Contrasting above‐ and belowground sensitivity of three Great Plains grasslands to altered rainfall regimes

Intensification of the global hydrological cycle with atmospheric warming is expected to increase interannual variation in precipitation amount and the frequency of extreme precipitation events. Although studies in grasslands have shown sensitivity of aboveground net primary productivity (ANPP) to both precipitation amount and event size, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and NPP. We conducted a 2‐year experiment in three US Great Plains grasslands – the C₄‐dominated shortgrass prairie (SGP; low ANPP) and tallgrass prairie (TGP; high ANPP), and the C₃‐dominated northern mixed grass prairie (NMP; intermediate ANPP) – to test three predictions: (i) both ANPP and BNPP responses to increased precipitation amount would vary inversely with mean annual precipitation (MAP) and site productivity; (ii) increased numbers of extreme rainfall events during high‐rainfall years would affect high and low MAP sites differently; and (iii) responses belowground would mirror those aboveground. We increased growing season precipitation by as much as 50% by augmenting natural rainfall via (i) many (11–13) small or (ii) fewer (3–5) large watering events, with the latter coinciding with naturally occurring large storms. Both ANPP and BNPP increased with water addition in the two C₄ grasslands, with greater ANPP sensitivity in TGP, but greater BNPP and NPP sensitivity in SGP. ANPP and BNPP did not respond to any rainfall manipulations in the C₃‐dominated NMP. Consistent with previous studies, fewer larger (extreme) rainfall events increased ANPP relative to many small events in SGP, but event size had no effect in TGP. Neither system responded consistently above‐ and belowground to event size; consequently, total NPP was insensitive to event size. The diversity of responses observed in these three grassland types underscores the challenge of predicting responses relevant to C cycling to forecast changes in precipitation regimes even within relatively homogeneous biomes such as grasslands.     

Convergence and contingency in production–precipitation relationships in North American and South African C4 grasslands

Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C₄ grasses and had similar growing season climates; thus, we expected convergence in precipitation–ANPP relationships. Lack of convergence, however, would support an alternative hypothesis—that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m² respectively) and growing season precipitation–ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of the growing season strongly affected annual ANPP in South Africa (P < 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal course of available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation in these two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site.     

Productivity responses to altered rainfall patterns in a C<Subscript>4</Subscript>-dominated grassland

Rainfall variability is a key driver of ecosystem structure and function in grasslands worldwide. Changes in rainfall patterns predicted by global climate models for the central United States are expected to cause lower and increasingly variable soil water availability, which may impact net primary production and plant species composition in native Great Plains grasslands. We experimentally altered the timing and quantity of growing season rainfall inputs by lengthening inter-rainfall dry intervals by 50%, reducing rainfall quantities by 30%, or both, compared to the ambient rainfall regime in a native tallgrass prairie ecosystem in northeastern Kansas. Over three growing seasons, increased rainfall variability caused by altered rainfall timing with no change in total rainfall quantity led to lower and more variable soil water content (0–30 cm depth), a ~10% reduction in aboveground net primary productivity (ANPP), increased root to shoot ratios, and greater canopy photon flux density at 30 cm above the soil surface. Lower total ANPP primarily resulted from reduced growth, biomass and flowering of subdominant warm-season C₄ grasses while productivity of the dominant C₄ grass Andropogon gerardii was relatively unresponsive. In general, vegetation responses to increased soil water content variability were at least equal to those caused by imposing a 30% reduction in rainfall quantity without altering the timing of rainfall inputs. Reduced ANPP most likely resulted from direct effects of soil moisture deficits on root activity, plant water status, and photosynthesis. Altered rainfall regimes are likely to be an important element of climate change scenarios in this grassland, and the nature of interactions with other climate change elements remains a significant challenge for predicting ecosystem responses to climate change.     

