Steven R. Archer

δ13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem

Abstract Plants with C 3 , C 4 , and CAM photosynthesis have unique δ 13 C values which are not altered significantly during decomposition and soil organic matter formation. Consequently, δ 13 C values of soil organic carbon reflect the relative contribution of plant species with C 3 , C 4 , and CAM photosynthetic pathways to community net primary productivity, and have been utilized to document vegetation change, to quantify soil organic matter turnover, and to refine our understanding of earth–atmosphere–biosphere interactions. Here, we review the basis of this methodology, and illustrate its use as a tool for studying grass–woody plant dynamics in a savanna ecosystem. In the Rio Grande Plains of southern Texas, C 4 grasslands and savannas have been largely replaced by C 3 subtropical thorn woodlands dominated by Prosopis glandulosa . We used δ 13 C values of soil organic matter, above- and belowground plant biomass, and litter in conjunction with radiocarbon dating and dendrochronology to test the hypotheses that: (1) C 3 Prosopis groves in uplands and C 3 Prosopis woodlands in low-lying drainages have been long-term components of the landscape; and (2) Prosopis woodlands of low-lying drainages have expanded up-slope since Anglo-European settlement. Current organic matter inputs were not in isotopic equilibrium with soil organic carbon in any of the patch types sampled. In upland grasslands, δ 13 C values of vegetation (−20‰) were lower than those of soil organic matter (−17‰), suggesting increased C 3 forb abundance in response to long-term, heavy grazing (herbaceous retrogression). In wooded landscape elements, δ 13 C values of current organic matter inputs were characteristic of C 3 plants (−28 to −25‰), while those of the associated soil organic matter were typically −20 to −15‰. These δ 13 C values indicate that woodlands, groves, and shrub clusters dominated almost exclusively by C 3 plants now occupy sites once dominated by C 4 grasses. A particularly strong memory of the C 4 grasslands that once occupied these sites was recorded in the δ 13 C values of organic carbon associated with fine and coarse clay fractions (−18 to −14‰), probably a consequence of the slow organic carbon turnover rates in those soil fractions. When δ 13 C values of soil organic carbon were evaluated in conjunction with radiocarbon measurements of that same carbon, it appeared that herbaceous retrogression and a shift from C 4 grassland to C 3 woodland occurred recently, probably within the last 50–100 years. Demographic characteristics of the dominant tree species corroborated the δ 13 C and 14 C evidence, and indicated widespread establishment of P. glandulosa and associated shrubs over the past 100 years. Together, these data provide direct, spatially explicit evidence that vegetation change has occurred recently across the entire landscape at this site. Environmental conditions where C 3 , C 4 , and CAM plants coexist (e.g., dry, alkaline soils) generally do not favor the preservation of pollen and phytoliths, and these same areas usually lack historical records of vegetation change. Consequently, vegetation dynamics have been difficult to quantify in grasslands, savannas, and woodlands. However, our results demonstrate clearly that δ 13 C values of soil organic matter afford a direct and powerful technique for reconstructing vegetation change in these areas.

GRASSLAND TO WOODLAND TRANSITIONS: INTEGRATING CHANGES IN LANDSCAPE STRUCTURE AND BIOGEOCHEMISTRY

In many of the worlds drylands, human-induced alteration of grazing and fire regimes over the past century has promoted the replacement of grasses by woody vegetation. Here, we evaluate the magnitude of changes in plant and soil carbon and nitrogen pools in a subtropical landscape undergoing succession from grassland to thorn woodland in southern Texas. Our approach involved linking a process-based ecosystem model to a transition matrix model. Grass and forest production submodels of CENTURY were parameterized with field data collected from herbaceous and wooded landscape elements broadly representative of habitats in global savanna systems. The Markov (transition matrix) model simulated the displacement of grassland communities under land use practices typical of many modern grasslands and savannas (heavy livestock grazing; no fire) and climate events. The modeled landscape was initialized for pre-Anglo-European settlement grassland conditions and then subjected to heavy, continuous livestock grazing an...

Woody plants in grasslands: post-encroachment stand dynamics.

