Fire mediated patterns of population densities in mountain big sagebrush bird communities

We employed a chronosequence approach to evaluate patterns of bird abundance in relation to post-fire vegetation recovery in mountain big sagebrush (Artemisia tridentata vaseyana). We estimated population density for 12 species of birds within the perimeters of 4 fires that had undergone 8–20 years of vegetation recovery and on adjacent unburned areas in the northwestern Great Basin, USA. Six species showed negative responses to fire persisting up to 20 years. Two species showed positive responses with effects persisting for <20 years. Understory vegetation was similar between burned and unburned areas irrespective of recovery time, and shrub canopy cover was similar between burned and unburned sites after 20 years of recovery. Persistent reductions in bird densities lead us to conclude that shrub canopy cover alone is not a sufficient metric for predicting recovery of songbird abundances following disturbance in mountain big sagebrush. © 2013 The Wildlife Society.     

Attempting to restore mountain big sagebrush (Artemisia tridentata ssp. vaseyana) four years after fire

Restoration of shrubs is needed throughout the world because of altered fire regimes, anthropogenic disturbance, and overutilization. The native shrub mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) is a restoration priority because of its value to wildlife in western North America. One of the principal threats to mountain big sagebrush is encroachment by western juniper (Juniperus occidentalis ssp. occidentalis Hook) and other conifers. Fire is frequently applied to control juniper; however, sagebrush recovery after fire can be variable. Seeding sagebrush postfire can hasten sagebrush recovery; however, seeding is not always necessary. Therefore, it may be advantageous to monitor postfire recovery to determine if seeding is needed. The effect of seeding sagebrush several years after fire is unknown. We evaluated the efficiency of seeding mountain big sagebrush four years after fire‐controlled junipers at five sites. Sagebrush cover (<0.5%) and density (<0.07 plants/m²) was low in seeded plots and did not differ from unseeded controls in the three postseeding years. We conclude that seeding sagebrush four years after fire did not accelerate sagebrush recovery. We speculate that seeded sagebrush failed to establish because of competition from herbaceous vegetation that had four years to recover after fire. Although it would be beneficial to seed sagebrush only when needed, our results suggest postponing seeding until monitoring has determined that recovery is inadequate may not be advisable. We suggest researchers investigate methods to improve predicting sagebrush recovery to allow for seeding, when needed, before the first postfire growing season.     

Restoring big sagebrush after controlling encroaching western juniper with fire: aspect and subspecies effects

The need for restoration of shrubs is increasingly recognized around the world. In the western United States, restoration of mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana [Rydb.] Beetle) after controlling encroaching conifers is a priority to improve sagebrush‐associated wildlife habitat. Conifers can be cost effectively removed with prescribed burning when sagebrush is codominant; however, burning removes sagebrush and natural recovery may be slow. We evaluated seeding mountain and Wyoming big sagebrush (A. tridentata Nutt. ssp. wyomingensis Beetle & Young) on north and south aspects after western juniper (Juniperus occidentalis ssp. occidentalis Hook) control with prescribed burning. We included seeding Wyoming big sagebrush, a more drought tolerant subspecies of big sagebrush, because it might grow better than mountain big sagebrush on hot, dry south slopes, during drought, or after juniper encroachment. Seeding mountain big sagebrush increased sagebrush cover and density compared to unseeded controls. In mountain big sagebrush‐seeded plots, sagebrush cover was 19 times greater on north compared to south aspects in the fourth year after seeding. At this time, sagebrush cover was also greater on mountain compared to Wyoming big sagebrush‐seeded plots. Natural recovery (i.e. unseeded) of sagebrush was occurring on north aspects with sagebrush cover averaging 3% 4 years after fire. Sagebrush was not detected on unseeded south aspects at the end of the study. These results suggest that postfire sagebrush recovery, with and without seeding, will be variable across the landscape based on topography. This study suggests seeding sagebrush after controlling junipers with burning may accelerate sagebrush recovery.     

