Late-successional Plant Research Articles

Soil Thresholds for Classification of Vegetation Types in Abandoned Cropland on the Loess Plateau, China

Soil erosion is a severe environmental problem on the Loess Plateau, China. Vegetation restoration is the most efficient method to control soil erosion and introducing late-successional plant species may accelerate natural succession. However, the progress is affected by soil conditions and the appropriate thresholds of soil condition for seed addition are needed. Our objectives were to identify the vegetation types, examine the key soil factors affecting the vegetation variation, and determine the thresholds of the soil factors for each vegetation type. Five vegetation types in secondary succession were identified: association (assoc.) Artemisia scoparia assoc. Bothriochloa ischaemum assoc. Artemisia gmelinii Artemisia giraldii assoc. Ostryopsis davidiana Rubus parvifolius L., Syringa oblate and assoc. Sophora viciifolia Years since abandonment (YEAR), alkali-hydrolysable nitrogen (AHN), soil water content (SW), and total phosphorus (TP) were the key factors used to discriminate among vegetation types. Assoc. A. scoparia developed in croplands that had been abandoned for less than 11 years. Assoc. B. ischaemum developed in croplands abandoned for more than 11 years with lower soil nutrient levels (0.30 ≤ TP ≤ 0.646 g kg−1 and 16.72 ≤ AHN ≤ 32.62 mg kg−1). Assoc. A. gmelinii + A. giraldii always required greater TP (0.646 ≤ TP ≤ 0.79 g kg−1) to develop. When soil water content was lower (6.6 ≤ SW ≤ 8.4%), the assoc. S. viciifolia developed; otherwise, other associations, such as assoc. O. davidiana, developed. These soil thresholds can be used as references for guiding restoration, such as specifying proper soil conditions for seed sowing.

Grazing impact on plant spatial distribution and community composition

Background and aims Re-introduction of large grazers in the few remaining natural and semi-natural grasslands are thought to be an effective management tool to prevent dominance of late successional plant species and restoration of plant biodiversity. The main objective of this study was to test whether the introduction of large herbivores retard the succession by reducing the abundance of highly competitive tall species and whether it is accompanied with changes in plant community composition and spatial distribution of plant species. Methods In order to test this hypothesis, we studied the effect of grazing by large herbivores on vegetation at three hierarchical levels: individual plant species, emergent groups of functionally similar herbaceous plant species, and the main gradients of plant community composition. Study sites were thirteen spatially separated, dry coastal dune grasslands in western Belgium and north-western France. Key results Grazing had a predominantly negative effect on high competitive dominant species and led to changes in composition of emergent groups toward less competitive plant species. Additionally, these changes in plant community composition were accompanied with changes in spatial distribution patterns of individual plant species and community richness. Conclusions Our results suggest that the current grazing management applied in these nature reserves is able to prevent the expansion of dominant highly competitive species and establishment of functionally different plant species.

Inoculation with a Native Soil Community Advances Succession in a Grassland Restoration

Restoration on post‐agricultural land may be hindered by the degradation of the soil community, which has been shown to contribute to structuring plant communities and driving succession. Our experiment tested the effect of inoculation with remnant grassland whole soil with or without nurse plants on the survival and growth of uninoculated early and late successional plant species. In 2007 and 2008, we planted uninoculated early, mid, and late successional plant species 0.25–2 m away from a central point of inoculated nurse plants. We found a negative response to inoculation on early successional plants and a positive response to inoculation on mid to late successional plants. This work suggests that the restoration of the soil community is critical to establishing a late successional plant community and that the benefit of inoculated plants can spread to neighbors.

Plant–microbe interactions and nitrogen dynamics during wetland establishment in a desert stream

