Ricardo Mata-González

The Inappropriate Use of Crop Transpiration Coefficients (Kc) to Estimate Evapotranspiration in Arid Ecosystems: A Review

ABSTRACT The transpiration coefficient (Kc) method estimates evapotranspiration as a proportion of the evapotranspiration of a reference crop growing in ideal conditions. This approach was designed for irrigated crops and assumes that plants are not subjected to resource limitations. Other assumptions are that plants have high leaf area index and little stomatal resistance to water loss. These conditions are not common for arid-land vegetation. However, mainly due to its simplicity, some studies have proposed the use of transpiration coefficients as a method of determining evapotranspiration in arid environments. In this article, the documented applications of the Kc method in arid environments and their accuracy are reviewed. We also critically discuss the physiological and agronomic concepts that support the Kc method as they relate to water-limited environments. The Kc method typically overestimates water use when plants encounter suboptimal conditions of soil water because it does not consider stomatal regulation and plant adaptations to drought. We conclude that, although the transpiration coefficient method is simple to implement and widely recognized, it is not suitable for determining evapotranspiration of vegetation adapted to arid conditions.

Physiological impacts of biosolids application in desert grasses

Abstract Although some studies have demonstrated that application of biosolids enhances biomass production of desert grasses, there is a lack of information regarding the physiological mechanisms of this enhanced production. The main objective of this study was to evaluate how gas exchange and leaf area production of blue grama ( Bouteloua gracilis ) and tobosagrass ( Hilaria mutica ) are affected by different rates and seasons of biosolids application and irrigation regimes. Plants of both species were transplanted in pots and maintained under a rain-out shelter. Pots were treated either in the spring or summer with biosolids at rates of 0, 7, 18, 34 and 90 dry Mg ha −1 and irrigated at either 40 or 80% field capacity soil water content. The increase in biosolids rates produced increases in leaf area that did not always correspond with increases in photosynthetic rates ( P n ). This suggests that achieving a well-developed canopy was more important than increasing P n per se. However, plant responses to biosolids application varied with irrigation regimes and between species. A decrease in P n of blue grama treated with 34 Mg ha −1 of biosolids was, in part, due to stomatal closure to modulate water loss and possibly to relocation of N for root growth. Under 80% field capacity, tobosagrass seemed to have a less conservative strategy in regulating g s than blue grama. Spring application of biosolids resulted in higher P n than summer application. Photosynthetic rates of blue grama and tobosagrass were not always related to tissue N concentrations.

Predicting foundation bunchgrass species abundances: model-assisted decision-making in protected-area sagebrush steppe

Foundation species are structurally dominant members of ecological communities that can stabilize ecological processes and influence resilience to disturbance and resistance to invasion. Being common, they are often overlooked for conservation but are increasingly threatened from land use change, biological invasions, and over-exploitation. The pattern of foundation species abundances over space and time may be used to guide decision-making, particularly in protected areas for which they are iconic. We used ordinal logistic regression to identify the important environmental influences on the abundance patterns of bluebunch wheatgrass (Pseudoroegneria spicata), Thurbers needlegrass (Achnatherum thurberianum), and Sandberg bluegrass (Poa secunda) in protected-area sagebrush steppe. We then predicted bunchgrass abundances along gradients of topography, disturbance, and invasive annual grass abundance. We used model predictions to prioritize the landscape for implementation of a management and restoration decision-support tool. Models were fit to categorical estimates of grass cover obtained from an extensive ground-based monitoring dataset. We found that remnant stands of abundant wheatgrass and bluegrass were associated with steep north-facing slopes in higher and more remote portions of the landscape outside of recently burned areas where invasive annual grasses were less abundant. These areas represented only 25% of the landscape and were prioritized for protection efforts. Needlegrass was associated with south-facing slopes, but in low abundance and in association with invasive cheatgrass (Bromus tectorum). Abundances of all three species were strongly negatively correlated with occurrence of another invasive annual grass, medusahead (Taeniatherum caput-medusae). The rarity of priority bunchgrass stands underscored the extent of degradation and the need for prioritization. We found no evidence that insularity reduced invasibility; annual grass invasion represents a serious threat to protected-area bunchgrass communities. Our study area was entirely within the Wyoming big sagebrush ecological zone, understood to have inherently low resilience to disturbance and resistance to weed invasion. However, our study revealed important variation in abundance of the foundation species associated with resilience and resistance along the topographic-soil moisture gradient within this zone, providing an important foothold for conservation decision-making in these steppe ecosystems. We found the foundation species focus a parsimonious strategy linking monitoring to decision-making via biogeographic modeling.

