50 FOREST REPORT 2005
2.3 The condition of tree crowns
The condition of tree crowns – an indicator of the state of the forest – is worse today than it was in 1985.
Although tree crowns show more gaps today than they did in the mid eighties, this does not mean that more trees are dying today. The annual rate is still 0.4%.
Not only did the storms “Vivian” (1990) and
“Lothar” (1999) damage tree crowns, but so too did the hot summer of 2003, but this was not visible until a year later.
WSL will be examining the effects of heat and drought on the condition of tree crowns more carefully in the next few years.
Condition
The condition of the crowns of trees – an indicator of the state of the forest –hasdeclinedsince theywere first surveyedin1985.
WSL’s studies of general defo- liation show that, particularly in the mid-nineties, gaps in the tree crownswere appearing.Be- tween1990 and1995 thepercent- age of treeswith onequarter few- er leaves or needles than a tree with full foliage rose frombelow 30 toalmost 40%.Since then the percentages have fluctuatedat a high level. Treecrowns in some years havea higher and in oth- eryearsalowerdensityof leaves or needles, but in general all of them have fewer needles and leaves than in1990.Although in 2003 experts again found more densely foliated tree crowns for the first time since the earlynine- ties,alreadyoneyear latercrown defoliation had almost reached the recordlevel of theyear2000.
Since observations began, trees havebeendying offat the same rate ofabout 0.4%annually.
Swiss forests havebeen struck bypowerful storms in recent dec- ades,especially“Vivian”in 1990 and “Lothar” nineyears later (>
2.4Forest damage).These led to the loss of manyleavesandnee- dles. The hot summer of 2003 alsodamaged treecrowns.
In addition to monitoring defoliation, researchers at WSL have been observing since 1993 if and how leaves and needles discolour. The Sanasilva surveys found that leaves and needles normallyhadnot discolouredby FURTHERINFORMATION
Swiss Federal Research Institute WSL 8903 Birmensdorf
Research Dept. FOREST Section Forest ecosystems and ecological risks (0) 44/ 739 25 95
51 2 HEALTH AND VITALITY
July or August, not even in the
“summer of the millennium” in 2003.However,byautumn2003 thepicture had changed. In the SwissCentralPlateau,7% of the broadleaf trees had lost at least 15% of their leaves or hadan un- usuallylarge number ofbrown or yellowleaves or needles.
Swiss forests have been struck by powerful storms in recent decades, especially “Vivian” in 1990 and “Lothar” nine years later. These led to the loss of many leaves and needles.
2.3.1
Development of crown defoliation in Switzerland
Proportionof trees withmore than 25% crown defoliation. Shown is the development of general defoliation andcrown defoliationof unknownorigin.
Crown defoliation[%]
–
–General defoliation –
–Crown defoliation of unknown origin
«Vivian»
«Lothar»
Dry summer of 2003 50
40 30 20 10 0
85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04
Causes of defoliation
Branches whipped by strong winds break off neighbouring twigs or knock off their leaves and needles. Leaves also fall in hailstorms or after frost. In ad- dition to these climatic causes therearealsobiological ones.In- sects eat theirway through tree crowns and fungi attack leaves and twigs.If trees receive insuffi- cient light in the shade of neigh- bouring trees, they form fewer newleaves or needles.When for- est workers fell trees,falling trees often tear twigsandbranches off the surrounding standing trees.
Drought toocanaffect thecondi- tion of treecrowns,although this is usually not noticeable at first sight.That iswhyWSLwill study the effects of heat and drought on the condition of tree crowns incomingyears.
2.3.2
Defoliation
The defoliated tree(left) hasnoticeably fewerneed- les than itsneighbour.
