Total Aboveground Net Primary Productivity Research Articles

Belowground bud banks of tallgrass prairie are insensitive to multi‐year, growing‐season drought

In tallgrass prairie plant communities, new shoots are recruited from belowground bud banks, often in response to disturbance. We explored the contribution of belowground bud banks to grassland stability when perturbed by severe drought. We sought to quantify changes in bud bank density and demography, assess the contribution of the bud bank to aboveground net primary productivity, and compare shifts in above‐ and belowground plant community structure in response to drought. We experimentally reduced precipitation 76% from ambient levels for two years, and compared responses of drought plots to ambient and irrigated controls during the two years of drought and two years of recovery. We measured belowground bud bank density and aboveground shoot density, canopy cover, and aboveground net primary productivity. While aboveground net primary productivity and C4 grass flowering shoot density declined during drought, bud bank density was insensitive to drought. Rapid resilience of the aboveground plant community following cessation of drought appears to be mediated by the resistance of bud banks to drought. Shoot density, but not bud bank density, was positively correlated with total aboveground net primary productivity. Furthermore, proportional reductions in aboveground net primary productivity increased as shoot density increased. Plant community structure shifted both above‐ and belowground in response to precipitation treatments. Our results suggest that the response of perennial grasslands to drought may be mediated by the stable belowground bud bank, with shifts in abundance of individual species maintaining overall bud bank density and ecosystem function.

Quantifying ecological drivers of ecosystem productivity of the early‐successional boreal Larix gmelinii forest

Wildfire area and severity are projected to increase in Eurasian larch (Larix spp.) forest, potentially altering ecosystem dynamics and carbon balance under warming climate. However, the ecological drivers that influence post‐fire ecosystem productivity in larch forest are poorly understood. Here we assess the influence of several factors controlling plant aboveground net primary productivity (ANPP) on the early successional stage of Dahurian larch (Larix gmelinii) forests following fire. We quantified the relative importance of fire severity, competition, topography, and soil inorganic N [NH4+ and NO3−] availability on ecosystem productivity 11 years post‐fire in a Dahurian larch forest of Northeast China using a boosted regression trees technique. Results showed that fire severity had the strongest negative influence on ANPP of saplings, followed by a positive influence of soil inorganic N availability and negative influence of elevation. Sapling density had the strongest negative influence on ANPP of understory (shrub and grass), followed by positive influence of aspect and inorganic N availability. N may limit ANPP of sapling, but not of understory vegetation. Fire severity had a dominant influence on the proportion of total ANPP contributed by the understory. Fire severity was positively correlated to understory productivity and proportion of total ANPP contributed by understory, but negatively correlated to sapling productivity. Our analysis suggests that increasing fire severity may increase understory productivity, while decreasing the sapling productivity, which may contribute to the potential changes of vegetation type and carbon cycling in Dahurian larch forests of Northeast China under warming climate.

Effects of artificially varying amounts of rainfall on two semi‐natural grassland types

AbstractQuestionIs a lowland grassland community with a lower amount of ambient precipitation and a high abundance of subdominant species more sensitive to altered rainfall input in comparison with mountain grassland with higher ambient precipitation and a low abundance of subdominant species?Location(1) Lowland Festuca grassland characterized by dry acidophilous vegetation, low mean annual precipitation and high species richness, and (2) mountain Nardus grassland with high mean annual precipitation and low species richness in the Eastern Czech Republic.MethodsWe conducted a manipulative experiment with rainout shelters and gravity irrigation systems. We simulated three levels of precipitation input (50%, 100% and 150%) and examined the treatment effects on above‐ground net primary productivity (ANPP) of the main plant functional groups (grasses, forbs, legumes, shrubs and mosses) and on plant species composition over three consecutive years.ResultsAltered rainfall input had a significant effect on total live ANPP in the drier, lowland grassland community, while no pronounced effect was observed in the mountain grassland characterized by higher ambient precipitation. In the lowland grassland, changes in total ANPP resulted primarily from enhanced growth of subdominant non‐N‐fixing forbs in response to the direct effect of higher artificial rainfall input and an indirect effect of reduced competition with grasses in drier treatments.ConclusionsWetter, species‐poor mountain grasslands will probably show smaller changes in response to expected climate changes in ANPP, species number and abundance than lowland grasslands, characterized by dry acidophilous vegetation, with higher species richness and higher abundance of subordinate species.

