Bruce E. Mahall

BIDIRECTIONAL FACILITATION AND INTERFERENCE BETWEEN SHRUBS AND ANNUALS IN THE MOJAVE DESERT

Composition and structure of plant communities can be strongly influenced by plant interactions. Interactions among plants commonly comprise positive and negative effects operating simultaneously and bidirectionally. Thus, a thorough understanding of plant interactions requires experimental separation and quantitative assessment of the bidirectional positive and negative effects that add up to the net effects of plant interactions. Using the close spatial association of annual plants with a desert shrub (Ambrosia dumosa) in a sandy area of the Mojave Desert of California as a test system, we separated and quantified negative and positive effects of annuals on shrubs and of shrubs on annuals. We achieved the separation of negative and positive effects with an experimental design that included reciprocal removals of neighbors and simulations of physical effects of neighbors using artificial structures. All experimental manipulations were conducted on space originally occupied by Ambrosia shrubs to focus on immediate effects of neighbors on water availability rather than on long-term microenvironmental effects (e.g., nutrient accumulation). We quantified positive effects by calculating the difference between performance parameters of neighbors growing with artificial structures (thatch to mimic the physical effects of the presence of annuals and artificial canopies to mimic the physical effects of shrubs) and those of neighbors growing alone (removals). We estimated negative effects by calculating the difference between plant performance on control plots (shrubs and annuals growing together) and performances of plants growing with the artificial structures. Annuals had simultaneously strong negative and weak positive effects on shrub water status, growth, and reproductive output. Annuals also had an impact on the sex expression of shrubs by inducing shifts toward a higher male proportion in inflorescences of monoecious Ambrosia. In contrast, we found strong positive and weak or no negative effects of the shrubs on survival, biomass production, and seed production of the entire annual community and of selected annual species (the abundant native Chaenactis fremontii and the two dominant introduced annual species Bromus madritensis ssp. rubens and Schismus barbatus). Overall, in net effect, the interaction between shrubs and annuals can be described as facilitation or positive net effects of shrubs on annuals, and interference or negative net effects of annuals on shrubs. However, during the growing season, the ratios between positive and negative effects shifted. Annual plants benefited from the presence of shrubs to the greatest extent early in the growing season, and initial negative effects of annuals on shrubs declined as annuals senesced later in the season. Results of this study support the view that an experimental resolution of bidirectional positive and negative effects is necessary to achieve an accurate, mechanistic understanding of species interactions.

Facilitation and Interference of Quercus Douglasii on Understory Productivity in Central California

Controversy over the effect of Quercus douglasii on the productivity of California grassland has been fueled by conflicting reports. In some studies, understory grassland productivity was < 25% of open grassland productivity, whereas in other studies understory productivity exceeded 200% of that of surrounding grassland. We examined light, temperature, soil nutrients, soil moisture, and fine tree root distributions under selected Q. douglasii trees (12 that appeared to have suppressive effects on understory productivity, and 12 that appeared to enhance understory productivity) in order to deter- mine how variations in these factors were associated with the differences in understory grassland productivities. We found that grassland productivity is likely to be facilitated by nutrient inputs via litterfall and throughfall under all trees, but that trees with low understory grassland productivities had substantially higher amounts of fine roots in the upper 50 cm of soil and much lower predawn xylem pressure potentials than trees with high understory productivities and presumably deeper root systems. Root exclosures reduced the negative effects of these trees on the dominant understory grass species, and further experiments indicated that the negative effects of the tree roots may partially result from allelopathic oak root exudates. Thus, shallow fine tree roots may inhibit understory productivity, and variations in Q. douglasii root morphology may explain the intertree variations in facili- tating/interfering effects on understory species.

