Key science findings

  • Droughts can alter the processes that govern terrestrial ecosystems, leading to changes in their species composition, structure and functioning, their interactions and benefits they provide to people.
  • Ecosystems in the south and east of England and Wales are more likely to be altered by climate change than those elsewhere in Great Britain.
  • In woodlands, beech and oak appear to be more susceptible drought than other tree species.
  • The projected leaf area index (LAI) and net primary productivity (NPP) generally increased over Britain, with a larger increment in northern Britain and west Wales.
  • The projected change of LAI and NPP varied from 5% – 100% of the mean change due to the uncertainty arising from climate variability (based on 100 MaRIUS scenarios).
  • Southeast England showed to greater climate variability, where net ecosystem change (NEE) might result in forests changing from being a carbon sink to a carbon source.
  • Southeast England is also most susceptible to drought, with the long-term drying effects reducing the suitability for beech, lime and hornbeam.

Introduction

Climate change is already affecting habitats and their species in several ways and future changes in temperature and water availability are likely to lead to further changes in the distribution of many species and thus in habitat composition and functioning . Both water scarcity and drought could also result in changes to the ecosystem services provided by the ecosystems and their species.

In MaRIUS we focused on selected wetland, grassland and woodland ecosystems. Woodlands are very important for biodiversity, carbon storage, water regulation and watershed protection. Recent literature has reported drought-induced mortality and dieback of forests in different regions across the world. In Great Britain, the potential impacts of drought on tree species and woodlands are not well understood. In MaRIUS, therefore, we have analysed the temporal and spatial patterns of the potential impacts of and vulnerability to drought for important British forest species.

Forests are the most important ecosystem for carbon storage, watershed protection and biodiversity. Recent studies have reported drought-induced mortality and dieback of forests across the world. In the UK, the projected mean summer temperature could rise by up to 3°C to 4°C in the 2080s, however, the potential impacts on tree species and forests are not well understood. Thus there is a need to assess the vulnerability of UK’s tree species to drought and analyse any significant mortality trend to provide scientific evidence for supporting climate adaptation through effective forest management and habitat conservation.

Research methods

For each of the wetland, grassland and woodland ecosystems, we selected three categories of species: dominant, functional and/or sensitive to climate change and of conservation importance. Then, using databases of species’ distribution or published maps, which we digitised, we applied an ensemble of species distribution models (BIOMOD) and also an artificial neural network model (SPECIES) to characterise the relationship between each distribution and baseline climate. We then applied two different (Low and High) UKCP09 emissions scenarios for the 2020s, 2050s and 2080s and five spatially consistent projections, as well as the MaRIUS future events dataset, to these relationships in order to project how the suitable climate space for the species might change. The potential impacts on the ecosystems were inferred by examining the changes in suitable climate space for its dominant and characteristic species.

For the analysis of tree species’ response to drought, we applied a version of the LPJ-GUESS (a process-based dynamic vegetation) model parameterized for 18 European tree species and/or plant functional types to simulate vegetation dynamics across Great Britain and the Thames basin for the period 1961 to 2011.

We developed and applied the tree-species based dynamic vegetation model LPJ-GUESS to Great Britain at a 5 km resolution. The model used the MaRIUS drought scenarios, which were based on 100 regional climate model outputs driven by boundary conditions from the HadAm3P under an RCP 8.5 emission scenario. The climate change impacts in terms of the change of leaf area index (LAI) and net primary productivity (NPP) were analysed by comparing the simulated future conditions (i.e. the 2030s and the 2080s) to those of baseline (i.e. 1975-2004). A drought vulnerability index was developed to assess the potential drought impact on the modelled tree species.

Results

Figure 1 shows that the hotspots of LAI change in the future appear at higher elevations in Scotland and northern England, as with the warming climate and rising CO2 levels conditions become more suitable for deciduous tree species growth. This also implies that a greater change in tree composition may occur here. Geographically, the uncertainty in response is larger in southern Britain in the 2080s (Fig 1d). The time series of simulated LAI shows that for the 50th percentiles, the LAI across Great Britain is projected to increase by around 30% for the highlands and 100% for lowlands from 1975 to the late 21st century.

 

 

 

 

 

 

 

 

 

Figure 1: The projected mean change of LAI relative to the baseline  (1975 to 2004) and its variability: a and c) 2030s; b and d) 2080s; simulated LAI e) Highlands  f) Lowlands.

