00531nas a2200121 4500008004100000245016500041210006900206490000800275100001800283700002300301700001800324856006700342 2024 eng d00aEvaluating methods for measuring the leaf area index of encroaching shrubs in grasslands: From leaves to optical methods, 3-D scanning, and airborne observation0 aEvaluating methods for measuring the leaf area index of encroach0 v3491 aTooley, E., G1 aNippert, Jesse, B.1 aRatajczak, Z. uhttps://linkinghub.elsevier.com/retrieve/pii/S016819232400079000531nas a2200133 4500008004100000245013000041210007100171300001400242490000800256100002100264700001800285700001800303856007600321 2024 eng d00aHigh among‐species variability in the context dependence of herbivory across disturbance, weather and topoedaphic gradients0 aHigh among‐species variability in the context dependence of herb a2237-22480 v1121 aSutton, Alex, O.1 aRatajczak, Z.1 aLouthan, A.M. uhttps://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2745.1439000574nas a2200121 4500008004100000245016900041210006900210260004300279490002100322100001800343700001900361856007200380 2024 eng d00aNorth American megafauna, friend or foe to woody encroachment across the Great Plains? Further exploring the American bison’s ecological role on plant communities0 aNorth American megafauna friend or foe to woody encroachment acr aManhattan, KSbKansas State University0 vPhD Dissertation1 aNoble, Sidney1 aRatajczak, Zak uhttps://krex.k-state.edu/items/2d3c8fa7-ad50-475d-a753-216ebeef5b0b00537nas a2200121 4500008004100000245012500041210006900166260004300235490001400278100001800292700001800310856008700328 2023 eng d00aEarly detection of wildfire risk in the Great Plains: merging machine learning, landscape metrics, and rich data sources0 aEarly detection of wildfire risk in the Great Plains merging mac aManhattan, KSbKansas State University0 vMS Thesis1 aNoble, Brynn.1 aRatajczak, Z. uhttps://krex.k-state.edu/bitstream/handle/2097/43444/BrynnNoble2023.pdf?sequence=300606nas a2200181 4500008004100000245009400041210006900135300001200204490000700216100002100223700002300244700001900267700001800286700001900304700001800323700001800341856006500359 2023 eng d00aImpacts of riparian and non-riparian woody encroachment on tallgrass prairie ecohydrology0 aImpacts of riparian and nonriparian woody encroachment on tallgr a290-3010 v261 aKeen, Rachel, M.1 aNippert, Jesse, B.1 aSullivan, P.L.1 aRatajczak, Z.1 aRitchey, Brynn1 aO’Keefe, K.1 aDodds, W., K. uhttps://link.springer.com/article/10.1007/s10021-022-00756-700659nas a2200193 4500008004100000245011800041210006900159300001000228490000700238100002200245700001800267700002100285700001800306700002400324700001900348700002400367700001700391856005700408 2023 eng d00aTrajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal0 aTrajectories and state changes of a grassland stream and riparia ae28300 v331 aDodds, Walter, K.1 aRatajczak, Z.1 aKeen, Rachel, M.1 aNippert, J.B.1 aGrudzinski, Bartosz1 aVeach, Allison1 aTaylor, Jeffery, H.1 aKuhl, Amanda uhttps://onlinelibrary.wiley.com/doi/10.1002/eap.283001697nas a2200181 4500008004100000245006900041210006800110300001200178490000700190520114100197100002301338700001801361700001801379700001901397700001801416700001901434856006201453 2021 eng d00aFire frequency, state change and hysteresis in tallgrass prairie0 aFire frequency state change and hysteresis in tallgrass prairie a636-6470 v243 a

Hysteresis is a fundamental characteristic of alternative stable state theory, yet evidence of hysteresis is rare. In mesic grasslands, fire frequency regulates transition from grass‐ to shrub‐dominated system states. It is uncertain, however, if increasing fire frequency can reverse shrub expansion, or if grass‐shrub dynamics exhibit hysteresis. We implemented annual burning in two infrequently burned grasslands and ceased burning in two grasslands burned annually. With annual fires, grassland composition converged on that of long‐term annually burned vegetation due to rapid recovery of grass cover, although shrubs persisted. When annual burning ceased, shrub cover increased, but community composition did not converge with a long‐term infrequently burned reference site because of stochastic and lagged dispersal by shrubs, reflecting hysteresis. Our results demonstrated that annual burning can slow, but not reverse, shrub encroachment. In addition, reversing fire frequencies resulted in hysteresis because vegetation trajectories from grassland to shrubland differed from those of shrubland to grassland.