Non-Native Plant Invasion along Elevation and Canopy Closure Gradients in a Middle Rocky Mountain Ecosystem

Mountain environments are currently among the ecosystems least invaded by non-native species; however, mountains are increasingly under threat of non-native plant invasion. The slow pace of exotic plant invasions in mountain ecosystems is likely due to a combination of low anthropogenic disturbances, low propagule supply, and extreme/steep environmental gradients. The importance of any one of these factors is debated and likely ecosystem dependent. We evaluated the importance of various correlates of plant invasions in the Wallowa Mountain Range of northeastern Oregon and explored whether non-native species distributions differed from native species along an elevation gradient. Vascular plant communities were sampled in summer 2012 along three mountain roads. Transects (n = 20) were evenly stratified by elevation (~70 m intervals) along each road. Vascular plant species abundances and environmental parameters were measured. We used indicator species analysis to identify habitat affinities for non-native species. Plots were ordinated in species space, joint plots and non-parametric multiplicative regression were used to relate species and community variation to environmental variables. Non-native species richness decreased continuously with increasing elevation. In contrast, native species richness displayed a unimodal distribution with maximum richness occurring at mid–elevations. Species composition was strongly related to elevation and canopy openness. Overlays of trait and environmental factors onto non-metric multidimensional ordinations identified the montane-subalpine community transition and over-story canopy closure exceeding 60% as potential barriers to non-native species establishment. Unlike native species, non-native species showed little evidence for high-elevation or closed-canopy specialization. These data suggest that non-native plants currently found in the Wallowa Mountains are dependent on open canopies and disturbance for establishment in low and mid elevations. Current management objectives including restoration to more open canopies in dry Rocky Mountain forests, may increase immigration pressure of non-native plants from lower elevations into the montane and subalpine zones.

Nitrogen in Desert Grasses as Affected by Biosolids, their Time of Application, and Soil Water Content

This study evaluated the interactive effect of biosolids, time of application, and soil water on plant N concentration and uptake by Bouteloua gracilis (blue grama) and Hilaria mutica (tobosagrass) grown in pots. Biosolids were surface-applied to the soil of the pots either in the spring or the summer at rates of 0, 7, 18, 34, and 90 dry Mg ha−1. All of the pots were irrigated weekly to achieve 40% or 80% of field capacity soil water. The maximum increase in B. gracilis tissue N concentration due to biosolids application with respect to control was 41% in plants grown under the higher irrigation regime and only 15% in those plants grown under the lower irrigation regime. In H. mutica, the higher irrigation level produced an average of 20% higher tissue N concentration than the lower irrigation level across all biosolids application rates. Both species had a much higher N uptake (3.5- to 6.3-fold) under the higher than under the lower irrigation regime due to the large increase in shoot biomass caused by the higher irrigation level, regardless of rate of biosolids. At low biosolids rates, soil NO3 −-N was higher under the lower irrigation regime than under the higher irrigation regime due to the lower N uptake in plants. Spring application of biosolids produced higher soil NO3 −-N levels than summer application only when irrigation level was high. The fertilizing effect of biosolids depended heavily on soil water availability.

Biosolids Effects in Chihuahuan Desert Rangelands: A Ten-Year Study

Arid and semiarid rangelands are suitable for responsible biosolids application. Topical application is critical to avoid soil and vegetation disturbance. Surface-applied biosolids have long-lasting effects in these ecosystems. We conducted a 10-year research program investigating effects of biosolids applied at rates from 0 to 90 dry Mg ha−1 on soil water infiltration; runoff and leachate water quality; soil erosion; forage production and quality; seedling establishment; plant physiological responses; nitrogen dynamics; biosolids decomposition; and grazing animal behavior and management. Biosolids increased soil water infiltration and reduced erosion. Effects on soil water quality were observed only at the highest application rates. Biosolids increased soil nitrate-nitrogen. Biosolids increased forage production and improved forage quality. Biosolids increased leaf area of grasses; photosynthetic rates were not necessarily increased by biosolids. Biosolids effects on plant establishment are expected only under moderately favorable conditions. Over an 82-mo exposure period, total organic carbon, nitrogen, and total and available phosphorus decreased and inorganic matter increased. Grazing animals spent more time grazing, ruminating, and resting in biosolids-treated areas; positive effects on average daily gain were observed during periods of higher rainfall. Our results suggest that annual biosolids application rates of up to 18 Mg ha−1 are appropriate for desert rangelands.

Effects of surface and subsurface water application on nitrogen and sodium relations of desert graminoids of different geographic origin

ABSTRACT This greenhouse study evaluated nitrogen and sodium relations of three desert graminoids (Distichlis spicata, Leymus triticoides, and Juncus arcticus) as affected by availability of surface water, subsurface water or both. These species are amply distributed in desert wetlands of western USA where surface and subsurface water are differentially available. Plants of the three species were collected from two areas of ecological distribution: Bishop, California and Burns, Oregon. Because nitrogen and sodium uptake by plants is highly linked to water availability we established three general hypotheses for this study: (1) nitrogen uptake would be greater when plants have surface water available, (2) sodium uptake would be greater when plants do not have surface water available, and (3) there are populations’ differences in the response of the species to water availability. We grew plants in two-layer pots in which soil water content in the upper and lower layers was controlled independently. The first hypothesis was partially supported as only Leymus triticoides seemed to preferentially depend on the top layer for nitrogen acquisition. With respect to the second hypothesis sodium concentration was indeed greatest when plants had no surface water, but only in D. spicata. The third hypothesis was also partially supported. The Oregon population of J. arcticus had 15% more nitrogen than the California population and the California population of D. spicata had 18% more sodium than the Oregon population. Our results underline the plant nutrient uptake implications of differential availability of water pools for common desert graminoid species.

