Seed Bank and Understorey Species Composition in a Semi-arid Environment: The Effect of Shrub Age and Rainfall

Understorey vegetation in patches of Retama sphaerocarpa shrubsin semi-arid environments is dependent on the overstorey shrublife history. Community structure changes with shrub age asa result of physical amelioration of environmental conditionsby the canopy and organic matter accumulation in the soil. Weinvestigated the effect of the canopy on understorey speciesdiversity in the field and its relationships with the soil seedbank under 50 shrubs from 5 to 25+ years old, and compared speciescomposition in the field in a wet and a dry year. Species compositionof the soil seed bank under R. sphaerocarpa shrubs did not differsignificantly with shrub age, but seed density increased asthe shrubs aged. In the field, community composition changedwith shrub age, increasing species richness in a process thatdepended on the amount of spring rainfall. Our results suggestthat the soil seed bank is rather uniform and that the shrubcanopy strongly selects which species appear in the understorey.There were seeds of many species present under both young andold shrubs but which only established under old shrubs. Thisshowed dispersal was not limiting species abundance and suggestedthat the canopy was an important sorting factor for speciespresent in the understorey. Less frequent species contributedthe most to patch diversity, and rainfall effectively controlledspecies emergence. Understorey community composition dependedon multiple interspecific interactions, such as facilitationby the shrub and competition from neighbours, as well as ondispersal processes. Facilitation in this environment is a keyfeature in the structuring of plant communities and in governingecosystem functioning. Copyright 2000 Annals of Botany Company Community structure, competition, dispersal, facilitation, species composition, rainfall variability, Retama sphaerocarpa, seed bank, semi-arid environments     

Facilitation in an unproductive boreal forest understorey community

Questions: We tested the hypothesis that if competition had a significant influence in structuring this boreal plant community, removal of neighbours, addition of fertilizer and addition of water would all benefit the transplanted seedlings. Alternatively, if facilitation had a greater influence, then removal of neighbours would be detrimental to the transplants but fertilization and watering would still be beneficial. Location: Understorey of the boreal forest in southwestern Yukon Territory, Canada (138°22′W; 61°02′N). Methods: Ten of the most common species were transplanted as seedlings into transects from which all neighbours had been removed, and also into transects with intact vegetation. We used a factorial design with two levels of watering and two levels of fertilization; this allowed us to test effects at both species and community level. Results: The summed survival and total biomass of all transplants was significantly higher in the presence of neighbours than without neighbours, indicating a facilitative effect of neighbouring plants, but there were significant increases in only six of the ten species. The combined survival and biomass of all species increased with watering, survival decreased and biomass increased with fertilization, but only two species had significant responses to fertilization: Anenome parviflora decreased and Mertensia paniculata increased in biomass. Watering increased the biomass of Achillea millefolium, Festuca altaica and Solidago multiradiata; there were also some interaction effects. Conclusions: (1) The presence of neighbours was generally facilitative. (2) Fertilization had negligible effects, and watering had minor beneficial effects. (3) This study demonstrates the importance of facilitation in structuring this boreal understorey community.     

A Test for the Resource Remobilization Hypothesis: Tree Sprouting using Carbohydrates from Above-ground Parts

To test the resource remobilization hypothesis, i.e. the hypothesisthat some trees sprout from root-collars or from the lower partof trunks using resources obtained from above-ground parts ratherthan from resources reserved in their roots, we conducted cuttingexperiments forEuptelea polyandra, a frequently sprouting treespecies with little carbohydrate reserves in its roots,Quercusserrata,a frequently sprouting tree species with large reservesin the roots, andMallotus japonicus, a rarely sprouting treespecies. Trees of each species were cut down in winter leavingtwo kinds of stumps, those approx. 1.5 m in height and thosecut off near the ground. The number and total dry weight ofnewly sprouted shoots per stump were compared between the twotreatments and among the three species at the end of the followinggrowing season. InE. polyandra,both the number and total dryweight of sprouts per stump were very small for both treatmentsand were similar to, or less than, those ofM. japonicus. Onthe other hand,Q. serratasprouted abundantly in both treatments.These results indicate thatE. polyandracannot sprout sufficientlywithout a considerably large volume of above-ground parts orthat additional structures such as foliage and branches maybe necessary for sprouting. We conclude that the resource remobilizationhypothesis is supported for this species.Copyright 1998 Annalsof Botany Company Euptelea polyandraSieb. et Zacc,Quercus serrataThunb,Mallotus japonicus(Thunb.) Muell. Arg., tree sprouting, cutting experiment, resprouter, resource movement, carbohydrate allocation, ground-surface disturbance, root stock, resource remobilization hypothesis.     

