Hot Topic

Climate Change: trees under pressure

Monday, 30 November 2015  | 

Global changes in climate are having a significant impact on forested ecosystems, causing increases in tree mortality rates, and decreases in tree growth and health. Besides changes in temperature and rainfall, climate change projections for Australia indicate an increase in the frequency and intensity of extreme climatic events. More intense extreme events will have a major impact on tree-dominated ecosystems. Declines in tree health and mass mortality will occur, and associated services, such as food provision and carbon sequestration, will be affected.
In the southwest of Western Australia, for example, 26% of mature trees across ~7,000 hectares of forest died in response to extreme drought and multiple heatwaves in 2010/2011. Similar levels of tree mortality were found across the 18.6 million hectare Mulga Lands bioregion in Queensland in response to the 2003-2007 drought. Dying and dead trees are not only visually disturbing; tree declines also affect species that are dependent on trees for food and shelter. For instance, the Australian Glossy Black-cockatoo showed less breeding success in a drought because their food, she-oak cones, was less available. A further negative effect of tree decline is a potential reduction in long-term carbon storage in forests. Forests play an important role in reducing the effects of climate change by capturing carbon from the atmosphere. Long-lived trees are extremely valuable in capturing and locking-in carbon for prolonged periods (>150 years). Increasing tree mortality rates with no additional seedling recruitment are therefore highly undesirable for the mitigation of climate change.
Whether climate change will have a permanent (undesirable) effect on forested ecosystems needs to be determined by consistent monitoring. Finding out where, when, and what kind of changes are likely to occur in the landscape through modelling will be critical to effectively adapt to the future climate.

Research Entries

Allen C. D., Macalady A. K., Chenchouni H., Bachelet D., McDowell N., Vennetier M., Kitzberger T., Rigling A., Breshears D. D., Hogg E. H., Gonzalez P., Fensham R., Zhang Z., Castro J., Demidova N., Lim J.-H., Allard G., Running S. W., 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, 660-84.
This paper provides an overview of recent tree mortality due to climatic water stress and warm temperatures in forests around the globe.
Based on the review, this study suggests that at least some of the world’s forested ecosystems already may be responding to climate change, showing higher background tree mortality rates and die-off in response to warming and drought. Overall, this review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.
Anderegg W. R. L., Hicke J. A., Fisher R. A., Allen C. D., Aukema J., Bentz B., Hood S., Lichstein J. W., Macalady A. K., McDowell N., Pan Y., Raffa K., Sala A., Shaw J. D., Stephenson N. L., Tague C. & Zeppel M. (2015) Tree mortality from drought, insects, and their interactions in a changing climate. New Phytologist, doi: 10.1111/nph.13477.
This paper review the evidence for cross-biome patterns in the relative importance of insect- and drought-triggered tree mortality
Drought, insects and their interactions are shown to be responsible for tree mortality around the world. However, a large research gap remains around the interactions between pests and pathogens under warmer temperatures and changing precipitation regimes, and how insects exacerbate drought-induced mortality.
Brouwers N., Matusick G., Ruthrof K., Lyons T. & Hardy G. (2013) Landscape-scale assessment of tree crown dieback following extreme drought and heat in a Mediterranean eucalypt forest ecosystem. Landscape Ecology 28, 69-80.
The objective of this study was to perform a landscape-scale assessment investigating the relationships between canopy dieback observed after a severe drought in combination with multiple heatwaves and landscape related factors focussing on the (i) geology, (ii) topography, (iii) climate, and (iv) fire history.
Trees on rocky soils with low water holding capacity were found to be more likely to have suffered dieback in this drought/heat event than on other soils in the research region. Other explanatory factors identified that dieback occurred (i) on sites that were close to rock outcrops, (ii) in areas that received a slightly higher amount of annual rainfall compared to the surrounding landscape, (iii) on sites at high elevations and (vi) on steep slopes, and (v) in areas that were generally slightly warmer than their surroundings.
Brouwers N. C., Mercer J., Lyons T., Poot P., Veneklaas E. & Hardy G. (2012) Climate and landscape drivers of tree decline in a Mediterranean ecoregion. Ecology and Evolution 3, 67-79.
