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Ben-Gurion University of the Negev" Vegetation Ecology Course 2015/16 Bertrand Boeken Net Primary Production (NPP) • Measurement • Plant growth • Global variation • Vegetation types • Resource limitation 1 Measurement Net primary production NPP = annual increase in biomass per unit area [g.m-2.yr -1] • Usually only aboveground • Though much production is allocated belowground Gross primary production GPP = NPP + R R = part of GPP used in respiration Standing mass (crop) = Total (bio-)mass present including new tissue, old living tissue and dead material US Environmental Protection Agency 2 Spatial scale • NPP is of interest on the local or regional scale, • But carried out by individual plants, • While photosynthesis occurs in the leaves at the molecular level. 3 Scaling-up Summation of single plant or leaf performance over a whole field, NPP = f (energy flux, growth conditions) with random spatial variation in performance and limited substrate heterogeneity or hierarchical complexity? NPP = f (energy flux, growth conditions, multiple species performance) with significant spatial heterogeneity, species differences, diversity, and biological interactions 4 Resources For primary production and growth • CO2 Air Light CO2 • Light • Water • Mineral nutrients • O2 Soil O2 Water Minerals 5 Whole plant growth • Leaf area index (LAI) • Leaf area duration (LAD) and phenology Size of photosynthetic surface Relative to canopy area per plant or vegetation stand with layers (LAI can be >1) Time that photosynthetic surface is available LAI and LAD can be combined by integration of LAI over time 6 Life form • Annuals invest most photosynthates in new leaves and stems, later in reproduction • Ephemeral perennials invest in storage and roots, stems and leaves, only much later in reproduction • Woody perennials also in support and long-term roots, stems and/or leaves Leaves Storage Leaves Stems Stems Roots RE Roots RE GR=dM/dt Annual plant Mass • Growth rate Perennial plant • Relative growth rate RGR=∆M/initial M Time 7 Global variation of NPP Latitudinal gradients • Light quality (PAR, angle) • Length of growing season • Low temperature • Aridity Latitudinal variation in NPP • • • • High near the equator Intermediate in temperate zones Low near poles But very low in the arid zones ca. 2200 t.km-2.yr-1 1200 t.km-2.yr-1 10-800 t.km-2.yr-1 90 t.km-2.yr-1 8 Vegetation types latitudinal trends • Forest Boreal coniferous forest Temperate deciduous forest Tropical rainforest 9 • Grassland Arctic tundra Temperate coastal grassland Tropical savanna 10 Vegetation-type effects on NPP NPP vs Standing mass NPP/B is greater for NPP Grasslands grasslands, especially if Forests dead material is removed (grazing) Standing mass (B) 11 Vegetation-type effects on NPP Coniferous forest Deciduous forest LAD long short LAI high low Chlorophyll content low high Light-use efficiency low high NPP high lower 12 Water limitation NPP vs Rainfall NPP Limited by transport and stomatal resistance Drylands Limited by water deficit, low vegetation cover 3000 mm/yr Precipitation However, short-term relative NPP is very high in deserts: 2.3 g.g-1 Coniferous forest: 1.6 Deciduous forest: 2.2 Temperate grassland: 1.2 13 NPP in water-limited systems High productivity per unit leaf mass, when and where water is available. Total biomass (g) However, growth is limited • Temporally, by low LAD due to short growing season; R2=0.277, P<0.0001 30 20 • Spatially, by low LAI due to incomplete ground cover Isolated shrub patches 10 0 0 50 100 150 200 Total plant density (per 600 cm2) Widely spaced annuals on biocrust Density-independent biomass production of annuals. 14 Spatial patterns in water-limited systems From closed canopy to contracted vegetation Humid Subhumid Vegetation cover Semiarid Arid “Bare” surface Lehavim (300 mm/yr) Makhtesh Ramon (<50 mm/yr) Park Shaked (150 mm/yr) 15