1 aCollins, Scott, L.1 aNippert, J.B.1 aBlair, J., M.1 aBriggs, J., M.1 aBlackmore, P.1 aRatajczak, Zak uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/ele.1367602579nas a2200169 4500008004100000245009300041210006900134300001400203490000700217520201300224100001802237700002502255700002202280700002102302700002402323856006202347 2019 eng d00aThe combined effects of an extreme heatwave and wildfire on tallgrass prairie vegetation0 acombined effects of an extreme heatwave and wildfire on tallgras a687 - 6970 v303 a

Questions: Climate extremes are predicted to become more common in many ecosystems. Climate extremes can promote and interact with disturbances, but the combined effects of climate extremes and disturbances have not been quantified in many ecosystems. In this study, we ask whether the dual impact of a climate extreme and concomitant disturbance (wildfire) has a greater affect than a climate extreme alone.
Location: Tallgrass prairie in the Konza Prairie Biological Station, northeastern Kansas, USA.
Methods: We quantified the response of a tallgrass prairie plant community to a 2‐year climate extreme of low growing‐season precipitation and high temperatures. In the first year of the climate extreme, a subset of plots was burned by a growing‐season wildfire. This natural experiment allowed us to compare community responses to a climate extreme with and without wildfire.
Results: In plots exposed to the climate extreme but not wildfire, community structure, diversity, and composition showed minor to insignificant changes, such as a 20% reduction in grass cover and a slight increase in species diversity. Plots exposed to both the climate extreme and wildfire underwent larger changes, including an 80% reduction in grass cover, 50% increase in forb cover, and increased plant diversity. Two years after the climate extreme, structural shifts in burned plots showed little sign of recovery, indicating a potentially lasting shift in plant community structure.
Conclusions: Our results suggest that community responses to climate extremes need to account for climate‐related disturbances — in this case, high temperatures, drought and wildfire. This response diverged from our expectation that heat, drought, and an additional fire would favor grasses. Although growing‐season wildfires in tallgrass prairie have been rare in recent decades, they will likely become more common with climate change, potentially leading to changes in grassland structure.

1 aRatajczak, Z.1 aChurchill, Amber, C.1 aLadwig, Laura, M.1 aTaylor, Jeff, H.1 aCollins, Scott., L. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.1275000637nas a2200145 4500008004100000245006600041210006500107260007800172300001300250100001800263700001500281700001500296700001500311856016500326 2019 eng d00aWill climate change push grasslands past critical thresholds?0 aWill climate change push grasslands past critical thresholds aCambridge, UK.bBritish Ecological Society and Cambridge University Press a98 - 1141 aRatajczak, Z.1 aLadwig, L.1 aGibson, D.1 aNewman, J. uhttps://www.cambridge.org/core/books/grasslands-and-climate-change/will-climate-change-push-grasslands-past-critical-thresholds/368C9316D9C5A8A3C6B6881779BA5EB502217nas a2200169 4500008004100000245012400041210006900165300001600234490000800250520162300258100001701881700002001898700001501918700001801933700002301951856007301974 2017 eng d00aAssessing the roles of fire frequency and precipitation in determining woody plant expansion in central U.S. grasslands0 aAssessing the roles of fire frequency and precipitation in deter a2683–26980 v1223 a

