Effects of flooding and artificial burning disturbances on plant species composition in a downstream riverside floodplain

Our objective was to detect environmental and disturbance factors that determine plant species composition in a downstream riverside floodplain of the Shinano River, where both natural flooding and artificial fire disturbances predominate. We classified the natural vegetation into five types by physiognomy, i.e., burnt field, intact (unburnt) field, early-successional forest, later-successional forest, and margin of channel. We deployed 14 transect belts containing 713 plots of 1 m × 1 m at these sites. In the plots, we determined plant occurrences and values of five environmental (soil moisture, soil texture, relative elevation above the river, horizontal distance from the river, and light conditions) and two disturbance factors (scouring of plants by flooding and fire). Plant species compositions and environmental and disturbance factors were different among the sites. Logistic regression analysis showed that burning stimulated the occurrences of herbaceous annuals and conversely constrained those of woody and fern species, suggesting that fire disturbance resets the succession. Scouring of plants by flooding also affected the occurrences of many species, and had a positive influence on that of herbaceous winter annuals in particular. Canonical correspondence analysis indicated that the distributional characteristics of the plant species in the riverside floodplain were influenced primarily by distance from the river, reflecting the magnitude of flood disturbance. We conclude that (1) magnitude of flood disturbance is the primary factor determining plant species composition, and (2) burning maintains early successional vegetation and simultaneously creates a unique plant species composition by stimulating the germination of buried viable seeds transported by flood.     

Influence of fire and soil nutrients on native and non-native annuals at remnant vegetation edges in the Western Australian wheatbelt

Abstract. The effect of fire on annual plants was examined in two vegetation types at remnant vegetation edges in the Western Australian wheatbelt. Density and cover of non-native species were consistently greatest at the reserve edges, decreasing rapidly with increasing distance from reserve edge. Numbers of native species showed little effect of distance from reserve edge. Fire had no apparent effect on abundance of non-natives in Allocasuarina shrubland but abundance of native plants increased. Density of both non-native and native plants in Acacia acuminata-Eucalyptus loxophleba woodland decreased after fire. Fewer non-native species were found in the shrubland than in the woodland in both unburnt and burnt areas, this difference being smallest between burnt areas. Levels of soil phosphorus and nitrate were higher in burnt areas of both communities and ammonium also increased in the shrubland. Levels of soil phosphorus and nitrate were higher at the reserve edge in the unburnt shrubland, but not in the woodland. There was a strong correlation between soil phosphorus levels and abundance of non-native species in the unburnt shrubland, but not after fire or in the woodland. Removal of non-native plants in the burnt shrubland had a strong positive effect on total abundance of native plants, apparently due to increases in growth of smaller, suppressed native plants in response to decreased competition. Two native species showed increased seed production in plots where non-native plants had been removed. There was a general indication that, in the short term, fire does not necessarily increase invasion of these communities by non-native species and could, therefore be a useful management tool in remnant vegetation, providing other disturbances are minimised.     

Successional trends and biomass of mosses on windthrow mounds in the temperate rainforests of Southeast Alaska

We investigated successional trends on windthrow mounds in two old-growth Tsuga heterophylla-Picea sitchensis forests in northern southeast Alaska to determine the influence of windthrow disturbance on the maintenance of plant diversity. We were particularly interested in assessing the value of mosses in detecting long-term effects of disturbance in temperate rainforests. Mosses established a dense carpet on windthrow mounds within the first few decades after the disturbance. No consistent changes were noted in total moss and vascular plant cover, moss biomass, or species diversity between young mounds (±50 yrs), intermediate mounds (±150 yrs) or old mounds (> 200 yrs), or between mounds and the undisturbed forest floor, despite consistent differences in soils development.Classification and ordination of the vegetation data did not show a consistent relationship between soil surface age or soil depth and overall species composition on the two sites. Young mounds were the most compositionally distinctive, primarily due to moss species. Pogonatum alpinum var. sylvaticum, P. contortum and Polytrichum formosum were generally confined to young mounds with unstable substrata, while Dicranum majus and Sphagnum girgensohnii were associated with old soil surfaces and deep organic soils. Vascular plant species with affinities for riparian or deep shade habitats (Tiarella trifoliata, Coptis asplenifolia and Dryopteris expansa) showed a general preference for the forest floor. Gymnocarpium dryopteris was the only vascular plant with a significant association with young mounds.Mosses comprised approximately 25% of understory plant biomass and as much as 50% of understory productivity. In cool temperate forests, the inclusion of mosses in vegetation analysis may provide valuable insights into the nature of vegetation patterns over subtle environmental gradients. The distinctiveness of the temperate rainforest type and the unique ecological effects of windthrow disturbance in this type are also suggested by this study.Abbreviations HR = Heintzleman Ridge (study site) - OP = Outer Point (study site)     

