Drought Risk

Photo: Kindly provided by Mark Harpur 


Context

Drought causes stress to trees that can impact on rates of growth and mortality, and thereby timber production and carbon sequestration values. Within the UK, for example, trees such as Sitka and Norway spruce, Larch and Beech are more vulnerable to drought than species such as Scots pine, Douglas-fir and Ash. Long-term drought can lead to loss of trees and also increases the susceptibility of trees to other risks especially fire and P&D. Extreme drought may also lead to reduced productivity and even large-scale die-offs on a regional scale.

One of the difficulties in predicting the impacts of droughts on forests is the uncertainty around forecasting future drought extent and severity. It is therefore useful to conceptually separate climate risk (here risk of drought) from the sensitivity of vegetation to that climate risk (Meir and Woodward, 2010). A recent IPCC report, for example, concluded that issues with existing drought models meant that there was only ‘medium confidence’ in projections for some regions including southern Europe and the Mediterranean region, central Europe, central North America, Central America and Mexico, northeast Brazil, and southern Africa. Elsewhere there was only a ‘low confidence’ in projections due to disagreement in projections (Seneviratne et al., 2012).

The susceptibility of trees to drought depends primarily not only on the ecological characteristics of the species but also on the soils present. The ability of soils to store and also to release water to plants is affected by a number of factors including soil (and rooting) depth, permeability, sand/silt and organic matter content, capillarity etc. A period of drought can have a greater or lesser impact depending on the type of soil and the pre-existing level of the water table.

Tropical forests are particularly vulnerable to drought as the severe droughts in Amazonia in 2005 and 2010, and others in SE Asia in earlier periods (e.g. 1982/3, 1997/8) highlighted. The two recent droughts in Amazonia were both described as ‘1 in a 100 year’ events according to preceding records and yet they occurred within 6 years of each other. The 2005 drought was estimated to have caused a total biomass carbon loss of 1.2 to 1.6 petagrams including estimates of below-ground losses (Phillips et al., 2009) . There is considerable uncertainty around how 21st century climate change will affect Amazonia but what is virtually certain is that there will be warmer temperatures and more extreme rainfall patterns. Extreme drought and rainfall events have both been observed in the last 7 years in the region consistent with this trend.

Whilst tropical forests are particularly vulnerable, temperate forests are not immune from drought. A study of the impacts of the 2003 heatwave and drought in Europe found that there was a 30% reduction in gross primary production[3] in the region that year, with a reduction in rainfall being the primary cause of loss in Eastern Europe (Ciais et al., 2005).


Approaches to assessing risk


Ecological Site Classification Tool (ESC) for tree species selection under current and future climate scenarios in Britain

Weblink: click here
The Ecological Site Classification Tool (ESC) was developed by Forest Research to assist forest practitioners in assessing the maximum yield expectations for a range of key broadleaved and conifer tree species for any potential site in Great Britain (Pyatt et al.,2001). The system uses four climatic variables and two edaphic (soil) properties to estimate potential yields. Climate variables are accumulated temperature; continentality; detailed aspect method scoring (DAMS – a measure of exposure and windiness); and moisture deficit. Pre-calculated climate variables have been loaded into ESC for the baseline period from 1961-90. Edaphic properties are soil moisture and soil nutrient regimes. Default values are contained within ESC but the user is recommended to input site specific values. The Tool is widely used within the UK forest sector. The Tool has also been developed to provide yield forecasts for Sitka Spruce, Scots Pine and Oak under different future climate scenarios (Petr et al., 2014). Moisture deficit is an indicator of ‘droughtiness’ under these scenarios. Work is currently underway to process this latter information into a form of use to end-users across the forest sector and to adapt the decision support tool to include it. A demonstration version of ESC can be accessed via the ‘Forest DSS’ tab on this site and clicking on the pull-down menu to select the Tool. Clicking on a site on the map provides sample outputs. Contact: Duncan Ray, Dr Michal Petr.


