Lower Net Primary Productivity Research Articles

Succession May Maintain High Leaf Area: Sapwood Ratios and Productivity in OldSubalpine Forests

Old forests are generally believed to exhibit low net primary productivity (NPP) and therefore to be insignificant carbon sinks. This relationship between age and NPP is based, in part, on the hypothesis that the biomass of respiratory tissues such as sapwood increases with age to a point where all photosynthate is required just to maintain existing tissue. However, this theoretical connection between respiration:assimilation ratios and forest productivity is based on age-dependent trends in the sapwood:leaf ratios of individual trees and even-aged stands; it does not take into account such processes in natural forests as disproportional increases in shade-tolerant species over time and multiple-age cohorts. Ignoring succession and structural complexity may lead to large underestimates of the productivity of old forests and inaccurate estimates of the ages at which forest productivity declines. To address this problem, we compared biomass allocation and productivity between whitebark pine, a shade-intolerant, early-successional tree species, and subalpine fir, a shade-tolerant, late-successional species, by harvesting 14 whitebark pines and nine subalpine firs that varied widely in dbh and calculating regression models for dbh vs annual productivity and biomass allocation to leaves, sapwood, and heartwood. Late-successional subalpine fir allocated almost twice as much biomass to leaves as early-successional whitebark pine. Subalpine firs also had a much lower allocation to sapwood and higher growth rates across all tree sizes. We then modeled biomass allocation and productivity for 12 natural stands in western Montana that were dominated by subalpine fir and whitebark pine varying in age from 67 to 458 years by applying the regressions to all trees in each stand. Whole-stand sapwood:leaf ratios and stand productivity increased asymptotically with age. Sapwood:leaf ratios and productivity of whitebark pine in these stands increased for approximately 200–300 years and then decreased slowly over the next 200 years. In contrast, sapwood:leaf ratios of all sizes of subalpine fir were lower than those of pine and productivity was higher. As stands shifted in dominance from pine to fir with age, subalpine fir appeared to maintain gradually increasing rates of whole-forest productivity until stands were approximately 400 years old. These results suggest that forests such as these may continue to sequester carbon for centuries. If shade-tolerant species that predominate late in succession maintain high assimilation-to-respiration ratios in other forests, we may be underestimating production in old forests, and current models may underestimate the importance of mature forests as carbon sinks for atmospheric CO2 in the global carbon cycle.

Biomass accumulation over the first 150 years in coastal Oregon Picea‐Tsuga forest

Abstract. Production and mortality are the component processes that together determine the biomass dynamics of forests. Due to the significant role of forests in the global carbon cycle, it is important to assess how these two processes affect the maximum biomass attained by forests, as well as the dynamics leading up to and following peak biomass. We address these questions for two sets of plots in Picea sitchensis‐Tsuga heterophylla forest on the northern Oregon coast that originated from a catastrophic wildfire in the 1840s, using new data on dynamics of live trees and stocks of coarse woody debris (CWD). The set of plots closest to the ocean and occupying steeper, more dissected terrain with areas of thin soils has lower biomass, lower net primary production (NPP) of bole wood and higher tree mortality as a fraction of standing biomass. The two sets of plots have similar CWD levels, most of which has accumulated in the last 25 yr. The present disparity in biomass between the two sets of plots appears to be the result of lower NPP on the low‐biomass plots for the entire 140+ yr history of the forest. Over the 58 yr that the high‐biomass plots have been measured (from stand age 85 to 143 yr), NPP of bole wood has declined by 41%. Only ca. 6% of this decline can be accounted for by an increase in maintenance respiration of woody tissues. For both sets of plots relative constancy of biomass in the long term appears likely, due to a short time lag in tree regeneration, asynchronous tree mortality and little overall decline in NPP of bole wood in recent decades. However, since tree mortality as a fraction of standing biomass is higher on the low‐biomass plots, and NPP of bole wood is slightly lower, the difference in biomass between the two sets of plots should increase if current rates of production and mortality persist.