The role of successional stage,vegetation type and soil disturbance in the invasion of the alien grass Cortaderia selloana

Question : This paper studies the establishment and performance of Cortaderia selloana (Pampas grass), an alien South American ornamental species that is invading many parts of the world. We asked whether (1) early successional stages were the most susceptible to C. selloana invasion; (2) soil microdisturbances increased invasion at any point of succession, and (3) C. selloana invasion of later successional stages was modulated by vegetation type Location : Delta del Llobregat (Catalonia, NE Spain). Methods : We monitored survival and growth of transplanted C. selloana seedlings in disturbed and non‐disturbed plots throughout a successional gradient with an age range of < 1 to > 10 years in different vegetation types and within the area of influence of coexisting species with similar growth form. Results : Although seedling survival was extremely low in all treatments, our results revealed that early successional stages were not the most easily invaded since we found no significant differences in the percentage survival of C. selloana along the successional gradient. However, survival and seedling bio‐mass were enhanced by soil disturbance at any seral stage. This result suggested that inhibition ruled C. selloana invasion. Invasibility neither depended on the invaded vegetation type nor on the co‐existing species with similar growth form. Finally, C. selloana invasion was not enhanced by decreasing competition with Phragmites australis, a native coexisting species because survival rates after a year were not significantly different. However, Phragmites increased C. selloana leaf length probably due to shading. Conclusions : C. selloana recruitment appears to be positively affected by soil disturbance but it is independent of successional stage or vegetation type.     

Effects of disturbance intensity and frequency on early old‐field succession

Abstract. Early old‐field succession provides a model system for examining vegetation response to disturbance frequency and intensity within a manageable time scale. Disturbance frequency and intensity can interact with colonization and competition to influence relative abundance of earlier and later successional species and determine, respectively, how often and how far succession can be reset. We tested the joint effects of disturbance frequency and intensity on vegetation response (species richness, abundance, canopy structure) during the first six years of succession by clipping the dominant species (D) or all species (T) in spring and fall of each year (S), once per year in summer (Y1), each two years in summer (Y2), or each four years in summer (Y4). Vegetation response reflected disturbance effects on expansion of a later monospecific dominant perennial herb, Solidago altissima, and persistence of the early, richer flora of annuals. A more abundant and taller top Solidago canopy developed on plots clipped each 2 yr or less frequently. Plots clipped yearly or seasonally were richer, but had less abundant, shorter, and differently stratified canopy. Disturbance mediated the relative abundance of early and later successional species; however, frequency and intensity effects were not completely congruent. Persistence of a richer early successional flora increased through the most frequent disturbance (S), and was magnified by disturbance intensity. Disturbance as extreme as clipping all vegetation twice yearly did not cause a drop in species richness, but maintained the early successional community over the first six years of succession. We conclude that clipping disturbance influenced the rate of succession, but the early community could rebound through the range of disturbance frequency and intensity tested.     

