Fire and browsing interact to alter intra-clonal stem dynamics of an encroaching shrub in tallgrass prairie.

The expansion of woody species into grasslands has altered community structure and ecosystem function of grasslands worldwide. In tallgrass prairie of the Central Great Plains, USA, decreased fire frequency and intensity have increased the cover and abundance of woody species. In particular, clonal shrub cover has increased at accelerated rates due to vegetative reproduction and resprouting after disturbance. We measured the intra-clonal stem demography and relative growth rates (estimated change in woody biomass) of the shrub Cornus drummondii in response to fire frequency (4 vs 20 year burn intervals) and simulated browsing during the 2018 and 2019 growing seasons at Konza Prairie Biological Station (Manhattan, Kansas). Overall, infrequent fire (4 year burn interval) increased intra-clonal stem relative growth rates and shrub relative growth rates. Intra-clonal stem relative growth rates were reduced in unbrowsed clones in 2018 due to drought and simulated browsing reduced intra-clonal stem relative growth rates in 2019. Additionally, simulated browsing nearly eliminated flower production within clones but did not affect intra-clonal stem mortality or recruitment within a growing season. Fire in conjunction with simulated browsing reduced estimated relative growth rates for entire shrub clones. Browsed shrubs that experienced prescribed fire in 2017 had reduced intra-clonal stem densities compared to unbrowsed shrubs and stem densities of browsed shrubs did not recover in 2018 or 2019. These results illustrate that infrequent fire alone promotes the expansion of clonal shrubs in tallgrass prairie and multiple interacting disturbances (e.g., fire and browsing) are required to control the spread of clonal shrubs into grasslands.

The ecology and significance of below-ground bud banks in plants.

BACKGROUND: Below-ground bud banks have experienced much recent interest due to discoveries that they (1) account for the majority of seasonal population renewal in many communities, (2) are crucial to regeneration following disturbance, and (3) have important consequences for plant population dynamics and plant and ecosystem function across a number of habitats. SCOPE: This review presents an overview of the role of bud banks in plant population renewal, examines bud bank life history, summarizes bud bank traits and their potential ecological implications, synthesizes the response of bud banks to disturbance, and highlights gaps to guide future research. The characteristics and life history of buds, including their natality, dormancy, protection and longevity, provide a useful framework for advancing our understanding of bud banks. The fate of buds depends on their age, size, type, location, and biotic and abiotic factors that collectively regulate bud bank dynamics. A bud bank can provide a demographic storage effect stabilizing population dynamics, and also confer resistance to disturbance and invasion. Regeneration capacity following disturbance is determined by interactions among the rates of bud natality, depletion and dormancy (meristem limitation), and the resources available to support the regeneration process. The resulting response of plants and their bud banks to disturbances such as fire, herbivory and anthropogenic sources determines the community's regenerative capacity. CONCLUSIONS: Vegetation responses to environmental change may be mediated through changes in bud bank dynamics and phenology. Environmental change that depletes the bud bank or prohibits its formation likely results in a loss of vegetation resilience and plant species diversity. Standardization of bud sampling, examination of bud banks in more ecosystems and their response to environmental variation and disturbance regimes, employment of stage-structured bud bank modelling and evaluation of the cost of bud bank construction and maintenance will benefit this expanding field of research.

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.

Belowground bud bank response to grazing under severe, short-term drought.

While the effects of drought and grazing are often studied separately, these disturbances co-occur in grasslands worldwide and interactively influence population, community, and ecosystem processes. The effects of drought and grazing on the belowground bud bank may dictate the trajectory of community recovery because new shoots arise from belowground buds after disturbance in perennial grasslands. We therefore investigated the separate and interactive effects of severe drought and grazing on the belowground bud bank and aboveground vegetation in the tallgrass prairie of northeast Kansas, USA. Contrary to our expectations, we observed changes in community structure and declines in species richness both above and below ground in response to drought and grazing. We also hypothesized that drought would reduce bud bank density of all taxonomic groups, but found that grass bud and shoot densities remained constant across all drought and grazing treatment combinations. While sedge and forb bud and shoot densities were reduced by drought, only sedge bud density declined to a greater extent when grazed under drought conditions. Live rhizome biomass did not vary by treatment and was highly correlated with bud bank density, suggesting that bud demography is tightly linked to the production and senescence of rhizomes. Despite the effects of drought and grazing on aboveground net primary productivity and community structure, our work suggests that grasses stabilize tallgrass prairie plant communities because their rhizomes and associated buds persist through co-occurring disturbances.

