Thomas H. Whitlow

Coupling biogeochemical cycles in urban environments: ecosystem services, green solutions, and misconceptions

Urban green space is purported to offset greenhouse-gas (GHG) emissions, remove air and water pollutants, cool local climate, and improve public health. To use these services, municipalities have focused efforts on designing and implementing ecosystem-services-based “green infrastructure” in urban environments. In some cases the environmental benefits of this infrastructure have been well documented, but they are often unclear, unquantified, and/or outweighed by potential costs. Quantifying biogeochemical processes in urban green infrastructure can improve our understanding of urban ecosystem services and disservices (negative or unintended consequences) resulting from designed urban green spaces. Here we propose a framework to integrate biogeochemical processes into designing, implementing, and evaluating the net effectiveness of green infrastructure, and provide examples for GHG mitigation, stormwater runoff mitigation, and improvements in air quality and health.

A unifying framework for biological invasions: the state factor model

Biological invasions are a fixture in our landscapes, with consequent losses in endemic biota and shifts in ecosystem function. Despite the historical recognition of exotic species success in novel environs, this phenomenon lacks a holistic-descriptive framework. Recent attempts to explain biological invasions are based largely on identifying the inherent invasive qualities of successful exotic species (i.e., invasiveness), or characterizing the susceptibility of a habitat to an introduced species (i.e., invasibility), with few studies examining their interaction or additional contributing factors (e.g., time since introduction). We propose unifying the ’points of entry’ into biological invasions with a state factor model that incorporates all contributing variables—not just species or habitats—into a quantifiable, factorial model amenable to hypothesis testing. State factors are phenomenological variables describing the state of a system—historically used in soil and vegetation science. Our state factor equation relates any quantifiable property of an invasion (i) as a function of propagule pressure (p), introduced habitat (h), invader autecology (a), source environment (s), and time since introduction (t). By manipulating state factors singly, or in interaction, targeted variation can be related to quantifiable properties of exotic species while controlling, or at minimum accounting for, remaining factors contributing variation to the system. This holistic factor-function paradigm extends research on invasions from beyond the limits imposed by current theory, fosters novel empirical approaches, elucidates knowledge gaps in our understanding of resident invasions, and allows for variable accounting via a factor matrix. Here we briefly outline the ontogeny of state factors in soil and vegetation science, detail our proposed ’phast’ framework for biological invasions, including notation, and examine a case study in state factor utility.

Effects of leaf and sap feeding insects on photosynthetic rates of goldenrod

SummaryHerbivory can alter the balance between sources and sinks within a plant, and changes in the source-sink ratio often lead to changes in plant photosynthetic rates. We investigated how feeding by three insect herbivores affected photosynthetic rates and growth of goldenrod (Solidago altissima). One, a phloem-sap feeding aphid (Uroleucon caligatum), creates an additional sink, and the other two, a leaf-chewing beetle (Trirhabda sp.) and a xylem-sap feeding spittlebug (Philaenus spumarius) both reduce source supply by decreasing leaf area. Plants were grown outside in large pots and insects were placed on them at predetermined densities, with undamaged plants included as controls. All insects were removed after a 12-day feeding period. We measured photosynthetic rates both of damaged leaves and of undamaged leaves that were produced after insect removal. Photosynthetic rates per unit area of damaged leaves were reduced by spittlebug feeding, but not by beetle or aphid feeding. Conductance of spittlebugdamaged leaves did not differ from controls, but internal carbon dioxide concentrations were increased. These results indicate that spittlebug feeding does not cause stomatal closure, but impairs fixation within the leaf. Effects of spittlebug feeding on photosynthetic rates persisted after the insects were removed from the plants. Photosynthetic rates per unit area of leaves produced after insect removal on spittlegug-damaged plants were lower than control levels, even though the measurements were taken 12 days after insect removal. The measurement leaf on spittlebugdamaged plants was reduced in area by 27% relative to the controls, but specific leaf area (leaf area/leaf weight) was increased by 18%. Because of the shift in specific leaf area, photosynthetic rates were also examined per unit leaf weight, and when this was done there were no significant differences between control and spittlebug-damaged plants. Beetle and aphid feeding had no effects on the photosynthetic rate of the leaves produced after insect removal. Plant relative growth rates (in terms of height) were reduced by spittlebugs during the period that the insects were feeding on the plants. Relative growth rates of spittlebug-damaged plants were increased above control levels after insect removal, but these plants were still shorter than controls 17 days after insect removal. Beetles and aphids did not affect plant relative growth rates or plant height. Feeding by both spittlebugs and beetles reduced leaf area, and the effect of the spittlebug was more severe than that of the beetle. These results show that effects of herbivory on photosynthetic rates cannot be predicted simply by considering changes in the source-sink ratio, and that spittlebug feeding is more damaging to the host plant than beetle or aphid feeding.

