Tissue water relations of three chaparral shrub species after wildfire

Summary We compared the tissue water relations among resprouts and seedlings of three chaparral species during the first summer drought after wildfire. Two of the species, Rhus laurina and Ceanothus spinosus recover after fire by a combination of resprouting and seedling establishment (facultative resprouters), whereas a third species, Ceanothus megacarpus recovers by seedling establishment alone (obligate seeder). Our objectives were to document any differences in tissue water characteristics that might arise between resprouts and seedlings and to test the hypothesis that seedlings of obligate seeders develop more drought tolerant characteristics of their tissues than seedlings of facultative resprouters. We found that resprouts had much higher predawn values of water potential, osmotic potential, and turgor potentials than seedlings. Predawn turgor potentials of resprouts were 1.5 MPa through July and August when turgor potentials for seedlings remained near 0 MPa. During summer months, midday water potentials were 2 to 3 MPa higher for resprouts than seedlings and midday conductances of resprouts were two to five fold greater than those of seedlings. Even though resprouts did not experience severe water stress like seedlings, their tissue water characteristics, as determined by pressure-volume curve analyses, were similar by the peak of the drought in August. Further-more, the tissue water characteristics of seedlings from the obligate seeder, C. megacarpus, were similar to those of facultative resprouters — R. laurina, and C. spinosus. We attribute the observed differences in plant water status between resprouts and seedlings to differences in rooting depths and access to soil moisture reserves during summer drought. We conclude that the higher growth rates, photosynthetic performance, and survivorship of postfire resprouts are primarily a result of higher water availability to resprouting tissues during summer months. It appears that the greater seedling survivorship during summer drought observed for the obligate seeder, C. megacarpus, is not associated with more favorable tissue water characteristics.     

Differential survival of chaparral seedlings during the first summer drought after wildfire

Summary Big Pod Ceanothus (Ceanothus megacarpus) is an obligate seeder after fire; Laurel Sumac (Rhus laurina) is primarily a resprouter after fire. Both species commonly occur together in mixed stands and are dominant members of the coastal chaparral of southern California. We compared the mean survival of post-fire seedlings of each species during the first summer drought after fire and found C. megacarpus to have a mean survival of 54% while R. laurina had a mean survival of only 0.1%. Rooting dephs were similar between species but predawn water potentials and leaf temperatures were higher for R. laurina seedlings. Leaf temperatures for R. laurina reached a mean value of 46.8° C on hot, summer days, about 5° C higher than seedlings of C. megacarpus. By the end of the first growing season, 92% of all C. megacarpus seedlings had suffered herbivory compared to only 17% of all R. laurina seedlings. Herbivory did not appear to be the immediate cause of seedling mortality. Transect data indicated that full recovery of prefire species composition and density at our study site was likely but the mode of recovery was different for the species examined. R. laurina recovered primarily by sprouting, C. megacarpus totally by seedling establishment and a third species, Adenostoma fasciculatum (chamise), by a combination of sprouting and seedling establishment. We attribute the higher mortality of R. laurina seedlings to the greater sensitivity of its tissue to water stress. It may be that differential survival of shrub seedlings and differential modes of reestablishment after fire play an important role in maintaining species diversity in the chaparral communities of coastal, southern California.     

A Population Model of Chaparral Vegetation Response to Frequent Wildfires

The recent increase in wildfire frequency in the Santa Monica Mountains (SMM) may substantially impact plant community structure. Species of Chaparral shrubs represent the dominant vegetation type in the SMM. These species can be divided into three life history types according to their response to wildfires. Nonsprouting species are completely killed by fire and reproduce by seeds that germinate in response to a fire cue, obligate sprouting species survive by resprouting from dormant buds in a root crown because their seeds are destroyed by fire, and facultative sprouting species recover after fire both by seeds and resprouts. Based on these assumptions, we developed a set of nonlinear difference equations to model each life history type. These models can be used to predict species survivorship under varying fire return intervals. For example, frequent fires can lead to localized extinction of nonsprouting species such as Ceanothus megacarpus while several facultative sprouting species such as Ceanothus spinosus and Malosma (Rhus) laurina will persist as documented by a longitudinal study in a biological preserve in the SMM. We estimated appropriate parameter values for several chaparral species using 25 years of data and explored parameter relationships that lead to equilibrium populations. We conclude by looking at the survival strategies of these three species of chaparral shrubs under varying fire return intervals and predict changes in plant community structure under fire intervals of short return. In particular, our model predicts that an average fire return interval of greater than 12 years is required for 50 % of the initial Ceanothus megacarpus population and 25 % of the initial Ceanothus spinosus population to survive. In contrast, we predict that the Malosma laurina population will have 90 % survivorship for an average fire return interval of at least 6 years.     

