On slip pulses at a sheared frictional viscoelastic/ non deformable interface

We study the possibility for a semi-infinite block of linear viscoelastic material, in homogeneous frictional contact with a non-deformable one, to slide under shear via a periodic set of ``self-healing pulses'', i.e. a set of drifting slip regions separated by stick ones. We show that, contrary to existing experimental indications, such a mode of frictional sliding is impossible for an interface obeying a simple local Coulomb law of solid friction. We then discuss possible physical improvements of the friction model which might open the possibility of such dynamics, among which slip weakening of the friction coefficient, and stress the interest of developing systematic experimental investigations of this question.Comment: 23 pages, 3 figures. submitted to PR

Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations

In order to predict accurately how elevated atmospheric CO2 concentrations will affect the global carbon cycle, it is necessary to know how trees respond to increasing CO2 concentrations. In this paper we examine the response over the period AD 1895 – 1994 of three tree species growing across northern Europe to increases in atmospheric CO2 concentrations using parameters derived from stable carbon isotope ratios of trunk cellulose. Using the isotope data we calculate values of intrinsic water-use efficiency (IWUE) and intercellular CO2 concentrations in the leaf (ci). Our results show that trees have responded to higher levels of atmospheric CO2 by increasing IWUE whilst generally maintaining constant ci values. However, the IWUE of most of the trees in this study has not continued to rise in line with increasing atmospheric CO2. This behaviour has implications for estimations of future terrestrial carbon storage