Colonization dynamics and facilitative impacts of a nitrogen‐fixing shrub in primary succession

Abstract. Whether nitrogen‐fixing plants facilitate or inhibit species change in primary succession is best resolved by examining their impacts throughout the plant's entire life cycle from arrival to senescence. We experimentally examined two aspects of the successional impacts of a nitrogen‐fixing shrub, Coriaria arborea, on Mt. Tarawera, a volcano in New Zealand: factors limiting Coriaria colonization and impacts of Coriaria‐induced soil changes on a later successional tree, Griselinia littoralis. Coriaria germination was promoted by artificial wind protection and by the presence of heath shrubs. Transplanted Coriaria seedlings survived only if nodulated with Frankia, and the addition of Coriaria‐enriched soils slowed Coriaria seedling growth and did not improve seedling survival. This explained why Coriaria seedlings were found mostly in protected habitats away from adult Coriaria, and suggested that Coriaria thickets are not self‐replacing. Coriaria increased soil fertility by developing a 4 cm thick organic soil horizon that was richer in nitrogen (tenfold) and phosphorus (threefold) than pre‐Coriaria stages. These soil changes resulted in three‐ to sixfold increases in growth of Griselinia when it was grown in Coriaria‐enriched soils in a glasshouse. Coriaria's net effect on primary succession is facilitative, but the establishment of Coriaria is itself facilitated by the amelioration of the physical habitat by earlier colonists, suggesting facilitation is important throughout the life cycle of Coriaria. Sequential facilitative events determine the order of species replacements in this study but inhibition, linked to the developmental stages of Coriaria, may determine the rate of species change.     

亚热带次级森林演替过程中模拟氮磷沉降对土壤微生物生物量及土壤养分的影响

氮（N）沉降深刻影响着森林生态系统的生物多样性、生产力和稳定性。亚热带地区森林土壤磷（P）的有效性较低,N沉降将更突显P的限制作用。N、P输入对亚热带次级森林土壤的影响是否依赖于森林演替阶段知之甚少。选取两种不同演替年龄阶段（年轻林：<40 a;老年林：>85 a）的亚热带常绿阔叶林,设置模拟N和/或P沉降（10 g m^-2 a^-1）4个处理（Ctrl、N、P、NP）,连续处理4.5年后采集表层、次表层和下底层（0-15、15-30、30-60 cm）土壤样品,综合分析了土壤微生物生物量碳（MBC）氮（MBN）和多种土壤养分含量。结果表明,MBC、MBN及土壤养分含量均随土壤深度增加而降低。N添加对两种演替阶段森林土壤中MBC和MBN均无显著影响。施P相关处理（P和NP）对年轻林表层土壤MBC和MBN无显著影响,但显著增加了老年林表层土壤MBC和MBN（P<0.05）,表明老年林可能比年轻林更易受P限制。N添加显著增加了两种演替森林表层土壤可溶性有机氮（DON）、氨态氮（NH₄⁺-N）和硝态氮（NO₃^--N）的含量（P<0.05）;P相关处理（P和NP）显著增加两种演替阶段表层和次表层土壤速效磷（AP）以及表层土壤全磷（TP）的含量（P<0.05）。土壤MBC和MBN与土壤中各养分指标（可溶性有机碳DOC、DON、NH₄⁺-N、NO₃^--N、AP、全碳TC、全氮TN和TP）呈显著正相关关系,土壤TC、TN和DOC是影响土壤微生物生物量的主要因子。研究可为评估和揭示未来全球环境变化背景下不同演替林龄亚热带森林的土肥潜力及土壤质量的演变提供一定的科学理论依据。     

