James L. Heilman’s research while affiliated with Texas A&M University and other places

Pigments are known to modify the spectral properties of foliage, which in turn affect the amount of radiant energy stored by the plant canopy. Studies have shown that red pigments (anthocyanin) increase leaf absorptivity of solar radiation, but little is known about their effect on canopy net radiation and temperature. We hypothesized that increased absorptivity of solar radiation caused by red pigments would result in higher canopy temperature when compared to that of a green canopy. To better understand the role of red pigments on canopy net radiation and temperature, we conducted a study where we measured leaf spectral properties, canopy spectral reflectivity, stomatal conductance, net radiation, and leaf and canopy temperature of red and green cotton (Gossypium hirsutum L.) canopies. On average, albedo of the red canopy was 0.02 lower than that of the green canopy, and most of the differences in reflected solar irradiance were in near-infrared wavelengths. Red canopy had greater net radiation than the green canopy throughout the measurement period, and that was due to its lower albedo. Red canopy was about 1 °C warmer than the green canopy on average; however, computer simulation indicates that albedo was of secondary importance in controlling canopy temperature. Contrary to our hypothesis, results suggest that lower stomatal conductance in the red leaves was the main driver of canopy temperature differences between red and green canopies.

Even though energy balance concepts are fundamental to solutions of problems in a number of disciplines in the agricultural and life sciences, they are seldom demonstrated in a laboratory activity. Here, we introduce a simple domeless net radiometer to demonstrate how the surface temperature of an object aboveground is regulated by the properties of the surfaces and environmental conditions. The device is based on the early designs of all-wave net radiometers and is composed of a foam disc with its opposing surfaces coated with either white or black paint. Temperatures of the disc’s surfaces are monitored using thermocouple temperature sensors. Using a combination of solar irradiance, albedo of the ground surface, air temperature, and wind speed measurements, the temperatures of the disc’s surfaces can be calculated by means of an energy balance model. We found good agreement between calculated and measured temperatures. In addition to demonstrate important physical concepts under natural outdoor conditions, we believe that the proposed laboratory activity will benefit students by allowing them to gain some experience and practical skills in working with environmental sensors, programming data acquisition systems, and analyzing data. Stimulating students’ creativity as well as developing their analytical and problem-solving skills is another goal of the proposed activity.

In an effort to conserve water, conversion of irrigated lawns to “water-saving” landscape designs are being promoted in urban areas. Different materials, with different radiative and thermal properties, are used to accomplish this. We conducted a field study where we measured reflected spectral irradiance, albedo, energy balance, and soil and surface temperatures of grass (GR), artificial turf (AT), decomposed granite (DG), and hardwood mulch (MU) to better understand how surface temperature is controlled in landscapes composed of these materials. DG had the highest albedo and a large fraction of the reflected solar energy was in the visible band. AT had the lowest albedo and highest net radiation of the materials we tested. DG was the most efficient material in conducting energy into the subsurface, whereas MU was the least. Both AT and MU concentrated most of their thermal energy in the surface, indicating poor ability to diffuse thermal energy into the subsurface. Latent heat flux was the major component of the energy balance of GR, as expected. After GR, DG maintained the lowest surface temperature due to a combination of high albedo and high heat flux into the subsurface.