Ecosystem functional changes associated with land clearing in NW Argentina

We assessed the extension of natural habitat conversion into croplands and grazing lands in subtropical NW Argentina and its impact on two key ecosystem functional attributes. We quantified changes in remotely sensed surrogates of aboveground net primary production (ANPP) and seasonality of carbon gains. Both functional attributes are associated with intermediate ecosystem services sensuFisher et al. (2009). Deforestation was estimated based on photointerpretation of Landsat imagery. The seasonal dynamics of the MODIS satellite Enhanced Vegetation Index (EVI) was used to calculate the EVI annual mean as a surrogate of ANPP, and the EVI seasonal coefficient of variation as an indicator of seasonal variability of carbon gains. The 2000–2007 period showed a high rate of land clearing: 5.9% of NW Argentina (1,757,600ha) was cleared for agriculture and ranching, corresponding to an annual rate of 1.15%. Dry forests experienced the highest rate and humid forests the lowest. Though land clearing for agriculture and ranching had relatively small impacts on total annual ANPP, once deforested, parcels significantly became more seasonal than the natural vegetation replaced. Such increase in seasonality is associated with a reduction of photosynthetic activity during a portion of the year (fallow). Direct consequences of this reduction can be expected on several ecosystem services such as erosion control and water regulation, due to greater exposure of bare soil, and biodiversity, due to the loss or decline in habitat quality and the decrease of green biomass availability for primary consumers during fallow. Land clearing also increased the magnitude of inter-annual differences in C gains, suggesting a greater buffer capacity against climate fluctuations of natural vegetation compared to croplands.

Forage production in natural and afforested grasslands of the Pampas: ecological complementarity and management opportunities

In managed rangelands periods of low primary productivity determine troughs of forage availability, constraining animal production year-round. Although alternative tools to increase forage availability during critical seasons exists, most of them are unaffordable and short-lived in marginal areas. We explore the potential benefits of deciduous tree plantations favoring winter forage productivity by comparing aboveground net primary productivity (ANPP) patterns in herbaceous understory to tree plantations and natural grasslands in the Pampas (Argentina). These temperate subhumid grasslands are characterized by the coexistence of winter species, mainly C3 grasses of the native genera Stipa, Piptochaetium, and Bromus and the exotic genera Lolium and Festuca) and summer species (mainly C4 grasses of the native genera Paspalum, Bothriochloa, and Stenotaphrum) that replace each other throughout the seasons, with domination of the latter. We hypothesize that the natural decoupling of growing seasons between winter deciduous trees and winter grasses could provide the basis for the sustainable promotion of winter forage. We measured ANPP on two 23-year-old Populus deltoides plantations and their understory and compared them with adjacent open grasslands. Afforested stands had 55–75% higher annual ANPP than their non-afforested neighbors, with trees contributing ~70% to total ANPP. Herbaceous canopies beneath plantations achieved about half of the ANPP observed in non-afforested situations with a contrasting seasonal distribution associated with shifts from C4 to C3 grass dominance. Winter ANPP, the most critical source of forage in these grazing systems, was similar or higher in the herbaceous understory of tree plantations to that on their non-afforested counterparts, suggesting that mixed systems involving deciduous trees and understory pastures are a valid and viable option in the region.

Long-term tree productivity of a South Carolina coastal plain forest across a hydrology gradient

