he Swiss forest currently shows no signs of being under immediate threat.
This welcome observation from for- est research does not mean, however , that all is well. Thus depositions of pollution from the air , especially of nitrogen oxides, continue to be a severe and long-term threat for our forests. It has not been possible to assess their affects fully yet. Bark beetles, especially the spruce bark beetle, are causing tremendous problems at the moment.
Drought periods, which have become noticeably more frequent in recent decades, are also a source of concern for Switzerland’s largest habitat.
Since the eighties, when the state of the forest became the focus of public attention, researchers have been able to clarify several processes tak- ing place in the forest. But many questions to do with the future of the forest remain unanswered.
There is still great need for more research, particu- larly long-term research on ecosystems. In order to grasp the complex relationships involved better , the scientific community is working more closely together than ever before. Researchers are observ- ing, for example, over 870 forest sites throughout Europe, of which 17 are in Switzerland, to explore the interconnections between the state of the for- est, environmental pollution and climate change.
Findings from research
Installation in the Seehorn forest (Davos,GR) to measure the gas exchange of a spruce tree:
“Health”and“vitality”are not directly measur- able parameters for trees, forests or people.
T
SUMMARY
rown defoliation is considered an indi- cator of stress ina tree.It does not,how- ever,provideany informationabout the source of stress. “Health” and“vitality”
are not directly measurable parameters for trees, forests or people.Unfortunately, we have not yet found any other indicators that canbe used to eas- ily assess the state of the forest onalarge scale.We do,however,know that trees grow more slowly and are more likely todie the moredefoliated they are.
Theannual representative survey ofcrowndefolia- tion hasbeen reduced toaminimum.It iscoordi- nated with theEuropean programme for surveying the state of the forest and usesdata collectedfrom roughly 1100 treesat 49 spot check points (seeIn- dicator 2.3).
Crowndefoliationcan neitherclarify the ques- tionabout the state of the forest nor provide any informationabout sources ofdamage.This is why WSLstarted doing long-term forest ecosystem re- search (LWF) in 1994, andimportant dataon the condition of the forest is now collected at 17 sites distributed throughout the four regions ofSwitzer- land.The plan is for the sites tobe monitoredforat least 30 years,recording thecrowndefoliationand the annual stem growth of the trees. In addition, climatedataiscontinuously recorded,fallen leaves andneedlesarecollected and chemically analysed, thechemicalandphysical properties of the soilare probed, the nutrient contents of leaves and nee- dles are determined, and thedevelopment of the vegetation is monitored.Thesedata are not only evaluatedin Switzerland but are also collectedin the framework ofa European programmeand ana- lysed together withdatafrom more than 870 other sites inEurope.
As wellas these surveys, theNationalForest In- ventory is performedevery ten yearsand thecan- tonsalso have permanent observation plots.The im- pact of insects,pathogensandgame iscontinually monitored through surveys,our own observations and case studies of the Phytosanitärer Beobach- tungs- und Meldedienst / Service phytosanitaire d’observation et d’information / Servizio fitosan- itario d‘osservazione e d‘informazione (PBMD/
SPOI / SFOI) inclose cooperation with the forest
C
Ozone and nitrogen depositions have reached levels today in many parts of Switzerland that exceed the inter- nationally established critical loads. If these threshold values are exceeded, then long-term detrimental changes can be expected, for example, nitrates seeping into the ground-water.
d‘ensemble /Situazione fitosanitaria deiboschi).
What is the condition of the Swiss forest today?
In many regions inEuropeand Switzerland an increasing trend for trees to grow taller hasbeen observed for several years. Thus the 50-year-old beechesat the LWF site “Othmarsingen” are now roughly 6metres higher than today’s 150-year-old beeches were 100 yearsago.The older treesarealso showing no signs of thedecline one might expect at that age.Various factors may explain these observa- tions: changes in forest management practices,less collecting of litterandpasturing which used to re- move nutrients, increases in nitrogen deposition, andhigher temperaturesand carbondioxidecon- centrations in theair.
It has not been possible inSwitzerland to link pollutantsdirectly withcrowndefoliation in trees.
In soils sensitive toacid deposition, an increase in soilacidification hasbeen observed.This makes nu- trients in the soil lessavailable for uptakeby plants
trations of toxic aluminiumand can hamper thede- velopment of the root system.
Ozoneandnitrogendepositions (seealsoIndi- cator 2.1) have reachedlevels today in many parts ofSwitzerland that exceed the internationally estab- lished critical loads/levels.If these threshold values are exceeded, then long-termdetrimentalchanges canbe expected,for example,nitrates seeping into the ground-water.Ozoneconcentrationsareanad- ditional stress factor for the forest ecosystem be- sides natural factors, such as insect attacks,path- ogens ordroughts each summer, that also havea negative impact.
There are, however, no signs that the forest’s future existence is in any immediate danger, al- thoughdepositions of pollutants still posealong- term risk.