广西马山岩溶次生林群落生物量和碳储量

岩溶植被在岩溶生态系统碳循环和全球碳平衡中具有重要的作用。通过对马山县岩溶次生林年龄序列(幼龄林、中龄林和老龄林)3个演替阶段9个样地(20 m×50 m)的系统取样调查,研究了停止人为干扰后岩溶次生林生物量和碳储量的变化。结果表明:沿幼林、中林和老林群落的顺向演替发展,群落生物量显著增加(P0.05),从幼林群落的48.17 t/hm2、到中林群落113.47 t/hm2,再到老林群落242.59 t/hm2。老林生态系统的碳储量较高,平均为236.69 t/hm2,中林和幼林较低且非常相近,分别为225.17 t/hm2和224.76 t/hm2,各次生林生态系统的碳储量差异不显著(P0.05)。土壤碳储量的大小顺序为幼林(198.44 t/hm2)中林(167.39 t/hm2)老林(113.43 t/hm2)。沿群落正向演替,各次生林生态系统中植物碳储量和土壤碳储量的比例发生明显的变化。幼林的土壤碳储量占生态系统碳储量的88.29%,植物碳储量只占11.71%;中林相应为74.34%和25.66%;而老林为47.92%和52.08%。可见,随着岩溶植被的正向演替,土壤碳转变为植物碳的趋势十分明显,这是岩溶森林不同于酸性土森林的一个显著特征。     

Ecosystem development in short‐term postglacial chronosequences: N and P limitation in glacier forelands from Santa Inés Island,Magellan Strait

Glacier foreland moraines provide an ideal model to examine the patterns of ecosystem development and the evolution of nitrogen and phosphorous limitation over successional time. In this paper, we focus on a 400‐year soil chronosequence in the glacier forelands of Santa Inés Island in the Magellan Strait, southern Chile by examining forest development on phosphorus (P)‐poor substrates in a uniquely unpolluted region of the world. Results show a steady increase in tree basal area and a humped trend in tree species richness over four centuries of stand development. The increase in basal area suggests that the late successional tree species were more efficient nutrient users than earlier successional ones. Total contents of carbon (C) and nitrogen (N) in soils increased during the chronosequence, reaching an asymptote in late succession. The net increases in soil C : N, C : P and N : P ratios observed over successional time suggest that nutrient limitation is maximal in 400‐year‐old substrates. Foliar C : N and C : P ratios also increased over time to reach an asymptote in old‐growth stages, following soil stoichiometric relationships; however the foliar N‐to‐P ratio remained constant throughout the chronosequence. Biological N fixation was greater in early postglacial succession, associated with the presence of the symbiotic N‐fixer Gunnera magellanica. Declining trends of δ¹⁵N in surface soils through the 400‐year chronosequence are evidence of decreasing N losses in old‐growth forests. In synthesis, glacier foreland chronosequences at this high South American latitude provide evidence for increasing efficiency of N and P use in the ecosystem, with the replacement of shade‐intolerant pioneers by more efficient, shade‐tolerant tree species. This pattern of ecosystem development produces a constant foliar N : P ratio, regardless of variation in soil N‐to‐P ratio over four centuries.     

Successional trends in standing dead biomass in Mediterranean basin species

Question: Landscape models of fire occurrence in ecosystems assume that the time since the last fire determines vegetation flammability by enabling the accumulation of dead biomass. In this study we ask if Mediterranean basin shrublands respond to these models or, on the contrary, if initial successional stages in these ecosystems could be more flammable than later stages. Location: Mediterranean shrubland in the Valencia region, eastern Spain. Methods: Using different stages of vegetation development (5, 9, 14 and 26 years since the last fire), we first study the structural comiosition of the above‐ground biomass in 375 individuals of nine woody species. Then, we measure how the standing dead biomass varies during succession, taking into account the surface cover of each species and the quantity of total dead biomass accumulated in different successional stages (3, 9, 14 and 26 years since the last fire). Results: The largest amount of standing dead biomass at the plant community level is observed in the middle stages of the succession. Early successional species, such as Cistus spp., Ulex parviflorus and Pinus halepensis, have a higher percentage of standing dead biomass at earlier stages in the succession than species typical of later successional stages, e.g. Juniperus oxycedrus, Quercus coccifera and Quercus ilex. Conclusions: The results suggest that monotonic increase in fire hazard with increasing stand age is not necessarily the rule in Mediterranean basin shrublands, since early successional species may accumulate large amounts of standing dead biomass and thus promote fire at early successional stages.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Soil organic matter accumulation and its implications for nitrogen mineralization and plant species composition during succession in coastal dune slacks

