Annual Plant Biomass Research Articles

Four ways to define the growing season.

What is addressed as growing season in terrestrial ecosystems is one of the main determinants of annual plant biomass production globally. However, there is no well-defined concept behind. Here, we show different facets of what might be termed growing season, each with a distinct meaning: (1) the time period during which a plant or a part of it actually grows and produces new tissue, irrespective of net carbon gain (growing season sensu stricto). (2) The period defined by developmental, that is, phenological markers (phenological season). (3) The period during which vegetation as a whole achieves its annual net primary production (NPP) or a net ecosystem production (NEP), expressed as net carbon gain (productive season) and (4) the period during which plants could potentially grow based on meteorological criteria (meteorological season). We hypothesize that the duration of such a 'window of opportunity' is a strong predictor for NPP at a global scale, especially for forests. These different definitions have implications for the understanding and modelling of plant growth and biomass production. The common view that variation in phenology is a proxy for variation in productivity is misleading, often resulting in unfounded statements on potential consequences of climatic warming such as carbon sequestration.

Effects of subsurface flow on dunefields: Doubling the amount of soil moisture and substantially increasing the biomass of annual plants

Plants in deserts exhibit heterogeneous distribution, which may stem from water redistribution following runoff. However, heterogeneous plant distribution is also common in dunefields regardless of runoff. In the Hallamish dune field in the hyper-arid Negev Desert, dense vegetation belts are noted in loci where an abrupt change in slope angles of uncrusted slopes (interface, IF) occurs, attributed to subsurface flow (SSF). While SSF was previously noted in dunefields, quantification of SSF is problematic, especially in arid regions where precipitation is low, evaporation is high, and the amount of SSF is not sufficiently large to facilitate direct measurement (in water volume). Here indirect measurements of SSF are reported based on near weekly measurements of available water content (AWC) during 2012/13, 2013/14 and 2015/16 at five slope positions along two dune catenas and the adjacent interdunes: flat semi-stable top (TOPs), flat crusted interdune (IDc), semi-stable (NFs) and crusted (NFc) north-facing slope, and IF. In addition, the hydrological rain and monthly surface stability were also measured, and additionally plant characteristics (cover, number of species, density, and biomass) were used as indicators for AWC. Higher AWC characterized the uncrusted slope positions (especially at IF) in comparison with crusted slope positions of similar slope angles. Although receiving the lowest amount of rain, AWC at IF was ∼ 2-fold higher than at most other slope positions, resulting in ∼ 3- to 5-fold higher cover and biomass, serving therefore as 'islands of fertility'. As verified by the Generalized Linear Regression Model, plant performance was mainly dictated by AWC and to a lesser degree by surface stability. Our findings may have broader implications for water redistribution in other dunefields, where a unique distribution of water has at times been attributed to factors such as the effect of biocrusts on infiltration rather than to SSF.

Altered precipitation has asymmetric impacts on annual plant communities in warm and cool growing seasons

While altered precipitation regimes can greatly impact biodiversity and ecosystem functioning, we lack a comprehensive view of how these impacts are mediated by changes to the seasonality of precipitation (i.e., whether it rains more/less in one season relative to another). Over 2 years, we examined how altered seasonal precipitation influenced annual plant biomass and species richness, Simpson’s diversity, and community composition of annual plant communities in a dryland ecosystem that receives both winter and summer rainfall and has distinct annual plant communities in each season. Using a rainfall exclusion, collection, and distribution system, we excluded precipitation and added water during each season individually and compared responses to control plots which received ambient summer and winter precipitation. In control plots, we found five times greater annual plant biomass, twice as many species, and higher diversity in winter relative to summer. Adding water increased annual plant biomass in summer only, did not change richness or diversity in either summer or winter, and modestly shifted community composition. Excluding precipitation in either season reduced annual plant biomass, richness, and Simpson’s diversity. However, in the second winter season, biomass was higher in the plots where precipitation was excluded in the previous summer seasons suggesting that reduced productivity in the summer may facilitate biomass in the winter. Our results suggest that increased precipitation in summer may have stronger short-term impacts on annual plant biodiversity and ecosystem function relative to increased winter precipitation. In contrast, decreasing precipitation may have ubiquitous negative effects on annual plants across both summer and winter but may lead to increased biomass in the following off-seasons. These patterns suggest that annual plant communities exhibit asymmetries in their community and ecosystem responses to altered seasonal precipitation and that considering the seasonality of precipitation is important for predicting the effects of altered precipitation regimes.

