Greater Net Primary Productivity Research Articles

Climate seasonality and extremes influence net primary productivity across California’s grasslands, shrublands, and woodlands

Terrestrial vegetation is a substantial carbon sink and plays a foundational role in regional and global climate change mitigation strategies. The state of California, USA, commits to achieving carbon neutrality by 2045 in part by managing terrestrial ecosystems to sequester more than 80 MMT of CO2. We used a 35-year net primary productivity (NPP) remote sensing product with gridded climate, soil, topography, and vegetation data to evaluate spatiotemporal drivers of NPP variation and identify drivers of NPP response to extremes in water availability in California’s major grasslands, shrublands, and woodlands. We used generalized boosted models (GBMs) and linear mixed effects models (LMMs) to identify influential predictors of NPP and characterize their relationships with NPP across seven major vegetation cover types: annual grasslands, blue oak, chamise-redshank chaparral, coastal scrub, coastal oak woodland, mixed chaparral, and montane hardwood. Climate seasonality, specifically greater precipitation and warmer minimum temperatures in early spring and winter, was associated with greater NPP across space, particularly in chaparral, blue oak, and grassland systems. Maximum annual temperature and climatic water deficit (CWD) showed a negative relationship with NPP in most vegetation cover types, particularly chaparral and coastal scrub. We found a significant decrease in NPP over time in most vegetation types, appearing to coincide with the 2012–2016 California mega-drought. However, response to water availability extremes differed by vegetation type. In most vegetation types, especially grasslands, increases in NPP in extreme wet years were greater than declines in NPP in dry years. Our analysis characterizes several climate risks and conservation opportunities in using California’s natural lands to store carbon. Namely, shifts in climate seasonality and water availability extremes threaten these systems’ ability to fix carbon, yet hotspots of NPP resilience may exist and could be enhanced through conservation and restoration. Additional mechanistic work can help illuminate these opportunities and prioritize conservation decision making.

No tillage with previous plastic covering increases water harvesting and decreases soil CO2 emissions of wheat in dry regions

In arid and highly populated areas, water shortage and soil CO2 emission (CE) are seriously threatening the sustainability of agriculture. Optimization of tillage, irrigation, and nitrogen (N) application within cropping systems are keys to reducing CE. We hypothesized that an integrated system could increase crop yields and improve water harvesting, while reducing CE and enhancing C sequestration potential from crop production. In 2016–2018, this hypothesis was tested under spring wheat of maize-wheat rotation in field experiments in the arid inland irrigation of China. This research evaluated how no tillage, reduced the supply levels of irrigation and N affect crop yield, water harvesting, and CE, C sequestration potential of following spring wheat after plastic-mulched maize. Across the three study years, tillage practice, irrigation level, and N level individually had a significant effect on grain yield, water harvesting, and CE characteristics of wheat in each year, but the interaction among the three factors was hardly significant. No tillage with previous residual plastic covering (NTP), reduced irrigation, and reduced N application increased grain yield, improved water harvesting of wheat, but decreased CE. On average, NTP practice combined with a reduction of 20 % in irrigation and N supply levels (i.e., the optimized system, NTPI2N2), increased grain yield, soil water storage between pre-sowing and post-harvesting (wheat harvesting), and the ratio of transpiration to evaporation (T/E) by 13.6, 150.0, and 79.7 % (P < 0.05), respectively, compared to conventional tillage with local conventional high supply level in irrigation and N (i.e., the control, CTI2N3). The optimized system reduced CE and carbon emission intensity (CEI) by 31.3 % and 39.0 % (P < 0.05) compared to the control treatment, respectively. However, the optimized system had 7.5, 12.6, 7.5, and 57.4 % (P < 0.05) greater net primary production (NPP), net ecosystem production (NEP), carbon in harvested wheat (harvest-C), and the ratio of NPP/CE than the control. It indicates that the optimized system had greater potential for enhancing soil C sequestration. Therefore, no tillage with previous residual plastic covering from the previous maize combined with a 20 % reduction in both irrigation and N levels is a promising strategy to improve water harvesting, reduce CE, and enhance soil C sequestration potential of wheat production in dry regions.