Vegetation and soil development during succession in coastal dune slacks on Terschelling island, the Netherlands, was investigated, by comparing neighbouring ecosystems on similar substrates that had been developing for 1, 5, 35 and 76 years since the vegetation and organic soil layer had been removed. In this successional sequence, soil organic matter accumulated rapidly due to the production of litter and dead roots. N mineralization was extremely low, increasing from 0.2 g m^-2 yr^-1 after 5 years to 0.8 g m^-2 yr^-1 after 76 years. It was accompanied by a decline in the pH (KCl) in the upper 10 cm of the soil from 6.8 to 4.4. Most of the above-ground biomass accumulated in the shrub species Oxycoccus macrocarpos and Salix repens. The 5- year-old plots harboured many plant species (18 species per 0.25 m²), but plant species diversity was much lower in the older plots. It is concluded that most changes in species composition and the decline in diversity occurred because early successional plant species were gradually outshaded by the thick litter layer and the accumulated shrub biomass.     

Changes of the relationships between soil and microbes in carbon,nitrogen and phosphorus stoichiometry during ecosystem succession

AimsThe carbon (C), nitrogen (N) and phosphorus (P) stoichiometry (C:N:P) of soil profoundly influences the growth, community structure, biomass C:N:P stoichiometry, and metabolism in microbes. However, the relationships between soil and microbes in the C:N:P stoichiometry and their temporal dynamics during ecosystem succession are poorly understood. The aim of this study was to determine the temporal patterns of soil and microbial C:N:P stoichiometry and their relationships during ecosystem succession.MethodsAn extensive literature search was conducted and data were compiled for 19 age sequences of successional ecosystems, including 13 forest ecosystems and 6 grassland ecosystems, from 18 studies published up to May 2016. Meta-analyses were performed to examine the sequential changes in 18 variables that were associated with soil and microbial C, N and P contents and the stoichiometry. Important findings (1) There was no consistent temporal pattern in soil C:N along the successional stages, whereas the soil C:P and N:P increased with succession; the slopes of the linear relationships between soil C:N:P stoichiometry and successional age were negatively correlated with the initial content of the soil organic C within given chronosequence. (2) There was no consistent temporal pattern in microbial C:N:P stoichiometry along the successional stages. (3) The fraction of microbial biomass C in soil organic C (qMBC), the fraction of microbial biomass N in soil total N, and the fraction of microbial biomass P in soil total P all increased significantly with succession, in consistency with the theory of succession that ecosystem biomass per unit resource increases with succession. (4) The qMBC decreased with increases in the values of soil C:N, C:P, or N:P, as well as the stoichiometric imbalances in C:N, C:P, and N:P between soil and microbes (i.e., ratios of soil C:N, C:P, and N:P to microbial biomass C:N, C:P, and N:P, respectively). The C:N, C:P, and N:P stoichiometric imbalances explained 37%-57% variations in the qMBC, about 7-17 times more than that explainable by the successional age, illustrating the importance of soil-microbial C:N:P stoichiometry in shaping the successional dynamics in qMBC. In summary, our study highlights the importance of the theories of ecosystem succession and stoichiometry in soil microbial studies, and suggests that appropriately applying macro-ecological theories in microbial studies may improve our understanding on microbial ecological processes.     

生态系统演替过程中土壤与微生物碳氮磷化学计量关系的变化

土壤碳(C)、氮(N)、磷(P)化学计量特征会显著影响微生物的生长、群落结构、生物量C:N:P化学计量及其代谢活动。然而生态系统演替过程中土壤-微生物C:N:P化学计量的时间格局及其协调关系还不明确。为此, 该研究收集了2016年5月以前发表的文献中19个生态系统演替序列(包括13个森林、6个草地生态系统)的土壤-微生物生物量C:N:P研究结果, 整合分析了其中土壤-微生物生态化学计量的时间动态, 结果表明: (1)生态系统演替过程中土壤C:N没有一致的时间格局, 而土壤C:P和N:P均随演替进程显著增加, 其中土壤C:N:P与演替时间之间线性关系的斜率与相应演替序列的初始土壤有机C含量呈负相关关系。(2)演替进程中土壤-微生物生物量C:N:P没有一致的时间格局。(3)微生物生物量C占土壤有机C百分比(qMBC)、微生物生物量N占土壤全N百分比、微生物生物量P占土壤全P百分比均随着演替进程而显著增加, 即单位资源所能支持的微生物生物量随着演替进程而增加, 这与宏观生态系统演替理论相符。(4) qMBC随着土壤C:N、C:P和N:P以及C:N、C:P和N:P化学计量不平衡性(即土壤C:N、C:P和N:P分别除以微生物生物量C:N、C:P和N:P)的增加而减小; 其中, C:N、C:P和N:P化学计量不平衡性解释了qMBC变异性的37%-57%, 是演替时间解释率的7-17倍, 表明土壤-微生物生态化学计量关系对qMBC演替动态有重要影响。该研究强调了生态化学计量学理论和生态系统演替理论在土壤微生物时间动态研究中的重要作用, 表明适当地融合生态学宏观理论于土壤微生物研究可以加深对土壤-微生物生态过程的认识。     