Disturbance Type and Plant Successional Communities in Bahamian Dry Forests

Different disturbances in similar habitats can produce unique successional assemblages of plants. We collected plant species composition and cover data to investigate the effects of three common types of disturbances—fire, anthropogenic clearing (‘cleared’), and clearing followed by goat grazing (‘cleared‐and‐grazed’)—on early‐successional coppice (dry forest) community structure and development on Eleuthera, Bahamas. For each disturbance type, both the ground layer (<0.5 m height) and shrub layer (>0.5 m height) were sampled in eight patches (>1 ha) of varying age (1–28 yr) since large‐scale mature coppice disturbance. Overall, plant communities differed among disturbance types; several common species had significantly higher cover in the shrub layer of fire patches, and cleared‐and‐grazed patches exhibited higher woody ground cover. Total percent cover in the shrub layer increased in a similar linear fashion along the investigated chronosequence of each disturbance type; however, cover of the common tree species, Bursera simaruba, increased at a notably slower rate in cleared‐and‐grazed patches. The pattern of increase and subsequent decrease in cover of Lantana spp. and Zanthoxylum fagara in the shrub layer was characterized by longer persistence and higher covers, respectively, in cleared‐and‐grazed patches, which also exhibited low peak cover and fast decline of nonwoody ground cover. Our results suggest that goats may accelerate some aspects of succession (e.g., quickly removing nonwoody ground cover) and retard other aspects (e.g., inhibiting growth of tree species and maintaining early‐successional shrubs in the shrub layer). These effects may lead to different successional trajectories, and have important conservation implications.     

Anthropogenic and Natural Disturbance Differentially Affect Sagebrush Bird Habitat Use

North American sagebrush (Artemisia spp.)-obligate birds are experiencing steep population declines due in part to increased disturbance, mainly human-caused, across their range. At the eastern edge of the sagebrush steppe, this issue may potentially be exacerbated because of natural disturbance by black-tailed prairie dogs (Cynomys ludovicianus). Our goal was to compare local and landscape models of habitat use by greater sage-grouse (Centrocercus urophasianus), Brewer's sparrow (Spizella breweri), and sage thrasher (Oreoscoptes montanus) with models including effects of natural (i.e., prairie dog) and anthropogenic disturbance. We used a combination of field data collection, and state and national datasets for the Thunder Basin National Grassland, eastern Wyoming, USA, to understand the factors that influence lek attendance by sage-grouse and habitat use by 2 passerines in this system. For all 3 species, models including big sagebrush (Artemisia tridentata) cover at local and landscape scales were the most competitive among univariate models, supporting the paradigm that sagebrush is key for these species. Models including anthropogenic disturbance (well density, road density) explained more variation than models of prairie dog disturbance alone for 2 of the 3 species, but long-term disturbance by prairie dogs did reduce abundance of Brewer's sparrows. Although long-term prairie dog disturbance has the potential to reduce sagebrush cover for sagebrush-obligate birds, such events are likely rare because outbreaks of plague (Yersina pestis) and lethal control on borders with private land reduce prairie dog disturbance. Conversely, anthropogenic disturbance is slated to increase in this system, suggesting potentially accelerated declines for sagebrush birds into the future. © 2020 The Wildlife Society.     

Avian Community Structure Along a Mountain Big Sagebrush Successional Gradient

Abstract: We compared vegetative structure and bird communities among 4 successional states in central Oregon representing a continuum from 1) postburn grassland, 2) mountain big sagebrush—Idaho fescue (Artemisia tridentatA—Festuca idahoensis) shrub—steppe, 3) sagebrush—steppe—juniper (Juniperus occidentalis), to 4) old-growth western juniper. Species richness, evenness, and diversity of bird communities were highest in old-growth and mid-successional juniper (22.9 species/transect and 23.6 species/transect, respectively) but lowest in the grasslands (17.6 species/transect). Bird species diversity was positively correlated with physiognomic cover diversity (r = 0.74, P = 0.001). Density of breeding birds was greatest in old-growth juniper (6.6 birds/ha) and lowest in postburn grasslands (3.6 birds/ha) but similar in shrub—steppe and sagebrush—steppe—juniper (6.0 birds/ha and 5.5 birds/ha, respectively). Old-growth juniper had the highest total densities of both tree and cavity nesters. Mountain chickadees (Parus gambeli), Cassin's finches (Carpodacus cassinii), chipping sparrows (Spizella passerina), brown-headed cowbirds (Molothrus ater), mountain bluebirds (Sialia currucoides), dark-eyed juncos (Junco hyemalis), Empidonax flycatchers, ash-throated flycatchers (Myiarchus cinerascens), and northern flickers (Colaptes auratus) were more abundant in cover types dominated by junipers. Vesper sparrows (Pooecetes gramineus), western meadowlarks (Sturnella neglecta), green-tailed towhees (Pipilo chlorurus), and horned larks (Eremophila alpestris) were associated with grassland communities. Brewer's sparrows (Spizella breweri), sage sparrows (Amphispiza belli), sage thrashers (Oreoscoptes montanus), and horned larks (Eremophila alpestris) were most abundant in sagebrush cover types. Management strategies should restore or maintain the desired proportions of the different successional states to maintain populations of grassland and sagebrush birds while providing habitat for tree and cavity nesting species.     