SMP: Solid matrix priming of seeds

The purpose of these studies was to develop the technique of solid matrix priming (SMP) and then evaluate its ability to improve seedling establishment of small-seeded vegetable crops under either sub- or supra-optimal temperatures. SMP is a process in which seeds are mixed with a solid material and water in known proportions. This mixture allows the seeds to imbibe and attain a threshold moisture content, but prevents radicle emergence. The solid material used in these studies was a ground Leonardite Shale, “Agro-Lig”, which was a friable material and had a high water-holding capacity. The water potential of the Agro-Lig was determined after SMP and the osmotic potential accounted for >97% of the total water potential, which was attributed to solutes from the Agro-Lig and seed leachates. The matric potential contributed <1.5% of the total water potential. Seedling emergence studies were conducted on primed tomato (Lycopersicon esculentum), carrot (Daucus carota) and onion (Allium cepa) seeds sown in an artificial soil media in flats maintained at 2010°C with a 12-h photoperiod. The priming treatments consisted of polyethylene glycol 8000, inorganic salts and SMP. All priming treatments decreased the time for 50% seedling emergence and increased the plant dry weight compared with the non-treated controls for each crop. The seedling emergence characteristics of SMP-sown seeds were superior to, or equal to, conventional solution priming treatments. In addition, lettuce (Lactuca sativa) seeds were primed with solutions or SMP and later germinated at 35°C in the dark. All priming treatments overcame thermodormancy.

Evolution of an invasive phenotype: shift to belowground dominance and enhanced competitive ability in the introduced range

In response to novel selection pressures in an introduced range, non-native species may evolve more competitive phenotypes unique from those of their native range. We examined the existence of an invasive phenotype in the herbaceous perennial Artemisia vulgaris, a frequent invader of the Northeast and Mid-Atlantic US. Populations from both the native (European) and the introduced (North American) ranges were grown in intra-specific competition (same population), inter-specific competition with the native perennial herb Solidago canadensis, and alone in a common garden to quantify shifts in resource allocation and neighbor effects on performance and competitive ability. Without competition, introduced A. vulgaris populations were much shorter than native populations, but germinated earlier, produced more ramets, more belowground and total biomass, and maintained higher root-to-shoot ratios. Under inter- and intra-specific competitions, introduced A. vulgaris populations were shorter, but produced more ramets, belowground, and total biomass than native populations. S. canadensis belowground and total biomass were more highly suppressed by introduced than native A. vulgaris. Our data suggest that since the introduction to North America, A. vulgaris has evolved a more competitive invasive phenotype characterized by many short ramets with more extensive root/rhizome networks. This rapid evolutionary shift likely benefits A. vulgaris in its introduced range by allowing establishment and subsequent dominance in dense stands of existing vegetation.

Impact of local traffic exclusion on near-road air quality: Findings from the New York City “Summer Streets” campaign

We monitored curbside airborne particulate matter (PM) concentrations and its proinflammatory capacity during 3 weekends when vehicle traffic was excluded from Park. Ave., New York City. Fine PM concentration peaked in the morning regardless of traffic while ultrafine PM was 58% lower during mornings without traffic. Ultrafine PM concentration varied linearly with traffic flow, while fine PM spiked sharply in response to random traffic events that were weakly correlated with the traffic signal cycle. Ultrafine PM concentrations decayed exponentially with distance from a cross street with unrestricted traffic flow, reaching background levels within 100 m of the source. IL-6 induction was typically highest on Friday afternoons but showed no clear relationship to the presence of traffic. The coarse fraction (>2.5 μm) had the greatest intrinsic inflammatory capacity, suggesting that coarse PM still warrants attention even as the research focus is shifting to nano-particles.

Revealing historic invasion patterns and potential invasion sites for two non-native plant species.

The historical spatio-temporal distribution of invasive species is rarely documented, hampering efforts to understand invasion dynamics, especially at regional scales. Reconstructing historical invasions through use of herbarium records combined with spatial trend analysis and modeling can elucidate spreading patterns and identify susceptible habitats before invasion occurs. Two perennial species were chosen to contrast historic and potential phytogeographies: Japanese knotweed (Polygonum cuspidatum), introduced intentionally across the US; and mugwort (Artemisia vulgaris), introduced largely accidentally to coastal areas. Spatial analysis revealed that early in the invasion, both species have a stochastic distribution across the contiguous US, but east of the 90th meridian, which approximates the Mississippi River, quickly spread to adjacent counties in subsequent decades. In contrast, in locations west of the 90th meridian, many populations never spread outside the founding county, probably a result of encountering unfavorable environmental conditions. Regression analysis using variables categorized as environmental or anthropogenic accounted for 24% (Japanese knotweed) and 30% (mugwort) of the variation in the current distribution of each species. Results show very few counties with high habitat suitability (≥80%) remain un-invaded (5 for Japanese knotweed and 6 for mugwort), suggesting these perennials are reaching the limits of large-scale expansion. Despite differences in initial introduction loci and pathways, Japanese knotweed and mugwort demonstrate similar historic patterns of spread and show declining rates of regional expansion. Invasion mitigation efforts should be concentrated on areas identified as highly susceptible that border invaded regions, as both species demonstrate secondary expansion from introduction loci.

