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Urbanization is homogenizing residential developments and soils across a wide climate gradient of the United States. Land development practices commonly install landscapes on low-quality compacted fill material. While previous research shows that soil quality beneath urban lawns improves over time, studies used golf courses to represent lawns or sampled across variable development and management styles. Limited data on sandy soil quality within residential landscapes in subtropical climate warranted investigation. To evaluate the effect of age on residential soil characteristics, two residential developments from Central and Southwest Florida were selected based on known lot ages up to 31 years. Soil samples (0 to 30 cm) were collected from landscape beds and lawns, along with lawn of common areas, to evaluate bulk density and organic matter over time. Though landscape beds typically require less management, soil characteristics were similar between lawns and landscape beds, suggesting similar processes. Bulk densities decreased by 22–34% and 9–20% within the upper (0–15 cm) and lower (15–30 cm) soil depths, respectively. Upper layer soil carbon accumulation rates ranged from 0.060 to 0.156 kg C m−2 yr−1, comparable to previous studies but did not exceed 0.021 kg C m−2 yr−1 or were not significant for greater depths. The results of this study are representative of past chronosequence studies conducted in cooler climates with fine soil texture. Sandy soil texture and warm climate do not govern carbon sequestration rates, but instead vegetation type and landscape management practices overcome these conditions. Article Highlights: Soil quality improves similarly over time within landscapes on sandy soils in separate developments.1 Bulk densities decrease within the top 30 cm while soil organic matter increases only within upper 15 cm. Results align with similar studies suggesting common drivers across broad spatial scales. [ABSTRACT FROM AUTHOR]

Wetlands are valuable ecosystems because they are highly productive, support a wide range of wildlife, and serve as hotspots for biogeochemical cycling. Historically, vast areas of wetlands in the United States (US) were drained and converted to agriculture. Efforts are currently underway to restore wetland and floodplain functioning across the US and elsewhere. Re-wetting historically drained and farmed soils can potentially liberate legacy phosphorus (P) to surface waters as soluble reactive P (SRP), offsetting P retained by sedimentation during floods. A better understanding of the controls on SRP release is needed to estimate net P retention in these settings. Soil P saturation ratio (PSR) and soil P storage capacity (SPSC) are two proxies for SRP runoff risk that have shown promise for characterizing restored wetlands but require further testing. In this study, we examined soils at 42 riparian sites ranging from active farms to mature wetlands in the Vermont portion of the Lake Champlain Basin (USA), where phosphorus load reduction is a critical goal to achieve in-lake water quality targets. We additionally quantified potential SRP release to overlying water using intact soil cores from 20 plots spanning 14 sites. Final SRP concentrations in intact cores spanned two orders of magnitude and were predicted well by SPSC and PSR. SRP release was greatest at more recently and frequently farmed sites. Several soil properties, including PSR and SPSC, were correlated with farming frequency and time since farming, indicating that SRP release could be mapped using existing geodata for soils, hydrology and land use. Our findings confirm that soil SRP release during flooding needs to be considered in estimates of net P balance for restored riparian wetlands in agricultural landscapes. [ABSTRACT FROM AUTHOR]