Interannual variability in primary production in tallgrass prairie: climate,soil moisture,topographic position,and fire as determinants of aboveground biomass

From 1975 to 1993, aboveground net primary production (NPP) at the Konza Prairie Research Natural Area in NE Kansas varied from 179 g/m to 756 g/m. Across a variety of sites, NPP was significantly related to precipitation (r = 0.37), but much variability was unexplained. Thus, we evaluated the relationship between NPP with meteorological variables and soil moisture measurements in tallgrass prairie sites that varied in fire frequency and topographic position. Annually burned lowland sites had significantly higher NPP than either annually burned upland or unbumed sites. NPP in burned sites was more strongly related to meteorological variables and soil moisture when compared to unbumed sites. The lack of significant correlation between soil moisture with NPP on unbumed sites suggests that factors other than water availability limit production in these sites. When NPP data were analyzed separately by life forms, interannual variability in forb NPP was not correlated with any meteorological variables, but was negatively correlated with grass NPP (r = -0.49). The inability of a single factor, such as precipitation to explain a large portion of the interannual variability in NPP is consistent with the concept that patterns of NPP in tallgrass prairie are a product of spatial and temporal variability in light, water, and nutrients, driven by a combination of topography, fire history, and climate.     

Drivers of Variation in Aboveground Net Primary Productivity and Plant Community Composition Differ Across a Broad Precipitation Gradient

Aboveground net primary production (ANPP) is a key integrator of C uptake and energy flow in many terrestrial ecosystems. As such, ecologists have long sought to understand the factors driving variation in this important ecosystem process. Although total annual precipitation has been shown to be a strong predictor of ANPP in grasslands across broad spatial scales, it is often a poor predictor at local scales. Here we examine the amount of variation in ANPP that can be explained by total annual precipitation versus precipitation during specific periods of the year (precipitation periods) and nutrient availability at three sites representing the major grassland types (shortgrass steppe, mixed-grass prairie, and tallgrass prairie) spanning the broad precipitation gradient of the U.S. Central Great Plains. Using observational data, we found that precipitation periods and nutrient availability were much stronger predictors of site-level ANPP than total annual precipitation. However, the specific nutrients and precipitation periods that best predicted ANPP differed among the three sites. These effects were mirrored experimentally at the shortgrass and tallgrass sites, with precipitation and nutrient availability co-limiting ANPP, but not at the mixed-grass site, where nutrient availability determined ANPP exclusive of precipitation effects. Dominant grasses drove the ANPP response to increased nutrient availability at all three sites. However, the relative responses of rare grasses and forbs were greater than those of the dominant grasses to experimental nutrient additions, thus potentially driving species turnover with chronic nutrient additions. This improved understanding of the factors driving variation in ANPP within ecosystems spanning the broad precipitation gradient of the Great Plains will aid predictions of alterations in ANPP under future global change scenarios.     

Contingent productivity responses to more extreme rainfall regimes across a grassland biome

Climate models predict, and empirical evidence confirms, that more extreme precipitation regimes are occurring in tandem with warmer atmospheric temperatures. These more extreme rainfall patterns are characterized by increased event size separated by longer within season drought periods and represent novel climatic conditions whose consequences for different ecosystem types are largely unknown. Here, we present results from an experiment in which more extreme rainfall patterns were imposed in three native grassland sites in the Central Plains Region of North America, USA. Along this 600 km precipitation–productivity gradient, there was strong sensitivity of temperate grasslands to more extreme growing season rainfall regimes, with responses of aboveground net primary productivity (ANPP) contingent on mean soil water levels for different grassland types. At the mesic end of the gradient (tallgrass prairie), longer dry intervals between events led to extended periods of below-average soil water content, increased plant water stress and reduced ANPP by 18%. The opposite response occurred at the dry end (semiarid steppe), where a shift to fewer, but larger, events increased periods of above-average soil water content, reduced seasonal plant water stress and resulted in a 30% increase in ANPP. At an intermediate mixed grass prairie site with high plant species richness, ANPP was most sensitive to more extreme rainfall regimes (70% increase). These results highlight the inherent complexity in predicting how terrestrial ecosystems will respond to forecast novel climate conditions as well as the difficulties in extending inferences from single site experiments across biomes. Even with no change in annual precipitation amount, ANPP responses in a relatively uniform physiographic region differed in both magnitude and direction in response to within season changes in rainfall event size/frequency.     

Intra-annual rainfall variability and grassland productivity: can the past predict the future?