Foraging and overwintering behavior of loggerhead sea turtles Caretta caretta in the western North Atlantic

Discerning the foraging habitat requirements of wildlife is key to providing for their conservation and management, especially with rare species. Sea turtles are slow-growing, late-maturing species that undertake wide-ranging migrations, making them especially susceptible to changes and disruptions in their environment. To protect and successfully manage these imperiled populations, an understanding of their spatial ecology is required; thus, characterizing critical habitats, identifying high-density areas, and identifying foraging regions is essential. We captured 30 loggerhead sea turtlesCaretta caretta(male and female; juvenile and adult) in the estuarine waters of North Carolina (USA) and tracked them in western North Atlantic neritic (nearshore and offshore) waters. Using a combination of satellite telemetry and spatial modeling techniques, we characterized their movements and identified foraging and overwintering sites. Average core-use areas in the north had greater net primary production (NPP) and were smaller than those in the south, indicating more abundant marine resources in northern foraging regions. In summer, loggerheads migrated to both northern and southern foraging grounds, but most (53%) resided within North Carolina neritic waters. Likewise, the majority of loggerheads (67%) we tracked in winter remained in North Carolina neritic waters, underscoring the importance of this area as year-round foraging habitat, and lending to its consideration as potential critical habitat for both juvenile and adult loggerheads. The change to foraging behavior mode was significantly influenced by day of the year, geographic location, and NPP; however, individual-specific factors influenced switching probabilities relative to other covariates. Data highlighting ‘hotspots’ or densely used areas by foraging sea turtles can thus be used by conservation managers to make informed decisions concerning sea turtle conservation measures.

Spatiotemporal variations in productivity and water use efficiency across a temperate forest landscape of Northeast China

BackgroundForests are increasingly valued for non-timber ecosystem services in place of conventional wood fiber production. Biomass carbon sequestration is one of the key ecosystem services forests are relied upon for mitigating global climate change. However, planning for large-scale tree planting and managing established forest stands for carbon sequestration require careful consideration of the gain in biomass production and tradeoff for other regulatory services. How a tradeoff between forest production and conservation of water resources is shaped by the condition of forest stand and environmental factors remains a question of broad interest in sustainable forest ecosystem management.MethodsWe studied the spatiotemporal patterns of net primary productivity (NPP), evapotranspiration (ET), and water use efficiency (WUE), and their relationships with local climatic and forest stand factors over a temperate forest landscape in Changbai Mountain, Northeast China. The time series of spatial data on NPP and ET were extracted from the global remote sensing datasets for the MOD16A3 and MOD17A3 products for the period 2000–2014. The time series of spatial patterns of annual precipitation and annual mean temperature were obtained as grid maps for regional meteorological variables. Stand patches were categorized into the types of conifers, broadleaves, and mixed-wood, as well as age-classes of young, mid-age, near mature, mature, and old-growth stands, and by establishment into natural and planted. Information on stands and selective site variables were compiled from the Forest Inventory Datasets of China. Analyses were performed with Arc-GIS.ResultsOver the study period of 2000–2014, the landscape-level annual NPP varied between 311.7 and 573.6 gC∙m− 2∙a− 1, ET between 559.9 and 603.0 mm∙a− 1, and WUE between 0.54 and 1.01 gC∙m− 2∙mm− 1. Across the forest landscape, the mean annual NPP varied between 205.0 and 639.4 gC∙m− 2∙a− 1, ET between 441.5 and 784.0 mm∙a− 1, and WUE in the range of 0.46–1.10 gC∙m− 2∙mm− 1. The spatial variations in NPP, ET, and WUE were commonly attributable to forest type, stand age class and density, establishment mode, and temperature variables, with some effects of other selective factors on ET and WUE. The three forest types were significantly (p < 0.05) differentiated in the mean annual NPP, ET, and WUE: the coniferous forests were highest in NPP (505.3 ± 1.4 gC∙m− 2∙a− 1; n = 1041) and WUE (0.872 ± 0.004 gC∙m− 2∙mm− 1; n = 1041), and lowest in ET (584.1 ± 1.6 mm∙a− 1; n = 1041), followed by the mixed-wood forests (NPP: 500.5 ± 0.8 gC∙m− 2∙a− 1; WUE: 0.856 ± 0.02 gC∙m− 2∙mm− 1; ET: 589.3 ± 0.9 mm∙a− 1; n = 2156); whereas the broadleaved forests were lowest in NPP (491.6 ± 0.6 gC∙m− 2∙a− 1; n = 4428) and WUE (0.832 ± 0.02 gC∙m− 2∙mm− 1; n = 4428), and highest in ET (594.7 ± 0.6 mm∙a− 1; n = 4428). The mean annual NPP, ET and WUE increased with stand age typically in coniferous forests, and weakly in mixed-wood forests. The natural stands had significantly (p < 0.001) greater NPP and WUE than planted stands in conifers and mixed-woods.ConclusionsForest type and stand-age class are factors of key consideration in planning and managing forest landscape for achieving the compromise between regional carbon balance and water usage. Facilitating the establishment of coniferous trees and conservation of mature forest stands, and near-natural management of planted forests, can be favorable options for delivering ecosystem services of carbon sequestration and water conservation in temperate regions.