Using Thermal Lidar to identify trees under drought and disease stress

Professor Juan Suárez is the UK contact for the ThermoLiDAR project (Link), funded under the EU 7th Framework Programme (finished in 2014), which aimed to improve the early detection of forest health through the provision of new tools for sustainable forest management based on LiDAR and THERMAL data integration. This project combined the use of LiDAR to retrieve estimates of canopy structure (height, fractional cover, biomass, etc), with thermal imagery to create estimates of relative temperature differences that can be used as a proxy for stomata conductance. The general principle is that under normal conditions, the stomata release water through respiration and therefore the canopy is kept relatively cool. However, at times of stress, stomata close earlier during the day and the canopy shows a relative higher temperature compared to healthier ones. Time series measuring thermal outputs at noon at the beginning, middle and end of a growing season can identify if trees are under drought stress. Link to thermolidar project. Contact: Prof. Juan Suárez-Minguez.


Susceptibility of different tree species to drought and drought-induced mortality – Northern Australia, Malaysia, Amazon

Prof. Patrick Meir, Prof. Oliver Phillips and Ms Adriane Esquivel Muelbert at the University of Leeds are investigating the underlying differences in response to drought of different tree species, their susceptibility to mortality, and suitability in different locations. They have a number of observational and experimental plots in Northern Australia, Malaysia and the Amazon. The plots in the Amazon complement Patrick’s work-to-date in eastern Amazonia focusing on experimental manipulation. Whilst around 150 species are included in the multiple-plot study, for focused physiological studies they have narrowed the list down to a small number of drought-tolerant and drought-intolerant taxa. Measurements are focused on 6 species, 3 of each tolerance group. This latter work is being conducted with Prof. Maurizio Mencuccini. Contact: Prof. Patrick Meir.


Susceptibility of different tree species to precipitation change  – Tropical dry forest biomes

The above  projects focus on investigating tolerances for tropical rain forests. However, more than 50% of the tropics globally have climates that are too seasonally dry to support rain forest. In these areas, two other major biomes are found: tropical dry forests and tropical savannas. Prof. Toby Pennington (Royal Botanic Garden Edinburgh) is investigating extending monitoring of biodiversity and ecological processes into dry forest biomes in Brazil with his co-investigator Dr Kyle Dexter (Edinburgh).   Tropical savannas and dry forests contain species already adapted to extreme droughts, and in the savannas, to frequent natural fires. Thus these species may be threatened by increased rainfall, which is predicted for some tropical regions. Conversely, they house species that could be useful resources under increasing drought. This project could provide information to assist in tree species selection to reduce risk of tree loss under future climate conditions.  Contact: Prof. Toby Pennington or Dr Kyle Dexter


Susceptibility of different species to drought  – Utility of Inga spp. for afforestation/reforestation and agroforestry

Prof Pennington and Dr Dexter also investigate the use of the genus Inga (around 300 species) commonly used in agroforestry, afforestation and reforestation projects. Ing species are particularly good at rejuvenating degraded soils. Planting species with a greater tolerance to a range of climates and thereby precipitation levels could significantly reduce risk to forest projects in an uncertain climate future. Contact: Prof. Toby Pennington and Dr Kyle Dexter


Susceptibility of different tree species to drought  – Mediterranean incl. Scots Pine

Prof. Mencuccini also leads on a project with Prof. Meir focusing on drought-induced mortality in the Mediterranean and in particular the susceptibility of Scots Pine to drought. (Salmon et al., 2015). Contact: Prof. Maurizio Mencuccini


Mechanisms and climate factors causing susceptibility to drought-induced mortality