Woody plant abundance is widely recognized to have increased in savannas and grasslands worldwide. The lack of information on the rates, dynamics, and extent of increases in shrub abundance is a major source of uncertainty in assessing how this vegetation change has influenced biogeochemical cycles. Projecting future consequences of woody cover change on ecosystem function will require knowledge of where shrub cover in present-day stands lies relative to the realizable maximum for a given soil type within a bioclimatic region. We used time-series aerial photography (1936, 1966, and 1996) and field studies to quantify cover and biomass of velvet mesquite (Prosopis velutina Woot.) following its proliferation in a semidesert grassland of Arizona. Mapping of individual shrubs indicated an encroachment phase characterized by high rates of bare patch colonization. Upon entering a stabilization phase, shrub cover increases associated with recruitment and canopy expansion were largely offset by contractions in canopy area of other shrub patches. Instances of shrub disappearance coincided with a period of below-average rainfall (1936-1966). Overall, shrub cover (mean +/- SE) on sandy uplands with few and widely scattered shrubs in 1902 was dynamically stable over the 1936-1996 period averaging approximately 35% +/- 5%. Shrub cover on clayey uplands in 1936 was 17% +/- 2% but subsequently increased twofold to levels comparable to those on sandy uplands by 1966 (36% +/- 7%). Cover on both soils then decreased slightly between 1966 and 1996 to 28% +/- 3%. Thus, soil properties influenced the rate at which landscapes reached a dynamic equilibrium, but not the apparent endpoint. Although sandy and clayey landscapes appear to have stabilized at comparable levels of cover, shrub biomass was 1.4 times greater on clayey soils. Declines in shrub cover between 1966 and 1996 were accompanied by a shift to smaller patch sizes on both sandy and clayey landscapes. Dynamics observed during the stabilization phase suggest that density-dependent regulation may be in play. If woody cover has transitioned from directional increases to a dynamic equilibrium, biomass projections will require monitoring and modeling patch dynamics and stand structure rather than simply changes in total cover.

Climate Change and Ecosystems of the Southwestern United States

Arid desert ecosystems of the western United States are particularly susceptible to climate change and climate variability. Plants and animals in this region live near their physiological limits for water and temperature stress. Slight changes in temperature or precipitation regimes or a change in the frequency and magnitude of extreme climatic events could therefore substantially alter the composition, distribution, and abundance of species, as well as the products and services that arid lands provide. In the United States, arid lands are located in the subtropical hot deserts of the Southwest, comprised of the Mojave, Sonoran, and Chihuahuan deserts, and the temperate cold deserts of the Intermountain West. Annual precipitation is low (<400 mm), but the seasonality of precipitation differs substantially among hot deserts (Fig. 1). The Mojave Desert is dominated by winter precipitation; thus biological activity is greatest during the cool season. The Chihuahuan Desert is dominated by summer precipitation with biological activity during hotter conditions. The hottest of the three deserts, the Sonoran, is intermediate, receiving both winter and summer precipitation. Each of these deserts is characterized by low productivity and slow plant growth, both of which are primarily water-limited. Vegetation communities are typically desert scrub, shrub– steppe, or desert grassland/savanna and are home to charismatic plants, including saguaro cacti, organ pipe cacti, and Joshua trees. The Chihuahuan Desert is the largest desert in North America, stretching from the southwestern United States deep into the Central Mexican Highlands. It has been classifi ed by the World Wildlife Fund as a Global 200 Ecoregion, a science-based global ranking of the Earth’s most biologically outstanding habitats. Arid lands currently provide a variety of products and services, including a large ranching industry, wildlife habitat, plant and animal diversity, regulation of water fl ow and quality, opportunities for outdoor recreation, and open spaces for expanding urban environments.