Precipitation quantity has been shown to influence grassland aboveground net primary productivity (ANPP) positively whereas experimental increases in of temporal variability in water availability commonly exhibit a negative relationship with ANPP. We evaluated long term ANPP datasets from the Konza Prairie Long Term Ecological Research (LTER) program (1984–1999) to determine if similar relationships could be identified based on patterns of natural variability (magnitude and timing) in precipitation. ANPP data were analyzed from annually burned sites in native mesic grassland and productivity was partitioned into graminoid (principally C₄ grasses) and forb (C₃ herbaceous) components. Although growing season precipitation amount was the best single predictor of total and grass ANPP (r ²=0.62), several measures of precipitation variability were also significantly and positively correlated with productivity, independent of precipitation amount. These included soil moisture variability, expressed as CV, for June (r ²=0.45) and the mean change in soil moisture between weekly sampling periods in June and August (%wv) (r ²=0.27 and 0.32). In contrast, no significant relationships were found between forb productivity and any of the precipitation variables (p>0.05). A multiple regression model combining precipitation amount and both measures of soil moisture variability substantially increased the fit with productivity (r ²=0.82). These results were not entirely consistent with those of short-term manipulative experiments in the same grassland, however, because soil moisture variability was often positively, not negatively related to ANPP. Differences in results between long and short term experiments may be due to low variability in the historic precipitation record compared to that imposed experimentally as experimental levels of variability exceeded the natural variability of this dataset by a factor of two. Thus, forecasts of ecosystem responses to climate change (i.e. increased climatic variability), based on data constrained by natural and recent historical rainfall patterns may be inadequate for assessing climate change scenarios if precipitation variability in the future is expected to exceed current levels.     

Feeding ecology and emergence production of annual cicadas (Homoptera: Cicadidae) in tallgrass prairie

The emergence phenology and feeding ecology of annual cicadas in tallgrass prairie are poorly documented. However, these large insects are abundant, and their annual emergence represents a potentially important flux of energy and nutrients from belowground to aboveground. We conducted a study at Konza Prairie Research Natural Area in eastern Kansas to characterize and quantify cicada emergence and associated energy and nutrient fluxes. We established emergence trap transects in three habitat types (upland prairie, lowland prairie, and riparian forest), and collected cicadas every 3 days from May to September. A subset of trapped cicadas was used for species- and sex-specific mass, nutrient, and stable isotope analyses. Five species were trapped during the study, of which three were dominant. Cicadetta calliope and Tibicen aurifera exhibited significantly higher emergence production in upland prairie than in lowland prairie, and were not captured in forested sites at all. T. dorsata emerged from all three habitat types, and though not significant, showed a trend of greater abundance in lowland grasslands. Two less abundant species, T. pruinosa and T. lyricen, emerged exclusively from forested habitats. Nitrogen fluxes associated with total cicada emergence were estimated to be ∼4 kg N ha^–1 year^–1 in both grassland habitats, and 1.01 kg N ha^–1 year^–1 in forested sites. Results of stable isotope analyses showed clear patterns of resource partitioning among dominant cicada species emerging from grassland sites. T. aurifera and C. calliope had δ¹³C and δ¹⁵N signatures indicative of feeding on shallowly rooted C₄ plants such as the warm-season grasses dominant in tallgrass prairie ecosystems, whereas T. dorsata signatures suggested preferential feeding on more deeply rooted C₃ plants. Received: 20 September 1999 / Accepted: 9 December 1999     

Above and belowground net primary productivity of grassland influenced by supplemental water and nitrogen in Inner Mongolia