Differences between understorey and canopy in herbivore community composition and leaf quality for two oak species in Missouri

1. From July 1994 to September 1995, at six censuses, the herbivore community associated with understorey (< 2.5 m height) and canopy (15–20 m) leaves of Quercus alba and Q. velutina was sampled in south-eastern Missouri, U.S.A. 2. Across all censuses, herbivore densities were not significantly different between canopy and understorey for Q. alba and Q. velutina, except in August 1994 when herbivore densities were 60% higher in the canopy on Q. alba. Little significant spatial variation in herbivore densities or community composition was found during the study years. 3. The herbivore community was diverse, consisting of 138 species of leaf-chewing insects. Species richness was significantly greater (by 5–20%) in the understorey than in the canopy for both tree species, and the relative abundance of the main families, different feeding guilds, and most common species differed significantly between understorey and canopy. 4. To determine the extent to which leaf quality explained the observed patterns, percentage nitrogen and protein binding capacity were measured in canopy and understorey leaves of Q. alba and Q. velutina. Per cent nitrogen was higher in canopy leaves for Q. velutina while protein binding capacity was higher in canopy leaves for Q. alba. 5. These results suggest that the herbivore community associated with these two species of Quercus comprises species that appear to respond individually to environmental and biological conditions encountered in the understorey and the canopy.     

Growth Consequences of Soil Nutrient Heterogeneity for two Old-field Herbs,Ambrosia artemisiifoliaandPhytolacca americana, Grown Individually and in Combination

Plant species can respond to small scale soil nutrient heterogeneityby proliferating roots or increasing nutrient uptake kineticsin nutrient-rich patches. Because root response to heterogeneitydiffers among species, it has been suggested that the distributionof soil resources could influence the outcome of interspecificcompetition. However, studies testing how plants respond toheterogeneity in the presence of neighbours are lacking. Inthis study, individuals of two species,Phytolacca americanaL.andAmbrosia artemisiifoliaL. were grown individually and incombination in soils with either a homogeneous or heterogeneousnutrient distribution. Above-ground biomass of individuallygrown plants of both species was greater when fertilizer waslocated in a single patch than when the same amount of fertilizerwas distributed evenly throughout the soil. Additionally, bothspecies proliferated roots in high-nutrient patches.A. artemisiifoliaexhibitedlarger root:shoot ratios, increased nitrogen depletion fromnutrient patches, and a higher growth rate thanP. americana,suggestingA. artemisiifoliais better suited to find and rapidlyexploit nutrient patches. In contrast to individually grownplants, soil nutrient distribution had no effect on final above-groundplant biomass for either species when grown with neighbours,even though roots were still concentrated in high nutrient patches.This study demonstrates that increased growth of isolated plantsas a consequence of localized soil nutrients is not necessarilyan indication that heterogeneity will affect interspecific encounters.In fact, despite a significant below-ground response, soil nutrientheterogeneity was inconsequential to above-ground performancewhen plants were grown with neighbours.Copyright 1999 Annalsof Botany Company Phytolacca americana, pokeweed,Ambrosia artemisiifolia, ragweed, nutrient heterogeneity, root proliferation, plasticity, foraging, nutrient patches.     

Do Sprouting Tree Species on Erosion-prone Sites Carry Large Reserves of Resources?