SOIL FUNGI AND THE EFFECTS OF AN INVASIVE FORB ON GRASSES: NEIGHBOR IDENTITY MATTERS

We studied the effects of soil fungi on interactions between Centaurea mel- itensis, an exotic invasive weed in central California, and two co-occurring grasses, Nassella pulchra and Avena barbata. The fungicide benomyl reduced the abundance of arbuscular mycorrhizal (AM) fungi in plant roots but did not affect non-AM fungi. Centaurea plants grown alone were >50% smaller with the resident microbial community intact than when benomyl was applied. When grown with Nassella, the effect of benomyl was reversed. Centaurea grew almost five times larger with the resident microbial community intact. Fungicide had no effect on the biomass of Centaurea grown with Avena, but biomass of Centaurea was significantly lower when grown with Avena than when grown with Nassella or alone. Photosynthetically fixed carbon may have been transferred from Nassella via soil fungi to Centaurea, constituting a form of soil fungi-mediated parasitism, but such a transfer did not occur from Avena to Centaurea. Second, Nassella may have been more inhibited by soil pathogens in the presence of Centaurea than when alone, and the inhibition of Nassella may have released Centaurea from competition. A third possibility is that Nassella has strong positive effects on the growth of soil fungi, but the positive feedback of beneficial soil fungi to Nassella is less than the positive feedback to Centaurea. Regardless of the mechanism, the difference in soil fungicide treatment effects on competition between Cen- taurea and Nassella vs. Centaurea and Avena has important implications for the invasion of California grasslands.

Community composition and photosynthesis by photoautotrophs under quartz pebbles, Southern Mojave Desert

We used 16s rDNA sequences to identify novel species of cyanobacteria beneath translucent quartz pebbles in the desert pavement on an alluvial piedmont of the Coxcomb Mountains in the southern Mojave Desert, California, USA. Transmission of light, as measured with an integrating sphere, was about 0.08% beneath the thickest pieces of quartz (25 mm) harboring these hypolithic autotrophs. The photosynthetic rate ranged from 0.1 to 1.0 μmol·m−2·s−1 in the linear range of its response to light (PAR of 0–50 μmol·m−2·s−1), over which the apparent quantum-use efficiency was 0.019. Light-saturated rates of 1.7–2.7 μmol·m−2·s−1 were recorded at light intensities of 200–400 μmol·m−2·s−1. The hypolithic community had an upper thermal tolerance of >90°C in laboratory conditions. The quartz pebbles confer a modest greenhouse effect that may be important for photosynthetic activity during cool, wet, wintertime periods that prevail in the Mojave Desert.

Ceanothus megacarpus chaparral: A synthesis of ecosystem processes during development and annual growth

