Effect of drought on biomass allocation in two invasive and two native grass species dominating the mixed-grass prairie
Control of exotic plant species invading the native prairie relies on our understanding of the eco-physio- logical mechanisms responsible for the spread of these species as they compete with native plants for soil resources. We used a greenhouse pot experiment to study vegetative biomass allocation in response to drought stress in two exotic grass species, Kentucky bluegrass (Poa pratensis L.) and smooth brome (Bromus inermis Leyss), and two native species, western wheatgrass (Pascopyrum smithii (Rydb.) A. Lo ¨ve) and (Stipa viridula Trin.). The experi- ment was conducted over 3 months in 2010 and again in 2011 in a factorial design of four species and two drought treatments. The proportional data of bio- mass allocation to shoots, roots, rhizomes and crowns (shoot base) of grass seedlings were analysed by both the nonparametric Mann-Whitney U-test on the original data and one-way ANOVA on the arcsine- transformed data. Our data suggest a clear distinction between the two invasive and two native species in potential competitiveness in soil resource use, with the two exotic species having higher biomass alloca- tion to roots than the two native species and the native species having a higher biomass allocation to crowns than the two exotic species. It is interesting to note that the strongly rhizomatous smooth brome did not produce rhizomes in the ﬁrst season's growth, regardless of the water stress level. The effect of drought stress on biomass allocation manifested itself more on rhizomes or crowns than on roots or shoots
of the four studied grass species, with the effects species-speciﬁc in nature.
Keywords: biomass partition, drought stress, invasive grasses, mixed-grass prairie, native grasses
The widespread invasion and dominance of exotic plants, such as Kentucky bluegrass (Poa pratensis L.) and smooth brome (Bromus inermis Leyss.), in the northern Great Plains rangelands are posing challenges to land management (Murphy and Grant, 2005; Dekeyser et al., 2009, 2010). Control of these invasive exotic plants relies on our understanding of the eco- physiological mechanisms responsible for the spread of these species as they compete with the co-occurring native plants for soil resources. However, the large variation in the eco-physiological traits within and between native and exotic plant groups makes it difﬁ- cult to detect statistical differences in measurements taken from the plants growing in ﬁeld conditions according to Smith and Knapp (2001), who found a similar photosynthetic resource use efﬁciency between the native and exotic plants growing in the tallgrass prairie and suggested that other traits, including the responses of plants to resource pulses, may explain why certain exotic species can invade the native prairie aggressively. As drought is the most limiting factor for the growth and production of the prairie plants (Sala et al., 1988), it is possible that the invasive grasses, such as Kentucky bluegrass and smooth brome, may employ some superior traits in terms of efﬁcient exploitation of soil water and drought resistance as compared to their native counterparts. A large root system in grass plants, either due to a greater rooting depth or due to a higher density, or both (Dong et al., 2010), can enhance their drought‐avoidance capacity. Smooth brome has a high drought‐resistance capacity with its deep root systems (Cook, 1943; Gist and Smith, 1948). While the shallow rooting depth of Kentucky bluegrass implies drought sensitivity (Peter- son et al., 1979; Jiang and Huang, 2001), its dense root mat in the shallow soil layer may facilitate rapid uptake of readily available rain water (Dong et al., 2011). Thus, an accurate assessment of the biomass allocation (out of the entire vegetative biomass) for the roots of these species is needed to better assess the role of root growth in soil water acquisition for these exotic species compared with native species. In grasses, food reserves in crowns and rhizomes support herbage production (Reece et al., 2002), promote resource sharing among ramets (Cullen et al., 2005) and allow regrowth in the spring (Kinmonth- Schultz and Kim, 2011). One important factor leading to the relative competitiveness of Kentucky bluegrass and smooth brome in the northern Great Plains range- lands is that they initiate their spring regrowth earlier than their native counterparts (Dekeyser et al., 2010). It still remains to be tested whether there is a greater biomass investment to crowns or rhizomes in the exo- tic grasses as compared to the native grasses in the mixed-grass prairie. We report on a study of the effect of drought stress on vegetative biomass allocation in two native grass species, western wheatgrass (Pascopy- rum smithii (Rydb.) A. Lo ¨ve) and green needlegrass (Stipa viridula Trin.), and two exotic species, Kentucky bluegrass and smooth brome. All four species are dominant in the mixed-grass prairie ecosystem. Green needlegrass is a bunch-type grass, while the remain- ing three species are strongly rhizomatous grasses (Sedivec et al., 2001). We tested the following two hypotheses: 1 The two exotic grass species may have a higher proportion of biomass allocated to roots, rhizomes or crowns than the two native species, which may facilitate the initiation of new tillers follow- ing defoliation or in early spring regrowth. 2 Compared with the native species, the two exotic species may have a greater increase in biomass allocation to roots, rhizomes or crowns under drought stress, so as to have a better chance of regrowth after drought. We also expected that the only bunch-type grass (green needlegrass) would have a greater fraction of biomass allocation to crowns than the three sod-form- ing grass species, in which both crowns and rhizomes serve as food reserves.
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