Claudia M. Tyler

DEMOGRAPHY AND RECRUITMENT LIMITATIONS OF THREE OAK SPECIES IN CALIFORNIA

We review published studies on the demography and recruitment of California oak trees and focus on the widespread dominant species of the foothill woodlands, Quercus douglasii, Q. lobata, and Q. agrifolia, to ascertain the nature and strength of evidence for a decline in populations of these species. The vast majority of studies have been of short duration (less than three years), focused on the acorn and seedling life stages, and conducted at few locations within each species’ geographic range. We summarize the extensive body of research that has been conducted on the biological and physical factors that limit natural seedling recruitment of oaks. The oak “regeneration problem” has largely been inferred from current stand structure rather than by demographic analyses, which in part reflects the short‐term nature of most oak research. When viewed over longer periods of time using field surveys or historical photos, the evidence for a regeneration problem in foothill oaks is mixed. Q. douglasii shows very limited seedling or sapling recruitment at present, but longer term studies do not suggest a decline in tree density, presumably because rare recruitment is sufficient to offset low rates of mortality of overstory individuals. Q. agrifolia appears to be stable or increasing in some areas, but decreasing in areas recently impacted by the disease Phytophthora ramorum. Evidence from the few available studies is more consistent in suggesting long‐term declines in foothill populations of Q. lobata. Long‐term monitoring, age structure analysis, and population models are needed to resolve the current uncertainty over the sustainability of oak woodlands in California.

The effects of neighbors on the growth and survival of shrub seedlings following fire

Where plant species vie for limited resources, disturbances might preclude competition by releasing a flush of nutrients, or by reducing biomass and thereby diminishing the consumption of resources. However, if new seedlings colonize in clumps, they may still deplete resources within the local aggregations, which may then reduce their growth and survivorship. We investigated competition among seedlings in a burned area by examining the relationship between the performance of newly established shrub seedlings of Ceanothus impressus and (1) the proximity and (2) the identity of their near neighbors. We also investigated the relationship between neighbor proximity and the availability of water. Both survivorship and growth of C. impressus were positively associated with increasing distance to near neighbors, in a manner consistent with resource competition. The availability of water (as determined by pre-dawn xylem pressure potentials) tended to be greater when neighbors were farther away, providing evidence that water was a resource for which plants were competing. This conclusion is reinforced by the finding that the effects of neighbors were stronger in drought years, suggesting that yearly variation in the availability of an important resource (water) can affect the strength of competitive interactions. This suggests that after disturbances, when some resources are apparently abundant on a large scale, competition may be important in determining the small scale patterns of seedling growth and survival.

Invasion of maritime chaparral by the introduced succulentCarpobrotus edulis

Invasion by the alien succulent,Carpobrotus edulis, has become a common occurrence after fire in maritime chaparral in coastal California, USA. We studied post-burnCarpobrotus establishment in chaparral that lackedCarpobrotus plants before the fire and compared seedbank and field populations in adjacent burned and unburned stands.Carpobrotus seeds were abundant in deer scat and in the soil before burning. Burning did not enhance germination: many seeds were apparently killed by fire and seed bank cores taken after fire revealed no germinable seeds. Laboratory tests showed that temperatures over 105°C for five minutes killedCarpobrotus seeds. In a field experiment involving use of herbivore exclosures, we found that herbivory was an important source of mortality for seedlings in both burned and unburned chaparral. All seedlings, however, died outside of the burn regardless of the presence of cages. Establishment there is apparently limited by factors affecting plant physiology. In the burned area, seedlings that escaped herbivory grew very rapidly. Overall, it appears that herbivory limited seedling establishment in both burned and unburned sites but that the post-burn soil environment supportedCarpobrotus growth in excess of herbivore use, thus promoting establishment.

Reproduction and growth of the chaparral geophyte, Zigadenus fremontii (Liliaceae), in relation to fire