This study involved a landscape-scale assessment investigating the relationships between the change in wandoo (Eucalyptus wandoo) canopy health assessed over a 6-year period and variables focussing on: (1) fragment metrics; (2) topography; (3) soil characteristics; and (4) climate.
Recent climate change related increases in autumn temperatures and decreases in annual rainfall were negatively affecting health of wandoo most prominently in the low rainfall zone of its climatic range.
Bureau of Meteorology. (2015) Climate change and variability. Australian climate variability & change - Trend maps. Commonwealth of Australia, URL:
Online mapping tool to show climate trends across Australia
Carnicer J., Coll M., Ninyerola M., Pons X., Sanchez G. & Penuelas J. (2011) Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought. Proceedings of the National Academy of Sciences 108, 1474-8.
In this study, crown defoliation data from forest inventory data was used to assess the macroecological impacts of drought on water-limited southern European forests over the 20-y period of 1987–2006.
A generalized increase in crown defoliation was observed in southern European forests occurring during 1987–2007. Forest tree species have consistently and significantly altered their crown leaf structures, with increased percentages of defoliation in the drier parts of their distributions in response to increased water deficits. This observations were specifically evident in Spain.
Dulamsuren C., Hauck M. & Leuschner C. (2010) Recent drought stress leads to growth reductions in Larix sibirica in the western Khentey, Mongolia. Global Change Biology 16, 3024-35.
This study composed of three parts: (1) temperature and precipitation trends during the late 20th century were analyzed, (2) tree-ring analyses were applied to test the hypothesis that recent drought stress led to reduced rates of growth and regeneration in Siberian larch, and (3) water relations were studied in some of the trees used for wood core sampling to test the hypothesis that trees currently suffer from drought stress.
Tree-ring width series from 4400 larch trees show a strongly decreasing annual increment since the 1940s. The onset of this decrease is independent of the age of the trees and, therefore, can be attributed to the increasing aridity in the 20th century. Simultaneously to the declining annual increment, regeneration of Siberian larch decreased as well.
Dwyer J. M., Fensham R. J., Fairfax R. J. & Buckley Y. M. (2010) Neighbourhood effects influence drought-induced mortality of savanna trees in Australia. Journal of Vegetation Science 21, 573-85.
This study employed a multi-faceted analytical approach to evaluate the following hypotheses of drought-related tree death in Eucalyptus savanna: A. Drought-induced stem death is randomly distributed in space; B. Stems are predisposed to drought-induced death through negative density-dependent effects; and C. Stems are predisposed to drought-induced death due to local deficits in plant available resources.
Mortality was concentrated in stems less than 15-cm diameter at breast height (DBH). There was evidence in support of hypotheses B (neighbour density) and C (resource heterogeneity)
Fensham R. J. (1998) The influence of cattle grazing on tree mortality after drought in savanna woodland in north Queensland. Austral Ecology 23, 405-7.
To investigate in Eucalyptus woodland in north Queensland after drought whether: (i) cattle grazing exacerbates drought death of trees; (ii) drought death preferentially affects individual species of trees; (iii) drought death preferentially affects individual size classes,
Twenty-seven per cent of all tree species were killed by the drought, but no significant differences were found between grazing treatment, species, and size classes.
Fensham R. J. & Fairfax R. J. (2007) Drought-related tree death of savanna eucalypts: Species susceptibility, soil conditions and root architecture. Journal of Vegetation Science 18, 71-80.
For eucalypt savanna in northeast Australia subject to multi-year rainfall deficits this paper asks whether (1) dominant tree species (Ironbarks, Boxes) are more drought susceptible than the sub-dominant Bloodwoods; (2) whether soil moisture is beyond wilting point in surface soil layers but available at depth; (3) soil conditions (moisture availability and texture) are related to tree death during drought; (4) the root systems of the Boxes and Ironbarks are shallower than the Bloodwoods; and the survivors of drought within species have deeper root systems than those that died.
Drought-induced tree death was higher for dominant Ironbark-Box than for sub-dominant Bloodwoods. Within species there was no significant difference in root basal area characteristics between trees that were recently killed by drought and those that remained relatively healthy. Surface soil moisture availability was lower where tree densities were high, and tree death increased as surface soil moisture became less available. Tree death was also greater as the clay content of sub-soils increased.