Aims In recent years, there has been an increased interest in examining changes in forest systems in response to drought, flooding, hurricanes and climate change. In the southern United States, forested wetlands are of special interest because of the extent of these forests. Coastal plain forested wetlands are among the most vulnerable to these climatic impacts. One of the problems in developing management practices for these coastal areas is the difficulty in adequately describing productivity relations and predicting how the structure and function of these communities might be affected by natural or anthropogenic disturbances. Community response to environmental change often occurs over a period of years, and the majority of reported studies are for 1–3 years in duration. This study documents long-term changes (10 years) in structure, composition and growth along a catena of high water table forested sites of an ancient beach ridge landscape in coastal South Carolina. Methods Aboveground net primary production (ANPP) of trees was monitored from 2000 to 2009 on three sites within a longleaf pine-swamp blackgum forest system on the southern end of the Waccamaw Neck area of Georgetown County, SC. Permanent study plots (20 3 25 m) were established across a moisture gradient (Dry, Intermediate, and Wet). Water levels were continuously monitored, litterfall was measured monthly and growth of trees >10 cm diameter at breast height was monitored on an annual basis. Annual litterfall and tree production values were summed to provide estimates of ANPP. Important Findings The study site was under severe drought conditions July 2001 through late summer 2002 and again in 2007. Diameter growth was affected in all three sites, but with different patterns. It seems that diameter growth in the Wet site was more sensitive to drought conditions in 2001–02 and 2007 than either Dry or Intermediate sites. While droughts did not seem to have a significant impact on litterfall in the Wet site, litterfall in the Intermediate site was more sensitive to the drought than either Dry or Wet sites. ANPP was significantly lower in both Intermediate and Wet sites in 2001 at 1 000 g/m) occurred in the Intermediate and Wet sites in 2003 following a return to more normal water levels at the end of the drought. Maximum tree production occurred on the Wet site in 2003 (657 g/m), which exceeded total ANPP of any site in 2001. In the Dry site, ANPP remained relatively consistent throughout the study when compared to Wet and Intermediate sites. While litterfall estimates are well defined with 3–5 years of data, data collection is continuing to assess impact of drought on stem growth across the gradient, which is still not clear with 10 years of data.

Effects of selective tree harvests on aboveground biomass and net primary productivity of a second-growth northern hardwood forest

Restoring structural features of old-growth forests, such as increased canopy gap sizes and coarse woody debris, is a common management goal for second-growth, even-aged stands. We experimentally manipulated forest structure by creating variable-size canopy gaps in a second-growth northern hardwood forest in north-central Wisconsin following two growing seasons of pre-treatment monitoring. The objectives of this study were to quantify the influence of canopy gaps of different sizes (50–380 m2) on aboveground biomass and productivity of each vegetation stratum two growing seasons following treatment. Two years after treatment, ground layer biomass in canopy openings increased significantly relative to surrounding undisturbed transition zones. The response of ground layer biomass was greatest in the large versus the medium and small gaps. Sapling aboveground net primary productivity was significantly greater in undisturbed transition zones than within gaps across gap sizes following the second post-treatment growing season. Annual stem diameter increment was greatest for trees along gap borders and was correlated with crown class, percentage of crown perimeter exposed, gap area, and shade tolerance. Total aboveground net primary productivity was significantly lower in the gap addition plots the first year but by the second post-treatment growing season no longer differed from that in the control plots.

Soil calcium, nitrogen, and water are correlated with aboveground net primary production in northern hardwood forests

Multiple soil resources could limit aboveground net primary production (ANPP) in temperate forests, but most studies have focused on nitrogen (N) availability. We tested a suite of resources including water, multiple measures of N, phosphate (PO 4), and extractable ions (calcium (Ca), potassium (K), magnesium (Mg)) as correlates of ANPP across a glacial landform gradient in northwestern Lower Michigan. Our goals were to identify resource correlates of ANPP that could potentially limit productivity, to characterize productivity partitioning to leaves (ANPP L) and wood (ANPP W) across the gradient, and to test soil resource correlates of partitioning. We measured soil resource availability and annual ANPP at 13 sites, and fit models of ANPP W and ANPP L as functions of each resource. We used Akaike's Information Criterion to assess empirical support for models with different functional forms. ANPP W was best correlated with Ca, with a relationship that was asymptotic beyond the four lowest-productivity sites. Both Mg and K covaried with Ca; Mg was also supported as an ANPP W correlate. Soil water and sum of ammonium (NH 4) and nitrate (NO 3) (ΣN) explained substantial ANPP W variance, but with weaker support than for Ca. ANPP L was best correlated with ΣN; NO 3 received support, while Ca received less support but still explained substantial ANPP L variance. These results suggest a potential role for Ca in regulating ANPP, at least in low-productivity sites, while reaffirming the importance of both N and water availability. Partitioning of annual production to ANPP W as a fraction of total ANPP was related to Ca and soil water, but only weakly to ΣN. Production was partitioned equally between ANPP L and ANPP W at low fertility, but ANPP W exceeded ANPP L by 24% at maximum productivity. Given this shift in partitioning, measuring only ANPP W but assuming that ANPP L scales proportionately would overestimate ANPP at higher fertility sites. Although correlative, our results suggest that ANPP is constrained by some combination of N, Ca, and water, and provide motivation for experimental manipulations of these resources to better understand forest responses to human influences. If northern temperate forests are strictly N-limited, as suggested by much of the literature, then anthropogenic N deposition should enhance ANPP. In contrast, if ANPP is limited, or co-limited, by Ca or by soil water, then anthropogenic change (altered precipitation, base cation leaching), could counteract production gains from N deposition.