Ecological Factors Influencing Nest Survival of Greater Sage-Grouse in Mono County,California

ABSTRACT We studied nest survival of greater sage-grouse (Centrocercus urophasianus) in 5 subareas of Mono County, California, USA, from 2003 to 2005 to 1) evaluate the importance of key vegetation variables for nest success, and 2) to compare nest success in this population with other greater sage-grouse populations. We captured and radiotracked females (n = 72) to identify nest sites and monitor nest survival. We measured vegetation at nest sites and within a 10-m radius around each nest to evaluate possible vegetation factors influencing nest survival. We estimated daily nest survival and the effect of explanatory variables on daily nest survival using nest-survival models in Program MARK. We assessed effects on daily nest survival of total, sagebrush (Artemisia spp.), and nonsagebrush live shrub-cover, Robel visual obstruction, the mean of grass residual height and grass residual cover measurements within 10 m of the nest shrub, and area of the shrub, shrub height, and shrub type at the nest site itself. Assuming a 38-day exposure period, we estimated nest survival at 43.4%, with percent cover of shrubs other than sagebrush as the variable most related to nest survival. Nest survival increased with increasing cover of shrubs other than sagebrush. Also, daily nest survival decreased with nest age, and there was considerable variation in nest survival among the 5 subareas. Our results indicate that greater shrub cover and a diversity of shrub species within sagebrush habitats may be more important to sage-grouse nest success in Mono County than has been reported elsewhere.     

Microsite conditions in retrogressive thaw slumps may facilitate increased seedling recruitment in the Alaskan Low Arctic

In Low Arctic tundra, thermal erosion of ice‐rich permafrost soils (thermokarst) has increased in frequency since the 1980s. Retrogressive thaw slumps (RTS) are thermokarst disturbances forming large open depressions on hillslopes through soil wasting and vegetation displacement. Tall (>0.5 m) deciduous shrubs have been observed in RTS a decade after disturbance. RTS may provide conditions suitable for seedling recruitment, which may contribute to Arctic shrub expansion. We quantified in situ seedling abundance, and size and viability of soil seedbanks in greenhouse trials for two RTS chronosequences near lakes on Alaska's North Slope. We hypothesized recent RTS provide microsites for greater recruitment than mature RTS or undisturbed tundra. We also hypothesized soil seedbanks demonstrate quantity–quality trade‐offs; younger seedbanks contain smaller numbers of mostly viable seed that decrease in viability as seed accumulates over time. We found five times as many seedlings in younger RTS as in older RTS, including birch and willow, and no seedlings in undisturbed tundra. Higher seedling counts were associated with bare soil, warmer soils, higher soil available nitrogen, and less plant cover. Seedbank viability was unrelated to size. Older seedbanks were larger at one chronosequence, with no difference in percent germination. At the other chronosequence, germination was lower from older seedbanks but seedbank size was not different. Seedbank germination was positively associated with in situ seedling abundance at one RTS chronosequence, suggesting postdisturbance revegetation from seedbanks. Thermal erosion may be important for recruitment in tundra by providing bare microsites that are warmer, more nutrient‐rich, and less vegetated than in undisturbed ground. Differences between two chronosequences in seedbank size, viability, and species composition suggest disturbance interacts with local conditions to form seedbanks. RTS may act as seedling nurseries to benefit many Arctic species as climate changes, particularly those that do not produce persistent seed.     

