H. (2011) Carbon gain, allocation and storage in rhizomes in response to elevated atmospheric carbon dioxide and nutrient supply in a perennial C3 grass, Phalaris arundinacea. Functional Plant Biology, 38, 797–807. MOORE D.S. and MCCABE G.P. (2006) Nonparametr

Introduction to the practice of statistics, Chap 15, 5th edn. pp. 15.1–15.40. New York, USA: W. H. Freeman. MURPHY R.K. and GRANT T.A. (2005) Land management history and floristics in mixed-grass prairie, North Dakota, USA. Natural Areas Journal, 25, 351–358. PERKINS S.R. and OWENS M.K. (2003) Growth and biomass allocation of shrub and grass seedlings in response to predicted changes in precipitation seasonality. Plant Ecology, 168, 107–120. PETERSON L.A., NEWMAN R.C. and SMITH D. (1979) Rooting depth of Kentucky bluegrass sod as measured by N absorption. Agronomy Journal, 71, 490–492. REECE P.E., NIXON J.S., MOSER L.E. and WALLER S.S. (2002) Seasonal dynamics of prairie sandreed rhizome

development. Journal of Range Management, 55, 182– 187. SALA O.E., PARTON W.J., JOYCE L.A. and LAUENROTH W.K. (1988) Primary production of the central grassland region of the United States. Ecology, 69, 40 –45. SEDIVEC K.K., TOBER D.A. and BERDAHL J. (2001) Grass varieties for North Dakota. Fargo, North Dakota: NDSU- Extension Service, North Dakota State University. SMITH M.D. and KNAPP A.K. (2001) Physiological and morphological traits of exotic, invasive exotic, and native plant species in tallgrass prairie. International Journal of Plant Science, 162, 785–792. TAYLOR J.R. (1982) An introduction to error analysis. A series of books in physics. Mill Valley, CA: University Science Books.