Nonvascular contribution to ecosystem NPP in a subarctic heath during early and late growing season

Bryophytes and lichens abound in many arctic ecosystems and can contribute substantially to the ecosystem net primary production (NPP). Because of their growth seasonality and their potential for growth out of the growing season peak, bryophyte and lichen contribution to NPP may be particularly significant when vascular plants are less active and ecosystems act as a source of carbon (C). To clarify these dynamics, nonvascular and vascular aboveground NPP was compared for a subarctic heath during two contrasting periods of the growing season, viz. early-mid summer and late summer-early autumn. Nonvascular NPP was determined by assessing shoot biomass increment of three moss species (Hylocomium splendens, Pleurozium schreberi and Dicranum elongatum) and by scaling to ecosystem level using average standing crop. For D. elongatum, these estimates were compared with production estimates obtained from measurements of shoot length increase. Vascular NPP was determined by harvesting shrub and herb apical growth and considering production due to stem secondary growth of shrubs. Hylocomium splendens and Pleurozium schreberi showed highest biomass growth in late summer, whereas for D. elongatum this occurred in early summer. Maximum relative growth rates were ca. 0.003–0.007 g g−1 d−1. For D. elongatum, production estimates from length growth differed from estimations from biomass growth, likely because of an uncoupling between length growth and biomass shoot growth. Nonvascular NPP was 0.37 and 0.46 g dry weight m−2 d−1, in early and late summer, respectively, whereas in the same periods vascular NPP was 3.6 and 1.1 g dry weight m−2 d−1. The contribution of nonvascular NPP to total aboveground NPP was therefore minor in early summer but substantial in late summer, when 25% of the C accumulated by the vegetation was incorporated into nonvascular plant tissue. The expected global change-induced reduction of nonvascular plant biomass in subarctic heath is likely therefore to enhance C release during the late part of the growing season.

Nutrient cycling in age sequences of two Eucalyptus plantation species

Accumulation and aboveground cycling of nitrogen, phosphorus, potassium, calcium and magnesium were quantified in two north coast New South Wales plantation sequences, Eucalyptus grandis (2–30 years of age) and E. pilularis (3–33 years of age). Maximum aboveground accumulated tree biomass of E. grandis and E. pilularis was 394 tonne ha −1 at 27 years of age and 270 tonne ha −1 at 33 years of age respectively and there was a further 42 and 6.7 tonne ha −1 respectively contained in the understorey. The estimated maximum current annual accumulation rate of tree biomass for E. grandis and E. pilularis was 16.4 tonne ha −1 year −1 at 5 years of age and 15.7 tonne ha −1 year −1 at 7 years of age, respectively, and maximum total aboveground net primary production (NPP) was 27.9 and 24.4 tonne ha −1 year −1, respectively. Total quantities of accumulated nutrients for E. grandis were much higher than for E. pilularis at comparable ages resulting from higher growth rates, higher tissue nutrient concentrations and different distributions of biomass components. The annual rate of nutrient accumulation for all nutrients reached a peak near the time of crown closure and then declined, the rate of decline differing between species. The annual rate of accumulation of nitrogen by the two species followed a very similar pattern but for all other nutrients was much lower for E. pilularis than for E. grandis. Nutrient requirement and uptake were higher for E. grandis than E. pilularis and however, reached a peak near crown closure in both species and then declined to a stable level in both species. Uptake of nitrogen, phosphorus and potassium was lower than requirement in both species indicating nutrient redistribution, while for calcium and magnesium, uptake was higher than requirement. Although a relatively minor component of the total biomass, the understoreys were a significant component of nutrient cycling. This was due to the higher rate of turnover of biomass and higher concentrations of nutrients in understorey than in trees.

