Stem radius changes of trees are mainly induced by tree water relations altering the pressure conditions and thus the size of living and woody tissues in the stem, wood and bark growth, the degradation of phloem cells, and thermal expansion and contraction of the stem. These stem radius fluctuations are measured by dendrometers.
Units, terms, definitions
Stem radii can either increase or decrease over time. This change in radius is abbreviated with DRt μm. t defines the time period over which the radius change is integrated.
Origin of stem radius fluctuations
Stem radius changes (DR) are determined by five components:
(i) tree water relations altering the pressure conditions and thus the water content and the size of living and woody tissues in the stem,
(ii) wood and bark growth (cambial activity),
(iii) the degradation of dead phloem cells,
(iv) freeze and thaw induced fluctuations of the stem, and
(v) temperature induced expansion or shrinkage of wood.
Swelling/shrinking or mechanical deformations of the dead outermost layer of the bark and temperature sensitivities of the dendrometers are (vi) artefacts and need to be reduced to a minimum when measuring DR.
(i) Plants in general and trees in particular are hydraulic systems in which all living cells are interconnected with each other by water columns (Lockhart, 1965; Molz & Klepper, 1973; Zweifel et al., 2007). Thus, pressure changes and water deficits in one part of the plant are transmitted into other plant parts and are, depending on water availability and hydraulic resistances, levelled off over time (van den Honert, 1948; Steppe et al., 2006). The living cells of the wood and bark act as a water storage location in the tree’s water flow and storage system and are hydraulically connected to the water flow in the dead conducting elements of the xylem (Perämäkiet al., 2001; Zweifel et al., 2001; Steppe et al., 2006; Sevanto et al., 2008). Negative pressures in the xylem induced by transpiration determine a contraction in the wood (Irvine & Grace, 1997; Sevanto et al., 2002; Daudet et al., 2005), in the cambium (Drew et al., 2010) and in the bark (Breda & Granier, 1996; Steppe et al., 2006; Zweifel et al., 2006; Drew et al., 2008). Depending on the tree species, the biggest fraction of this reversible, pressure induced DR is usually originated in the bark (Zweifel & Häsler, 2001) but also the cambium and the xylem including its cell walls undergo a volume change depending on the actual tree water status at the point of measurement (Sevanto et al., 2002; Daudet et al., 2005; Sevanto et al., 2005; Sevanto et al., 2008). The loss of volume is proportional to the loss of water out of the respective tissues and therefore proportional to the measured DR (Zweifel et al., 2000; Steppe et al., 2006 ).
The reversible fraction of DR, which is induced by tree water relations, lasts from minutes to weeks and can either be positive or negative, depending on the changing turgor of the stem tissues and the respective drought conditions of the plant’s environment (Zweifel et al., 2006; Zweifel et al., 2007; De Swaef & Steppe, 2010; Zweifel et al., 2010). In dry periods, the daily balance between water uptake and water loss can be negative, and therefore lead to tree water deficits lasting several weeks with shrinking stems even during the wood growth period (Zweifel et al., 2005; Drew et al., 2008; De Swaef et al., 2009). On a daily scale, cells lose water during periods of transpiration (daytime) and are replenished during night and rainy or foggy periods. Accordingly, stems shrink during the day and expand at night (Steppe et al., 2006; Steppe et al., 2008b).
(ii) In addition to this rather short-term rhythm of water induced stem radius changes, seasonal growth periods contribute to the dynamics of DR (Kozlowski & Winget, 1964; Downes et al., 2009). New xylem and phloem cells are built and elongated to their predisposed size (Lockhart, 1965). Both processes of cell division and cell elongation are turgor-pressure dependent and therefore also affected by tree water relations (De Schepper & Steppe, 2010). In the succeeding wood formation process, the juvenile xylem cells become lignified and die when mature (Drew et al., 2010) and the stem size is only little altered by these woody structures (see above). By contrast, newly built phloem cells remain living and are not lignified. Thus, phloem cells remain elastic and undergo diurnal water-related size changes even when mature.
(iii) In contrast to the lignified xylem elements, phloem cells shrink, break down, die and, finally, are shed (Lockhart, 1965; Rossi et al., 2008; Gricar et al., 2009). This fraction of DR is not well understood so far but might significantly contribute to shrinking stems particularly during wintertime and in tree species with low (wood) growth rates.
(iv) Freeze and thaw events of tree stems are a special aspect of water induced stem radius changes since the effect on DR is induced by a rapid dehydration of living tissues (mainly phloem cells) short before ice is built in the bark and by a rehydration of these tissues with thawing (Zweifel & Häsler, 2000; Ameglio et al., 2001; Ameglio et al., 2002; Daudet et al., 2005). A potential mechanism assumes that water is withdrawn from living cells to increase the osmotic potential, thus, decreasing the freezing point and therefore protect the living cells from freezing damages. DR strongly decreases during these freezing events and this despite the fact that ice needs more volume than liquid water. This finding indicates that the water removed from living cells freezes in gas-filled inter-cellular spaces with no effects on DR. As soon as the temperature returns above the freezing point the contracted tissues are re-hydrated and DR increases in the same magnitude as it has decreased before.
(v) The coefficient of thermal expansion of dry wood varies between 15-35*10-6 K-1. For a tree stem with 25 cm in diameter, this results in 4-9 μm expansion per K temperature increase, what is mostly relatively small compared to diurnal amplitudes of DR.
(vi) The most common artefact in DR measurements is the temperature sensitivity of the dendrometer itself. The mechanical frame, the anchoring in the stem, and also the electronic devices and their power sources are potentially temperature sensitive. It is therefore essential to test any type of dendrometers for its temperature sensitivity by mounting the device on a stone column or a similar temperature-insensitive object and compare the measurements with the temperature readings. Other sources of measurement artefacts are the swelling or shrinkage of the dead outermost layer of the bark. Generally, any changes in DR induced by dead tissue outside the cork cambium are to be judged as artefacts, e.g. mechanical bending of drying and shed pieces of he bark.
Point dendrometers automatically detect stem radius changes on a single point on the stem. There exist different anchoring and frame approaches. Usually a frame with the electronic device attached to it (e.g. a linear variable displacement transducer (LVTD) or a potentiometer) is anchored with three rods in the heartwood of the stem. The sensor head is placed on the bark surface where the dead outermost layer of the bark has been removed. A good point dendrometer has a temperature sensitivity < 1 μm ∘C-1 and a resolution of < 1 μm.
The frame can also enclose the entire stem (often used for small-diameter stems) enabling the mounted sensor (either point dendrometer or LVDT) to measure whole stem diameter variations (Steppe & Lemeur, 2004).
Band dendrometers detect stem circumference changes and are available for manual and automated readings. Since the electronic devices of band dendrometers and the long metal bands are more temperature sensitive than point dendrometers the readings are usually less precise but include the entire circumference and not just a single spot of the stem. Another source of noise of the band dendrometer is the shrinking/swelling or mechanical bending of the dead outermost layer of the bark since it is hard to properly remove this dead layer all around the stem under the band without injuring the living tissue.
Measured fractions and ranges of DR
Fagus sylvatica (Steppe et al., 2006; Steppe et al., 2008b)
Quercus robur (Steppe et al., 2008b; De Schepper & Steppe, 2010)
Malus domestica (Steppe et al., 2008a; De Swaef et al. 2009)
Solanum lycopersicum (De Swaef & Steppe, 2010)
- Tree water relations (conductance, water flow and storage, tree water deficit, water potentials, winter dehydration = frost shrinkage, etc.)
- Wood growth (cambial activity, wood anatomy, tree ring development, etc.)
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