Poplars are one of the woody plants that are very sensitive to water stress, which may reduce the productivity of fast-growing plantations. Poplars can exhibit several drought tolerance strategies that may impact productivity differently. Trees from two improved hybrids, Populus balsamifera × Populus trichocarpa Torr. & Gray (clone B × T) and P. balsamifera × Populus maximowiczii A. Henry (clone B × M), having P. balsamifera L. as a parent and trees from native and unimproved P. balsamifera were subjected to a 1-month drying cycle in a growth chamber and then rewatered. The unimproved and native B clone maintained higher stomatal conductance (g(s)) than the hybrids, and high photosynthetic activity and transpiration, even when soil water content was nearly zero. As a result, both instantaneous water use efficiency (WUE(i)) and leaf carbon isotope composition (δ(13)C) indicated that this clone was less affected by drought than both hybrids at maximal drought stress. However, this clone shed its leaves when the drought threshold was exceeded, which implied a greater loss of productivity. The B × M hybrid showed a relatively conservative response to water stress, with the greatest decrease in transpiring versus absorbing surface (total leaf area to root biomass ratio). This clone was also the only one to develop new leaves after rewatering, and its total biomass production was not significantly decreased by drought. Among the two hybrids, clone B × T was the most vigorous, with the greatest transpiration (E(i)) and net CO(2) assimilation (A) rates, allowing for high biomass production. However, it had a more risky strategy under drought conditions by keeping its stomata open and high E(i) rates under moderate drought, resulting in a lower recovery rate after rewatering. The opposite drought response strategies of the two hybrids were reflected by clone B × T having lower WUE(i) values than clone B × M at maximal drought, with a very low Ψ(min) value of -3.2 MPa, despite closed stomata and stopped photosynthetic activity. Positive linear relationships between A and g(s) for the three hybrids indicated strong stomatal control of photosynthesis. Moreover, the three poplar clones showed anisohydric behaviour for stomatal control and their use under long-term drought should be of interest, especially the B × M clone.

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Estimation of litter colonization by fungi, using ergosterol, an indicator of fungal biomass, is a reliable way to describe the process of leaf litter decomposition. This litter colonization by fungi is regulated both by exogenous or environmental factors, and endogenous factors, i.e. litter chemistry. In this work, we have examined the effects of some of these factors on litter fungal colonization in a Mediterranean ecosystem, by determining ergosterol content of Quercus coccifera leaf litter. Environmental factors have been studied through the fertility of the soil, by comparing plots amended with two rates of compost and plots without amendment. Results indicated that (i) compost had a significant effect on soil fertility but did not increase ergosterol content of leaf litter and (ii) soil humidity improved leaf litter colonization by fungi. Endogenous factors have been studied through measurements of total phenolic and ergosterol concentrations of seven shrub species leaf litter. We have shown (i) a negative significant correlation between total phenolic compounds and ergosterol concentrations of leaf litter and (ii) a positive significant correlation between total phenolic compound concentrations in green leaves and in leaf litter. We conclude that, in this Mediterranean shrub ecosystem, leaf litter colonization by fungi is controlled by soil moisture and plant leaf litter quality.