Plants for a changing landscape

The current plant palette for each climatic zone may no longer be resilient enough to cope with the effects of climate change, nor provide the appropriate services in the most effective manner.

Ross Cameron

University of Sheffield

Research at the Department of Landscape Architecture, University of Sheffield, is closely aligned with the UN Sustainability Goals on Climate Action and Sustainable Cities. Landscape architects have a huge role to play in adapting our cities to climate change. Specifically, this means using landscape and its associated vegetation to cool our cities, mitigate floods, trap atmospheric carbon and enable our built infrastructure to save energy.

Green infrastructure is fundamental in ensuring our cities remain liveable in the forthcoming decades, and green infrastructure largely means ‘plants’ – but which plants? Sadly, the current plant palette for each climatic zone may no longer be resilient enough to cope with the effects of climate change, nor provide the appropriate services in the most effective manner. We need to think more imaginatively to ensure our green infrastructure actually survives and remains functional in the future.

Our research has been evaluating the impacts of climate change on landscape plants. We have tried to identify future ‘winners’ and ‘losers’ through a better understanding of how key plant traits affect both a plant’s capacity to survive (resilience) and to continue to provide the services we desire (functionality), as the climate warms. Understanding these traits may help us identify appropriate landscape taxa for future use, without the need for extensive, long-term trialling (Cameron and Blanusa, 2016). Traits include obvious factors such as leaf size, but also aspects such as the capacity to regulate their internal life systems when under stress. These approaches are analogous to food crops, where science is trying to identify those traits that ensure survival but also underpin a viable crop yield.

Predicting The Future?

It is a brave person that predicts the future, but powerful modelling based on CO2 emissions has allowed us to develop different future scenarios; so called Representative Concentration Pathways (RCP). Most scientists believe we are on the RCP6 pathway, which represents an increase in mean global temperature of between 1.4 and 3.1oC. As this is mean temperature, that equates to a UK climate by 2100 roughly equivalent to northern Spain today.

What does this mean for landscape plants?

If climate change meant a smooth transition from one climate to another, and that this transition was slow enough, some existing plant tapa would adapt.

The current situation though means that climates are changing very rapidly and this puts pressure on the adaptation processes. For example, it is asking a lot of an individual native rowan (Sorbus aucuparia, for instance) street-planted in 2005 to deal with the conditions prevalent in 2065. Moreover, these transitions are unlikely to be smooth at all, and we will see much more unpredictable weather. For example, a very warm February which causes plants to rapidly de-harden (from their protective winter state), followed by a cold March, can subsequently kill plants that would survive a more conventional spring. It is this latter point (the rapid oscillations between one weather pattern and the next) that will challenge certain landscape plants, and result in only those resilient taxa surviving.

The ‘wild’ native primrose has traits and strategies that helped it cope with a range of stress factors associated with a changing climate.

© Ross Cameron

Within a genus such as Viola, cultivars with smaller and less complex flowers (e.g. bottom left) were often the most resilient.

© Ross Cameron

What determines resilience?

Our research implies plants that have a wide natural range, or that are found naturally across different environmental conditions, may do best in future. Having traits that allow plants to persist (e.g. large roots for starch storage, or possess the capacity to exploit a new niche) are likely to do best – dandelions (Taraxacum spp.) will continue to do well! We used Primula as a model genus to better understand the concept of resilience, using controlled environments to represent future climate scenarios. The results showed that the widelydistributed primrose (Primula vulgaris) did better than ‘more specialised’ species, like cowslip (Primula veris) – adapted to dry soils – and oxlip (Primula elatior) – adapted to damp conditions. It was P. vulgaris’ capacity to cope with both a degree of ‘wet’ and ‘dry’ that gave it greater resilience. Interestingly, some cultivated forms of P. vulgaris also outperformed the two other species. It was observed, however, that the most ornamental, ‘showy’ hybrids demonstrated least resilience overall. We found similar trends in Viola. This suggests taxa that are investing a lot of resources in aesthetics, such as large flowers, are the most susceptible. If we take another genus, Rosa for example, we might find that the robust Rosa rugosa continues to do well, new English shrub roses do OK, but weakergrowing hybrid teas suffer.

Although traits that confer resilience will be one aspect, we can still learn much by studying current native distributions and linking that to key environmental parameters – temperature, annual rainfall and soil type. Based on this, Rhododendron and Hydrangea will find the drier east more gruelling, whereas Bougainvillea and Lagerstroemia become common in the south. Again, species that demonstrate competitive advantage across environmental variables, and feature genes from different provenances, are those to consider first for future plantings. Ideally, from a biodiversity perspective, we might suggest native plants, but future rapid warming may force us to rely on nonnatives, particularly in cities with enhanced heat islands (Lewis et al., 2019; Sjoman et al., 2015).

Remaining Functional?

The challenge for future selection is not only to ensure plants survive, but that they also enhance their functional uses. These have traditionally been to look good and attract wildlife. Increasingly though, landscape species are considered because of the additional ‘services’ they provide. For example, their capacity to hold back rainwater, improve air quality, insulate buildings and enhance human wellbeing. Plants need to be used more effectively for these purposes, and that involves careful plant selection. Thus, we are trying to link traits to ecosystem services too, so we can accurately predict which genera to utilise in future. For example, to insulate buildings from excessive heat, Clematis, Lonicera or Actinidia Kolomikta or Hydrangea Quercifolia are considered optimal, as they provide shade but also high evaporative cooling.

Conclusions

Green infrastructure will be vital for the future viability of our cities, but we need to be sure now that we are selecting the right plants. Simply speaking, we need taxa that will survive, but also mitigate the effects of climate change itself. Research aims to assist, but we also require discussion with respect to policy and practice. We cannot assume that our native stock will adapt in time, nor that we can put a pin in the map and say that flora will be well-adapted simply because it currently grows 2000km south of us.

Dr Ross Cameron is Director of Research at the Department of Landscape Architecture at the University of Sheffield. He specialises in Landscape Science and Management.

References

Cameron, R.W. and Blanuša, T., 2016. Green infrastructure and ecosystem services–is the devil in the detail? Annals of Botany, 118(3), pp.377-391. Lewis, E., Phoenix, G.K., Alexander, P., David, J. and Cameron, R.W., 2019. Rewilding in the Garden: are garden hybrid plants (cultivars) less resilient to the effects of hydrological extremes than their parent species? A case study with Primula. Urban Ecosystems, 22(5), pp.841-854. Sjöman H, Hirons AD, Bassuk NL (2015) Urban forest resilience through tree selection—variation in drought tolerance in Acer. Urban For Urban Green 14(4):858–865 some existing plant taxa would adapt.