Priority effects occur when species that arrive first in a habitat significantly affect the establishment, growth, or reproduction of species arriving later and thus affect functioning of communities. However, we know little about how the timing of arrival of functionally different species may alter structure and function during assembly. Even less is known about how plant density might interact with initial assembly. In a greenhouse experiment legumes, grasses or forbs were sown a number of weeks before the other two plant functional types were sown (PFT) in combination with a sowing density treatment. Legumes, grasses or non-legume forbs were sown first at three different density levels followed by sowing of the remaining PFTs after three or six-weeks. We found that the order of arrival of different plant functional types had a much stronger influence on aboveground productivity than sowing density or interval between the sowing events. The sowing of legumes before the other PFTs produced the highest aboveground biomass. The larger sowing interval led to higher asymmetric competition, with highest dominance of the PFT sown first. It seems that legumes were better able to get a head-start and be productive before the later groups arrived, but that their traits allowed for better subsequent establishment of non-legume PFTs. Our study indicates that the manipulation of the order of arrival can create priority effects which favour functional groups of plants differently and thus induce different assembly routes and affect community composition and functioning.

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In this study two crop species, winter wheat (<i>Triticum aestivum</i>) and sugar beet (<i>Beta vulgaris</i>), were monitored over the course of five days during the entire season. We investigated the link of the main physiological leaf-level mechanisms, stomatal conductance and efficiency of photosynthetic energy conversion on canopy transpiration and photosynthetic CO₂ uptake. The physiological status of 900 leaves of different plants in a natural canopy was characterized on the leaf level using chlorophyll fluorescence. Gas exchange measurements were performed at leaves of 12 individual plants of each species. Eddy covariance flux measurements provided information on CO₂, water and energy fluxes on the field scale. The diurnal pattern of stomatal resistance on the leaf level was especially for sugar beet similar to the canopy resistance, which indicates that stomatal resistance may have a large impact on the bulk canopy resistance. The diurnal changes in canopy resistance appeared to have less effect on the evapotranspiration, which was mainly dependent on the amount of incoming radiation. The similar diurnal pattern of water use efficiency on the leaf level and on the canopy level during the day, underline the influence of physiological mechanisms of leaves on the canopy. The greatest difference between water use efficiency on leaf and canopy occurred in the morning, mainly due to an increase of stomatal resistance. Limitation of CO₂ uptake occurred in the afternoon when water vapor pressure deficit increased. Maxima of net ecosystem productivity corresponded to the highest values of photosynthetic capacity of single leaves, which occurred before solar noon. Within the course of a few hours, photosynthetic efficiency and stomatal resistance of leaves varied and these variations were the reason for diurnal variations in the carbon fluxes of the whole field. During the seasonal development, the leaf area index was the main factor driving carbon and water exchange, when both crops were still growing. During senescence of winter wheat these structural parameters did not account for changes in canopy fluxes and remaining high green leaf material of sugar beet did not present the reduction in canopy fluxes due to beginning dormancy. We thus hypothesize that the functional status of plants is also important to correctly predict carbon and water fluxes throughout the season. We propose to additionally include the physiological status of plants in carbon flux models in order to improve the quality of the simulation of diurnal patterns of carbon fluxes

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