SummaryThe evergreen sclerophyll shrub,Ceanothus megacarpus, dominates the chaparral in many areas of the Santa Ynez Mountains, Santa Barbara County, California, USA. Often occurring in pure stands,C. megacarpus chaparral accumulates large amounts of aboveground live biomass during growth after fire as a result of high rates of net primary productivity (850 g/m2/yr) compared to other chaparral areas and Mediterranean scrublands of the world. Populations ofC. megacarpus are even-aged, growing from buried seed which requires heat treatment for germination. During the early years of stand development after fire, competition for water among the shrubs results in mortalities of up to 50% of the initial population. This competition is evident by changes in the population size-structure and dispersion pattern of the living shrubs. Water potential measurements during the summer drought show that smaller individuals are more severely stressed than larger individuals. Similarly, shrubs in younger stands develop lower water potentials than shrubs in older stands. There is little mortality after 15 to 20 years of growth; competition for water is alleviated in older stands presumably because the surviving shrubs have developed more extensive root systems. Leaf area reaches a steady-state of 1.6 m2/m2 in 20-year-old stands. At this stage of development, competition for light is manifest in shrub physiognomy, particularly in the death of lower branches.Nutrient losses occur as a result of volatilization and erosion due to fire, and long periods of time are apparently necessary to replace nutrients such as nitrogen from atmospheric sources. Nevertheless, stands ofC. megacarpus show little evidence of nutrient deficiencies. Decomposition of plant detritus is rapid and this species shows efficient internal nutrient use. Up to 46% of the nitrogen and 56% of the phosphorus needed for annual growth are supplied by reabsorption from senescing foliage before abscission. These aspects of nutrient cycling may help explain the long-term persistence and sustained productivity of chaparral dominated byC. megacarpus in this area of California.RésuméL’arbuste sclérophylle sempervivantCeanothus megacarpus domine la végétationchaparral de beaucoup de régions des montagnes Santa Ynez du comté de Santa Barbara dans la Californie des Etats-Unis. Souvent en futaies pures, ce chaparral àC. megacarpus produit une très grande biomasse aérienne vivante pendant un accroissement qui suit un feu et qui résulte d’une productivité primaire nette de 850 g/m2/an en comparaison d’autres régions dechaparral et de formations ligneuses basses du type méditerrané du monde. Les arbustes des peuplements deC. megacarpus sont tous du même âge parce qu’ils sont nés de graines dans le sol dont la germination exige la chaleur de feu. Pendant les premières années d’accroisement après un feu, la moitié des arbustes meurt à cause d’une compétition pour l’eau mise en évidence par des modifications aportées à la distribution des classes des diamètres des bases dans le peuplement et à la distribution spatiale des arbustes vivants. Des mesures de potentiel hydrique pendant la sécheresse d’été montrent que les petits individus sont plus affectés que les grands. Les arbustes de futaies plus jeunes ont de potentiel hydrique plus bas que ceux de futaies plus âgées. Il y a moins de mortalité à partir de 15 à 20 ans; la compétition pour l’eau ralentit chez les arbustes plus âgés, probablement parce que ceux-ci ont une plus grande biomasse racinaire. La surface de feuilles des arbustes de futaies âgées de 20 ans atteint 1,6 m2/m2, étage de développement où la compétition pour la lumière se manifeste par la mort des branches les plus basses.Les éléments minéraux se perdent à cause de volitilization et de feu, et il semble qu’il faille beaucoup de temps pour remplacer de l’atmosphère des minéraux tels que l’azote. Néanmoins les futaies deC. megacarpus ne semblent pas avoir un minéralomasse insuffisante. La matière organique d’origine végétale se décompose rapidement etC. megacarpus utilise efficacement les minéraux produits par ce moyen. Jusqu’à 45% de l’azote et 56% du phosphore pour l’accroissement annuel se fournissent par une réabsorption de produits dans des feuilles en sénescence avant la perte de ces structures. Ces aspects de cycles d’éléments minéraux peuvent contribuer à expliquer la longue persistence et la productivité soutenue du chaparral àC. megacarpus de cette région de la Californie.

Quantitative Phenology and Water Relations of an Evergreen and a Deciduous Chaparral Shrub

Quantitative phenology and pre—dawn xylem pressure potentials (XPP) of an evergreen shrub, Ceanothus megacarpus, and a deciduous shrub, Salvia mellifera, were compared on pairs of plants in shared microsites in southern Californian chaparral for 2 yr. Shoot elongation of both species occurred at essentially the same time during both growing seasons. Leaf maturation also occurred at essentially the same time for both species during the 1977—1978 season, but commenced ≈1 mo earlier on S. mellifera than on C. megacarpus during the 1978—1979 season. No substantial differences in phenology were found among leaves of C. megacarpus canopy, subcanopy, or short shoots. On S. mellifera, however, leaf maturation was more rapid and was completed earlier in the season on flowering canopy shoots than on nonflowering canopy and subcanopy shoots, and although no mature leaves on flowering canopy shoots persisted beyond June during either year, a substantial number of mature leaves on subcanopy shoots remained well into their second growing season. Leaf curling, associated with changes in XPP, was significant in both species, and allowed rapid adjustment of leaf area without the abscission or production of leaves. Flowering and seed production occurred early in the growing season for C. megacarpus and late in the growing season for S. mellifera. This difference in flowering phenology corresponded to the more rapid vegetative development of S. mellifera flowering canopy shoots. In both species subcanopy or short shoots made up a much larger fraction of the total number of shoots per plant, and consequently contributed relatively more to whole—plant leaf phenology and annual shoot production, than did canopy shoots. There were only subtle differences between species in the timing of increases in whole—plant mature leaf area (MLA). Maximum rates of increase of whole—plant MLA occurred in both species when soil temperature (ST) was ≈13.5°C and XPPs were ≈—0.60 MPa, and these rates declined sharply at STs above 15° and XPPs below —1.5 MPa. Ceanothus megacarpus and S. mellifera had nearly indistinguishable XPPs throughout both years. Leaf shedding by S. mellifera was more related to photoperiod and flowering than to low XPP, and was clearly not a successful drought—avoiding mechanism. Like some evergreen species, S. mellifera tolerated long periods of extremely low XPP. Important differences between the species were that over a 12—mo period the integrated total of MLA—days for S. mellifera was 73% of that for C. megacarpus, and leaves produced during the current growing season contributed 71% of the total MLA—days in S. mellifera and only 37% in C. megacarpus. The differences between the results of this study and several accepted generalizations regarding growth and water relations of deciduous and evergreen chaparral shrubs may be due to the observation of these growth forms in different microsites and the lack of truly quantitative phenological measurements in previous studies.

