• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

IMMEDIATE FIRE EFFECT ON PLANT:

Heat stimulation or scarification (5 min at 158 degrees Fahrenheit (70 ^oC)) is required for germination of desert ceanothus seed [12,27,49,91,143,144]. Seed mortality varies with fire severity and depth of burial [54]. Quinn [104] observed 90% mortality in hoaryleaf ceanothus (C. crassifolius) seed 1 week after wildfire. 

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

PLANT RESPONSE TO FIRE:

Most seeds germinate the 1st spring after fire [50], although some seedling establishment has been observed in subsequent springs [53,91,93,94]. Fire must be severe enough to scarify seed, although a case was reported in which seed was not scarified by fire, but by temperatures reached by sun on the blackened soil the 2nd postfire season [53]. 

The 1st 1 to 3 years after fire are critical in determining composition of  mature chaparral communities [2,72]. Seedling mortality affects the species balance of shrub seedlings and stump sprouts, is generally greatest during the 1st postfire year, and varies as a result of herbivory, competition and climate [72]. Mortality of desert ceanothus may be high in early postfire years when compared with other shrubs [48,62,93]. Although Keeley and Zedler [55] report only 2% mortality of desert ceanothus seedlings in the 1st postfire year at a California chaparral site, while chamise and manzanita species experienced 39 to 71% mortality. Abundant seedlings usually provide more than enough plants to insure perpetuation even after high initial mortality [62].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

Fires of greatest intensity would occur at some intermediate fire-free interval, as fuel loads increase up to a point and then decrease as more and larger plants die and decompose. This is also the time when the soil seed pool would be the greatest [81].

Pase [91] observed greater seedling establishment in Arizona chaparral following a severe burn (2,618 seedlings/acre (6466/ha)) than following a less severe burn (636 seedlings/acre (1571/ha)). Moreno and Oechel [81] found that seedling abundance of desert ceanothus was positively correlated with fire severity. Variability in distribution of desert ceanothus may be due to local fluctuations in fire severity [76]. Prescribed burns conducted when soils are moist may reduce the response of  desert ceanothus because the intensity and duration of heat may be too low to stimulate germination [89]. 

Large fires may harm population levels of nonsprouting shrubs that have suffered heavy soil seed predation [48]. However, a larger burn may help protect seedlings from herbivory by small mammals that cannot reach them due to lack of cover [72,128].

Season of burn may affect establishment of desert ceanothus. Following early spring (low severity) fires, establishment of ceanothus was poor compared with more severe summer season fires [2,89]. A chaparral stand dominated by desert ceanothus and chamise was burned in the early winter, and desert ceanothus seedlings failed to germinate the 1st postfire spring. Seedlings were unable to compete with established vegetation the following spring, resulting in a stand dominated by chamise [89].

Drought following seedling establishment may favor ceanothus over chamise [25,71,80]. However, Moreno and Oechel [80] found  mortality of desert ceanothus seedlings was strongly related to depletion of soil moisture the first few months after germination. While desert ceanothus demonstrates a greater adaptability to the physical environment, it is also subject to a greater degree of interspecific interference by plant and animal interaction than chamise. 

After a December prescribed burn in 54-year-old California chaparral desert ceanothus seedlings numbered 4 seedlings per foot² (44 per m²) in May. Most of these seedlings died in May and June, leaving 7.8% alive in December. Mortality was not affected by stump sprouters, but was likely due to drought and/or herbivory by rabbits [62].  

Seedlings of desert ceanothus are lacking physical and chemical defenses against herbivores that may develop later and are especially vulnerable to herbivory [104]. Mills [71] observed that preferential consumption of desert ceanothus by  brush rabbits could eliminate its competitive advantage over chamise. In  the absence of herbivory, chamise experienced higher mortality. Desert ceanothus plants that were located among the dense branches of chamise were taller than plants in uncovered microsites on open plots. This suggests that chamise may act as a nurse plant, protecting desert ceanothus from herbivory by rabbits. Ceanothus seedlings were also heavily infected by psyllids during the 2nd and 3rd postfire years. This infection significantly (p<0.01) reduced growth and survivorship of ceanothus seedlings, killing around 13% of seedlings [72].

FIRE MANAGEMENT CONSIDERATIONS:

Desert ceanothus requires 5 to 15 years after fire before substantial seed crops are produced, so fires at more frequent intervals can produce localized extinctions [54,98]. A diversity of fire-free intervals at one site is suggested for maintenance of community diversity. Season of burn is also an important consideration. The observation that fewer species germinate in chaparral communities after winter and spring burns may be explained by the fact that these communities evolved in a fire regime with late summer and fall fires [89].  Since soil seed bank density is rarely as high as a single years seed rain, a fire that occurs closer to the time of seed dispersal (usually late spring or early summer), operates on a larger seed bank and will likely result in a greater number of seedlings than would a fire later in the season [104]. It is important to consider the relationship between prefire shrub age, vigor and seed production, and postfire seedling establishment [119].

Fire must be severe enough to scarify seed [81]. Similarly, prescribed burns conducted when soils are moist may reduce the response of the seed bank, if the intensity and duration of the heat is too low to stimulate germination [89].  High severity fires favor desert ceanothus over chamise. A high severity spot occupied by a burned out chamise plant will likely succeed to desert ceanothus due to its greater resistance to fire severity [81]. 

Size of fire may also affect survival of desert ceanothus. The interior of large burns may be relatively free of small mammals, thereby favoring desert ceanothus. On smaller burns access may be immediate and frequent, resulting in reduced  cover of desert ceanothus [9,71].  

Seeding with grasses after fire may impede establishment of ceanothus seedlings [2]. In the absence of herbivory, desert ceanothus exhibits a competitive advantage over chamise [9,72]. Crown cover of desert ceanothus was unchanged in an Arizona chaparral community that was seeded to weeping lovegrass (Eragrostis curvula) and grazed by cattle after a severe wildfire [99].

There is increasing dead biomass in a stand as shrubs age and die, resulting in increased flammability [119]. Brush management burning guidelines are available [8,19]. Some flammability characteristics of desert ceanothus are given in Rundel [110]. Chaparral types dominated by ceanothus will burn only under conditions of very low fuel moisture or high winds and it is not unusual to find stands over 100 years old in this type [19].

Related categories for SPECIES: Ceanothus greggii | Desert Ceanothus