In late-successional steady state ecosystems, plants and microbes compete for nutrients and nutrient retention efficiency is expected to decline when inputs exceed biotic demand. In carbon (C)-poor environments typical of early primary succession, nitrogen (N) uptake by C-limited microbes may be limited by inputs of detritus and exudates derived from contemporaneous plant production. If plants are N-limited in these environments, then this differential limitation may lead to positive relationships between N inputs and N retention efficiency. Further, the mechanisms of N removal may vary as a function of inputs if plant-derived C promotes denitrification. These hypotheses were tested using field surveys and greenhouse microcosms simulating the colonization of desert stream channel sediments by herbaceous vegetation. In field surveys of wetland (cienega) and gravelbed habitat, plant biomass was positively correlated with nitrate (NO3 −) concentration. Manipulation of NO3 − in flow-through microcosms produced positive relationships among NO3 − supply, plant production, and tissue N content, and a negative relationship with root:shoot ratio. These results are consistent with N limitation of herbaceous vegetation in Sycamore Creek and suggest that N availability may influence transitions between and resilience of wetland and gravelbed stable states in desert streams. Increased biomass in high N treatments resulted in elevated rates of denitrification and shifts from co-limitation by C and NO3 − to limitation by NO3 − alone. Overall NO3 − retention efficiency and the relative importance of denitrification increased with increasing N inputs. Thus the coupling of plant growth and microbial processes in low C environments alters the relationship between N inputs and exports due to increased N removal under high input regimes that exceed assimilative demand.

Predicting root defence against herbivores during succession

Summary 1. Root herbivores and pathogens interfere with basic below‐ground plant function, and can thereby affect plant fitness and spatial and temporal patterns in natural plant communities. However, there has been little development of concepts and theories on below‐ground plant defence, a deficit that is in contrast to the abundance of theorizing for above‐ground plant parts. 2. A review of the past 10 years of research on below‐ground plant–herbivore interactions has revealed that, similar to above‐ground tissues, root defences can be expressed constitutively or induced upon herbivore attack, and can be classified into direct and indirect traits, tolerance, and escape. Indeed, it has been shown that roots tolerate herbivory by outgrowing or re‐growing lost tissues, or resist it by producing secondary metabolites that are toxic to herbivores or attract natural enemies of herbivores. 3. We propose that, similar to above‐ground plant–herbivore theories, the partition of abiotic and biotic factors over ecological succession can serve as the basis for predicting investment in defence strategies below‐ground. 4. Investigation of herbivore pressure and root responses along primary and secondary successional gradients suggests that: (i) roots are often fast growing, thinner and softer in early compared to later succession. (ii) Insect and nematode herbivore pressure increases until mid‐succession and later decreases. (iii) Mycorrhizal abundance increases with succession, and the composition of fungal species changes through succession, often shifting from arbuscular mycorrhizae to ecto‐mycorrhizae. 5. Based on these findings, and on classical (above‐ground) plant defence theory, we suggest the following set of testable hypotheses for below‐ground plant defence: (i) During succession, early plants invest most of their resources in growth and less in defences (associated with a general lack of herbivores and pathogens, and with limited availability of resources in the system), therefore relying more on re‐growth (tolerance) strategies. (ii) During mid‐succession, a buildup of herbivore pressure facilitates replacement by plant species that exhibit greater direct and indirect defence strategies. (iii) Constitutive and inducible levels of defences may trade‐off, and early successional plants should rely more on induction of defences after herbivore attack, whereas late successional plants will increasingly rely on constitutively produced levels of physical and chemical defence. (iv) Successional changes in microbial associations have consequences for root defence by improving plant nutrition and defence expression as well as directly competing for root space; however, toxic or impenetrable root defences may also limit association with root symbionts, and so may constrain the expression of root defence.

Inhibition of an invasive plant (Mikania micrantha H.B.K.) by soils of three different forests in lower subtropical China

Biological invasion represents one of the most serious threats to biodiversity, and invasion ecology research has become one of the central issues of contemporary environmental science. However, the relative role of soil development as correlated with succession in influencing variation in invasion resistance has seldom been examined. We hypothesized that the invasion potential of exotic plants depends on soil conditions. In this study, we explored variation among soils of three forest types in their resistance to invasion by Mikania micrantha H.B.K. in lower subtropical China; this species is one of the top ten most invasive weeds in the world. In model systems of soils from different forests, M. micrantha performed worst on undisturbed late-successional stage forest (monsoon evergreen broadleaf forest, MEBF) soils and did not appear to take advantage of abundant soil nutrients. These results could reflect that a potential inhibitory effect was driven by MEBF soil. Plant litter is an important component of soils, litter bioassay and addition experiments demonstrated native plant species from different forests differentially confer invasion resistance and affect the performance of exotic plants. Naturally, other factors such as the relative abundances of nutrients, microbial feedbacks, could have contributed to these results as well. Our results challenge the notion that forests are not immune to invasion and suggest that maintaining late-successional plant communities in lower subtropical China, which could be related to increased productivity and stability, should be a primary objective of land managers.