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

SummaryPredawn 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.

Plant ecology: Family roots

Experiments in which related and unrelated plants were grown together reveal the ability of roots to recognize their kin. The ecological and evolutionary implications are tantalizing topics for future studies.

Effects of irradiance on growth, photosynthesis, and water use efficiency of seedlings of the chaparral shrub, Ceanothus megacarpus

SummaryThe effects of irradiance during growth on biomass allocation, growth rates, leaf chlorophyll and protein contents, and on gas exchange responses to irradiance and CO2 partial pressures of the evergreen, sclerophyllous, chaparral shrub, Ceanothus megacarpus were determined. Plants were grown at 4 irradiances for the growth experiments, 8, 17, 25, 41 nE cm-2 sec-1, and at 2 irradiances, 9 and 50 nE cm-2 sec-1, for the other comparisons.At higher irradiances root/shoot ratios were somewhat greater and specific leaf weights were much greater, while leaf area ratios were much lower and leaf weight ratios were slightly lower than at lower irradiances. Relative growth rates increased with increasing irradiance up to 25 nE cm-2 sec-1 and then leveled off, while unit leaf area rates increased steeply and unit leaf weight rates increased more gradually up to the highest growth irradiance.Leaves grown at 9 nE cm-2 sec-1 had less total chlorophyll per unit leaf area and more per unit leaf weight than those grown at 50 nE cm-2 sec-1. In a reverse of what is commonly found, low irradiance grown leaves had significantly higher chlorophyll a/b than high irradiance grown leaves. High irradiance grown leaves had much more total soluble protein per unit leaf area and per unit dry weight, and they had much higher soluble protein/chlorophyll than low irradiance grown leaves.High irradiance grown leaves had higher rates of respiration in very dim light, required higher irradiances for photosynthetic saturation and had higher irradiance saturated rates of photosynthesis than low irradiance grown leaves. CO2 compensation irradiances for leaves of both treatments were very low, <5 nE cm-2 sec-1. Leaves grown under low and those grown under high irradiances reached 95% of their saturated photosynthetic rates at 65 and 85 nE cm-2 sec-1, respectively. Irradiance saturated rates of photosynthesis were high compared to other chaparral shrubs, 1.3 for low and 1.9 nmol CO2 cm-2 sec-1 for high irradiance grown leaves. A very unusual finding was that leaf conductances to H2O were significantly lower in the high irradiance grown leaves than in the low irradiance grown leaves. This, plus the differences in photosynthetic rates, resulted in higher water use efficiencies by the high irradiance grown leaves. High irradiance grown leaves had higher rates of photosynthesis at any particular intercellular CO2 partial pressure and also responded more steeply to increasing CO2 partial pressure than did low irradiance grown leaves. Leaves from both treatments showed reduced photosynthetic capability after being subjected to low CO2 partial pressures (≃100 μbars) under high irradiances. This treatment was more detrimental to leaves grown under low irradiances.The ecological implications of these findings are discussed in terms of chaparral shrub community structure. We suggest that light availability may be an important determinant of chaparral community structure through its effects on water use efficiencies rather than on net carbon gain.

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.