Sustainable goat grazing for managing abandoned fields: Dynamics of vegetation quality, quantity and nutritional status of goats over five years

AbstractGrazing goats have potential for use in managing abandoned fields. However, it is unclear how long‐term grazing affects goats as a result of vegetational changes in abandoned fields. This study aimed to assess the dynamics of vegetation quality and quantity and the nutritional status of goats for five years in an abandoned field at different stocking rates: high stocking rate (HS: 30–33 goats/ha) and low stocking rate (LS: 14 goats/ha). In five years, the dominant plant species changed from bamboo (Phyllostachys edulis) to tufted grass species at the HS, whereas dwarf bamboo (Pleioblastus argenteostriatus) dominated throughout the five years at the LS. The annual plant biomass at both stocking rates was similar and varied from year to year. Crude protein content in the goats' diet did not show any seasonal or annual trend regardless of the stocking rate. Neutral detergent fiber content in the HS diet did not show any seasonal or annual trend; however, that in the LS diet linearly increased with the grazing year. The dry matter (DM) intake for goats at the LS decreased linearly with the grazing year (p < 0.05), whereas that for goats at the HS did not show any trend. The DM digestibility was higher for goats at the HS than for goats at the LS (p < 0.05). Serum parameters did not show a negative nutritional status of the goats over the study years. Annual daily gain was positive throughout the grazing years regardless of the stocking rate. Consequently, within the stocking rate range studied here, long‐term goat grazing changes the quality of vegetation in an abandoned field; however, goats can maintain their nutritional status and body weight for five years. Thus, goat grazing is a feasible way to control abandoned vegetation over the long term.

Climate warming accelerates temporal scaling of grassland soil microbial biodiversity.

Determining the temporal scaling of biodiversity, typically described as species-time relationships (STRs), in the face of global climate change is a central issue in ecology because it is fundamental to biodiversity preservation and ecosystem management. However, whether and how climate change affects microbial STRs remains unclear, mainly due to the scarcity of long-term experimental data. Here, we examine the STRs and phylogenetic-time relationships (PTRs) of soil bacteria and fungi in a long-term multifactorial global change experiment with warming (+3 °C), half precipitation (-50%), double precipitation (+100%) and clipping (annual plant biomass removal). Soil bacteria and fungi all exhibited strong STRs and PTRs across the 12 experimental conditions. Strikingly, warming accelerated the bacterial and fungal STR and PTR exponents (that is, the w values), yielding significantly (P < 0.001) higher temporal scaling rates. While the STRs and PTRs were significantly shifted by altered precipitation, clipping and their combinations, warming played the predominant role. In addition, comparison with the previous literature revealed that soil bacteria and fungi had considerably higher overall temporal scaling rates (w = 0.39-0.64) than those of plants and animals (w = 0.21-0.38). Our results on warming-enhanced temporal scaling of microbial biodiversity suggest that the strategies of soil biodiversity preservation and ecosystem management may need to be adjusted in a warmer world.

Increased fire risk in Mojave and Sonoran shrublands due to exotic species and extreme rainfall events

AbstractHistorically, desert ecosystems in the southwestern United States have been characterized by low incidence of fire. However, widespread wildfires in these systems have been recently observed. Large areas of creosote bush (Larrea tridentata) scrub scattered in the Mojave and Sonoran deserts were impacted by wildfires in 2005, after a series of years with above‐average precipitation. It has been hypothesized that exotic invasive grasses, for example, Schismus arabicus, are responsible as they are able to grow in the open and may produce sufficient biomass to carry fires during high‐rainfall years. However, there has been little experimental evidence to support this hypothesis. We conducted a field experiment exploring the response of the annual plant community to increased rainfall in two creosote‐dominated sites in the Sonoran and Mojave deserts by manipulating precipitation levels, fire history, soil disturbance, and exotic‐annual‐plant seed availability. We examined how these treatments impacted fuel loads (biomass) and species distributions as a function of distance from shrubs. We found that enhanced rainfall produces a general increase in biomass and density of annual plants. At the Sonoran site, rainfall tends to increase the density of exotic annuals. At the Mojave site, it is the opposite, where native annuals benefit more from higher rainfall. However, it is important to note that in the Mojave site, native annuals produce higher biomass under shrubs and exotic annuals produce higher biomass in the open in response to increased rainfall. The introduction of fire at both sites slightly increased biomass production as well. Soil disturbance and seed addition had only subtle effects. From our results, it is clear that increased rainfall in southwestern U.S. deserts is conducive to increasing biomass of annual plants, especially in the inter‐shrub areas, which in turn can lead to enhanced fire risk. The potential to shift toward higher dominance by exotic annuals with fire at the Sonoran site could further enhance this risk. With several consecutive years of high rainfall, increased seed production by exotics would potentially amplify this effect.

