Variation in Leaf Structure and Function in Quercus Douglasii Trees Differing in Root Architecture and Drought History

Seasonal changes in leaf specific mass, nitrogen, chlorophyll, and photosynthetic properties were measured for two groups of spatially intermixed Quercus douglasii trees with different drought histories and apparently different root architectures. One group, referred to as \u27\u27high-psi(pd) trees, included trees with low amounts of fine root biomass in the upper 50 cm of soil and high predawn xylem pressure potentials (psi(pd)) during summer drought. These two characteristics indicate that trees in this group have deep roots, which may reach the water table. The second group, referred to as \u27\u27low-psi(pd) trees, had three to five times higher fine root biomass in the upper 50 cm of soil and low psi(pd) during summer drought. These two characteristics indicate that these trees may not have access to the water table and are dependent on shallow soil moisture, which decreases rapidly during the rainless summers of central California. In the spring, after the full expansion of new leaves, but prior to significant divergence in psi(pd) between the groups, leaf area per leaf, leaf specific mass, chlorophyll per leaf area, incident quantum yield, leaf respiration rate, and irradiance at light compensation were lower for low-psi(pd) trees than for trees with high psi(pd). Nitrogen per leaf area did not differ between the groups. Net photosynthetic capacity at 2000 mu mol m(-2) s(-1) (A(max)) per leaf area was similar among all trees in the spring, but A(max)/leaf mass during the spring was higher for trees that eventually would develop low seasonal psi(pd). Since differences existed between new cohorts of leaves produced in the spring before summer drought, when psi(pd) was similar, we suggest that some leaf characteristics of Q. douglasii trees are determined by the de ree of drought exposure experienced in previous years, or by genetic variation within the species. During the rainless summer and fall seasons, A(max)/leaf area, A(max)/leaf mass, and total leaf chlorophyll/leaf mass decreased more rapidly in trees with low psi(pd) than in trees with high psi(pd), so that from August to the beginning of leaf senescence in October, leaves of high-psi(pd) trees had higher A(max)/leaf area, A(max)/leaf mass, and total leaf chlorophyll/leaf mass than those of low-psi(pd) trees. Overall, variations in root architecture and summer psi(pd), for Q. douglasii were correlated with substantial differences in morphological and physiological leaf characteristics. This apparent coordination of aboveground and belowground organs may explain, in part, how Q. douglasii tolerates the exceptionally broad range of topography and soil moisture conditions in which it occurs

Effects of Regional Origin and Genotype on Intraspecific Root Communication in the Desert Shrub Ambrosia Dumosa (Asteraceae)

Previous work has shown that the contact inhibition that occurs among roots of Ambrosia dumosa shrubs has a self/nonself recognition capability. In the current study, we investigated some of the geographic and genotypic dimensions of this recognition capability by using root observation chambers to observe the effects of encounters of individual roots on root elongation rates. We measured such effects in encounters between roots of plants from the same region and compared these to effects in encounters between roots of plants from two different regions. We also measured effects of encounters between roots of plants from the same clones and compared these to effects of encounters of roots of plants from different clones. Roots of plants from the same region (population) showed the usual \u27\u27nonself\u27\u27 precipitous decline in elongation rates following contact, but when roots of plants from different regions contacted each other, elongation rates continued unchanged. When roots of separate plants from the same clone contacted each other, the same \u27\u27nonself\u27\u27 precipitous decline in elongation rates as seen in encounters between roots of plants of different clones from the same region occurred. Meanwhile, in these same experiments \u27\u27self\u27\u27 contacts between sister roots connected to the same plants resulted in no changes in elongation rates. Thus, differences between individuals from two geographically separate populations of Ambrosia dumosa may be sufficient to thwart the \u27\u27nonself,\u27\u27 population-level recognition of similarity apparently necessary for contact inhibition. Furthermore, the \u27\u27self\u27\u27 recognition mechanism, which precludes contact inhibition between two roots on the same plant, appears to be physiological rather than genetic in nature

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 melitensis, 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 \u3e50% 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 Centaurea and Nassella vs. Centaurea and Avena has important implications for the invasion of California grasslands

Spatial ecology of a small desert shrub on adjacent geological substrates

1. Spatial pattern analyses were used to generate hypotheses about the processes that shape the structure of a plant community in the Mojave Desert of North America, with a focus on the semi-shrub Ambrosia dumosa. We analysed spatial distributions and sizes of this species relative to other semi-shrubs, shrubs and annuals, and the relationships between spatial patterns and abiotic and biotic habitat characteristics. 2. The analyses were based on maps of sample plots placed along a transect spanning two adjacent geological substrates: aeolian sand and gravelly, sandy to loamy alluvium. Of these two substrates, sand supported higher total biomasses of Ambrosia and of all woody perennials, while alluvium supported on average higher biomasses of winter annuals. 3. Annuals and seedlings of Ambrosia were much more strongly aggregated with Ambrosia canopies on sand than on alluvium, suggesting that these small plants were more strongly facilitated by Ambrosia on sand than on alluvium. 4. Ambrosia semi-shrubs were spatially segregated on sand but aggregated on alluvium, and the degree of segregation on sand increased with the total above-ground biomass of Ambrosia per unit area, indicating that negative interactions between Ambrosia plants were stronger in more productive habitats. Canopy sizes of Ambrosia in all mapped plots increased with distance to the nearest conspecific neighbour, which suggests that neighbour interactions negatively affected plant sizes. 5. Ambrosia plants on sand were spatially aggregated with Acamptopappus sphaerocephalus semi-shrubs, suggesting that at least one of these species may benefit from the association. Ambrosia plants were spatially segregated from Larrea tridentata shrubs on both substrates, possibly due to negative effects of Larrea roots on Ambrosia roots reported in previous studies. 6. Subtle differences in substrate characteristics were correlated with strong differences in the spatial distribution of Ambrosia plants relative to their neighbours, which suggests that edaphic conditions may affect the spatial structure of the community by modifying complex positive and negative interactions between neighbouring plants