Xylem dysfunction caused by water stress and freezing in two species of co-occurring chaparral shrubs

Water transport from the roots to leaves in chaparral shrubs of California occurs through xylem vessels and tracheids. The formation of gas bubbles in xylem can block water transport (gas embolism), leading to shoot dieback. Two environmental factors that cause gas embolism formation in xylem conduits are drought and freezing air temperatures. We compared the differential vulnerabilities of Rhus laurina and Ceanothus megacarpus, co-dominant shrub species in the coastal regions of the Santa Monica Mountains of southern California, to both water stress-induced and freezing-induced embolism of their xylem. Rhus laurina has relatively large xylem vessel diameters, a deep root system, and a large basal burl from which it vigorously resprouts after wildfire or freezing injury. In contrast, Ceanothus megacarpus has small-diameter vessels, a shallow root system, no basal burl and is a non-sprouter after shoot removal by wildfire. We found that R. laurina became 50% embolized at a water stress of –3 MPa and 100% embolized by a freeze–thaw cycle at all hydration levels. In contrast, C. megacarpus became 50% embolized at a water stress of –9 MPa and 100% embolized by freeze–thaw events only at water potentials lower than –3 MPa. Reducing thaw rates from 0·8 °C min^?1 to 0·08 °C min^?1 (the normal thaw rate measured in situ) had no effect on embolism formation in R. laurina but significantly reduced embolism occurrence in well-hydrated C. megacarpus (embolism reduced from 74 to 35%). These results were consistent with the theory of gas bubble formation and dissolution in xylem sap. They also agree with field observations of differential shoot dieback in these two species after a natural freeze–thaw event in the Santa Monica Mountains.     

Factors controlling postfire seedling establishment in southern California chaparral

Summary This paper discusses the interactions among prefire shrub abundance, soil moisture, and plant and animal species on postfire seedling establishment in mixed chparral in southern California. Postfire germination and survival of seedlings in a stand dominated by a facultative seeder (Adenostoma fasciculatum) and by an obligate seeder (Ceanothus greggii) were monitored for 2 years. Relative to prefire abundance, germination of C. greggii was higher than that of A. fasciculatum. Survival during the first year was also higher in C. greggii than in A. fasciculatum. During the second year, however, mortality of C. greggii was greater than that of A. fasciculatum, mostly due to a psyllid infection. Germination of A. fasciculatum was negatively related to prefire shrub abundance. C. greggii germination was not associated with prefire shrub abundance. Seedling mortality of both species was very strongly related to the depletion of soil moisture the first few months after germination. A. fasciculatum was more sensitive than C. greggii to the drying of the soil, especially in the upper levels. C. greggii seedlings had longer roots, greater root/shoot biomass ratios, higher water potentials, and a later peak in seasonal growth activity compared to A. fasciculatum. Herbs promoted greater survival of A. fasciculatum. Our results indicate that the obligate seeder species, C. greggii, is better adapted to establish seedling in chaparral by producing greater relative germination and greater seedling survival than the facultative seeder species A. fasciculatum. The greater adaptability of C. greggii to the physical environment is counteracted by interspecific interference by plant and animal interactions which tend to favor A. fasciculatum over C. greggii.     