Woody plant expansion into grasslands and savannas is occurring and accelerating worldwide and often impacts ecosystem processes. Understanding and predicting the environmental and ecological impacts of encroachment has led to a variety of methodologies for assessing its onset, transition, and stability, generally relying on dynamical systems approaches. Here we continue this general line of investigation to facilitate the understanding of the roles of precipitation frequency and intensity and fire frequency on the conversion of grasslands to woody-dominated systems focusing on the central United States. A low-dimensional model with stochastic precipitation and fire disturbance is introduced to examine the complex interactions between precipitation and fire as mechanisms that may suppress or facilitate increases in woody cover. By using Lyapunov exponents, we are able to ascertain the relative control exerted on woody encroachment through these mechanisms. Our results indicate that precipitation frequency is a more important control on woody encroachment than the intensity of individual precipitation events. Fire, however, exerts a much more dominant impact on the limitation of encroachment over the range of precipitation variability considered here. These results indicate that fire management may be an effective strategy to slow the onset of woody species into grasslands. While climate change might predict a reduced potential for woody encroachment in the near future, these results indicate a reduction in woody fraction may be unlikely when considering anthropogenic fire suppression.

1 aBrunsell, N.1 avan Vleck, E.S.1 aNosshi, M.1 aRatajczak, Z.1 aNippert, Jesse, B. uhttps://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017JG00404603195nas a2200181 4500008004100000245008100041210006900122300001200191490000800203520260500211100001802816700002202834700002302856700002402879700001702903700001302920856008002933 2017 eng d00aChanges in spatial variance during a grassland to shrubland state transition0 aChanges in spatial variance during a grassland to shrubland stat a750-7600 v1053 a
  1. State transitions are changes in ecosystem structure and self-reinforcing feedbacks that are initiated when an exogenous driver variable crosses a threshold. Reversing state transitions is difficult and costly. While some state transitions are relatively rapid, many take years to decades. Outside of theoretical models, very little is known about slower state transitions and how they unfold in time and space.
  2. We quantified changes in spatial variance as a mesic grassland ecosystem shifts to a shrub-dominated state, using long-term experiments and simulations that maintain grasslands with annual fires or initiate a state transition to shrub dominance by decreasing fire frequency.
  3. In the experiments, the susceptibility to state transitions varied substantially in space. In the less frequent fire treatment, some plots became shrub-dominated around year 20 and grass extirpations began in year 25, but a third of the plots were still grass-dominated in year 37. Variable rates of state transition resulted in increasing spatial variance of grass cover over time, whereas shrub cover variance decreased. In the annually burned treatment, grasses remained dominant and the spatial variance of grass cover declined.
  4. In a separate experiment, less frequent fires were maintained for 23 years and then switched to annual fires. The switch to annual fires occurred shortly after grass variance started to increase and a majority of these plots quickly returned to a grass dominated state.
  5. In simulations, spatial variance remained low and average grass cover was high under frequent fires. If fire frequency decreased below a threshold, the ecosystem transitioned to shrubland, with a transient increase in the spatial variance of grass cover during the transition between states.
  6. Synthesis. Spatial variability in the rate and susceptibility to state transitions is indicative of a system with a patchy spatial structure, high spatial heterogeneity and low connectivity between patches. Increases in spatial variance can serve as an indication that some patches have begun a state transition and that management interventions are needed to avoid widespread transitions. This is one of the first empirical examples where altering management after an increase in spatial variance prevented state transitions.

 

1 aRatajczak, Z.1 aD’Odorico, P.D.1 aNippert, Jesse, B.1 aCollins, Scott., L.1 aBrunsell, N.1 aRavi, S. uhttps://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2745.1269603030nas a2200181 4500008004100000245010200041210006900143300001200212490000700224520241900231100001802650700002102668700002402689700002202713700001702735700002302752856007302775 2017 eng d00aThe interactive effects of press/pulse intensity and duration on regime shifts at multiple scales0 ainteractive effects of presspulse intensity and duration on regi a198-2180 v873 a