While water availability determines grassland productivity in semiarid regions, nutrient availability is the main limiting factor under wet conditions. An experiment was conducted in 2008 at two sites in Inner Mongolia with histories of heavy grazing (HG) and moderate grazing (MG) to study the interactive effects of water and nitrogen on above- and belowground net primary productivity (ANPP and BNPP), biomass partitioning, and plant species composition. The study comprises two water treatments (no irrigation and irrigated when soil water content was below 70% of the field capacity), and two nitrogen (N) levels (0 and 100 kg N ha^?1). Mean values of ANPP at the peak biomass time reached 1,028?±?95 SD g m^?2 at the HG site and 568?±?32 SD g m^?2 at the MG site in irrigated and fertilized treatment. Nitrogen use efficiency (NUE) was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites. Water use efficiency (WUEt) based on total water input and ANPP decreased with irrigation at the HG site. Meanwhile, N application significantly increased WUEt, WUEp (based on precipitation), and WUEi (based on irrigation water) at both sites. BNPP was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites, and it tended to decrease with N addition. However, the fraction of belowground to total biomass (f _BNPP = BNPP/(ANPP+BNPP) decreased with the addition of supplemental resources and exhibited a negative correlation with ANPP. Species diversity remained lower at the HG site compared to the MG site; it decreased with the addition of supplemental resources at the latter site. The annual Salsola collina contributed the most to the total biomass under irrigation. Based on global climate models, more frequent extreme climates are predicted in the future, which can result in changes in resource availabilities. Therefore, our research results have important implications for predicting the production and other properties of grassland ecosystems.     

Soil C and N responses to woody plant expansion in a mesic grassland

Changes in land management and reductions in fire frequency have contributed to increased cover of woody species in grasslands worldwide. These shifts in plant community composition have the potential to alter ecosystem function, particularly through changes in soil processes and properties. In semi-arid grasslands, the invasion of shrubs and trees is often accompanied by increases in soil resources and more rapid N and C cycling. We assessed the effects of shrub encroachment in a mesic grassland in Kansas (USA) on soil CO₂ flux, extractable inorganic N, and N mineralization beneath shrub communities (Cornus drummondii) and surrounding undisturbed grassland sites. In this study, a shift in plant community composition from grassland to shrubland resulted in a 16% decrease in annual soil CO₂ flux(4.78 kg CO₂ m^–2 year^–1 for shrub dominated sites versus 5.84 kg CO₂ m^–2 year^–1 for grassland sites) with no differences in total soil C or N or inorganic N. There was considerable variability in N mineralization rates within sites, which resulted in no overall difference in cumulative N mineralized during this study (4.09 g N m^–2 for grassland sites and 3.03 g N m^–2 for shrub islands). These results indicate that shrub encroachment into mesic grasslands does not significantly alter N availability (at least initially), but does alter C cycling by decreasing soil CO₂ flux.     

Interannual variability in responses of belowground net primary productivity (NPP) and NPP partitioning to long‐term warming and clipping in a tallgrass prairie

The dynamics of belowground net primary productivity (BNPP) is of fundamental importance in understanding carbon (C) allocation and storage in grasslands. However, our knowledge of the interannual variability in response of BNPP to ongoing global warming is limited. In this study, we explored temporal responses of BNPP and net primary productivity (NPP) partitioning to warming and clipping in a tallgrass prairie in Oklahoma, USA. Infrared heaters were used to elevate soil temperature by approximately 2 °C since November 1999. Annual clipping was to mimic hay harvest. On average from 2005 to 2009, warming increased BNPP by 41.89% in the unclipped subplots and 66.93% in the clipped subplots, with significant increase observed in wet years. Clipping also had significant positive impact on BNPP, which was mostly found under warming. Overall, f_BNPP, the fraction of BNPP to NPP, increased under both warming and clipping treatments, more in dry years. Water availability (either precipitation or soil moisture) was the most limiting factor for both BNPP and f_BNPP. It strongly dominated the interannual variability in NPP, f_BNPP, and their responses to warming and clipping. Our results suggest that water availability regulates tallgrass prairie's responses to warming and land use change, which may eventually influence the global C cycle. With increasing variability in future precipitation patterns, warming effects on the vegetation in this region may become less predictable.     

Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO₂ enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO₂ stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C₄ grass species were grown for 5 years along a field CO₂ gradient (250–500 μL L^?1) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO₂ enrichment increased community ANPP by 0–117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO₂‐induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP‐CO₂ regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30–60% of the CO₂‐mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively. By contrast, species change had little influence on the ANPP‐CO₂ response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C₄ grass species. By favoring a mesic C₄ tall grass, CO₂ enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO₂‐stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.     