INTERRELATIONSHIPS AMONG SHRUB ENCROACHMENT, LAND MANAGEMENT, AND LITTER DECOMPOSITION IN A SEMIDESERT GRASSLAND

Encroachment of woody plants into grasslands, and subsequent brush management, are among the most prominent changes to occur in arid and semiarid systems over the past century. Despite the resulting widespread changes in landcover, substantial uncertainty about the biogeochemical impacts of woody proliferation and brush management exists. We explored the role of shrub encroachment and brush management on leaf litter decomposition in a semidesert grassland where velvet mesquite (Prosopis velutina) abundance has increased over the past 100 years. This change in physiognomy may affect decomposition directly, through altered litter quality or quantity, and indirectly through altered canopy structure. To assess the direct and indirect impacts of shrubs on decomposition, we quantified changes in mass, nitrogen, and carbon in litterbags deployed under mesquite canopies and in intercanopy zones. Litterbags contained foliage from mesquite and Lehmann lovegrass (Eragrostis lehmanniana), a widespread, nonnative grass in southern Arizona. To explore short- and long-term influences of brush management on the initial stages of decomposition, litterbags were deployed at sites where mesquite canopies were removed three weeks, 45 years, or 70 years prior to study initiation. Mesquite litter decomposed more rapidly than lovegrass, but negative indirect influences of mesquite canopies counteracted positive direct effects. Decomposition was positively correlated with soil infiltration into litterbags, which varied with microsite placement, and was lowest under canopies. Low under-canopy decomposition was ostensibly due to decreased soil movement associated with high under-canopy herbaceous biomass. Decomposition rates where canopies were removed three weeks prior to study initiation were comparable to those beneath intact canopies, suggesting that decomposition was driven by mesquite legacy effects on herbaceous cover-soil movement linkages. Decomposition rates where shrubs were removed 45 and 70 years prior to study initiation were comparable to intercanopy rates, suggesting that legacy effects persist less than 45 years. Accurate decomposition modeling has proved challenging in arid and semiarid systems but is critical to understanding biogeochemical responses to woody encroachment and brush management. Predicting brush-management effects on decomposition will require information on shrub-grass interactions and herbaceous biomass influences on soil movement at decadal timescales. Inclusion of microsite factors controlling soil accumulation on litter would improve the predictive capability of decomposition models.

Biogenic hydrocarbon emissions and landcover/climate change in a subtropical savanna

Abstract Biogenic non-methane hydrocarbon (NMHC) emissions strongly influence the chemical composition of the troposphere. Thus, variations in emissions of these compounds are expected to cause changes in concentrations of important atmospheric trace gases. Here, we assess the relative magnitude of potential changes in NMHC (e.g., isoprene and monoterpene) emissions using field flux measurements from a subtropical savanna parkland/thorn woodland site in conjunction with model predictions of climate and landcover change. NMHC emissions of about 40 plant species were characterized. Grasses, as a group, had low emission rates. Several common woody species had high emission rates. However, there was little evidence of emissions being consistently related to woody plant taxonomy, growthform or functional groups. Above-canopy measurements were used to validate modeled isoprene flux predictions of about 2 mg C m−2 h−1 for savanna parkland/thorn woodland and ca. 0.7 mg C m−2 h−1 for the regional landscape, which is a mixture of savanna parkland/thorn woodland and cropland. Linkage of the biogenic emissions model with a plant succession model indicated that landcover change since the early 1800s has elicited a 3-fold increase in total NMHC emissions. This increase reflected changes in vegetation species composition (from domination by grasses which were typically ‘low emitters’, to shrubs and trees, many of which were ‘high emitters’) and increases in foliar density. Field measurements on two common shrub species indicated that isoprene emission increased exponentially with increases in leaf temperature from 20 to 40° C and were not suppressed by drought stress. Accordingly, our model predicted that projected increases in ambient temperature (3 to 6°C) emissions could produce a 2-fold increase in biogenic NMHC emissions. Cloud cover, precipitation, relative humidity, and winds also exerted some control over NMHC emissions, but their influence was highly variable and difficult to estimate. Although our results are specific to southern Texas USA, they indicate the magnitude of change in NMHC emissions that could occur at other locations when climate and vegetation composition are altered.

Spatial variability in the potential for symbiotic N2 fixation by woody plants in a subtropical savanna ecosystem

1. Root infection by symbiotic N 2 -fixing Frankia and Rhizobium strains was quantified in relation to light and soil properties for seedlings of 12 woody species from a subtropical savanna in southern Texas, USA. 2. None of four rhamnaceous species nodulated, despite the fact that bioassays with a known actinorhizal species yielded 13 nodules per seedling. Celtis pallida (Ulmaceae), Acacia greggii and Acacia berlandieri (Leguminosae) also failed to nodulate even though field populations of these species were characterized by high (2.7-4.2%) foliar nitrogen concentration. 3. Infective rhizobia occurred in all soils studied regardless of soil depth, distance from a host plant or type of plant cover. Plant growth in N-free media and acetylene reduction activity suggested that all nodules were capable of N 2 -fixation. 4. The extent of nodulation varied by species. However, nodulated seedlings were taller, produced more biomass and allocated less biomass to root systems than their non-nodulated counterparts. 5. Numbers of nodules on seedlings of Prosopis glandulosa, the dominant woody species in this subtropical savanna and throughout the south-western USA, were reduced by low light (15% full sunlight) regardless of soil N level; at medium and full sunlight nodule biomass expressed as a fraction of whole plant biomass decreased with increasing soil N. Nodulation of field-grown P. glandulosa appears to be ephemeral, apparently varying with changes in soil moisture. 6. Nodulation and N 2 fixation among woody legumes in subtropical savannas can occur across a broad range of soil conditions and depths with significant impacts on local and regional N-cycles. 7. Field levels of foliar N in species that failed to nodulate in the laboratory were comparable to or greater than those in species capable of nodulation, suggesting that leaf N is not a reliable indicator of N 2 fixation.