Long‐term seed bank dynamics in a temperate forest under conversion from coppice‐with‐standards to high forest management

Questions: How do changes in forest management, i.e. in disturbance type and frequency, influence species diversity, abundance and composition of the seed bank? How does the relationship between seed bank and vegetation change? What are the implications for seed bank dynamics? Location: An ancient Quercus petraea — Carpinus betulus forest in conversion from coppice‐with‐standards to regular Quercus high forest near Montargis, France. Methods: Seed bank and vegetation were sampled in six replicated stand types, forming a chronosequence along the conversion pathway. The stand types represented mid‐successional stages of stands in transition from coppice‐with‐standards (to high forest (16 plots) and early‐ and mid‐successional high forest stands (32 plots). Results: Seed bank density and species richness decreased with time since last disturbance. Adjusting for seed density effects obscured species richness differences between stand types, but species of later seres were nested subsets of earlier seres, implying concomitant shifts in species richness and composition with time since disturbance. Later seres were characterized by species with low seed weight and high seed longevity. Seed banks of early seres were more similar to vegetation than to later seres. Conclusions: Abandonment of the coppice‐with‐standards regime altered the seed bank characteristics, as well as its relationship with vegetation. Longer management cycles under high forest yield impoverished seed banks. For their persistence, seed bank species will increasingly rely on management of permanently open areas in the forest landscape. Thus, revegetation at the beginning of new high‐forest cycles may increasingly depend on inflow from seed sources.     

Changes in two Minnesota forests during 14 years following catastrophic windthrow

Abstract. We measured tree damage and mortality following a catastrophic windthrow in permanent plots in an oak forest and a pine forest in central Minnesota. We monitored changes in forest structure and composition over the next 14 years. Prior to the storm, the oak forest was dominated by Quercus ellipsoidalis, and the pine forest by Pinus strobus. The immediate impacts of the storm were to differentially damage and kill large, early‐successional hardwoods and pines. Subsequent recovery was characterized by the growth of late‐successional hardwoods. In both forests the disturbance acted to accelerate succession. Ordination of tree species composition confirmed the trend of accelerated succession, and suggested a convergence of composition between the two forests.     

Biomass production of the last remaining fen with Saxifraga hirculus in Switzerland is controlled by nitrogen availability

Olde Venterink H. and Vittoz P. 2008. Biomass production of the last remaining fen with Saxifraga hirculus in Switzerland is controlled by nitrogen availability. Bot. Helv. 118: 165 – 174. For conservation management of endangered plants it is important to know which nutrient(s) control growth of the vegetation, because maintenance of low nitrogen (N), phosphorus (P) or potassium (K) availability requires different management measures. The aim of this study was to determine the type of nutrient limitation for the vegetation in the last remaining site with Saxifraga hirculus in Switzerland, using nutrient ratios in the aboveground vegetation as an indicator. We made vegetation relevees, collected biomass of the vascular plants, and took soil samples in three plots at this site. The biomass was very low (152–231 g m ^-2), and all three plots were clearly N-limited with N:P ratios of 7– 8. Soil extractable N concentrations were generally low, and P and K concentrations were moderate to high, which was consistent with the indicated N limitation. Hence conservation management first of all needs to prevent N-enrichment, and needs to avoid increased mineralization rates through drainage, or the accumulation of N in the system from atmospheric deposition. Therefore N output seems required through for instance grazing or mowing. The current grazing management seems to function well, since total aboveground biomass is very low and S. hirculus has a high abundance in this last remnant. Submitted 5 June 2008; Accepted 14 October 2008 Subject editor: Sonja Wipf     

Long-term effects of fire on ant communities (Hymenoptera: Formicidae) in Pinus brutia forests of south-western Turkey

We studied how forest fire affects the structure of ant communities in the long term and how microhabitat variables that influence communities of ants in different succession stages change. For this purpose, we selected five sites burnt 3 to 26 years prior to the study and a control site unburnt for at least 50 years. Sampling of ants was conducted in four pit-fall traps in four transects in three replication plots at every successional site and in two plots at the control site. Microhabitat variables regarding the vegetation structure and litter layers were recorded and related to the abundances of ants. The results show that subshrubs, leaves, and needles were the most important microhabitat variables that affected the ant communities. In certain ant genera, significant changes depending upon successional gradient were determined. While the genera Aphaenogaster and Cataglyphis had non-linear relationships with successional gradient, negative linear relationships were found in Crematogaster and Prenolepis. Messor is the only genus caught in high numbers in the earliest successional stage. It showed a decrease with successional gradient. Significant changes in ant communities along the successional gradient were associated with the characteristics of vegetation and the litter layer.     

