TreeNet

The biological drought and growth indicator network

Current drought effects on forest trees in Switzerland (1)

Trees in the Northern Alps and in the Valais show distinctly higher tree water deficit-induced stem shrinkage (TWD) than in the years before. We conclude that the investigated tree species in these areas are more heavily affected by drought than in any of the years measured before (since 2011). Forest trees in the Swiss lowlands are not strongly affected by drought yet.

TreeNet detects how much tree stems shrink during dry periods with point dendrometers. This so-called tree water deficit (TWD) indicates how much trees are suffering from dry conditions. The higher the value is the more a tree is thirsty. We compared the average TWDs of the past five days (11-16 July 2018) with TWDs of the same period of the years before (starting in 2011) in order to quantify current levels of drought stress.

 

The figure shows three regions of Switzerland: the region in the northern part of the Alps (Alp Nord), the Valais, a valley located in the central Alps, and the lowlands north of the Alps (Mittelland). The red columns indicate the mean tree water deficit (TWD) of the five days from 11-16 July 2018 and the orange columns indicate the mean value of the years 2011-2017. The black symbols indicate the range of values occurring during this time period. The figure includes several tree species.

The figure shows TWDs of different tree species within each of the three regions. In the northern Alps fir (Tanne), pine (Föhre), and spruce (Fichte) currently show higher mean TWDs than in the years before. The same is true for the species beech (Buche), oak (Eiche), pine (Föhre), and spruce (Fichte) in the central Alpine valley Wallis. Somewhat increased but not yet beyond the range of past years are TWDs measured in the Swiss lowlands (Mittelland).

 

TreeNet Switzerland collects continuous data on stem radius fluctuations measured with point dendrometers from trees all over Switzerland and estimates drought and growth indicators for Swiss forest ecosystems. We closely collaborate with the Long-term Forest Ecosystem Research Programme (LWF/WSL), the ETHZ and the Institute for Applied Plant Biology (IAP).

For more information about the actual drought in Switzerland refer to drought.ch

See also:

Current drought effects on forest trees in Switzerland (2)

Current drought effects on forest trees in Switzerland (3)

TreeNet.info in the News 

 


New quickviews of current raw data available

TreeNet raw data of the past 7 days are now freely browsable. Try the following links to see the latest data of

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Are there growth trends of the Swiss tree species over the past 5 years?

We are currently analyzing the drivers of wood growth of Swiss forest trees over the past five years (preliminary results).

Figure 1  Growth trends of the main tree species in Switzerland. Radial growth: radial increment per year; Cross section area growth: annual increment of the trees’ cross section area; Relative growth: Annual increment divided by the stem radius (preliminary results).

Wood growth is the result of determining factors at the time of growing but also a response to environmental conditions in the past (carry-over effects).  Carry-over effects include conditions of hours, days or even years ago. It includes e.g., the determination of buds in autumn of the past year but also the life-span of different organs of a tree like the sapwood, the leaves or the carbon storage pool.

No general growth trend was found (Fig. 1). However, there appear decreasing trends for some species over the past four years (2012-2015). Generally, the standard deviation from the respective means is large and there appear different patterns for different species.


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Are trees able to grow in periods of stem shrinkage?

Publication in New Phytologist about separating stem radius measurements into growth and tree water deficit

Summary

  • Separating continuously measured stem radius (SR) fluctuations into growth-induced irreversible stem expansion (GRO) and tree water deficit-induced reversible stem shrinkage (TWD) requires a conceptualization of potential growth processes that may occur during periods of shrinking and expanding SR below a precedent maximum. Here, we investigated two physiological concepts: the linear growth (LG) concept, assuming linear growth, versus the zero growth (ZG) concept, assuming no growth during periods of stem shrinkage.
  • We evaluated the physiological mechanisms underlying these two concepts and assessed their respective plausibilities using SR data obtained from 15 deciduous and evergreen trees.
  • The application of the LG concept produced steady growth rates, whereas growth rates varied strongly under the ZG concept, more in accordance with mechanistic expectations. Further, growth increased for a maximum of 120 min after periods of stem shrinkage, indicating limited growth activity during those periods. However, this extra growth was found to be a small fraction of total growth only. Furthermore, TWD under the ZG concept was better explained by a hydraulic plant model than TWD under the LG concept.
  • We conclude that periods of stem shrinkage allow for very little growth in the four tree species investigated. However, further studies should focus on obtaining independent growth data to ultimately validate these findings.