Dendrometer and meteorological data from the TreeNet network, year 2014
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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明确，因此前尚未获取高时间分辨率的实测数据。因此，昼夜环境条件对树木生长的影响仍不明确。本研究基于长达8年、累计达5700万条原始数据点，首次对7个温带树种的逐小时径向茎干生长开展了系统性野外监测研究。研究结果表明，树木主要于夜间生长，在午夜后出现生长峰值，此时的水汽压亏缺（VPD）处于最低水平之一。较高的水汽压亏缺会严格抑制径向茎干生长，使得树木在日间除清晨外几乎无法生长。令人意外的是，当水汽压亏缺较低时，即使处于中度干旱的土壤中，树木仍可进行生长。不同树种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，其年累计生长时长最长，年生长量也最大。由此我们得出结论：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，本研究还证实，相较于碳吸收，树木生长对干燥空气更为敏感——因为生长会在气孔闭合前就已停止。
