Why Intra-Annual Density Fluctuations Should Be Formed at Night? Implications for Climate–Growth Relationships in Seasonally Dry Conifer Forests

Abstract

Trees grow at night, when the vapor pressure deficit (VPD) is low enough. Therefore, intra-annual density fluctuations (IADFs) should be formed when the VPD drops below a certain threshold. This idea is tested by assessing climate-latewood IADF relationships in six conifer species under Mediterranean climate conditions. Hourly climate and dendrometer data were analyzed for years with elevated IADF production in two species (Pinus halepensis, Juniperus thurifera). Lastly, climate–growth relationships were evaluated in two drought-prone sites to assess the relative role of minimum vs. maximum temperatures as growth drivers. Latewood IADF production was positively related to growth rate. IADFs were more abundant when monthly or 10-day long precipitation was high in the late growing season (August and September). According to dendrometer data, growth mainly occurred in early night (20–2 h) and early morning (6–8 h). This growth window corresponded to rainy periods with VPD below a minimum threshold associated with summer storms. Latewood IADFs are produced in response to these wet late-summer conditions, which could be related to bimodal growth. These associations are in line with correlations showing that high minimum (night) rather than maximum (day) temperatures reduce growth. This last idea should be further checked in drought-prone forests using global tree-ring databases.