Historical Cover Trends in a Sagebrush Steppe Ecosystem from 1985 to 2013: Links with Climate,Disturbance, and Management

Understanding the causes and consequences of component change in sagebrush steppe is crucial for evaluating ecosystem sustainability. The sagebrush (Artemisia spp.) steppe ecosystem of the northwest USA has been impacted by the invasion of exotic grasses, increasing fire return intervals, changing land management practices, and fragmentation, often lowering the overall resilience to change. We utilized contemporary and historical Landsat imagery, field data, and regression tree models to produce fractional cover maps of rangeland components (shrub, sagebrush, herbaceous, bare ground, and litter) through the last 30 years. Our main goals were to (1) investigate rangeland component trends over 30 years, (2) evaluate the magnitude and direction of trends in components and climate drivers and their relationship, and (3) assess component trends influenced by climate. Results indicated that over the study period, shrub, sage, herbaceous, and litter cover decreased, while bare ground cover increased. Measured rates of change ranged from ? 0.14% decade^?1 for shrub cover to 0.05% decade^?1 for bare ground, whereas herbaceous and litter cover trends were negligible. Net landscape cover changes were consistent with expectations of climate change and disturbance producing a loss of biotic cover, and converting a portion of shrub and sagebrush to herbaceous cover. Overall, fire and related successional recovery was the greatest change agent for all components in terms of area and cover change, while increasing minimum temperature, at a rate of 0.66°C decade^?1, was found to be the most significant climate driver.     

Bromus tectorum Response to Fire Varies with Climate Conditions

The invasive annual grass Bromus tectorum (cheatgrass) forms a positive feedback with fire in some areas of western North America’s sagebrush biome by increasing fire frequency and size, which then increases B. tectorum abundance post-fire and dramatically alters ecosystem structure and processes. However, this positive response to fire is not consistent across the sagebrush steppe. Here, we ask whether different climate conditions across the sagebrush biome can explain B. tectorum’s variable response to fire. We found that climate variables differed significantly between 18 sites where B. tectorum does and does not respond positively to fire. A positive response was most likely in areas with higher annual temperatures and lower summer precipitation. We then chose a climatically intermediate site, with intact sagebrush vegetation, to evaluate whether a positive feedback had formed between B. tectorum and fire. A chronosequence of recent fires (1–15 years) at the site created a natural replicated experiment to assess abundance of B. tectorum and native plants. B. tectorum cover did not differ between burned and unburned plots but native grass cover was higher in recently burned plots. Therefore, we found no evidence for a positive feedback between B. tectorum and fire at the study site. Our results suggest that formation of a positive B. tectorum-fire feedback depends on climate; however, other drivers such as disturbance and native plant cover are likely to further influence local responses of B. tectorum. The dependence of B. tectorum’s response to fire on climate suggests that climate change may expand B. tectorum’s role as a transformative invasive species within the sagebrush biome.     

Time‐since‐fire and climate interact to affect the structural recovery of an Australian semi‐arid plant community

How does time‐since‐fire influence the structural recovery of semi‐arid, eucalypt‐dominated Murray‐Mallee shrublands after fire, and is recovery affected by spatial variation in climate? We assessed the structure and dynamics of a hummock grass, Triodia scariosa N.T. Burb, and mallee eucalypts – two key structural components of mallee shrublands – using a >100 year time‐since‐fire chronosequence. The relative influence of climatic variables, both individually and combined with time‐since‐fire, was modelled to account for spatial variation in the recovery of vegetation structural components. Time‐since‐fire was the primary determinant of the structural recovery of T. scariosa and eucalypts. However, climate, notably mean annual rainfall and rainfall variability, also influenced the recovery of the eucalypt overstorey, T. scariosa cover and mean hummock height. We observed that (i) the mean number of live eucalypt stems per individual decreased while mean individual basal area increased, (ii) cover of T. scariosa peaked at ~30 years post‐fire and gradually decreased thereafter, and (iii) the ‘hummock’ form of T. scariosa occurred throughout the chronosequence, whereas the ‘ring’ form tended not to occur until ~30 years post‐fire. Time‐since‐fire was the key determinant of the structural recovery of eucalypt‐dominated mallee shrublands, but there is geographical variation in recovery related to rainfall and its variability. Fire regimes are likely to have different effects across the geographic range of mallee shrublands.     

