Climate change is increasingly changing the living conditions of trees. Rising temperatures and longer dry phases are causing the upper layers of the forest floor in particular to dry out – exactly where trees normally absorb water and the nutrients dissolved in it. Can they move to deeper soil layers – and do the nutrients reach the treetops? This is exactly what researchers have investigated for the first time on fully grown trees.
Trees absorb water from the soil through their roots, which is then transported through the trunk and branches to the leaves. There it is needed for photosynthesis and cools the tree through evaporation. Important nutrients, especially nitrogen, also enter the plant with the water. It is usually absorbed as nitrate or ammonium and is essential for plant growth as it is needed to build proteins, enzymes and chlorophyll.
However, if the soil dries out, this supply becomes more difficult. Because many vital nutrients only reach the tree dissolved in water. If there is no supply, growth can be slowed down and the health of the trees can be weakened in the long term.
On the trail of water and nitrogen
A research team led by Klara Mrak from the University of Göttingen has now investigated how trees absorb water and nitrogen under dry conditions. To do this, the researchers analyzed 18 Douglas firs and beech trees in two forests in northwest Germany. While the soil at the first location was clayey and nutrient-rich, at the second it consisted predominantly of sandy, nutrient-poor material.
In order to track the uptake of water and nutrients, the researchers introduced isotope-labeled hydrogen and nitrogen into the soil at a depth of 60 centimeters. To do this, they used stable, non-radioactive versions of these elements – the rare form of deuterium for hydrogen and a heavier version than usual for nitrogen. These remain detectable in the tree and make it possible to trace their path through the plant. The researchers also measured the speed of water transport using a sap flow measuring device. At the expected time of arrival of the marked liquid, they climbed into the treetops and took samples from leaves and water-conducting tissue over a period of two months.
A race to the top
The result: The hydrogen isotopes reached the crowns of the 20 to 30 meter tall trees within four weeks or sooner. Nitrogen isotopes, on the other hand, were only detectable after four to six weeks. “Water flows quickly through the channels of the trees, as a constant stream of water from the roots to the leaves. Nitrogen is carried by the water, but is slowed down by biological processes along the way. The fact that the transport of the two resources is so decoupled has so far only been proven in the laboratory, but not in the forest on fully grown trees,” explains Mrak.
The study also showed which layers the trees get their water from: trees on sandy soils, where water seeps away quickly, absorbed more resources from deeper soil layers than trees on clayey soils. “Douglas firs can absorb resources from the depths on sandy soils, even more than beech trees. This observation underlines their ability to adapt under drought stress,” says co-author Christina Hackmann from the University of Gölttingen. “These are promising properties for climate-resilient forests in times of increasing drought.”
Source: Georg August University of Göttingen; Specialist article: Plant, Cell & Environment, doi: 10.1111/pce.70327