So wait, climate change could be good for trees?

Duveneck, MJ and JR Thompson. 2017. Climate change imposes phenological trade-offs on forest net primary productivity. Journal of Geophysical Research Biogeosciences 122. doi:10.1002/2017JG004025

Many politicians lacking knowledge about the effects of climate change like to point to the fact that increased CO2 concentrations in our atmosphere will be good for plants and, by default, conclude that climate change is good for humans. But will climate change actually be good for plants? Well maybe. Like all predictions in this world there is quite a bit of uncertainty in the answer to that question; however, quite a few recent studies indicate that due to a longer growing season this may be true at least for some tree species. Trees are extremely important to humans’ and other species’ ability to exist on this planet. We need oxygen and they giveth. Trees also store a lot of carbon, but predicting how this will change in the future is important as we try to prepare for a changing climate.

Back to the basics

We all know that deciduous trees put on leaves in the spring and shed them in the fall. During the growing season, leaves use sunlight to convert CO2 and water into biomass through the process of photosynthesis. Environmental scientists call the biomass that is produced across a landscape over a certain period of time gross primary production or GPP. Trees also simultaneously take part in a process known as respiration, whereby cells take up oxygen to produce energy from stored sugars and release CO2. Healthy trees produce more biomass than they use throughout a year. This increase in biomass is referred to as net primary production (NPP) or the total amount of biomass produced by a tree after accounting for respiration (for a good explanation and diagram look here and here). That means that forests are currently net carbon sinks, but how could this change in the future under a different climate?

Effects of increased CO2 concentrations on tree growth

Longer growing seasons controlled by elevated spring and fall temperatures give temperate trees more time to produce biomass. At the same time, increased­ atmospheric CO2 concentrations may act to further enhance a tree’s efficiency (Keenan et al. 2013). This basically means that trees can produce more biomass with less water. Unfortunately, extremely warm temperatures in the summer will likely increase respiratory demand during this season as temperatures rise above the optimal temperature thresholds for photosynthesis. The main question is, will elevated summer respiration cancel out increases in gross production that result from a longer growing season and increased efficiency?

A couple of recent studies using a variety of methods have indicated that, in fact, gross production increases may mitigate carbon losses due to increased respiration. Richardson et al. (2010) found that during abnormally long growing seasons, gross production exceeded respiration, resulting in increased productivity. Similarly, Wolf et al. (2016) found that increased gross production from a longer growing season in 2012 exceeded carbon lost from extreme drought during that same year. A computer simulation showed that under even the IPCC’s (Intergovernmental Panel on Climate Change) most extreme predictions for climate change, net primary production increased overall across New England relative to current climate scenarios even though summer respiration also increased (Duveneck et al. 2017). This sounds like good news, but even with these studies a large amount of uncertainty remains in how forests will respond to climate change.

The plot thickens

One complicating variable is water. Some climate change models predict a reduction in annual precipitation for certain regions. This possibility combined with a longer growing season (longer time for growth = larger volume of water required per year) could easily lead to both water limitation and as a side effect nutrient limitation. This can lead to a reduction in predicted biomass production and increased respiration. To make matters even more confusing, nutrient and water abundance is spatially variable even within a stand of trees. This means that tree response to increased temperatures and CO­2 may be much more variable both across landscapes and across species. For instance, Duveneck et al. (2017) found that the two model scenarios that resulted in the largest increases in annual NPP relative to today across New England had distinctively different spatial patterns (Fig. 1). The first model scenario (HADGE) that corresponded with the highest increases for both temperature and precipitation indicated that the coastal regions of New England would see the largest increase in annual NPP. Conversely, the other model scenario (MPIMLR), which also corresponded to increased temperatures but much less precipitation, predicted that the northern regions of New England would see the largest increase in annual NPP above present. Thus precipitation and temperature seem to both be important. Species composition is another factor to remember. Species that do not require as much water may have a positive response to a longer growing season (higher temperatures), but species that need a lot of water may not do as well in terms of increased productivity. In the long run, these drought tolerant trees may out-compete less drought tolerant species, leading to an entirely different forest community.

Figure from Duveneck et al. (2017) showing monthly net primary productivity (g m-2 month-1) at year 2100 of the New England region under current climate and four different climate change model scenarios. The “current” scenario shows similar patterns in NPP that we see in New England today with a peak around June and July. The other scenarios show alterations in the temporal distribution of NPP from current. The HADGE and MPIMLR scenarios have the highest NPP but show spatial difference with coastal areas displaying the highest NPP in HADGE and inland areas displaying the highest NPP in MPIMLR.

So what does all this mean?

From the information we currently have, it seems that generally trees growing in temperate forests might be more productive in the warmer climates of the future. However, the uncertainty in climate model predictions – how temperature and precipitation distribution will change across space and time – corresponds to large uncertainty in our predictions of how forests will respond to climate change. The future of other biological factors such as herbivore and insect interactions with tree growth remains relatively inconclusive as well. Lastly, spatial scale must be considered. For instance, if forests of boreal and temperate regions are able to store more carbon in the future, but all tropical forests are lost to desertification, net carbon storage in forests will decline globally. Even if biomass storage increases locally, the global perspective is really what determines whether forests will both remain carbon sinks and increase in abundance as the climate changes. So will climate change be good for trees? Unfortunately, I do not think we know the complete answer as of now, but we are well on our way.

Additional References:

Richardson, AD, et al. 2010. Influence of spring and autumn phenological transitions on forest ecosystem productivity. Philosophical Transactions of the Royal Society of London B, 365: 3227-3246. doi:10.1098/rstb.2010.0102

Wolf, S, et al. 2016. Warm spring reduced carbon cycle impact of 2012 US summer drought. Proceedings of the National Academy of Sciences USA, 113:5880-5885. doi:10.1073/pnas.1519620113

Keenan, TF, et al. 2013. Increase in forest water use efficiency as atmospheric carbon dioxide concentrations rise. Nature, 499:324-327. doi:10.1038/nature12291

Category/Keywords: Science through time, climate

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Mary Grace Lemon

Mary Grace Lemon

I am currently a PhD student in the School of Renewable Natural Resources at Louisiana State University. My dissertation focus is forested wetland hydrology. I use an array of hydrological research tools to try and improve our understanding of water movement through large floodplain forests of the southeastern United States. Before starting my PhD I earned a Masters degree from the University of North Carolina Wilmington. My masters research involved investigation of sediment transport around oyster reefs in tidal creeks. From then on, I have had a passion for understanding how biological systems interact with hydrological processes. Outside of work, I spend the majority of my time exploring the swamps and culture of Louisiana.

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