Ecological dynamics

Ecological dynamics:
The plant communities of this site are dynamic in response to changes in disturbance regimes and weather patterns. Fire plays an important role in all forest ecosystems. Important processes that are regulated by fire include regeneration and reproduction, seedbed preparation, competition reduction and thinning to maintain stand health (Spurr and Barnes 1964). Rocky Mountain ponderosa pine is found throughout the West and grows on discontinuous mountains, plateaus, and canyons. A possible explanation for its limited and spotty distribution is correlated to the distribution of sites that receive summer rainfall (Oliver and Ryker 1992). Ponderosa pine seedlings are able to grow taproots that extend greater than 20 inches within a few months of germination (Oliver and Ryker 1992). This ability is essential for survival in desert ecosystems. Rocky Mountain ponderosa pine differs from typical ponderosa pine with its shorter needles, fewer needles per fascicle, germination rate and root growth capacity (Krugman and Jenkinson 2008). Investigation to the vast genetic diversity of P. ponderosa suggests there are five major geographic races, including two distinct races in variety scopulorum. Northern sources of variety scopulorum are characterized by relatively good growth and frost resistance. The southern sources, which occur in Nevada, are slower growing but also frost resistant (Krugman and Jenkinson 2008). Rocky Mountain ponderosa pine has migrated into the Great Basin following the ice ages by way of the Southern Rocky Mountains. Rocky Mountain ponderosa pines never attain the size of the typical variety (P. ponderosa var. ponderosa).
Soils provide physical support, moisture and nutrients to the forest community. Trees have reciprocating effects on the soil. Since they tend to exist on site for extended periods of time, their roots typically extend deep into the subsoil and even into fractured bedrock influencing the rate of soil development. Considerable amounts of organic material are returned to the soil in the form of fallen litter and decaying roots. Increased organic matter on the soil surface, or litter layer, helps to keep moisture conditions more uniform. Insulation provided the tree canopy and litter layer also reduces the temperature fluctuation from day to night (Fisher and Binkley 2002). Seedlings of ponderosa pine grow best with warm days and warm nights. Root growth of ponderosa pine is highly dependent on soil temperature. Low soil temperatures result in low metabolic activity and membrane permeability which limit water and nutrient uptake (Spurr and Barnes 1964).
Biogeochemical cycling in semi-arid forest system is controlled by moisture availability and fire. The role of water is more important in humid systems, which are largely affected by leaching. A review of the literature shows the pos-fire nitrogen fixation is more important than atmospheric deposition and leaching in semi-arid systems. Fire affects the nitrogen pool of a forest in two ways: 1) volatilization during the fire and 2) the influx of nitrogen fixing vegetation following the fire. Increases of nitrogen immediately after fire are attributed to the release of ammonium (NH4+) which takes place at temperatures above 100°C (Johnson et al. 1998). Nitrogen is considered to be a limiting nutrient in this system and the plant community will rapidly respond to inputs of nitrogen. Temporary increases in available nitrogen decrease to pre-burn levels within 5 to 12 years after fire (Johnson et al. 1998).
Ponderosa pine is shade intolerant and becomes more shade intolerant with age. The successional status of Ponderosa pine can be expressed in terms of its successional role, which ranges from seral to climax depending on specific site conditions. It plays a climax role on sites toward the extreme limits of its environmental range and becomes increasingly seral with more favorable conditions. On sites with more favorable moisture, pine encounters greater competition and must establish itself opportunistically. On such sites, establishment is likely to be highly dependent upon the cyclical nature of large seed crops and favorable weather conditions.

Fire Ecology:
Rocky Mountain ponderosa pine is considered very resistant to fire. Fire adaptations include self pruning branches, thick bark, thick bud scales, widely spaced branches that do not favor combustion and a deep rooting habit (Howard 2003). This species has evolved with frequent fires of mixed severity. Surface fires reoccurred every 5 to 30 years, this maintained open-growth, park-like stands. Stand replacing fires were less common, estimates range from 60 to 160 years.
Under pristine conditions fire creates openings for seedling establishment, thus maintaining its persistence. These fires also thinned saplings and maintained the relatively open understories. Ponderosa pine is fire-adapted in all stages of its life history and is especially well adapted to light, regular surface fires. The seeds prefer openings with mineral soil usually prepared by fire for seedbed. For the seedlings and young trees, early development of insulative bark, shielded meristems, high moisture content in living needles, and rapid extension of taproots reduce their mortality from fire. For mature trees, thick bark, deep roots, and low- flammability crown structures help them survive most fires.
Mountain big sagebrush is highly susceptible to injury from fire. It is often top-killed by fire and will not resprout. Utah serviceberry may be slightly harmed by fire, depending on moisture conditions, but is generally considered to be fire tolerant. Antelope bitterbrush is considered a weak sprouter and is often killed by summer or fall fires. Antelope bitterbrush in some areas may sprout after light-severity spring fires. High fuel consumptions increase antelope bitterbrush mortality and therefore favors seedling establishment. Rubber rabbitbrush is top killed by fire, but recovers rapidly and is often one of the first species to return following fire. Greenleaf manzanita is top killed by low to moderate severity fire. It can reestablish by sprouting from dormant buds in the root burl or from fire-stimulated germination of residual seeds in the soil. Desert snowberry is generally top killed by fire, but sprouts from the root crown and rhizomes. Gambel’s oak is fire adapted and sprouts from the lignotuber and rhizomes. Muttongrass is unharmed to slightly harmed by light-severity fall fires. Muttongrass appears to be harmed by and slow to recover from severe fire. Needleandthread is top-killed by fire. It may be killed if the aboveground stems are completely consumed. Needleandthread is slightly to severely damaged by fire. Needleandthread sprouts from the caudex following fire, if heat has not been sufficient to kill underground parts. Recovery usually takes 2 to 10 years. Blue grama has variable fire tolerance; it has fair tolerance when dormant but experiences some damage if burned during active growth, especially during drought. Fire generally favors blue grama, generally increasing its occurrence, production, and percent cover. Indian ricegrass can be killed by fire, depending on severity and season of burn. Indian ricegrass reestablishes on burned sites through seed dispersed from adjacent unburned areas.