Regime shifts are difficult to reverse transitions that occur when an ecosystem reorganizes around a new set of self-reinforcing feedbacks. Regime shifts are predicted to occur when the intensity of some exogenous driver variable—such as temperature, annual harvest rate or nutrient addition rate—gradually approaches and crosses a threshold value, initiating a transition to an alternative state. However, many driver variables now change rapidly as presses or pulses, not gradually, requiring new conceptual frameworks for understanding and predicting regime shifts. We argue that identifying and controlling regime shifts in response to presses and pulses will require a greater focus on the duration, not just intensity, of changes in driver variables. In ecosystems with slower dynamics, transitions to an alternative state can take years to decades and as a result, a driver press with an intensity capable of resulting in a regime shift over long time-spans may fail to cause a regime shift when applied for shorter durations. We illustrate these ideas using simulations of local-scale alternative stable state models and preliminary evidence from long-term grazing and eutrophication experiments. The simulations also suggest that small changes in the duration of driver presses or pulses can determine whether an ecosystem recovers to its original state. These insights may extend to larger scales. In spatially extended simulations that included patchiness, spatial heterogeneity, and spatial connectivity, all patches recovered to their original state after shorter presses. However, once press duration exceeded a threshold, growing proportions of the landscape shifted to an alternative state as press duration increased. We observed similar patchy transitions in a catchment-scale experiment that reinstated frequent fires approximately halfway through a regime shift from grassland to shrubland, initiated by fire suppression. In both the local- and larger-scale models, the threshold duration needed to elicit regime shifts decreased as press intensity increased or when factors counteracting regime shifts weakened. These multiple lines of evidence suggest that conceptualizing regime shifts as an interactive function of the intensity and duration of driver changes will increase understanding of the varying effects of driver presses, pulses, and cycles on ecosystem dynamics.

1 aRatajczak, Z.1 aD'Odorico, Paolo1 aCollins, Scott., L.1 aBestelmeyer, B.T.1 aIsbell, F.L.1 aNippert, Jesse, B. uhttps://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/ecm.124902806nas a2200253 4500008004100000245012700041210006900168300001400237490000700251520198600258653002402244653001602268653002102284653001802305653001502323653001902338100001802357700001902375700001702394700001502411700002302426700002002449856008302469 2016 eng d00aAssessing the potential for transitions from tallgrass prairie to woodlands: are we operating beyond critical transitions?0 aAssessing the potential for transitions from tallgrass prairie t a280–2870 v693 a

A growing body of evidence suggests humans are pushing ecosystems near or beyond key ecological thresholds, resulting in transitions to new, sometimes undesirable phases or states that are costly to reverse. We used remotely sensed fire data to assess if the Flint Hills—a landscape of tallgrass prairie in the Central Great Plains, United States—is operating beyond fire frequency thresholds. Long-term fire experiments and observational evidence suggests that applying prescribed fire at return intervals > 3 yr can lead to transitions from grassland to shrubland. Fire return intervals > 10 yr and complete fire suppression, in particular, can result in transitions to woodlands over 30 − 50 yr. Once shrublands and woodlands are established, restoration back to grassland is difficult with prescribed fires. We applied these fire frequency cutoffs to remotely sensed fire data from 2000 to 2010 in the Flint Hills, identifying the extent of tallgrass prairie susceptible to shrub and tree expansion. We found that 56% (15 620 km2) of grasslands in this region are burned less than every 3 yr and are therefore susceptible to conversion to shrub or tree dominance. The potential effects of this large-scale shift are greater risk for evergreen (Juniperus virginiana) woodland fires, reduced grazing potential, and increased abundance of woodland adapted species at the expense of the native grassland biota. Of the 12 127-km2 area likely to remain grassland, 43% is burned approximately annually, contributing to vegetative homogenization and potential air-quality issues. While this synthesis forecasts a precarious future for tallgrass prairie conservation and their ecosystem services, increases in shrub or tree dominances are usually reversible until fire frequency has been reduced for more than 20 yr. This delay leaves a small window of opportunity to return fire to the landscape and avoid large-scale transformation of tallgrass prairie.

