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

2

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.

4

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

5

Resources For primary production and growth

•  CO2

•  Light

•  Water

•  Mineral nutrients

•  O2

Light CO2

Water

Minerals

O2

Air

Soil

6

•  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

7

•  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

8

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

9

Boreal coniferous forest

Temperate deciduous forest

Tropical rainforest

Vegetation types latitudinal trends

•  Forest

10

•  Grassland

Temperate coastal grassland

Arctic tundra

Tropical savanna

11

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)

12

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

14

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

)

15

Vegetation cover “Bare” surface

Subhumid Semiarid Arid Humid

From closed canopy to contracted vegetation

Lehavim (300 mm/yr)

Park Shaked (150 mm/yr)

Makhtesh Ramon (<50 mm/yr)

Spatial patterns in water-limited systems