Saplings ofEuptelea polyandra were studied to determine whethertree species found on unstable hillslopes of temperate, old-growthforests in Japan carry substantial storage materials for sproutingreplacement genets, as is the case with resprouter species offire-prone areas. Concentrations (% d. wt basis) of carbohydrates(starch, sucrose, glucose and fructose) contained in roots,stems and leaves were measured in summer and winter.E. polyandrasaplings were compared with those ofQuercus serrata (a frequentlysprouting tree), and those ofMallotus japonicus andIdesia polycarpa(rarely sprouting trees) in the same forest. Total concentrationsof carbohydrates (the sum of starch, sucrose, glucose and fructose)in roots were lowest inE. polyandra in both summer and winter.In addition,E. polyandra had a lower ratio of root biomass tototal plant biomass thanQ. serrata , but similar to that ofthe non-sprouting species,M. japonicus andI. polycarpa . Onthe other hand, the total concentration of carbohydrates inthe above-ground parts were similar in the four species in bothsummer and winter. These results indicate thatE. polyandra hadless long-term storage resources to implement sprouting, inspite of its apparent effectiveness in sprouting. We proposehypotheses to explain the reason whyE. polyandra stores a relativelysmall amount of resources for sprouting. Carbohydrate concentration; Euptelea polyandra Sieb. et Zacc; ground-surface disturbance; Idesia polycarpa Maxim; Mallotus japonicus (Thunb.) Muell. Arg.; Quercus serrata Thunb.; resprouter; root dry weight ratio; soluble sugars; sprouting; starch     

Potential Nitrification as an Indicator of Preferential Uptake of Ammonium or Nitrate by Plants in an Oak Woodland Understorey

The preferences of some woodland understorey species for ammoniumand nitrate were investigated by measuring the potential nitrification(conversion of ammonium to nitrate) in the rhizosphere comparedwith the bulk soil. Less acid-tolerant species, which usuallyprefer nitrate or a mixture of ammonium and nitrate in hydroponicculture, should have a higher potential nitrification in therhizosphere compared to the bulk soil due to a low uptake ofammonium (since ammonium is relatively immobile). Acid-tolerantspecies should have a high uptake of ammonium and thereby loweror equal potential nitrification in the rhizosphere comparedto the bulk soil. The hypothesis was tested in a field investigationof five understorey herb species, Deschampsia flexuosa, Convallariamajalis, Poa nemoralis, Geum urbanum andAegopodium podagrariaperformed in oak forests in southern Sweden. Overall, the twoless acid-tolerant species, Geum urbanum and Aegopodium podagraria,had high potential nitrification in the rhizosphere comparedto the bulk soil (indicating a relatively low uptake of ammonium),whilst the acid tolerant species, Deschampsia flexuosa andConvallariamajalis , had approximately equal potential nitrification inthe rhizosphere compared to the bulk soil (indicating a relativelyhigh uptake of ammonium). In the case of Poa nemoralis, a specieswhich grows in both acid and less acid soils, we found the potentialnitrification in the rhizosphere and in the bulk soil to besimilar at low inorganic nitrogen concentrations, but the difference(rhizosphere > bulk) increased when nitrification in thebulk soil was enhanced (i.e. when the nitrogen availabilityincreased). The potential nitrification in the bulk soil variedbetween 0 and 16 nmol g^-1h^-1and was positively correlated withpH. When species occurred at the same site, the potential nitrificationin the bulk soil tended to be lower for the acid tolerant species.Despite a large variation in potential nitrification, the methodoffers a possibility of measuring the preference of plants forammonium/nitrate in a soil system, under natural conditions.Copyright 2000 Annals of Botany Company Ammonium uptake, nitrate uptake, nitrogen preference, potential nitrification, rhizosphere, Deschampsia flexuosa, Convallaria majalis, Poa nemoralis, Geum urbanum, Aegopodium podagraria     

Microcrustacean communities of high-elevation lakes in the Sierra Nevada, California

