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The vegetation of Niwot Ridge has a rich history of study, beginning with phytosociological studies directly on the Ridge and in the surrounding mountains and incorporating more experimental and dynamic approaches in later years. This chapter provides an overview of the spatial patterns of Niwot Ridge plants and plant communities relative to the primary controlling environmental gradients at scales from the individual to the landscape. The spatial patterns of vegetation at all scales are dominated by physical forces, particularly the interaction of wind, snow, and topography. The controls of biotic factors on the distribution and abundance of plant species on Niwot Ridge have received considerably less attention than have physical factors, but recent studies have revealed the importance of competition and certain mutualisms in structuring community composition. Community research on Niwot Ridge has been organized around a hierarchy of spatial scales, from the plot to the region. Plot-based studies have focused on physiological and ecological dynamics of specific species and communities, and more spatially extensive studies have provided a hierarchical framework for the plot studies. In this chapter, we first present an overview of the broader patterns in the vegetation, followed by descriptions of the communities, and then the specifics of physical and biotic controls on species and plant growth that drive the community patterns. The landscape-scale patterns in the Niwot vegetation are driven by a complex elevation gradient, which is a combination of temperature and snow regime, with wind modifying and interacting with temperature and snow at all points along the gradient (chapter 2). Certainly the most critical boundary in the system is the upper tree limit, which defines the alpine system and which lies roughly between 3400 and 3600 m elevation on Niwot Ridge. Billings (1988) provided a climatic-floristic-physiographic review of major North American alpine systems that helps to place Niwot Ridge into a larger perspective. Climatically, Niwot is intermediate between the dry Sierras, which have greater precipitation but almost none of it falling during the summer, and the wetter northern Appalachians (Mt. Washington), which have fairly even annual precipitation and no drought.

The Pleistocene Epoch, often referred to as the Ice Age, lasted from approximately 2.6 million to 11,700 years ago. The last major ice advance began about 110,000 years ago, and the most recent episode of maximum ice coverage, the Last Glacial Maximum, began about 26,500 years ago and ended approximately 19,000 years ago. Thereafter, glacier retreat began, largely ending by about 11,700 years ago. That marked the beginning of the Holocene interglacial geologic epoch, which continues to the present. During the last glacial period, sea level was much lower because so much water was locked up in ice sheets, largely at the poles. This lowering of the sea level exposed the margins of the continents (the continental shelves) around the world. When the Ice Age ended, sea level started to rise during the deglacial period, a process that continued into the Holocene. Deltaic regions received meltwaters from the thawing glaciers, along with glacier- derived sediments. Of particular note in the late Holocene is a climate episode called the Medieval Warm Period, originally identified by the English botanist Hubert Lamb. The Medieval Warm Period was a time of warm climate in the North Atlantic region and may have also impacted other areas around the world. It lasted from about the years 950 to 1250. Later in this chapter, I will discuss this climate anomaly, along with something called the “Hockey Stick” debate, which relates to exceptional warming during recent centuries of the Holocene (i.e., global warming). In any case, all modern and paleodeltas formed during periods of peak sea level in the Holocene. These new deltas had fertile soils that were constantly irrigated by the flow of fresh water, which promoted early settlement by humans. So, the Holocene started near the end of the retreat of the Pleistocene glaciers, and human civilizations arose entirely in the Holocene Epoch. To view the Holocene, simply look around you today. In this chapter, I will explore the natural and human-induced causes of global climate change and how they impact deltaic regions.