Growth, water relations and photosynthesis of seedlings and resprouts after fire

Seasonal patterns of growth, water relations, photosynthesis and leaf characteristics were compared between obligate seeders (Cistus monspeliensis and Cistus ladanifer) and resprouters (Arbutus unedo and Pistacia lentiscus) from the first to the second year after fire. We hypothesized that seedlings would be more water-limited than resprouts due to their shallower root systems. Regarding water use strategies, Cistus species are drought semi-deciduous and A. unedo and P. lentiscus are evergreen sclerophylls, therefore, comparisons were based on the relative deviation from mature conspecific plants. Seedlings and resprouts had higher shoot elongation and leaf production than mature plants, and over an extended period. Differences from mature plants were larger in resprouts, with two-fold transpiration, leaf conductance and photosynthesis in late spring/early summer. Seedlings of C. monspeliensis exhibited higher transpiration and leaf conductance than mature plants, while those of C. ladanifer only exhibited higher water potential. Growth increments and ameliorated water relations and photosynthesis after fire were attributed to an increase in water and nutrient availability. The small differences in water relations and photosynthesis between seedlings and mature conspecifics are in accordance with the prediction of seedlings experiencing higher water limitation than resprouts. We attribute these results to differences in root systems: resprouters benefited from an increase in root/shoot ratios and the presence of deep roots whereas Cistus seedlings relied on very shallow roots, which cannot provide assess to deep water during summer. Nevertheless, seedlings did not show evidence of experiencing a more severe water limitation than mature conspecifics, which we attributed to the presence of efficient mechanisms of avoiding and tolerating water stress. The results are discussed in relation to post-fire demography of seeders and resprouters in Mediterranean communities.     

Comparative physiology of burned and unburned Rhus laurina after chaparral wildfire

Summary Laurel Sumac (Rhus laurina) is a dominant member of the coastal chaparral community of southern California that survives periodic burning by wildfires by resprouting from a lignotuber (root crown). We investigated the physiological basis for resprouting by comparing shoot elongation, leaf nitrogen content, tissue water status, leaf conductance to water vapor diffusion, and photosynthetic rates of post-fire R. laurina to those of adjacent unburned shrubs. Resprouts had higher rates of shoot elongation, leaf conductance, and photosynthesis than mature, unburned shrubs. Leaf nitrogen contents were elevated in burned shrubs even though their leaves developed interveinal chlorosis. A comparison of soil water potential to predawn water potential indicated that roots of R. laurina remain active below 2 m during the first summer drought after wildfire. Our results support the hypothesis that lignotubers not only contain dormant buds that develop into aerial shoots after wildfire but they also supply nutrient resources that enhance shoot elongation. Because R. laurina is relatively sensitive to drought, yet very successful in its rapid recovery after fire, maintaining an active root system after shoot removal may be the primary function of the massive lignotuber formed by this species.     

Maritime climate influence on chaparral composition and diversity in the coast range of central California

We investigated the hypothesis that maritime climatic factors associated with summer fog and low cloud stratus (summer marine layer) help explain the compositional diversity of chaparral in the coast range of central California. We randomly sampled chaparral species composition in 0.1‐hectare plots along a coast‐to‐interior gradient. For each plot, climatic variables were estimated and soil samples were analyzed. We used Cluster Analysis and Principle Components Analysis to objectively categorize plots into climate zone groups. Climate variables, vegetation composition and various diversity measures were compared across climate zone groups using ANOVA and nonmetric multidimensional scaling. Differences in climatic variables that relate to summer moisture availability and winter freeze events explained the majority of variance in measured conditions and coincided with three chaparral assemblages: maritime (lowland coast where the summer marine layer was strongest), transition (upland coast with mild summer marine layer influence and greater winter precipitation), and interior sites that generally lacked late summer water availability from either source. Species turnover (β‐diversity) was higher among maritime and transition sites than interior sites. Coastal chaparral differs from interior chaparral in having a higher obligate seeder to facultative seeder (resprouter) ratio and by being dominated by various Arctostaphylos species as opposed to the interior dominant, Adenostoma fasciculatum. The maritime climate influence along the California central coast is associated with patterns of woody plant composition and β‐diversity among sites. Summer fog in coastal lowlands and higher winter precipitation in coastal uplands combine to lower late dry season water deficit in coastal chaparral and contribute to longer fire return intervals that are associated with obligate seeders and more local endemism. Soil nutrients are comparatively less important in explaining plant community composition, but heterogeneous azonal soils contribute to local endemism and promote isolated chaparral patches within the dominant forest vegetation along the coast.     