Seventy-five high-elevation lakes in the Sierra Nevada mountainsof California were sampled for microcrustacean species and majorion compositions. Cluster analysis was used to delineate fiverecurrent community types. Distributions of both individualspecies and of community types were related to chemical, morphometric,topographic and geologic variables, as well as fish presenceor absence, by stepwise logistic multiple regression. Fish distributionwas an important predictor of the distributions of all individualspecies for which significant regression models could be built;other common predictors included nitrate concentration, elevation,basin area and lake depth. One common community type (n = 22)consisted of Dapbnia rosea, Diaptomus signicauda, Bosmina longirostrisand Holopedium gibberum. A second, more species-rich, community(n = 27) included many of these same species, in addition toCyclopsvemalis, Diaphanasoma brachyurum, Polyphemus pediculis and/orCeriodaphnia affinis. Two further communities (n = 9 and n =6 respectively) contained Daphnia middendorffiana with eitherDiaptomus shoshone or Diaptomus eiseni. A fifth community type(n = 11) either lacked microcrustaceans or contained only Chydorusand/or Alona spp. The distributions of the first four of thesecommunity types could be well predicted on the basis of fishpresence and elevation (community types I and II), or fish absenceand lake depth (community types III and IV). The distributionof the fifth community type was independent of fish presenceor absence; phosphate concentration was the only significantpredictor of this community. ¹Present address: New York City Department of EnvironmentalProtection, PO Box 184, Valhalla, NY 10595, USA     

Temporal patterns and variations in phytoplankton community organization and abundance in Narragansett Bay during 1959-1980

Incident irradiance, surface water temperature and phytoplanktonspecies abundances were measured at weekly intervals in NarragansettBay from 1959 through 1980. Stepwise discriminant analyses (SDA)of this 22-year data set indicate that fundamental ecosystemchanges occurred between the 1960s and 1970s, with 1969 beingthe key transitional year in these decadal shifts in phyto planktontaxonomic structure and seasonal abundance. This decadal shiftwas accompanied by the increased summer abundance of small Thalassiosiraspp., which first appeared in 1966 and by 1969 became the sixthmost important phytoplankton component in this bay. Decadaltrends in phyto plankton community organization and abundancewere also accompanied by distinct long-term climatological gradientsof temperature and light. The 1960s were generally colder andbrighter than the 1970s. Prior to 1969, the annual phytoplanktonmaximum occurred most commonly during winter; in the 1970s,the annual maximum generally shifted to a summer event. Three5-year phytoplankton cycles occurred between 1959 and 1974.During each pentade, the phytoplankton community returned toa similar taxonomic organization and abundance cycle after divergingin the intervening years. Pentade cycles did not occur after1974; the phytoplankton community thereafter diverged significantlyfrom each preceding year. Five species [Skeletonema costatum,Detonula confervacea, Asterionellopsis glacialis, Hererosigmaakashiwo (= Olisthodiscus lureus) and Thalassiosira nordenskioeldii]dominated the phytoplankton over the 22-year period. SDA revealeda high degree of similarity and constancy in the annual occurrencepatterns of these taxa. The decadal shifts revealed by SDA weremore directly related to the considerable interannual variabilitythat characterized the abundance and seasonality of the lessabundant species.     

Plastic Response of Crown Architecture to Crowding in Understorey Trees of Two Co-dominating Conifers

Crown architecture and growth rate of trunk height, trunk diameterand lateral branches of understorey trees (5–10m tall)were compared between two co-dominating conifers,Abies sachalinensisandPiceaglehnii, in relation to the index of local crowding intensity,W,represented as a function of density, distance and basal areaof taller neighbours. For the two conifers, the growth of trunkheight and diameter was decreased and crowns became flat withincreasingW, keeping crown projection area. Self-pruning oflower branches was more intense inAbiesthan inPiceain crowdedconditions, while both conifers showed similar crown forms inless crowded conditions. These results suggest that the growthin lateral branches exceeded that in height in crowded conditions,especially inAbies. Tree age of both conifers increased withincreasingW, resulting from the low growth rate in crowded conditions.The age of the longest and lowest branch ofPicea, up to 150years, was positively correlated with the tree age ranging from70 to 250 years, whereas that ofAbieswas constant at around30 years irrespective of tree age varying from 40 to 140 years.This result agrees with the observation that agedAbieshad moreflat-shaped crowns than in agedPiceain crowded conditions. Theseresults suggest that each conifer adapted to crowding in differentways: high elongation of branches with high turnover rate forAbiesandviceversaforPicea. Abies; crown form; neighbourhood interference; Picea; plasticity     

Understorey gaps influence regeneration dynamics in subtropical coastal dune forest

Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in coastal dune forests, and the effect that its cover has on forest regeneration. We used line transects to quantify the area of the forest understorey with I. woodii cover and with gaps in the cover. Paired experimental plots were established in semi-permanent understorey gaps with I. woodii naturally absent and in adjacent areas with I. woodii present to compare plant community composition, soil, and light availability between the two habitats. Isoglossa woodii was widespread, covering 65–95% of the understorey, while gaps covered the remaining 5–35% of the area. The spatial distribution of this species was strongly related to tree canopy structure, with I.␣woodii excluded from sites with dense tree cover. Seedling establishment was inhibited by low light availability (<1% of PAR) beneath I.␣woodii. When present, I. woodii reduced the density and species richness of tree seedlings. The tree seedling community beneath I. woodii represented a subset of the seedling community in gaps. Some species that were found in gaps did not occur beneath I. woodii at all. There were no significant differences between the sapling and canopy tree communities in areas with I. woodii gaps and cover. In the coastal dune forest system, seedling survival under I. woodii is dependent on a species’ shade tolerance, its ability to grow quickly during I. woodii dieback, and/or the capacity to regenerate by re-sprouting and multi-stemming. We propose a general conceptual model of forest regeneration dynamics in which the abundant understorey species, I. woodii, limits local tree seedling establishment and survival but gaps in the understorey maintain tree species diversity on a landscape scale.     

Spatial and temporal patterns of phytoplankton composition in subtropical coastal lagoon, the Indian River Lagoon, Florida, USA

A 2 year study of the phytoplankton community was carried outin the Indian River Lagoon, USA. In terms of biovolume, thephytoplankton community was generally dominated by dinoflagellates,diatoms or cyanobacteria. Mean phytoplankton standing cropswere highest in the most flow-restricted regions of the lagoon,which had the lowest mean salinity values and comparativelyhigh total nitrogen:total phosphorus ratios. In this region,blooms of dinoflagellates were common in the first year of thestudy, which was characterized by an El Niño event thatyielded exceptionally high rainfall levels and freshwater outflow.Picoplanktonic cyanobacteria blooms became more prominent inthe second year of the study, which was characterized by belowaverage rainfall conditions. In unrestricted flow regions ofthe lagoon, located near inlets to the Atlantic Ocean, diatomswere most often the dominant taxa. Regions of intermediate waterturnover rates and high external loading of phosphorus had aprevalence of diatom blooms. However, the average phytoplanktonstanding crops in the latter regions did not reach the levelsexperienced in the flow-restricted parts of the lagoon. In termsof individual phytoplankton taxa, the most common bloom-formingdiatoms in the Indian River Lagoon system included: Skeletonemacostatum, Dactyliosolen fragilissimus, Skeletonema menzelii,Cerataulina pelagica, Odontella regia, Chaetoceros lorenzianus,Rhizosolenia setigera and Thalassionema nitzschioides. The majorbloom-forming dinoflagellate species included: Pheopolykrikoshartmannii, Akashiwo sanguinea, Prorocentrum micans, the potentiallytoxic species Pyrodinium bahamense var. bahamense and Prorocentrumminimum. Several picoplanktonic cyanobacteria were also prominentmembers of the phytoplankton community, including Synechococcuselongates. The spatial and temporal patterns observed in someof these dominant species were attributable to patterns in keyenvironmental variables, including salinity, temperature andnutrient concentrations.     

How Tough are Sclerophylls?

Fracture toughness was estimated for a 'least tough' path inthe leaves of woody species from three sclerophyllous plantcommunities. Most of the species from Mediterranean, tropicalheath forest and lowland tropical rain forest habitats had verytough leaves, with toughness generally 600-1300 J m^-2, whichis two to four times higher than soft-leaved tropical pioneertrees. The toughest leaf (2032 J m^-2), Parishia insignis, camefrom the canopy of the lowland rain forest. Leaves from theshaded understorey of the rain forest did not appear any lesstough than those from the canopy.Copyright 1993, 1999 AcademicPress Leaf fracture toughness, sclerophylly, Mediterranean vegetation, tropical forest     