Aboveground production and nutrient circulation along a flooding gradient in a South Carolina Coastal Plain forest

Relative to effects of flooding, little is known about the influence of hydrology-nutrient interactions on aboveground net primary production (NPP) in forested wetlands. We found that nutrient circulation and NPP were closely related along a complex physical, chemical, and hydrologic gradient in a bottomland hardwood forest with four distinct communities. Aboveground biomass, NPP, biomass partitioning to stem production, growth efficiency, and soil macronutrient availability were greatest in the flooded zone, possibly because of the stable hydrologic regime. In the wet transition zone, trees were least productive, nutrient use efficiency was highest, and N retranslocation from foliage before abscission was "complete." Wet and dry transition zones had the lowest litterfall quality. Soil organic matter was negatively correlated with extractable NH4-N plus NO3-N before in situ incubations and positively correlated with litterfall lignin/N ratios. Lignin/P and C/N ratios were positively correlated with exchangeable soil Ca and Mg, cation exchange capacity, and clay content and negatively correlated with extractable soil P. We concluded that periodic flooding and associated widely fluctuating soil chemistry resulted in disequilibrium between the plant community and environmental conditions, which led to nutrient deficiency and low NPP in the transition zones compared with the continuously flooded and mesic zones.

Pelagic processes in extensive shrimp ponds of the Mekong delta, Vietnam

Rates of pelagic primary production, respiration, and bacterioplankton dynamics were measured in relation to water quality parameters in three extensive shrimp ponds in the Mekong delta, Vietnam. There were few consistent differences in pelagic characteristics among different locations within these ponds, among the three ponds, or between the ponds and adjacent river water. Rates of primary production ( 14 C uptake) ranged from <1 to 94 mg C m −3 d −1. Rates of gross primary production (light–dark bottle technique) ranged from −184 to 2697 mg C m −3 d −1. Rates of pelagic respiration ranged from 60 to 3783 mg C m −3 d −1. Primary production rates measured by oxygen flux were greater than those measured via 14 C uptake, with P/ R ratios varying widely (−0.45 to 3.4), but the mean P/ R at each site was <1 with a grand mean among ponds of 0.7. Bacterioplankton numbers (mean range: 0.2 to 27.2×10 8 cells ml −1) and productivity (range: 2.5 to 297.3 mg C m −3 d −1) did not vary consistently among ponds with season. Mean bacterioplankton growth rates were fastest ( μ=0.29 d −1) in the pond with highest shrimp production and slowest in the poorest yielding pond ( μ=0.08 d −1). The primary production and respiration rates, and bacterioplankton dynamics, indicate that these pond waters are net heterotrophic. Plankton measurements rarely correlated with changes in physicochemistry (pH, dissolved O 2, salinity, temperature) or nutrient concentrations as within-site and temporal variability were large for most parameters. The diurnal cycles of physicochemical characteristics were similar to those measured in other unfertilized, low-alkalinity ponds, but these cycles were dampened by intense rainfall. Our data indicate that low net primary production, high rates of respiration, moderate rates of bacterioplankton production, high suspended solid and nutrient concentrations, low and very variable pH and dissolved O 2 concentrations, and variations in salinity due to intense rainfall episodes limit shrimp production in these extensive ponds.

Comparing global models of terrestrial net primary productivity (NPP): overview and key results

SummarySeventeen global models of terrestrial biogeochemistry were compared with respect to annual and seasonal fluxes of net primary productivity (NPP) for the land biosphere. The comparison, sponsored by IGBP‐GAIM/DIS/GCTE, used standardized input variables wherever possible and was carried out through two international workshops and over the Internet. The models differed widely in complexity and original purpose, but could be grouped in three major categories: satellite‐based models that use data from the NOAA/AVHRR sensor as their major input stream (CASA, GLO‐PEM, SDBM, SIB2 and TURC), models that simulate carbon fluxes using a prescribed vegetation structure (BIOME‐BGC, CARAIB 2.1, CENTURY 4.0, FBM 2.2, HRBM 3.0, KGBM, PLAI 0.2, SILVAN 2.2 and TEM 4.0), and models that simulate both vegetation structure and carbon fluxes (BIOME3, DOLY and HYBRID 3.0). The simulations resulted in a range of total NPP values (44.4–66.3 Pg C year–1), after removal of two outliers (which produced extreme results as artefacts due to the comparison). The broad global pattern of NPP and the relationship of annual NPP to the major climatic variables coincided in most areas. Differences could not be attributed to the fundamental modelling strategies, with the exception that nutrient constraints generally produced lower NPP. Regional and global NPP were sensitive to the simulation method for the water balance. Seasonal variation among models was high, both globally and locally, providing several indications for specific deficiencies in some models.