Relationships between ericaceous vegetation and soil nutrient status in a post-fire Kalmia angustifolia-black spruce chronosequence

Post-fire nutrient flushes are an important precursor to secondary succession in fire-driven boreal forest. We studied the magnitude of changes in post-fire soil nutrient status across a chronosequence of ericaceous shrub-dominated boreal forest stands in eastern Newfoundland, Canada. The chronosequence comprised nine stands burned between 1 and 38 years prior to the study. These sites have resisted tree reestablishment following forest fire-induced mortality of black spruce and a concomitant increase in dominance of the ericaceous dwarf shrub Kalmia angustifolia L. Our objectives were: (1) to identify the factors driving soil nutrient status in these post-fire stands dominated by ericaceous plants, and (2) to test hypotheses that specific relationships exist among environmental factors, dominant vegetation and indicators of soil nutrient status. Macronutrients such as NH₄⁺, total organic N and mineral soil P concentrations showed non-linear declines with time since fire. These parameters were also negatively associated with cover of ericaceous plants. Potential phytotoxins such as total phenolics and aluminium concentrations increased with increasing cover of K. angustifolia. Variability in net ammonification, total P and total phenolic acids in organic soils were strongly related to ericaceous dominance even when the effect of time since fire was partialled out using regression analysis. These findings suggest a strong capacity for ericaceous vegetation to have top-down effects on soil chemical property particularly in the organic horizon with the increase in its post-fire dominance.     

Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.     

Variegated desert vegetation: Covariation of edaphic and fire variables provides a framework for understanding mulga‐spinifex coexistence

Abstract Mulga (Acacia aneura Mimosaceae) and spinifex (Triodia spp. Poaceae) habitats together characterize a large part of arid central Australia. Often very abrupt boundaries form between these two habitats, giving rise to a mosaic pattern of contrasting shrub‐grass alterations across the landscape. Reasons for such patterning remain poorly understood though current niche‐based views relate species' distributions to spatial resource gradients or to fire effects. Field survey work was conducted on central Australian mountain ranges to further quantify floristic, regeneration traits, and structural patterning across mulga‐spinifex transitions and to test resource‐ and disturbance‐models that explain these patterns. Compositional analysis demonstrated variability in transition type – in certain cases boundaries denoted true floristic discontinuity and in others, somewhat more of a structural shift. Moreover, it was shown that minimal between‐habitat floristic overlap coincided with the occurrence of distinct edaphic changes, while greater compositional commonality occurred when soil gradients were more diffuse. This indicated that floristic patterning cannot be ascribed to any one single process. In the case of strong soil gradients, between‐habitat segregation most likely resulted from resource‐based niche differentiation; for weaker gradients, fire‐frequency assumed greatest importance. Disturbance theory most readily accounted for the distribution of woody species' post‐fire regeneration traits across habitat boundaries. The results also suggested that biotic factors –viz competition, facilitation and animal‐mediated dispersal – may be of additional consequence for mulga‐spinifex coexistence. Overall, the study served to emphasize the importance of multi‐factor explanation for within‐ and between‐habitat patterning in these mosaics. It also highlighted the need for experimentation to facilitate distinction between cause and correlation.     

Vegetation establishment, succession and microsite frost disturbance on glacier forelands within patterned ground chronosequences

Aim The impact of microscale frost disturbance on vegetation colonization and successionary trends was examined within patterned ground features of Little Ice Age chronosequences. The goal was to investigate and compare vegetation response to micro‐site frost disturbance with that of previous studies done at a coarser landscape scale. Location The study sites occur on Little Ice Age glacier forelands within Jotunheimen, Norway (61°–62° N). The forelands of the glaciers Slettmarkbreen, Styggedalsbreen and Vestre Memurubreen have been well studied providing chronological controls for landscape studies. Sorted patterned ground features are found within the chronosequences, typically declining with frost intensity and disturbance with increasing terrain age. Methods Micro‐plots (8.3 × 8.3 cm) were placed at the inner borders and centres of patterned ground features. Species were identified and per cent species cover and per cent cover of life‐form category were noted. Nonparametric Kruskal–Wallis and Mann–Whitney U‐tests were used to test for differences between percent cover of life‐form categories within patterned ground features as well as to identify thresholds of successional change across the chronosequences. Results Significant relationships between life‐from groups and patterned ground positions of varying ages were deduced using nonparametric statistics. Findings were then used to discuss trends of succession within patterned ground features and across the chronosequences. Vegetation establishment occurs at the border positions of young (< 30 years) patterned ground features. With time and distance from the ice margin, vegetation encroaches inwards toward the disturbed centres. Succession within patterned ground exhibits several stages: (1) bryophytes/crusts and lichens, (2) grasses/sedges and (3) woody shrubs. The occurrence of forbs was sporadic and generally non‐significant. Main conclusions Frost disturbance in patterned ground appears to delay successional trends of vegetation communities when compared with previous studies on ‘stable’ terrain, producing micro‐site lag effects. These small patches of disturbed ground are therefore important regarding vegetation assemblages across the landscape and are unlikely to be detected at the landscape scale.     

