Why trees grow at night

Summary The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day–night conditions on tree growth remained uncertain. Here we present the first comprehensive field study of hourly‐resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 yr. We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Species‐specific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close.

. Scheme of processes determining stem radius changes (SRC) measured by point dendrometers mounted on the stem surface, and a data example.      Table S1. Tree species and its occurrence at the different sites with different expositions and vegetation compositions. Table S2. Species-specific averages of vapor pressure deficit (VPD) and soil water potential (SWP) for hours with stem growth. Table S3. Explanatory power of vapour pressure deficit (VPD) and soil water potential (SWP) for diel growth with multiple linear regression models. Table S4. Explanatory power of environmental variables for hourly-resolved diel growth with multiple linear regression models. induce an irreversible expansion of the stem radius, here called growth (GRO). Water-induced changes in the water potential in the stem lead to reversible shrinkage and expansion mainly of the bark. Stem shrinkage is thus the result of water withdrawal from the bark into the xylem and is therefore called tree water deficit-induced shrinkage of the stem (or tree water deficit, TWD). This process is completely reversible, as the water returns to the bark and the stem expands again when the water potential gradient changes. According to the turgor threshold theory (Lockhart, 1965)   show hourly-resolved radial stem growth over 24 hours in relation to soil water potential data from the topsoil and from 2 m soil profiles. Growth is shown as relative contribution of a specific hour to annual growth. The bold black line indicates the median SWP measured during the growth period, the thin black lines correspond to the 25-and 75% quantiles. The temporal growth response pattern largely remained the same, whereas the values on the y-axis of the topsoil profiles were 200-300 kPa higher than in the 2 m profiles. The interpolation output was restricted to the effectively measured range of SWP. Blue areas indicate no data.  S3. Species-specific, hourly-resolved, radial stem growth in the measured space of vapor pressure deficit (VPD) and soil water potential (SWP). Growth was quantified as the relative hourly contribution to the total annual growth (per calculated grid element) and ranged from white (no growth, 0%), over red (marginal growth, 0.02%) to dark green (high growth, 0.06%). The interpolation output of the contour diagram was restricted to the effectively measured range of environmental variables. Grey areas indicate no data. . Growth was quantified as the relative hourly contribution to the total annual growth (per calculated grid element) and ranged from white (no growth, 0%), over red (marginal growth, 0.02%) to dark green (high growth, 0.06%). The bold black line indicates the median VPD and SWP conditions in the stem growth period (thin black lines indicate 25-and 75%-quantiles). In contrast, the bold red line shows the same but for hours with growth only. Data shown are in an hourly resolution and pooled for all species. Contour diagrams (right panels) are based on a local polynomial regression (loess) function that interpolates growth in relation to time of day and environmental conditions. Adj. R 2 give the percentage of variation the model covered.

Fig. S5. Growth response patterns in relation to vapour pressure deficit (VPD) and soil water potential (SWP)
grouped for the months April to August. The growth response patterns appeared to be largely independent from the time within the growth period. Growth was quantified as the relative hourly contribution to the total annual growth (per calculated grid element) and ranged from white (no growth, 0%), over red (marginal growth, 0.02%) to dark green (high growth, 0.06%). The bold black line indicates the median VPD and SWP conditions in the stem growth period (thin black lines indicate 25-and 75%-quantiles). In contrast, the bold red line shows the same but for hours with growth only. Data shown are in an hourly resolution and pooled for all species. Contour diagrams (right panels) are based on a local polynomial regression (loess) function that interpolates growth in relation to time of day and environmental conditions.

Table S1. Tree species and its occurrence at the different sites with different expositions and vegetation
compositions (see also Fig. 1). Sites: number of sites the species is occurring. Trees: number of trees equipped with a dendrometer. RD: underlying raw data points in a 10 min resolution (source: www.treenet.info, data set L2M_2020), ALT: range of altitudes, MAT: range of mean annual temperatures of the sites (sources: www.treenet.info, www.lwf.ch, www.meteoswiss.admin.ch), MAP: range of mean annual sums of precipitation at the sites (source: MeteoSwiss as 'Combiprecip' data combining lidar data with ground-based measurements1), DBH: range of tree stem diameters at breast height (source: www.treenet.info, metadata set L2M_2020), TH: range of tree heights (source: www.treenet.info, metadata set L2M_2020).

Species
Sites [ Table S3. Explanatory power of vapour pressure deficit (VPD) and soil water potential (SWP) for diel growth.
Listed are the results of species-specific multiple linear regression models for growth with the variables VPD, SWP, and the respective interaction (VPD:SWP). adj.R 2 = explanatory power of the model. DR 2 = the explanatory power added by SWP. Significance of results: ns = not significant, * p < 0.05, ** p < 0.01, *** p < 0.001.  Table S4. Explanatory power of environmental variables for hourly-resolved diel growth (relative contribution to annual growth). Listed are the results of species-specific multiple linear regression models for diel, hourly-resolved growth with the variables relative air humidity (RelH), air temperature (Temp), net radiation (Rad), soil water potential (SWP), and the respective interactions rad:swp and temp:swp. Other interactions fell out of the model. R 2 = explanatory power of the growth model. Significance of results: ns = not significant, * p < 0.05, ** p < 0.01, *** p < 0.001.