Chris A. Martin

Socioeconomics drive urban plant diversity

Spatial variation in plant diversity has been attributed to heterogeneity in resource availability for many ecosystems. However, urbanization has resulted in entire landscapes that are now occupied by plant communities wholly created by humans, in which diversity may reflect social, economic, and cultural influences in addition to those recognized by traditional ecological theory. Here we use data from a probability-based survey to explore the variation in plant diversity across a large metropolitan area using spatial statistical analyses that incorporate biotic, abiotic, and human variables. Our prediction for the city was that land use, along with distance from urban center, would replace the dominantly geomorphic controls on spatial variation in plant diversity in the surrounding undeveloped Sonoran desert. However, in addition to elevation and current and former land use, family income and housing age best explained the observed variation in plant diversity across the city. We conclude that a functional relationship, which we term the “luxury effect,” may link human resource abundance (wealth) and plant diversity in urban ecosystems. This connection may be influenced by education, institutional control, and culture, and merits further study.

Ecosystem services and urban heat riskscape moderation: water, green spaces, and social inequality in Phoenix, USA

Urban ecosystems are subjected to high temperatures--extreme heat events, chronically hot weather, or both-through interactions between local and global climate processes. Urban vegetation may provide a cooling ecosystem service, although many knowledge gaps exist in the biophysical and social dynamics of using this service to reduce climate extremes. To better understand patterns of urban vegetated cooling, the potential water requirements to supply these services, and differential access to these services between residential neighborhoods, we evaluated three decades (1970-2000) of land surface characteristics and residential segregation by income in the Phoenix, Arizona, USA metropolitan region. We developed an ecosystem service trade-offs approach to assess the urban heat riskscape, defined as the spatial variation in risk exposure and potential human vulnerability to extreme heat. In this region, vegetation provided nearly a 25 degrees C surface cooling compared to bare soil on low-humidity summer days; the magnitude of this service was strongly coupled to air temperature and vapor pressure deficits. To estimate the water loss associated with land-surface cooling, we applied a surface energy balance model. Our initial estimates suggest 2.7 mm/d of water may be used in supplying cooling ecosystem services in the Phoenix region on a summer day. The availability and corresponding resource use requirements of these ecosystem services had a strongly positive relationship with neighborhood income in the year 2000. However, economic stratification in access to services is a recent development: no vegetation-income relationship was observed in 1970, and a clear trend of increasing correlation was evident through 2000. To alleviate neighborhood inequality in risks from extreme heat through increased vegetation and evaporative cooling, large increases in regional water use would be required. Together, these results suggest the need for a systems evaluation of the benefits, costs, spatial structure, and temporal trajectory for the use of ecosystem services to moderate climate extremes. Increasing vegetation is one strategy for moderating regional climate changes in urban areas and simultaneously providing multiple ecosystem services. However, vegetation has economic, water, and social equity implications that vary dramatically across neighborhoods and need to be managed through informed environmental policies.

Urbanization and warming of Phoenix (Arizona, USA): Impacts, feedbacks and mitigation

This paper examines the impacts, feedbacks, and mitigation of the urban heat island in Phoenix, Arizona (USA). At Sky Harbor Airport, urbanization has increased the nighttime minimum temperature by 5°C and the average daily temperatures by 3.1°C. Urban warming has increased the number of “misery hours per day” for humans, which may have important social consequences. Other impacts include (1) increased energy consumption for heating and cooling of buildings, (2) increased heat stress (but decreased cold stress) for plants, (3) reduced quality of cotton fiber and reduced dairy production on the urban fringe, and (4) a broadening of the seasonal thermal window for arthropods. Climate feedback loops associated with evapotranspiration, energy production and consumption associated with increased air conditioning demand, and land conversion are discussed. Urban planning and design policy could be redesigned to mitigate urban warming, and several cities in the region are incorporating concerns regarding urban warming into planning codes and practices. The issue is timely and important, because most of the worlds human population growth over the next 30 years will occur in cities in warm climates.

Drivers of Spatial Variation in Plant Diversity Across the Central Arizona-Phoenix Ecosystem

ABSTRACT We examined how growth of the Phoenix urban landscape has changed spatial patterns in native Sonoran desert plant diversity. Combining data from the U.S. Census with a probability-based field inventory, we used spatial and multivariate statistics to show how plant diversity across the region is influenced by human actions. Spatial variations in plant diversity among sites were best explained by current and former land use, income, housing age, and elevation. Despite similar average diversity in perennial plant genera between desert and urban sites, numerous imported exotics have significantly increased variation in plant generic composition among urban sites, with a “luxury effect” of higher plant diversity at sites in wealthier neighborhoods. We conclude that controls on natural spatially autocorrelated desert plant diversity are replaced by a variable suite of site-specific human factors and legacy effects, which require an integration of ecology and social science to be fully understood.

