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1
Net Primary Production (NPP)
• Measurement • Plant growth • Global variation • Vegetation types • Resource limitation
Ben-Gurion University of the Negev"
Vegetation Ecology Course 2015/16 Bertrand Boeken
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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
3
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.
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Summation of single plant or leaf performance over a whole field, or hierarchical
complexity?
Scaling-up
NPP = f (energy flux, growth conditions) with random spatial variation in performance and limited substrate heterogeneity
NPP = f (energy flux, growth conditions, multiple species performance) with significant spatial heterogeneity, species differences, diversity, and biological interactions
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Resources For primary production and growth
• CO2
• Light
• Water
• Mineral nutrients
• O2
Light CO2
Water
Minerals
O2
Air
Soil
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• Leaf area index (LAI)
Whole plant growth
• 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
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• Growth rate GR=dM/dt
• Relative growth rate
RGR=∆M/initial M
Mas
s
Time
Annual plant
Perennial plant
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
Stems
Storage
Roots Roots RE
RE
Leaves
Stems
Leaves
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Latitudinal gradients
• Light quality (PAR, angle)
• Length of growing season
• Low temperature
• Aridity
Latitudinal variation in NPP • High near the equator ca. 2200 t.km-2.yr-1 • Intermediate in temperate zones 1200 t.km-2.yr-1 • Low near poles 10-800 t.km-2.yr-1 • But very low in the arid zones 90 t.km-2.yr-1
Global variation of NPP
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Boreal coniferous forest
Temperate deciduous forest
Tropical rainforest
Vegetation types latitudinal trends
• Forest
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Vegetation-type effects on NPP NPP vs Standing mass
Standing mass (B)
NPP
Grasslands
Forests
NPP/B is greater for grasslands, especially if dead material is removed (grazing)
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Coniferous forest Deciduous forest
Light-use efficiency
LAD
Chlorophyll content low high
low high
long short
NPP lower high
LAI high low
Vegetation-type effects on NPP
13
Water limitation
Precipitation
NPP
3000 mm/yr
Limited by transport and stomatal resistance
Limited by water deficit, low vegetation cover
Drylands
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
NPP vs Rainfall
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NPP in water-limited systems High productivity per unit leaf mass, when and where water is available.
However, growth is limited • Temporally, by
low LAD due to short growing season;
• Spatially, by low LAI due to incomplete ground cover
Isolated shrub patches Widely spaced annuals on biocrust Density-independent biomass production of annuals.
0 50 100 150 200
R2=0.277, P<0.0001
0
10
20
30
Total plant density (per 600 cm2)
Tota
l bio
mas
s (g
)