For Better For Worse: A Humidity and Plant Relationship
Our climate is changing. Whilst rising global temperatures and greenhouse gas emissions plague our thoughts and understanding of the effects that climate change is having on our environment, there are other key players that appear to be lost in a sea of uncertainty.
Relative humidity trends are changing on a global scale with some countries observing an increase (central and eastern United States, western China, central Europe, eastern Africa) others a decrease (UK, South Africa). This could have a drastic impact on the health of our planet from a biological perspective. Whilst the impacts of increases in humidity on humans are relatively well understood (see BBC news article http://www.bbc.co.uk/news/science-environment-40793019), the understanding of the responses of plants to such changes leaves a lot to be desired.
Plants are the backbone of life on Earth. Not only do they provide us with oxygen, food and medicine, but are key players in the regulation of water cycles, redistributing water from the soil back up to the atmosphere via transpiration. Plants also capture carbon, storing it within their tissue, a trait that is inadvertently helping us alleviate the effects of CO2 emissions, by removing them from the atmosphere. They are the silent regulators and protectors of our planet. Though unable to move when conditions become unfavourable, they possess a wide range of responses and adaptations to help them survive. We need to understand these responses if we are to understand how their productivity, survival, distribution and subsequent effects on our planet may change when faced with a change in relative humidity, a relationship which is not yet fully understood.
What is relative humidity?
Relative humidity is how much water is in the atmosphere relative to how much the air could hold at its current temperature, expressed as a percentage. The warmer the air is, the more water vapour it can hold.
Potential responses to a high humidity environment
We do not yet know how exactly plants will respond to an increase in relative humidity, but a change in overall transpiration rates could be on the cards. Transpiration is the process where plants lose water (through tiny pores called stomata) to the surrounding air, cooling them down - a similar concept to us sweating to regulate our temperature. Transpiration (see diagram) is driven by concentration gradients, there is more water in the leaf (high relative humidity) and less in the atmosphere (lower relative humidity), water therefore moves from high humidity to low humidity, so water escapes from the leaves. However, if the surrounding air increases in humidity, it will be more like humidity levels in the leaf so the concentration gradient decreases therefore transpiration rates could decrease. If the plants are unable to ‘sweat’ as efficiently they are at risk of overheating which could be detrimental to their health.
It’s not only over-heating the plant should worry about in a more humid environment, with high humidity also comes a higher likelihood and susceptibility of some fungal diseases that thrive in moist environments. Unless the plants are suitably adapted to deal with such diseases, they could have a drastic impact on plant health and mortality.
Potential responses to a low humidity environment
An environment with lower humidity (drier air) could be of significant stress to any plants not sufficiently adapted to it. Interestingly, a study in 2016 has suggested drier air to be more of a stress to plants than dry soil conditions. This is an interesting concept that a plant can be showing signs of drought stress, despite growing in ample soil water conditions, which could lead to a lot of confusion amongst farmers and gardeners alike! Also, during low humidity conditions the plant is at risk of losing too much water from its leaves to the surrounding air. To reduce the amount of water lost the plant closes stomata and reduces transpiration rates. However, this reduction could then lead to insufficient cooling and overheating of the plant.
Conclusion
These are just some of the potential responses of plants to changes in relative humidity. We need to understand how plants will respond to changes in humidity. For better or for worse, their responses could affect their overall health, distribution and even their productivity. Unfavourable effects on productivity will be detrimental to food production, which is under increasing pressure from booming populations that don’t look like slowing down anytime soon.
An understanding of the responses will also go a long way in providing us with valuable information to be included in designing predictive climatic models and give us a better insight into how our planet will be affected by the next chapter in its life, 21st century climate change.
Further reading/references
Collier, P., Conway, G. & Venables, T., 2008. Climate change and Africa. Oxford Review of Economic Policy, 24(2), pp.337–353. Available at: https://academic.oup.com/oxrep/article-lookup/doi/10.1093/oxrep/grn019 [Accessed July 10, 2017].
Dai, A., 2006. Recent climatology, variability, and trends in global surface humidity. Journal of Climate, 19(15), pp.3589–3606. Available at: http://journals.ametsoc.org/doi/abs/10.1175/JCLI3816.1 [Accessed July 10, 2017].
Jones, P.D. & Moberg, A., 2003. Hemispheric and large-scale surface air temperature variations: An extensive revision and an update to 2001. Journal of Climate, 16(2), pp.206–223. Available at: http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282003%29016%3C0206%3AHALSSA%3E2.0.CO%3B2 [Accessed July 10, 2017].
Leuschner, C., 2002. Air humidity as an ecological factor for woodland herbs: leaf water status, nutrient uptake, leaf anatomy, and productivity of eight species grown at low or high vpd levels. Flora - Morphology, Distribution, Functional Ecology of Plants, 197(4), pp.262–274. Available at: http://www.sciencedirect.com/science/article/pii/S0367253004700262 [Accessed November 3, 2017].
Met Office, 2012. Observed trends Relative humidity. UK Climate Projections, p.http://ukclimateprojections.metoffice.gov.uk/23069.
Novick, K.A. et al., 2016. The increasing importance of atmospheric demand for ecosystem water and carbon fluxes. Nature Climate Change, 6(11), pp.1023–1027. Available at: http://www.nature.com/nclimate/journal/v6/n11/pdf/nclimate3114.pdf [Accessed June 8, 2017].
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