An Evaluation of Reclamation Success on Idaho's Phosphate Mines

To evaluate reclamation success on the Wooley Valley phosphate mine in southeastern Idaho, we compared vegetation structure and soil physical, chemical, and elemental properties of several different reclamation treatments with those of a nearby reference area (a native Artemisia tridentata vaseyana/Festuca idahoensis association) after 14 years. Vegetation data had been collected four years after reclamation, and we were able to compare differences in biomass and species composition between dates on the reclaimed area. Four years after reclamation there were no differences in total biomass between topsoil or spoil or between seed only, seed + mulch, or control treatments on the different soil types. Most treatments were dominated by seeded perennial grasses. Fourteen years after reclamation there were no differences in biomass or cover between spoil and topsoil plots, but on spoil plots the seeded and mulched treatment had higher total biomass and vegetation cover than on control or seed-only treatments. The seeded perennial legume Medicago sativa was codominant with the seeded forage grasses on all of the treatments. High initial fertilization rates probably facilitated the early establishment and dominance of the forage grasses; once nutrient levels, especially nitrogen, began to decline, the legume increased in abundance. Similarity between the reclaimed area and the reference or native area was low. Reclaimed treatments had higher biomass but lower species richness. The topsoil and spoil plots had similar soil texture, bulk density, pH, cation exchange capacity, electrical conductivity, and phosphorus. Differences in organic carbon, total nitrogen, carbon: nitrogen ratios, and available moisture were related more to treatments than to soil type. High biomass and, thus, litter input on the seed + mulch treatment on spoil plots resulted in both higher OC and TN than any on other soil/treatment combination. The reclaimed area had lower OC, TN, and available moisture than did the reference area on all but seed + mulch spoil plots. Bulk density was higher on reclaimed plots. The long-term differences observed between the reclaimed and reference areas parallel those obtained for other western reclamation sites. Although successional trajectories depend on the attribute measured, similarity to native reference areas depends on the initial reclamation methods. We discuss reclamation methods that would increase the structural and functional similarity of reclaimed and reference areas on the Wooley Valley phosphate mine.     

Flood‐deposited wood creates regeneration niches for riparian vegetation on a semi‐arid South African river

Question : The formation of large woody debris (LWD) piles during floods has significant impacts on riparian succession through pioneering plants often establishing in association with wood. We assess the importance of LWD for seed regeneration of riparian plants after a century‐scale flood disturbance in a semi‐arid environment. Location : The Sabie River within Kruger National Park in the semi‐arid northeast of South Africa. Methods : Our approach was to quantify the riparian soil seed bank, to record the frequency of establishment of riparian plants in woody debris piles, and to conduct experimental out‐plantings of common riparian trees in plots with and without LWD. Results : We found the abundance and diversity of seedlings were higher in soils taken from wood piles than from open reference areas, and most seedlings were herbaceous species. Surveys indicated that numbers of seedlings recorded within woody debris were significantly greater than in open reference areas or within established vegetation. Seedling establishment in various cover‐types also varied for different riparian tree species. Experimental out‐planting of seedlings of two riparian tree species (Philenoptera violacea and Combretum erythrophyllum) revealed that, after 433 days, planted seedlings survived only in woody debris piles. Conclusion : LWD formed after a large flood creates heterogeneous patches that may influence post‐disturbance regeneration of riparian vegetation by providing a variety of environmental niches for seedlings establishment. We suspect that higher seedling survival in LWD is due to increased moisture (particularly in the dry season) and nutrients, and protection from seasonal flooding and herbivory.     

Fire-tolerance mechanisms of common woody plant species in a semiarid savanna in south-western Zimbabwe

The frequency of fire has increased in savannas yet few studies have assessed how plants persist when subjected to long‐term disturbance by fire. We investigated the contributions of bark thickness and resprouting to the persistence of woody plants in two fire trials that were started in 1948 and 1949. The number of resprouts per individual, bark thickness, basal diameter and height of woody plants were measured in unburnt plots and those burnt annually, triennially and quinquennially during the late dry season. Changes in tree density, number of resprouts and individuals in different height classes between 1963 and 2002 were assessed. Bark thickness varied among species and also increased with increases in basal diameter. Generally, plants with thick bark survived fire more than those with thin bark. Resprouting was the major fire survival strategy for most species. The number of resprouts produced per plant ranged from 4 ± 3 (Acacia rehmanniana) to 14 ± 9 (Pseudolachnostylis maprouneifolia). Fire reduced species richness in plots burnt annually and triennially by 47% and 6% respectively. Species richness increased in unburnt plots (5%) and those burnt quinquennially (16%). Most woody species survived fire through a combination of traits.     

