• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

Desert ceanothus contains nitrogen-fixing bacteria within its root nodules, which may increase its abundance on nitrogen poor sites. Based on fairly limited observations of root nodules in desert ceanothus in southern California chaparral, Kummerow and others [61] estimated N fixation at the rate of 0.09 pounds N per acre (0.1 kg per ha) per year in a stand with 32% cover. Desert ceanothus has a relatively low photosynthetic rate 90.85nM/cm²/sec^-1 in full sunlight [88]. 

RAUNKIAER [105] LIFE FORM:

REGENERATION PROCESSES:

Seed production: Desert ceanothus plants reach sexual maturity and begin producing seed between 3 and 15 years of age [12,54,145].  Obligate seeding species of ceanothus produce several times the number of seeds as sprouting ceanothus plants. Quantity of seed produced varies significantly (p<0.01), sometimes by orders of magnitude, from year to year [48], and may be influenced by plant size and density, stand location and fluctuations in climate [48,144]. One-year annual seed  production at a California site was estimated at 33.8 million seeds per acre (83 million per hectare) [70]. Seed production is thought to be related to the amount of precipitation received in the previous year, with high precipitation leading to greater seed production [48,145]. Zammit and Zedler [145] found that the rate of seed production increases with shrub height and was maximized within two decades after fire. They observed no evidence of decline in seed production with age of shrub up to 86 years. Average annual seed production ranged from 0 seeds up to 21,092 seeds per plant. Seed production requires flower pollination and is therefore dependent on insect populations. 

Seed dispersal: Seeds are propelled explosively as the capsules mature and dry. This mechanism aids dispersal as seeds are sent outward from the parent plants, and may represent an antipredator strategy [144]. Rodents, birds, and ants may also play a role in seed dispersal [13].

Seed banking:  As many as 1.0 to 1.4 million seeds per acre (2.6 to 3.7 million seeds per hectare) have been observed in the soil at a given time [48]. Seed reserves are influenced primarily by site-specific patterns of seed production and by the intensity of postdispersal seed predation and are not correlated with time since fire [143,144]. Zammit and Zedler [143] observed a significant (p<0.05) increase in density of germinable seeds with increasing live crown cover, and found highest density (180/ft² (2000/m²)) in stands of intermediate age. Density of the soil seed bank is almost always lower than the density of annual seed rain [48,104,144]. Failure of seeds to accumulate over time may be due to losses by erosion, predation or infestation, and severe fire [48,54,143]. Seed of desert ceanothus stored in the soil may remain viable for decades [27,91,92]. Keeley [48] observed about 50% viability of  desert ceanothus seeds stored in the soil. 

Seed predation: Most seeds produced in a year are consumed or removed within a few months of dispersal. Desert ceanothus seeds may be used by harvester ants that rapidly and selectively remove seeds from the soil surface [144],  or they may be infested by the phytophagous chalcid wasps [48]. In some areas rodents may consume up to 99 % of annual ceanothus seed production [13]. An estimate of seed loss is 34% of annual production [145].

Germination:  Germination between fires is negligible, but after fire, seedlings are produced in abundance [14,48,51,144], assuming a quantity of viable seed is present in the soil. Heat stimulation or scarification (5 min at 158 degrees Fahrenheit (70 ^oC)) is required for germination of desert ceanothus [12,27,49,91,143,145]. Severe fires can kill more shallowly buried seed, while stimulating those buried more deeply [144]. Increasing fire severity resulted in better and earlier germination of desert ceanothus seeds compared with chamise seeds and sprouts. Fewer seeds germinated in the field when compared with greenhouse germination studies, suggesting that the number of seedlings emerging in the field is a fraction of the postfire readily germinable seeds, and that suitable microsites for germination are an important factor [81]. 

Seedling establishment:  Establishment of desert ceanothus seedlings occurs during the 1st year postfire, although a few seedlings may continue to emerge during the 2nd season or later [53,94,107]. Competition for light and water is often intense [47]. Seedlings are vulnerable to the effects of drought and damage by insects, rabbits or rodents [72]. Early mortality is often high. Keeley and Zedler [55] report 95% seedling survival during the 1st  year of regrowth at a southern California chaparral site, while Kummerow and others [62] report only 7.8% survival of desert ceanothus seedlings the 1st year.

SITE CHARACTERISTICS:

Chaparral occurs on rocky, nutrient-poor soils and is best developed on steep slopes [54]. California chaparral is characterized by a Mediterranean climate with cool, moist winters, and hot, dry summers. Water availability is likely the primary determinant of community structure in areas where desert ceanothus grows [10]. In general, obligate-seeding species such as desert ceanothus increase in abundance, diversity and longevity with increasing aridity. Seedlings of these species are also more tolerant to drought than those of associated sprouting species [47,54]. Poole and Miller [100] found desert ceanothus to maintain leaf conductance at very low water potentials compared with associated chaparral shrub species. However, Zammit and Zedler [145] found that shrubs on north aspects tend to grow larger and produce more seed than those on south aspects, likely due to better moisture availability on these aspects. Also, nonsprouting ceanothus may experience mass mortality within a stand in severe drought years [50]. Desert ceanothus seems to capture and use more water by using it lavishly while it is readily available, and still maintaining conductance at very low water potentials [90,101]. Data suggest that water stress may inhibit nodulation in ceanothus, which may help explain the low densities of N-fixing nodules found in desert ceanothus compared with densities found under other ceanothus species in northern California [103]. Availability of phosphorus may also limit growth of ceanothus [42]. Desert ceanothus is a poor competitor under shaded conditions [50].

