biomass partitioning, leaf area index, and canopy greenness: the good, the baad and the ugly

Biomass partitioning, leaf area index, and canopy greenness:

the Good, the BAAD and the Ugly

HIE, 28 October 2015

Remko Duursma(and many collaborators)

Why are we obsessed with the Earth’s carbon balance?

Source: IPCC

Climate warming is proportional to cumulative CO2 emissions

Source: IPCC

Why are we obsessed with the Earth’s carbon balance?A large fraction of CO2 emissions is sequestered by the land sink

But difficult to predict the change in carbon sequestration with climate change

Global vegetation models are used for these predictions.

Friend et al. 2014 (PNAS)

Large uncertainty!

Very large disagreement between models in the residence time of carbon in ecosystems

Global vegetation models make different assumptions about most processes

Medlyn et al. 2015 (Nature Climate Change)

Improving representation of residence time

• The residence time will depend in part on the partitioning of carbon to long-lived stems vs. short-lived foliage

• Leaves turn over quickly, enter the soil carbon cycle where most of the carbon is released back to the atmosphere

• Woody biomass persist for many years.

• For biomass allocation, current-generation GVMs are highly simplified and based on very sparse input data or often 'best guesses‘

• We need data

Will Cornwell (Twitter, 10 June 2014; @will_cornwell)

Another problem is the huge diversity in plants, and lack of data on most species

Plant functional types: simple classification to avoid species-level data

Ca. 45% of the world’s plant species are woody (Fitzjohn et al. 2014)De

cidu

ous

Ever

gree

n

Angiosperm Gymnosperm

Global distribution of four major woody plant functional types

Based on data from ESA Climate Change Initiative Land Cover project (http://www.esa-landcover-cci.org/)

But the major PFTs have much overlap in climate space

BioScience

Gary Hincks

Angiosperms evolved later, are far more diverse than gymnosperms

(copyrighted image)

Functional significance of the angiosperm / gymnosperm divide

• Angiosperms differ from gymnosperms in water transport vessel anatomy (both in stems and in leaves)

• This has consequences for water relations (drought tolerance) and is also reflected in wood density

Based on data by Zanne et al. 2009 & TRY Categorical Plant Traits

Enquist & Niklas 2002 (Science)

Gymnosperms have more foliage biomass than angiosperms

(Although these figures hide this difference very well)

• But, surprisingly, we don’t know whether leaf area differs between PFTs• Leaf area is relevant because it drives light interception and thus photosynthesis

“… some gymnosperms attain a higher productivity than co-occurring angiosperm trees by accumulating several cohorts of leaves with a higher total leaf area.”

1989

Based on data from Wright et al. 2004 (Nature; GlopNET)

Plant functional types differ strongly in leaf mass per area (LMA)

0.25-0.75 quantiles

• In turn, LMA correlates with leaf lifespan, and thus residence time of foliage.

Can lower leaf mass in deciduous angiosperms be compensated by LMA?

Questions

• How does biomass partitioning (leaf vs. stem) differ between

• Angiosperms vs. Gymnosperms• Deciduous vs. Evergreen

• Does higher leaf mass per area (LMA) lead to higher plant leaf mass, or lower leaf area?

• Does biomass partitioning depend on climate (mean annual rainfall, mean annual temperature)?

The Biomass and Allometry Database (BAAD)

• data from published and unpublished sources, containing biomass and size metrics for woody plants

• Authors were contacted directly, and were asked for raw data + metadata• Individual plants, destructive harvest (not from allometric estimates)

Raw data Manipulate data (if needed) Extract variables included in BAAD (and assign unified variable names) Add new data (e.g. latitude, longitude, species) Store metadata (methods for data collection) Store study contacts

Clean data • Repeat for each separate study• Combine all clean datasets• Post-process (calculate derived

variables, check species names against databases, etc.)

BAAD

See also our post on https://ropensci.org/blog/

See also our post on https://ropensci.org/blog/

Data are available without restrictions (CC0 License)

BAAD in numbers 20950 individual woody plants176 published or unpublished studies674 species from 120 taxonomic families

Height range from <1cm to 112m, weight from <1g to >300t.