Context-dependent changes in the resistance and resilience of soil microbes to an experimental disturbance for three primary plant chronosequences

The extrinsic factors that regulate soil microbial stability (resistance and resilience) are little understood, even though soil microbes are important drivers of ecosystem function and their stability is likely to affect soil carbon storage and plant nutrient availability. Soils were collected across three primary plant chronosequences (two in New Zealand and one in Hawaii) that differed in climate, parent material and time spans to test the following hypotheses: i) there is a tradeoff between the resistance and resilience of key soil microbial response variables, ii) this tradeoff is related to the relationship of soil microbial resistance and resilience to soil resources, iii) resources change predictably during different primary plant chronosequences, and iv) if the first three hypotheses hold and are consistent for all three chronosequences, then soil microbial resistance and resilience should change predictably across different chronosequences. Results showed that although there was a tradeoff between resistance and resilience, the role of resources in determining this was unclear. Within each chronosequence, resources that were positively related to resistance were negatively related to resilience and vice versa, consistent with our second hypothesis. However, the direction and strength of correlations between stability and soil resources depended strongly on which soil microbial response variable was measured, and the chronosequence it was measured in. Total amounts of resources often showed consistent trends with ecosystem development for each chronosequence, but the way that resource quality changed varied between chronosequences. At least partly because of the variable nature of these relationships, the trajectory of resistance and resilience during ecosystem development varied considerably across chronosequences. Thus, although consistent trends were found within each chronosequence, the relationships between the stability of different soil microbial response variables, resources and ecosystem development depended strongly on which chronosequence was considered.     

N and P in New Zealand Soil Chronosequences and Relationships with Foliar N and P

The growth of forest species in soil development chronosequences becomes increasingly phosphorus (P)-limited with time, as P is weathered, eroded and leached from soil. Foliar nitrogen (N) concentrations also tend to decrease with soil age when vegetation may be limited in both N and P. Here we report on soil development in temperate rain forests along three New Zealand chronosequences that have minimal pollution and disturbance from human activities, at Franz Josef, Waitutu and Central Volcanic Plateau, and on factors influencing soil net N mineralization (aerobic; 56 days) and foliar N and P concentrations. Except in very young soils (<500 years), at least 85% of total-P in mineral soil (0–10 cm) was transformed to organic-P. In each chronosequence, total-P declined with time, and foliar N:P ratios (mass) generally increased from 8 to 15–18, suggesting P was more limiting than N in the oldest soils of the chronosequence. There was a negative relationship between net N mineralization and C:N ratio for mineral soil. For the FH (organic) layer, net N mineralization had the strongest relationships with total-N concentration (positively) and C:organic-P ratio (negatively); however, relationships varied with forest group, suggesting that other factors were also important. Foliar P of kamahi (Weinmannia racemosa Linn. f.), a dominant canopy species, was related to soil organic-P, suggesting mineralization was an important process for tree nutrition.Foliar N was positively related to N concentration in the FH layer, but was not significantly related to any measured property in mineral soil, possibly because of the wide range of soils. The consistent declines in both soil and foliar P across the contrasting chronosequences strongly suggest that vegetation becomes progressively P-limited during long-term ecosystem development.     

Consequences of shrub expansion in mesic grassland: Resource alterations and graminoid responses

Abstract. In the mesic grasslands of the central United States, the shrub Cornus drummondii has undergone widespread expansion in the absence of recurrent fire. We quantified alterations in light, water and N caused by C. drummondii expansion in tall‐grass prairie and assessed the hypothesis that these alterations are consistent with models of resource enrichment by woody plants. Responses in graminoid species, particularly the dominant C₄ grass Andropogon gerardii, were concurrently evaluated. We also removed established shrub islands to quantify their legacy effect on resource availability and assess the capability of this grassland to recover in sites formerly dominated by woody plants. The primary effect of shrub expansion on resource availability was an 87% reduction in light available to the herbaceous understorey. This reduced C uptake and N use efficiency in A. gerardii and lowered graminoid cover and ANPP at the grass‐shrub ecotone relative to undisturbed grassland. Shrub removal created a pulse in light and N availability, eliciting high C gain in A. gerardii in the first year after removal. By year two, light and N availability within shrub removal areas returned to levels typical of grassland, as had graminoid cover and ANPP were similar to those in open grassland. Recovery within central areas of shrub removal sites lagged behind that at the former grass‐shrub ecotone. These results indicate that the apparent alternative stable state of C. drummondii dominance in tall‐grass prairie is biotically maintained and driven by reductions in light, rather than resource enrichment. Within areas of shrub removal, the legacy effect of C. drummondii dominance is manifest primarily through the loss of rhizomes of the dominant grasses, rather than any long‐term changes in resource availability. C. drummondii removal facilitates grassland recovery, but the effort required to initiate this transition is a significant cost of woody plant expansion in mesic grasslands. Prevention of woody plant expansion in remnant tall‐grass prairies is, therefore, a preferred management option.     