10acatastrophic shifts10aforecasting10amesic grasslands10aregime shifts10aresilience10atipping points1 aRatajczak, Z.1 aBriggs, J., M.1 aGoodin, D.G.1 aMohler, R.1 aNippert, Jesse, B.1 aObermeyer, B.K. uhttps://www.sciencedirect.com/science/article/pii/S1550742416300021?via%3Dihub02089nas a2200253 4500008004100000245008600041210006900127300001400196490000700210520133400217100001501551700001801566700002001584700001701604700002001621700001701641700001501658700001901673700001701692700001701709700001901726700001601745856007401761 2016 eng d00aBeyond arctic and alpine: the influence of winter climate on temperate ecosystems0 aBeyond arctic and alpine the influence of winter climate on temp a372 - 3820 v973 a

Winter climate is expected to change under future climate scenarios, yet the majority of winter ecology research is focused in cold-climate ecosystems. In many temperate systems, it is unclear how winter climate relates to biotic responses during the growing season. The objective of this study was to examine how winter weather relates to plant and animal communities in a variety of terrestrial ecosystems ranging from warm deserts to alpine tundra. Specifically, we examined the association between winter weather and plant phenology, plant species richness, consumer abundance, and consumer richness in 11 terrestrial ecosystems associated with the U.S. Long-Term Ecological Research (LTER) Network. To varying degrees, winter precipitation and temperature were correlated with all biotic response variables. Bud break was tightly aligned with end of winter temperatures. For half the sites, winter weather was a better predictor of plant species richness than growing season weather. Warmer winters were correlated with lower consumer abundances in both temperate and alpine systems. Our findings suggest winter weather may have a strong influence on biotic activity during the growing season and should be considered in future studies investigating the effects of climate change on both alpine and temperate systems.

1 aLadwig, L.1 aRatajczak, Z.1 aOcheltree, T.W.1 aHafich, K.A.1 aChurchill, A.C.1 aFrey, S.J.K.1 aFuss, C.B.1 aKazanski, C.E.1 aMuñoz, J.D.1 aPetrie, M.D.1 aReinmann, A.B.1 aSmith, J.G. uhttps://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/15-0153.102215nas a2200241 4500008004100000022001400041245012300055210006900178300001400247490000800261520148100269653001801750653000901768653001401777653001201791653001401803100001601817700002301833700001801856700001501874700001801889856006601907 2016 eng d a0029-854900aTight coupling of leaf area index to canopy nitrogen and phosphorus across heterogeneous tallgrass prairie communities0 aTight coupling of leaf area index to canopy nitrogen and phospho a889 - 8980 v1823 a

Nitrogen (N) and phosphorus (P) are limiting nutrients for many plant communities worldwide. Foliar N and P along with leaf area are among the most important controls on photosynthesis and hence productivity. However, foliar N and P are typically assessed as species level traits, whereas productivity is often measured at the community scale. Here, we compared the community-level traits of leaf area index (LAI) to total foliar nitrogen (TFN) and total foliar phosphorus (TFP) across nearly three orders of magnitude LAI in grazed and ungrazed tallgrass prairie in north-eastern Kansas, USA. LAI was strongly correlated with both TFN and TFP across communities, and also within plant functional types (grass, forb, woody, and sedge) and grazing treatments (bison or cattle, and ungrazed). Across almost the entire range of LAI values and contrasting communities, TFN:TFP ratios indicated co-limitation by N and P in almost all communities; this may further indicate a community scale trend of an optimal N and P allocation per unit leaf area for growth. Previously, results from the arctic showed similar tight relationships between LAI:TFN, suggesting N is supplied to canopies to maximize photosynthesis per unit leaf area. This tight coupling between LAI, N, and P in tallgrass prairie suggests a process of optimal allocation of N and P, wherein LAI remains similarly constrained by N and P despite differences in species composition, grazing, and canopy density.

