Abstract
The quantification of cambial growth over short time periods has been hampered by problems to discern between growth and the swelling and shrinking of a tree stem. This paper presents a model, which separates cambial growth and reversible water-potential induced diurnal changes from simultaneously measured whole stem and xylem radial variations, from field-measured Scots pine trees in Finland. The modelled growth, which includes osmotic concentration changes, was compared with (direct) dendrometer measurements and microcore samples. In addition, the relationship of modelled growth and dendrometer measurements to environmental factors was analysed. The results showed that the water-potential induced changes of tree radius were successfully separated from stem growth. Daily growth predicted by the model exhibited a high correlation with the modelled daily changes of osmotic concentration in phloem, and a temperature dependency in early summer. Late-summer growth saw higher dependency on water availability and temperature. Evaluation of the model against dendrometer measurements showed that the latter masked a true environmental signal in stem growth due to water-potential induced changes. The model provides better understanding of radial growth physiology and offers potential to examine growth dynamics and changes due to osmotic concentration, and how the environment affects growth.
Highlights
Tree growth is probably the single most studied process in forest science, but we still do not fully understand its exact dependence upon environmental conditions, at short-term timescales
If the water-induced component of the inner-bark radius, which is related to changes in the xylem water potential, can be separated successfully, we can claim that its remaining increment can be used as a proxy for growth and its changes due to osmotic concentration
Reversible swelling and shrinking Increment and/or change Whole stem radial thickness Inner-bark radial thickness Xylem radial thickness Inner-bark radial change due solely to movement of water between the xylem and inner bark Modelled radial cambial growth and change of radius due to osmotic concentration movement Parameter related to the radial hydraulic conductance between the xylem and inner-bark The ratio of the diurnal amplitudes of Db and Dx when xylem and inner-bark pressure changes are identical Reversible peak in Δ Gm due to changes in osmotic concentration Linear-derived estimate of daily growth from Δ Gm (Db), xylem radius (Dx), xylem radial hydraulic conductance (α) and the ratio of the elastic properties of inner bark to xylem (β)
Summary
Tree growth is probably the single most studied process in forest science, but we still do not fully understand its exact dependence upon environmental conditions, at short-term timescales. Much of the reversible change arise from sap movement from higher to lower water potential along the xylem tract and the exchange of water between the xylem and phloem tissues (Whitehead & Jarvis 1981). These changes can mask short-term growth and hamper our ability to use dendrometer measurements for assessing actual cambial growth. This is especially evident during drought periods, when the stem dehydrates and shrinks and rehydrates after rainfall (Buell et al 1961; Bordiert 1994). Measured stem diameter may change gradually due to change in the moisture content of the bark (Gall et al 2002)
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