The lichen woodland (LW) is an open-crown subarctic forest distributed principally in North America where it extends from Newfoundland in Atlantic Canada to the Yukon and Alaska. It is the main tree ecosystem of the LW zone north of the closed-crown boreal forest zone, and south of the forest-tundra zone where its cover diminishes progressively toward the Arctic tree line. Growth and development of LWs are closely dependent on dry-mesic, nutrient-poor podzolic soil environments largely distributed on the Canadian Precambrian Shield. The sun-exposed open structure and dominance of lichen species on the dry-mesic soils of LWs determine many of their functions. A diversified cryptogamic flora is thriving in LWs due to reduced competition from vascular plants unable to grow and survive on dry, nutrient-poor soils. Because of the reduced greenhouse effect caused by the open structure and dominance of pale-color lichen mats inducing a greater albedo, LWs exerts a negative microclimatic impact on the environment culminating in the reduction of the frost-free growth season and increase and intensity of frost events. A suite of common, often recurrent, sometime compounding, fine- and large-scale disturbances (including climate change) activates the successional dynamics of LWs and also their historical and present expansion and contraction across the boreal biome. Post-disturbance chronosequences induced by fire, wind and caribou trampling and grazing are producing similar seral communities converging toward the self-maintenance of the LW ecosystem. Long-term succession in southernmost LW sites possibly converges toward the closing of tree canopy and recovery of closed-crown conifer forests (CCCF) in absence of fire. The creation of LW occurred repeatedly during the late Holocene, as well as its extirpation from the northernmost sites (forest-tundra zone) with wildfire as the principal triggering mechanism. LWs are presently unable to transgress the tree line, except for minor advances in small confined sites. Current evidence of the dual distribution of LWs and CCCF in eastern Canada shows that the LW zone is expanding southward into the CCCF zone, where compound disturbances associated with epidemics followed by small to extensive fires are transforming dense forest stands to LWs, a process most likely independent of climate. In terms of ecosystem management, there are concerns about the maintenance of the southernmost LWs within the CCCF zone as poor carbon sinks. Projects involving LW plantation sites in the CCCF zone in eastern Canada are proposed to offset the increased emission of atmospheric gases and thus mitigate climate change. [ABSTRACT FROM AUTHOR]

Questions As changing wildfire regimes modify North American deserts, can fires of greater severity and frequency negatively impact the recovery of native bryophyte communities, which are not adapted to such disturbances? Does post-fire recovery result from the survival of existing surface plants, dormant propagules in sub-surface soil banks (dispersal in time) or aerial immigration into burned sites (dispersal in space)? Do wildfires negatively affect the survival of propagules in bryophyte soil banks? Location Continental arid shrublands, Mojave Desert, southwestern US. Methods We characterized bryophyte communities along a post-fire chronosequence spanning three decades across sites where fires had different severities, ages and potential fuels. Three community profiles (surface, soil and aerial) were surveyed with a combination of on-site surveys and emergence germination techniques. We tested for differences in beta diversity, species composition and richness with PERMDISP, PERMANOVA and linear mixed models, respectively. Results Burn severity was associated with differences in beta diversity, species composition and richness, while burn age was associated only with different composition and richness. No effect of potential fuel availability was found. More variation in composition was explained by significant differences among profiles than by other fire attributes. Species richness (but not beta diversity) was higher in soil profiles than in aerial spore rain or existing surface communities. Soils from the oldest and least severe burns had a greater number of species than soils from recent and more severe burns. Conclusions Bryophyte soil banks are common elements of desert soils that facilitate post-disturbance recovery of communities, but soil banks are themselves threatened by the intensifying frequency and severity of wildfires in North American deserts. Recovery of desert bryophyte communities seems to begin (but not necessarily conclude) within 30 yr after wildfires. In the near future, communities may become perturbed from historical patterns as contemporary fire regimes undergo extensive changes. [ABSTRACT FROM AUTHOR]

