Ground Net Primary Production Research Articles

Relationship between Productivity, and Species and Functional Group Diversity in Grazed and Non-Grazed Pampas Grassland

Most hypotheses addressing the effect of diversity on ecosystem function indicate the occurrence of higher process rates with increasing diversity, and only diverge in the shape of the function depending on their assumptions about the role of individual species and functional groups. Contrarily to these predictions, we show that grazing of the Flooding Pampas grasslands increased species richness, but drastically reduced above ground net primary production, even when communities with similar initial biomass were compared. Grazing increased species richness through the addition of a number of exotic forbs, without reducing the richness and cover of the native flora. Since these forbs were essentially cool-season species, and also because their introduction has led to the displacement of warm-season grasses from dominant to subordinate positions in the community, grazing not only decreased productivity, but also shifted its seasonality towards the cool season. These results suggest that species diversity and/or richness alone are poor predictors of above-ground primary production. Therefore, models that relate productivity to diversity should take into account the relative abundance and identity of species that are added or deleted by the specific disturbances that modify diversity.

Remote sensing of biophysical variables in boreal forest stands of Picea mariana

Using two hybrid radiative transfer models to represent conifer canopies and stands, algorithms to infer several important structural parameters of stands of black spruce (Picea mariana), the most common boreal forest dominant, were developed and evaluated. Spectral mixture analysis and multi-spectral reflectance data for 31 black spruce stands of varying density and structure were used to infer the values for the areal proportions of sunlit canopy, sunlit background and shadow fraction, which we call radiometric elements, and the areal proportions of these radiometric elements were strongly related to leaf area index, biomass density, and annual above ground net primary productivity. The best overall correspondence between the radiometric elements and biophysical variables was found from the shadow fraction obtained with the cone-based canopy reflectance model corrected for variations in solar zenith angle.

Primary production and nitrogen allocation of field grown sugar maples in relation to nitrogen availability

Above ground net primary production (NPP), nitrogen (N) allocation, and retranslocation from senescing leaves were measured in 7 sugar-maple dominated sites having annual net N mineralization rates ranging from 26 to 94 kg · ha−1 · yr−1. The following responses were observed: (1) Green sun leaves on richer sites had higher N mass per unit leaf area than sun leaves on poorer sites; (2) Total canopy N varied much less than annual net mineralization, ranging from 81 to 111 kg · ha−1; (3) This was due to the existence of a large and relatively constant pool of N which was retranslocated from senescing leaves for use the following year (54 to 80 kg · ha−1); (4) The percentage of canopy N retranslocated by sugar maple was also relatively constant, but was slightly higher on the richer sites. Percent N in leaf litter did not change across the gradient; (5) Above ground NPP increased linearly in relation to N allocated above ground. Therefore, N use efficiency, expressed as above ground NPP divided by N allocated above ground was constant; (6) N use efficiency expressed as (NPP above ground/total N availability) was a curvilinear function of N availability; and (7) This pattern reflected a decreasing apparent allocation of N below ground with decreasing N availability.

Influence of floristic composition on the net primary production and dry matter turnover in a tropical grassland

AbstractPlant biomass, net primary productivity and dry matter turnover were studied in a grassland situated in a tropical monsoonal climate at Kurukshetra, India (29°58′N, 76°51′E). Based on differences in vegetation in response to microrelief, three stands were distinguished on the study site. The stand I was dominated by Sesbania bispinosa, stand II represented mixed grasses and stand III was dominated by Desmostachya bipinnata. Floristic composition of the three stands revealed the greatest number of species on stand II (75). The study of life form classes indicated a thero‐cryptophytic flora. The biomass of live shoots in all the three stands attained a maximum value in September (424–1921 g m‐2) and below ground plant biomass in November (749–1868 g m‐2). The annual above ground net primary production was greatest on stand I (2143 g m‐2) and lowest on stand II (617 g m‐2). The rate of production was highest during the rainy season (15.34 to 3.18 g m‐2 day‐2). Below ground net production ranged from 1592 to 785 g m‐2 y‐2 and the rates were high in winter and summer seasons. Total annual net primary production was estimated to be 3141, 1403, 2493 and 2134 g m‐2 on stands I, II, III and on the grassland as a whole, respectively. The turnover of total plant biomass plus below ground biomass indicated almost a complete replacement of phytomass within the year. The system transfer functions showed greater transfer of material from total net primary production to the shoot compartment during rainy season and to the root compartment during winter and summer seasons.

Above Ground Food Resources and Herbivory in a Beech Forest Ecosystem

In a 90 yr old beech stand the apparent beech leaf consumption 1967-1974 ranged from 7 to 20% (average 12.3%). Phyllophages actually removed 3-9% (average 5.7%) of leaf area index (LAI) an average of about 9 g m-2 yr=1 (d.w.) of beech leaves (about 180 kJ m-2 yr-'), corresponding to 2-5% (average 3%) of annual leaf production and about 4% of available food. Due to wasteful feeding, average consumption of phyllophages on beech was only about 145 kJ m-2 yrc. Leaves and mast were the above ground components of beech trees mainly attacked by herbivorous insects. Beech net assimilation was only insignificantly reduced by leaf consumption. Insect herbivory on beech represented 0.7-0.8% of beech net primary production; above ground insect herbivory in the beech forest ecosystem was < 1 % of total above ground net primary production. Aspects of selective feeding of herbivores and the general role of phyllophages as primary consumers, secondary producers, and regulators in the beech forest ecosystem are discussed.