Post-fire vegetation recovery at El Malpais National Monument

Bleakly, David; Lightfoot, David

INTRODUCTION Forest fires are an important disturbance or perturbation in forest ecosystems that influence the taxonomic composition and physical structure of vegetation (Wright & Bailey 1982). Following fires, forest vegetation goes through a change or succession over time, usually resulting in vegetation similar to that present prior to the fire. Forest managers now use fire as a tool to maintain the condition of forests (New Mexico Forestry and Resources Conservation Division 1992). Predictive models have been developed to help managers predict how vegetation succession may proceed to yield desired results (Kessell & Fischer 1981, Fischer & Clayton 1983). Ponderosa pine forests of the southwest United States are fire-adapted ecosystems where forest fire frequency was on the order of 8-12 years prior to human fire suppression beginning in the early 1900's (Potter & Foxx 1984, Kozlowski & Ahlgren 1974, Wright & Bailey 1982). The pre-1900 fire frequency on the lava flows at El Malpais National Monument was about 1 0 years (Henri Grissino-Mayer, pers. comm. ). Ponderosa pine is fire adapted, and seedling establishment is generally dependent upon forest fires (Dick-Peddie 1993, Potter & Foxx 1984, Kozlowski & Ahlgren 1974). Fires remove or reduce some vegetation which may compete with pine seedlings for moisture and light and other vegetation which may allelopathically inhibit seedling growth (USFS 1974). Many herbaceous plant species in ponderosa pine forests are also adapted to fire and increase in numbers and biomass following fires (Fischer & Clayton 1983, Oppenheimer 1978). In New Mexican ponderosa pine forests, perennial grasses remain abundant in open parkland environments that are maintained by frequent fire (Potter & Foxx 1984), and native forbs tend reach maximum abundance in open burned areas 2 to 3 years after fires (Oppenheimer 1978). Post-fire vegetation consists of foliage that was not killed by the fire, new growth from perennial plants that survived the fire, and seedlings of plants the survived the fire either as seeds present in the soil, or that germinate from seeds that dispersed to the burned area after the fire (Kozlowski & Ahlgren 197 4 ). The composition of perennial plants that survive a fire versus new plants resulting from seedling establishment depends upon the initial forest stand conditions and fire intensity (Fischer & Clayton 1983). Generally, any one fire produces a mosaic of different burn intensities across the landscape resulting in a mosaic of different successional patches. An important component of the post-fire flora are forbs that are characteristically weedy and colonized intensely burned areas via seed dispersal. These plants tend to be annuals that have good dispersal capabilities and are able to colonized disturbed sites where other plants have been eliminated or reduced. Such weedy plants tend to be poor competitors, and, over time, are generally replaced by long-lived perennial plants in the absence of further disturbance. Examples of such plants colonizing recently burned ponderosa pine forest in New Mexico include mullein ( Verbascum thaspus; Oppenheimer 1978) and pigweed (Chenopodium spp., Potter & Foxx 1984) both of which were found at ElMA during this study. The potential for forest fires to increase the distribution and abundance of exotic weedy plant species is a concern of land managers. As a disturbance, forest fires may create environmental conditions that are favorable to the introduction and establishment of exotic weeds into areas that were previously not occupied by such weeds, or where exotic weeds were present but not common. Substrate type has not been treated as a variable in the study of fires in Southwestern ponderosa forests and there is essentially no literature available on the subject. Ponderosas grow in soils derived from all rock types and basalts are common substrates for Southwestern ponderosa forests (USFS 197 4 ). Rocks of volcanic origin (basalts, tuf