Abstract: Highly-weathered clay mineralogical suites of Southeastern (U.S.) soils, which are exemplified in portions of upland, well-drained environments of the Piedmont and Atlantic and Gulf Coastal Plain, consist of kaolinite, hydroxy-interlayered vermiculite, gibbsite, iron oxides, and to a lesser extent, quartz, halloysite and mica. The development of these mineral assemblages is often coincident with Ultisol formation in these landscapes. By Soil Taxonomy, these soils often have kandic horizons or are in subactive CEC activity classes. Hydroxy-interlayered vermiculite, a mineral considered to be pedogenic and ubiquitous within these systems, has been evaluated in past studies through depth distribution trends, thermodynamics-solubility, and alluvial chronosequences, to develop a more thorough understanding of mineral transformations and pedogenesis in these landscapes. The average formula for hydroxy-interlayered vermiculite developed from several regional studies (M+0.28(Si3.46Al0.54)(Al1.38Fe0.31Mg0.31)[(Al1.31(OH)3.35)]O10(OH)2) suggests the 1/2 cell layer charge to be approximately 0.85, with approximately 67% of the charge offset by non-exchangeable Al-hydroxy interlayers. Thermodynamic evaluations indicate a relatively more stable pedogenic environment for hydroxy-interlayered vermiculite in near-surface and eluvial environments (dependent on the degree of hydroxy-Al interlayer infilling), with a more stable environment for kaolinite in illuvial subsoils. This corresponds with observations of mineral distribution with depth in representative soils of the region, although other mechanisms and pathways, which have been elucidated through several studies, likely play a larger role. Quantification of mineral constituents in these systems faces challenges, but satisfactory results can be obtained using a variety of techniques. Deconvolution of thermal (DSC and TGA) output shows promise for quantifying discrete minerals that have overlapping temperatures of reactions. Chronosequence studies in the region have improved our understanding of the temporal state factor associated with the development of this mineralogy. For example, a study on the Tallapoosa River of Central Alabama illustrated the presence of vermiculite and smectite and the relative absence of hydroxy-interlayered vermiculite within the solum on Holocene terraces, but its presence on significantly older (Pleistocene) terraces. Another study evaluated 29 pedons on seven Alabama River terrace levels (late Pliocene to late Holocene) in Central and South Alabama, and found clay mineralogy on the oldest terrace levels was similar to adjacent upland Coastal Plain landscapes. The aggregate of findings indicate that given sufficient time in a leaching environment, highly weathered mineralogical suites form in a variety of landscapes in this region. [Copyright &y& Elsevier]

Aim The assembly of a forest flora on sites abandoned from agriculture is potentially limited by the physical and chemical suitability of soils. This paper describes the post-agricultural development of soil in deciduous forest sites to gauge its importance in limiting a second-growth forest flora. Location Northern Delaware and south-eastern Pennsylvania, USA. Methods A chronosequence of 99 second-growth forest sites was assembled in the Piedmont region. Stand age ranged from 0–20 to > 100 years since canopy closure, as determined from historical aerial photographs. Seventy-four sites are likely to have experienced cultivation before regeneration of forest. The presence of large rocks or pit-and-mound microtopography suggests that the remaining 25 sites were not cultivated, although they were probably used as pasture and/or woodlot. Soil organic content, moisture, pH, and concentrations of extractable potassium, phosphorous, calcium, magnesium and sulphur were measured in samples from each site. Coarse woody debris, size of the largest tree, canopy openness and several microtopographic features were also recorded. Results Most soil variables fell into two separate but internally correlated groups: the first group including organic matter, moisture and sulphur; the second including pH, calcium, potassium and magnesium. In formerly cultivated sites, organic matter, moisture and sulphur increased gradually through the chronosequence, approaching values recorded in uncultivated sites at 81–100 years. Calcium, potassium, magnesium and pH declined sharply between the 21–40- and 41–60-year classes – probably an indirect effect related to geographic location and selective abandonment. Phosphorous also declined with time, but the decline was not as clearly defined. Main conclusions The progress of soil recovery observed here is faster than rates implied in other studies, possibly because these sites experienced a period as pasture before reversion to forest. Sites appear to have returned to undisturbed-forest levels as fast as or faster than the predicted accumulation of a forest flora, suggesting that assembly of second-growth forest communities is not limited by the rate of soil development in this region. [ABSTRACT FROM AUTHOR]

