Jacqueline P. Ott

Bud production and dynamics of flowering and vegetative tillers in Andropogon gerardii (Poaceae): the role of developmental constraints

PREMISE OF THE STUDY Perennial grasses maintain aboveground tiller populations through vegetative reproduction via belowground buds and sexual reproduction via seed. The maintenance of a bud bank has important demographic consequences for perennial grasses. A tradeoff between these reproductive modes would be expected for a plant with limited resource availability. However, the ontogeny of the tiller could affect its ability to allocate between these two modes of reproduction. METHODS Vegetative bud production and dynamics and tiller production were examined biweekly through an annual cycle on vegetative and flowering tillers of Andropogon gerardii. KEY RESULTS Andropogon gerardii maintains a large reserve of dormant buds. Although vegetative and flowering tillers had similar bud phenology, flowering tillers produced larger numbers of buds of larger size, and transitioned a larger proportion of their buds to tiller, than did vegetative tillers. Therefore, a negative consequence of sexual reproduction on vegetative reproduction was not evident at the tiller level. A size threshold for floral induction likely exists that results in flowering tillers having more buds per tiller than vegetative tillers. The increased bud outgrowth of flowering tillers could be a result of their larger bud size or weaker apical dominance as compared to vegetative tillers. CONCLUSIONS Plant development can place significant constraints on tradeoffs between the reproductive modes in perennial grasses and could affect their plasticity in plant reproductive allocation. Differences in developmental phenology and bud production between flowering and vegetative tillers may influence grass responses to environmental changes such as altered precipitation regimes or resource availability.

Vegetative Reproduction and Bud Bank Dynamics of the Perennial Grass Andropogon gerardii in Mixedgrass and Tallgrass Prairie

Abstract Plant species with wide distributions may differ in their population dynamics across their range, especially in contrasting habitats. Most tiller recruitment of perennial grasses occurs vegetatively from the belowground bud bank rather than from seed. Seed reproduction often occurs under a narrower range of environmental conditions than vegetative reproduction. As a result flowering and seedling recruitment patterns of a species often differ between contrasting habitats and across its range. How vegetative reproduction and bud bank dynamics of a species vary between contrasting habitats has not been well studied and could explain the differences in its persistence and productivity between habitats. Therefore, the vegetative reproduction and dynamics of Andropogon gerardii, a dominant C4 perennial grass of the Great Plains of North America, were compared between tallgrass and northern mixedgrass prairie habitats. Bud production and tiller recruitment in 10 populations were examined throughout an annual growing cycle in the northern mixedgrass prairie of South Dakota. Bud bank characteristics, and individual and population performance were compared with previous work conducted in Kansas tallgrass prairie. Stage-structured matrix models examined population growth rates. Andropogon gerardii tillers produced lower numbers of buds and had lower flowering rates in mixedgrass prairie populations. The annual phenology of bud and tiller development was also contracted to fit within the shorter growing season in northern mixedgrass prairie. However, bud longevity and bud bank age structure were similar between habitats, both having buds that lived for > 2 y and multi-aged bud banks. Similar population growth rates occurred in both habitats despite lower individual performance of both flowering and vegetative reproductive capacity (i.e., bud production) in mixedgrass prairie populations. Lower regional productivity of A. gerardii in northern mixedgrass prairie than in tallgrass prairie does not appear to be due to differences in bud and tiller population growth. Instead, sparse or patchy suitable habitat and/or reduction in tiller size may explain its reduced productivity. Lower population growth rates may be observed in other habitats or in years with harsher environmental conditions that further lower individual performance.

Bud bank dynamics and clonal growth strategy in the rhizomatous grass, Pascopyrum smithii

Abstract Recruitment of rhizomatous perennial grass ramets primarily occurs from the belowground bud bank. Investment in guerilla versus phalanx growth is determined by bud availability, development, and spatial distribution. The tiller and bud bank dynamics of Pascopyrum smithii, a dominant rhizomatous grass of the northern mixed-grass prairie, were examined in South Dakota throughout an annual cycle to assess the investment in guerilla versus phalanx growth and the role of different bud populations in renewal versus regenerative functions and vegetative spread via rhizomes. Pascopyrum smithii invested substantially in both phalanx and guerilla tiller production. However, investment in rhizome production before tiller recruitment prioritized guerilla over phalanx growth. Annual tiller recruitment of P. smithii was capable of flexible timing, occurring in either spring or fall. Renewal buds, from which rhizomes and tillers were recruited, primarily consisted of the youngest generation of buds borne at the base of tillers. Although rhizome axillary buds and older tiller axillary buds were rarely used in annual tiller recruitment, they provided a sizable reserve (regenerative) bud bank. The spatial distribution of bud development produced the mixed guerrilla–phalanx growth pattern and flexible tiller recruitment timing of P. smithii. Therefore, P. smithii is capable of employing both conservative and foraging growth strategies which will facilitate its persistence under local neighborhood variability and changing resource availability associated with environmental change. Understanding the spatial distribution of buds as determined by rhizome architecture is essential to understanding the distribution and composition of species within plant communities dominated by clonal species.

Bud-bank and tiller dynamics of co-occurring C3 caespitose grasses in mixed-grass prairie

PREMISE OF THE STUDY Tiller recruitment from the belowground bud bank of caespitose grasses influences their ability to monopolize local resources and, hence, their genet fitness. Differences in bud production and outgrowth among tiller types within a genet and among species may explain co-occurrence of caespitose grasses. This study aimed to characterize genet bud-bank and tiller production and dynamics in two co-occurring species and compare their vegetative reproductive strategies. METHODS Bud-bank and tiller dynamics of Hesperostipa comata and Nassella viridula, dominant C3 caespitose grasses in the northern mixed-grass prairie of North America, were assessed throughout an annual cycle. KEY RESULTS The two species showed similar strategies, maintaining polycyclic tillers and thus creating mixed-age genet bud banks comprising multiple bud cohorts produced in different years. Vegetative tillers produced the majority of buds, whereas flowering tillers contributed little to the bud bank. Buds lived for at least 2 yr and were maintained in multiple developmental stages throughout the year. Because bud longevity rarely exceeded tiller longevity, tiller longevity drove turnover within the bud bank. Tiller population dynamics, more than bud production per tiller, determined the differential contribution of tiller types to the bud bank. Nassella viridula had higher bud production per tiller, a consistent annual tiller recruitment density, and greater longevity of buds on senesced and flowering tillers than H. comata. CONCLUSIONS Co-occurring C3 caespitose grasses had similar bud-bank and tiller dynamics contributing to genet persistence but differed in bud characteristics that could affect genet longevity and species coexistence.

Greater bud outgrowth of Bromus inermis than Pascopyrum smithii under multiple environmental conditions

Tiller recruitment of perennial grasses in mixed-grass prairie primarily occurs from belowground buds. Environmental conditions, such as temperature, soil moisture and grazing can affect bud outgrowth of both invasive and native perennial grasses. Differential bud outgrowth responses of native and invasive species to climate change and grazing could alter competitive interactions that have implications for future land management. The aims of this work were to (i) compare how spring temperature altered bud outgrowth of native Pascopyrum smithii (Rydb.) A. Love (western wheatgrass) and introduced Bromus inermis Leyss.(smooth brome), (ii) compare how watering frequency altered bud outgrowth of these two species and (iii) evaluate how clipping interacts with spring temperature or watering frequency to affect P. smithii bud outgrowth.