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.

Soil aggregation and carbon sequestration are tightly correlated with the abundance of arbuscular mycorrhizal fungi: results from long-term field experiments.

We examined the role of arbuscular mycorrhizal fungi (AMF) in ecosystems using soil aggregate stability and C and N storage as representative ecosystem processes. We utilized a wide gradient in AMF abundance, obtained through long-term (17 and 6 years) large-scale field manipulations. Burning and N-fertilization increased soil AMF hyphae, glomalin-related soil protein (GRSP) pools and water-stable macroaggregates while fungicide applications reduced AMF hyphae, GRSP and water-stable macroaggregates. We found that AMF abundance was a surprisingly dominant factor explaining the vast majority of variability in soil aggregation. This experimental field study, involving long-term diverse management practices of native multispecies prairie communities, invariably showed a close positive correlation between AMF hyphal abundance and soil aggregation, and C and N sequestration. This highly significant linear correlation suggests there are serious consequences to the loss of AMF from ecosystems.

Responses of two bunchgrasses to nitrogen addition in tallgrass prairie: the role of bud bank demography.

Growth of tallgrass prairie plants, many of which maintain substantial bud banks, can be limited by nitrogen (N), water, and/or light. We hypothesized that tallgrass prairie plants respond to increases in N through demographic effects on the bud bank. We tested the effects of a pulse of N on (1) bud bank demography, (2) plant reproductive allocation, and (3) ramet size. We parameterized matrix models, considering each genet as a population of plant parts. Nitrogen addition significantly impacted bud bank demography in two subdominant species of bunchgrass: Sporobolus heterolepis (a C(4) grass) and Koeleria macrantha (a C(3) grass), but had no effect on the size of individual ramets. Emergence from the bud bank and ramet population growth rates (λ) were significantly higher in S. heterolepis genets that received supplemental N. Nitrogen addition also affected the bud demography of K. macrantha, but N addition decreased rather than increased λ. Prospective and retrospective demographic analyses indicated that bud bank dynamics were the most important demographic processes driving plant responses to nutrient availability. Thus, the variation in productivity in these tallgrass prairie species is driven principally by the demography of the bud bank rather than by the physiology and growth of aboveground tillers. Improved understanding of bud bank dynamics may lead to improved predictive models of grassland responses to environmental changes such as altered N deposition and precipitation.

Below-ground bud banks increase along a precipitation gradient of the North American Great Plains: a test of the meristem limitation hypothesis.

In perennial grasslands, the below-ground population of meristems (bud bank) plays a fundamental role in plant population dynamics. Here, we tested the 'meristem limitation hypothesis' prediction - that bud banks increase along an increasing precipitation/productivity gradient in North American grasslands - and assessed the seasonal dynamics of bud banks. We sampled bud and stem populations quarterly at six sites across a 1100 km gradient in central North America. Bud banks increased with average annual precipitation, which explained 80% of the variability between the sites. In addition, seasonal changes in grass bud banks were surprisingly similar across a 2.5-fold range in precipitation and a 4-fold range of productivity: densities peaked in March, decreased in June and increased slightly in September. Increasing meristem limitation may constrain vegetation responses to inter-annual changes in resources. An important consequence of this is that biomes with large bud banks may be the most responsive to environmental change. If meristem limitation represents an important constraint on productivity responses to environmental variability, then bud banks must be considered in developing predictive models for grassland responses to environmental change.

Belowground bud banks and meristem limitation in tallgrass prairieplant populations.