10aCo-limitation10afire10agrassland10agrazers10anutrients1 aKlodd, A.E.1 aNippert, Jesse, B.1 aRatajczak, Z.1 aWaring, H.1 aPhoenix, G.K. uhttps://link.springer.com/article/10.1007%2Fs00442-016-3713-302244nas a2200145 4500008004100000245012200041210006900163300001500232490000700247520170900254100001801963700002301981700002002004856007402024 2014 eng d00aAbrupt transition of mesic grassland to shrubland: evidence for thresholds, alternative attractors, and regime shifts0 aAbrupt transition of mesic grassland to shrubland evidence for t a2633 -26450 v953 a

Ecosystems with alternative attractors are susceptible to abrupt regime shifts that are often difficult to predict and reverse. In this study, we quantify multiple system dynamics to determine whether the transition of mesic grassland to shrubland, a widespread phenomenon, represents a linear reversible process, a nonlinear but reversible threshold process, or a transition between alternative attractors that is nonlinear and prone to hysteresis. Using a 28-yr data set with annual resolution and extensive spatial replication, we found that shrub cover is correlated with distinct thresholds of fire and C4 grass cover, resulting in temporal bimodality of shrub cover and abrupt shifts of shrub cover despite gradual changes in grass cover. These abrupt increases in shrub cover are the most rapid ever reported in grasslands, and illustrate internal thresholds that separate grasslands and shrublands. Nonlinear transitions from low to high shrub cover were also closely associated with positive feedback mechanisms that alter fire and competition (r2 = 0.65), suggesting that grasslands and shrublands could show hysteresis, and by definition exist as alternative attractors. Thus, the response of this ecosystem to anthropogenic activity should tend to be rapid, nonlinear, and perhaps difficult to reverse. Regime shifts in this mesic grassland were predictable: we found that grassland and shrubland attractors were differentiated by critical thresholds of ∼50–70% grass cover, 5–10% shrub cover, and a fire return interval of ∼3 yr. These thresholds may provide adaptive potential for managing nonlinear behavior in socio-ecological systems in a changing environment.

1 aRatajczak, Z.1 aNippert, Jesse, B.1 aOcheltree, T.W. uhttps://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/13-1369.101971nas a2200133 4500008004100000245009800041210006900139260004300208490002300251520148500274100001801759700002301777856003701800 2014 eng d00aEcological thresholds and abrupt transitions of tallgrass prairie to shrublands and woodlands0 aEcological thresholds and abrupt transitions of tallgrass prairi aManhattan, KSbKansas State University0 v PhD. Dissertation3 a

Ecological thresholds are breakpoints where small increases in external pressure can generate rapid and difficult to reverse ecological transitions. Often, ecological thresholds are not recognized until they are crossed at a large-scale, leading to unintended and lasting externalities. In tallgrass prairie, we identified ecological thresholds of 3-year fire returns and ~60% grass cover, based on mechanistic field studies and long-term fire and grazing experiments. When tallgrass prairie is pushed passed these thresholds, it makes an abrupt transition to a self-reinforcing shrubland state. Demographic bottlenecks, niche partitioning, and altered fire feedback mechanisms account for both the non-linear nature of grassland-shrubland transitions and the resistance of established shrublands to fire and drought. In the last decade, only ~27% of Central Great Plains tallgrass prairie was burned every 1-2 years, and therefore ~73% of this region is susceptible to shrubland and woodland transitions in the next two to three decades. If transitions to shrublands and woodlands do occur, we expect a multi-trophic loss of grassland biodiversity, decreased cattle production, and the potential for damaging woodland fires in close proximity to human development. However, knowledge of fire thresholds, adaptive management tools, and bottom-up citizen action campaigns are creating a rare window of opportunity to avoid transformation of the remaining tallgrass prairies.

1 aRatajczak, Z.1 aNippert, Jesse, B. uhttp://hdl.handle.net/2097/1766102633nas a2200253 4500008004100000245013900041210006900180300001500249490000800264520182600272653001002098653001502108653001202123653001502135653001202150653003102162653001002193653001502203100001802218700002302236700001902259700002002278856008102298 2014 eng d00aFire dynamics distinguish grasslands, shrublands, and woodlands as alternative attractors in the Central Great Plains of North America0 aFire dynamics distinguish grasslands shrublands and woodlands as a1374 -13850 v1023 a