Effect of earthworm on growth of late succession plant species in postmining sites under laboratory and field conditions

Earlier studies of postmining heaps near Sokolov, Czech Republic (0–46 years old) showed that massive changes in plant community composition occur around 23 year of succession when the heaps are colonized by the earthworms Lumbricus rubellus (Hoffm.) and Aporrectodea caliginosa (Savigny). The aim of the current study was to test the hypothesis that the introduction of earthworms into a postmining soil enhances growth of late succession plant species. In a laboratory experiment, earthworms significantly increased biomass of Festuca rubra and Trifolium hybridum grown in soil from a 17-year-old site. The biomass increase corresponded to a significant decrease in pH and an increase in oxidable C, total N, and exchangeable P, K, and Ca content. A second laboratory experiment showed higher biomass production of late successional plant community (Arrhenatherum elatius, Agrostis capillaris, Centaurea jacea, Plantago lanceolata, Lotus corniculatus, and Trifolium medium) in soil from late successional stage (46 years old); the introduction of earthworms into soil from an early successional stage (17 years old) increased biomass production. In a field experiment, introduction of L. rubellus to enclosures containing a 17-year-old soil not colonized by earthworms significantly increased the biomass of grasses after 1 year. The results support the hypothesis that colonization of postmining areas by earthworms can substantially modify soil properties and plant growth.

Effects of Artificial Roosts for Frugivorous Bats on Seed Dispersal in a Neotropical Forest Pasture Mosaic

In the Neotropics ongoing deforestation is producing open and heavily fragmented landscapes dominated by agriculture, mostly plantations and cattle pastures. After some time agriculture often becomes uneconomical and land is abandoned. Subsequent habitat regeneration may be slow because seed inputs are restricted by a lack of incentives--such as suitable roost sites--for seed dispersers to enter deforested areas. Increasing environmental awareness has fostered growing efforts to promote reforestation. Practical and cost-efficient methods for kick-starting forest regeneration are, however, lacking. We investigated whether artificial bat roosts for frugivorous bat species can attract these key seed dispersers to deforested areas, thereby increasing seed rain. We installed artificial bat roosts in a forest-pasture mosaic in the Costa Rican Atlantic lowlands and monitored bat colonization and seed dispersal. Colonization occurred within a few weeks of installation, and 10 species of bats occupied the artificial roosts. Five species of frugivorous or nectarivorous bats colonized artificial roosts permanently in both primary habitat and in deforested areas, in numbers similar to those found in natural roosts. Seed input around artificial roosts increased significantly. Sixty-nine different seed types, mostly of early-successional plant species, were transported by bats to artificial roosts in disturbed habitats. The installation of artificial bat roosts thus successfully attracted frugivorous bats and increased seed inputs into degraded sites. This method is likely to speed up early-vegetation succession, which in turn will attract additional seed dispersers, such as birds, and provide a microhabitat for seeds of mid- and late-successional plants. As well as supporting natural forest regeneration and bat conservation, this cost-efficient method can also increase environmental awareness among landowners.

Do late-successional tannin-rich plant communities occurring on highly acidic soils increase the DON/DIN ratio?

Previous studies suggested that late-successional tannin-rich plant communities increase the amount of dissolved organic N (DON) relative to dissolved inorganic N (DIN) in decomposing litter. We devised an experiment to test this hypothesis by adding varying proportions of black spruce (Picea mariana) and tannin-rich Kalmia angustifolia litter to forest floor samples collected on six black spruce cutovers. An increasing proportion of Kalmia litter increased condensed tannin and total phenolic concentrations over the course of a 46-week incubation period. Mineral N concentrations did not vary among treatments in spite of much higher total N concentrations in Kalmia litter. This was more likely due to the formation of protein–tannin complexes rather than microbial immobilization of N, as indicated by the decline in available C with increasing Kalmia litter on two of the five sampling dates. There was a significant positive linear trend between the proportion of Kalmia litter and the DON/DIN ratio on one sampling date (week 13) only. Results suggest that the DON/DIN ratio is controlled by confounding factors (e.g., tannins bonding with non-extractable humus particles) and has limited value for describing ecological succession.

