Why trees grow at night

Abstract

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.

Keywords: cell turgor threshold; climate change; day-night radial stem growth; dendrometer; ecophysiology; photoperiod; wood and bark formation; xylogenesis.

Fig. 1

Location and characteristics of the 50 TreeNet sites in Switzerland (www.treenet.info). Seven tree species including data of 170 individually measured trees (Supporting Information Table S1) were investigated with a total of 57 million underlying data points at a 10‐min resolution (species‐specific sums, panel top right) that were aggregated to hourly averages for analyses. The background colour of the map indicates the average annual precipitation. The site dots are colour‐coded according to the mean annual temperature shown in the boxplot panel (lower left). The boxplots indicate the median (horizontal bar) mean annual temperature, with the boxes representing the 25–75% quartile range and the whiskers the lowest and highest value within 1.5× the interquartile (IQR, 75–25%) range. Circles define outliers beyond the 1.5×IQR. Mean annual temperature and precipitation cover a large gradient from 4.5 to 11.9°C and from 540 to 1700 mm, respectively. The photograph shows a point dendrometer mounted on a beech tree and the panel below gives the corresponding growth performance over 8 yr, highlighting the years with the lowest and the largest stem radial growth.

Fig. 2

Diel growth of seven temperate tree species. Boxplots show median, 25 and 75 percentiles and the SD of the pooled data for all species (a–c). Lines show the medians of each species (d–f). The relative contribution of a specific hour to the annual growth is shown in (a, d). The probability for growth quantified as number of hours with growth relative to the total number of hours in the stem growth period is shown in (b, e) and the median hourly growth rates of hours with growth are shown in (c, f).

Fig. 3

Hourly‐resolved, radial stem growth and the corresponding uncertainty analysis in the measured space of vapour pressure deficit (VPD) and soil water potential (SWP) across all species. (a) Growth was quantified as the relative hourly contribution to the total annual growth (per grid element) and ranged from white (no growth, 0%), over red (marginal growth, 0.02%) to dark green (high growth, 0.06%). (b) The coefficient of variation (CoV) of the uncertainty analysis indicated the robustness of the results between very good (values < 10, dark blue), good (10 to < 20), and satisfactory (20–50), to poor (> 50, light blue to white). The interpolation output of the contour diagram was restricted to the effectively measured range of environmental variables. Grey areas indicate no data. Species‐specific growth responses can be found in the Supporting Information Fig. S3.

Fig. 4

Diel radial stem growth in relation to (a) vapour pressure deficit (VPD) and (b) soil water potential (SWP). Growth is colour‐coded and ranges from white (no growth, 0%), through red (little growth, 0.02%) to dark green (high growth, 0.06%). The main panel shows the general response over all species whereas the small panels show the species‐specific responses (same axis). The bold black line indicates the median VPD and SWP conditions in the stem growth period (thin black lines indicate 25%‐ and 75%‐quantiles). By contrast, the bold red line shows the same but for hours with growth only. A plot with the underlying measurement data can be found in Supporting Information Fig. S4. Growth is quantified as the relative growth contribution to the total annual growth and is based on aggregated data sets considering the nested design of trees and species within sites. The interpolation output of the contour diagram was restricted to the effectively measured range of VPD and SWP. Grey areas indicate no data.

Fig. 5

Time of day of maximum growth in relation to (a) the number of hours with growth and (b) the radial stem increment per year. Shown are the medians (circles) and the range between the 25%‐ and 75%‐quantiles (lines) of each species (colour‐coded). Dashed lines show the linear regression. **, P < 0.01; ^□, P < 0.1.