Forests and Water: A State of the Art Review for Colorado

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are being managed for recreation, wildlife, and scenic benefits (DNR, 2002). As noted by Veblen and Lorenz (1991), the overall trend in Colorado is for forests to be used less for timber and grazing and more for recre- ational and residential purposes. The following sections discuss the effects of climatic fluctuations, timber har- vest, fire regimes, grazing, insects and other disturbances on forests, and how these disturbances might affect past, present, and future water yields. 4.1.1. Effects of climatic fluctuations and climate changes on forests and runoff. Changes in climate will affect the distribution, compo- sition, and density of forests over both short and long- term time scales (Joyce and Birdsey, 2000; Veblen et al., 2000). Wetter conditions generally will lead to an increase in forest density, but this effect will occur over decades or even centuries. Increases in forest density and a shift to more mesic species can directly affect the amount of runoff. However, changes in climate can also affect forests and runoff by altering the disturbance regime (Dale et al., 2001), and the resulting changes in forest composition can have a proportionally larger ef- fect on water yields over much shorter time scales. For example, a severe drought can increase tree mortality, which can quickly increase water yields. These increases in water yield will persist until the forest returns to its previous condition, and the time scale for this recovery is much longer than the length of the drought needed to initiate that change. Short- and long -term changes in climate also can affect the risk of wildfires. Fire is an important source of for- est disturbance in Colorado, and both short- and long- term climatic fluctuations can affect the frequency and intensity of fires by altering the frequency and intensity of drought conditions as well as the type and amount of vegetation (Keane et al., 2002). Climatic fluctuations also can affect the susceptibility of forests to beetles and other insects, and this can directly affect the amount of runoff (Love, 1955). These indirect effects of climatic fluctuations and climate change must be considered when assessing the likely response of Colorado's forests to both short- and longer -term changes in climate. Tree -ring records and other physical evidence suggest that the treeline in Colorado shifted upwards around 1250 A.D. (Brown and Shepperd 1995). Growth rates also increased at treeline, but there is not a clear explana- tion for this increase in growth. The resultant increase in vegetation cover and density has probably caused a Proportional reduction in water yields from higher eleva- tion areas. 31 On a shorter time scale, Mast (1993) has documented an increase in the density of ponderosa pine stands in the Front Range as a result of two recent wet periods. The first wet period was from the 1870s through the 1890s, and second was from the 1970s through the 1980s. Fire suppression and reductions in grazing also may have contributed to the observed increase in the density of ponderosa pines, particularly during the second wet pe- riod (Keane et al., 2002). This work and other data confirm that the presence and productivity of forest ecosystems in Colorado are gener- ally limited by water rather than light (Cooper 1960). Wet periods lead to increased stand densities, particular- ly in more water - limited areas such as the lower and mid - elevation forests in the Colorado Front Range (Veblen et al., 2000). However, moderate droughts do not necessar- ily cause a corresponding increase in tree mortality. The trees established during wet periods often can survive subsequent dry periods. At least in the more fire -prone forests such as ponderosa pine, short-term fluctuations in precipitation are important because the amount of fine fuels increases rapidly during wet periods. Subsequent dry periods then have a substantially greater higher fire risk (Veblen et al., 2000), and the historic record indi- cates that the most widespread fires in the Front Range occurred as a result of dry periods following wet periods (Romme et al., 2002). In the more mesic, higher- eleva- tion forests, short-term changes in precipitation directly affect the amount of runoff, but have relatively little ef- fect on fuel loadings. Fire frequency in these more mesic forests is driven primarily by the occurrence of severe drought (Romme et al., 2002). On a broader scale, it is increasingly recognized that humans are having a direct effect on the global climate as summarized by Intergovernmental Panel on Climate Change (IPCC, 2001). The primary cause is an increase in greenhouse gases, particularly carbon dioxide, but the increased concentration of particulates, sulfur dioxide, and other contaminants may also be affecting the climate at the regional or global scale. At this point the most clearly documented anthropogenic change on climate is a net increase in temperature. Long -term meteorological data from Colorado confirm the prediction of an overall warming trend, and this trend is due primarily to higher minimum temperatures in winter (N. Doesken, Assistant State Climatologist, pers. comm., 2001). An increase in air temperatures due to global warming is likely to affect the direction, number, and strength of storms, and thus the amount, type and timing of pre- cipitation in Colorado. A change in precipitation will