Fensham R. J., Fairfax R. J. & Archer S. R. (2005) Rainfall, land use and woody vegetation cover change in semi-arid Australian savanna. Journal of Ecology 93, 596-606.
Regression analysis and aerial photography were used to assess the relative importance of land-use history (fire and grazing), rainfall and initial woody cover (woody cover at the beginning of a sample period relative to the range of woody cover expressed within a land type) in accounting for rates of change in overstorey and understorey cover between the 1940s and 1990s in Australian tropical savannas in central Queensland, Australia.
On average, overstorey and understorey cover increased over the second half of the 20th century. This pattern is consistent with the first half of the 20th century having more intense droughts and being drier overall than the relatively wet second half.
Fensham R. J., Fairfax R. J. & Dwyer J. M. (2012) Potential aboveground biomass in drought-prone forest used for rangeland pastoralism. Ecological Applications 22, 894-908.
The current paper analyzes an extensive data set recording the structure and aboveground biomass of mulga (Acacia aneura) dry forest vegetation from throughout the Mulga Lands biogeographic region in Queensland. The surveys were conducted after a recent drought (2003–2007), and allow for the assessment of associated tree mortality. It further addresses the following questions: (1) How accurately can the aboveground woody biomass of mature mulga forest be modelled in relation to climate and soil attributes? (2) How does the magnitude of recent drought, soil attributes, and tree density influence dead biomass? And (3) what influence does the range of management activities within the region have on live and dead biomass?
The data were collected after a recent drought, and the amount of recent tree mortality was weakly positively related to a measure of three-year rainfall deficit, and maximum temperature (positive), soil depth (negative), and coarse sand (negative). There was no evidence of density-dependent effects on tree mortality. Climate change scenarios represented by the coincidence of historical extreme rainfall deficit with extreme temperature suggest mortality of 30.1% of aboveground biomass, compared to 21.6% after the recent (2003–2007) drought.
Fensham R. J., Fairfax R. J. & Ward D. P. (2009) Drought-induced tree death in savanna. Global Change Biology 15, 380-7.
This paper improves our understanding of the role of rainfall variability as an influence on savanna tree dynamics by addressing two questions: (1) Is tree death a response to multi-year drought in savanna environments? (2) Is there an underlying trend of increasing woody cover that is independent of the rainfall patterns?
This study shows that extreme droughts (less than two-thirds expected rainfall over 3 years) occur in the drier half of the savanna biome and can cause substantial tree death. An Australian case study reveals that a net increase in tree cover over five decades of above-average rainfall was offset by sudden tree death during drought. The results are not supportive of a sustained increase in the woody component of xeric savannas resulting from CO2 fertilization or land management. Extensive tree death in savanna regions will become a stark consequence of climate change if predictions of increasing severity and frequency of drought are realized.
Fensham R. J. & Holman J. E. (1999) Temporal and spatial patterns in drought-related tree dieback in Australian savanna. Journal of Applied Ecology 36, 1035-50.
This study analyses drought both spatially and temporally across Queensland, and collates evidence of previous dieback events as a consequence of drought. We document the effects of a severe drought during the 1990s to provide quantification of the proportion of dieback across a large area of tropical savanna. We then address the hypothesis that dieback is primarily a consequence of drought by relating death to rainfall patterns at different spatial scales.
Approximately 29% of trees were dead or nearly dead over a sampled area of about 55 000 square kilometres. Of the widespread dominants, the Eucalyptus crebra - E. xanthoclada species complex was highly susceptible to dieback, E. brownii and E. melanophloia - E. whitei moderately affected, and Corymbia clarksoniana and Melaleuca nervosa less severely affected. The 1990s drought was especially intense in the vicinity of the North Queensland study area. However, within the study area there were only weak correlations between dieback and rainfall deficits as derived from modelled data. The substantial patchiness in dieback within the study area was only poorly related to local rainfall patterns.
IPCC (2013) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
To provide the physical science basis for evidence of human induced climate change
Warming of the climate system is undeniable, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased. These change processes are projected to continue into the future, and extreme climatic events like droughts are projected to become more frequent and severe around the world.