Human disturbance and environmental factors as drivers of long‐term post‐fire regeneration patterns in Mediterranean forests

Question: What are the main forces driving natural regeneration in burned mature Mediterranean forests in the medium‐long term and what are the likely successional trajectories of unmanaged vegetation? Location: Valencia Region, eastern Spain. Methods: A wildfire burned 33 000 ha of Pinus halepensis and P. pinaster forest in 1979, and subsequent smaller wildfires took place between 1984 and 1996. The study was designed to sample the range of environmental and disturbance (fire recurrence and land use) conditions. The territory was classified into 17 different geomorphological and fire‐recurrence units. Vegetation cover and floristic composition were measured on a total of 113 plots (1000 m² each) randomly selected within these units. Results: The results show that 23 years after the fire the regenerated vegetation consists of successional shrublands, and that forest ecosystem resilience can be very low. The vegetation presents a strong correlation with most of the environmental variables, but fire (one or two fires), soil type and land use (in that order) are the main drivers of vegetation composition. Quercus coccifera shrublands persist on limestone soils while diverse types of other shrublands (dominated by seeder species) are found on marl soils. Conclusions: The results of this study indicate that disturbance factors strongly coupled to human activities, such as land use and fire, play a critical role in the current state of vegetation. Fire creates vegetation patches in different successional states while land use and soil type define the different types of shrubland in terms of their specific composition.     

Tropical forest structure and diversity in relation to altitude and disturbance in a Biosphere Reserve in central India

Questions: Is species diversity affected in protected areas where human activities are permitted or tolerated? On plots of a fixed size, does stem density alone predict number of species? Are differences in density related to disturbance and altitude? Location: Achanakmar‐Amarkantak Biosphere Reserve, central India. Methods: 42 sites, each with three replicate 10‐m radius plots, were examined. All trees (≥ 30 cm GBH) in each plot were measured for girth at breast height. α‐diversity, species richness and evenness were calculated for each site. The sites were ordinated by Nonmetric Multidimensional Scaling (NMS) using relative importance values of component species. Correspondence Analysis was used to broadly delineate communities. Anthropogenic disturbances were recorded in terms of percentage of trees lopped, scale of lopping, number of domestic livestock dung piles and foot trails (both livestock and people) for each plot. Results: The NMS analysis exhibited a near linear arrangement of sites with no evidence of discrete vegetation zones. NMS axes were significantly related to altitude and disturbance scores. With increasing elevation, basal area increased but number of species, α‐diversity and its components declined monotonically. The number of species and indices of species diversity were positively associated with tree lopping and also with total disturbance. Number of species was controlled by stem density only in plots not dominated by Shorea robusta. Conclusions: Recent levels of human disturbance are associated with higher species diversity in this biosphere reserve. There is some evidence that stands at all altitudes follow the same successional pattern to dominance by Shorea, a successional pattern that also results in decreased diversity without disturbance.     

Modelling the recovery of an annual savanna grass following a fire-induced crash

Abstract The native annual Sorghum populations of the Australian wet-dry tropics are highly resilient to dry season fires. During the early wet season, however, fires that occur after the new grass population has emerged can cause catastrophic population crashes. We examined savanna plots that had been burnt in this way, and compared them with adjacent unburnt plots. We found that Sorghum densities in the burnt plots were lower on average by a factor of 10, but that some fires had reduced the density only to one-third of the unburnt plots. It is not clear whether these differences relate directly to site or seasonal factors, or to differences in the way the burning was carried out. Other vegetation components responded to the fires differently: forbs (dicotyledonous herbs) increased in cover, while perennial grasses, woody plants, and overall species richness, were not significantly affected. The amount of leaf litter declined. A population model for Sorghum based on the demography of unburnt populations predicted that they should recover from a wet season burn, taking 7–16 years to return to normal densities. However, the actual field populations did not seem to be recovering, suggesting that wet season fires not only lower densities, but may also fundamentally change population processes in these annual grasses.     