Response of net primary production to land use and climate changes in the middle-reaches of the Heihe River Basin.

Net primary production (NPP) supplies matter, energy, and services to facilitate the sustainable development of human society and ecosystem. The response mechanism of NPP to land use and climate changes is essential for food security and biodiversity conservation but lacks a comprehensive understanding, especially in arid and semi‐arid regions. To this end, taking the middle‐reaches of the Heihe River Basin (MHRB) as an example, we uncovered the NPP responses to land use and climate changes by integrating multisource data (e.g., MOD17A3 NPP, land use, temperature, and precipitation) and multiple methods. The results showed that (a) land use intensity (LUI) increased, and climate warming and wetting promoted NPP. From 2000 to 2014, the LUI, temperature, and precipitation of MHRB increased by 1.46, 0.58°C, and 15.76 mm, respectively, resulting in an increase of 14.62 gC/m2 in annual average NPP. (b) The conversion of low‐yield cropland to forest and grassland increased NPP. Although the widespread conversion of unused land and grassland to cropland boosted both LUI and NPP, it was not conducive to ecosystem sustainability and stability due to huge water consumption and human‐appropriated NPP. Urban sprawl occupied cropland, forest, and grassland and reduced NPP. (c) Increase in temperature and precipitation generally improved NPP. The temperature decreasing <1.2°C also promoted the NPP of hardy vegetation due to the simultaneous precipitation increase. However, warming‐induced water stress compromised the NPP in arid sparse grassland and deserts. Cropland had greater NPP and NPP increase than natural vegetation due to the irrigation, fertilizers, and other artificial inputs it received. The decrease in both temperature and precipitation generally reduced NPP, but the NPP in the well‐protection or less‐disturbance areas still increased slightly.

Conservation Systems for Positive Net Ecosystem Carbon Balance in Semiarid Drylands

Core Ideas The soil organic C dynamics and net ecosystem C balance of five dryland cropping systems were compared. Conservation systems stored up to 15% more soil organic C than conventional system. Net ecosystem C balance was positive with cover cropping. Cover crops and conservation tillage are crucial for soil C storage in drylands. Biomass C inputs often limit agroecosystem C dynamics, nutrient cycling, and soil organic carbon (SOC) storage in semiarid drylands. This study evaluated SOC and net ecosystem carbon balance (NECB) of five cropping systems in the drylands of the Southern Great Plains. Cropping systems evaluated included corn (Zea mays)–sorghum [Sorghum bicolor (L.) Moench] rotation with conventional tillage without cover cropping (CTNC), strip tillage with and without cover cropping (STCC and STNC, respectively), and no tillage with and without cover cropping (NTCC and NTNC, respectively). After 4 yr of experimental tillage, we measured CO2 emissions, soil and soil surface air temperatures, soil moisture content, potentially mineralizable carbon (PMC), total SOC, total nitrogen (TN), and net primary productivity (NPP). Conservation systems (any treatments including no‐till, strip till, or cover crops) had 5 to 6°C lower soil temperature and 2.8 to 4.9°C lower soil surface air temperature and stored 2.3 to 3.9% more soil moisture content than CTNC. Conservation systems also stored 15.2% more SOC than CTNC. Cropping systems that integrated cover crops in the rotation (STCC and NTCC) had greater NPP and positive NECB. Regardless of tillage management, cover cropping had a greater NECB, including SOC (NECBSOC) than CTNC. Reducing tillage and diversifying cropping systems through cover cropping can benefit semiarid dryland agroecosystems by increasing SOC storage and maintaining positive NECB.