Matusick G, Ruthrof KX, Brouwers NC, Dell B, Hardy GSJ (2013) Sudden forest canopy collapse corresponding with extreme drought and heat in a mediterranean-type eucalypt forest in southwestern Australia. European Journal of Forest Research 132(3): 497-510
This paper addresses the canopy collapse in the Northern Jarrah Forest (NJF) observed after the drought and heat of 2010/11 in southwest Western Australia (SWWA) and aims to: (1) estimate the incidence (spatial scale) and severity of canopy collapse across the NJF; (2) define associations between canopy collapse and forest stand variables; and (3) investigate the variation in crown collapse and regrowth in relation to tree species and size.
Canopy mortality was found to be concentrated in distinct patches within the landscape (see further Brouwers et al., 2013). Within these patches, 74 % of all measured tree stems ([1 cm DBHOB) had dying or recently killed crowns, leading to 26 % stem mortality six months following the collapse. Patches of canopy collapse were more densely stocked with the dominant species, Eucalyptus marginata, and lacked the prominent midstorey species Banksia grandis, compared to the surrounding forest. A differential response to the disturbance was observed among co-occurring tree species, which suggests contrasting strategies for coping with extreme water stress.
Matusick G, Ruthrof KX, Hardy GESJ (2012) Drought and heat triggers sudden and severe dieback in a dominant Mediterranean-type woodland species. Open Journal of Forestry 2(4): 183-186
This study hypothesizes that extreme heat and dry conditions in 2010 and the beginning of 2011 in southwest Western Australia (SWWA) contributed to an observed dieback event in an Eucalyptus gomphocephala forest. This research aimed to estimate the incidence of damage, describe the symptomology and severity of the dieback, as well as highlight potential site differences between affected and unaffected woodland.
An estimated 500 ha of woodland was severely affected between February and March 2011. Tree foliage rapidly discolored and died over this period. In the affected portion of the woodland, approximately 90% of trees greater than 20 cm DBH were impacted, while in the adjacent unaffected woodland 6% showed signs of damage. Dropping groundwater levels, a relatively shallow soil profile, and extreme drought and heat in 2010-2011 are thought to predispose water-shedding sites to drought-triggered canopy dieback during extended periods of dryness.
Ruthrof K., Matusick G. & Hardy G. (2015) Early Differential Responses of Co-dominant Canopy Species to Sudden and Severe Drought in a Mediterranean-climate Type Forest. Forests 6, doi:10.3390/f6062082.
The study aims to determine the susceptibility and response pattern of the two co-occurring dominant overstory tree species Eucalyptus marginata and Corymbia calophylla, to the extreme drought (and heat) of 2010 and early 2011in the Northern Jarrah Forest region in southwest Western Australia. Specifically, the study examined the relative probability of these species experiencing: (a) partial crown dieback, (b) complete crown dieback; (c) resprouting; and (d) tree mortality, assessed 16 months following the drought.
Eucalyptus marginata was more susceptible to partial and complete crown dieback compared to C. calophylla, three months after the drought. Overall, E. marginata trees were more likely to die from the impacts of drought, assessed at 16 months. These short-term differential responses to drought may lead to compositional shifts with increases in frequency of drought events in the future.
Jump AS, Hunt JM, Penuelas J (2006) Rapid climate change-related growth decline at the southern range edge of Fagus sylvatica. Global Change Biology 12: 2163-2174
To determine whether there is evidence for changes in growth trends of mature individuals of Fagus sylvatica between 1952-2003 in relation to temperature and rainfall.
Growth rates of mature F. sylvatica have rapidly declined since ~1975, particularly at the lower range edge limit. Age was not driving this, however, with equal amounts of average annual rainfall, the gradual increasing temperatures since 1975 seems to be driving this reduction in growth.
Peng C, Ma Z, Lei X, Zhu Q, Chen H, Wang W, Liu S, Li W, Fang X, Zhou X (2011) A drought-induced pervasive increase in tree mortality across Canada's boreal forests. Nature Climate Change 1(9): 467-471
The study sought to determine whether systematic changes in tree recruitment and mortality have occurred in Canada's boreal forests from 1963 to 2008, and, if any changes have occurred, whether they could be attributed to changes in climate and/or other potential contributing factors.