What is the role of perennial plants in semi-steppe rangelands? Direct and indirect effects of perennial on annual plant species

Biodiversity–productivity relationships have widely been studied in natural herbaceous communities but remain debated. Here, we hypothesized that species richness of perennial plants would increase species richness of annual plants but aboveground biomass of perennial plants would restrict the species richness and aboveground biomass of annual plants across plant functional types (i.e. shrubs, forbs and graminoids) in natural rangelands. We quantified the species richness of annual and perennial plants across plant functional types, and then measured aboveground biomass destructively in 735 quadrats of semi-steppe rangelands. We used linear path analysis model to test for the multivariate effects of perennial on annual plant species using species richness and aboveground biomass datasets, in addition to the influences of grazing disturbances and soil textural properties. Our results showed the species richness of perennial plants (forbs and graminoids) generally promoted the species richness of annual plants (forbs and graminoids). However, the aboveground biomass of perennial species (shrubs, forbs and graminoids) imposed negative or negligible (i.e. nonsignificant negative) influences on the species richness and aboveground biomass of annual species (forbs and graminoids). Hence, perennial species have probably restricted the aboveground biomass of annual plant species due to the resource acquisition, but protected annual plants from grazing and other anthropogenic disturbances. This study suggests that the relationship between species richness and aboveground biomass is positive in general, regardless of the confounding effects of perennial on annual species across plant functional types. This study highlights that simply by integrative modelling of perennial and annual plant species across plant functional types for the associations between species richness and aboveground biomass, better insights can be gained into ecological mechanisms playing a role in the biodiversity conservation of natural rangelands and other natural ecosystems in general.

Diet of the Mohave Ground Squirrel (Xerospermophilus mohavensis) in Relation to Season and Rainfall

The Mohave ground squirrel (Xerospermophilus mohavensis) is endemic to the western Mojave Desert of California. It is listed as threatened under the California Endangered Species Act, yet there is little published information on its habitat requirements. We studied the diet of Mohave ground squirrels at 4 sites in desert scrub habitat in Inyo County, California, primarily by microhistological analysis of 754 samples of fecal pellets collected from live-trapped animals. Over all sites and seasons, shrub foliage was the largest component of the diet (39.8% relative density) and mainly derived from several taxa of Chenopodiaceae: winterfat (Krascheninnikovia lanata), spiny hopsage (Grayia spinosa), and saltbushes (Atriplex spp.). Forb leaves were next in importance (34.1% relative density), especially from Fabaceae (Astragalus and Lupinus), Polemoniaceae (Gilia and Linanthus), and Asteraceae. Flowers, pollen, and seeds were also major components (20.3% relative density). Leaves composed nearly all of the diet in spring, whereas pollen, flowers, and seeds made up about a third of the diet in summer. Following dry winters when annual forbs were limited, Mohave ground squirrels depended primarily on foliage from perennial shrubs and forbs. Following wet winters when spring annuals were abundant and most plant species flowered and set seed prolifically, squirrels consumed a high proportion of leaves plus flowers, pollen, and seeds of annual forbs. Mohave ground squirrels reproduced only after winter rainfall >80 mm that resulted in a standing crop of herbaceous annuals ≥100 kg · ha-1. Mohave ground squirrels consumed very little of the nonnative annual plant biomass present on our study sites (Erodium, Salsola, Bromus, and Schismus contributed <3% overall to the diet). Conservation implications include the following: (1) priority should be given to protecting habitats supporting preferred perennial forage plants, including winterfat and spiny hopsage; (2) habitats with an understory dominated by native annual forbs have higher value than those dominated by nonnative plants, especially annual grasses; and (3) if climate change results in lower and less regular winter precipitation, suitable habitat for Mohave ground squirrels may be reduced and fragmented in the drier portions of the geographic range.