Spatial ecology of a small desert shrub on adjacent geological substrates

Summary 1 Spatial pattern analyses were used to generate hypotheses about the processes that shape the structure of a plant community in the Mojave Desert of North America, with a focus on the semi-shrub Ambrosia dumosa . We analysed spatial distributions and sizes of this species relative to other semi-shrubs, shrubs and annuals, and the relationships between spatial patterns and abiotic and biotic habitat characteristics. 2 The analyses were based on maps of sample plots placed along a transect spanning two adjacent geological substrates: aeolian sand and gravelly, sandy to loamy alluvium. Of these two substrates, sand supported higher total biomasses of Ambrosia and of all woody perennials, while alluvium supported on average higher biomasses of winter annuals. 3 Annuals and seedlings of Ambrosia were much more strongly aggregated with Ambrosia canopies on sand than on alluvium, suggesting that these small plants were more strongly facilitated by Ambrosia on sand than on alluvium. 4 Ambrosia semi-shrubs were spatially segregated on sand but aggregated on alluvium, and the degree of segregation on sand increased with the total above-ground biomass of Ambrosia per unit area, indicating that negative interactions between Ambrosia plants were stronger in more productive habitats. Canopy sizes of Ambrosia in all mapped plots increased with distance to the nearest conspecific neighbour, which suggests that neighbour interactions negatively affected plant sizes. 5 Ambrosia plants on sand were spatially aggregated with Acamptopappus sphaerocephalus semi-shrubs, suggesting that at least one of these species may benefit from the association. Ambrosia plants were spatially segregated from Larrea tridentata shrubs on both substrates, possibly due to negative effects of Larrea roots on Ambrosia roots reported in previous studies. 6 Subtle differences in substrate characteristics were correlated with strong differences in the spatial distribution of Ambrosia plants relative to their neighbours, which suggests that edaphic conditions may affect the spatial structure of the community by modifying complex positive and negative interactions between neighbouring plants

Data from: A 37-year experimental study of effects of structural alterations on a shrub community in the Mojave Desert, California

1. In 1977 an experiment was initiated in the Mojave Desert to investigate how shrub interactions affect structure in a community dominated by Ambrosia dumosa and Larrea tridentata. Here, as in much of the Mojave, Larrea were regularly distributed, Ambrosia occurred in aggregations, and the two were randomly distributed relative to each other. Pre-dawn xylem pressure potentials (PDXPPs) of single Ambrosia or Larrea in centers of 100m2 circular plots were monitored to assess effects of intraspecific, interspecific, and total removals of neighboring shrubs. Contrary to theory, results over the next two years indicated interspecific interference was more intense than intraspecific interference in both species. 2. These plots were maintained through 2014. Measurements of seedling recruitment from 1980 to 2014, and of PDXPP, aboveground biomass, and canopy senescence from 2003 to 2014 were conducted. 3. Recruitment of both species was substantial immediately after the removals, but declined to very low levels after 1983. Ambrosia recruited into all Ambrosia and Total-Removal plots, but Larrea recruited only into plots that contained mature Ambrosia. 4. PDXPPs of monitored shrubs continued to be enhanced in removal plots for at least 27 years, but this changed from most being due to interspecific removals in both species to intraspecific removals causing most enhancement in Ambrosia and inter- and intraspecific removals causing nearly equal enhancements in Larrea. 5. Aboveground biomasses of monitored shrubs of Ambrosia and Larrea were 2.1X and 2.8X larger in Total-Removal plots, 1.6X and 1.7X larger in intraspecific removal plots, respectively, and 1.1X larger in interspecific removal plots for both species than those in Control plots, indicating the absence of intraspecific interference had the dominant long-term effect. 6. Canopy senescence differed between Ambrosia and Larrea in extent, timing and effect of specific removal treatments; it was greatest for both species in Controls, averaging 75% and 34%, respectively. 7. Synthesis. Shrub interactions and their relations to community structure are mechanistically and spatially complex. Differences between short-term and long-term responses to removals reveal multi-tiered, temporally dynamic feedback loops between shrub interactions and community structure driven by demographics, species-specific root growth, resource competition, communications, and territoriality

LTDS - KEY for plots and treatments

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