K. J. McInnes’s research while affiliated with Texas A&M University and other places

Woody plant areal encroachment is pervasive throughout the Southern Great Plains, USA. The ability of woody plants to dissipate excess solar radiation - dynamically over the day and sustained periods without recovery overnight -is key for maintaining photosynthetic performance during dry stretches, but our understanding of these processes remains incomplete. Photosynthetic performance and energy dissipation were assessed for co-occurring encroachers on the karst Edwards Plateau (Juniperus ashei, Prosopis glandulosa, and Quercus fusiformis) under seasonal changes in water status. Only J. ashei experienced mild photoinhibition from sustained energy dissipation overnight while experiencing the lowest photochemical yields, minimal photosynthetic rates, and the highest dynamic energy dissipation rates at midday during the dry period - indicating susceptibility to photosynthetic downregulation and increased dissipation under future drought regimes. Neither other encroacher experienced sustained energy dissipation in the dry period, though P. glandulosa did experience marked reductions in photosynthesis, photochemical yields, and increased regulatory dynamic energy dissipation.

Understanding the dynamics of cracking and swelling of clayey soils improves the ability to predict the impact of land use on the hydrologic response of watersheds containing shrink-swell soils. The objective of this research was to characterize the impact of land use on spatial and temporal shrink-swell dynamics of a Vertisol. The land uses are grazing land (GL), native prairie (NP), and cropland (CL). The research was conducted at the USDA-ARS Grassland, Soil and Water Research Laboratory near Riesel TX. The soil at the site is Houston Black (Fine, smectitic, thermic Udic Haplustert). To monitor vertical soil movements, five measurement sites each on a GL and NP and four measurement sites on a CL (corn) were selected. Bi-weekly measurements of vertical soil movements and soil water were made beginning in June 2008. The change in absolute heights of soils at the surface and subsurface (0, 30, 60 and 90 cm) was used to track the temporal trends in thickness of soil layers. Near each set of soil height measurement, soil water content was measured using a neutron moisture meter. The study showed that maximum soil subsidence and the time of its occurrence varied with land use. The maximum soil subsidence ranged from 91 to 120 mm in the sites of GL; from 71 to 75 mm in the sites of NP, and from 67 to 76 mm in the sites of CL. The maximum soil subsidence in the CL was within the same range as that of the prairie (NP), but the maximum shrinkage occurred in mid August 2008 in the cornfield whereas at the end of August 2009 in the prairie. Grass roots, size, and shape of gilgai likely influence the observed variation and further studies are underway. Knowledge gained in these studies may be used to modify and refine hydrology models that simulate runoff, infiltration and solute transport across different land uses in a Vertisol landscape.

Woody encroachment into karst grasslands and savannas is presumed to reduce water availability and aquifer recharge, in part, because deep roots extract large quantities of water from perennial sources within the fractured bedrock underlying shallow soils. If true, energy balance partitioning and transpiration in woody ecosystems should be decoupled to an extent from rainfall, and sensitivity of the energy balance and evapotranspiration (ET) to rainfall and water deficits should be dampened. We evaluated responses of energy and water vapor fluxes to rainfall and water deficits in a live oak (Quercus virginiana)-Ashe juniper (Juniperus ashei) woodland on the karst Edwards Plateau, TX, USA, over a 2-year period using eddy covariance measurements of the turbulent fluxes. Total ET during the two years was 1416 mm, 92% of total rainfall. We observed large and rapid reductions in λE and increases in H during drying cycles, and high correlation between ET and soil water content in the upper 20 cm of the root zone. In most cases, ET declined at the same time as soil water content, indicating that the woodland relied heavily on water from recent rainfall events, rather than antecedent water. We found no evidence that deep roots were extracting significant amounts of water from a perennially stable supply of water. Excavations at the woodland site revealed a rock layer at 20 cm below the soil surface, with a dense root mat above the rock and penetration of relatively few roots into the rock through cracks and fissures. Thus, the most likely sources of water for trees were soil water and a limited supply of water stored in near-surface fractured rock layers.

Eddy covariance measurements were begun in late April 2004 to quantify CO2 exchange in a perennial C3/C4 grassland on the Edwards Plateau near San Marcos, TX. Objectives were to document how net ecosystem exchange of CO2 (NEE) and its components, gross photosynthesis (GPP) and ecosystem respiration (Re), vary on a seasonal and interannual basis, and to examine how environmental factors affect C exchange. Described here are the first 32 months of measurements. The grassland was intermittently grazed in 2004 and 2005, and heavily grazed during the spring and summer of 2006. Total rainfall from May through December 2004 was 1378 mm, well above the 858 mm annual mean, whereas rainfall in 2005 and 2006 was near normal. The grassland was dominated by C4 grasses when measurements began, but C3 grasses and forbs became dominant as the study progressed. The shift from a C4- to a C3-dominated ecosystem was accompanied by a 24% decline in light use efficiency. Water deficits were a frequent occurrence, even during 2004 when rainfall was high, causing large reductions in Re, GPP, and light use efficiency, and temporary shifts in the grassland from C sink to C source. Our measurements showed the grassland was a small C sink over the 32 months, gaining 170 g m−2 of C, due in large part to drought-induced suppression of Re during the winter of 2005–2006, and to a pulse of growth that occurred during the last 3 months of 2006. Total GPP and Re were 2081 and 1911 g m−2, respectively. The grassland accumulated 1037 g m−2 of C during the daytime, and lost 867 g m−2 at night. Rates of C uptake were highest in the spring, and were higher when grazing was heaviest because growth of new leaves having a high photosynthetic efficiency, and reductions in Re, compensated for loss of leaf area.