Tree mortality rates increased by an overall average rate of 4:7% per year from 1963 to 2008 irrespective of tree age. Increasing temperature and water deficits were both significantly positively correlated with tree mortality rates for all plots combined and for western Canada, whereas the correlation was only significant for temperature in eastern Canada.
van Mantgem PJ, Stephenson NL (2007) Apparent climatically induced increase of tree mortality rates in a temperate forest. Ecology Letters 10(10): 909-916
This study addressed three questions. First, have tree mortality or recruitment rates in old-growth forest in the Sierra Nevada, California, changed systematically over the last two decades? Second, if demographic rates have changed, are the changes consistent among different taxonomic groups and forest types? Finally, what are probable causes of the changes?
Mortality rate, but not the recruitment rate, increased significantly over the 22 years of measurement (1983–2004). Mortality rates increased in both of two dominant taxonomic groups (Abies and Pinus) and in different forest types (different elevational zones). The increase in overall mortality rate resulted from an increase in tree deaths attributed to stress and biotic causes, and coincided with a temperature-driven increase in an index of water deficit. Other factors, such as changes in air pollution concentrations or internal stand conditions, were not correlated with mortality rates. It is concluded that these forests may be experiencing increasing deaths related to temperature-driven evaporative stress, potentially making them more vulnerable to extensive dieback during otherwise normal periods of reduced precipitation.
van Mantgem PJ, Stephenson NL, Byrne JC, Daniels LD, Franklin JF, Fulé PZ, Harmon ME, Larson AJ, Smith JM, Taylor AH, Veblen TT (2009) Widespread increase of tree mortality rates in the western United States. Science 323(5913): 521-524
To determine whether systematic changes in tree demographic (mortality and recruitment) rates have occurred recently in coniferous forests of the western United States, and if so, to identify possible causes of those changes.
Between 1955 – 2007 tree mortality rates increased and temperature and water deficit were positively correlated with tree mortality rates.
Vilà-Cabrera A, Martínez-Vilalta J, Vayreda J, Retana J (2011) Structural and climatic determinants of demographic rates of Scots pine forests across the Iberian Peninsula. Ecological Applications 21(4): 1162-1172
To explore the dynamics of mortality and growth throughout the distribution of Scots pine in the Iberian Peninsula (Spain). The main objective was to evaluate the responses of mortality and growth rates to spatial variability of climate, climatic anomalies, and forest structure over the period of 1986 to 2007.
Over the period of ~10 years between individual surveys, the amount of standing dead trees increased 11-fold. Higher mortality rates were related to increasing dryness, and growth was reduced with increasing dryness and temperature. Additionally, regeneration tended to be lower in plots with higher mortality. The results suggest a process of large-scale self-thinning enhanced by increasing drier conditions.
Cameron M. (2006) Distribution and cone production in Allocasuarina diminuta and A. gymnanthera (Casuarinaceae) in central New South Wales. The Rangeland Journal 28, 153-61.
Investigations focused on distribution and reproduction of Allocasuarina diminuta subsp. diminuta and A. gymnanthera in relation to climate conditions, which would contribute to an understanding of the food resources available to glossy black-cockatoos.
Cone production appeared to be linked to rainfall, with fewer cones produced in dry years. Drought conditions resulted in the failure of plants to produce seed and caused the death of significant numbers of plants. It is suggested that a decrease in moisture balance or increase in drought frequency/length due to global warming is likely to have negative consequences for Allocasuarina and glossy black-cockatoo populations. See further Cameron (2009).
Cameron M. (2009) The influence of climate on Glossy Black-cockatoo reproduction. Pacific Conservation Biology 15, 65-71.
There was a strong positive relationship between the proportion of juveniles in the population and total annual rainfall in the preceding year. Breeding success decreased with decreasing rainfall and after drought no nests were established resulting in no recruitment. Lesser food availability in and after the drought (Cameron, 2006) is the likely driver of lesser nesting attempts and success.
There was a strong relationship between cockatoo productivity and rainfall in the preceding year. Breeding success decreased with decreasing rainfall and after drought no nests were established resulting in no recruitment. Lesser food availability in and after the drought (Cameron, 2006) is the likely driver of lesser nesting attempts and success.