No stimulation in root production in response to 4 years of in situ CO2 enrichment at the Swiss treeline

Summary 1Plants are frequently observed to increase carbon allocation to below-ground sinks and particularly, to accelerate fine root turnover in response to rising atmospheric CO2 concentration. While these strong below-ground responses have predominantly been observed in rapidly expanding systems, late successional plant communities have rarely been studied. 2In an ongoing free air CO2 enrichment (FACE) experiment, we assessed below-ground responses to elevated CO2 after 4 years, in a treeline ecosystem in the Swiss Central Alps (2180 m a.s.l.) dominated by a late successional ericaceous dwarf shrub community (Vaccinium myrtillus, V. uliginosum, Empetrum hermaphroditum), and a sparse overstorey of 30-year-old Larix decidua and Pinus uncinata trees. Measurements included quantification of fine root growth using ingrowth root cores and parallel standing crop harvests and decomposition of roots using litter bags. 3Elevated CO2 did not stimulate root growth of the treated vegetation (although some significant above-ground growth responses were observed), nor did altered root decomposition occur. Root quality measurements indicated that elevated CO2 resulted in significantly higher starch concentrations, but no change in N concentration, or root dehydrogenase activity. 4The use of the stable isotope δ13C permitted us to trace the new carbon entering the system through our CO2 enrichment treatment. We observed that only c. 30% of new root biomass (< 2 mm) was formed by new carbon indicating a rather slow root turnover in this system. 5Our data show that fine root growth may be much less stimulated by elevated CO2 in systems with late successional elements than has been reported in ecosystems with a rapidly expanding plant community biomass.

Temporal variation in plant–soil feedback controls succession

Soil abiotic and biotic factors play key roles in plant community dynamics. However, little is known about how soil biota influence vegetation changes over time. Here, we show that the effects of soil organisms may depend on both the successional development of ecosystems and on the successional position of the plants involved. In model systems of plants and soils from different successional stages, we observed negative plant-soil feedback for early-successional plant species, neutral feedback for mid-successional species, and positive feedback for late-successional species. The negative feedback of early-successional plants was independent of soil origin, while late-successional plants performed best in late- and worst in early-successional soil. Increased performance of the subordinate, late-successional plants resulted in enhanced plant community diversity. Observed feedback effects were more related to soil biota than to abiotic conditions. Our results show that temporal variations in plant-soil interactions profoundly contribute to plant community assemblage and ecosystem development.

Diet composition and foraging ecology of Asian elephants in Shangyong, Xishuangbanna, China

The composition of the diet and the foraging ecology of the Asian elephant in its natural habitat were studied from 1998 to 2000 in the Shangyong National Natural Reserve located at Xishuangbanna, China, using field observation and dung analysis. A total of 106 plant species were recorded as being eaten by Asian elephants, 83 of which were identified in the elephants' dung. The plant families that contributed to a major proportion of the elephants' diet in the study area were: Gramineae (8 spp., 10.0%), Moraceae (7 spp., 9.9%), Papilionaceae (4 spp., 8.4%), Araliaceae (3 spp., 6.6%), Vitaceae (3 spp., 5.7%), Apocynaceae (3 spp., 4.6%), Musaceae (1 spp., 4.2%), Zingiberaceae (3 spp., 3.7%), Myrsinaceae (3 spp., 3.6%), Rosaceae (3 spp., 3.6%), Euphorbiaceae (5 spp., 3.3%), Ulmaceae (2 spp., 3.0%) and Mimosaceae (4 spp., 2.9%). The most important plants in the elephants' diet were Ficus spp. (Moraceae, 9.0%), Dendrocalamus spp. (Gramineae, 4.5%), Musa acuminata (Musaceae, 4.2%), Microstegium ciliatum (Gramineae, 3.5%) and Amalocalyx yunnanensis (Apocynaceae, 3.1%). Asian elephants consumed a variety of plants in terms of life forms, including trees, vines, shrubs and herbs. Early successional plant species constitute a higher proportion of the diet than late successional plants (42 spp. taking 59% vs. 32 spp. taking 37%). Browsing species accounted for a larger proportion of the diet compared to grazing species (77 spp. taking 91% vs. 6 spp. taking 9%). The number of plant taxa (species, genus, family) in elephants' diet each month negatively correlated with monthly rainfall and mean temperature. The study may help to develop proper strategies for wildlife management especially with regard to the human-elephant conflict, which is now a serious issue in the conservation of Asian elephants in this area.