Synergistic and antagonistic effects of mixing monospecific soils on plant-soil feedbacks

Background and aims

Disturbance affects the ability of organisms to persist on a site, and disturbance history acts as a filter of community composition. This is true for vascular plants and morphological groups of biocrusts, which respond differently to disturbance. Although functioning arid ecosystems include both groups, filtering of morphological groups of biocrusts has not previously been compared simultaneously with the responses of vascular plants.

Methods

Using a chronosequence approach, cover of vascular plants and biocrusts was examined across chronic disturbance gradients related to invasion by exotic species and grazing by livestock, following the acute disturbance of fire using paired burned and unburned plots in Wyoming big sagebrush on 99 plots.

Results

Cover of vascular plants and biocrusts was related to disturbance more so than abiotic factors of precipitation following fire, soil chemistry, percent coarse fragment and heat load index. Over time since fire of 12–23 years, we saw recovery of early successional groups: short mosses, shallow-rooted perennial grasses and annual forbs. Cover of deep and shallow-rooted perennial grasses and annual forbs increased in cover with intermediate levels of disturbance. Perennial forbs lacked a clear relationship with disturbance. Biocrusts decreased in cover with less disturbance when compared with perennial herbaceous plants but differed in sensitivities. Tall mosses were less sensitive to disturbance compared with lichens. Short mosses increased with some disturbance.

Conclusions

Morphological groups of biocrusts and vascular plants are eliminated with increasing variability in the size of gaps between perennials represented by the standard deviation of gaps between perennials. The inclusion of both groups in assessments of ecosystem recovery following disturbance addresses the fact that recovery of either group does not happen in isolation from the other but with interacting contributions to ecosystem functions.

     

Grass-Shrub Associations over a Precipitation Gradient and Their Implications for Restoration in the Great Basin,USA

As environmental stress increases positive (facilitative) plant interactions often predominate. Plant-plant associations (or lack thereof) can indicate whether certain plant species favor particular types of microsites (e.g., shrub canopies or plant-free interspaces) and can provide valuable insights into whether “nurse plants” will contribute to seeding or planting success during ecological restoration. It can be difficult, however, to anticipate how relationships between nurse plants and plants used for restoration may change over large-ranging, regional stress gradients. We investigated associations between the shrub, Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), and three common native grasses (Poa secunda, Elymus elymoides, and Pseudoroegneria spicata), representing short-, medium-, and deep-rooted growth forms, respectively, across an annual rainfall gradient (220–350 mm) in the Great Basin, USA. We hypothesized that positive shrub-grass relationships would become more frequent at lower rainfall levels, as indicated by greater cover of grasses in shrub canopies than vegetation-free interspaces. We sampled aerial cover, density, height, basal width, grazing status, and reproductive status of perennial grasses in canopies and interspaces of 25–33 sagebrush individuals at 32 sites along a rainfall gradient. We found that aerial cover of the shallow rooted grass, P. secunda, was higher in sagebrush canopy than interspace microsites at lower levels of rainfall. Cover and density of the medium-rooted grass, E. elymoides were higher in sagebrush canopies than interspaces at all but the highest rainfall levels. Neither annual rainfall nor sagebrush canopy microsite significantly affected P. spicata cover. E. elymoides and P. spicata plants were taller, narrower, and less likely to be grazed in shrub canopy microsites than interspaces. Our results suggest that exploring sagebrush canopy microsites for restoration of native perennial grasses might improve plant establishment, growth, or survival (or some combination thereof), particularly in drier areas. We suggest that land managers consider the nurse plant approach as a way to increase perennial grass abundance in the Great Basin. Controlled experimentation will provide further insights into the life stage-specific effectiveness and practicality of a nurse plant approach for ecological restoration in this region.