Influence of summer marine fog and low cloud stratus on water relations of evergreen woody shrubs (Arctostaphylos: Ericaceae) in the chaparral of central California

Mediterranean-type climate (MTC) regions around the world are notable for cool, wet winters and hot, dry summers. A dominant vegetation type in all five MTC regions is evergreen, sclerophyllous shrubland, called chaparral in California. The extreme summer dry season in California is moderated by a persistent low-elevation layer of marine fog and cloud cover along the margin of the Pacific coast. We tested whether late dry season water potentials (Ψ(min)) of chaparral shrubs, such as Arctostaphylos species in central California, are influenced by this coast-to-interior climate gradient. Lowland coastal (maritime) shrubs were found to have significantly less negative Ψ(min) than upland interior shrubs (interior), and stable isotope (δ(13)C) values exhibited greater water use efficiency in the interior. Post-fire resprouter shrubs (resprouters) had significantly less negative Ψ(min) than co-occurring obligate seeder shrubs (seeders) in interior and transitional chaparral, possibly because resprouters have deeper root systems with better access to subsurface water than shallow-rooted seeders. Unexpectedly, maritime resprouters and seeders did not differ significantly in their Ψ(min), possibly reflecting more favorable water availability for shrubs influenced by the summer marine layer. Microclimate and soil data also suggest that maritime habitats have more favorable water availability than the interior. While maritime seeders constitute the majority of local Arctostaphylos endemics, they exhibited significantly greater vulnerability to xylem cavitation than interior seeders. Because rare seeders in maritime chaparral are more vulnerable to xylem cavitation than interior seeders, the potential breakdown of the summer marine layer along the coast is of potential conservation concern.     

An individual-based model of chaparral vegetation response to frequent wildfires

The Santa Monica Mountains are home to many species of chaparral shrubs that provide vegetative cover and whose deep roots contribute to the stability of the steep slopes. Recently, native chaparral have been threatened by an unprecedented drought and frequent wildfires in Southern California. Besides the damage from the wildfires themselves, there is the potential for subsequent structural losses due to erosion and landslides. In this paper, we develop a mathematical model that predicts the impact of drought and frequent wildfires on chaparral plant community structure. We begin by classifying chaparral into two life history types based on their response to wildfires. Nonsprouters are completely killed by a fire, but their seeds germinate in response to fire cues. Facultative sprouters survive by resprouting but also rely on seed germination for post-fire recovery. The individual-based model presented here simulates the growth, seed dispersal, and resprouting behavior of individual shrubs across two life history types as they compete for space and resources in a rectangular domain. The model also incorporates varying annual rainfall and fire frequency as well as the competition between plants for scarce resources. The parameters were fit using seedling and resprout survivorship data as well as point quarter sampling data from 1986 to 2014 at a biological preserve within the natural landscape of the Malibu campus of Pepperdine University. The simulations from our model reproduce the change in plant community structure at our study site which includes the local extinction of the nonsprouter Ceanothus megacarpus due to shortened fire return intervals. Our simulations predict that a combination of extreme drought and frequent wildfires will drastically reduce the overall density of chaparral, increasing the likelihood of invasion by highly flammable exotic grasses. The simulations further predict that the majority of surviving shrubs will be facultative sprouting species such as Malosma laurina.     

Drought and changes in leaf orientation for two California chaparral shrubs: Ceanothus megacarpus and Ceanothus crassifolius

Summary Predawn xylem pressure potentials were measured on two California chaparral shrubs, Ceanothus megacarpus and Ceanothus crassifolius, throughout the winter and spring growing season and into the summer drought. On the days xylem pressure potentials were measured, leaf orientation measurements were made on a population of marked leaves from the same shrubs. Predawn xylem pressure potentials decreased from -0.1 MPa in both species to -7.8 and -6.6 MPa in C. megacarpus and C. crassifolius, respectively, between May and August, 1981. Leaf inclinations became more vertical during this period with the greatest change observed in C. crassifolius. This change in leaf inclination was reversible, and, in the late winter and early spring, one year old leaves became more horizontal. Leaf azimuths were random and did not change seasonally. Simulations of solar radiation interception indicated that the increase in leaf inclination associated with summer drought reduced the absorption of solar radiation in August by 6% for C. megacarpus and 20% for C. crassifolius. Standard leaf energy budget calculations suggest that steep leaf inclinations would result in slightly lower leaf temperatures and transpiration rates under summer conditions.     