Effects of rainfall patterns on annual plants in Horqin Sandy Land, Inner Mongolia of China

Growth of annual plants in arid environments depends largely on rainfall pulses. An increased understanding of the effects of different rainfall patterns on plant growth is critical to predicting the potential responses of plants to the changes in rainfall regimes, such as rainfall intensity and duration, and length of dry intervals. In this study, we investigated the effects of different rainfall patterns (e.g. small rainfall event with high frequency and large rainfall event with low frequency) on biomass, growth characteristics and vertical distribution of root biomass of annual plants in Horqin Sandy Land, Inner Mongolia of China during the growing season (from May to August) of 2014. Our results showed that the rainfall patterns, independent of total rainfall amount, exerted strong effects on biomass, characteristics of plant growth and vertical distribution of root biomass. Under a constant amount of total rainfall, the aboveground biomass (AGB), belowground biomass (BGB), plant cover, plant height, and plant individual and species number increased with an increase in rainfall intensity. Changes in rainfall patterns also altered the percentage contribution of species biomass to the total AGB, and the percentage of BGB at different soil layers to the total BGB. Consequently, our results indicated that increased rainfall intensity in future may increase biomass significantly, and also affect the growth characteristics of annual plants.

Transformation of shrublands to forests: The role of woody species as ecosystem engineers and landscape modulators

Trees in forests and shrubs in shrublands can be considered physical ecosystem engineers since they modify their environment by creating patches that differ in their ecosystem properties from un-engineered patches. The objective of this study was to evaluate the effects of replacing one functional group of woody species (shrubs) with another group (trees) that differs in its ecosystem engineering (EE) mode on soil and vegetation as emerging ecosystem properties in the northern Negev, Israel. The soil quality index (SQI) and the aboveground net primary productivity (ANPP) of herbaceous plants were used as emergent ecosystem properties for characterizing woody and non-woody patch states. The SQI integrates 14 physical, biological, and chemical soil properties, indicating the soil state in a patch. The ANPP of herbaceous plants was estimated as the annual plant biomass accumulation, indicating the vegetation state in a patch. Relationships between the SQI and ANPP properties in different patch states were calculated in terms of the magnitude (MG) and direction of the change (θ). The results show an overall conservation of the collective ecosystem properties on the landscape level, but an inversion in the ecosystem functions of the two patch types arising from the replacement of the woody EEs. A significant correlation between SQI and ANPP was found in the forest and shrubland system, with R2=0.89 and R2=0.82, respectively. In the forest, higher SQI and herbaceous plant ANPP scores were found in the non-woody patches than in the tree understory with MG=0.27 and θ=214.61. The opposite trend existed in the shrubland where higher SQI and ANPP scores were found under the shrub canopy than in the non-woody patches with MG=0.34 and θ=43.65. The conclusions are: (1) the engineering properties induced by the dominant woody plants through patch formation are important in driving ecosystem modulation; (2) the SQI and ANPP trajectories represent the magnitude of change between patch type, and (3) SQI and ANPP are reliable emergent ecosystem properties for evaluating changes of patches formed by woody plants as EEs.

Models of Invasion and Establishment for African Mustard (Brassica tournefortii)

AbstractIntroduced exotic plants can drive ecosystem change. We studied invasion and establishment ofBrassica tournefortii(African mustard), a noxious weed, in the Chemehuevi Valley, western Sonoran Desert, California. We used long-term data sets of photographs, transects for biomass of annual plants, and densities of African mustard collected at irregular intervals between 1979 and 2009. We suggest that African mustard may have been present in low numbers along the main route of travel, a highway, in the late 1970s; invaded the valley along a major axial valley ephemeral stream channel and the highway; and by 2009, colonized 22 km into the eastern part of the valley. We developed predictive models for invasibility and establishment of African mustard. Both during the initial invasion and after establishment, significant predictor variables of African mustard densities were surficial geology, proximity to the highway and axial valley ephemeral stream channel, and number of small ephemeral stream channels. The axial valley ephemeral stream channel was the most vulnerable of the variables to invasions. Overall, African mustard rapidly colonized and quickly became established in naturally disturbed areas, such as stream channels, where geological surfaces were young and soils were weakly developed. Older geological surfaces (e.g., desert pavements with soils 140,000 to 300,000 years old) were less vulnerable. Microhabitats also influenced densities of African mustard, with densities higher under shrubs than in the interspaces. As African mustard became established, the proportional biomass of native winter annual plants declined. Early control is important because African mustard can colonize and become well established across a valley in 20 yr.