Effects of soil organic matter and nitrogen supply on competition between Festuca ovina and Deschampsia flexuosa during inland dune succession

We tested the prediction that the successional replacement of plant species during succession on inland sand dunes results from the effects of an increase in nitrogen mineralization on competitive interactions. The growth and competitive strength of Festuca ovina and Deschampsia flexuosa on soil substrates with different amounts of soil organic matter or nitrogen supply were measured. Small tillers of Festuca ovina and Deschampsia flexuosa were grown in monocultures and 1:1-mixtures on soil columns with undisturbed layers of soil organic matter from different successional age. There was (a) no visible soil organic matter, (b) a thin soil organic layer (0.5 cm) and (c) a thicker soil organic layer (6.0 cm) present on the soil columns. The species were also grown on columns with no visible soil organic matter (bare sand) with two different levels of N fertilization to mimic the increased N mineralization in the older successional stages.In monoculture, Festuca produced more biomass on the substrates with a soil organic layer compared to the unfertilized sand substrate. It also produced more biomass on sand substrates with N fertilization. Deschampsia produced more biomass in treatments with a soil organic layer compared to the bare sand treatments, but did not respond to the ammonium-nitrate addition. In competition, Festuca seemed to be the stronger competitor on the unfertilized sand substrate. Festuca was also the better competitor on the N fertilized sand treatments, while on the treatments with a soil organic layer Deschampsia was the winning species. Our results do not support the hypothesis that an increase in N supply is responsible for the replacement of Festuca by Deschampsia that concur with the accumulation of soil organic matter during succession in inland dunes.     

Biomass distribution of two subalpine dwarf‐shrubs in relation to soil moisture and nutrient content

Question: Do soil water content and/or soil nitrogen (N) content and/or soil phosphorus (P) content affect the biomass of Vaccinium myrtillus and V. vitis‐idaea in a sub‐alpine heath? Location: Dolomites, northern Italy, 1800 m a.s.l. Methods: We determined above‐ground and below‐ground biomass of the shrubs at three sites, each on a different substrate type. At each site, we determined soil N‐ and P‐contents. We also determined leaf water potential (Psi;₁), N‐ and P‐concentrations in plant tissues and litter, as well as δ¹³C and δ¹⁵N in mature leaves. Results: V. myrtillus biomass was highest at the silicate site, V. vitis‐idaea biomass was highest at the carbonate site. Both shrubs had low biomass at the peat site, possibly due to a toxic effect of waterlogging in wet soils. For both species, pre‐dawn Psi;₁ indicated optimal hydration and midday Psi;₁ did not show any sign of water stress. Water use efficiency (WUE) did not differ among sites for any species. Whole‐plant nutrient concentrations showed that, with increasing biomass, N was diluted in V. myrtillus tissues while P was diluted in V. vitis‐idaea tissues. Foliar N‐concentration was higher overall for V. myrtillus. Foliar P‐concentration in V. myrtillus peaked at the silicate site. Foliar N : P ratios suggested that V. myrtillus was primarily P‐limited and V. vitis‐idaea primarily N‐limited. Conclusions: Water content affected the distribution of the two shrubs in a similar way, higher P‐availability in the soil enhanced V. myrtillus rather than V. vitis‐idaea.     