Zigadenusfremontii is often a striking component of the flora following firein the chaparral. Like other geophytes, it produces large numbers of flowers inthe first spring after a burn. Although these plants are most conspicuous intheearly postfire environment, the question that remains is, how do they persistinthe interval between fires? To address this we investigated differences in thegrowth and reproduction of Z. fremontii in burned andunburned chaparral. We monitored marked individuals for nine years at threesites: two that were burned in 1990 and one in the same area that was inunburned mature chaparral. We measured leaf area, and production of flowers andfruits. We also conducted seed experiments in the field to determine the ratesand timing of germination. We found that reproduction occurs only in theimmediate postfireperiod: flowering and production of fruits and seeds in the first yearfollowingfire, and seedling establishment by year 3. There was a cost of reproduction;plants that flowered (in the burn area) had negative growth rates the followingyear. In contrast, plants in unburned chaparral, which did not flower, hadpositive growth rates over the same period. Moreover, plants that produced themost flowers had the lowest growth rates. In the unburned chaparral site,plantswere not dormant as predicted from previous literature; instead they producedleaves nearly every year. In most years the average leaf area per plant wasgreater than that in the burned sites. Our results indicate that postfirereproduction depends on growth and carbohydrate storage in the inter-fireperiod. We also suggest that this species is relatively long-lived for aherbaceous perennial.

Are Long Fire-free Periods Needed to Maintain the Endangered, Fire-recruiting Shrub Arctostaphylos morroensis(Ericaceae)?

Morro manazanita (Arctostaphylos morroensis) is a distinctive shrub restricted to a small area along the coast of California, USA. This endangered species faces two opposing fire-related extinction risks: (1) adults are killed by fire, and (2) recruitment opportunities only occur with fire. These strongly limit the capacity of this, as well as other obligate-seeding species, to recover from a population decline, which may result if there is an inadequate amount of time between fires for replenishment of sufficient seed populations. Using a prescribed burn, we tested whether the size of the seed bank that had accumulated in a 40-yr-old stand would prove adequate for maintaining A. morroensis population sizes through fire. Prior to the burn, we found ~11,000 seeds/m 2 in the soil, mostly in the top 5 cm. However, the number of viable seeds was substantially lower (334 seeds/m 2 ), and less than one-third of these survived the experimental fire (99 seeds/m 2 ). Germination occurred only in the first two wet seasons after the fire, and may have been adequate to replace the number of A. morroensis present before the burn. However, most seedlings did not survive their initial summer drought. After three years, the new population of A. morroensis was less than half the size of the parent population. Further mortality is expected because the remaining seedlings are highly clumped. We conclude that A. morroensis may require considerably longer than 40 years to establish an adequate seed bank to compensate for mortality and prevent population decrease or local extinction. This prolonged risk is perhaps explained by specialization of this species to a historic regime of relatively infrequent fire. There are many obligate-seeding species in fire-prone shrublands that may not be resilient to a regime of fire more frequent than that with which they evolved.

Consumer control of oak demography in a Mediterranean-climate savanna

Tree recruitment is rare and oak populations are declining in many Mediterranean-climate oak savannas. Factors affecting acorn production, seedling establishment and initial seedling growth have been much studied in short-term experiments. However, fecundity and early survival rates have not been placed in a demographic framework that also considers sapling survival and growth and adult tree mortality. We use matrix models and life table response experiments to analyze long-term experimental and observation data on California valley oak (Quercus lobata Nee´) near its southern range limit in Santa Barbara County to answer three questions: (1) How sensitive is oak population growth rate to variation in acorn production and initial seedling establishment vs. sapling survivorship and growth? (2) How do mammalian consumers—specifically, cattle, deer and rodents—impact valley oak seedling establishment and sapling survival and growth? (3) Can vertebrate consumers account for the observed population decline of valley oak in savannas in its southern range? We find that population growth is far more sensitive to consumer- mediated variation in sapling survivorship and growth than to variation in fecundity or seedling establishment and that consumers exert strong influence on the demography of the species. Deterministic, finite population growth rate (k )i s,1 for unprotected plants and for plants that are protected from cattle but still exposed to mule deer and rodents. Population growth rate increases to 1.03 with protection from both cattle and deer, mainly because plants are able to quickly reach the tree layer when they are protected from ungulate browsing. Population growth rate jumps to 1.15 for plants protected from both ungulates and rodents as a result of increased survivorship and height growth of established individuals and because of increased seedling establishment during the first year. Our experimental findings are consistent with observed patterns in natural populations in Santa Barbara County, where tree recruitment is rare in both grazed and non-grazed savannas but more common in areas such as roadsides that are refuges from cattle and deer.