Effect of Phosphate Fertilization on Flooding Pampa Grasslands (Argentina)

Abstract We postulate that phosphorus (P) fertilization may increase above-ground net primary productivity ( ANPP ) of rotationally grazed rangelands without reducing the legume component, as does N fertilization. In doing so, we evaluated the effect of phosphate fertilization on the production and relative contribution of legumes and grasses of native and old tall fescue ( Festuca arundinacea Schreb) grasslands; we recorded annual production, seasonal productivity, and biomass contribution of each component. The experiment was conducted in a commercial farm located in the Flooding Pampa and managed under rotational grazing. Treatments consisted of two fertilization programs (66 (P 66 ) and 29 (P 29 ) kg P · ha -1 supplied as rock phosphate and/or monoammonium phosphate from 1997 to 1999) and a nonfertilized control. A paddock dominated by native grassland and another dominated by old tall fescue grassland were selected. Nine 5-ha plots were established in each paddock, and treatments were randomly assigned. During the experimental period, from October 1998 to October 1999, total above-ground biomass was harvested from each plot before and after each grazing period and separated into components: tall fescue, other C 3 perennial grasses, legumes, C 3 annual grasses, C 4 grasses, forbs, and standing dead material. ANPP of each component was estimated during the warm (October 1998–February 1999) and the cool (March 1999–September 1999) season. In native grassland, phosphate fertilization increased ANPP of C 3 annual grasses and legumes during both the warm and the cool seasons; therefore annual ANPP of the grassland under P 66 was 40% higher than under P 29 and doubled ANPP of nonfertilized plots. Phosphate fertilization didn't increase total annual ANPP of old tall fescue grassland, but it did increase ANPP of legumes during both seasons.

EXPERIMENTAL WARMING, NOT GRAZING, DECREASES RANGELAND QUALITY ON THE TIBETAN PLATEAU

We investigated experimental warming and simulated grazing (clipping) effects on rangeland quality, as indicated by vegetation production and nutritive quality, in winter-grazed meadows and summer-grazed shrublands on the Tibetan Plateau, a rangeland system experiencing climatic and pastoral land use changes. Warming decreased total aboveground net primary productivity (ANPP) by 40 g x m(-2) x yr(-1) at the meadow habitats and decreased palatable ANPP (total ANPP minus non-palatable forb ANPP) by 10 g x m(-2) x yr(-1) at both habitats. The decreased production of the medicinal forb Gentiana straminea and the increased production of the non-palatable forb Stellera chamaejasme with warming also reduced rangeland quality. At the shrubland habitats, warming resulted in less digestible shrubs, whose foliage contains 25% digestible dry matter (DDM), replacing more digestible graminoids, whose foliage contains 60% DDM. This shift from graminoids to shrubs not only results in lower-quality forage, but could also have important consequences for future domestic herd composition. Although warming extended the growing season in non-clipped plots, the reduced rangeland quality due to decreased vegetative production and nutritive quality will likely overwhelm the improved rangeland quality associated with an extended growing season. Grazing maintained or improved rangeland quality by increasing total ANPP by 20-40 g x m(-2) x yr(-1) with no effect on palatable ANPP. Grazing effects on forage nutritive quality, as measured by foliar nitrogen and carbon content and by shifts in plant group ANPP, resulted in improved forage quality. Grazing extended the growing season at both habitats, and it advanced the growing season at the meadows. Synergistic interactions between warming and grazing were present, such that grazing mediated the warming-induced declines in vegetation production and nutritive quality. Moreover, combined treatment effects were nonadditive, suggesting that we cannot predict the combined effect of global changes and human activities from single-factor studies. Our findings suggest that the rangelands on the Tibetan Plateau, and the pastoralists who depend on them, may be vulnerable to future climate changes. Grazing can mitigate the negative warming effects on rangeland quality. For example, grazing management may be an important tool to keep warming-induced shrub expansion in check. Moreover, flexible and opportunistic grazing management will be required in a warmer future.

Changes in aboveground primary production and carbon and nitrogen pools accompanying woody plant encroachment in a temperate savanna

AbstractWhen woody plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with woody plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis+herbaceous) of sites with the highest Prosopis basal area were 1.2 × and 4.0 × greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0–10 cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional plant traits

Season‐long elevation of ozone concentration to projected 2050 levels under fully open‐air conditions substantially decreases the growth and production of soybean