Plant succession and rhizosphere microbial communities in a recently deglaciated alpine terrain

This study describes how early and late successional plant species affect soil microorganisms in alpine ecosystems. We quantify the relative importance of plant species and soil properties as determinants of belowground microbial communities . Sixteen plant species were selected from six successional stages (4–14–20–43–75–135 years) within the foreland of the Rotmoosferner glacier , Austria, and at one (reference) site outside the foreland. The size, composition and function of the communities of microorganism in the bulk soil and the rhizosphere were characterized by ninhydrin-reactive nitrogen, phospholipid fatty acids and enzyme activities ( β -glucosidase, β -xylosidase, N -acetyl- β -glucosaminidase, leucine aminopeptidase , acid phosphatase , sulphatase). The results show that the microbial data could be grouped according to early (up to 43 years) and late-colonizing plant species (75 or more years). In early succession, no plant species or soil age effect was detected on the microbial biomass , phospholipid fatty acids, or enzyme activity. The rhizosphere microbial community was similar to that in the bulk soil, which in turn was determined by the abiotic environmental conditions. In late succession, improved soil conditions probably mediated plant species effects on the belowground microbial community. The most pronounced rhizosphere effects were attributed to plant species of the 75- and 135-year-old sites. The microbial colonization (size, composition, activity) of the bulk soil predominantly followed changes in vegetation cover, plant life forms and soil organic matter . In summary, the observed successional pattern of the above- and belowground communities provides an example of the facilitation models of primary succession. Ziel dieser Arbeit ist die Charakterisierung der Rhizosphärenmikroorganismen von Pionier- und Folgearten entlang des Rotmoosferner Gletschervorfeldes im Ötztal, Österreich. Sechs Standorte innerhalb (Alter: 4–14–20–43–75–135 Jahre) und ein Standort ausserhalb des Gletschervorfeldes (Referenz) und 16 Pflanzenarten wurden unter dem Aspekt der fortschreitenden Sukzession ausgewählt, und Rhizosphärenboden der für das jeweilige Sukzessionsstadium representativen Pflanzenarten, sowie der Gesamtboden beprobt. Die mikrobielle Gemeinschaftsstruktur wurde anhand von Phospholipidfettsäuremustern, mikrobielle Umsetzungsraten anhand von Enzymmessungen ( β -Glucosidase, β -Xylosidase, N -Acetyl- β -Glucosaminidase, Leucin-Aminopeptidase, saure Phosphatase, Sulfatase) mittels fluorogener Substrate charakterisiert. Innerhalb der ersten 43 Sukzessionsjahre konnte kein artspezifischer Effekt auf die mikrobielle Rhizosphärengemeinschaft nachgewiesen werden. Die Aktivität und Zusammensetzung wurde überwiegend von den extremen abiotischen Standortsbedingungen geprägt. Ab 75 Sukzessionsjahren konnte aufgrund verbesserter Boden- und Nährstoffbedingungen ein pflanzenspezifischer Einfluss auf die mikrobielle Gemeinschaft nachgewiesen werden. Der stärkste Rhizosphäreneffekt wurde auf den 75- und 135 Jahre alten Standorten beobachtet. Die mikrobielle Kolonisierung des Bodens wurde durch die Vegetationsbedeckung, die Lebensformen und vom Humusgehalt bestimmt.