The importance of climate and soils for estimates of net primary production: a sensitivity analysis with the terrestrial ecosystem model

AbstractWe used the Terrestrial Ecosystem Model (TEM) to investigate how alternative input data sets of climate (temperature/precipitation), solar radiation, and soil texture affect estimates of net primary productivity (NPP) for the conterminous United States. At the continental resolution, the climates of Cramer and Leemans (C&amp;L) and of the Vegetation/ Ecosystem Modelling and Analysis Project (VEMAP) represent cooler and drier conditions for the United States in comparison to the Legates and Willmott (L&amp;W) climate, and cause 5.2% and 2.3% lower estimates of NPP. Solar radiation derived from C&amp;L and given in VEMAP is 32% and 60% higher than the solar radiation data derived from Hahn cloudiness. These differences cause ∼ 8% and 10% lower NPP because of radiation‐induced water stress. In comparison to the FAO/CSRC soil texture, which represents most biomes with loam soils, the soil textures are finer (more silt and clay) in the Zobler and VEMAP data sets. The use of VEMAP soil textures instead of FAO/CSRC soil textures causes ∼ 3% higher NPP because enhanced volumetric soil moisture causes higher rates of nitrogen cycling, but use of the Zobler soil textures has little effect. In general, NPP estimates of TEM are more sensitive to alternative data sets at the biome and grid cell resolutions than at the continental resolution. At all spatial resolutions, the sensitivity of NPP estimates represents the impact of uncertainty among the alternative data sets we used in this study. The reduction of uncertainty in input data sets is required to improve the spatial resolution of NPP estimates by process‐based ecosystem models, and is especially important for improving assessments of the regional impacts of global change.

Effect of Grazing by Greater Snow Geese on the Production of Graminoids at an Arctic Site (Bylot Island, NWT, Canada)

1 Arctic ecosystems are sensitive to grazing because of their low overall net primary production. We therefore studied the effect of greater snow geese on the production of arctic graminoids. 2 At Bylot Island, NWT (73?N), breeding snow geese graze Eriophorum scheuchzeri and Dupontiafisheri in lowland polygon fens. Vegetation in seasonal exclosures was sampled at 2-week intervals from mid-June to mid-August 1990, 1991 and 1993 (ungrazed areas) and compared with plant growth in short-term exclosures set up over grazed areas. Standing crop (above-ground biomass), net above-ground primary production (NAPP) and nitrogen content were determined for both plant species. 3 Goose faeces were used as an index of grazing intensity. Cumulative faeces counts increased from 2.2 faeces m-2 in 1990-9.9 faeces m-2 in 1993. 4 Peak above-ground dry biomass of ungrazed areas averaged 33 g m2. In all years, goose grazing significantly reduced the above-ground biomass of Eriophorum and Dupontia. Over the 3 years, we estimated that geese consumed from 65 to 113% of the cumulative NAPP of Eriophorum, and from 30 to 78% of the cumulative NAPP of Dupontia. Consumption of Dupontia, but not Eriophorum, was related to grazing intensity. 5 Cumulative NAPP of grazed areas was slightly lower than that of ungrazed areas at the end of the summer. On an individual plant basis, cumulative NAPP of Dupontia, but not of Eriophorum, was reduced by grazing. Nitrogen content of plants after grazing was higher than in ungrazed plants. 6 Although grazed plants were able to grow new foliage, goose grazing did not enhance NAPP at Bylot Island as has been reported elsewhere. This could be either because grazing occurs too late in the season when the regrowth capacity of plants is low or because nutrients released from goose faeces are absorbed by mosses and are not immediately available to graminoids.

Diurnal Changes in the Partial Pressure of Carbon Dioxide in Coral Reef Water

Coral reefs are considered to be a source of atmospheric carbon dioxide because of their high calcium carbonate production and low net primary production. This was tested by direct measurement of diurnal changes in the partial pressure of carbon dioxide (Pco(co2)) in reef waters during two 3-day periods, one in March 1993 and one in March 1994, on Shiraho reef of the Ryukyu Islands, Japan. Although the Pco(co2) values in reef waters exhibited large diurnal changes ranging from 160 to 520 microatmospheres, they indicate that the reef flat area is a net sink for atmospheric carbon dioxide. This suggests that the net organic production rate of the reef community exceeded its calcium carbonate production rate during the observation periods.

Explaining global termite diversity: productivity or history?

A map of termite generic richness worldwide shows that the Ethiopian biogeographical region is most genus-rich, while the Neotropical and Indo-Malayan regions are less rich. Net primary productivity (NPP) has been postulated as a possible explanation for some diversity patterns (energy-diversity theory). We reject this as the primary explanation for termite generic richness patterns because the Ethiopian region has low NPP and a high generic richness while the Neotropical and Indo-Malayan regions have high NPP and low generic richness. We explore an alternative historical hypothesis, that differing levels of Quaternary climatic disturbance are responsible for the differences in termite generic richness between the three tropical regions.

