Climate change: warming may extend some tree growing seasons and compensate for reduced carbon uptake during drought

The researchers suggest that carbon uptake will likely be enhanced in some species, in particular, though the exact mechanisms that trigger earlier leaf-out (when the leaves first appear) and delayed leaf fall are unknown.

Reduced precipitation and rising temperatures associated with climate change will be critical stresses for trees because of their impacts on growth. For example, warmer summers may cause leaves to senesce (or age) earlier in autumn if further photosynthesis and growth are unnecessary or if summer drought has dried leaves.

In this study, researchers investigated the effects of dry weather and high temperatures on the length of time a tree is active and its ability to take up carbon. They studied European beech (Fagus sylvatica) and downy oak (Quercus pubescens). Oak is known to be more tolerant to low moisture and heat, whilst growth is impeded in beech at temperatures over 40°C.

In open-top chambers, the researchers exposed downy oak and beech seedlings to four combinations of heat and low soil moisture over four years. In the control condition, soil was at field capacity, and air temperature was ambient with mean annual temperatures of 11.6–16.5°C. Air temperature inside the warming treatment was maintained at 5°C above the control. Soil moisture in the dry soil treatment was about half of the control level. Both treatments were also combined.

Leaf development was tracked in spring and leaf senescence from September¹. Growing season length was calculated as the number of days between leaf-out and leaf senescence. Other measurements were taken to enable simulations of photosynthesis and carbon uptake.

Warming, irrespective of soil moisture, caused leaf-out to occur earlier than in the control, for both species – by about five days for beech and two weeks for oak. Time of leaf senescence was not significantly affected for either species. Reduced moisture alone had little impact on phenology of either species, but this factor advanced leaf-out in oak by 4.9 days and prolonged leaf senescence by 20 days.

Warmed oak showed the highest annual carbon uptake: 1 760 grammes (g) per square-metre leaf area. In the dry and warm treatment, higher photosynthetic efficiency also increased oak’s annual carbon uptake and compensated for the stress caused by reduced moisture. The lowest carbon uptake in oak was 1 359 – under the dry soil condition. For beech, yearly carbon uptake was lower in all treatments compared to the control, which took up 637 g. The lowest uptake was 467 g, in the warming and dry soil treatment.

The prolonged growing season under the warming and warm/dry conditions led to carbon gains of about 4% and 8% in beech and oak, respectively, prior to the time that activity started in the control trees. During the simultaneous growing season, beech in the warm/dry treatment took up about a third less carbon than the control; oak about 12% less.

The results indicate that earlier leaf flushing (the production of new leaves often occurring on all branches of a bare tree or plant) driven by warming leads to higher carbon capture particularly in downy oak, compensating for reduced photosynthesis during hot and dry periods. The researchers note that this effect is context and species dependent: delayed leafing-out has been found in semi-arid systems warmed by 5°C². Temperate ecosystems are, therefore, more likely to gain growth potential from warming. Meanwhile, the different reactions in beech and oak point to different underlying processes driving leafing-out and senescence (e.g. photoperiod or length of daylight). Downy oak is a drought-tolerant species, but some studies suggest beech may be less able to adapt to global warming.

The researchers conclude that shifts in plant phenology due to global warming (i.e. earlier leaf-out) may mitigate the negative effect of drought on carbon uptake. However, more research is needed to validate findings – for example, if data could also be collected on the reaction of other tree species, it would feed into knowledge of annual carbon fixation. Also, the treatments were applied for the entire growing season, but the timing of drought or warming could have different effects, say the researchers. The researchers add that a longer growing season could also make trees vulnerable to other climate-linked stressors such as frost and herbivore damage.

Further Information

Please also see Science for Environment Policy’s recent article, ‘Europe’s beech forests threatened by climate change’ which discusses how climate change could significantly reduce beech trees’ growth across most of the continent this century: https://environment.ec.europa.eu/news/europes-beech-forests-threatened-climate-change-2022-10-26_en

Footnotes:

1. The researchers estimated the percentage of coloured and dropped leaves of each tree. They then calculated the date when 50% of the leaves were either coloured or had fallen. The speed of senescence was assessed by the number of days between 10% of leaves being coloured and dropping and 80% having turned.
2. Adams, H. D., Collins, A. D., Briggs, S. P., Vennetier, M., Dickman, L. T., Sevanto, S. A., Garcia-Forner, N., Powers, H. H. and Mcdowell, N. G. (2015) Experimental drought and heat can delay phenological development and reduce foliar and shoot growth in semiarid trees. Global Change Biology, 21: 4210–4220.

Source:

Grossiord, C., Bachofen, C., Gisler, J., Mas, E., Vitasse, Y. and Didion‐Gency, M. (2022) Warming may extend tree growing seasons and compensate for reduced carbon uptake during dry periods. Journal of Ecology 110:1575–1589. Available from: https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2745.13892

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.