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Trees and the vagaries of climate.

Trees and the vagaries of climate.

by The blog at woodlands.co.uk, 20 October, 2023, 0 comments

During a drought, the trees in a woodland or forest become 'stressed' and may die.  The  reason for their death is not immediately obvious (beyond lack of water), and  it is not possible to ‘transplant’ a mature tree and its complete root system to a lab for detailed investigations.  However, recently, researchers at the University of Innsbruck have taken ‘the lab’ to a set of mature pine and pine trees. The trees were fitted with rugged and waterproof ultra-sound detectors.  Some of the trees had their canopies covered by a ‘roof’ so that the summer rain was denied to the trees, and they essentially experienced a ‘drought’.   Drought stressed trees produce ultrasound ‘clicks’ (faint acoustic waves that bounce off of air bubbles) that can be picked up by the detectors.  Air bubbles or emboli form in the vascular system of the trees when they are struggling for water.  Water is drawn up the xylem vessels by the evaporation of water (via the stomata) from the leaves, there is a continuous column of water.  When the column of water breaks, bubbles form with the xylem vessels and the transport of water to the leaves is reduced.  If the flow of water is substantially reduced the tree will die. The sound detectors found that the spruces produced more clicks than the beeches when water stressed, suggesting more emboli were formed within their xylem tissues.  It may be that the beeches were able to access the deeper reserves of water in the soil, whereas the spruces had a shallower root system. Trees can, of course, reduce water loss from their leaves by closing down their stomates.  But when their stomates are closed, they cannot take in carbon dioxide for photosynthesis and make the sugars / starch that they need for their metabolism.  At the end of the experiment, the trees that experienced ‘drought’ were drenched with water and most recovered well, and their rates of photosynthesis caught up with the ‘control’ groups of trees (those with summer rain).  However, the spruces’ water reserves were somewhat depleted; this was determined by measuring the resistance the tissues offered to an electrical current. The ability to withstand / recover from drought could over time affect the make up of woodlands and forests,  particularly if the trend for hotter and drier summers continues. Interestingly, some work in the United States (at University of Wisconsin–Madison) suggests that young tree saplings that have experienced drought or heat are more likely to survive when transplanted into more challenging areas.  It seems that the soil microbes that young saplings experience can help young trees establish themselves.  Saplings grown in soil (and microbes) that have experienced drought / cold / heat are more likely to survive when later transplanted and faced with similar conditions.  Trees with ‘cold-adapted’ microbes survived better when experiencing Wisconsin’s winter temperatures. The work was conducted with different species of tree in a variety of locations in Wisconsin and Illinois. The transplant locations varied in temperature and rainfall.  It may be that fungi that inhabit the roots of the saplings are involved in these ‘responses’, though the microbial population of the soil is diverse. For more details of this work, follow the link here.
Masting

Masting

by The blog at woodlands.co.uk, 10 December, 2021, 0 comments

‘The Fall’ in the eastern United States has been colourful and plentiful this year.  There have been bumper crops of acorns, maple seeds and pine cones.  It is a Mast Year.  The trees have produced enormous numbers of potential offspring. These seeds and fruits will have significant 'knock on effects' in the ecosystems for some years.   Beeches and oaks can release so many seeds that they significantly increase the organic content of the soil and its nutrient value.  This fuels fungal and microbial growth. Small mammals feast on the acorns / mast and their numbers increase.  They, in turn, are food for foxes, owls and other predators *.   Quite what drives a mast year has long been a cause of speculation.  Ideas have included  masting evolved to overwhelm seed predators (mice, squirrels etc.) and thus ensure that at least some seeds survive to germinate and grow on.  fluctuations in nutrient availability affect the trees and flower / fruit production environmental prediction - that masting occurs in those years when seeds are likely to have good weather for sprouting in the following Spring.   even sunspot activity has been invoked Recently, a database [MASTREE] was created of mast years (for Beech and Norway Spruce) that extends back centuries.  This has enabled scientists to explore the environmental prediction idea, that is, whether masting is correlated with climatic events and occurs when seeds are likely to have favourable weather for germination and growth in the Spring after their production. On comparing the data with climate records, they found masting events [in beeches] correlated with climate patterns associated with the NAO - North Atlantic Oscillation, i.e. changes in air pressure between Iceland (low) and the Azores (high).  A “positive” NAO phase favours both masting and subsequent seedling growth; that is warm wet winters promote seed production and dry springs favour seedling growth.  Quite how the trees turn such climatic events into ‘signals’ for masting is another matter. Not all are convinced however. Some argue that the resources used up in producing so many seeds / fruits mean that the trees are exhausted and it takes time for these resources to be replaced and for the tree to flower and fruit fully again.   Professor David Kelly has a somewhat different hypothesis related to weather .  He suggests greater warmth in the previous growing season(s) may be the trigger.  Quite how the trees ‘remember’ the warmth that they have experienced is not known; but one thought is that it is due to what is termed ‘epigenetic marking’.  It is possible that the DNA of the genes that affect flowering is changed by the warm temperatures.   The activation of particular genes can be altered by their DNA undergoing methylation - a process where methyl (-CH3) groups are added (or removed) from the DNA.  Further information on masting and climatic effects on trees - visit science.org * [Sadly, a Swiss study found good masting years were later associated with a rise in tick-borne disease.]  

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