Bioenergetic prediction of climate change impacts on northern mammals

Climate change will likely alter the distribution and abundanceof northern mammals through a combination of direct, abioticeffects (e.g., changes in temperature and precipitation) andindirect, biotic effects (e.g., changes in the abundance ofresources, competitors, and predators). Bioenergetic approachesare ideally suited to predicting the impacts of climate changebecause individual energy budgets integrate biotic and abioticinfluences, and translate individual function into populationand community outcomes. In this review, we illustrate how bioenergeticscan be used to predict the regional biodiversity, species rangelimits, and community trophic organization of mammals underfuture climate scenarios. Although reliable prediction of climatechange impacts for particular species requires better data andtheory on the physiological ecology of northern mammals, tworobust hypotheses emerge from the bioenergetic approaches presentedhere. First, the impacts of climate change in northern regionswill be shaped by the appearance of new species at least asmuch as by the disappearance of current species. Second, seasonallyinactive mammal species (e.g., hibernators), which are largelyabsent from the Canadian arctic at present, should undergo substantialincreases in abundance and distribution in response to climatechange, probably at the expense of continuously active mammalsalready present in the arctic.     

The roles of stochasticity and biotic interactions in the spatial patterning of plant species in alpine communities

Questions

Plant community composition can be influenced by multiple biotic, abiotic, and stochastic factors acting on the local species pool to determine their establishment success and abundance and subsequently the diversity of the community. We asked if the influences of biotic interactions on the composition of plant species in communities, as indicated by patterns of plant species spatial associations (independent, positive or negative), vary across a productivity gradient within a single ecosystem type. Do dominant species of communities show spatial patterning suggestive of competitive interactions with interspecific neighbors? Do species that span multiple community types exhibit the same heterospecific interactions with neighbours in each community?

Location

Three alpine communities in the southern Rocky Mountains.

Methods

We measured the occurrence of species in a 1‐cm spatial grid within 2 m × 2 m plots to determine the spatial patterns of species pairs in the three communities. A null model of independent species spatial arrangements was used to determine whether species pairs were positively, negatively or independently associated, and how these patterns differed among the communities across the gradient of resource supply and environmental stress.

Results

Positive associations, indicative of facilitation between species, were most common in the most resource‐poor and least productive community. However negative associations, suggestive of competitive interactions among species, were not more common in the two more resource‐rich, productive communities. The dominant species of these communities did exhibit higher negative than positive associations with neighbours relative to positive patterning. Independent interspecific patterning was equally common relative to positive and negative patterns in all communities. Species that previously were shown to either facilitate other species or compete with neighbours exhibited spatial patterning consistent with the earlier experimental work.

Conclusions

A large number of species exhibit a lack of net biotic interactions, and stochastic factors appear to be as important as competition and facilitation in shaping the structure of the three alpine plant communities we studied.

     

Architectural strategies of Cornus sericea, a native but invasive shrub of Southern Quebec, Canada, under an open or a closed canopy

Background and Aims

Qualitative and quantitative studies of the pattern of invasive plant development is considered a key aspect in understanding invasiveness. An architectural analysis was therefore performed in order to understand the relationship between shoot architecture and invasiveness in red-osier dogwood, Cornus sericea (Cornaceae).

Methods

The structural and ontogenic characteristics of individuals in invading and non-invading populations in the native range of the species were compared to test the implication of developmental plasticity on invasiveness.

Key Results and Conclusions

The results show that the shrub has a modular architecture governed by strong developmental rules. Cornus sericea is made up of two levels of organization, each with its own intrinsic sequence of differentiation. These intrinsic mechanisms were used as a framework for comparison and it was found that, in response to the light environment, developmental plasticity was elevated, resulting in two architectural strategies. This developmental plasticity concerns the growth direction and the size of the modules, the speed of their time-course changes, their branching and flowering. Under an open canopy, C. sericea rapidly develops large vertical structures and abundant flowering. This strategy leads the plant to be invasive by excluding competitors and disseminating in the landscape. In the understorey, C. sericea slowly develops long horizontal structures which creep across the soil surface, while assimilating structures are poorly developed. This strategy does not lead to invasiveness but may allow the plant to survive in the understorey and reach sunny patches.     