Rhizome meristem populations were sampled in tallgrass prairie to quantify the size, grass : forb composition, and temporal and spatial variability of the soil bud bank and to compare fire effects on bud bank and seed bank composition. Soil cores (10.5 cm diameter, 15 cm deep) were collected from replicate annually and infrequently burned tallgrass prairie sites, and intact rhizomes and rhizome buds were censused. Bud bank densities ranged from approximately 600 to 1800 meristems/m(2) among sites and had high spatial and seasonal variability. In annually burned prairie, the total bud bank density was two-fold greater and the grass : forb meristem ratio was more than 30-fold greater than that of infrequently burned prairie. These patterns are opposite those observed in soil seed banks at this site. The rhizome population in annually burned prairie was 34% larger than the established aboveground tiller population. By contrast, the bud bank density in unburned prairie was significantly lower than aboveground stem densities, indicating possible belowground meristem limitation of stem density and net primary production on infrequently burned prairie. The patterns observed in this study suggest that the densities and dynamics of tallgrass prairie plant populations, as well as their response to disturbance (e.g., fire and grazing) and climatic variability, may be mediated principally through effects on the demography of belowground bud populations. Patterns of seed reproduction and seed bank populations have little influence on short-term aboveground population dynamics of tallgrass prairie perennials.

Variation in grazing tolerance among three tallgrass prairie plant species.

Three tallgrass prairie plant species, two common perennial forbs (Artemisia ludoviciana and Aster ericoides [Asteraceae]) and a dominant C(4) perennial grass (Sorghastrum nutans) were studied under field and greenhouse conditions to evaluate interspecific variation in grazing tolerance (compensatory growth capacity). Adaptation to ungulate grazing was also assessed by comparing defoliation responses of plants from populations with a 25-yr history of no grazing or moderate ungulate grazing. Under field conditions, all three species showed significant reductions in shoot relative growth rates (RGR), biomass, and reproduction with defoliation. In the two forbs, clipping resulted in negative shoot RGR and reductions in both number and length of shoot branches per ramet. Sorghastrum nutans maintained positive RGR under defoliation due to a compensatory increase in leaf production. Defoliation reduced rhizome production in A. ericoides and S. nutans, but not in A. ludoviciana. Clipping significantly reduced sexual reproductive allocation in all three species, although S. nutans showed a smaller reduction than the forbs. All three species showed similar responses to defoliation in burned and unburned sites. Under greenhouse conditions, a similar clipping regimen resulted in smaller reductions in growth and reproduction than those observed in the field. For all three species, the grazing tolerance indices calculated under natural field conditions were significantly lower than those estimated from greenhouse-grown plants, and the interspecific patterns of grazing tolerance were different. Aster ericoides exhibited the highest overall defoliation tolerance under greenhouse conditions, followed by S. nutans. Artemisia ludoviciana, the only study species that is typically not grazed by ungulates in the field, showed the lowest grazing tolerance. In the field experiment S. nutans showed the highest grazing tolerance and the two forbs had similar low tolerance indices. These patterns indicate that, despite high compensatory growth potential, limited resource availability and competition in the field significantly reduce the degree of compensation and alter interspecific differences in grazing tolerance among prairie plants. In all three species, defoliation suppressed sexual reproduction more than growth or vegetative reproduction. Significant interactions between plant responses to defoliation and site of origin (historically grazed or ungrazed sites) for some response variables (root/shoot ratios, rhizome bud initiation, and reproductive allocation) indicated some degree of population differentiation and genetic adaptation in response to a relatively short history of ungulate grazing pressure. The results of this study indicate that patterns of grazing tolerance in tallgrass prairie are both genetically based and also environmentally dependent.

Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps.

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (Ψ). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem Ψ than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher Ψ, g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and Ψ, but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO2 concentration relationships were consistent with the explanation that increased Ψ allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low-Ψ limitations on non-stomatal components of A. Our working hypothesis is that galls increase Ψ and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased Ψ and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.