Maurizio Mencuccini

FORESTS, DROUGHT AND GLOBAL CHANGE

School of GeoSciences, University of Edinburgh, (UK)ICREA at CREAF (Barcelona, Spain)

• plants, trees, forests • water• carbon• nutrients

• Tropics• Amazon• Africa• Borneo

• physiology• modelling

-Water and climate change globally

-Characteristics of soils and plants that affect the water cycle: plant functional traits

- Causes of drought-induced mortality around the word

- Case studies of local-regional mortality

Structure

Forests, drought and global change

Bonan (Science, 2008)

Important forest-atmosphere feedbacks: energy budget, hydrology, C cycle, land use

Water as greenhouse gas

• Most important GH gas (more than CO2)

• More CO2 → More heat → More vapour → More heat (esp. tropics)

• Small GH effect compared to CO2

Global water cycle

Consequences of increased air vapour pressure :• Greater greenhouse effect (positive feedback)• More clouds (more reflection of radiation – negative feedback)• Positive feedback > negative feedback• More transport of heat (clouds more buoyant, more wind, more storms)• More vapour for rain and snow (more storms, more floods, heavier rains,

more snow in places)

Drought predictions worldwideChange in annual runoff for the period 2090-2099 relative to 1980-1999:

IPCC (2008)

Water in soil

• Soil organic matter affect water holding capacity of soils

• → link between C, nutrient and H2O cycles

Water in plants

• Stomata in leaves• → inescapable trade-off between CO2

uptake and water loss • Water use efficiency

Water in atmosphereParadox of warming:• More water in atmosphere (in absolute terms)• But plants perceive less water in atmosphere (in

relative terms!!!• Deficit in vapour pressure increases with T• Atmospheric drought• Increased plant water loss

Strategies to cope with drought• Drought sensitive

• Drought tolerant (e.g., agave, cacti)

• Drought avoiders (e.g., Tamarix)

The concept of stress

A load on biological systems that modifies metabolic processes and produces a strain in the organism.

Definitions

1. Functional traits : organismal features (e.g., leaf size, rooting depth, leaf [N], phenology, seed size, etc.) relevant for the species’ response to the environment and for biotic interactions.

2. Functional diversity : the value, abundance and distribution of organismal functional traits in a community.

1) Wood economics spectrum

Water transport / Mechanical support / Storage / Decay resistance

Angiosperms Gymnosperms

Example: Effect of drought on wood hydraulics

• Development of emboli inside a Scots pine stem as drought increases (left to right)

Xylem water potential

Free water with no solutes at sea level: WP=0Typical values: well watered: WP=-1.5 MPa= -15 atmDrought-stressed: WP=-3.0 / -6.0 MPa= -30 / -60 atm

Global synthesis• 190 angiosperms,

32 gymnosperms

• Vertical distance from diagonal is hydraulic safety margin from hydraulic failure

• No differences across major biomes

Choat et al (2012)

wet

dry

• low LMA → high %N → high Amass• low LMA → low leaf lifespan → low resistance to herbivory

About 3,000 spp.(Wright et al 2004, Nature)

1) Leaf economics spectrum

Leaf structure

Angiosperms

Gymnosperms

water and oxygen

Carbon

dioxide

Leaf mass per area

Leaf mass / leaf area

Sources of variability in MLA:- Leaf thickness- Volume of air space- Number/size of mesophyll cells- Thickness/density epidermis

Main objectives•Provide a global archive of plant traits

•Promote trait-based approaches in ecology and biodiversity science•Support the design of a new generation of global vegetation models

Current state of database and network•3 million trait records for about 69000 plant species

•591 participants from 207 scientific institutes worldwide•256 scientific projects requesting plant trait data from TRY

Quantifying and scaling global plant trait diversityTRY is a network of vegetation scientists headed byDIVERSITAS, IGBP, the Max Planck Institute for Biogeochemistry and an international Advisory Board.

Dynamic Global Vegetation Models

• Combined with global atmospheric circulation models• DGVM predict major processes of terrestrial biosphere at

temporal scales varying from months to centuries

Plant functional types in DGVMs• PFT= Broad groups of species with common

physiological/morphological traits (e.g., needleleaf evergreen or deciduous trees, broadleaf evergreen or deciduous trees, shrubs, grasses, crops)

• Look-up tables of main traits for each group

Functional traits and response to stresses

• In plants, we measure strain, not stress directly• When elastic strain stops, metabolism recovers• When plastic strain stops, metabolism is modified

Example: Effects of high temperatures on photosynthetic traits

Short-term repair after heat stress

Response of plants to stresses

• Over longer time scales, plants respond to stress• Repair, hardening, acclimation, evolution

Acclimation at multi-annual time scale: plasticity

� Plasticity is genetically controlled� Species differ in how plastic they are (there is a cost)� The environment as inducer of phenotypes

