1918] ROBBINS—BOULDER PARK 521 
Succession involves the glacial lakes, and the flood plains with 
deposition banks of silt and sand; the xerarch succession involves 
the gravels laid bare by the rapid drainage of a glacial lake which 
at one time occupied the greater part of the Park, and the flood 
plains with shores of coarse gravel or shingle. The stages in these 
series lead to a temporary. meadow climax (subclimax). The 
climax is reached much sooner by the hydrarch than by the xerarch 
series. In fact, much of the high terrace is now in the dry grassland 
stage, which under existing environmental conditions, incident 
to the topographical setting of the Park, promises to be long stand- 
ing. However, there is ample evidence that even under these 
circumstances it is being slowly replaced by meadow. More- 
over, the physiographic operations now in progress, namely, the 
erosion of Boulder, Meadow, and Trestle Creeks, and the 
accumulation of wash material at the bases of slopes, look toward 
the disappearance of the dry grassland habitat and the ushering in 
of meadowland. 
The question now arises, are trees advancing upon the meadow ? 
The forest associations bordering the Park are as follows: (1) aspen 
(Populus tremuloides Michx.), (2) lodgepole pine, (3) (Pinus Mur- 
rayana Oreg. Com.), and (3) Engelmann spruce-subalpine fir 
(Picea Engelmannii [Parry] Engelm.—Abies lasiocarpa (Hook.] 
Nutt.). Aspen often forms a fringe between the coniferous asso- 
ciations and the meadow of the open Park. This relation prevails 
throughout the Rocky Mountain region. Wherever the develop- 
mental series has led up to the meadow stage, however, as it has 
in Boulder Park, this stage bids fair to hold its ground against the 
invasion of trees, thus constituting a climax (subclimax) of long 
duration. The principal factor involved here is competition. It 
may occur to one that, although the competition of meadow species 
prevents the forestation of the open Park, there is nothing to prevent 
the dry grassland with its abundance of open ground being invaded 
by trees. In this connection it may be said that the exposure of 
the dry grassland to excessive evaporation as conditioned by wind, 
temperature, and the lack of a snow cover makes a situation in 
which trees find it impossible to get a start. The climatic climax 
of the region is a forest of Englemann spruce-subalpine fir. 