Abiotic and biotic predictors of macroecological patterns in bird and butterfly coloration

AbstractAnimal color phenotypes are invariably influenced by both their biotic community and the abiotic environments. A host of hypotheses have been proposed for how variables such as solar radiation, habitat shadiness, primary productivity, temperature, rainfall, and community diversity might affect animal color traits. However, while individual factors have been linked to coloration in specific contexts, little is known about which factors are most important across broad taxonomic and geographic scales. Using data collected from 570 species of birds and 424 species of butterflies from Australia, which inhabit an area spanning a latitudinal range of 35° and covering deserts, tropical and temperate forests, savannas, and heathlands, we test multiple hypotheses from the coloration literature and assess their relative importance. We show that bird and butterfly species exhibit more reflective and less saturated colors in better‐lit environments, a pattern that is robust across an array of variables expected to influence the intensity or quality of ambient light in an environment. Both taxa display more diverse colors in regions with greater net primary production and longer growing seasons. Models that included variables related to energy inputs and resources in ecosystems have better explanatory power for bird and butterfly coloration overall than do models that included community diversity metrics. However, the diversity of the bird community in an environment was the single most powerful predictor of color pattern variation in both birds and butterflies. We observed strong similarities across taxa in the covariance between color and environmental factors, suggesting the presence of fundamental macroecological drivers of visual appearance across disparate taxa.

Climate change imposes phenological trade‐offs on forest net primary productivity

AbstractClimate warming is expected to lengthen growing seasons of temperate forest ecosystems and increase gross primary productivity. Simultaneously, warming is expected to increase summer ecosystem respiration, which could offset gains accrued from longer growing seasons. These responses have been observed during anomalously warm years, but the role of future climate change on phenological trade‐offs and how they affect net primary productivity (NPP) at regional scales in temperate forests remain unexplored. We simulated scenarios of climate change on monthly forest NPP throughout 18 million hectares of temperate forests in New England, USA, through year 2100. Using an ecophysiological model coupled to a forest landscape model, we simulated scenarios of climate change on monthly NPP. A high emission scenario (RCP 8.5), resulted in longer growing seasons that offset midsummer ecosystem respiration costs and produced greater annual NPP throughout the study landscape compared to simulations using the current climate. In spring and autumn months, temperature was positively associated with greater NPP; in summer months, the relationship was negative. Spatially, the greatest increase in NPP occurred in the warmer southern region under a warm climate scenario with increased precipitation. Under a warm scenario with drier conditions, the greatest increase in NPP occurred in the cooler northern region. Phenological trade‐offs will affect NPP of future forests and their potential to serve as a negative feedback to climate change. Barring other limitations, longer growing seasons will offset greater respiratory demands and contribute to increases in NPP throughout the temperate forests of New England in the future.

Emissions intensity and carbon stocks of a tropical Ultisol after amendment with Tithonia green manure, urea and biochar

Biochar has been shown to reduce soil emissions of CO2, CH4 and N2O in short-term incubation and greenhouse experiments. Such controlled experiments failed to represent variable field conditions, and rarely included crop growth feedback. The objective of this study was to assess the effect of biochar, in comparison to green manure and mineral nitrogen, on greenhouse gas Emissions Intensity (EI=emissions in CO2 equivalents per ton of grain yield) in a low-fertility tropical Ultisol. Using a field trial in western Kenya, biochar (0 and 2.5tha−1; made from Eucalyptus wood) was integrated with urea (0 and 120kgNha−1) and green manure (Tithonia diversifolia; 0, 2.5 and 5tha−1) in a factorial design for four consecutive seasons from October 2012 to August 2014. Compared to the control, biochar increased soil CO2 emissions (9–33%), reduced soil CH4 uptake (7–59%) and reduced soil N2O emissions (1–42%) in each season, with no seasonal differences. N2O emissions increased following amendment with T. diversifolia (6%) and urea (13%) compared to the control. Generally, N2O emissions decreased where only biochar was applied. The greatest decrease in N2O (42%) occurred where all three amendments were applied compared to when they were added separately. EI in response to any of the amendments was lower than the control, ranging from 9 to 65% (33.0±3.2=mean±SE). The amendments increased SOC stocks by 0.1–1.2tha−1year−1 (mean±SE of 0.8±0.09tha−1year−1). The results suggest decreased net EI with biochar in low fertility soils mainly through greater net primary productivity (89% of the decrease).