Falster et al. 2015 (Ecology)

MAP and MAT of studies in BAAD compared to global land cover

Duursma & Falster in revision

Komiyama et al. 2002, 2003

Komiyama et al., Japan

Ribeiro et al. 2011

Australia

Canada

Spain

Malaysia

Congo Estonia

Spain Argentina

Also : N content, wood density by component (limited)

Different scaling for leaf and woody biomass with plant height

Duursma & Falster in revision

Sequioa sempervirensEucalyptus

regnans

Terminology

• We here considered aboveground biomass only(Analysis of root data showed no differences between PFTs)

Leaf Mass Fraction (LMF) = leaf mass / aboveground biomass

Leaf Area Ratio (LAR) = leaf area / aboveground biomass

Leaf Mass per Area (LMA) = leaf mass / leaf area

Least-square means

Leaf mass fraction : proportional to leaf mass per area across PFTs

PFTs have similar leaf area per unit biomass

Leaf area ratio does not differ between PFTs

Duursma & Falster in revision

Pipe model: every leaf is connected to a unit sapwood which supplies water to the leaves

When pipes die, they turn into heartwood, which stays on the plant

When leaves die, they fall off

So we expect leaf / stem ratio to decline over time, as plants grow in size

Valentine 1988 (AnnBot)

Why does the leaf mass fraction decrease as plants grow?

• LMF and LAR are strongly dependent on plant height

• Leaf mass fraction can be further decomposed into

where AS is basal stem area

• Similar to LMF, foliage biomass per unit stem area was proportional to LMA

• These variables are only very weakly dependent on plant height

Weak and inconsistent effects of climate

• Either by biome (boreal, temperate, tropical) or mean annual precipitation and mean annual temperature

Duursma & Falster in revision

Conclusions• Three plant functional types differ strongly in leaf mass supported at a

total aboveground biomass or basal stem area

• At given plant height, LMF was proportional to LMA across PFTs• This also to some extent across species, although there is much

variation within PFTs not accounted for

• As a result, leaf area ratio was not different between PFTs

• No clear effects of climate on biomass partitioning

• These results can be used to constrain biomass partitioning estimates in global vegetation models, which routinely predict differences between PFTs

But what about leaf area of plant communities?

Leaf area index (LAI) : amount of leaf area per unit ground area.

LAI = stocking * tree leaf area(stocking: number of trees per unit ground area)

Based on data compiled by Luyssaert et al. 2007

n = 943, ‘Natural’ vegetation only. Based on data by Iio et al. 2013

Evergreen gymnosperms have higher LAI than evergreen angiosperms

But low LAI in evergreen angiosperms seems largely driven by Eucalyptus!

‘Natural’ vegetation only. Based on data by Iio et al. 2013

Leaf area index at the EucFACE : response to elevated CO2 and/or soil water?

• Low LAI for Eucalyptus suggests a response to CO2 is possible• CO2 enhances photosynthesis, we could expect increased leaf growth?

Review of Free-Air CO2 Enrichment (FACE) sites by Norby & Zak 2011

Models largely also predict a positive response to CO2

Model simulations at the EucFACE, over 12 years with variable rainfallBased on simulation data by Medlyn et al. in revision (Application of ecosystem models to EucFACE)

(%)

Largely because increased productivity leads to more leaf growth

EucFACE• Six 'rings' of 25m diameter• 3 at ambient [CO2], 3 at ambient + 150ppm• 'Fully' instrumented• Supersite nearby • Eucalyptus tereticornis

High variability in rainfall : frequent water limitation

LAI estimates via canopy transmittance

Monsi and Saeki 1953

Very long history of estimating LAI based on measurements of light intensity

(The slope of this relationship is the ‘extinction coefficient’)

• 3 sensors below the canopy, one above, each ring.• PAR is logged every minute since October 2012.

Almost 40 million readings of PAR to date.