Many exotic plants utilize early successional traits to invade disturbed sites, but in some cases these same species appear able to prevent re-establishment of late-successional and native species. Between 2002 and 2004, I studied 25 fields that represent a 52-year chronosequence of agricultural abandonment in a shrub-steppe ecosystem in Washington State, USA, to determine if exotic plants behaved as early successional species (i.e., became less abundant over time) or if they established persistent communities. Exotics maintained dominance in tilled (73% of total cover) relative to never-tilled (6% of total cover) fields throughout the chronosequence. Exotic community composition, however, changed on annual and decadal timescales. Changes in exotic community composition did not reflect typical successional patterns. For example, some exotic perennial species (e.g., Centaurea diffusa and Medicago sativa) were less common and some exotic annual species (e.g., Sissymbrium loeselii and S. altissimum) were more common in older relative to younger fields. Exotics in the study area appeared to establish communities that are resistant to re-invasion by natives, resilient to losses of individual exotic species, and as a result, maintain total exotic cover over both the short- and long-term: exotics replaced exotics. Exotics did not invade native communities and natives did not invade exotic communities across the chronosequence. These results suggest that, in disturbed sites, exotic plants establish an alternative community type that while widely variable in composition, maintains total cover over annual and decadal timescales. Identifying alternative state exotic communities and the mechanisms that explain their growth is likely to be essential for native plant restoration. [ABSTRACT FROM AUTHOR]

Abstract: Following severe wildfires in southwestern ponderosa pine forests, dead trees remain on the landscape and eventually fall, but relatively little is known about the quantity and quality of post-wildfire coarse woody debris (CWD). To describe post-fire conditions, we measured snags, CWD, and fine woody debris and forest floor depth on seven fires in a chronosequence from 3 to 27 years old in northern Arizona. Snags declined in density with increasing time since fire and generally followed expected patterns of decay, except that few snags stood long enough to reach a clean-bark state. The mean biomass of the surface CWD ranged from as low of 3.3Mgha−1 to a high of 41.3Mgha−1. Total CWD biomass in the surface fuel load remained roughly comparable from 8–9-year-old fires to a 27-year-old fire but the state of the CWD changed from sound to rotten. The change to a rotten condition suggests an increase in ignitability of the post-fire fuel load, but fine fuels that could support high fireline intensity were relatively low. The number of “jackstraws,” points where intersecting downed logs could create a hot spot if reburned, was slightly higher in the oldest fire. Few fire-created snags remained by the 27th year post-fire. Management options to reduce fuels after severe wildfire, such as salvage logging, prescribed burning, or passive management, should be addressed in a broad ecological context. [Copyright &y& Elsevier]

The article provides information on the process used in converting soil organic matter (SOM) to perennial vegetation. The researchers measured SOM accumulation and changes in soil properties on a replicated chronosequence of former agricultural fields in the U.S. As seen in the recorded rate, soil organic carbon contents will be equivalent to unplowed native prairie sites after years of cultivation. A model used that investigates the mechanisms of SOM formation suggests that rapid formation of stable carbon resulted from biochemically resistant microbial products and plant material.

Chronosequences of ∼250 to ∼130 years were established on two late Holocene glacier forelands in the Jotunheimen region of southern Norway. Patterned features occurring within chronologically established time units were studied. Young patterned features, forming ∼10–20 years after deglaciation, are frost active. Vegetation cover is minimal within the young patterned features, consisting of bryophytes/organic crusts. Soil development within patterned ground is also minimal/absent because frost action retards horizonization. With time and distance from the glaciated ice margin, frost activity declines within the patterned features, suggesting that a thin, active “periglacial zone” exists near the ice margin. Initially, frost activity decreases at the borders of the features with the centers stabilizing later in time. This results in fine-scale soil heterogeneity and variations of soil development. Fine-scale pedogenic development is first encountered at the borders of patterned ground that has developed on terrain exposed since the ∼1930s, yet soil development is predominantly absent at the centers of patterned ground. With time and distance from the ice margin, frost activity declines and allows patterned features to homogenize from border to center positions in regard to soil characteristics. Across the chronosequences, soils within patterned features pedogenically follow previous soil chronosequence studies, evolving from USDA classifications of Entisols into Inceptisols. Frost disturbance within patterned ground, however, produces a lag effect, that results in longer periods of time for pedogenesis to occur and thinner soils than that of the surrounding terrain. [Copyright &y& Elsevier]

Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest clearing in the coming decades. Using paired forest-field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3 kg m−2 lower than C stocks in original forest soils. This suggests that soil C losses from land-use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60-year-old grassland showing net ecosystem C gain of 2.1 kg m−2 over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land-use change were smaller than reductions in C stocks (34% or 0.31 kg m−2), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high-C forest-floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, the resulting C losses (51% or 11.2 kg m−2 at the ecosystem level in this study) will induce a positive feedback to climatic warming and additional land-use change. However, by selecting relatively C-poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced. [ABSTRACT FROM AUTHOR]

A chronosequence of 22 sites in Durham, NH, was used to study upland successional patterns of species diversity over a span of 14–209 years of forest development and to test the hypothesis that diversity is maximized at mid-succession. Nested quadrat sampling was used to estimate the relative importance values of vascular plant species in the tree, shrub, and herb strata. The successional status of each site was estimated using two dendrochronological indices: oldest tree index and abandonment index. Four indices of species diversity were calculated for each site and stratum: species richness, the Shannon index, Hurlbert’s probability of interspecific association, and Pielou’s evenness. Within the quadrats of all 22 sites, total richness was 168 species of vascular plants. Richness decreased linearly over the 200-year chronosequence in all strata for both successional indices, from a maximum of 74 species at 14 years to a low of 13 species at 209 years. Evenness showed no evidence of a successional pattern, so the combined indices of diversity (Shannon and Hurlbert) displayed only weak trends. Multiple regression showed that successional age was always a significant predictor of species richness in every stratum (except the shrub stratum when using the abandonment index), and that stem density (i.e., the “sample size effect”) was not significant in the tree stratum when using the oldest tree index. Age indices held comparable or lower P-values than stem density in most strata. It is likely that richness increased from a low value immediately following farm abandonment to a peak around the age of the youngest sites sampled (over a span of 15–30 years), the rate of increase being governed by the availability of dispersing seeds. The observed decline in richness over the rest of succession was probably caused in part by a decrease in available light. The rate of decline was apparently determined by the rate of competitive elimination of shade-intolerant species. [Copyright &y& Elsevier]

This paper provides an overview of the work reported at a symposium on age-related changes in the structure and function of forests in the United States Pacific Northwest. Some of the work presented at this meeting is reported in the peer-reviewed papers comprising this journal issue. Age-related changes in leaf structure, CO2 assimilation rate, stable carbon isotope ratio, nitrogen concentration and stomatal limitation were demonstrated at many organizational scales. At larger scales, age-related changes were reported in canopy structure and light profile, stand productivity, tree mortality and respiration. These data raise new questions about the potential interaction among the structural and functional changes in aging forests, and indicate many avenues for future research concerning tree growth and ecosystem functioning. [ABSTRACT FROM PUBLISHER]

Field and laboratory data combined with geochronological information were used to investigate the nature and rates of pedogenesis of three late Holocene soils at the Lubbock Lake archaeological site (Southern High Plains of Texas). A variety of pedologic characteristics become better expressed with time, and aerosolic additions and burial of the soils significantly affect the development of these characteristics. Organic carbon accumulates rapidly in initial stages of develop- ment of the A horizon: mollic epipedons form in about 100 years and steady-state conditions of organic carbon content are attained in about 1000 years. Upon burial, organic carbon content of the A horizon quickly decreases. Calcic horizons form in 200 years and minimally developed argillic horizons form in 450 years, the result of infiltration of clay and carbonate derived from aerosolic dust. Horizonation is better expressed with time, and B-horizon hues redden from 10YR to 5YR in 3500 years. Locally, landscape position is an important factor: along the valley axis A horizons become cumulic and mixed-layer illite-smectite clays form at the expense of illite because more moisture is available and there are strong seasonal contrasts in wetting and drying. [ABSTRACT FROM AUTHOR]