Patterns of Arthropod Diversity in Contrasting Habitats of El Pedregal de San Angel, a Preserve in Mexico City

We studied the community of arthropods inhabiting the ecological preserve El Pedregal de San Angel, in Mexico City. We estimated morphospecies richness, abundance, and diversity patterns in two contrasting habitats, open and closed, during four seasons of the year. Because closed-habitat sites have greater net primary productivity than open habitats, we predicted arthropod diversity would be greater in closed than in open habitats. Morphospecies richness of Acari and Arachnida was similar in both types of habitats throughout the year, whereas richness of some orders of Insecta such as Coleoptera, Diptera, Hemiptera, and Hymenoptera was greatest in open habitats. Mites were the most abundant arthropods and were always more abundant in open than closed habitats. Diversity of arthropods was greater in closed habitats during fall and spring, suggesting that habitat characteristics related to primary productivity might affect this community. We suggest that studies of patterns of arthropod diversity and factors associated with the maintenance of such diversity should be considered in the preservation of these organisms and of their natural habitats.

Decomposition kinetics of soil carbon of different age from a forest exposed to 8 years of elevated atmospheric CO 2 concentration

Ecosystem exposure to elevated atmospheric CO 2 concentration can often leads to increased ecosystem carbon (C) fluxes, as well as greater net primary production. Changes in the soil C pool with elevated [CO 2] are more difficult to measure and therefore remain poorly understood. In this study, we carried out a series of laboratory soil incubations, in order to determine whether 8 years of ecosystem exposure to elevated [CO 2] altered decomposition dynamics of two age classes of soil C in a temperate coniferous forest. Our objectives were to determine whether there were differences in the decomposition kinetics of soil C up to 8 years old (C post-tr) and soil C older than 8 years (C pre-tr), in the absence of concurrent plant activity. We collected soil from the Duke Forest Free Air CO 2 Enrichment site in North Carolina and incubated whole and crushed (all macroaggregates dispersed) soil from two depth increments (0–5 cm and 5–15 cm) for 102–127 days. We found that mineral soil from the treatment plots had higher respiration rates in the absence of concurrent plant activity than mineral soil from plots under ambient CO 2 conditions. These differences in respiration rate were only significant in 0–5 cm soil and could be largely explained by higher initial respiration rates of soil collected from the CO 2-treated plots. Disruption of soil macroaggregates did not result in a difference in efflux rate in soil from this forest under ambient or elevated CO 2 conditions at either depth. The specific respiration rate of C post-tr was higher than that of C pre-tr in the top 5 cm of soil, while the opposite was true for 5–15 cm of soil. Even though C post-tr was assimilated by the ecosystem more recently than C pre-tr, their decay constants were similar at both depths. These results suggest that, in the absence of plant activity, the mineralization of soil C of different ages in this forest may be under similar biological and/or biochemical control. Therefore, if the higher initial rates of decomposition of C post-tr seen in these experiments are sustained in the field, greater labile pool size of recently added C, and potentially faster cycling of this pool, may in part explain higher soil respiration rates and limited soil C accumulation under elevated [CO 2] in this forest.

Canopy leaf area constrains [CO 2 ]-induced enhancement of productivity and partitioning among aboveground carbon pools

Net primary productivity (NPP) is enhanced under future atmospheric [CO2] in temperate forests representing a broad range of productivity. Yet questions remain in regard to how elevated [CO2]-induced NPP enhancement may be affected by climatic variations and limiting nutrient resources, as well as how this additional production is distributed among carbon (C) pools of different longevities. Using 10 years of data from the Duke free-air CO2 enrichment (Duke FACE) site, we show that spatially, the major control of NPP was nitrogen (N) availability, through its control on canopy leaf area index (L). Elevated CO2 levels resulted in greater L, and thus greater NPP. After canopy closure had occurred, elevated [CO2] did not enhance NPP at a given L, regardless of soil water availability. Additionally, using published data from three other forest FACE sites and replacing L with leaf area duration (LD) to account for differences in growing season length, we show that aboveground NPP responded to [CO2] only through the enhancement of LD. For broadleaf forests, the fraction of aboveground NPP partitioned to wood biomass saturated with increasing LD and was not enhanced by [CO2], whereas it linearly decreased for the conifer forest but was enhanced by [CO2]. These results underscore the importance of resolving [CO2] effects on L to assess the response of NPP and C allocation. Further study is necessary to elucidate the mechanisms that control the differential allocation of C among aboveground pools in different forest types.