Maintenance Respiration and Stand Development in a Subalpine Lodgepole Pine Forest

We examined a chronosequence of subalpine lodgepole pine stands to test the hypothesis that low net primary production in older forest stands is caused by higher maintenance respiration costs of woody tissue. We predicted that respiration of woody tissue (particularly stem sapwood) would be greater in older stands and that the higher maintenance costs would account for observed low wood production. For a unit of ground surface, the carbon flux involved in wood production and association constructed respiration was 210 g.m—2.yr—1 in a 4—yr—old stand, but declined to 46 g.m—2.yr—1 in a 245—yr—old stand. However, maintenance respiration of woody tissue in stems and branches consumed only 61 g.m—2.yr—1 in the 40—yr—old stand and 79 g.m—2.yr—1 in the 245—yr—old stand. The slight, nonsignificant increase in maintenance respiration of woody tissues could not explain the dramatic decline in aboveground wood production in the old—growth stand.

Use of Scattered Resources in Rain Forest of Humid Tropical Lowlands

Well developed, complex rain forests of humid tropical lowlands (RFHTL) do not seem to support large populations of animals compared with other types of forest. Recent studies of terrestrial and understory species of tropical and subtropical forests indicate that the resources available to animals of RFHTL are generally scattered and uncommon. On the forest floor only ants and termites, among insects, are consistently common; they are utilized as food by a variety of specialized ground insectivores. Terrestrial herbivores that eat large fallen fruit are also generally scarce as are the often specialized insectivores that seek typically uncommon understory foliage insects. The low densities and high degrees of specialization that accommodate animal species to resources of RFHTL may be ultimately due to a combination of relatively low net primary production of undisturbed mature rain forest and the rapid rate of microbial decomposition of forest litter. -from Authors

Elemental dynamics in forested bogs in northern Minnesota

Dynamics of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) were determined for three perched bogs, formed by lake filling, and three raised bogs, formed by landscape swamping. N and K concentrations were higher in the undergrowth of perched bogs, and Ca and Mg concentrations were higher in subsurface anaerobic peat of raised bogs. Elemental pools in vegetation were in the order N &gt; Ca &gt; K &gt; Mg &gt; P; in surface peat, N &gt; Ca &gt; Mg &gt; P = K. Differences in elemental mass between the bog types were closely related to biomass differences. The atmosphere potentially supplied from 3% of annual plant uptake of K to 20% of Mg; this fraction was inversely related to uptake as a proportion of the surface peat. Vegetation on raised bogs had a greater proportion of uptake from the atmosphere (15 vs. 12%), a faster rate of elemental turnover (3.8 vs. 4.8 years), and lower net primary productivity (NPP) than on perched bogs, all indicative of a lower nutrient status. The annual mineralization rate of the surface peat for both bog types was estimated at 1.5% year−1; NPP predicted from N mineralized at this rate agrees well with observations. The better nutritional status of perched bogs may be related to landscape position, with potential inputs via runoff from adjacent uplands. The nutrient capital in both bog vegetation and substrate was similar to that in upland northern conifer forests. Key words: acrotelm, ombrotrophic, raised bogs, nutrients, peatlands, nutrient cycling.

Salt marsh productivity with natural and altered tidal circulation.

The effects of altered tidal circulation on southern California salt marshes are investigated by comparing a well-flushed wetland and two modified wetlands which have reduced tidal flow. The Tijuana Estuary had continuous exchange of seawater but relatively low net aerial primary productivity (0.4-1.0 kg m-2yr-1) of vascular plants. Low productivity (0.6 kg m-2yr-1) was also found in the Flood Control Channel of the San Diego River, where tidal exchange was restricted to flow through a riprap dike. High productivity (1.2-2.9 kg m-2yr-1) in Los Penasquitos Lagoon was attributed to the influences of freshwater impounded behind a sand bar which blocked the mouth of the lagoon during much of the study period.It is hypothesized that elimination of tidal flow during the growing season increased primary productivity of vascular plants because freshwater runoff decreased soil salinity and because nutrients were retained within the marsh. However, we predict that sand bar obstruction can decrease productivity if below-average rainfall leads to hypersalinity of closed lagoons. Comprehensive evaluation of the effects of altered tidal circulation requires longterm study and examination of the total ecosystem.