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Leaf rot slows after droughts, hindering plant growth

By Martin Leggett - 06 Apr 2011 12:25:0 GMT
Leaf rot slows after droughts, hindering plant growth

The effects of droughts on trees carry on long after the rains have returned, according fresh research - directed by Jeff Dukes, at Purdue University, Indianapolis - into climate stress for woodlands in the north-eastern US. Water-stress triggers physical changes in tree leaves, as the trees seek to protect themselves from water loss; and those changes impact the ways in which leaf litter is decomposed, which could then cause plant growth to slow down.

The study was run as part of the Boston Area Climate Experiment, set up specifically to look at how the trees and plants of the Massachusetts region would respond to drier, hotter summers. Several experimental field plots have been divided up, and scientists from the project are able to control the heat, precipitation and humidity experienced by using heaters and sprinklers.

One way I which trees adapt to a lack of water is to produce more tannins. These are normally produced to protect leaves from being grazed, as well as attacks by diseases and pests. But under drought conditions, it is known that the tannin levels rise - and this research, published in New Phytologist, has confirmed that tannin levels double under hot dry conditions, for the Red Maple tree.

The study also confirms that there are changes in the make-up of the leaf's tannin complex, which produces more 'condensed tannins'. Together, these two drought-related effects have big knock-ons for the rotting of the leaves, the nutrients in the soil, and so future plant growth.

Firstly, the high levels of tannin mean that leaves, gathered on the forest floor, decompose more slowly. That reduces the nutrient flow back into the soil, and slackens the pace of the CO2 being released from the leaf litter. And secondly, those extra 'condensed tannins' soaked up to 5 time more, of important soil enzymes, than normal tannins.

Those soil enzymes are a great aid plant growth, so less of them will probably mean less growth for plants and trees alike - so prolonging the influence of the drought. But it also means less CO2 is absorbed from the atmosphere as plants grow - so balancing out, somewhat, the lower carbon-dioxide released from slower rotting leaves.

How these two hooks into the carbon cycle, due to the drought-increased leaf tannins, will play out long-term is unknown. Dukes reflected the uncertainty of what might happen, saying that ''this is an issue that could affect many natural processes. We just don't know what the net result will be.''