SOIL NITROGEN CYCLING UNDER ELEVATED CO2: A SYNTHESIS OF FOREST FACE EXPERIMENTS

The extent to which greater net primary productivity (NPP) will be sustained as the atmospheric CO2 concentration increases will depend, in part, on the long‐term supply of N for plant growth. Over a two‐year period, we used common field and laboratory methods to quantify microbial N, gross N mineralization, microbial N immobilization, and specific microbial N immobilization in three free‐air CO2 enrichment experiments (Duke Forest, Oak Ridge, Rhinelander). In these experiments, elevated atmospheric CO2 has increased the input of above‐ and belowground litter production, which fuels heterotrophic metabolism in soil. Nonetheless, we found no effect of atmospheric CO2 concentration on any microbial N cycling pool or process, indicating that greater litter production had not initially altered the microbial supply of N for plant growth. Thus, we have no evidence that changes in plant litter production under elevated CO2 will initially slow soil N availability and produce a negative feedback on NPP. Understanding the time scale over which greater plant production modifies microbial N demand lies at the heart of our ability to predict long‐term changes in soil N availability and hence whether greater NPP will be sustained in a CO2‐enriched atmosphere.

Direct costs of forest reproduction, bee-cycling and the efficiency of pollination modes

The bee guild represents direct primary costs of angiosperm reproduction. Tropical flower visitors take an amount comparable to herbivores, exceeding 3% of net primary production energy. Therefore herbivory and aboveground net primary production have been underestimated. Comparing pollinators to other herbivores, harvest in mature forest by tropical bees is greater than leafcutter ants, game animals, frugivores, vertebrate folivores, insect defoliators excluding ants, flower-feeding birds and bats, but not soil organisms. The ratio of total aboveground net primary production to investment in pollen, nectar and resin used by pollinators suggests wind pollination is several times more efficient in temperate forests than is animal pollination in neotropical moist forest. Animal pollination may be favoured by habitat mosaics and an unpredictable or sparse dispersion of conspecifics — consequences of fluctuating abiotic and biotic environments. Natural selection evidently favours diminished direct reproductive costs in forests, for example by wind pollination, regardless of latitude and disturbance regime. An example is “wind pollination by proxy” of dominant trees in seasonal southeast Asian forests. They flower only occasionally and their pollen is dispersed by tiny winged insects that are primarily carried by the wind — rather than the nectar-hungry bees, bats, birds and moths used by most tropical flora. Increasing evapotranspiration is associated with greater net primary production; I show its correlation with species richness of social tropical bees across the isthmus of Panama, which may indicate increasing forest reproductive effort devoted to flowering, and its monopolization by unspecialized flower visitors in wetter and less seasonal lowland forests.

Rainfall and Grazing Effects on Net Primary Productivity in a Tropical Savanna, India

The effects of rainfall amount, rainfall distribution, and grazing intensity on net primary production and herbivory were examined for a dry tropical savanna. Aboveground net primary production in permanently protected savanna ranged from 329 to 841 g.m—2.yr—1 and was positively related to annual rainfall. It was not related to early rainy season rainfall, but was related to rainfall of the later part of the rainy season. Grazing simulated 4—45% greater net primary production, the simulation being higher at lightly and moderately grazed sites. Consumption, as estimated by comparing net primary production inside temporarily fenced and outside the fenced plots, was 476— 34 g.m—2.yr—1. Consumption in the present savanna was positively related to net primary production, and to annual rainfall. Green biomass, being in function of soil moisture, was greater in the high rainfall year. Belowground net primary production in permanently protected savanna (BnpP) ranged from 404 to 838 g.m—2.yr—1 and was significantly related to annual rainfall. BnpP was not related to early rainy season rainfall but was related to late rainy season rainfall. Total net primary production ranged from 836 to 1579 g.m—2.yr—1 and was also related to annual rainfall than early rainy season rainfall. Grazing reduced belowground net primary production by 25—65% in grazed savanna (BnpG). Maximum reduction occurred in heavily grazed savanna. BnpG was inversely related with rainfall. Total net primary production (herbaceous + woody vegetation) of the savanna ranged from 11.9 to 19.1 Mg.ha—1.yr—1. The total net primary production of the savanna was similar to that of the native dry deciduous forest; however, the production of herbaceous species was 61% lower in the forest. The total net primary production of savanna was higher than that of the dry tropical forest when the belowground net primary production of the 0—50 cm layer was considered.