Water use efficiency of annual‐dominated and bunchgrass‐dominated savanna intercanopy space

ABSTRACTIn semi‐arid savannas, dominance of intercanopy space by annual or perennial grasses may alter partitioning of ecosystem water and carbon fluxes and affect ecosystem water use efficiency (WUEe), the ratio of net ecosystem carbon dioxide exchange (NEE) to evapotranspiration (ET). To establish if these contrasting growth habits changed controls to WUEe, we tracked volumetric soil moisture (θ25cm), ET and transpiration (T), NEE and its constituent ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) fluxes, and community water use efficiency (WUEc = GEP : T) in annual‐dominated and bunchgrass‐dominated plots in a southern Arizona, United States, savanna. Annual and bunchgrass plots had similar θ25cm, ET, and T, suggesting the similarity in ET was due to higher soil evaporation in annual plots. Seasonal NEE was delayed and lower in annual plots compared with that in bunchgrass plots, owing to higher Reco in annual plots. Transpiration, GEP, and Reco in both vegetation types increased following late‐season rain, indicating similar late‐season phenological constraint. WUEe was lower in annual plots, but with similar WUEc between plot types. These results suggest that differences in annual plant biomass allocation and plot‐level leaf area distribution increased proportional soil evaporation and aboveground Reco contributions, reducing plot‐level WUEe, not lowering plant WUE typical of arid‐land annuals. Lower plot‐level WUEe suggests that any increase in annual plant dominance would increase interannual variation of productivity in savanna intercanopy spaces, which could enhance the negative effects of predicted higher temperatures, greater aridity, and larger and more widely spaced storms on arid‐land watershed processes. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Fallen logs as sources of patchiness in chenopod shrublands of South Australia

While numerous studies focus on heterogeneity created by living plants and animal diggings in arid lands, little information exists on the pattern and role of heterogeneity created by large woody debris. We studied soil nutrient content, volumetric water content, and vegetation associated with the presence of logs in sites subject to different grazing intensity in a chenopod shrubland of South Australia. Soil volumetric water content was lower close to logs, and the soil there dried faster after rains. Organic carbon was higher, but available phosphorus was lower in soil associated with logs. We documented greater seedling emergence, species richness and diversity, and greater annual plant biomass next to logs. Our results were similar in the sites with high and low grazing pressure, the latter having higher numbers of seeds and species in the germinable soil seed bank. Logs in this system create patches that differ in properties from other patches in the system (i.e. those created by perennial plants, depressions, or diggings). Logs constitute a unique microenvironment which affects the plant community structure and may enhance the diversity of the system, and should be considered in management schemes.

Nutrient retention in plant biomass and sediments from the salt marsh in Hangzhou Bay estuary, China

Nutrient load into the ocean can be retained during the process of plant uptake and sedimentation in marshes along the bay zone. Seasonal variations of biomass and nutrient concentration in three dominated plant assemblages and associated sediments were monitored in this study area to determine effects of salt marsh on nutrient retention. Results showed that plant aboveground biomass displayed a unimodal curve with nutrient concentration generally decreased from spring to winter. Belowground biomass was relatively low during the rapid growth period with nutrient concentration tending to decrease and then increase during this period. Plant total nitrogen (TN) pools are higher than total phosphorus (TP) pools, and both pools showed significant seasonal variations. Water purification coefficients (WPC) of nutrients by plant assimilation were 34.4/17.3, 19.3/24.0, and 5.14/6.04t/(m(2)year) (TN/TP) for Phragmites australis, Spartina alterniflora, and Scirpus mariqueter, respectively. Overall, these results suggest that higher annual plant biomass and nutrient assimilation contribute to greater nutrient retention capacity and accumulation in sediments, thereby enabling reduced eutrophication in transitional waters.

