Vegetation phenology is an important indicator of climate change and climate variability and it is strongly connected to biospheric-atmospheric gas exchange. We aimed to evaluate the applicability of phenological information derived from digital imagery for the interpretation of CO 2 exchange measurements. For the years 2005-2007 we analyzed seasonal phenological development of 2 temperate mixed forests using tower-based imagery from standard RGB cameras. Phenological information was jointly analyzed with gross primary productivity (GPP) derived from net ecosystem exchange data. Automated image analysis provided reliable information on vegetation developmental stages of beech and ash trees covering all seasons. A phenological index derived from image color values was strongly correlated with GPP, with a significant mean time lag of several days for ash trees and several weeks for beech trees in early summer (May to mid-July). Leaf emergence dates for the dominant tree species partly explained temporal behaviour of spring GPP but were also masked by local meteorological conditions. We conclude that digital cameras at flux measurement sites not only provide an objective measure of the physiological state of a forest canopy at high temporal and spatial resolutions, but also complement CO 2 and water exchange measurements, improving our knowledge of ecosystem processes.

 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. Speciesspecific 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.

Summary• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales.• Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms.• We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration.• Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

Summary• Continuous stem radius changes (DR) include growth and water-related processes on the individual tree level. DR is assumed to provide carbon turnover information complementary to net ecosystem productivity (NEP) which integrates fluxes over the entire forest ecosystem. Here, we investigated the unexpectedly close relationship between NEP and DR and asked for causalities.• NEP (positive values indicate carbon sink) measured by eddy covariance over 11 yr was analysed at three time scales alongside automated point dendrometer DR data from a Swiss subalpine Norway spruce forest.• On annual and monthly scales, the remarkably close relationship between NEP and DR was positive, whereas on a half-hourly scale the relationship was negative. Gross primary production (GPP) had a similar explanatory power at shorter time scales, but was significantly less correlated with DR on an annual scale.• The causal explanation for the NEP-DR relationship is still fragmentary; however, it is partially attributable to the following: radial stem growth with a strong effect on monthly and annual increases in NEP and DR; frost-induced bark tissue dehydration with a parallel decrease in both measures on a monthly scale; and transpiration-induced DR shrinkage which is negatively correlated with assimilation and thus with NEP on a half-hourly scale.

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