Growth Rings of Roots in Perennial Forbs in Duolun Grassland, Inner Mongolia, China

Annual growth rings of roots in perennial forbs have been used in studies of climate change and the ecology of grasslands. However, little has been done in this aspect of research in China. In this study, we report the characteristics of growth rings in the main roots of 13 herb species sampled in Duolun of the Inner Mongolia grassland in northern China. The results show that around two thirds of the species possess clearly demarcated annual rings in the root xylem. Some species of the same genera show different patterns in anatomical structure of the root xylem. Standardized annual ring widths of three species, Potentilla anserine L., Cymbaria dahurica L. and Lespedeza daurica （Laxm.） Schindh, show a common linear trend, indicating a continued favorable growth condition in the sampling sites. Our results provide evidence that growth rings in roots of some perennial forbs in the Inner Mongolia grassland can serve as a new and useful indicator of past changes in the grassland environment.     

Comparative analysis of annual rings of perennial forbs in the Loess Plateau,China

Although recent studies have demonstrated that annual growth rings are present among perennial forbs species at high northern latitudes, little is known about whether there are demarcated growth rings of perennial forbs in the Loess Plateau of China where plant growth is strongly limited by dry climate conditions and severe soil erosion. In this study, we collected the main roots of 11 perennial forbs species along the precipitation gradient in the Loess Plateau, and analyzed the growth rings in the secondary root xylem. We found that ten species showed distinct annual growth rings, and the anatomical patterns, including vessel size and density, varied considerably among different families. Our results suggest, for forbs species in the Loess Plateau, that vessel diameter in the root xylem was strongly correlated with growth rate of the forb’s roots. Ring widths of the forbs showed a significant declining trend, reflecting the deteriorating signal of growth condition with age. In comparison to other families, forb species of Fabaceae usually have the evidently larger vessels that link directly to higher hydraulic capacity and growth rate. In terms of annual ring width patterns, this study provides an applicable approach to detecting effects of limited climatic conditions and life history strategies on herbaceous vegetation in the Loess Plateau.     

Growth rings in the roots of temperate forbs are robust annual markers

There is increasing interest in the analysis of annual growth rings in the secondary root xylem of perennial forbs (herb-chronology). Therefore, we need to verify whether these growth rings are always formed annually. To investigate the formation of root rings we performed common garden experiments at two distinct sites in Switzerland. We grew nine unrelated forb species from seed and subjected them to competition and clipping treatments. Anatomical developments in the roots of the individuals were tracked during five growing seasons. Across all species and treatments at least 94 % of the expected growth rings associated with full growing seasons were identifiable and the development of the anatomical patterns was consistently seasonal. While the distinctness of annual rings varied somewhat between species and sites, the treatments had no effect on the presence of annual rings. In no case were false rings developed. The results of this study demonstrate that the growth rings in the roots of northern temperate forbs represent robust annual growth increments and, hence, can reliably be used in herb-chronological studies of age- and growth-related questions in plant ecology.     

Climatic signals in tree‐rings of Araucaria angustifolia in the southern Brazilian highlands

Araucaria angustifolia (Bertol.) O. Kuntze (Araucariaceae) is a Neotropical tree, widely distributed in subtropical mountain rain forests and nearby natural grasslands of Southern Brazil. This species produces annual growth rings, but its dendroclimatic potential is barely known. In the present paper, the long‐term growth patterns of A. angustifolia were investigated using annual growth ring time series and association to climate over the last century. Wood cores of A. angustifolia trees growing in forest and grassland habitats were obtained with an increment borer. The cores were surfaced, measured and cross‐dated. The dated ring‐width time series were standardized and submitted to correlation and principal component analysis to verify growth trends among sites and trees. Growth‐climate relationships were investigated using correlation and regression analyses, comparing the ordination axes scores to regional time series of precipitation and temperature. Due to anatomical irregularities, mainly partial rings, only 35 out of 60 trees were cross‐dated. The correlation and ordination analyses showed common tree‐growth trends within and between sites, indicative of a regional environmental force determining inter‐annual cambial activity variation. Despite growing in distinct habitats and disturbance regimes, A. angustifolia trees share a common long‐term growth pattern, which is significantly related to thermal conditions during the current and previous growing seasons. Moreover, site‐specific characteristics may have influenced opposite growth responses and association to climate conditions between forest and grassland trees.     