Patterns of Water Use by Great Basin Plant Species Under Summer Watering

We analyzed temporal and spatial patterns of water use by a functionally-diverse group of Great Basin plant species and determined their water use rates at the whole-plant and individual-leaf scales under variable summer watering. Species studied were the desert grasses Distichlis spicata and Sporobolus airoides, the desert shrubs Artemisia tridentata, Ericameria nauseosa, and Atriplex confertifolia; the wetland/riparian plants Juncus arcticus, Leymus triticoides, and Salix exigua; and the annual exotic Salsola tragus. Plant species were individually grown in 5.8 m2 plots in a common garden in eastern California. Three irrigation treatments in the form of monthly pulses were applied during the summer: low (1.3 cm), medium (2.6 cm), and high (3.9 cm), in addition to a nonirrigated control. Whole-plant water uptake characteristics were determined by soil water depletion at different soil depths, while leaf transpiration was determined by gas exchange. Whole-plant water extraction and leaf transpiration varied similarly among species. Desert shrubs had low water extraction (35 to 395 g m−2 day−1) and were not affected by irrigation. The desert grasses and riparian/wetland species had higher water extraction, increasing with irrigation levels. L. triticoides and J. arcticus had the highest water extraction overall (>2,000 g m−2 day−1). Desert shrubs relied 10 times more on deeper water sources than herbaceous species. The average T/ET was 31%, but varied by species. Summer available water in environments such as the Great Basin favors desert grasses and riparian/wetland species, but not desert shrubs. The observed species differences provide alternatives for water and vegetation management.

Growth and leaf chemistry of Atriplex species from Northern Mexico as affected by salt stress

ABSTRACT Atriplex species are tolerant to salinity and water stress and thus they are suitable for restoration of many degraded ecosystems. In addition, many Atriplex species offer good value as forages. We compared growth and leaf chemistry of Atriplex canescens, a well-known halophyte, and A. acanthocarpa, a poorly-studied species, as affected by salinity in a greenhouse study. Seeds and soil were collected in northern Mexico, the native range of these species. Plants were grown in pots containing native soil and irrigated with NaCl solutions of 0, 50, and 100 mM. Shoot growth of A. canescens declined 37% as NaCl treatments increased from 0 to 100 mM while shoot growth of A acanthocarpa was not significantly affected by salinity. The high salt tolerance of A. acanthocarpa was linked to a high accumulation of leaf sodium (Na) (7- to 13-fold higher than A. canescens). A. acanthocarpa had also higher growth rate than A. canescenes, making the former species a good candidate for cultivation, especially under saline conditions. Tissue concentration of potassium (K) in both species was minimally affected by the salinity treatments. Leaf nitrogen (N) concentration increased as plants faced higher salinity treatments, especially in A. canescens. The high salt tolerance and higher Na accumulation of A. acanthocarpa make this species an attractive choice for reclamation of saline areas. We suggest A. acanthocarpa should be explored as viable forage for cultivation and for reclamation of degraded areas just as A. canescens has been throughout the world.

Long-Term Fire Effects on Native and Invasive Grasses in Protected Area Sagebrush Steppe☆

ABSTRACT Following western settlement, fire was suppressed directly and indirectly by Euro-American land management practices. Currently, reintroduction of fire into sagebrush steppe systems may be desirable, but long-term fire effects are not well-known. In this 15-year study we used a generalized linear mixed modeling approach to analyze the response of native and invasive grass species to fire in anan Artemisia tridentata subsp. wyomingensis (Wyoming big sagebrush) community in north-central Oregon, United States. This study examined responses of Bromus tectorum (cheatgrass), Pseudoroegneria spicata (bluebunch wheatgrass), and Poa secunda (Sandberg bluegrass) along gradients of community type and topography through time post fire. Community types were identified as either A. tridentata subsp. wyomingensis dominant (brush plots) or Juniperus occidentalis (western juniper) dominant (woodland plots). Cover of B. tectorum was greatest in brush plots. B. tectorum cover increased significantly 5 yr post burn and stabilized. At 5 yr, postburn cover of B. tectorum was 135% in brush and 301% in woodland plots of preburn cover. P. spicata was more abundant in woodland plots than in brush plots. In woodland plots, P. spicata cover decreased by 49% 1 yr post burn but returned to preburn cover by 5 yr post burn. On northern exposures recovery of P. spicata cover occurred between 1 and 2 yr post burn, whereas on southern exposures recovery occurred between 2 and 5 yr post burn. The cover of P. secunda did not show a significant response to fire. These results suggest the importance of topography and plant community in determining postfire community response and underscores the importance of place-based studies to guide management and conservation actions.