Leaf and Shoot Demography of an Arctic Stoloniferous Grass, Puccinellia Phryganodes, in Response to Grazing

(1) Swards ( < 2 5 cm in height) of Puccinellia phryganodes are heavily grazed by lesser snow geese (Chen caerulescens caerulescens) on intertidal flats on the shores of Hudson Bay. This paper describes the morphological changes that account for the increased net primary production due to grazing. (2) Demographic techniques were used in 1983 to monitor the births and deaths of leaves and shoots in grazed and ungrazed swards (exclosures) between mid-June ('spring melt') and early September (before 'freeze-up'). (3) Overall, the mean number of axillary shoots produced per main shoot of Puccinellia was significantly higher in grazed plots (2 03) than that in ungrazed plots (1 47). Shoots remained alive beyond one growing season. (4) There was no significant difference in the mean number of leaves produced per main shoot by plants from grazed and ungrazed swards. Leaf births per axillary shoot were significantly greater in grazed plots compared with ungrazed plots. Overall 64% of all leaves produced between June and September were grazed or partially grazed. (5) Within-season cumulative deaths were 6 9 and 6 0 leaves per main shoot and 2 2 and 1 1 leaves per axillary shoot, respectively, in grazed and ungrazed plots. Life expectancies for leaves exserted between June and early August were as follows: partially grazed leaves 24 9-30 2 days, ungrazed leaves in grazed plots 27 5-34 4 days, leaves in ungrazed plots 32 7-42 5 days. This indicates a rapid turnover of leaves in this arctic salt marsh. Leaves produced late in the season survived the winter but died in early summer of the following year. (6) These results are compared with those of Carex subspathacea, the other important forage species. In C. subspathacea the greater net primary production in response to grazing is achieved by the growth of existing shoots and not by the production of new shoots.

The Leaf and Shoot Demography of Grazed and Ungrazed Plants of Carex Subspathacea

(1) Dense swards of aerial shoots (< 2 5 cm in height) of Carex subspathacea are heavily grazed by lesser snow geese on intertidal flats on the shores of Hudson Bay. This paper describes the morphological changes in plants which account for the greater net primary production when these swards are grazed. (2) Demographic techniques have been used to monitor the births and deaths of leaves and shoots in grazed and ungrazed swards. Between mid-June ('spring thaw') and early September 1984 (just prior to 'freeze-up'), shoots in grazed plots produced an average of 8 2 new leaves, compared with an average of 5 7 leaves per shoot in ungrazed plots. This difference accounted for the greater net annual primary production in grazed swards. (3) Within-season deaths of leaves totalled 8 0 and 5 7 leaves per shoot, respectively, in grazed and ungrazed plots. Life expectancies were as follows: grazed leaves, 33 3 days; ungrazed leaves in grazed plots, 34 2 days; leaves in ungrazed plots 45 1-472 days. This indicates a very rapid turnover of leaves in this arctic salt marsh where the growing season is about 110 days. Some leaves produced late in the season survived the winter but died in early summer of the following year. (4) No differences were detected in the number of new shoots in grazed and ungrazed swards, in contrast to a higher significant within-season mortality of shoots in grazed plots. Turnover of shoots is very slow compared with leaves. Shoots remain alive beyond one growing season. (5) The geese graze selectively. Only 20%/, of shoots in grazed plots were removed and only 30% of leaves were removed completely, although between 420% and 56%/o of leaves were grazed. Meristems are inot destroyed and the continuous production of leaves occurs throughout the season. (6) The growth responses of the sedge to grazing are probably dependent on goose faeces which provide a source of soluble nitrogen in the nitrogen-deficient habitat.