Patterns of post-fire flowering and fruiting in Chlorogalum pomeridianum var. pomeridianum (DC.) Kunth in southern California chaparral

For many geophytes living in Mediterranean ecosystems, the passage of fire can produce bursts of flowering, seed production, vegetative growth, and seedling recruitment. In the present study, we investigated patterns of flowering and fruit production of the chaparral geophyte Chlorogalum pomeridianum (common soap plant) at two sites: one burned in a prescribed fire and one in nearby unburned chaparral. Both sites burned in a wildfire 2 years later, and we continued monitoring marked plants for an additional 6 years, enabling us to observe the effects of recent reburning on reproduction and growth. We found that flowering was stimulated by fire but was not strictly fire-dependent. There was a positive relationship between bulb size and leaf area, as well as between these two characteristics and flower and fruit production. Flower stalk initiation occurred when plants reached a minimum leaf area of ~1000 cm2, indicating that a minimum bulb size must be reached before reproductive effort is initiated. Direct herbivory of flowering stalks reduced fruiting and leaf herbivory indirectly prevented the initiation of flowering stalks. In the first several years after fire, flower and fruit production could be explained by resource matching but in subsequent years, resource matching was replaced by resource switching.

Desiccation sensitivity and heat tolerance of Prunus ilicifolia seeds dispersed by American black bears ( Ursus americanus )

ABSTRACT. Carnivore consumption of fruit is a principle means by which many fleshy-fruited plant species achieve long-distance seed dispersal. We examined carnivore dispersal of hollyleaf cherry (Prunus ilicifolia) seeds, specifically assessing the survival, desiccation sensitivity, and germination of seeds found in bear scats. Studies were conducted both in the laboratory and in 2 burn areas in Los Padres National Forest, California. Bear scats containing P. ilicifolia seeds were collected in burned and unburned chaparral. We counted seeds in each scat and noted whether endocarps had tooth punctures or rattled audibly when shaken. For comparative germination trials, we also collected fruits and seeds directly from mature shrubs. In the laboratory, following a cold-moist stratification period, seeds were assessed for germinability. In the field, we compared desiccation rates and germinability of seeds from bear scats and freshly collected seeds. We compared rates of moisture loss and germination for seeds subjected to several different conditions, including 25 °C (room temperature), 30 °C and 65 °C (in the lab), and placement on exposed soil in a burn area (in the field), where midday temperatures were approximately 45 °C but likely much higher by late afternoon. Prunus ilicifolia seeds collected from bear scats were largely undamaged; the vast majority of these seeds germinated. In some cases, germination rate was higher for seeds from scats than for seeds from intact fruits. Several results indicate that desiccation is an important cause of reduced germinability. First, seeds that rattled audibly germinated poorly; and the louder the rattle, the lower the germination percentage. Second, seeds (both fresh and those from bear scats) placed in the field under protective screens had greatly reduced levels of germination (an 84% decline) after only 7 days. Third, seeds dried in the lab, even at relatively moderate temperatures, showed a decline in germination with seed moisture loss. The addition of high temperatures accelerated this decline in germination. We discuss the relevance of heat and desiccation sensitivity of seeds dispersed by bears to successful seed germination in burned and late-seral mesic and xeric chaparral.

Livestock Exclusion Impacts on Oak Savanna Habitats—Differential Responses of Understory and Open Habitats ☆

ABSTRACT Large grazing animals can have profound impacts on plant communities and soil properties; however, these impacts are not always uniform across or within regions. The distribution of features such as forage quality, water, or shade within a pasture can change the behavior of grazers and thus, the impact of their grazing. Where managed livestock grazing has been proposed as a conservation tool to enhance or maintain desirable plant communities, understanding how spatial variation between tree and intertree habitats within a savanna landscape affects the response of vegetation and soil properties to grazing will be critical for designing management plans for different sites. In this study, we used a previously established, long-term livestock grazing experiment in California oak [Quercus L.] savannas to investigate how the removal of grazing affected plant communities and soil characteristics underneath and outside of isolated tree canopies. In the oak understory, plant community composition shifted in response to livestock removal, largely due to a 68–400% increase in the relative cover of native species. Overall plant community composition in open grassland neighboring trees changed little in response to livestock grazing removal, yet we did see a decrease in species richness and diversity surrounding deciduous oaks as the dominance of the exotic annual Bromus diandrus Roth increased. The depth of plant litter increased 1–2 cmin both habitat types when livestock grazing was absent, along with minor changes in soil carbon, nitrogen, and bulk density. These results highlight how different habitat patches within savanna landscape can have varying responses to grazing removal and illustrate how challenging it will be to use grazing as a management tool to enhance the diversity of native species. In the oak understory, native species that are tolerant of herbivory may be absent or unable to coexist with non-native annual grasses. The abundance of understory habitat at a particular site may therefore be an important variable predicting the outcome of livestock grazing.