Dendrometer and meteorological data from the TreeNet network, year 2004

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 years. 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.成熟林木的昼夜茎干生长节律迄今仍未被充分探明，因目前尚未获取高时间分辨率的实测数据。由此，昼夜环境条件对树木生长的影响始终不明确。本研究首次针对7个温带树种开展每小时分辨率径向茎干生长的综合野外监测研究，依托最长达8年周期内的5700万条原始数据点完成分析。研究结果显示，树木主要于夜间生长，生长峰值出现在午夜过后，此时水汽压亏缺（VPD）处于最低水平区间。高水汽压亏缺会严格抑制径向茎干生长，使得白昼时段几乎无生长活动，仅清晨为例外。值得注意的是，当水汽压亏缺处于较低水平时，即使土壤处于中度干旱状态，树木仍可正常生长。不同树种间的昼夜生长动态存在特异性差异：能够在夜间更早启动生长的树种，其年有效生长时长最长，年生长量也最大。据此我们得出结论：能够更快克服每日水分亏缺的树种，具备更高的生长潜力。此外，本研究证实，树木生长对干燥空气的敏感性高于碳吸收过程，因为生长活动会在已知的气孔关闭节点到来前就已停止。