Alnus (alder), and Populus (poplar). These plants react upon the habitat by producing shade, lower the water table still further by transpiration, build up the soil, and lead to the accumulation of humus with associated microorganisms. This type of wet woodland is also known as carr. Climax stage Finally a self-perpetuating climax community develops. It may be a forest if the climate is humid, grassland in case of sub-humid environment, or a desert in arid and semi-arid conditions. A forest is characterized by presence of all types of vegetation including herbs, shrubs, mosses, shade-loving plants and trees. Decomposers are frequent in climax vegetation. The overall changes taking place during development of successional communities are building up of substratum, shallowing of water, addition of humus and minerals, soil building and aeration of soil. As the water body fills in with sediment, the area of open water decreases and the vegetation types moves inwards as the water becomes shallower. Many of the above-mentioned communities can be seen growing together in a water body. The center is occupied by floating and submerged plants with reeds nearer the shores, followed by sedges and rushes growing at the edges. Still further are shrubs and trees occupying the dry land. Examples An example is a small kettle lake called Sweetmere, in Shropshire, UK. Sweetmere is one of many small kettle lakes which formed at the end of the last glacial period when the temperatures began to increase. The ice began to melt and retreat approximately 10,000 years ago. As the climate slowly began to warm this allowed algae, water lilies and floating aquatic plants to begin to colonise the lake. These, in essence, were the pioneer species. Once these began to die it provided organic matter to the lake bed sediment and therefore increased fertility and reduced depth. As a result, this allowed deeper rooted species to develop such as reed, bulrush and reedmace. At this point there is a growing floating raft of thick organic matter within the lake. Because the bulrushes and reeds have relatively deep roots, this encouraged bioconstruction which traps more sediment, allowing sedges, willow and alder to become established. This process further decreased the water depth and raised the lakebed thus making it drier. Drier conditions now meant that a wider range of species could inhabit the area. Birch and alder came into dominance. All species which have grown have occurred