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.     

Post-fire mortality and water relations of three congeneric shrub species under extreme water stress — a trade-off with fecundity?

Mortality and water relations of three cooccurringHakea species were studied over the first year following a wildfire in scrub-health in southwestern Australia.Hakea ruscifolia regenerated from both resprouting adults and seedlings, whereasH. smilacifolia andH. polyanthema regenerated only from seedlings. We monitored seedling and resprout survival relative to pre-fire numbers and water relations (shoot water potential, transpiration, pressure-volume parameters) in order to determine the relationship between plant mortality and water relations over the critical first summer drought. Seedlings ofH. polyanthema (few initial seedlings per parent) showed little mortality (8%), achieved through both drought avoidance (low transpiration, high predawn water potential and relative water content (RWC), substantial osmotic adjustment) and drought tolerance later in the season. Seedlings ofH. smilacifolia andH. ruscifolia (high seedling/parent ratios) showed little drought avoidance and high mortality (54–68%). The remaining seedlings spent 3–4 months in a wilted condition (up to 3.6 MPa and 45% RWC below the turgor loss point inH. ruscifolia) indicating marked drought tolerance of the survivors. In contrast to its seedlings,H. ruscifolia resprouts were successful drought avoiders and experienced no mortality. The high level of survival and drought resistance ofH. polyanthema was consistent with its large seedlings (via large seeds) and low initial fecundity. The study highlights the importance of the interaction between ecophysiology-morphology and demography in determining the recruitment strategies of plants.     

What predicts the richness of seeder and resprouter species in fire‐prone Cape fynbos: Rainfall reliability or vegetation density?

In ecosystems subject to regular canopy fires, woody species have evolved two general strategies of post‐fire regeneration. Seeder species are killed by fire and populations regenerate solely by post‐fire recruitment from a seed bank. Resprouter species survive fire and regenerate by vegetative regrowth from protected organs. Interestingly, the abundance of these strategies varies along environmental gradients and across regions. Two main hypotheses have been proposed to explain this spatial variation: the gap dependence and the environmental‐variability hypotheses. The gap‐dependence model predicts that seeders are favoured in sparse vegetation (vegetation gaps allowing effective post‐fire recruitment of seedlings), while resprouters are favoured in densely vegetated sites (seedlings being outcompeted by the rapid crown regrowth of resprouters). The environmental‐variability model predicts that seeders would prevail in reliable rainfall areas, whereas resprouters would be favoured in areas under highly variable rainfall that are prone to severe dry events (leading to high post‐fire seedling mortality). We tested these two models using distribution data, captured at the scale of quarter‐degree cells, for seeder and resprouter species of two speciose shrub genera (Aspalathus and Erica) common in fire‐prone fynbos ecosystems of the mediterranean‐climate part of the Cape Floristic Region. Contrary to the predictions of the gap‐dependence model, species number of both resprouters and seeders increased with values of the Normalized Difference Vegetation Index (a widely used surrogate for vegetation density), with a more marked increase for seeders. The predictions of the environmental‐variability hypothesis, by contrast, were not refuted by this study. Seeder and resprouter species of both genera showed highest richness in environments with high rainfall reliability. However, with decreasing reliability, seeder numbers dropped more quickly than those of resprouters. We conclude that the environmental‐variability model is better able to explain the abundance of woody seeder and resprouter species in Southern Hemisphere fire‐prone shrublands (fynbos and kwongan) than the gap‐dependence model.     