Epicuticular wax is thought to enable sorghum [Sorghum bicolor (L.) Moench] plants to cope with drought. Increased reflectivity of solar radiation and reduced conductance of water vapor are mechanisms responsible for aiding plants in water conservation. Increased reflectivity should lead to a decrease in canopy temperature and water use, whereas decreased conductance should lead to an increase in canopy temperature because of a decrease in evaporative cooling. It is not clear how these competing effects exert control over water use in a crop such as sorghum. To better understand the role of epicuticular waxes on the energy balance of sorghum, experiments were conducted to determine the effects of waxes on field-scale energy fluxes using near-isogenic lines of grain sorghum having different levels of epicuticular wax loading. Waxes caused an overall 2% increase in albedo, and about 86% of the reflected energy between 400 and 1100 nm was from near-infrared wavelengths. This is at variance with recent reports that suggest that waxes can substantially increase the reflectivity of sorghum. Instead, our results indicated that the amount of reflected radiant energy due to waxes was small. When water was non-limiting, waxes caused a 22% decrease in canopy conductance compared to a 2% increase in albedo on average. Consequently, at the expense of higher canopy temperatures, waxes caused a 5% reduction in latent heat flux when water was not limiting. Without rain, water became limiting to plants with lower wax load sooner than it did for plants with higher wax load. The high wax load canopy then was on average 0.4 °C cooler, had 24% greater canopy conductance and 13% greater latent heat fluxes compared to the low wax one. Results suggested that the primary mechanism through which waxes affect the energy balance of sorghum is by means of reduced conductance of water vapor.

Core Ideas Time for water to fully drain from cracks and mesopores was 24 to 72 h. In moist, cracked soil, water moved to 60 cm after 2 h. On dry, cracked soil, water moved to >120 cm in <1.5 h. The mesopore infiltration module improved estimation of soil profile water content. At the pedon scale, the mesopore module improved ponding predictions. Water is preferentially conducted away from the soil surface through large cracks formed in shrink–swell soils, which complicates our ability to calculate the partitioning of water into infiltration and runoff. Preferential flow paths affect the hydrology of a landscape but often are not included in hydrology models. The Precision Agricultural‐Landscape Modeling System (PALMS) contains a Mesopore and Matrix (M&M) module that allows preferential flow and was tested on cracking soil at the pedon and small watershed scale for this study. Four irrigation events were conducted on 10‐m by 10‐m plots of a cracking soil, and volumetric water content (VWC) output for PALMS with and without the M&M module was compared with that measured by a neutron moisture meter. Additionally, measurements of VWC on a 4.4‐ha small watershed were compared with PALMS predictions. At both scales, the M&M module simulated water movement down the soil profile more quickly and eliminated unobserved ponding at the pedon scale relative to the PALMS matrix only. Simulations of water content of the soil profile were generally improved when the M&M module was used. Furthermore, PALMS M&M was relatively easy to parameterize using obtainable and physically relevant parameters, rendering it applicable to shrink–swell soils in a variety of systems.

Upland cotton (Gossypium hirsutum L.) is an important socioeconomic crop throughout most of the southern U.S. In Texas, cotton is the lead cash crop and its productivity is often limited by abiotic stress events such as drought and elevated ambient temperatures. The objective of this study was to assess the effects of 1-methylcyclopropene (1-MCP) applications triggered by canopy temperature and forecasted ambient temperatures on field-grown cotton plants. Yield and crop morphological responses to 1-MCP applications were investigated in field studies conducted during the summers of 2012 to 2014 at the Texas A&M University Field Laboratory in Burleson County, TX. Positive effects of 1-MCP were found for fruit retention in 2013 and 2014 for both irrigated and dryland studies; however, a negative impact was found in the 2012 irrigated study. By harvest, 1-MCP applications had no effect on final cotton yield or fiber quality parameters. Applications of 1-MCP affected some morphological characteristics of cotton plants; however, it did not improve crop yield.