Mean surface ozone concentration is predicted to increase 23% by 2050. Previous chamber studies of crops report large yield losses caused by elevation of tropospheric ozone, and have been the basis for projecting economic loss. This is the first study with a food crop (soybean, Glycine max) using free-air gas concentration enrichment (FACE) technology for ozone fumigation. A 23% increase in ozone concentration from an average daytime ambient 56 p.p.b. to a treatment 69 p.p.b. over two growing seasons decreased seed yield by 20%. Total above-ground net primary production decreased by 17% without altering dry mass allocation among shoot organs, except seed. Fewer live leaves and decreased photosynthesis in late grain filling appear to drive the ozone-induced losses in production and yield. These results validate previous chamber studies suggesting that soybean yields will decrease under increasing ozone exposure. In fact, these results suggest that when treated under open-air conditions yield losses may be even greater than the large losses already reported in earlier chamber studies. Yield losses with elevated ozone were greater in the second year following a severe hailstorm, suggesting that losses caused by ozone might be exacerbated by extreme climatic events.

NET PRIMARY PRODUCTIVITY OF A WESTERN MONTANE RIPARIAN FOREST: POTENTIAL INFLUENCE OF STREAM FLOW DIVERSION

ABSTRACT We estimated aboveground and belowground net primary productivity (NPP) for two reaches of a montane riparian ecosystem in the eastern Sierra Nevada Mountains of California with differing stream flow regimes resulting from varying degrees of stream flow diversion for hydroelectric power generation. Total understory productivity (herbaceous and shrub) was 2.5 times higher (P < 0.001) at the high-flow site than at the low-flow site (22.4 and 8.9 g C m−2 yr−1, respectively). Annual litterfall was also higher (P = 0.03) at the high-flow (235 g C m−2 yr−1) than at the low-flow site (180 g C m−2 yr−1). However, tree and total aboveground NPP, as well as annual soil respiration and belowground NPP, were all statistically similar between sites. Furthermore, total (above- plus below-ground) NPP was statistically similar between sites (903 and 643 g C m−2 yr−1 for the high- and low-flow sites, respectively). These productivity estimates are, to our knowledge, the first ever reported for a western montane r...

PH and nutrient effects on above-ground net primary production in a Minnesota, USA bog and fen

Nutrient limitation is often assumed to be similar among the species of a plant community. However, limitation can differ among ecosystems and among life forms and individual species within a particular ecosystem. Peatlands have some of the lowest nutrient availabilities and highest acidities among wetland types, but the relative roles of nutrient limitation and pH stress in structuring peatland plant communities are unknown. Accordingly, we measured changes in above-ground net primary production (ANPP) and percent cover of plants to additions of low levels of N, P, and calcium carbonate in a bog and fen in northern Minnesota, USA. Plots were treated for three years with a combination of 2 or 6 g N m−2 yr−1 as ammonium, 0.67 or 2 g P m−2 yr−1, and/or calcium carbonate to raise the pH of the bog from 3.8 to 4.9 and the pH of the fen from 4.9 to 6.4. In the bog, the low N treatment increased ANPP, whereas the high N treatment inhibited ANPP. Lime addition also stimulated ANPP. The whole-community bog response was largey due to bryophytes, which accounted for 76% of ANPP on average. However, the productivity of the shrub community (18% of total ANPP) increased with P additions but only during the third year of fertilizer application. Productivity of the bog graminoids did not respond significantly to any addition. Fen ANPP was stimulated by P addition, but the effect was isolated to graminoids (95% of total ANPP), and this was largely due to the response of Carex exilis. Our results suggest that low nutrient availability does not necessarily imply nutrient limitation of peatland plant communities. Furthermore, life forms and individual species responded differently, indicating that there are several levels of nutrient limitation within each peatland community. In particular, bog Sphagnum mosses appear to have a very low tolerance for N. Production and community structure were controlled by N-availability and pH in the bog and by P-availability in the fen.

Response of an understory plant community to elevated [CO2 ] depends on differential responses of dominant invasive species and is mediated by soil water availability.

• Rising atmospheric CO2 concentrations are likely to have direct effects on terrestrial ecosystems. Here, we describe effects of elevated concentrations of CO2 on an understory plant community in terms of production and community composition. • In 2001 and 2002 total and species-specific above-ground net primary productivity (ANPP) were estimated by harvesting above-ground biomass within an understory community receiving ambient [CO2 ] and elevated [CO2 ] at Oak Ridge National Laboratory's free-air carbon dioxide enrichment (FACE) facility. • During a wet year, community composition differed between plots receiving ambient [CO2 ] and elevated [CO2 ], but total ANPP did not differ. By contrast, during a drier year, community composition did not differ, but total ANPP was greater in elevated than ambient [CO2 ] plots. These patterns were driven by the response of two codominant species, Lonicera japonica and Microstegium vimineum, both considered invasive species in the south-eastern United States. The ANPP of L. japonica was consistently greater under elevated [CO2 ], whereas the response of M. vimineum to CO2 enrichment differed between years and mediated total community response. • These data suggest that community and species responses to a future, CO2 -enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses.