Changes in Forest Soil Properties in Different Successional Stages in Lower Tropical China

Background

Natural forest succession often affects soil physical and chemical properties. Selected physical and chemical soil properties were studied in an old-growth forest across a forest successional series in Dinghushan Nature Reserve, Southern China.

Methodology/Principal Findings

The aim was to assess the effects of forest succession change on soil properties. Soil samples (0–20 cm depth) were collected from three forest types at different succession stages, namely pine (Pinus massoniana) forest (PMF), mixed pine and broadleaf forest (PBMF) and monsoon evergreen broadleaf forest (MEBF), representing early, middle and advanced successional stages respectively. The soil samples were analyzed for soil water storage (SWS), soil organic matter (SOM), soil microbial biomass carbon (SMBC), pH, NH₄ ⁺-N, available potassium (K), available phosphorus (P) and microelements (available copper (Cu), available zinc (Zn), available iron (Fe) and available boron (B)) between 1999 and 2009. The results showed that SWS, SOM, SMBC, Cu, Zn, Fe and B concentrations were higher in the advanced successional stage (MEBF stage). Conversely, P and pH were lower in the MEBF but higher in the PMF (early successional stage). pH, NH₄ ⁺-N, P and K declined while SOM, Zn, Cu, Fe and B increased with increasing forest age. Soil pH was lower than 4.5 in the three forest types, indicating that the surface soil was acidic, a stable trend in Dinghushan.

Conclusion/Significance

These findings demonstrated significant impacts of natural succession in an old-growth forest on the surface soil nutrient properties and organic matter. Changes in soil properties along the forest succession gradient may be a useful index for evaluating the successional stages of the subtropical forests. We caution that our inferences are drawn from a pseudo-replicated chronosequence, as true replicates were difficult to find. Further studies are needed to draw rigorous conclusions regarding on nutrient dynamics in different successional stages of forest.     

Differential recovery of above‐ and below‐ground rich fen vegetation following fertilization

Abstract. For seven years we studied the recovery of vegetation in a Belgian P limited rich fen (Caricion davallianae), which had been fertilized with nitrogen (200 g.m^?2) and phosphorus (50 g.m^?2) in 1992. The vegetation in this fen has low above‐ground biomass production (< 100 g m^?2) due to the strong P limitation. Above‐ground biomass was harvested from 1992 to 1998 and P and N concentrations measured. In 1998, below‐ground biomass was also harvested. The response to fertilization differed markedly between below‐ and above‐ground compartments. Above‐ground, P was the single most important factor that continued to stimulate growth 7 yr after fertilization. Below‐ground, N tended to decrease live root biomass and increase dead root biomass and seemed to have a toxic effect on the roots. In the combined NP treatment the stimulating effect of P (an increase of live root biomass) was countered by N. The 1998 soil analysis showed no difference in soil P in the plots. Thus, Fe hydroxides are not capable of retaining P in competition with fen vegetation uptake. However, higher capture of P in root Fe coatings from N plots may partially explain this negative N effect. The results suggest that N root toxicity will be amplified in strongly P limited habitats but that its persistence will be less influenced by P availability. This mechanism may be a competitive advantage for N₂ fixing species that grow in strongly P limited wetlands.     

Impacts of moso bamboo (Phyllostachys pubescens) invasion on dry matter and carbon and nitrogen stocks in a broad‐leaved secondary forest located in Kyoto,western Japan

In western and central Japan, the expansion of exotic moso bamboo (Phyllostachys pubescens Mazel ex J. Houz.) populations into neighboring vegetation has become a serious problem. Although the effects of bamboo invasion on biodiversity have been well studied, shifts in nutrient stocks and cycling, which are fundamental for ecosystem functioning, are not fully understood. To explore the effects of P. pubescens invasion on ecosystem functions we examined above‐ and below‐ground dry matter and carbon (C) and nitrogen (N) stocks in a pure broad‐leaved tree stand, a pure bamboo stand, and two tree–bamboo mixed stands with different vegetation mix ratios in the secondary forest of Kyoto, western Japan. In the process of invasion, bamboo shoots offset broad‐leaved tree deaths; thus, no clear trend was apparent in total above‐ or below‐ground biomass or in plant C and N stocks during invasion. However, the ratio of above‐ground to below‐ground biomass (T/R ratio at the stand level) decreased with increasing bamboo dominance, especially in the early stages of invasion. This shift indicates that rapid bamboo rhizomatous growth is a main driver of substantial changes in stand structure. We also detected rises in the C/N ratio of forest‐floor organic matter during bamboo invasion. Thus major impacts of P. pubescens invasion into broad‐leaved forests include not only early shifts in biomass allocation, but also changes in the distribution pattern of C and N stored in plants and soil.     