Effects of prairie restoration on weed invasions

Restoration of late-successional plant communities may reduce weed invasion in non-cropland. In studies of natural succession and invasion, however, the presence of late-successional vegetation is often confounded with lower weed propagule pressure and greater time since disturbance. This paper reports a study in which propagule pressure and time since disturbance were controlled by adding weed species to experimental plots dominated by early and late-successional perennial grass species. Seeds of 12 weed species were added to randomized, 6-year-old plots of restored prairie and non-restored, old-field vegetation. Restoration reduced added weed biomass by 92% in year one and 72% in year two, and reduced biomass of four individual weed species, each by more than 82%. Likely mechanisms through which restoration may have reduced invasion include competition and establishment limitation. To examine these mechanisms, subplots were treated with N addition and burning, to reduce competition and remove litter, respectively. Several patterns suggest that competition may have mediated reductions in weed invasion: restoration increased biomass of resident vegetation, decreased light levels, and decreased weed size. Furthermore, adding N reduced effects of restoration on resident and added weeds. The importance of establishment limitation in reducing invasion is suggested by increased litter mass and decreased weed density with restoration. Burning removed litter, and reduced effects of restoration on added weeds. These results suggest that tallgrass prairie restoration can reduce weed invasion, and that this reduction may involve multiple mechanisms, including competition and establishment limitation.

Successional trends in Sonoran Desert abandoned agricultural fields in northern Mexico

Excessive ground-water use and saline intrusion to the aquifer led, in less than three decades, to an increase in abandoned agricultural fields at La Costa de Hermosillo, within the Sonoran Desert. Using a chronosequence from years since abandonment, patterns of field succession were developed. Contrary to most desert literature, species replacement was found, both in fields with and without saline intrusion. Seasonal photosynthetic capacity as well as water and nitrogen use efficiencies were different in dominant early and late successional plant species. These ecological findings provided a framework for a general explanation of species dominance and replacement within abandoned agricultural fields in the Sonoran Desert.

Management of plant communities on set-aside land and its effects on earthworm communities

Plant communities of set-aside agricultural land in a European project were managed in order to enhance plant succession towards weed-resistant, mid-successional grassland. Here, we ask if the management of a plant community affects the earthworm community. Field experiments were established in four countries, the Netherlands, Sweden, the UK, and the Czech Republic. High (15 plant species) and low diversity (four plant species) seed mixtures were sown as management practice, with natural colonization as control treatment in a randomized block design. The response of the earthworms to the management was studied after three summers since establishment of the sites. Samples were also taken from plots with continued agricultural practices included in the experimental design and from a site with a late successional plant community representing the target plant community. The numbers and biomass of individuals were higher in the set-aside plots than in the agricultural treatment in two countries out of four. The numbers of individuals at one site (The Netherlands) was higher in the naturally colonized plots than in the sowing treatments, otherwise there were no differences between the treatments. Species diversity was lower in the agricultural plots in one country. The species composition had changed from the initial community of the agricultural field, but was still different from a late successional target community. The worm biomass was positively related to legume biomass in Sweden and to grass biomass in the UK.

Green Tree Retention in Fennoscandian Forestry

Boreal forest management has changed rapidly during recent years, and new forestry practices, such as green tree retention (GTR), which aim at ecological sustainability have been developed. With our present knowledge, we cannot fully assess the ecological benefits of prevailing Fennoscandian GTR levels (5-10 trees/ha). Small retention tree groups cannot be expected to provide habitat for interior forest species because of edge effects. However, even individual trees will increase the amount of snags and logs, which are important habitats for many species. According to our own results, the response of understory vegetation depends clearly on retention level. With 7% retention of stand volume, or about 50 trees per ha, we found no remarkable differences in vegetation response as compared with clear-felling. This indicates that a higher level of retention is needed to maintain late successional plant species on the site. Our data also show that there is much within-stand type variation, which correlates with the amount of CWD and stand characteristics. By locating larger retention tree groups in species-rich areas, and avoiding disturbance to CWD, a higher proportion of the resident species diversity may be ''lifeboated'' over the regeneration phase on the stand level. In this paper we review existing literature on green tree retention, with special emphasis on Fennoscandian forest management, biodiversity and productivity issues. We present results of two ongoing Finnish experimental studies that focus on the effects of various forestry practices on understory vegetation including epixylic taxa.

Acclimation to sudden increase in light favoring an invasive over native trees in subtropical islands, Japan.