Influence of fire severity on the regeneration,recruitment and distribution of eucalypts in the Cotter River Catchment,Australian Capital Territory

Abstract Plant responses to fire are variable between and within species and are influenced by numerous factors including fire severity. This study investigated the effects of fire severity on the regeneration and recruitment of forest eucalypts in the Cotter River Catchment, Australian Capital Territory (ACT). This study also examined the potential for the obligate seeder Eucalyptus delegatensis R.T. Baker (Myrtaceae) to expand into adjacent stands dominated by the facultative resprouter Eucalyptus fastigata H. Deane & Maiden (Myrtaceae) by seed shed and seedling establishment beyond the pre‐fire boundary. Sites were located in areas of either higher or lower fire severity, and transects were placed across the boundary of stands of E. delegatensis and E. fastigata. Species distributions, tree survival and seedling densities and heights were recorded, and the location of each boundary was determined as the region of maximum change in species composition along the transects. Eucalyptus delegatensis was the only eucalypt killed by higher severity fire. However, E. delegatensis seedling density was greater at higher severity sites than lower severity sites. Eucalyptus fastigata seedling density was low across all sites, with other eucalypts producing few, if any, seedlings. There was no evidence that E. delegatensis had increased its range into downslope stands dominated by E. fastigata. Patterns of vegetative recovery and seedling recruitment may be related to a number of factors, including differences in allocation patterns between seeders and sprouters, and the effects of overstory and understory competition. It is unclear what processes impede E. delegatensis seedling establishment beyond the stand boundary, but may involve an inability of E. delegatensis to shed seed sufficiently far downslope; unsuitable conditions for germination beyond the boundary; or, competition from a retained or resprouting overstory, despite the potential for increased dispersal distance soon after fire.     

Seedling Growth and Storage Characteristics of Seeder and Resprouter Species of Mediterranean-type Ecosystems of S. W. Australia

Juvenile (2–4 years old) plants of a taxonomically diverserange of dicotyledonous species were examined following recruitmentfrom seed in recently burnt habitats in S.W. Australia. Obligateseeder species (those succumbing to fire) had on average, analmost threefold greater total plant d. wt and more than a fourfoldgreater shoot: root d. wt ratio than comparably-aged, cohabiting,resprouter species (those capable of surviving fire). Starchwas generally much more concentrated in root dry matter of resproutersthan seeders, and both categories exhibited greater starch storagecapacity in roots than shoots. Members of the Myrtaccae wereexceptional in not showing a greater root starch reserve inresprouter than in seeder species. and in carrying as high,or higher, starch levels in shoots as in roots. Anatomical investigationson roots provided instances of zero starch storage, storage,only in rays or in cortex, in rays and in xylem parenchyma,in rays and in cortex, or in all three locations. High starchratings of resprouter roots related mostly to higher starchgrain packing density at storage sites, but in certain instancesthese also reflected proportionally greater areas of tissuespecifically devoted to storage. Dry matter of shoots of bothseeders and resprouters generally contained higher levels ofN, P, K, Ca and Mg than that of roots, but there was no significantevidence of elements being more concentrated in resproutersthan in seeders. Fire response, seedlings, resprouter, obligate seeder, shoot: root ratio, starch storage, mineral nutrition     

Seed bank patterns in Restionaceae and Epacridaceae after wildfire in kwongan in southwestern Australia

Abstract. Post-fire seed germination, seedling mortality and seed banks were investigated in scrub-heath (kwongan) in SW Australia. Study species included herbaceous and woody, obligate seeders and resprouters in two non-bradysporous but significant plant families (Restionaceae and Epacridaceae). In all species, seedlings were recruited only in the first autumn-spring after fire and occurred in similar densities as the estimated germinable annual seed input. Seedlings were absent from unburnt vegetation. Although most species retained some residual seeds after fire, tests (excised embryo culture) indicated that a negligible number of seeds were germinable. Regardless of fire response or species, there appeared to be a large loss of seeds each year and in most cases, only a small proportion of the annual seed production was used in post-fire recovery of plants. Based on seedling: parent ratios, all species had the capacity to reconstitute parent densities from germinants in the first year after fire, but high seedling mortality and no further recruitment resulted in less seedlings than replacements for four resprouter Restionaceae and three Epacridaceae (all obligate seeders) at the end of the third year after fire.     