Figure 2 shows the simulated NPP varies from around 0.1 to 0.5 kgC/m2/year over Britain, with an average of 0.36 kgC/m2/year. The NPP is projected to increase by 0.03-0.18 kgC/m2/year in the 2030s and 0.05-0.39 kgC/m2/year in the 2080s, showing larger increases in the northern half of Britain and west Wales. The fertilization effect of rising CO2 and temperatures  are likely to increase the suitable conditions for most deciduous trees in northern Britain, with the potential for them to replace the grassland and coniferous species. The spatial patterns of NEE change show a trend of greater carbon accumulation in Scotland, north England and west Wales with the strength of carbon sink increasing in the 2080s. Southeast Britain has a projected negative NEE and thus it could become a carbon source. Also, it appears to be the most uncertain region in terms of carbon flux due to climate variability.

Figure 2: The modelled present-day NPP and NEE averaged over the period from 1975 to 2004 driven by the historical climate dataset and the future conditions of NPP and NEE

Figure 3 shows that in most regions the drought vulnerability of the tree species increases slightly. This suggests that the temperature increase and precipitation reduction are likely to lead to more potential drought stress for tree growth. Among the tree species, downy birch, ash, oak and elm are projected to be more vulnerable to drought in southeastern England (i.e. Anglian, Thames and SE England) in the 2030s, with vulnerable regions spreading across Britain in the 2080s. The drought vulnerable regions for silver birch are in the Thames, SE England and Severn basins and for hazel in the Thames, SE England and northern Scotland. Those for hornbeam, aspen and lime are more in northern England and southern Scotland. The beech shows stress across almost the whole Britain in the 2030s.

Figure 3: The drought vulnerability of 12 tree species in the 19 river basin regions

 

Changes in lowland hay meadow species under different UKCP09 scenarios

This grassland type is currently found almost entirely within England, with only a few sites in the east of Wales. The species that characterise it are projected to progressively lose suitable climate space in southern, eastern and central England and in Wales. One species, great burnet (Sanguisorba officinalis) is modelled to lose all suitable climate space in Great Britain under one projection and bird’s-foot trefoil (Lotus corniculatus) to lose none. This suggests that in the southern half of England there could be changes in the composition of this habitat.

There are important concentrations of lowland meadows in the Thames Basin, but by the 2050s some of it is likely to become climatically unsuitable for these species. Under the high emissions scenario, all suitable climate space for most of them what_are_droughts/is projected to be lost by the 2050s, leading to a potential loss of this habitat which is important for conservation.

 

Changes in suitable climate space in Great Britain (upper) and Thames basin (lower) for MG 4 Alopecurus pratensis–Sanguisorba officinalis Lowland hay meadows species under different UKCP09 scenarios.

Changes in suitable climate space in Great Britain (upper) and Thames basin (lower) for MG 4 Alopecurus pratensis–Sanguisorba officinalis Lowland hay meadows species under different UKCP09 scenarios.

The spatial pattern of drought-induced tree mortality levels across the Thames Basin in the 2030s and 2080s shows that generally, beech is more susceptible to drought than oak especially in the southeast Thames Basin. The level of drought impact on beech is slightly increased in the 2080s, whereas the similar trend is not evident for oak. Amongst the sub-catchments, the Wey and Colne appear to be particularly affected under almost all climate scenarios, followed by London for oak. The western sub-catchments (e.g. Kennet and Cotswolds) are less affected.

 

Vulnerability levels of beech and oak in the Thames Basin to drought relative to baseline.

Vulnerability levels of beech and oak in the Thames Basin to drought relative to baseline.

Further information

  • Yu, J.J.; Berry, P. (2017). Potential Effects of Drought on Tree Dieback in Great Britain and Implications for Forest Management in Adaptation to Climate Change (abstract). The EGU General Assembly 2017. Vienna, Austria, 23-28, April, 2017
  • Yu, J.J.; Berry, P; Guillod, B; Hickler, T. Climate Change Impacts on Terrestrial Ecosystem and Drought Vulnerability of Tree Species in Great Britain. (In production)

Research Team

Dr Pam Berry Dr Pam Berry Dr Jianjun Yu Dr Jianjun Yu