Photosynthetic Response to Flooding of Acer Rubrum Seedlings from Wet and Dry Sites

Abstract Seeds were collected from three red maple swamps (wet sites) and three uplands (dry sites) near Ithaca, New York, and the resulting seedlings were used in flood tolerance studies to investigate if red maples broad habitat range is due to ecotypic differentiation. One-year-old seedlings were flooded while still dormant (spring flooding study) and net photosynthesis, growth and chlorophyll levels were measured at 1 mo intervals for 3 mo. Flooding reduced net photosynthesis, growth and chlorophyll levels in seedlings from both sites, but survival of both wet and dry site seedlings was near 100%. After 1 mo of flooding net photosynthesis of wet and dry site seedlings were similar, but after 3 mo, flooded wet site seedlings had higher photosynthetic rates than did dry site seedlings. Control wet site seedlings were significantly larger than dry site seedlings and had significantly higher photosynthetic rates and chlorophyll levels. These differences suggest either genetic variation between seedlings from the two habitats in response to the growing conditions or the influence of seed size differences and confound the spring flooding study results. Flooded wet site seedlings had higher, final, net photosynthetic rates than did dry site seedlings, but the response to flooding was greater for wet site seedlings than it was for dry site seedlings. In a second study (summer flooding study), 1-y-old seedlings in full leaf were flooded for 22 d and then drained to determine if recovery from flooding stress differed for wet and dry site seedlings. Again, flooding decreased net photosynthesis for seedlings from both habitats but, when the trees were drained, net photosynthesis for wet site seedlings recovered more quickly and to a higher level than it did for dry site seedlings. Flooding also caused a drop in chlorophyll levels for seedlings from both habitats, but chlorophyll levels of seedlings from neither habitat recovered when the seedlings were drained. There were no significant differences between wet site and dry site control seedlings for net photosynthesis; therefore, the quicker and larger recovery of photosynthetic potential in wet site seedlings in the summer flooding study suggests that ecotypic differentiation has occurred and that genetic differences, in part, account for red maples occurrence on contrasting edaphic sites.

A laser-diode-based system for measuring sap flow by the heat-pulse method

Transpiration, the movement of water through plants from the soil to the atmosphere, is an important process in plant physiology, the hydrologic cycle, and the global energy balance. Transpiration at the scale of individual plants can be measured with weighing lysimetry, but this technique is limited to small plants and is generally impractical for trees and many field crops. Heat-pulse velocity methods offer an alternative, and several plant sap flow gauges have been marketed. Because these gauges use electrical resistance heaters to heat the stem, they present several problems: they are invasive, typically bulky, provide poor temperature control (killing the cambium), and have lengthy response times, so they cannot measure short-term transients. In this report, we describe a system for measuring sap flow in real-time and without the need to puncture the stem. Instead of a resistance heater, it uses a laser beam as a heat source, and instead of contact thermometers, it uses non-contact infrared thermometers. Used with a precision-mounting unit that insures constant alignment, it determines whole-plant transpiration. The laser has the added advantage of delivering a precisely controlled amount of heat for a discrete time period. The end product is a more accurate, less invasive way to gauge water flow through herbaceous plant stems.

Turgor maintenance in leaves and roots of ‘Colt’ cherry trees (Prunus avium X pseudocerasus) in response to water stress

SummaryPressure-volume methodology was used to evaluate the components of plant water potential in expanding leaves, mature leaves and roots of well watered and water stressed plants of Prunus avium x pseudocerasus ‘Colt’. Under well watered conditions, expanding leaves, mature leaves and roots lost turgor at water potentials of -1.37, -1.84, and -1.12 MPa, respectively. Comparable tissue from water stressed plants maintained turgor to^water potentials of —2.08, —2.09, and —1.85 MPa. This improved capacity for maintaining turgor resulted primarily from changes in tissue osmotic potential and was not due to changes in tissue elasticity. Regardless of the imposed water regime, roots lost less turgor for a given change in tissue water potential than did leaves. As compared with leaves, the more elastic root tissue compensated for higher tissue osmotic potentials at full turgor by allowing for a greater reduction in relative water content and concentration of tissue solutes as water potentials decreased, ther...