Population dynamics of Echinops gmelinii Turcz. at different successional stages of biological soil crusts in a temperate desert in China

• The effects of biological soil crusts (BSC) on vascular plant growth can be positive, neutral or negative, and little information is available on the impacts of different BSC successional stages on vascular plant population dynamics.
• We analysed seedling emergence, survival, plant growth and reproduction in response to different BSC successional stages (i.e. habitats: bare soil, cyanobacteria, lichen and moss crusts) in natural populations of Echinops gmelinii Turcz. in the Tengger Desert of northwest China. The winter annual E. gmelinii is a dominant pioneer herb after sand stabilisation.
• During the early stages of BSC succession, the studied populations of E. gmelinii were characterised by high density, plant growth and fecundity. As the BSC succession proceeded beyond moss crusts, the fecundity decreased sharply, which limited seedling recruitment. Differences in seedling survival among the successional stages were not evident, indicating that BSC have little effect on survival in arid desert regions. Moreover, E. gmelinii biomass allocation exhibited low plasticity, and only reproductive allocation was sensitive to the various habitats. Our results further suggest that the negative effects of BSC succession on population dynamics are primarily driven by increasing topsoil water‐holding capacity and decreasing rain water infiltration into deeper soil.
• We conclude that BSC succession drives population dynamics of E. gmelinii, primarily via its effect on soil moisture. The primary cause for E. gmelinii population decline during the moss‐dominated stage of BSC succession is decreased fecundity of individual plants, with declining seed mass possibly reducing the success of seedling establishment.

     

高寒矮嵩草群落退化演替系列氮、磷生态化学计量学特征

运用历史资料与实地调查相结合的方法,以多元数量统计为手段确定采样地点,以空间尺度代替时间尺度,确定演替系列,以生态化学计量学为基础探讨了高寒矮嵩草草甸退化演替系列氮(N)磷(P)含量及化学计量学特征,发现:1)高寒矮嵩草草甸土壤全量N、P含量随退化演替程度的加深而呈倒"V"字形变化趋势,速效N、P含量随退化程度的加深呈降低趋势,但土壤草甸全量及速效N/P化学计量学特征则呈现降低趋势;2)地上植物N/P化学计量学特征在整个退化演替过程没有明显的差异。说明高寒矮嵩草群落退化改变了土壤中全量及速效N、P的积累和分解速率,打破了土壤系统养分平衡模式,但并没有明显改变植物地上部分整体的N/P化学计量学特征,因此在退化演替过程中植物N/P比为草地退化诊断的惰性指标;土壤N/P化学计量学特征变化同草地退化演替过程具有较好的同步性,其对草地退化演替的敏感性较高,有可能成为未来草地退化诊断的生态指示指标。     

Facilitation‐ vs. competition‐driven succession: the key role of resource‐ratio

Symbiotic nitrogen (N)‐fixing plants are abundant during primary succession, as typical bedrocks lack available N. In turn, fixed N accumulates in soils through biomass turnover and recycling, favouring more nitrophilous organisms. Yet, it is unclear how this facilitation mechanism interacts with competition for other limiting nutrients such as phosphorus (P) and how this affects succession. Here, we introduce a resource‐explicit, community assembly model of N‐fixing species and analyze successional trajectories along resource availability gradients using contemporary niche theory. We show that facilitation‐driven succession occurs under low N and high enough P availabilities, and is characterised by autogenic ecosystem development and relatively ordered trajectories. We show that late facilitation‐driven succession is sensitive to catastrophic shifts, highlighting the need to invoke other mechanisms to explain ecosystem stability near the climax. Put together with competition‐driven succession, these results lead to an enriched version of Tilman's resource‐ratio theory of succession.     