Tree diseases and forest insect pestscontinu- ously affect the forest’s health inanatural way and are favoured by extreme weatherconditions.The dry summer of 2003 led toamarkedincrease in sil- ver firbarkbeetlesand the storm “Lothar” inDe- cember 1999, the storm of thecentury,resultedin a massive proliferation of the spruce barkbeetle, which is still continuing. It has far exceeded all previous epidemics.In 2003 there wasarecordin- festation of over 2millioncubicmetres of infested wood.This decreased, as expected, to 1.3 million cubicmetres of spruce woodin 2004, which is still not less than the high infestation levels recordedin the years 2001 and 2002.Between 2001 and 2004 a total of 6 million cubic metres of spruce wood fell prey tobarkbeetles, whichcorresponds to the amount of spruce timberblowndown in the storm
“Lothar”.Reasons for the unusually high level of in- fested woodhave todo with the exceptional force of the storm, changes in forest protection strategies and weatherconditions that favoured beetle prolif- eration–in particular the extraordinarily hot and dry summer in 2003.
SUMMARY
Impact of the summer of the century
Thedry year in 2003 affected theSwiss forest in different ways. Stem growth in treesat 15 out of the 17LWFsites clearly reacted to the estimat- ed drought stress.Thedrier it wasbetweenMarch and August 2003, the less stem growth there was in comparison with the previous year.Aridity de- creased,however, withaltitude.At sitesabove 1300 metresa.s.l., trees even grew better.This hadmostly todo with the higher temperatures in 2003, ascli- mateanalyses inAlpine regionsand comparisons with satellite images wereable todemonstrate.On most of these sites trees grew again normally in 2004.Unlike with growth,however, thecondition of the tree crowns was different. In July and Au- gust 2003 no increase incrowndefoliation was ob- served, althoughat the most aridsites some prema- turecolourchange occurred andleaves were shed early in late summerand autumn.In summer 2004 amarkedincrease incrown defoliation was regis- tered at many sites.Tree mortality does not appear to have risen markedly at any of theLWFsitesapart from theVisp site inCantonValais.
It was expected that there would be similar loss- esamongbroadleaf trees to those that occurred after thedry years in 1947and1949.Up until now,how- ever,no markedincrease indirect drought damage hasbeen observed, nor in the root andstemdis- eases that thrive on lack of water.This shows once again thecomplexity ofboth the influences on the forest and the forest’s reactions to them.
Pine mortality in Canton Valais
InValaisalarge-scale landscapechange iscur- rently taking place. Oneaspect of this is that the mortality rates of forest pines have increasedgreat- ly over the past few decades.In 2004,for example, mortality at theLWFsite inVisp wasaround 20%, which isabout 20 times theSwissaverage!Another aspect is that an extensivechange in treecomposi- tion of the forest is taking placeaway from pine to- wardsdifferent broadleaf species,especially the pu- bescent oak.Initial results ofan ongoing research project, which is part of the research programme Recording plant locations on a windthrow site, Field observations during theThirdNational For- est Inventory:The researchers compile valuable data on the state of the forest and changes that have occurred.
“Forest Dynamics”,suggest the followingcausation scenario:
Valais isan innerAlpinedry valley with gener- ally low precipitationand agreat deal of sunshine.
Theclimate inValais, as elsewhere inSwitzerland, hasbecomeconsiderably warmer in the past centu- ry,particularly since the eighties, whereas precipita- tion levels have remainedmuch the same.This has led toan increase in evapotranspirationand thus in aridity.Although the subboreal forest pinecancope with singledrought years,severaldrought years in arow can cause it tobecome permanently weak- ened.Asaresult,it seems tobe more susceptible to pine-specificstress factors such as mistletoe in- fection,stand competition,phytopathogens (plant diseases), insects and nematodes (roundworms).
These may in the end actually killa tree.Thecur- rently very noticeable expansion of the pubescent oak has mostly todo with thechanges in forest use in recent decades (theabandonment of forest pas- turing for sheepandgoatsas wellas of littercollec- tion, andreduced timber harvesting).
The large-scalechanges in the pine forests in Valais according to WSL’s research cannot be ex- plained by any single triggering factor.Rather they involvean interaction of many mutually influenc- ing (interdependent) factors.
Our current level of research is not enough for us to understand the importance of individual impact factors in the interactions within the ecosystem. The influence of the different factors can be enhanced or cancelled out in the interplay of such forces as weather condi- tions, site characteristics and air pollutants.
scientists have not yet been able to fully explain the infl uence of any single variable in the forest ecosystem.
SUMMARY
Acting according to
the precautionary principle
The forest has many functions that are impor- tant for people.That is why it is important tocon- tinuously monitor thecondition of the ecosystem
“forest” and to understand the causes of chang- es.Ourcurrent level of research is not enough for us to understand the importance of individual im- pact factors in the interactions within the ecosys- tem.The influence of the different factorscan be enhancedorcancelledout in the interplay of such forces as weather conditions, site characteristics and air pollutants.