Grasslands are threatened globally due to the expansion of woody plants. The few remaining headwater streams within tallgrass prairies are becoming more like typical forested streams due to rapid conversion of riparian zones from grassy to wooded. Forestation can alter stream hydrology and biogeochemistry. We estimated the rate of riparian woody plant expansion within a 30 m buffer zone surrounding the stream bed across whole watersheds at Konza Prairie Biological Station over 25 years from aerial photographs. Watersheds varied with respect to experimentally-controlled fire and bison grazing. Fire frequency, presence or absence of grazing bison, and the historical presence of woody vegetation prior to the study time period (a proxy for proximity of propagule sources) were used as independent variables to predict the rate of riparian woody plant expansion between 1985 and 2010. Water yield was estimated across these years for a subset of watersheds. Riparian woody encroachment rates increased as burning became less frequent than every two years. However, a higher fire frequency (1–2 years) did not reverse riparian woody encroachment regardless of whether woody vegetation was present or not before burning regimes were initiated. Although riparian woody vegetation cover increased over time, annual total precipitation and average annual temperature were variable. So, water yield over 4 watersheds under differing burn frequencies was quite variable and with no statistically significant detected temporal trends. Overall, burning regimes with a frequency of every 1–2 years will slow the conversion of tallgrass prairie stream ecosystems to forested ones, yet over long time periods, riparian woody plant encroachment may not be prevented by fire alone, regardless of fire frequency.

10abison10aEcosystems10aForests10aGrasslands10aGrazing10aLinear regression analysis10aTrees10aWatersheds1 aRatajczak, Z.1 aNippert, Jesse, B.1 aBriggs, J., M.1 aBlair, John, M. uhttps://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.1231102826nas a2200277 4500008004100000245010400041210006900145300001200214490000600226520201300232653001502245653001502260653001202275653001102287653002102298653001202319653001102331653002002342100002302362700002002385700001702405700001802422700001302440700001702453856007802470 2013 eng d00aEvidence of physiological decoupling from grassland ecosystem drivers by an encroaching woody shrub0 aEvidence of physiological decoupling from grassland ecosystem dr a81630 -0 v83 a

Shrub encroachment of grasslands is a transformative ecological process by which native woody species increase in cover and frequency and replace the herbaceous community. Mechanisms of encroachment are typically assessed using temporal data or experimental manipulations, with few large spatial assessments of shrub physiology. In a mesic grassland in North America, we measured inter- and intra-annual variability in leaf δ13C in Cornus drummondii across a grassland landscape with varying fire frequency, presence of large grazers and topographic variability. This assessment of changes in individual shrub physiology is the largest spatial and temporal assessment recorded to date. Despite a doubling of annual rainfall (in 2008 versus 2011), leaf δ13C was statistically similar among and within years from 2008-11 (range of −28 to −27‰). A topography*grazing interaction was present, with higher leaf δ13C in locations that typically have more bare soil and higher sensible heat in the growing season (upland topographic positions and grazed grasslands). Leaf δ13C from slopes varied among grazing contrasts, with upland and slope leaf δ13C more similar in ungrazed locations, while slopes and lowlands were more similar in grazed locations. In 2011, canopy greenness (normalized difference vegetation index – NDVI) was assessed at the centroid of individual shrubs using high-resolution hyperspectral imagery. Canopy greenness was highest mid-summer, likely reflecting temporal periods when C assimilation rates were highest. Similar to patterns seen in leaf δ13C, NDVI was highest in locations that typically experience lowest sensible heat (lowlands and ungrazed). The ability of Cornus drummondii to decouple leaf physiological responses from climate variability and fire frequency is a likely contributor to the increase in cover and frequency of this shrub species in mesic grassland and may be generalizable to other grasslands undergoing woody encroachment.

