Light capture and Plant architecture determine Co-existence and Competitive Exclusion in Grassland Succession how grazing modifies succession Marinus J.A. Werger, Utrecht University

plant + lightphotosynthesis growth light harvesting is important !

BUT: plants evolved in dense stands, NOT in open stands with competing neighbours without competing neighbours

THUS: efficiency of light harvesting was strong selection pressure throughout plant evolution THUS: differences in efficiency of light harvesting may determine competitive success of species AND may explain succession

This study: temperate grassland Climax: broad-leaved deciduous forest or mixed deciduous forest Forest destruction or long-time overgrazing: Grassland Grassland: grazedGrassland Grassland: grazing excluded scrub & forest

which mechanisms regulate this succession? differences in light harvesting efficiencies plant’s physiological constraints plant’s architectural design these interact

How to measure light harvesting efficiency? Light climate in grassland canopies Light absorbed by leaf area

Light decreases exponentionally with leaf area passed

Thus: Tall plants high in canopy more light but high investments in biomass Short plants down in canopy little light but low investments in biomass

dense stand open stand LAI leaf N (gN/m2)

July June May Nitrogen gradient during the growth season

leaf N (mmol/m2) Maximum photosynthetic rate increases with leaf N Respiration increases with leaf N but Photosynthetic rate increases much stronger than Respiration

INVESTMENT, LIGHT HARVESTING & EFFICIENCY

MEASUREMENT AT PEAK STANDING CROP LOWER LEAVES ALREADY DEAD

light Total leaf area vs light absorption

LAI TOTAL sp Leaf area distribution of 11 species in the stand

TOTAL sp light interception Light interception by 11 species in the stand

Light harvesting as a function of leaf area and of mass

Efficiency of light harvesting as a function of leaf area and as a function of plant mass efficiency increases with more investment in leaf area efficiency does not increase with more investment in mass

plant height cm Light harvesting efficiency of 5 species Co-existence seems possible

grazing lawn

exclosure from grazing

Exclosure > 4.5 yrs

climax forest

Only the 4.5 yrs exclosure (dotted line) shows increasing mass use efficiency with increasing plant mass

plant height cm Light capture efficiency per unit leaf area increases with plant height (but not in the grazing lawn)

Grazing lawn 2.5 yrs 4.5 yrs dry 4.5 yrs moist In successional vegetation light harvesting efficiency increases with plant height

Some species persist in all 4 successional stages Some species in 3 or 2 or just 1 successional stage(s) e.g. Zoysia dominates the lawn and and disappears in later stages Miscanthus is rare in the lawn and dominates in later stages Carex stays constant in all stages WHY ?

species are restricted by their architectural and physiological constraints

Stage veg. height Grazing lawn 8 cm Excl. 2 yrs 40 cm Excl. dry 4.5 yrs 65 cm Excl. moist 4.5 yrs 180 cm Plant height ΦmassPlant height ΦmassPlant height ΦmassPlant height Φmass Zoysia830 - Carex Miscan- thus Stage veg. height Grazing lawn 8 cm Excl. 2 yrs 40 cm Excl. dry 4.5 yrs 65 cm Excl. moist 4.5 yrs 180 cm Plant height ΦmassPlant height ΦmassPlant height ΦmassPlant height Φmass Zoysia Carex Miscan

Why goes Zoysia extinct? Why becomes Miscanthus dominant? because Zoysia cannot grow tall (it makes max. 6 short internodes) and it is not shade tolerant Miscanthus can grow tall

Zoysia: Its architecture prevents it to grow tall with the vegetation This strongly reduces its Φmass (efficiency) It has no shade tolerance, therefore Its physiology makes it impossible to survive in the shade deep in the vegetation

Miscanthus: its architecture allows it to grow tall with the vegetation it harvests plenty of light therefore reduction in Φmass is not important, because its Φarea stays high it can grow to dominate But why does it not dominate in the grazing lawn?

Why does Miscanthus not dominate the grazing lawn? its architecture prevents it to grow small: it can only make long internodes grazing destroys the growing points (meristems) too frequently

Which other plant types are successful? LIANAS: use support of other plants to reach top of canopy relatively less investment of mass per height this is favourable for Φmass (efficiency)

Which other plant types are successful? SHRUBS: initially no high efficiency (low Φmass) individuals start every year a bit higher above the ground (height advantage) efficiency becomes higher finally tall and outshading other plants RESULT: succession to forest