One key area of focus in relation to drought and forestry is the causal mechanisms behind drought-induced mortality. There are two main hypotheses of causation: the first is that drought causes the water column to ‘snap’ under negative pressure, resulting in irreversible hydraulic failure and leaf desiccation; the second relates to leaf stomata closing to prevent water loss and thereby preventing sufficient CO2 to be taken in through the same stomata for photosynthesis, with the result that there is a progressive decline in the availability of carbon to metabolism, ultimately resulting in death by ‘carbon starvation’. Both mechanisms cause physiological weakening and potentially mortality, either directly through hydraulic failure or carbon starvation, or indirectly through greater susceptibility to pest and diseases or windthrow. Prof. Meir is working on how such mechanisms might be modelled (Meir et al., 2015) using input from sample plots. There is evidence for both mechanisms in different parts of the world. In particular, if the point at which carbon starvation or hydraulic failure occurs can be isolated this could in future be used in conjunction with probabilistic climate forecasts to derive mortality estimates. Prof Meir’s group published a significant advance in this area in Dec 2015 (Rowland 2015, Nature), showing data which favoured the hydraulic explanation rather than the carbon starvation explanation as the ‘trigger’ for drought-induced mortality. Contact: Prof. Patrick Meir.


Sensitivity of Amazonian forest ecosystems to drought – General expertise

Prof. Patrick Meir co-leads a large NERC-funded project to understand and model the drought impacts on rainforests pan-tropically. The work combines work in SE Asia with long-term ecosystem-scale experimental work in Amazonia. The latter project in eastern Amazonia involves using field data and other field-based sources of evidence to develop understanding and to allow the estimation of drought risk. The work also builds towards advancing the UK’s land surface model that is used by the Hadley Centre in the IPCC reports on the future of the Earth system. For this work he is joined by a colleague at Exeter University (Prof Stephen Sitch). More recently he also led the University of Edinburgh’s contribution to the EU’s AmazAlert consortium looking at critical land-climate feedbacks in the region, and related policy responses. He led and edited a special feature in the subject-leading journal New Phytologist summarising the state of knowledge on Amazonian rainforests and drought (Meir and Woodward, 2010) and co-authored a paper overviewing existing modelled predictions of the response by tropical rainforests globally to 21st century climate change (Huntingford et al., 2013). Contact: Prof. Patrick Meir.


Using Dynamic Global Vegetation Models to model the sensitivity of Amazonian forest ecosystems to severe drought

Prior to the experimental work in Amazonia (see section above on Sensitivity of Amazonian forest ecosystems to drought), Prof Patrick Meir led a small team comparing the ability of different Dynamic Global Vegetation Models (DGVMs) to model the sensitivity of Amazonian forest ecosystems to climate, especially severe drought (Galbraith et al., 2010). This work demonstrated the need for the current experimental work. His team have more recently published ground-breaking work from the Amazon experiment to show that tropical rainforests are not resistant to long term drought, and that the very large loss of biomass that occurs through increased mortality associated with drought is triggered by deterioration in the transport of water from soil to leaves, rather than through reduced metabolic capacity (Rowland et al. 2015, Nature). This result is now being used to improve the predictive capacity of DGVMs by improving their mechanistic representation of the response to drought. Statistical models can help us predict future change, but this approach is considered unreliable under future climates which don’t replicate the combination of preceding climate variables from which statistical models are derived. For this reason the advance made by Rowland et al. 2015 is of particular note, as it helps modellers focus their development of their models on a particular group of processes. Contact: Prof. Patrick Meir.