Consequences of shrub expansion in mesic grassland: Resource alterations and graminoid responses

Abstract. In the mesic grasslands of the central United States, the shrub Cornus drummondii has undergone widespread expansion in the absence of recurrent fire. We quantified alterations in light, water and N caused by C. drummondii expansion in tall‐grass prairie and assessed the hypothesis that these alterations are consistent with models of resource enrichment by woody plants. Responses in graminoid species, particularly the dominant C₄ grass Andropogon gerardii, were concurrently evaluated. We also removed established shrub islands to quantify their legacy effect on resource availability and assess the capability of this grassland to recover in sites formerly dominated by woody plants. The primary effect of shrub expansion on resource availability was an 87% reduction in light available to the herbaceous understorey. This reduced C uptake and N use efficiency in A. gerardii and lowered graminoid cover and ANPP at the grass‐shrub ecotone relative to undisturbed grassland. Shrub removal created a pulse in light and N availability, eliciting high C gain in A. gerardii in the first year after removal. By year two, light and N availability within shrub removal areas returned to levels typical of grassland, as had graminoid cover and ANPP were similar to those in open grassland. Recovery within central areas of shrub removal sites lagged behind that at the former grass‐shrub ecotone. These results indicate that the apparent alternative stable state of C. drummondii dominance in tall‐grass prairie is biotically maintained and driven by reductions in light, rather than resource enrichment. Within areas of shrub removal, the legacy effect of C. drummondii dominance is manifest primarily through the loss of rhizomes of the dominant grasses, rather than any long‐term changes in resource availability. C. drummondii removal facilitates grassland recovery, but the effort required to initiate this transition is a significant cost of woody plant expansion in mesic grasslands. Prevention of woody plant expansion in remnant tall‐grass prairies is, therefore, a preferred management option.     

The effects of fire frequency and grazing on tallgrass prairie productivity and plant composition are mediated through bud bank demography

Periodic fire, grazing, and a variable climate are considered the most important drivers of tallgrass prairie ecosystems, having large impacts on the component species and on ecosystem structure and function. We used long-term experiments at Konza Prairie Biological Station to explore the underlying demographic mechanisms responsible for tallgrass prairie responses to two key ecological drivers: fire and grazing. Our data indicate that belowground bud banks (populations of meristems associated with rhizomes or other perennating organs) mediate tallgrass prairie plant response. Fire and grazing altered rates of belowground bud natality, tiller emergence from the bud bank, and both short-term (fire cycle) and long-term (>15 year) changes in bud bank density. Annual burning increased grass bud banks by 25% and decreased forb bud banks by 125% compared to burning every 4 years. Grazing increased the rate of emergence from the grass bud bank resulting in increased grass stem densities while decreasing grass bud banks compared to ungrazed prairie. By contrast, grazing increased both bud and stem density of forbs in annually burned prairie but grazing had no effect on forb bud or stem density in the 4-year burn frequency treatment. Lastly, the size of the reserve grass bud bank is an excellent predictor of long-term ANPP in tallgrass prairie and also of short-term interannual variation in ANPP associated with fire cycles, supporting our hypothesis that ANPP is strongly regulated by belowground demographic processes. Meristem limitation due to management practices such as different fire frequencies or grazing regimes may constrain tallgrass prairie responses to interannual changes in resource availability. An important consequence is that grasslands with a large bud bank may be the most responsive to future climatic change or other global change phenomena such as nutrient enrichment, and may be most resistant to exotic species invasions.     

Influence of management and precipitation on carbon fluxes in great plains grasslands