Acclimation of photosynthesis to growing temperature in oak and birch

� The thermal optimum for photosynthesis increased with the increase in growth temperature The absolute values of respiration declined with increasing growth temperature

Growth T: Blue=ambient; orange=+2degree C; red=+4 degree C

Plasticity along geographical clines� Study of the gas exchange and hydraulics of

Scots pine across Eurasia

Poyatos et al (Oecologia 2007)

Trailing edge of distribution

Within-species plasticity: the case of Scots pine

AL:AS = area leaves / area stem (Corner’s rule)KS = stem hydraulic conductance (Wood economics)P50 = vulnerability to embolism (Wood economics)∆∆∆∆13C = Stomatal aperture (Leaf economics)

Martinez-Vilalta et al (New Phytol 2009)

Mediterranean Europe: The case of Scots pine

NE Spain

Valais(Alps)

Martínez-Vilalta & Piñol (For Eco Man, 2002)

Bigler et al. (Ecosystems, 2006)

Limits to plasticity: mortality

1) Direct water stress (hydraulic failure)2) Carbon starvation

MacDowell et al. ( New Phyt, 2008)

Drought-induced mortality

Hydraulic failure

• Development of emboli inside a Scots pine stem as drought increases (left to right)

Long-term starvation due directly to higher T?

Adams et al. (PNAS, 2009)Pinon pine at biosphere 2

Carbon starvation

current

future

Impacts at community level

• Species filtering• Trait filtering• Trait convergence

� Drought as a phenotypic filter

Funk et al (Trends Ecol Evol 2008)

Not intense / infrequent

Intense / Frequent

DR

OU

GH

T

AcclimationPlasticity

Phenotypic selection

Genotypic selection

Hydrological consequences

• Radiation regime

• Water regime

• (Also carbon cycle)

• (Also nutrients cycles)

Kurz et al. (Nature, 1998)

-Mountain pine beetle- British Columbia, Canada- ~80,000 km 2 affected.-Carbon losses equivalent to 75% of all Canada fire emissions for 1 year.- 10x larger than any previous known outbreak

Evidence of large scale mortality. Boreal.

Causes:- reduced winter T- increased summer T- reduced summer rainfall

Records of mortality in large-scale undisturbed forest reserves in the old-growth stage of development

Van Mantgemet al. (Science, 2009)

Evidence of large scale mortality: temperate

1994 1994

1994, 1998…2005

Quercus ilex

Pinus sylvestris

Evidence of large scale mortality:Mediterranean Europe

MacDowell et al. (New Phytologist, 2008)

Pinus edulis

Juniperus monosperma

Evidence of large scale mortality: subdeserts

Piñon – juniper woodlands (SW USA)Up to 95% mortality in some areas

Evidence of large scale mortality: tropics

How much has gone? ~ 16 % (depends on estimate)

Current rate? Approx. 1.9 million ha yr -1 (=19 000 km 2):~0.5 % yr -1

Laurance et al. 2001, Environmental Conservation, 28, 305; Fearnside 1999, Environmental Conservation, 26, 305

de Fries et al. 2002, Proceedings of the National Academy for Sciences (PNAS)

5 m km2 of which 4 m km2 originally forested

Impacts

Logging: reduces canopy cover (50%), increases litt er…fire removes larger treescreates access (agriculture, hunting), affects eros ion

Fire: low resistance among rain forest treesincreases likelihood of short return time….total los ssynergism with climate and ignition sources (e.g., roads)

Fragmentation: edge effects on microclimate, fire v ulnerabilityeffect of patch size on population viabilitydistance between patches (dispersal)invasion by exotic species (e.g., grasses)

Climatic / hydrologic: Temperature changeRegional and local rainfallSmoke and dustImportance of large scale events (e.g., El Niño)

ImpactsFragment edges; micro climate

> 150 % of deforested area in 1988 (<100km2 block, <1km to edge)- increased turbulence, temp., drought stress, competition

→→→→ higher mortality, esp. big trees

Laurance et al. 2000, Nature, 404, 836

Flammability feedbacks

• Drought or logging may increase dry fuel load

• Ground fires increase the probability of return fires

• Additional human occupation increases number of ignition sources

→ potential for much larger forest losses

Climatic feedbacks

• Reduced rainfall from conversion to pasture

• Effect magnified by dust from intense dry season fires

• Interaction with global climate

→ increased local fire vulnerability, global costs

Carvalho et al. 2001, Nature, 409, 131

-Water cycle central to understanding of climate change

-Predictions of rainfall changes are highly uncertain but likely very region specific

-Water cycle in forests depend on characteristics of soils, plants and atmosphere; plant functional traits are central.

- Concerns about species distribution have begun to appear for several regions around the world

-Several case studies of local-regional mortality

Conclusions