Savannas in central Texas are dominated by live oak (Quercus virginiana) and Ashe juniper (Juniperus asheii) underlain by perennial, C3/C4 grasslands, and are increasingly becoming juniper and mesquite dominated due to overgrazing and suppression of wildfires. Since 2004, we have been investigating how carbon, water and energy exchange in these rain-limited savannas respond to rainfall variability and this observed vegetation change. In semi-arid regions, rainfall pulses provide inputs of soil moisture and trigger biotic activity in the form of plant gas exchange and microbial metabolism as well as water dependent physical processes in the soil. Each of these components has a different characteristic response curve to soil moisture and integrates soil water content over a different range of depths. Here we focus on examining how the observed increase of woody species in central Texas savannas alters the response of net ecosystem exchange and its components, ecosystem respiration and gross ecosystem exchange, to rain pulses. Using data we have collected over the last three years from three Ameriflux tower sites at Freeman Ranch near San Marcos, TX (C3/C4 grassland, juniper/mesquite savanna with 50 percent woody cover, and oak/juniper woodland), we quantify the responses of both ecosystem respiration and daily carbon uptake to rainfall pulses throughout the year. Specifically, we look at the enhancement and persistence of ecosystem respiration and carbon uptake responses following a pulse, and isolate the main controlling factors on the observed response: seasonality, antecedent soil moisture and temperature, or previous pulses. In all three land covers, the general response to precipitation pulses is a respiration pulse followed by an increase in total carbon uptake. Differences in pulse responses observed at the savanna site compared to the grassland and woodland sites can be explained, in part, by the observed differences in rooting structure and photosynthetic capacity due to differences in plant functional groups and leaf area index. The woodland site is most sensitive to winter pulses in terms of enhanced sink strength following pulses and is dependent on both temperature and pulse size. Both the grassland and shrubland sites show greater sink strength following summer pulses, rather than winter pulses. Both the ecosystem respiration and net uptake responses in all three sites are dependent upon whether there was shallow versus deep soil moisture recharge following pulses. Implications for the influence of future climate change on carbon dynamics in these savanna ecosystems will be discussed.

The spatial variability of preferential pathways for water and chemical transport in a field soil, as visualized through dye infiltration experiments, was studied by applying multifractal and wavelet transform analysis (WTA). After dye infiltration into a 4m2 plot located on a Vertisol soil near College Station, Texas, horizontal planes in the subsoil were exposed at 5 cm intervals, and dye stain patterns were photographed. Box-counting methods and WTA were applied to all of the 16 digitalized high-resolution dye images and to the dye-mass image obtained merging all sections. The wellknown Devil’s staircase multifractal was also used to illustrate wavelet-based analysis. Our results suggest that wavelet methods can complement box-counting analysis in the context of multiscaling structure analysis.

Rangeland ecosystems account for almost two thirds the total land area in Texas. Over the past century, heavy livestock grazing and fire suppression coupled with changes in climate have facilitated the expansion of woody species into rangelands throughout the state. Based in part on the assumption that woody species use more water than their herbaceous counterparts, land managers have used a variety of techniques to reduce tree and shrub abundance to combat the loss of forage for cattle. As a result, the structure of rangelands in Texas is complex, characterized by woody vegetation that is patchy in distribution, and continually changing between grassland, savanna and woodland. Despite the large areal extent of Texas rangelands, very little is known about how the observed changes in ecosystem structure impact carbon cycle dynamics and surface energy exchange. To reduce these uncertainties, we explored explicit relationships between structure and function in these ecosystems by comparing tower-based measurements of carbon and water vapor exchange made simultaneously from July 2004-Dec 2005 across three representative land covers in central Texas: open grassland, savanna with 30% Ashe juniper and honey mesquite cover, and closed canopy woodland. Here we report our findings on what impact the type and pattern of woody plant cover has on biological controls and patterns of carbon sequestration, evapotranspiration, and sensitivity to precipitation pulses. Monthly measurements of leaf level gas exchange, soil respiration rates, herbaceous net ecosystem exchange, and sap flow measurements on dominant woody species were used to augment eddy covariance estimates of ecosystem-atmosphere exchange. The addition of woody species significantly increased carbon sequestration in these ecosystems. Net ecosystem production from July 05-Jun 05 in the grassland, savanna and forest ecosystems was -14 g C m-2, -413 g C m-2, -450 g C m-2, respectively. Evapotranspiration was less sensitive to changes in ecosystem structure. Woody encroachment is widely presumed to reduce sensitivity of carbon and energy exchange to rainfall and drought because deep root systems of woody plants provide access to water stored at depths unavailable to grasses. Data from our three sites challenge this presumption. Our data suggest that in karst terrain characterized by shallow soils underlain by limestone bedrock, effective root systems of dominant woody plants are shallow. The differential responses of the herbaceous component and the woody species to temperature extremes, summer droughts and large summer precipitation events provide a mechanistic understanding of carbon cycling in these ecosystems, and how woody species, in particular, alter both seasonal and annual carbon dynamics in Texas savanna rangelands.

