Fossil wood is abundant in sediments that were deposited at high latitudes during the Cretaceous. The wood provides evidence for forests growing in high latitudes in the past, at times when climate was warmer during greenhouse periods. However, computer climate models that try to simulate past climates cannot reproduce the polar warmth necessary for the existence of forests but more commonly generate ice-covered polar regions. This may be due to the lack of a realistic vegetation component in the models. The aim of this project is to determine the leaf habit and leaf life-spans (LLSs) of conifers growing at high latitudes during the mid-Cretaceous. The results will be used to test the outputs of the University of Sheffield Conifer Model (USCM), a model that will produce a new reconstruction of high latitude vegetation for coupling with computer climate models to determine the effects of high latitude vegetation on climate systems in the past. Fossil conifer wood is studied from mid-Cretaceous sediments from the Canadian Arctic, Svalbard, Australia and Antarctica, all high latitude sites during the mid-Cretaceous. Identification of the woods indicates that Northern Hemisphere forests were composed of Piceoxylon, Laricioxylon, Cedroxylon, Taxodioxylon, Pinuxylon, Palaepiceoxylon, Taxoxylon, Juniperoxylon, Protocedroxylon, Araucariopitys, Xenoxylon and Cupressinoxylon. The Canadian Arctic was dominated by Pinuxylon and Svalbard by Taxodioxylon. In the Southern Hemisphere the conifers include Podocarpoxylon, Taxodioxylon, Araucariopitys, Cupressinoxylon, Agathoxylon, Protophyllocladoxylon, Sciadopityoxylon and Circoporoxylon. Antarctica was dominated by Taxodioxylon and in Australia the woods were dominated by Podocarpoxylon. Palaeoclimate analysis using nearest living relative and growth ring analyses indicate that the mid-Cretaceous climate in the Canadian Arctic was cool temperate and Svalbard was cool to warm temperate. In the Southern Hemisphere Australia appears to have been cool temperate whilst in Antarctica it is only possible to define the climate as temperate. A new technique was developed to determine the leaf habit (evergreen or deciduousness) and LLSs of modern conifers using wood anatomy. This method quantitatively characterises cell patterns within growth rings as the Ring Markedness Index (RMI, a measure of the markedness of the ring boundaries). The RMI has been shown to have a strong logarithmic relationship to leaf life-span and can therefore be used to determine the leaf life-span of fossil woods. Results of RMI analysis on fossil conifer woods provide an indication of the LLSs of mid-Cretaceous conifers for the first time. Both northern and southern hemisphere forests were dominated by evergreen conifers, with only a small component of deciduous conifer types. The dominant LLS in both Northern Hemisphere sites (the Canadian Arctic and Svalbard) was 33 months. In the Southern Hemisphere the dominant LLS in Australia was 63 months, on James Ross Island, Antarctica 48 months and on Alexander Island, Antarctica it was 151 months. Comparison with leaf habit and LLS predictions from USCM indicate a reasonable match, verifying model output. The Conifer Model will now be coupled with regional and global climate models to examine the magnitude of the effect of conifer forests in the polar regions during the mid-Cretaceous greenhouse climate