Four years of simulated climate change reduces above‐ground productivity and alters functional diversity in a grassland ecosystem

AbstractQuestionsHow does above‐ground grassland biomass production respond to change in multiple climate drivers over a 4‐yr period? Can climate‐induced patterns of biomass response be explained by shifts in plant community structure? Does sustained climate change affect the relationships between abundance of functional groups, community‐scale leaf traits and above‐ground production?LocationPerennial grassland in the French Massif Central.MethodsMonoliths extracted from the study grassland were exposed to a simulated climate change corresponding to the air temperature, atmospheric CO 2 and summer rainfall conditions projected for 2080. We examined impacts of climate treatments on above‐ground biomass and community structure for 4 yr, and investigated the relationship between biomass production, species diversity and three key functional traits: specific leaf area, leaf dry matter content and leaf N content.ResultsBoth warming and simultaneous application of warming, summer drought and elevated CO 2 were associated with an increase in annual above‐ground biomass at the start of the study, but biomass responses became progressively negative over the course of the experiment. Decreases in vegetation N exports were also observed over time, possibly due to reduced soil N availability under climate change. Taxonomic diversity showed no response to climate treatments, but the relative abundance of grasses decreased under both warming and simultaneous application of warming, summer drought and elevated CO 2 after 3 yr. In parallel, legume relative abundance increased in all warmed treatments. Functional diversity responses varied depending on climate treatment and leaf trait. In the control treatment, patterns of variation in annual plant biomass were best explained by functional diversity during the study period. However, in warmed treatments, variation in annual plant biomass was more closely linked to the functional traits of dominant species.ConclusionsContinuous, multi‐year exposure to projected climate conditions has a negative impact on above‐ground biomass in our grassland study system. Our data suggest that climate‐induced decreases in above‐ground biomass may be driven by changes in the relative abundance of plant functional groups, and could also reflect changes in soil nutrient availability. Unlike species diversity, community‐level leaf traits and functional diversity appear to play an important role for above‐ground biomass production, and may have indirect effects on ecosystem stability in changing climates.

Desert shrubs have negative or neutral effects on annuals at two levels of water availability in arid lands of South Australia

Perennial plants have been shown to facilitate understorey annual plant species in arid lands through the modification of spatial patterns of resources and conditions. This effect can result from a balance between simultaneously positive and negative interactions, both direct and indirect. This balance may shift with temporal variability in water availability. 2. We conducted a field experiment in a chenopod shrubland in South Australia to separate the effects of shade, below-ground competition, and soil modification by shrubs on the performance of annual plants, and to determine if the strength and direction of the interaction shifted with changes in water availability. 3. Annual plant diversity and seedling density was highest in plots established in open sites away from the dominant shrubs ( Maireana sedifolia ). Experimental removal of M. sedifolia increased seedling density compared to plots under undisturbed shrubs and plots where the removed shrub was replaced with artificial shade. Shading of open plots also reduced seedling density. Annual plant biomass was highest in areas where shrubs had been removed and was reduced by artificial shading. Biomass was higher in open plots than under intact shrubs. Experimental water addition did not alter plant density, but increased biomass across all treatments, particularly in artificially shaded bush plots. 4. Synthesis. Our results show that the overall effect of shrubs on the annual plant community in the system is negative under the range of water availabilities experienced during the experiment. This negative net-effect results from a combination of simultaneous facilitation via soil modification, and above- and below-ground competition. Assessment in different systems of different combinations of mechanisms that have simultaneously positive and negative effects will allow us to refine hypotheses seeking to explain the relative importance of facilitation across spatial and temporal gradients.

Topographic controls on snow distribution, soil moisture, and species diversity of herbaceous alpine vegetation, Niwot Ridge, Colorado

The nature of the snowpack has the potential to strongly influence the patterns of alpine plant productivity and composition by governing soil moisture levels, growing season duration and the thermal regime of alpine soils. This study evaluates these relationships by modeling the interrelationships of snow depth, snow water equivalent (SWE), snow disappearance rate, soil moisture, attributes of the alpine plant community and selected terrain factors using decision‐tree techniques at Niwot Ridge, Colorado Front Range. The modeling results showed a strong correlation (r2 &gt; 0.9, P &lt; 0.001) between the snow disappearance rate and SWE and terrain factors that control the degree of shelter and exposure of a given local and elevation. The model was sufficiently robust to predict the spatial distribution of the snowpack for 12 years that exhibited average snow fall (r2 = 0.8, P &lt; 0.001), but yielded lower correlation (r2 = 0.2, P &lt; 0.001) in drought years. Soil moisture was significantly correlated (r2 = 0.7, P &lt; 0.001) with snow‐fall amounts and terrain factors; however, meltwater and summer rain offset the potential soil moisture deficit in windward sites. Annual plant biomass did not correlate well with snow attributes and soil moisture because the cascading impact of topography on snowpack and soil moisture was not well captured by measurements of aboveground biomass. In contrast, the species richness index was significantly correlated with snow depth and soil moisture (r2 = 0.7, P &lt; 0.001), thereby demonstrating the importance of snow on some attributes of the alpine plant community.