Productivity responses to altered rainfall patterns in a C4-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), an approximately 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.

SOIL RESOURCES REGULATE PRODUCTIVITY AND DIVERSITY IN NEWLY ESTABLISHED TALLGRASS PRAIRIE

In native tallgrass prairie, soil depth and nitrogen (N) availability strongly influence aboveground net primary productivity (ANPP) and plant species composition. We manipulated these factors in a newly restored grassland to determine if these resources similarly constrain productivity and diversity during the initial three years of grassland establishment. Four types of experimental plots with six treatment combinations of deep and shallow soil at reduced-, ambient-, and enriched-N availability formed the basis of this study. The soil responses to the experimental treatments were examined over three years, and patterns in diversity and productivity were examined in year 3. The soil depth treatment did not significantly affect soil carbon (C) and N pools or ANPP and diversity. A pulse amendment of C added to the soil prior to planting increased soil microbial biomass and decreased potential net N mineralization rates to effectively reduce N availability throughout the study. Nitrogen availability declined over time in nonamended soils as a result of plant establishment, but adding fertilizer N alleviated the increasing immobilization potential of the soil. The level of ANPP was lowest and diversity highest in the reduced-N treatment, whereas the enriched-N treatment resulted in high productivity, but low diversity. As a result, diversity was inversely correlated with productivity in these newly established com- munities. The same mechanism invoked to explain decreased diversity under nutrient en- richment in old-field ecosystems and native grasslands (e.g., reduced light availability with increased production) was supported in the restored prairie by the positive relationship between ANPP and intercepted light, and a strong correlation between light availability and diversity. The effects of nutrient availability on plant community composition (diversity and richness) were due primarily to the responses of prairie species, as the productivity of early successional, nonprairie species was less than 1% of total ANPP after three years of establishment. These results show that the effects of resource availability on productivity and diversity are similar in young and mature grasslands, and that manipulation of a limiting nutrient during grassland establishment can influence floristic composition, with conse- quences for long-term patterns of diversity in restored ecosystems.

Effects of nutrient and paper mill biosolids amendments on the growth and nutrient status of hardwood forests

Aboveground net primary productivity (ANPP) and soil and vegetation nutrient concentrations were measured in pole-sized northern hardwood and aspen forests to quantify the potential for various nutrient amendments to increase tree growth, as well as potential deleterious effects on vegetation. Four blocks were installed in each forest type containing the following treatments: control, N+Ca+Mg+K+P+S (complete), wood-fired boiler ash (ash), N+wood-fired boiler ash (N+ash), and two rates of paper mill sludge (sludge and 2× sludge). The northern hardwood forests had three additional treatments: N, Ca+Mg+K (base cations), and N+Ca+Mg+K (N+base cations). Fertilizer treatments were designed to provide N at a rate of 100 kg ha −1. Other nutrients were applied in proportion to N so that N, P, K, S, Mg, and Ca were applied at a ratio of 100:50:33:9:5:4, respectively. Total ANPP was not significantly different among treatments for the maple stands, but plots treated with wood ash were 10% greater than controls. ANPP was greater in 1998 and 1999 for aspen plots receiving N+ash, but results were only statistically significant ( p=0.013) in 1999. Basal area differed by 40–60% among experimental plots within any given block and was positively correlated to wood increment. Wood increment, adjusted for basal area, did not differ significantly among treatments for the aspen stands, but the N+ash treatment was 30% greater than the control. A two-way factorial comparison indicated that ash had a significant ( p=0.024) positive effect on wood increment and N had a significant ( p=0.044) negative effect on wood increment for the maple stands. Tree growth was not correlated to foliar nutrient concentrations, but ratios of K to other elements (K:Ca, K:Mg, K:N, and K:P) were highest, on average, for wood ash plots. These differences were not statistically significant with the exception of K:N for the aspen stands and K:Ca for the maple stands. We did not observe any beneficial effects of paper mill sludge on forest growth, but there was some evidence that wood ash increased annual wood production.