Properties of native plant communities do not determine exotic success during early forest succession

Considerable research has been devoted to understanding how plant invasions are influenced by properties of the native community and to the traits of exotic species that contribute to successful invasion. Studies of invasibility are common in successionally stable grasslands, but rare in recently disturbed or seral forests. We used 16 yr of species richness and abundance data from 1 m² plots in a clearcut and burned forest in the Cascade Range of western Oregon to address the following questions: 1) is invasion success correlated with properties of the native community? Are correlations stronger among pools of functionally similar taxa (i.e. exotic and native annuals)? Do these relationships change over successional time? 2) Does exotic abundance increase with removal of potentially dominant native species? 3) Do the population dynamics of exotic and native species differ, suggesting that exotics are more successful colonists? Exotics were primarily annual and biennial species. Regardless of the measure of success (richness, cover, biomass, or density) or successional stage, most correlations between exotics and natives were non‐significant. Exotic and native annuals showed positive correlations during mid‐succession, but these were attributed to shared associations with bare ground rather than to direct biotic interactions. At peak abundance, neither cover nor density of exotics differed between controls and plots from which native, mid‐successional dominants were removed. Tests comparing nine measures of population performance (representing the pace, magnitude, and duration of population growth) revealed no significant differences between native and exotic species. In this early successional system, local richness and abundance of exotics are not explained by properties of the native community, by the presence of dominant native species, or by superior colonizing ability among exotics species. Instead natives and exotics exhibit individualistic patterns of increase and decline suggesting similar sets of life‐history traits leading to similar successional roles.     

N2 fixing alder (Alnus viridis spp. fruticosa) effects on soil properties across a secondary successional chronosequence in interior Alaska

Green alder (Alnus viridis ssp. fruticosa) is a dominant understory shrub during secondary successional development of upland forests throughout interior Alaska, where it contributes substantially to the nitrogen (N) economy through atmospheric N₂ fixation. Across a replicated 200+ year old vegetation chronosequence, we tested the hypotheses that green alder has strong effects on soil chemical properties, and that ecosystem-level N inputs via N₂ fixation decrease with secondary successional stand development. Across early-, mid-, and late-successional stands, alder created islands of elevated soil N and carbon (C), depleted soil phosphorus (P), and more acidic soils. These effects translated to the stand-level in response to alder stem density. Although neither N₂ fixation nor nodule biomass differed among stand types, increases in alder densities with successional time translated to increasing N inputs. Estimates of annual N inputs by A. viridis averaged across the upland chronosequence (6.6 ± 1.2 kg N ha^?1 year^?1) are substantially less than inputs during early succession by Alnus tenuifolia growing along Alaskan floodplains. However, late-succession upland forests, where densities of A. viridis are highest, may persist for centuries, depending on fire return interval. This pattern of prolonged N inputs to late successional forests contradicts established theory predicting declines in N₂-fixation rates and N₂-fixer abundance as stands age.     

Primary succession on lava domes on Terceira (Azores)

Questions: Are the vegetation attributes significantly different among lava domes and among geomorphologic units as a result of age and soil features? Are the successional rates equal in all the geomorphologic units of the domes? Are the colonizer species of lava domes totally replaced by other species in the late successional stages? Location: Terceira Island, Azores (Portugal). Methods: Three comparable domes of 240, 370 and 2080 yr old were selected. Data on floristic composition, vegetation bioarea (area occupied by plant species in transects), structure, demography and soil nutrients were collected. Quantitative and qualitative changes along the succession gradient were also analyzed. Results: Vegetation attributes were consistent with the successional stage of each dome in the primary sere; however, the geomorphologic units did not follow the same pattern. The influence of the rates of plant colonization and soil formation are responsible for the decrease of the successional rates from footslopes, to summits, to slopes. The vegetation successionally changes from Juniperus scrub, to Juniperus wood and forest, and there is little species replacement since the similarity in species composition is high between the 3 domes. Conclusions: This is a special type of direct succession that takes place mainly through an increase in biomass and structural complexity. We observed small wooded areas in the fissures of very young domes that are samples of later successional forests — a phenomenon that we call ‘zoom effect’.