Bischofia javanica Blume, an exotic tree, dominates many forest areas of the Bonin Islands in the western Pacific of Japan. The aim of this study was to test the hypothesis that the success of B. javanica (a mid-successional plant species) is related to its high acclimation capacity to sudden light increase due to canopy gap formation. We compared its ecophysiological response to simulated canopy opening with those of native species of different successional status: Trema orientalis Blume, Schima mertensiana (Sieb, et Zucc.) Koidz, Elaeocarpus photiniaefolius Hook.Et Arn. and Ardisia sieboldii Miquel. In all species, transfer of leaves developed in shade (5.3% of full sun) to full sun resulted in a substantial initial reduction in the dark-adapted quantum yield of photosystem II (Fv/Fm). T. orientalis, a pioneer plant species, showed the least reduction (38%), whereas E. photiniaefolius and A. sieboldii, both late-successional plant species, demonstrated large reductions (about 80%). In all four native species, Fv/Fm in shade leaves gradually recovered following transfer, but B. javanica recovered more fully and rapidly than the other species. Unlike Fv/Fm, the chlorophyll content in all species did not recover following the initial decline. This indicates that the recovery of quantum yield (Fv/Fm) was independent of the reduction in chlorophyll. Among all the species, B. javanica showed the highest (1) increase in maximum photosynthetic rate of shade leaves after transfer, (2) production of newly formed sun leaves, and (3) increase in relative growth rate. Ecophysiological characters of B. javanica in simulated canopy openings indicated rapid photosynthetic acclimation in existing shade leaves by minimizing photoinhibition and a rapid deployment of new sun leaves with high photosynthetic capacity. Because its habitats on these Pacific Islands are prone to typhoon disturbance, the successful invasion of B. javanica may lie in the congruence of its acclimation potential and the frequent gap events.

The Forest Regeneration Puzzle

Forest ecosystem dynamics have long been explained by changes in floristic composition (Foster 1988) and nutrient availability (Vitousek 1984). The passage from pioneer to late-successional plant communities has frequently been modeled (McCook 1994) and, in a few instances, has been studied experimentally (Facelli and Facelli 1993). Interactions between plants (including their immediate environment as a passive partner) have generally been considered to be responsible for forest succession. However, the belowground components of any given ecosystem must also be taken into account before successional and steady-state patterns in plant communities can be fully understood (Cromack 1981, Miles 1985, Nilsson 1994). For example, managed coniferous forests

The responses of alpine grassland to four seasons of CO 2 enrichment: a synthesis

Alpine grassland at 2 470 m altitude in the Swiss Central alps was exposed to elevated CO 2 by using open top chambers (16 ambient, 16 elevated CO 2). Some plots received mineral fertilizer at a rate of N-deposition commonly measured in low altitude parts of Europe. Here we present a summary of results and data from the final harvest. Above-ground biomass measured after the completion of growth in the fourth season of treatment was not affected by CO 2 enrichment as was found by previous biometric estimates, but mean below-ground biomass was slightly stimulated (+ 12%, n.s.). In contrast, net CO 2 uptake per unit land area was strongly stimulated by CO 2 enrichment at the beginning of the experiment, and during the early part of each season. However, the CO 2 stimulation decreased during the later part of each growing season. By year four, also mid-season differences in CO 2 uptake per unit land area had disappeared. Neither microbial biomass, soil respiration in the laboratory, nor in situ land-area-based CO 2 evolution during the 10 week growing season increased under elevated CO 2. The total biomass N-pool and free soil nitrate and ammonium (capture by ion exchange resin bags) remained unaffected, whereas leaf nitrogen concentration was reduced and nonstructural carbohydrate concentration increased under elevated CO 2 in forbs. These differences in tissue composition largely disappeared during senescence and litter formation. Despite low CO 2 responsiveness at ecosystem level, species responses differed in terms of nitrogen, carbohydrates, tillering and flowering, suggesting the possibility for long-term changes in community structure. Addition of NPK equivalent to 40 kg N ha −1 a −1 had massive effects on all plant traits studied, but did not enable stimulated growth under CO 2 enrichment. However, when fertilizer and CO 2 enrichment were provided jointly, soil microbes were stimulated, indicating a co-limitation by carbon and nutrients (most likely nitrogen). Since responses to elevated CO 2 were absent in both warm and cold growing seasons, we conclude that this late successional plant community is carbon saturated at current atmospheric CO 2 concentrations for reasons not directly related to nutrient supply and climate. Perhaps, contrary to our expectation, evolutionary adjustments of this “old” ecosystem to the life conditions at high altitudes caused carbon to become a surplus resource today.