Forest Mortality and Australian Terrestrial Carbon Stores

Project ObjectivesRising atmospheric CO2 is driving changes in climate already being observed across Australia. Across southern Australia rainfall is declining, temperatures have increased (mean increase~ 0.9ºC since 1910) and the frequency of heat waves has increased in each decade since the 1950’s. These all affect ecosystem function and the capacity to store carbon in terrestrial ecosystems.

Forest mortality events associated with changing climates have been observed in most of the world’s biomes (Allen et al. 2010) including Australia (Fensham and Fairfax 2007; Mitchell et al. under review) and affect forest productivity and ecosystem processes. Predictive modelling is a key tool for examining the net primary production and carbon sequestration of Australia forests. Yet commonly used tools do not account for forest mortality (Roxburgh et al. 2005). Key reasons for this include: (1) lack of a common understanding of thresholds in physiological responses to changing climates; and (2) lack of appropriate data from field observations and experiments for model validation. This makes it extremely difficult to address issues such as vulnerability of forest ecosystems to future climates; the consequences of increasing mortality for carbon sequestration; and how biodiversity might be affected by future mortality events.

Our Working Group addressed these gaps by bringing together field and experimental researchers, modellers, and end-users focussed on native and managed Australian forest/woodland ecosystems. Undertaking a multidisciplinary analysis, we have integrated our current understanding of forest mortality with stand and landscape-level processes to determine forest vulnerability under future climates. Importantly, this exercise will increase our capacity to predict future climate related mortality events through development of a living-database which is continually expanded and utilised. Potential strategic applications include; improved climate mitigation strategies, improved modelling of terrestrial C stores and better definition of the role of forests in climate change mitigation.

MethodsTwo workshops were convened and concepts and ideas were explored in an effort to:

• Define drought in a way that facilitates comparison of responses across ecosystems and biomes in an ecologically relevant manner

• Determine how ecosystem resistance and resilience will contribute to ecosystem responses to extreme climatic events

• Determine the relationship between plant water relations, drought tolerance traits and climate through the compilation of existing data.

A review of historical mortality events in Australia was conducted and compiled. The relationship between climate and existing observed mortality events was explored by examining the climatic conditions that contributed to mortality using the standardised precipitation Evapotranspiration Index (SPEI) and relationships with high temperature stress (Maximum Temperature Index MTI).

Development of conceptual frameworks and the acquisition of a substantial data base of water relations traits for Australian species was accomplished in the two meetings. This compliments a global data base of plant hydraulic traits developed by ARC Veg Network Working group (Choat et al. 2012) and compliments a data base being developed as part of the E-Mast initiative within TERN.

Principal Investigator: Anthony O’Grady1

Group Members: Patrick Mitchell1, Libby Pinkard1, David Tissue2, Remko Duursma2, Stephen Roxburgh1, Jaymie Norris3, Katinka Ruthroff 4, Rod Fensham5&6, Craig Nitschke7, David Hilbert1, Chris Blackman8, Tim Brodribb9

Participant’s institutions: 1 CSIRO Ecosystem Sciences 2 Hawkesbury Institute for the Environment, University of Western Sydney 3 Victorian Department of Sustainability and Environment 4 State centre of excellence for climate change, woodland and forest health, Murdoch University 5 Queensland Herbarium, 6 University of Queensland 7 University of Melbourne 8 Macquarie University 9 University of Tasmania

Major Findings

Figure 1: Relationship between the joint probabilities of SPEI and MT for a range of mortality events in different ecosystems around Australia. In all mortality events, the probability of these climatic conditions occurring was less than two percent. Under future warming and drying, the probability of these climatic conditions occurring dramatically increases and places forests and woodlands at increased risk of climatic stress.

There is strong evidence for a global convergence in the vulnerability of forests to future climate extremes. Mitchell et al. (in review Global Change Biology) demonstrate a common climatic threshold beyond which all forests are at increased risk of mortality from extreme heat and drought. The probability of this threshold being exceeded will increase under climate scenarios that predict a warmer future with increasingly intense droughts. Australian eucalypt forests are well adapted to extreme climates, and the frequency of these extremes plays an important role in determining species distributions.

• Many eucalypts succumb to hydraulic failure during drought, and existing water relations data suggests that this is a key resilience strategy for coping with regular water deficits.

• Carbohydrate reserves are relatively stable during drought and these large carbohydrate reserves in adult trees form the basis for canopy recovery or re-sprouting following the return of more favourable conditions.

