Using simple causal modeling to understand how water and temperature affect daily stem radial variation in trees

Variation in tree stem diameter results from reversible shrinking and swelling and irreversible radial growth, all processes that are influenced by tree water status. To assess the causal effects of water and temperature on stem radial variation (ΔR) and maximum daily shrinkage (MDS), the diurnal cycle was divided into three phases: contraction, expansion and stem radius increment. Diurnal cycles were measured during 1996-2004 in Picea abies (L.) Karst., Pinus cembra L. and Larix decidua Mill. in a timberline ecotone to understand the links between stem diameter variation (v; defined as MDS or ΔR), phase duration (h), and weather or sap flow descriptors (d). We demonstrated that a high proportion of MDS and ΔR was explained by h because of the nonlinearity of the physiological responses to weather d. By causal modeling, we tested whether the relationship between d and v was due to h (lack of causal relationship between d and v) or to both d and h (double cause). The results of this modeling added new physiological insight into daily growth-climate relationships. Negative correlations were found between ΔR and air temperature owing to the negative effect of temperature on h only, and did not correspond to a direct effect on tree growth mediated by an alteration in metabolic activities. Precipitation had two main effects: a direct effect on ΔR and an indirect effect mediated through an effect on h. A reduction in sap flow at night led to an increase in ΔR for P. abies and L. decidua, but not for P. cembra