Measurements of light above and below the canopy

PAR = photosynthetically active radiationSunny

Time (hours)

PP

FD

m

olm

2s

1

0 4 8 12 16 20 24

050

010

0015

0020

00

Cloudy

Time (hours)

PP

FD

m

olm

2s

1

0 4 8 12 16 20 240

200

400

600

800

PAR

PAR

Canopy transmittance : ratio of below / above canopy PAR

Sunny

Time (hours)

Tran

smitt

ance

P

PFD

belo

wP

PFD

abov

e

0 4 8 12 16 20 24

0.0

0.2

0.4

0.6

0.8

1.0 Cloudy

Time (hours)

Tran

smitt

ance

P

PFD

belo

wP

PFD

abov

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0 4 8 12 16 20 240.

00.

20.

40.

60.

81.

0

‘SunShine’ sensor tells us how cloudy it is (‘fraction diffuse radiation’)

LAI from canopy transmittance reveals high temporal and among-ring variability

An uninvited guest : psyllids are affecting leaf area dynamics

Cardiaspina sp. Photos: Aidan HallHall et al. 2015Gherlenda et al. submitted

LerpPsyllid

Duursma et al. accepted, GlobChangeBiol

No effect of CO2 on LAI at the EucFACE

Why is there no effect of elevated CO2 on LAI at the EucFACE?

1. Leaf growth may not be limited by availability of carbon• Water limitation• Nutrient limitations

2. Extra carbon from increased photosynthesis may be allocated elsewhere• Roots? Storage?

3. It’s the statistics, dummy : not detecting a difference is no evidence for absence of a difference

Least-square means : fitted LAI at a common pre-treatment basal area

Confidence intervals are very small : we could pick up a difference of 6% in LAI between treatments

Duursma et al. accepted, GlobChangeBiol

'Flat canopy' photos (canopy cover photography)

ca. 30 degrees

• Automated tresholding (blue channel)• Ca. 21 photos per ring, ca. monthly, when

cloudy

Teresa Gimeno, Matthias Boer

Good correspondence between independent methods

Photography-based method also showed no response to elevated CO2

LAI f

rom

can

opy

tran

smitt

ance

Duursma et al. accepted, GlobChangeBiol

EucFACE: no effect of elevated CO2 on LAI

Eucalyptus

Norby & Zak 2011 + EucFACE

Leaf growth observed

19 Dec 2013 6 Jan 2014

maps.nearmap.com

= Leaf production – Leaf shedding

Litter production and dynamics of leaf area index

Litter production shows interesting dynamics:when new leaves are growing, old leaves are shedM

onth

ly n

et c

hang

e in

LAI

Monthly litter production

Duursma et al. accepted, GlobChangeBiol

So we now how much leaf area there is, but what does it look like?Is all leaf area functional all of the time?

The cameras are programmed to take six photos of each ring, 3 times a day

So far, more than 25,000 photos have been taken since Nov . 2014

Canopy greenness

‘Green chromatic coordinate’ (GCC) = R / (R + G + B)

Has been used extensively to study phenology of deciduous canopiesUseful for ‘evergreen’ Eucalyptus canopies?

Richardson et al. 2009 (Ecol. App.)

Link to VideoNov. 2014 Jan. 2015

Sep. 2015 Oct. 2015

Ring 3

Duursma, unpublished

Ring 6, sunny days

The Good : Leaf area index at the EucFACE can be measured accurately with canopy transmittance, and we can confidently conclude no response to CO2

The BAAD: a global Biomass And Allometry Database reveals consistent patterns among major woody plant functional types.

… but these do not easily translate to patterns in leaf area index.

The Ugly: Canopy greenness as measured by automated cameras reveals when the canopy is ugly, but analysis will be difficult

AcknowledgmentsBiomass And Allometry Database:Daniel Falster, Masae Ishihara, Diego R. Barneche, Rich G. FitzJohn,Angelica Vårhammar, and 86 data contributors

EucFACETeresa Gimeno, Matthias Boer, Kristine Crous, Mark Tjoelker, David Ellsworth, Steven Wohl, Vinod Kumar, Craig McNamara, Craig Barton, Andrew Gherlenda, Jeff Powell

OtherBelinda Medlyn, Martin De Kauwe

www.remkoduursma.com