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Yorkshire Naturalists at Thornton Dale. 
The valley slopes bear three main types of woodland, (1) oak woods, 
(2) ash -oak woods, (3) beech plantations. The last are confined to the 
gentler slopes near Thornton Dale, the oak woods occur higher up the 
valley (High Dalby). In general, the oaks and birches shew a parallel 
distribution. Quercus robur and Betula verrucosa are most abundant at 
the southern end of the valley, while Q. sessilis and B. pubescens are 
almost confined to the more acid soils at the higher northern end. 
The variation in the composition of the woods appears to be due to 
changes in the underlying strata, and variations in the depth of the 
soil. The operation of these factors is well illustrated in Dalby Nut 
Wood. While most of this wood is developed over a slighly calcareous 
sandstone, the upper part reaches the cap of oolitic limestone which here 
covers the plateau. Owing to soil movement on the steep slopes, the 
soil is deepest and least calcareous towards the base of the wood — it 
is shallowest and contains . most lime near the top. The upper part of 
the wood contained (in round figures) 60% ash and 25% oak, with a 
ground flora of Mercurialis and garlic. The lower part 60% oak, 15% 
ash, and about 15% birch, with a rather open ground flora of Anemone, 
Oxalis, Luzula pilosa and Mnium hornum. The soil in the last case was 
distinctly acid. This wood is unfortunately being cut down. 
A similar effect, due to the arrangement of the strata, is seen south 
of this point, where the wood had long been removed. The lower half 
of the slop is clothed with Pteris on Agrostis — Anthoxanthum grass- 
land — on fairly deep acid soil. The upper half of the slope is dry 
oolitic rubble, on which great colonies of Mercurialis, Convallaria, 
Hypericum hirsutum, Inula Conyza and columbine still persist. 
Around Ellerburn there are definite evidences of the succession of 
woodland vegetation. In some of the woods there were practically no 
oaks. These woods had been cut about twenty years previously, and ash 
had recolonised the site more quickly than the oak which formerly had 
been present in fair quantity. On the quarry wastes ash is usually the 
most abundant tree, but sycamore runs it very close. The relative 
preponderance of these two trees (as seedling colonists) seemed to be 
determined very largely by the amount of light. In the beech plantations 
sycamore was much the most abundant, but round the wood margins 
(in high light intensity) ash was more successful. 
The woods north of High Dalby are developed on soils derived from 
slightly calcareous sandstone. The soil is open, dry and thoroughly 
leached (acid). Here are the oak woods in which Quercus sessilis is 
abundant, birches (chiefly B. pubescens) being only frequent round the 
edges of the wood. The ground flora is of the Anthoxanthum-Dicranum- 
Pteris type, and where the woods have been cut, Calluna rapidly becomes 
sub-dominant. This. type of woodland now remains as a whole only on 
the south-facing slopes, the north-facing slopes being usually covered 
with heather. A striking exception, however, is the wood on the Crosscliff 
escarpment, a north-facing slope of one in three. This wood is composed 
of Oak (60% both species), Birch (20% both species) and Mountain 
Ash (20%). The ground flora is unusual — principally Dryopteris 
dilatata, V accinium Myrtillus , Oxalis and Dicranum ma,jus with other 
heathy species. Although the soil is an open peat, and very acid, it 
only shews. a very slight deficiency of bases (Comber’s test). The light 
intensity was only 17-20% of full sunlight at 2 p.m. Calculation shews 
that on a similar south slope, the light intensity under the same trees 
would be about 40%, a value actually obtained in the High Dalby Wood 
(at 10 a.m.). The difference in the amounts of radiation thus indicated 
would, of course, be much more striking earlier or later in the day — when 
a north slope may receive no direct radiation, and as a result, not only 
will the daily light conditions be much less favourable, but the soil 
would also be colder and moister. 
The upper edge of the Crosscliff escarpment is crowned by a very 
Naturalist 