Annual burning of semi‐natural grasslands for conservation: winners and losers among plant species

Species‐rich semi‐natural grasslands in Europe are a main target for conservation efforts, and alternative methods to the traditional management of mowing or grazing would be welcome due to the difficulties in maintaining traditional management practices. One such method proposed is burning of grassland vegetation during late winter or spring. To evaluate the effects of annual spring burning vs annual mowing on semi‐natural grassland vegetation, we compared the frequency of species in eleven field experiments in southern Sweden after ca 14 years. Out of the 88 species analyzed, five were more frequent in burnt plots compared with mowed plots (Vicia cracca, Cirsium arvense, Urtica dioica, Galium verum, Convallaria majalis). In contrast, 37 species were significantly less frequent in burnt plots compared with mowed ones, those with the largest differences being Ranunculus acris, Briza media, Veronica chamaedrys, Festuca ovina, Plantago lanceolata and Anthoxanthum odoratum. Tall‐grown species and those with preferences for N‐rich soils increased in frequency under an annual spring‐burn regime, compared with annual mowing, as did species producing larger amounts of nectar. Hence, although vegetation composition becomes more trivial with annual spring burns, there might be long‐term benefits for nectar‐feeding insects.     

Managing open habitats by wild ungulate browsing and grazing: A case‐study in North‐Eastern Germany

Question: Can wild ungulates efficiently maintain and restore open habitats? Location: Brandenburg, NE Germany. Methods: The effect of wild ungulate grazing and browsing was studied in three successional stages: (1) Corynephorus canescens‐dominated grassland; (2) ruderal tall forb vegetation dominated by Tanacetum vulgare; and (3) Pinus sylvestris‐pioneer forest. The study was conducted over 3 yr. In each successional stage, six paired 4 m²‐monitoring plots of permanently grazed versus ungrazed plots were arranged in three random blocks. Removal of grazing was introduced de novo for the study. In each plot, percentage cover of each plant and lichen species and total cover of woody plants was recorded. Results: Wild ungulates considerably affected successional pathways and species composition in open habitats but this influence became evident in alteration of abundances of only a few species. Grazing effects differed considerably between successional stages: species richness was higher in grazed versus ungrazed ruderal and pioneer forest plots, but not in the Corynephorus sites. Herbivory affected woody plant cover only in the Pioneer forest sites. Although the study period was too short to observe drastic changes in species richness and woody plant cover, notable changes in species composition were still detected in all successional stages. Conclusion: Wild ungulate browsing is a useful tool to inhibit encroachment of woody vegetation and to conserve a species‐rich, open landscape.     

Vegetation change in a man‐made salt marsh affected by a reduction in both grazing and drainage

Abstract. In order to restore natural salt marsh in a 460‐ha nature reserve established in man‐made salt marsh in the Dollard estuary, The Netherlands, the artificial drainage system was neglected and cattle grazing reduced. Vegetation changes were traced through two vegetation surveys and monitoring of permanent plots over 15 yr after the management had been changed. Exclosure experiments were started to distinguish grazing effects from effects of increased soil waterlogging caused by the neglect of the drainage system. Both vegetation surveys and permanent plots demonstrated a dichotomy in vegetation succession. The incidence of secondary pioneer vegetation dominated by Salicornia spp. and Suaeda maritima increased from 0 to 20%, whereas the late‐successional (Phragmites australis) vegetation from 10 to 15%. Grazing intensity decreased towards the sea. The grazed area contracted landward, which allowed vegetation dominated by tall species to increase seaward. Grazing and increased waterlogging interacted in several ways. The impact of trampling increased, and in the intensively grazed parts soil salinity increased. This can probably be explained by low vegetation cover in spring. Framework Ordination, an indirect‐gradient‐analysis technique, was used to infer the importance of environmental factors in influencing changes in species composition. Many changes were positively or negatively correlated with soil aeration and soil salinity, whereas elevation was of minor importance. Grazing accounted for only a few changes in species frequency. Changes in permanent plots were greater during the first than during the second half of the study period. In exclosures that were installed halfway through the study period, there was a relatively rapid recovery of previously dominant species that had decreased during the first half of the study period. Species richness per unit area in the reserve increased. At the seaward side of the marsh, the altered management allowed succession to proceed leading to establishment of stands of Phragmites australis, whereas on the landward side, the combination of moderate grazing with neglect of the drainage system appeared an effective measure in maintaining habitats for a wider range of halophytic species.     