The influence of canopy cover on understorey development in forests of the western Cascade Range,Oregon, USA

Natural disturbances, especially fire and treefalls, influence tree canopy composition in the Pseudotsuga menziesii forests of the western Cascade Range, Oregon. The composition of tree, shrub, and herb assemblages in the understorey of stands with different canopy types, such as maturing Pseudotsuga, Tsuga heterophylla, or mixed species stands, also differs.Differences in both canopy type and the prevalence of canopy openings correlated with different degrees of understorey development in stands of similar ages. This suggests that understorey assemblages also reflect disturbance history. Before understorey assemblages can be used to relate community samples to community or habitat types, the extent to which their composition reflects long term influences of stand history vs. differences in site potential must be determined.     

Tree seed germination and seedling establishment in treefall gaps and understorey in a subtropical forest of northeast India

Abstract Seed germination, and survival and growth of seedlings of four dominant tree species, Quercus dealbata, Quercus griffithii, Quercus glauca and Schima khasiana were studied in the treefall gaps and forest understorey of an undisturbed mature-phase humid subtropical broadleaved forest in northeast India. Three important microenvironmental factors namely photosynthetically active radiation (PAR), soil moisture and litter depth, were also measured in the forest understorey and gaps and correlated with seedling mortality. Seed germination of S. khasiana was significantly higher in the treefall gaps than in the understorey; among the tree species studied, it had the highest germination. Quercus seedlings were abundant in the understorey and small gaps, while S. khasiana seedlings were more numerous in the large gaps. The survivorship curves for the seedling populations revealed that the three Quercus species survived better in the understorey, while S. khasiana did so in the gaps. PAR and soil moisture were positively correlated with tree seedling mortality, which occurred mainly during the winter months. The Quercus seedlings grew better in the forest understorey and small gaps and S. khasiana seedlings in the large gaps. The differential performance of the tree seedlings to the conditions prevailing in the understorey and gaps of two sizes indicates that different species were adapted to different light environments depending upon their optimum requirements. This could be an effective mechanism for promoting species coexistence in the forest community.     

How old are Wet Forest understories?

Abstract Radiocarbon ages were derived from 12 individuals representing four species of woody understorey plants growing in the Eucalyptus regnans dominated Wet Forests of Victoria's Central Highlands. Cyathea australis, Dicksonia antarctica and Olearia argophylla are common and often dominate the understorey. The fourth species, Persoonia arborea, is a small understorey tree endemic to this region. Field observations and radiocarbon ages for these four species (up to 370 ± 70 years BP) indicate they are capable of surviving fire and are long-lived in this environment.     

Vegetation type determines heterotrophic respiration in subalpine Australian ecosystems

Soils are the largest store of carbon in the biosphere and cool‐cold climate ecosystems are notable for their carbon‐rich soils. Characterizing effects of future climates on soil‐stored C is critical to elucidating feedbacks to changes in the atmospheric pool of CO₂. Subalpine vegetation in south‐eastern Australia is characterized by changes over short distances (scales of tens to hundreds of metres) in community phenotype (woodland, shrubland, grassland) and in species composition. Despite common geology and only slight changes in landscape position, we measured striking differences in a range of soil properties and rates of respiration among three of the most common vegetation communities in subalpine Australian ecosystems. Rates of heterotrophic respiration in bulk soil were fastest in the woodland community with a shrub understorey, slowest in the grassland, and intermediate in woodland with grass understorey. Respiration rates in surface soils were 2.3 times those at depth in soils from woodland with shrub understorey. Surface soil respiration in woodlands with grass understorey and in grasslands was about 3.5 times that at greater depth. Both Arrhenius and simple exponential models fitted the data well. Temperature sensitivity (Q₁₀) varied and depended on the model used as well as community type and soil depth – highlighting difficulties associated with calculating and interpreting Q₁₀. Distributions of communities in these subalpine areas are dynamic and respond over relatively short time‐frames (decades) to changes in fire regime and, possibly, to changes in climate. Shifts in boundaries among communities and possible changes in species composition as a result of both direct and indirect (e.g. via fire regime) climatic effects will significantly alter rates of respiration through plant‐mediated changes in soil chemistry. Models of future carbon cycles need to take into account changes in soil chemistry and rates of respiration driven by changes in vegetation as well as those that are temperature‐ and moisture‐driven.