In Arizona, chaparral sites are most common on coarse textured, poorly developed soils with granitic parent material [6]. Desert ceanothus is usually found on the upper portions of slopes and /or relatively level sites where other shrubs are scarce [12]. It is commonly found on north slopes at lower elevations in the desert zone and south slopes at upper elevations, increasing in abundance with increasing elevation up to 5,000 feet (1515 m) in Arizona [24,69]. In the desert plains grassland in southern New Mexico, Arizona and southwestern Texas, desert ceanothus can be found along ravines and similarly favorable sites [141].

Generalized elevation and precipitation ranges by state are as follows:

Ceanothus greggii var. vestitus is found on dry slopes from 3,500-7,500 feet (1067-2286 m) in California, Utah and Arizona. Ceanothus greggii var. perplexans is found on dry slopes from 3,465 to 8,085 feet (1050 to 2450 m) in California [84].

SUCCESSIONAL STATUS:

Desert ceanothus produces a large number of seeds and, with fire, a small number of mature plants are replaced by an abundance of seedlings which require 3 to 15 years to reach sexual maturity [12,48]. Parker [90] found that maximum transpiration rates are consistently higher for shallow-rooted obligate-seeding Ceanothus species than for deeper-rooted sprouting shrub species and that this results in more rapid seedling growth for these species [54]. As seedlings grow and shrub cover increases, there is intense intra- and interspecific competition for light and moisture resulting in heavy mortality [47,50,55]. Ceanothus dominated chaparral accumulates aboveground biomass at an exponential rate through at least 2 decades after stand establishment [108]. Ceanothus greggii var. vestitus was found to have greatest canopy coverage in chaparral stands 22 to 40 years after disturbance [47], and desert ceanothus was most abundant in 10-22 year old burns [76]. Mortality of ceanothus was observed to consistently decrease over time up to 120 years [50]. Hanes [31] observed that over one-half of the population of C. crassifolius and C. greggii var. vestitus was dead in stands older than 40 years. Desert ceanothus plants are well adapted to competing in mature chaparral and are seldom entirely eliminated from a stand [108]. If these "short-lived" species escape close competition they can be quite long-lived [47,113]. Keeley [47] observed that the C. greggii var. perplexans was still an important constituent of southern California chaparral which had remained undisturbed for 90 years. Similarly, Zammit and Zedler [145] showed no decline in seed production in desert ceanothus 6-82 years after fire. 

It has been suggested that senescence occurs in older chaparral due to the formation of allelochemicals and/or sequestering of nutrients in biomass and recalcitrant soil compounds. Larigauderie and others [63] observed little change in growth rate or vigor of desert ceanothus with stand age, suggesting that desert ceanothus is not a short-lived species and its elimination from older stands is likely due to reasons such as decline in nutrient availability, rather than a physiological decline of the shrub. Fenn and others [23] found no evidence that N stored in the microbial biomass in soils under desert ceanothus is involved in nutrient deficiencies in older California chaparral stands. Marion and Black [68] observed an accumulation of available N over time up to 60 years, after which it declined; and a steady decline in soil P over time. 

In pinyon-juniper woodlands of southern California, desert ceanothus is part of the understory shrub component that will dominate a site for about 50 years following fire, followed by a slow recolonization of singleleaf pinyon [135]. Wangler and Minnich [135] observed skeletons of desert ceanothus but no living stems on burns greater than 47 years old in these communities. Cover of desert ceanothus peaked at about 18-33 years after fire at 10-13% cover and 3,232-4,094 stems per ha. It is considered a perennial nurse shrub for single leaf pinyon. Desert ceanothus was present only in the earliest successional stage (grass-forb) as described by Koniak and Everett [58] in another southern California pinyon-juniper woodland.

SEASONAL DEVELOPMENT:

Desert ceanothus has the capacity to fix carbon year-round [54], and data indicate that it will grow whenever conditions are favorable [122]. Stomatal conductances are high (0.5 cm sec^-1) in desert ceanothus in the winter when compared with other chaparral evergreen shrubs. It is also likely to maintain more active photosynthesis into the drought than many deep-rooted shrubs, but later in the season may have complete stomatal shut-down for a month or more [54,88]. Sparks and others [119] measured seasonal photosynthate allocation in desert ceanothus and chamise and observed relatively low structural allocation in spring and higher in summer; and increased proportional allocation to storage in the fall. 

Specific phenological development of desert ceanothus by location is as follows [46,60,106,122,145]:

Related categories for SPECIES: Ceanothus greggii | Desert Ceanothus