Climate is the average weather in a location over a long period of time. As some climate scientists like to say, 'Climate is what you expect, weather is what you get'. Climate is not only the measure of average conditions but also the characteristic range of variability from those conditions. Therefore, climate change refers to shifts in the average state of the climate and in its variability. For example, climate change may be evident in changes to expected temperature or precipitation, but also to the occurrence of variable weather events such as heat waves, floods, or drought. Changes in the frequency and severity of climate variability (weather events) often cause more damage and are more difficult to adapt to than gradual change.
Globally, changes in climate are already impacting forests through higher average temperatures, altered precipitation patterns, and more extreme weather events. Changes in temperature and precipitation can influence the occurrence and intensity of forest disturbances such as wildfire, introduced species, and pathogen (disease) outbreaks. Forests will play a crucial role in adapting to a changing climate because they will continue to serve as natural reservoirs for high-quality water and as refuges for biodiversity. Therefore, it is important to have healthy and resilient forests that rebound after disturbance.
By looking at historical climate data and prevalent climate trends, we can assess what changes Western North Carolina might expect, and begin to understand how to prepare for possible changes.
The climate in Western North Carolina is largely defined by the region’s topographical features. The elevation gradients of the Southern Appalachian Mountains range from around 1,500 feet along the eastern boundary to 6,684 feet at the summit of Mount Mitchell (the highest point east of the Mississippi River). Some of the valleys in the Southern Appalachians drop to 1,000 feet above sea level, while some 82 peaks exceed 5,000 feet and 43 tower above 6,000 feet. These elevation gradients result in varying average temperature and precipitation, creating a unique range of microclimates, each of which supports unique forest types and globally rare species. Due to the different climates in the region, the effects of climate change will vary and impact the forests in different ways.
Why is temperature important? Temperature and forest types are closely related. This relationship shows the great extent of acclimation of forests to the temperature zones in which they are found. Temperature directly affects biotic processes and water requirements, thus organisms, species, and forest types respond to changes in temperature. Temperature, along with sunlight, is a driving force behind photosynthesis and temperatures regulate the demand plants have for water. Forests depend on certain soil moisture levels for growth and respiration, and air temperature affects soil moisture through evaporation as well.
Temperature in Western North Carolina
The terrain of the Southern Appalachians creates a range of average temperatures in Western North Carolina. Average temperatures generally decrease with an increase in elevation. On any day, from mountain top to valley floor, the difference in temperature across the region may be as much as 20º F.
The cold average temperatures in the highest elevations of the Southern Appalachians serve as refuges for several forest types, ecological communities, and many species. Changes to these temperatures will have devastating impacts to high-elevation forests types. Forest types, such as the spruce-fir and northern hardwoods, are currently at the limit of their suitable habitat range. The unique temperatures and acclimated forest types at high elevations (>5,500 feet) in Western North Carolina are isolated in the southeast – where the climate resembles that of southern Canada and the most northeastern United States.
Maximum temperatures decrease with increasing elevation. Average maximum temperatures also tend to be higher on south-facing slopes, due to sun exposure. Trees and forest types have specific maximum temperature tolerances, and the relatively low maximum temperatures in high elevation areas account for the unique forest types and species. Maximum temperatures regulate important factors, such as soil moisture through evaporation. Forests depend on soil moisture levels for uptake and to establish fire regimes – drier soil moisture results in higher fire risk.
Minimum temperatures generally decrease with increasing elevation. But unlike average maximum temperatures, average minimum temperatures are not as closely related to aspect or cardinal direction. Minimum temperatures are important to forests because they limit biological activity of pests, such as invasive insects. Like maximum temperature, minimum temperature is important to natural disturbance regimes and overall health of the forest.
Why is precipitation important? Precipitation is essential to ecosystem processes and to water supplies for municipalities. Precipitation is any form of water - rain, snow, or hail - that falls from the atmosphere and reaches the ground. Upon reaching the ground, precipitation enters the hydrologic cycle. This cycle involves water traveling from the land to the ocean, during which water molecules evaporate back into the atmosphere. A portion of precipitation soaks into the ground and moves downward through small openings in the soil and rock. This water that soaks into the ground becomes groundwater or is available to be taken up by vegetation; thus, the amount of precipitation is essential to forest growth. The water that does not soak into the ground runs off into the streams and rivers.
Precipitation in Western North Carolina
Western North Carolina has some of the most variable average precipitation in the southeastern United States. In fact, the wettest and driest points in Western North Carolina are separated by only 45 miles. Elevation gradients of the mountains contribute to the differences in precipitation. Consider average rainfall in two towns, Highlands and Marshall. Highlands sits at the southern edge of the Blue Ridge Escarpment, so moist air lifted over the mountains drops heavy amounts of rain on this high-elevation town. Nearby areas are wet enough to be considered rain forests, and waterfalls abound. The town of Marshall, however, sits at the north end of the French Broad River basin. Shielded from the prevailing moist winds from the south and west by the Balsam and Smoky Mountains (where most of the rainfall is squeezed out), this area is the driest in the entire state of North Carolina.
Elevation gradients also contribute to the variable forms of precipitation the region receives. Lower elevations receive most precipitation as rainfall. On the other hand, the tallest mountain peaks receive up to 50 inches of snowfall each year. The range of precipitation zones create suitable conditions for the diversity of forest types and species found in the region.
The seasonal distribution or timing of precipitation is an important characteristic of climate. While average annual precipitation tells how much rainfall an area receives in a normal year, the seasonal distribution tells when it arrives. The timing of precipitation also indicates how often an area experiences drought and flooding. Both drought and flooding are natural occurrences in Western North Carolina, but changes in the frequency of these events have the potential to greatly impact water resources for forest ecosystems.
Western North Carolina receives most of its rainfall during the spring and summer months, while the fall and winter months are comparatively dry. This pattern in the distribution of rain throughout the year is essential to biological processes. Forests need more water in the warmer growing season. Deciduous trees, especially, take up much more water in spring and summer when leaves are on the trees due to evapotranspiration (the process of water coming through the leaves of trees and then evaporating into the air). The timing of precipitation in Western North Carolina allows trees to leaf out in the spring and sustains the flow of water in streams.
Wet and dry periods
The Palmer Hydrological Drought Index (PHDI) is influenced by precipitation and shows long-term dry and wet conditions. The PHDI reflects how precipitation affects groundwater, water tables, and reservoirs. Uncharacteristically frequent or extended dry or wet periods indicate an interruption in the seasonal pattern of precipitation.
With climate change, there may not be any change in total precipitation, but there may be extended wet and dry periods that might result in more flooding or drought. The PHDI for Western North Carolina shows that the longest dry periods over the past century have occurred in the past three decades. While flooding and drought are natural disturbances in Western North Carolina, increased frequency or severity of these disturbances will negatively impact the forests.