Suitable management and sufficient precipitation on grasslands can provide carbon sinks. The net carbon accumulation of a site from the atmosphere, modeled as the Net Ecosystem Productivity (NEP), is a useful means to gauge carbon balance. Previous research has developed methods to integrate flux tower data with satellite biophysical datasets to estimate NEP across large regions. A related method uses the Ecosystem Performance Anomaly (EPA) as a satellite-derived indicator of disturbance intensity (e.g., livestock stocking rate, fire, and insect damage). To better understand the interactions among management, climate, and carbon dynamics, we evaluated the relationship between EPA and NEP data at the 250 m scale for grasslands in the Central Great Plains, USA (ranging from semi-arid to mesic). We also used weekly estimates of NEP to evaluate the phenology of carbon dynamics, classified by EPA (i.e., by level of disturbance impact). Results show that the cumulative carbon balance over these grasslands from 2000 to 2008 was a weak net sink of 13.7 g C m⁻² yr⁻¹. Overall, NEP increased with precipitation (R² = 0.39, P < 0.05) from west to east. Disturbance influenced NEP phenology; however, climate and biophysical conditions were usually more important. The NEP response to disturbance varies by ecoregion, and more generally by grassland type, where the shortgrass prairie NEP is most sensitive to disturbance, the mixed-grass prairie displays a moderate response, and tallgrass prairie is the least impacted by disturbance (as measured by EPA). Sustainable management practices in the tallgrass and mixed-grass prairie may potentially induce a period of average net carbon sink until a new equilibrium soil organic carbon is achieved. In the shortgrass prairie, management should be considered sustainable if carbon stocks are simply maintained. The consideration of site carbon balance adds to the already difficult task of managing grasslands appropriately to site conditions. Results clarify the seasonal and interannual dynamics of NEP, specifically the influence of disturbance and moisture availability.     

Contrasting Effects of Precipitation Manipulations on Production in Two Sites within the Central Grassland Region, USA

In grassland ecosystems, where soil water most frequently controls ecosystem processes, expected changes in precipitation and temperature may have dramatic effects on ecosystem dynamics. Previous observational studies have reported that aboveground net primary production (ANPP) in grasslands is very sensitive to changes in precipitation. Yet, we lack experimentally based evidence to support these observations. Further, most of the studies have focused solely on ANPP, neglecting belowground production (BNPP). This is an important gap in our knowledge, as BNPP represents 50% or more of total net primary production (NPP) in grasslands. Here, we present results from a 3-year water manipulation experiment (2008–2010) at two sites in the central grassland region of North America, USA. We were successful in changing the soil water content in our treatments, but these changes resulted in different, but significant responses in ANPP and BNPP at our two sites. At the shortgrass steppe, we found that neither NPP nor ANPP were sensitive to treatment precipitation, and although we found BNPP was sensitive to changes in treatment precipitation, the direction of the response varied between years. In contrast, ANPP was very sensitive to treatment precipitation on the mixed-grass prairie, whereas BNPP was insensitive. Based on our finding that two grassland ecosystems showed dramatically different above and belowground production responses to soil water manipulations, we cannot assume that predicted changes in climate will cause similar above- and belowground production responses. Second, our results demonstrated that sites within the same region may differ markedly in the sensitivity of ANPP to changes in growing season precipitation.     

Assessing the Rate, Mechanisms, and Consequences of the Conversion of Tallgrass Prairie to Juniperus virginiana Forest

We assessed the determinants and consequences of the expansion of Juniperus virginiana L. (red cedar) populations into central US grasslands using historical aerial photos and field measurements of forest extent, tree growth, fire-induced mortality, and responses in herbaceous species diversity and productivity. Photos from northeast Kansas dating back to 1956 indicate that native tallgrass prairie can be converted to closed-canopy red cedar forest in as little as 40 years (a 2.3% increase in forest cover per year). Mean tree density in 21 forested sites ranged from 130 to 3500 trees/ha, with most sites at more than 800 trees/ha. In younger stands, maximum growth rates of individual red cedar trees exceeded 20 cm/y in height. Land management practices were critical to the establishment and growth of red cedar forest. Grazing reduced the fuel loads by more than 30% in tallgrass prairie. Based on measurements of mortality for more than 1800 red cedar trees, fire-induced mortality in grazed areas averaged 31.6% versus more than 90% at ungrazed sites. When tallgrass prairie was converted to red cedar forest, herbaceous species diversity and productivity were drastically reduced, and most grassland species were virtually eliminated. Consequently, community structure shifted from dominance by herbaceous C₄ species to evergreen woody C₃ species; this shift is likely to be accompanied by alterations in carbon storage and other ecosystem processes in a relatively short time period. Here we present a conceptual model that integrates the ecological and socioeconomic factors that underlie the conversion of grassland to red cedar forest. Received 29 August 2001; accepted 5 February 2002.     

