Jennings : A Botanical Survey of Presque Isle. 341 



The Toxicodendron Thicket-formation. 



The effect of the Andropogo'n formation upon the the habitat is, likely, 

 the addition of considerable humus, not at the surface of the soil, but 

 by the decay of roots and buried limbs, etc., in the lower layers. At 

 the same time the soil is made firmer by the binding action of the 

 roots, and the surface becomes more protected by the growth of the 

 grass. 



Under these conditions there soon appears a new set of species con- 

 stituting the Toxicodendron thicket-formation having the following 

 structure : 



Fades. — 



Toxicodendron pubescens. 



Principal Species. — 



Celastrus scande?is, Vitis vulpina. 



Primus serotina, Rubus allegheniensis, 



Priinus pennsylvnnica, Popultis deltoides. 



Secondary Species. — 



Rubus occidenialis, Poa compressa, 



Fragaria virginiana, Andropogon furcatus, 



Juniperus virginiana. 



This formation is typically represented in the southeastern half of 

 Long Ridge. The Toxicodendron (poison ivy) occupies almost the 

 entire habitat by a dense thicket composed of stiffly upright, closely 

 growing shrubs, two to five feet high. To the writer the species was 

 quite poisonous, and the thickets impede progress considerably by being 

 bound together by Rubus and Celastrus. Populus deltoides is quite 

 abundant on Long Ridge, but the trees are dying and appear to play a 

 relatively insignificant part in the structure of the formation and they 

 may here be considered relicts of the preceding formation. 



The Arctostaphylos-Juniperus Heath-formation (on ridges). 



Towards the Light House the Toxicodendroti thicket-formation gives 

 way quite suddenly to heath, apparently exactly the same structure on 

 the ridge as on the former sand-plain adjoining (see Plates XXVIII 

 and XXIX). The only difference apparent is the presence on the 

 ridge of the relict Populus and a greater amount of Celastrus scandens. 

 There is no such broad mixed zone on Long Ridge between the Toxico- 