Water use by woody plants on contrasting soils in a savanna parkland: assessment with δ2H and δ18O

In savanna parklands of southern Texas, patches of grassland and ‘discrete clusters’ of small trees and shrubs occur on sandy loam surface soils underlain by an argillic horizon (claypan) at 40 cm. Large trees and shrubs in ‘groves’ occur on deep (2 m) sandy loam soils without an argillic horizon. δ2H and δ18O of rainfall, groundwater, and soil and plant water were measured to: (1) determine if coexistence in woody patches occurs via vertical stratification of soil water uptake; (2) document differences in plant water acquisition on contrasting soil types; and (3) evaluate recharge and evaporative losses of soil moisture from grassland vs. wooded landscape elements. Groundwater was isotopically similar to weighted rainfall, suggesting local recharge at this site. Linear regressions of soil water δ2H on δ18O yielded slopes less than the meteoric water line, indicating significant evaporative losses of soil moisture in all landscape elements. Interspecific differences in root density distribution were significant; some woody species had roots well below 1.6 m, while others had few roots below 0.8 m. δ2H and δ18O values of stem water from all plants in groves were lower than those of soil water in the upper 1.5 m of the profile, suggesting all species obtained their water from depths >1.5 m. Deep roots of trees and shrubs at this savanna parkland site thus appeared to have a functional significance that was not revealed by biomass or density determinations. Root densities of species in discrete clusters (claypan present) were typically greater than those of the same species in groves (claypan absent), especially in the upper 80 cm of the soil profile. Consistent with rooting profiles, δ2H and δ18O values of plant water indicated that trees and shrubs in discrete clusters with fine- textured subsoils obtained most of their water at depths <1.5 m. As with groves, there was no indication of water resource partitioning between species. In summary, we saw no isotopic evidence that co- occurring woody plants at this savanna parkland site were partitioning soil moisture vertically during late summer/early fall, despite marked differences in their root density distributions. This supports other lines of evidence which indicate that species interactions in tree/shrub clumps are competitive, and that species composition is therefore unstable in those landscape elements.

Desertification, land use, and the transformation of global drylands

Desertification is an escalating concern in global drylands, yet assessments to guide management and policy responses are limited by ambiguity concerning the definition of “desertification” and what processes are involved. To improve clarity, we propose that assessments of desertification and land transformation be placed within a state change–land-use change (SC–LUC) framework. This framework considers desertification as state changes occurring within the context of particular land uses (eg rangeland, cropland) that interact with land-use change. State changes that can be readily reversed are distinguished from regime shifts, which are state changes involving persistent alterations to vegetation or soil properties. Pressures driving the transformation of rangelands to other types of land uses may be low, fluctuating, or high, and may influence and be influenced by state change. We discuss how the SC–LUC perspective can guide more effective assessment of desertification and management of drylands.

Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture

BACKGROUND AND AIMS Plasticity in structural and functional traits related to water balance may determine plant performance and survival in ecosystems characterized by water limitation or high levels of rainfall variability, particularly in perennial herbaceous species with long generation cycles. This paper addresses whether and the extent to which several such seasonal to long-term traits respond to changes in moisture availability. METHODS Using a novel approach that integrates ecology, physiology and anatomy, a comparison was made of lifetime functional traits in the root xylem of a long-lived perennial herb (Potentilla diversifolia, Rosaceae) growing in dry habitats with those of nearby individuals growing where soil moisture had been supplemented for 14 years. Traditional parameters such as specific leaf area (SLA) and above-ground growth were also assessed. KEY RESULTS Individuals from the site receiving supplemental moisture consistently showed significant responses in all considered traits related to water balance: SLA was greater by 24 %; roots developed 19 % less starch storing tissue, an indicator for drought-stress tolerance; and vessel size distributions shifted towards wider elements that collectively conducted water 54 % more efficiently - but only during the years for which moisture was supplemented. In contrast, above-ground growth parameters showed insignificant or inconsistent responses. CONCLUSIONS The phenotypic changes documented represent consistent, dynamic responses to increased moisture availability that should increase plant competitive ability. The functional plasticity of xylem anatomy quantified in this study constitutes a mechanistic basis for anticipating the differential success of plant species in response to climate variability and change, particularly where water limitation occurs.