A Benchmark Test for Ecohydrological Models of Interannual Variability of NDVI in Semi-arid Tropical Grasslands

Pulses of aboveground net primary productivity (ANPP) in response to discrete precipitation events are an integral feature of ecosystem functioning in arid and semi-arid lands. Yet, the usefulness of nonlinear, ecohydrological pulse response functions to predict regional-scale patterns of annual ANPP at decadal scales remains unclear. Here, we assessed how different pulse response (PR) models compete with simple linear statistical models to capture variability in yearly integrated values of Normalized Difference Vegetation Index (NDVI_int), a remotely sensed proxy of annual ANPP. We examined 24-year-long time series of NDVI_int calculated from Advanced Very High Resolution Radiometer (AVHRR) NDVI for 350,000 km² of tropical grasslands in northern Australia. Based on goodness-of-fit statistics, PR models clearly outperformed statistical models when parameters were optimized for each site but all models showed the same error magnitude when all sites were combined in ensemble simulations or when the models were evaluated outside the calibration period. PR models were less biased and their performance did not deteriorate in the driest areas compared to linear models. Increasing the complexity of PR models to provide a better representation of soil water balance and its feedback with plant growth did not improve model performance in ensemble simulations. When error magnitude, bias, and sensitivity to parameter uncertainty were all considered, we concluded that a low-dimensional PR model was the most robust to capture NDVI_int variability. This study shows the potential of long time series of AVHRR NDVI to benchmark process-oriented models of interannual variability of NDVI_int in water-controlled ecosystems. This opens new avenues to examine at the global scale and over several decades the causal relationships between climate and leaf dynamics in the grassland biome.     

Above- and Belowground Net Primary Production in a Temperate Mixed Deciduous Forest

Our current ability to detect and predict changes in forest ecosystem productivity is constrained by several limitations. These include a poor understanding of belowground productivity, the short duration of most analyses, and a need for greater examination of species- or community-specific variability in productivity studies. We quantified aboveground net primary productivity (ANPP) over 3 years (1999–2001), and both belowground NPP (BNPP) and total NPP over 2 years (2000–2001) in both mesic and xeric site community types of the mixed mesophytic forest of southeastern Kentucky to examine landscape variability in productivity and its relation with soil resource [water and nitrogen (N)] availability. Across sites, ANPP was significantly correlated with N availability (R² = 0.58, P = 0.028) while BNPP was best predicted by soil moisture content (R² = 0.72, P = 0.008). Because of these offsetting patterns, total NPP was unrelated to either soil resource. Interannual variability in growing season precipitation during the study resulted in a 50% decline in mesic site litter production, possibly due to a lag effect following a moderate drought year in 1999. As a result, ANPP in mesic sites declined 27% in 2000 compared to 1999, while xeric sites had no aboveground production differences related to precipitation variability. If global climate change produces more frequent occurrences of drought, then the response of mesic sites to prolonged moisture deficiency and the consequences of shifting carbon (C) allocation on C storage will become important questions.     

Tallgrass prairie pollen assemblages in mid-continental North America

The mid-continent of North America has experienced dramatic and abrupt climate change during the Holocene, but the response of grassland vegetation to past climate change has been difficult to quantify. To improve interpretation of tallgrass prairie vegetation from pollen assemblages, we acquired and analysed a surface sample set collected from 25 small ponds (less than 10 ha surface area) in the largest contiguous remnant of tallgrass prairie in the USA. We compared these tallgrass prairie assemblages to 476 modern pollen samples classified as “prairie” in the North American Surface Sample database. We then compared the surface pollen assemblages with fossil pollen assemblages from sediment cores at two sites in Kansas—Cheyenne Bottoms and Muscotah Marsh—using the modern analog technique. Pollen assemblages in the Flint Hills surface samples were very similar to each other, with an average squared chord distance of 0.19. They were different than other modern grassland pollen assemblages mainly due to higher percentages of pollen from six woody taxa: Carya, Cornus, Juniperus, Juglans, Maclura, and Platanus. Arboreal pollen percentages ranged from 17 to 62 % and did not correlate with woody cover among sites. Cheyenne Bottoms was open grassland for the past 25,000 years, but it did not have many tallgrass prairie analogs. Muscotah Marsh did not have many grassland analogs over the past 30,000 years, possibly due to its position on the prairie-forest border or its surrounding wetland vegetation.