Effects of three species of Chihuahuan Desert ants on annual plants and soil properties

We tested the hypothesis that ant species, which occupy the same nest for a decade or longer, would modify nest soils by increasing soil nutrients and microorganisms resulting in increased biomass, density, cover and species richness of annual plants. We measured soil properties and annual plants on nest soils of three species of Chihuahuan Desert ants ( Pogonomyrmex rugosus—seed harvester, Aphaenogaster cockerelli—generalist forager, and Myrmecocystus depilis—liquid collector-insect scavenger) in comparison to paired reference soils at several locations. There were no differences in nest and reference total soil nitrogen of M. depilis and of P. rugosus on three catena soils. Total soil nitrogen of nest-modified soils was higher than of reference soils of A. cockerelli and P. rugosus in a desert grassland site. Soil microbial biomass and respiration were not significantly different among ant species at most locations with the exception of P. rugosus at the base of the catena. Annual plant biomass was higher on M. depilis and A. cockerelli nest soils than on the reference soils. Annual plant biomass was higher on P. rugosus nest soils than on reference soils at the base of the catena and in the grassland but not at the mid-slope and top of the catena. The effects of long-lived ant colony nests on soil properties and vegetation vary in time and space but are independent of the feeding behavior of the ant species.

Mechanical Disturbance of Microbiotic Crusts Affects Ecohydrological Processes in a Region of Revegetation-fixed Sand Dunes

ABSTRACT Microbiotic crusts (MC), also called biological soil crusts or cryptogamic crusts, are formed by cyanobacteria, fungi, blue-green algae, lichens, and mosses, and are widespread in arid and semiarid zones such as the area of sands fixed in place by revegetation near Shapotou at the southeastern edge of China's Tengger Desert. We studied mechanically disturbed and 40-year-old intact MC in this area using a series of soil hydrological experiments and ecological investigations to examine the ecohydrological responses after mechanical disturbance of the MC. Once the MC was disturbed, the MC layer became ore primitive or disappeared entirely, resulting in a 32.8% decrease in the fine particles content of the upper soil layers, a 28.7% increase in soil albedo and a 168.9% decrease in the topsoil's water-retention capacity. These changes greatly altered soil hydrological processes in the disturbed soils: the steady-state infiltration rates in the upper soil layer increased by 693%, the precipitation recharge layer deepened, and the surface evaporation rate decreased by 20.3%. Moreover, the disturbance increased storage of plant-available water in the herbaceous rooting zone and improved the environment for germination and subsequent growth of annual herb species, as shown by a notable increase in the coverage, density, frequency, and biomass of annual plants. We conclude that MC represent a major component of Shapotou's regional ecosystem and that disturbance will significantly alter local ecohydrological processes.

PEAK FIRE TEMPERATURES AND EFFECTS ON ANNUAL PLANTS IN THE MOJAVE DESERT

Very little is known about the behavior and effects of fire in the Mojave Desert, because fire was historically uncommon. However, fire has become more frequent since the 1970s with increased dominance of the invasive annual grasses Bromus rubens and Schismus spp., and land managers are concerned about its ecological effect. In this paper, I describe patterns of peak fire temperature and their effect on annual plants in creosote bush scrub vegetation of the Mojave Desert. Temperatures were monitored among microhabitats and distances from the soil surface, and between spring and summer. Microhabitats ranged from high amounts of fuel beneath creosote bush (Larrea tridentata) canopies, to intermediate amounts at the canopy drip line, to low amounts in the interspaces between them. Distances from the soil surface were within the vertical range where most annual plant seeds occur (−2, 0, 5, and 10 cm). I also compare temperature patterns with postfire changes in soil properties and annual plant biomass and spe...