POST-FIRE REGENERATION OF SOUTHERN CALIFORNIA CHAPARRAL

Succession of chaparral shrubs was studied for the first 4 years after fire on the four major slope faces at three elevations in southern California. Although total cover fluctuated from year to year, shrub cover increased annually through the third year. There was little or no increase in shrub cover between the third and fourth years. Four years after fire the shrubs covered 55% ground surface at the highest elevation site but only 28% at the lowest elevation site. Shrub cover was similar between slope faces with one exception; at the lowest elevation site shrub cover was twice as great on the north and east-facing slopes as on the south and west-facing slopes. Post-fire recovery of shrubs was by seedlings and/or resprouts. Yucca whipplei was an exception in that it did not resprout once the aboveground parts were killed nor did seedlings establish after fire, however the aboveground parts of many Y. whipplei survived the fire. Species which reproduced entirely by seed did so in the first post-fire year from soil-stored seed with the exception of Ceanothus greggii at the highest elevation site. This species was entirely absent the first year after fire but abundant in the second year. Species producing both resprouts and seedlings varied from site to site in the proportion of resprouts:seedlings. Between 83–100% of the post-fire populations of Cercocarpus betuloides, Arctostaphylos glandulosa, and Xylococcus bicolor were resprouts whereas 12–13% of Ceanothus tomentosus were resprouts. For Adenostoma fasciculatum, resprouts constituted 27–54% of the population at the lowest elevation and 65–94% at the highest elevation; whereas, the Quercus dumosa population was 100% resprouts at the lowest elevation and 31–67% resprouts at the highest elevation. Data are presented on the height of resprouts and seedlings for all species at the end of the first post-fire year.     

Spatially variable propagule pressure and herbivory influence invasion of chaparral shrubland by an exotic grass

Although numerous studies have identified mechanisms that either resist or facilitate biological invasions, few studies have explicitly tested how resisting and facilitating mechanisms interact to drive invasion success. In California, USA, undisturbed Mediterranean-type shrublands have resisted invasion by the perennial tussock grass Cortaderia jubata. In some cases, however, this resistance has been spectacularly breached even in the absence of large-scale disturbance. I tested the hypothesis that these invasions are facilitated by local reductions in the strength of biotic resistance. I evaluated invasive success using C. jubata seed and seedling additions at different microhabitats: the edge of a chaparral stand, under shrub canopy at different distances from the stand edge, and in canopy gaps within the stand. When left exposed to mammalian herbivores, seedling survivorship decreased sharply from nearly 40% on the stand edge to zero just 10 m into the stand. When transplants were protected from herbivory, however, distance from the edge had no significant influence on transplant survivorship. Seedling emergence was also greater on the edge and in canopy gaps than under the canopy, but these differences were not caused by differences in herbivory. The flux of invasive propagules reaching the soil surface was immense and greater along the edge and within gaps than under the stand canopy. Mirroring these patterns, naturally occurring seedling abundance declined dramatically with distance from the stand edge, and seedlings were far more common within stand gaps than would be expected given gap frequency within the stand. Despite strong biotic resistance to invasion within the stand, the cover of C. jubata has increased 20% over the last 9 years. These results suggest that the relative amount of susceptible edge habitat and the supply of invasive propagules can facilitate invasion even in the face of strong local biotic resistance.     

Water use patterns of four co-occurring chaparral shrubs

Summary Mixed stands of chaparral in California usually contain several species of shrubs growing close to each other so that aerial branches and subterranean roots overlap. There is some evidence that roots are stratified relative to depth. It may be that root stratification promotes sharing of soil moisture resources. We examined this possibility by comparing seasonal water use patterns in a mixed stand of chaparral dominated by four species of shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a neutron probe and soil phychrometers to follow seasonal depletion and recharging of soil moisture and compared these patterns to seasonal patterns of predawn water potentials, diurnal leaf conductances, and diurnal leaf water potentials. Our results indicated that 1) Quercus was deeply rooted, having high water potentials and high leaf conductances throughout the summer drought period, 2) Heteromeles/Adenostoma were intermediate in rooting depth, water potentials, and leaf conductances, and 3) Rhamnus was shallow rooted, having the lowest water potentials and leaf conductances. During the peak of the drought, predawn water potentials for Quercus corresponded to soil water potentials at or below a depth of 2 m, predawn water potentials of Heteromeles/ Adenostoma corresponded to a depth of 0.75 m, and predawn water potentials of Rhamnus corresponded to a depth of 0.5 m. This study supports the concept that co-occurring shrubs of chaparral in California utilize a different base of soil moisture resources.