• Seedlings may be particularly vulnerable to these future extremes and further work needs to be done to assess the vulnerability of forests to this bottleneck.

There are strong correlations between many drought tolerance traits and climate at the hottest and driest end of a species distribution. This provides predictive power for examining vulnerability to future climate regimes.

A conceptual model is proposed of how climate, ecosystem processes and ecosystem resilience might be analysed under future climate regimes (Figure 2). This model suggests that the response of many ecosystem processes can be described by common climatic thresholds when climate is addressed within a probabilistic framework.

• There is compelling evidence for a common climatic threshold associated with stand mortality. However, evidence for commonality in other ecosystem processes needs further exploration.

• We present a framework for comparing ecosystem responses across ecosystems in a manner that facilitates a risk assessment of vulnerability of forests to future climate change in a consistent manner.

• This framework also allows for the comparison of ecosystem or species resilience as a measure of resilience that can be obtained from an estimate of the half time for recovery of the respective processes; thereby, enabling a comparison of resilience amongst species and communities.

Publications and Outcomes

We are now a step closer to having the capacity to assess the risk to ecosystems or species under a range of future climate scenarios. To date, the vulnerability of Australian ecosystems has been difficult to assess. The development of a probabilistic approach to estimating exposure and sensitivity of ecosystems is an important step forward. In addition, the data compiled in this working group compliments global and Australian efforts to synthesise and analyse existing information. To our knowledge, the water relations data base compiled here represents the largest data base of water relations traits in Australia and provides insight into the sensitivity of many Australian species to drought. This data base will become an essential resource in the design and development of carbon and biodiversity plantings around the country.

How will this affect Australian ecosystem science & management?

References

A data base of leaf water relations traits for Australian species with trait water relations and hydraulic traits for over 300 species. A living data base for existing observed mortality events currently containing 17 mortality events and metadata. Mitchell P.J.M., O’Grady A.P. (2013) Drought, tree death and the carbon balance. Ecos: CSIRO Publishing. O’Grady A.P., Mitchell P.J.M., Pinkard E.A., Tissue D.T. (2013) Thirsty roots and hungry leaves: unravelling the roles of carbon and water dynamics in tree mortality. New Phytologist 200(2): 294-297. (invited commentary). O’Grady et al. Towards a consistent approach for assessing drought risk to terrestrial carbon stores (invited speaker Annual meeting of the Ecological Society of America Minneapolis MN August 2013). A draft manuscript is nearing submission: Mitchell et al. “Toward an ecologically relevant definition of drought and its impact” and another manuscript O’Grady et al. “Defining the limits to plant function using drought tolerance traits” is in preparation.

Allen CD, Macalady AK, Chenchouni H, Bachelet D, McDowell N, Vennetier M, Kitzberger T, Rigling A, Breshears DD, Hogg EH, Gonzalez P, Fensham R, Zhang Z, Castro J, Demidova N, Lim J-H, Allard G, Running SW, Semerci A, Cobb N. (2010) A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259(4): 660-684.

Choat B, Jansen S, Brodribb TJ, Cochard H, Delzon S, Bhaskar R, Bucci SJ, Feild TS, Gleason SM, Hacke UG, Jacobsen AL, Lens F, Maherali H, Martinez-Vilalta J, Mayr S, Mencuccini M, Mitchell PJ, Nardini A, Pittermann J, Pratt RB, Sperry JS, Westoby M, Wright IJ, Zanne AE. (2012) Global convergence in the vulnerability of forests to drought. Nature 491(7426): 752-755.

Fensham RJ, Fairfax RJ. (2007) Drought-related tree death of savanna eucalypts: Species susceptibility, soil conditions and root architecture. Journal of Vegetation Science 18(1): 71-80.

Roxburgh SH, Berry SL, Buckley TN, Barnes B, Roderick ML. (2005) What is NPP? Inconsistent accounting of respiratory fluxes in the definition of net primary production. Functional Ecology 19(3): 378-382.

Major Findings (continued)

Figure 2: Conceptual diagram of the responses of key ecosystem processes to drought. The figure proposes that there is a common climatic threshold where drought impacts key ecosystems processes. Characterising these responses provides an index of the resistance of the community to stress (i.e. the amplitude of the response) and an index of the resilience of the community to climate stress (measured as the half-time for recovery of the process being considered). The framework provides an approach for comparing and contrasting ecosystem exposure and sensitivity to climatic stresses in a consistent manner across ecosystems, and a pathway for formal assessments of risks to existing terrestrial carbon stores.