INHIBITION OF NITRIFICATION BY CLIMAX ECOSYSTEMS

Three plots representing two stages of old-field succession and the climax were selected in each of three vegetation types in Oklahoma: oak-pine forest, post oak-blackjack oak forest, and tall grass prairie. Soil samples from the 0–15 and 45–60 cm levels were analyzed every other month for 1 yr for exchangeable ammonium nitrogen and for nitrate. On alternate months numbers of Nitrosomonas and Nitrobacter were determined in the 0–15 cm level. The amount of ammonium nitrogen was lowest in the first successional stage, intermediate in the second successional stage, and highest in the climax stand. This trend was remarkably consistent throughout all sampling periods, all vegetation types, and both sampling levels in the soil. The amount of nitrate was highest in the first successional stage, intermediate in the second successional stage, and lowest in the climax stand in both sampling levels, all vegetation types, and virtually all sampling periods. The numbers of nitrifiers were high in the first successional stage, generally, and decreased to a very low level in the climax. In fact, there was often no Nitrobacter in the climax stands. These results indicate that the nitrifiers are inhibited in the climax so that ammonium nitrogen is not oxidized to nitrate as readily in the climax as in the successional stages. Evidence from other geographic areas and vegetation types strongly supports this conclusion. This would certainly appear to be a logical trend in the evolution of ecosystems because of the increased conservation of nitrogen and energy. The ammonium ion is positively charged and is adsorbed on the negatively charged colloidal micelles, thus preventing leaching below the depth of rooting. On the other hand, nitrate ions are negatively charged, are repelled by the colloidal micelles in the soil, and thus readily leach below the depth of rooting or are washed away in surface drainage. There is growing evidence also that many plant species can use ammonium nitrogen as effectively or more so than nitrate nitrogen. If ammonium nitrogen is used directly, this eliminates four chemical steps because nitrogen which is oxidized to nitrite and then to nitrate must be reduced back to nitrite and then to ammonium nitrogen before it can react with keto-acids in the formation of amino acids. The two reduction reactions require considerable expenditure of energy.     

Effects of fire disturbance on the soil physical and chemical properties and vegetation of Pinus massoniana forest in south subtropical area

A variety of studies on the impact of fire disturbance on ecosystems has shown that the physical and chemical properties of soil after fire disturbance change notably. Meanwhile, little is known about the effects of different fire intensities on the soil properties and vegetation after fire disturbance, especially in the south subtropical area. In this paper, we analyzed the soil physical and chemical properties, vegetation species and species diversity of fire center, fire edge (which was burned a year ago) and non-burned Pinus massoniana plantation in Gaoyao, Guangdong province, China. The results showed that the soil conductivity, water content, total nitrogen, total potassium, and available potassium content of fire center were significantly higher than those of the non-burned land, and pH was higher than that of fire edge, whereas the available nitrogen, total phosphorus and organic matter content were much lower, which were generally existed in 0–10 cm soil layer and 10–30 cm soil layer. Changes of the soil properties of fire edge were similar with those of fire center, but less significant, and seemed to be more complex. Effects of burning on the vegetation of fire disturbance plots were found to be notable, species number and average height of plants of fire disturbance plots were lower than those of the non-burned plots, a difference of species diversity and uniformity were also shown, and finally, the composition of plant community also changed, e.g., pioneer species such as D. dichotoma, etc., dominated, and drought-resistant plants, heat-resistant plants and positive plants increased after burning.