The spatial variability of preferential pathways for water and chemical transport in a field soil, as visualized through dye infiltration experiments, was studied by applying configuration entropy and multifractal analysis. After dye infiltration into a 4 m plot located on a Vertisol soil near College Station, TX, horizontal planes in the subsoil were exposed at 5-cm intervals, and dye stain patterns were photographed. Each of the digitized high-resolution dye images obtained were analysed calculating the maximum configuration entropy (H(L)), the characteristic length (L), and the generalized dimensions (Dq). The results indicate that H(L) and L are two useful descriptors that give an optimal scale of discrimination in the spatial arrangement of the dye tracer at each horizontal section. In addition, L can be used to choose the scale range at which the multifractal analysis should be applied. It has been showed that Dq, being q>0, depend much more on the percentage of black pixels than on the image structure when a box-counting method is used.Finally, a multifractal analysis was applied to maximum dye infiltration depth and amount of dye pixels bellow the area studied, obtained by merging images from the 16 exposed planes. The results show a multiscaling structure and a consistent Dq for both measures. This could be useful for statistically describing preferential flow path geometry and flow processes under field conditions.

Ashe juniper (Juniperus asheii) and plateau live oak (Quercus virginiana var fusiformis) have encroached into the historical grasslands of the Edwards Plateau of central Texas. The increased tree density may impact local water budgets because the area is the recharge zone for the Edwards Aquifer, the drinking water source for large municipalities like Austin and San Antonio. On the other hand, the trees have the capability of sequestering a greater amount of carbon than the historic grasslands. This study is a part of a larger NIGEC project examining the energy fluxes of the Oak-Juniper ecosystem. Four trees of each species were permanently marked and sampled with a leaf-level gas exchange system every 5 to 6 weeks throughout an entire year. Each tree was sampled on the northwest and southeast sides of the canopy, and at each location both sun-lit and shaded leaves were sampled. Averaged (± SE) over the entire year, live oak had significantly greater carbon assimilation rates than Ashe juniper (13.12 ± 0.6 vs. 6.47 ± 0.4 µmol CO2/m2/s, respectively). Oak trees exhibited a greater seasonal flux in carbon assimilation than juniper. Carbon assimilation was least in October 2005 for both species (2.47 and 6.46 µmol CO2/m2/s for juniper and oak, respectively) and greatest in November 2004 for juniper (13.02 µmol CO2/m2/s) and in April 2005 for oak (21.64 µmol CO2/m2/s). Sun-lit leaves also had a consistently greater assimilation rate (P

The Edwards Plateau (Texas Hill Country) is a large (93,000 km2), distinct ecoregion in south and west central Texas that is a biological crossroads for three major biomes of North America (grassland, desert, deciduous forest). Extensive portions of the Plateau are dominated by live oak/ashe juniper savannas with increasing encroachment by invasive junipers over the underlying grasslands. A multi-year, multi-disciplinary study was begun in 2004 to quantify fluxes, source and sinks of atmospheric CO2 on the Edwards Plateau, and to determine how encroachment by ashe juniper alters CO2 fluxes and CO2 source/sink relationships. Flux towers utilizing eddy covariance measurements of CO2 exchange were installed in a typical savanna dominated by grasses (both C3 and C4) and forbs, and in a live oak/ashe juniper forest representing the worst case scenario of juniper encroachment. Measurements at the savanna began on 29 April `04 and at the forest on 18 July `04. The first year of the study was unusually wet, while 2005 was drier than normal through much of the year. The savanna was a net carbon sink during the spring and early summer during both years, and a net source the remainder of the year. Net gain of carbon on an annual basis was near zero. The forest was a net sink for carbon throughout the year because live oak and ashe juniper, the dominant species, are evergreen and the winter was warm. Light response curves showed that the forest was as responsive to drought as the savanna, in spite of root systems capable of penetrating the limestone bedrock and extracting water stored in cracks and fissures.