References

CIAIS, P., REICHSTEIN, M., VIOVY, N., GRANIER, A., OGEE, J., ALLARD, V., AUBINET, M., BUCHMANN, N., BERNHOFER, C., CARRARA, A., CHEVALLIER, F., DE NOBLET, N., FRIEND, A. D., FRIEDLINGSTEIN, P., GRUNWALD, T., HEINESCH, B., KERONEN, P., KNOHL, A., KRINNER, G., LOUSTAU, D., MANCA, G., MATTEUCCI, G., MIGLIETTA, F., OURCIVAL, J. M., PAPALE, D., PILEGAARD, K., RAMBAL, S., SEUFERT, G., SOUSSANA, J. F., SANZ, M. J., SCHULZE, E. D., VESALA, T. & VALENTINI, R. 2005. Europe-wide reduction in primary productivity caused by the heat and drought in 2003. Nature, 437, 529-533.
HUNTINGFORD C, ZELAZOWSKI P, MERCADO, LM, SITCH, S, GALBRAITH, D, FISHER R, LOMAS M, WALKER A, JONES CD, BOOTH BBB, MALHI Y, COX PM, HEMMING D, KAY G, GOOD P, LEWIS S, ATKIN OK, LLOYD J, GLOOR M, ZARAGOZA-CASTELLS J, MEIR P & BETTS R. 2013. Simulated resilience of tropical rainforest to CO2–induced climate change. Nature Geoscience6, 268-273
MEIR, P. & WOODWARD, F. I. 2010. Amazonian rain forests and drought: response and vulnerability. New Phytologist, 187, 553-557.
PETR, M., BOERBOOM, L. G. J., VAN DER VEEN, A. & RAY, D. 2014. A spatial and temporal drought risk assessment of three major tree species in Britain using probabilistic climate change projections. Climatic Change, 124(4), pp.791–803.
PYATT, G., RAY, D. & FLETCHER, J. 2001. An Ecological Site Classification for Forestry in Great Britain, Norwich, HMSO, Crown Copyright. Link
ROWLAND L, DA COSTA ACL, MENCUCCINI M, GALBRAITH DR, OLIVEIRA RS, BINKS OJ, OLIVEIRA AAR, PULLEN AMM, DOUGHTY CE, METCALFE DB, VASCONCELOS SS, FERREIRA LV, MALHI Y, GRACE J and MEIR P. 2015.  ‘Death from drought in tropical forests is triggered by hydraulics not carbon starvation’. Nature, 528, 119-125.
SALMON Y, TORRES-RUIZ JM, POYATOS R, MARTINEZ-VILALTA J, MEIR P, COCHARD H, MENCUCINNI M. (2015). Balancing the risks of hydraulic failure and carbon starvation: a twig scale analysis in declining Scots pine. Plant Cell and Environment, 38, 2575-2588
SENEVIRATNE, S. I., NICHOLLS, N., EASTERLING, D., GOODESS, C. M., KANAE, S., KOSSIN, J., LUO, Y., MARENGO, J., MCINNES, K., RAHIMI, M., REICHSTEIN, M., SORTEBERG, A., VERA, C. & X. ZHANG, X. 2012. Changes in climate extremes and their impacts on the natural physical environment. In:. Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (IPCC).. Cambridge University Press, Cambridge, UK, and New York, NY, USA, pp. 109-230.
PHILLIPS, O. L., ARAGAO, L., LEWIS, S. L., FISHER, J. B., LLOYD, J., LOPEZ-GONZALEZ, G., MALHI, Y., MONTEAGUDO, A., PEACOCK, J., QUESADA, C. A., VAN DER HEIJDEN, G., ALMEIDA, S., AMARAL, I., ARROYO, L., AYMARD, G., BAKER, T. R., BANKI, O., BLANC, L., BONAL, D., BRANDO, P., CHAVE, J., DE OLIVEIRA, A. C. A., CARDOZO, N. D., CZIMCZIK, C. I., FELDPAUSCH, T. R., FREITAS, M. A., GLOOR, E., HIGUCHI, N., JIMENEZ, E., LLOYD, G., MEIR, P., MENDOZA, C., MOREL, A., NEILL, D. A., NEPSTAD, D., PATINO, S., PENUELA, M. C., PRIETO, A., RAMIREZ, F., SCHWARZ, M., SILVA, J., SILVEIRA, M., THOMAS, A. S., TER STEEGE, H., STROPP, J., VASQUEZ, R., ZELAZOWSKI, P., DAVILA, E. A., ANDELMAN, S., ANDRADE, A., CHAO, K. J., ERWIN, T., DI FIORE, A., HONORIO, E., KEELING, H., KILLEEN, T. J., LAURANCE, W. F., CRUZ, A. P., PITMAN, N. C. A., VARGAS, P. N., RAMIREZ-ANGULO, H., RUDAS, A., SALAMAO, R., SILVA, N., TERBORGH, J. & TORRES-LEZAMA, A. 2009. Drought Sensitivity of the Amazon Rainforest. Science, 323, 1344-1347.