Alterations in growth and crassulacean acid metabolism (CAM) activity of in vitro cultured cactus

Unlike C-3 plants, cacti possess a crassulacean acid metabolism (CAM) physiology that can alter the pattern of carbon uptake and affect plant growth under artificial environmental conditions, especially in tissue culture. In vitro-derived plantlets of Coryphantha minima grew 7-fold larger than plants cultured under similar ex vitro conditions. Growth regulators incorporated into the culture media during shoot proliferation stage of micropropagation had a strong influence on this increased growth. Other important factors that contributed to increased growth under in vitro conditions were high relative humidity and sugar in the culture medium. An analysis of gas exchange and daily fluctuations of malic acid levels revealed an increase in net photosynthetic rate, in terms of carbon assimilation, by in vitro plants compared with that of ex vitro plants. This stimulated photosynthesis in the presence of an external carbon source was unexpected but apparently true for cacti exhibiting CAM physiology. Unlike CAM plants grown in ex vitro conditions, net CO2 uptake by in vitro-cultured cacti occurred continuously in the light as well as the dark. Once regenerated, cacti were transferred to ex vitro conditions where the normal CAM pathway resumed with a concomitant reduction in growth and CO2 uptake. These results showed that growth of cacti can be considerably accelerated by in vitro culture.

Geographic isolates of Glomus increase root growth and whole-plant transpiration of Citrus seedlings grown with high phosphorus

Abstract Four Glomus species/isolates from arid, semi-arid and mesic areas were evaluated for their effects on growth and water use characteristics of young Citrus volkameriana (′Volkamer′ lemon) under well-watered conditions, followed by three soil-drying episodes of increasing severity (soil moisture tensions of –0.02, –0.06, and –0.08 MPa) and recovery conditions. Arbuscular mycorrhizal (AM) plants were also compared to non-AM plants given extra phosphorus (P) fertilizer. AM plants and non-AM plants had similar shoot size (dry weight and canopy area), but all AM fungus treatments stimulated root growth (dry weight and length). Leaf P concentrations were 12–56% higher in AM plants than non-AM plants. Enhanced root growth was positively correlated with leaf P concentration. In general, AM plants had greater whole-plant transpiration than non-AM plants under well-watered conditions, under mild water stress and during recovery from moderate and severe soil drying. This suggests a faster recovery from moisture stress by AM plants. AM plants had lower leaf conductance than non-AM plants when exposed to severe soil drying. Although the greatest differences were between AM and non-AM plants, plants treated with Glomus isolates differed in colonization level, leaf P concentration, root length, transpiration flux and leaf conductance.

Element absorption and hydration potential of polyacrylamide gels

Abstract Element absorption and hydration potential (HP) of two polyacrylamide gels (Aa and Bb) were studied after incubation in Hoaglands nutrient solution concentrations of either 2X, 1X, 0.5X, 0.25X, 0.125X, or 0X (deionized water). Element absorption was observed and analyzed by making transects from the gel granule surface to center on a Phillips CM12S scanning transmission electron microscope (STEM) equipped with an EDAX 9800 plus EDS unit for x‐ray micro analysis. Thick sections were cut on dry glass knives using an RMC MT6000 ultramicrotome. Surface analysis of bulk specimens was also made with an AMR 1000A scanning electron microscope plus PGT1000 EDS unit. Overall, gel HP decreased curvilinearly as solution concentration increased linearly; however, HP was generally higher for gel Bb than for Aa. Surface analysis of both gels compounds (1X concentration) revealed the presence Ca, Zn, and K. Additionally, Mn, S, and P in trace amounts, and Fe were found at the surface of gel Aa. Furthermore, Fe ...

Response of southern magnolia to supraoptimal root-zone temperatures

Abstract 1. 1.|Rooted cuttings of southern magnolia (Magnolia grandiflora Hort. ‘St. Mary’) were exposed for 6 h daily to root-zone temperatures (RZT) of 28, 35 or 42°C for 8 weeks during the spring or fall of 1988. 2. 2.|Length of survival for 8-month old rooted cuttings was shortened when roots were exposed during the spring to 42 or 35°C compared to 28°C. Survival of 13-month old trees was unaffected by RZT treatments applied during the fall. 3. 3.|Electrolyte leakage from excised root tissue exposed for 30 min to temperatures ranging from 25 to 70°C, was used to assess cellular injury of 13-month old rooted cuttings after RZT treatments. 4. 4.|The critical killing temperatures (CT50) of root tissue pretreated at 28, 35 or 42°C RZT were 52.5 ± 0.9°C, 54.0 ± 0.4°C, respectively, and indicated differences in root membrane thermostability. 5. 5.|In another experiment, histological and scanning electron microscopy studies showed the proliferation of cells that was probably a callus response to temperature-mediated root injury induced by 38 or 42°C RZT after 20 or 8 days, respectively.