Depending on severity, wildfire alters stand biomass, tree species distribution, and age, which may modify stand transpiration (Et) and the amount of water available to other parts of the hydrologic cycle. Our objective was to determine how wildfire severity affected Et in mixed pine/oak (Pinus taeda L./Quercus stellata Wangehn., Quercus marilandica Muenchh.) stands in the Lost Pines eco‐region (Bastrop, TX, USA). Transpiration was estimated for mature pines and oaks at unburned and moderately burned sites, and oak resprouts and pine saplings at a severely burned plot. On average, mature pines had 36% greater sap flux rates (Js) than mature oaks in the unburned and moderately burned stands. Under low moisture stress, regenerating pines had greater Js than resprouting oaks, but Js quickly decreased as soil moisture declined. By contrast, mature pines were unaffected by dry periods. Pines contributed most to Et at the unburned and moderate stands. Conversely, oak Et dominated the severely burned stand, contributing over 95%. Transpiration was greatest at the moderately burned stand (2.02 mm day‐1), followed by the unburned (1.44 mm day‐1), and the severely burned stands (0.46 mm day‐1). Despite greater Js in resprouts and saplings, reductions in total sapwood area resulted in lower stand‐level daily Et at the severe site. Although severe fire decreased stand transpiration through reductions in vegetation density, individual oak resprouts appear to thrive, undeterred by high vapor pressure deficit. Without pine planting, oaks will likely dominate severely burned stands which could result in shifts to local hydrology and microclimate.

Increased rainfall variability due to climate change significantly impacts carbon and water cycling in ecosystems, but these impacts may be masked when using arbitrary annual reporting periods such as the calendar year which may not have any relevance to natural annual ecosystem processes. A variety of alternative annual integration periods have been described for specific purposes or locations, but are of limited general applicability. Here we present an eddy covariance data‐driven empirical method to determine a locally relevant annual time period. The method selects a start date for a locally relevant water year (LRWY) that maximizes correlation between annual precipitation (AP), and annual evapotranspiration (AET) and annual gross primary production (AGPP). The method was tested using data from 2004 – 2013 for two Ameriflux sites (woodland, grassland) in Central Texas. The timeframe included periods of unusually high rainfall, and periods of extreme drought. Highest correlation between AP, and AET and AGPP was obtained with an LRWY beginning in mid‐September. Use of the LRWY better captured the impact of soil water recharge in the autumn on photosynthesis the following spring than did calendar years. Use of the LRWY also identified more annual periods in which AET exceeded AP, which more accurately reflected the impact of drought on ecosystems processes than did analysis based on calendar years.

Cotton (Gossypium hirsutum L.) is the lead cash crop in Texas, and its productivity is often challenged by stressful environmental conditions such as high temperatures and sub-optimal water supply. The objective of this investigation was to assess the impact of 1-methylcyclopropene (1-MCP) applications triggered by canopy temperature and forecasted ambient temperatures on field-grown cotton plants. Physiological responses to 1-MCP applications were investigated in field studies conducted during the summers of 2012-2014 at the Texas AandM University Field Laboratory in Burleson County, TX. During all three growing seasons, more than 65% of the days reached temperatures above 28 °C, which indicated great potential for high temperature stress. Daily plant canopy temperature, net photosynthesis, transpiration, and photosystem II quantum yield were affected by 1-MCP treatment when plants were irrigated, but not under dryland conditions. Positive effects of 1-MCP were found for fruit retention in 2013 and 2014, for both irrigated and dryland studies, although a negative impact was found in the 2012 irrigated study. Applications of 1-MCP affected physiological characteristics; however, it did not affect crop yield.

Background and aims We quantified spatial variability in water storage and plant access to water in the rocky soils of a karst savanna dominated by Ashe juniper (Juniperus ashei) and honey mesquite (Prosopis glandulosa). Methods In a 25×25 m grid with 5-m node spacing, water content and bulk density profiles were measured to a depth of 1.6 m by a combination of time domain reflectometry, neutron thermalization and gamma ray densitometry. Changes in water content were used to infer recharge and plant uptake of soil water. Predawn water potentials of trees were sampled periodically to evaluate individual differences in water access. Results Pore volume and maximum water storage varied between 0.24 and 0.42 m3 m−3, and 198 to 431 mm, respectively, across the 36 individual profiles. Porosity accounted for 19 and 20%and depth for 35 and 61% ofthe variation in uptake and recharge, respectively.Predawn water potentials were consistently different among individual trees over multiple dry seasons. Conclusions Unequal water status among trees was consistent with the variability of recharge and uptake in the rooting zone, suggesting that trees cannot fully compensate for spatial variability in soil properties by roots foraging for water.