10aEcosystems10aGrasslands10aGrazing10aLeaves10aPlant physiology10aSeasons10aShrubs10aWater resources1 aNippert, Jesse, B.1 aOcheltree, T.W.1 aOrozco, G.L.1 aRatajczak, Z.1 aLing, B.1 aSkibbe, A.M. uhttps://journals.plos.org/plosone/article?id=10.1371/journal.pone.008163000924nas a2200133 4500008004100000245009000041210006900131300001000200490000800210520047400218100001800692700002300710856005700733 2012 eng d00aComment on "Global Resilience of Tropical Forest and Savanna to Critical Transitions"0 aComment on Global Resilience of Tropical Forest and Savanna to C a541 -0 v3363 a

Hirota et al. (Reports, 14 October 2011, p. 232) used spatial data to show that grasslands, savannas, and forests represent opposing stable states. Reanalyzing their data and drawing from temporal studies, we argue that spatial analyses underestimate the bistability of grasslands and savannas due to limitations of substituting space for time. We propose that temporal and spatial data are needed to predict critical transitions between grasslands and savannas.

1 aRatajczak, Z.1 aNippert, Jesse, B. uhttp://science.sciencemag.org/content/336/6081/541.301717nas a2200145 4500008004100000245008900041210006900130300001300199490000700212520121200219100001801431700002301449700002401472856007501496 2012 eng d00aWoody encroachment decreases diversity across North American grasslands and savannas0 aWoody encroachment decreases diversity across North American gra a697 -7030 v933 a

Woody encroachment is a widespread and acute phenomenon affecting grasslands and savannas worldwide. We performed a meta-analysis of 29 studies from 13 different grassland/savanna communities in North America to determine the consequences of woody encroachment on plant species richness. In all 13 communities, species richness declined with woody plant encroachment (average decline = 45%). Species richness declined more in communities with higher precipitation (r2 = 0.81) and where encroachment was associated with a greater change in annual net primary productivity (ANPP; r2 = 0.69). Based on the strong positive correlation between precipitation and ANPP following encroachment (r2 = 0.87), we hypothesize that these relationships occur because water-limited woody plants experience a greater physiological and demographic release as precipitation increases. The observed relationship between species richness and ANPP provides support for the theoretical expectation that a trade-off occurs between richness and productivity in herbaceous communities. We conclude that woody plant encroachment leads to significant declines in species richness in North American grassland/savanna communities.

1 aRatajczak, Z.1 aNippert, Jesse, B.1 aCollins, Scott., L. uhttps://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/11-1199.102412nas a2200157 4500008004100000245008600041210006900127300001000196490000600206520188500212100001802097700002302115700001802138700002002156856007802176 2011 eng d00aPositive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie0 aPositive feedbacks amplify rates of woody encroachment in mesic  a121 -0 v23 a

Over the last century, many grasslands worldwide have transitioned from a graminoid to a tree/shrub-dominated state in a short period of time, a phenomenon referred to as woody encroachment. Positive feedbacks and bi-stability are thought to be important drivers of woody encroachment, but there is little empirical evidence to suggest that positive feedbacks accelerate the woody encroachment of mesic grasslands. In mesic tallgrass prairie, shrub establishment does not directly facilitate seedling establishment. Yet, shrub establishment may facilitate the clonal spread of existing shrubs into nearby patches, because clonal reproduction might circumvent barriers that typically limit woody seedlings. Our results show that when Cornus drummondii (the predominate encroacher of mesic tallgrass prairie) extends rhizomatous stems into open grasslands, these stems use the same deep soil water sources as mature stems—thereby avoiding competition with grasses and gaining access to a reliable water source. In addition, herbaceous fuel concentrations are lower at the shrub/grass interface than in open grasslands, reducing the potential impacts of subsequent grassland fires. We propose that the release from resource and fire limitation results in a positive feedback loop as clonal stems are able to extend into surrounding patches, circumvent demographic barriers, mature, and spread by developing their own clonal stems. Long-term data on site (26 years) corroborates this interpretation: the size of deep-rooted clonal shrub species has increased 16-fold and their cover has increased from 0 to 27%, whereas the cover of shallow-rooted species (both clonal and non-clonal) has only increased marginally. Together, these results suggest that (1) positive feedbacks can facilitate mesic woody encroachment and (2) bi-stability exists in mesic tallgrass prairie.

1 aRatajczak, Z.1 aNippert, Jesse, B.1 aHartman, J.C.1 aOcheltree, T.W. uhttps://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/ES11-00212.1