Age-determination of Dicotyledonous Herbaceous Perennials by Means of Annual Rings: Exception or Rule?

Thirty-five herbaceous dicotyledonous perennial plant species,with permanent root systems, from 16 families, were examinedfor the presence of growth rings in the secondary root xylem.Most of the species surveyed showed ring zonations in the roots,and these could be verified as annual growth rings in the tenspecies for which plants of known age were available. The potentialvalue of ‘herbchronology’ as a tool in ecologicalinvestigations of species and stands of perennial herbs of temperatezones is discussed. Annual rings; age-determination; dendroecology; herbaceous perennials; anatomical patterns; secondary root xylem     

Nonannual tree rings in a climate‐sensitive Prioria copaifera chronology in the Atrato River,Colombia

In temperate climates, tree growth dormancy usually ensures the annual nature of tree rings, but in tropical environments, determination of annual periodicity can be more complex. The purposes of the work are as follows: (1) to generate a reliable tree‐ring width chronology for Prioria copaifera Griseb. (Leguminoceae), a tropical tree species dwelling in the Atrato River floodplains, Colombia; (2) to assess the climate signal recorded by the tree‐ring records; and (3) to validate the annual periodicity of the tree rings using independent methods. We used standard dendrochronological procedures to generate the P. copaifera tree‐ring chronology. We used Pearson correlations to evaluate the relationship of the chronology with the meteorological records, climate regional indices, and gridded precipitation/sea surface temperature products. We also evaluated 24 high‐precision ¹⁴C measurements spread over a range of preselected tree rings, with assigned calendar years by dendrochronological techniques, before and after the bomb spike in order to validate the annual nature of the tree rings. The tree‐ring width chronology was statistically reliable, and it correlated significantly with local records of annual and October–December (OND) streamflow and precipitation across the upper river watershed (positive), and OND temperature (negative). It was also significantly related to the Oceanic Niño Index, Pacific Decadal Oscillation, and the Southern Oscillation Index, as well as sea surface temperatures over the Caribbean and the Pacific region. However, ¹⁴C high‐precision measurements over the tree rings demonstrated offsets of up to 40 years that indicate that P. copaifera can produce more than one ring in certain years. Results derived from the strongest climate–growth relationship during the most recent years of the record suggest that the climatic signal reported may be due to the presence of annual rings in some of those trees in recent years. Our study alerts about the risk of applying dendrochronology in species with challenging anatomical features defining tree rings, commonly found in the tropics, without an independent validation of annual periodicity of tree rings. High‐precision ¹⁴C measurements in multiple trees are a useful method to validate the identification of annual tree rings.     

Comparative analysis of growth rings in perennial forbs grown in an Alpine restoration experiment

Recent studies have demonstrated that growth rings are widespread in the roots of forbs, and there is evidence that the rings are formed annually. However, the annual nature and development of the growth rings has not yet been examined in comparative experimental studies. In this study growth rings were analysed in the main roots of four alpine forbs (Lotus alpinus, Trifolium thalii, Silene willdenowii and Potentilla aurea) that were grown in an alpine restoration experiment for 6 years. All individuals of L. alpinus and T. thalii, and some individuals of S. willdenowii showed six clearly demarcated growth rings, demonstrating that the rings were formed annually. P. aurea did not show distinguishable growth rings. In L. alpinus and T. thalii there were fluctuations in growth ring width that were consistent between individuals and also between species, and matched variations in climatic growth conditions. Results of the present study indicate that conclusions drawn from previous studies suggesting that growth rings in the roots of forb species are most likely formed annually are also valid for alpine plants. In terms of annual ring width patterns, this study also provides the first strong evidence for consistent responses of different forb species and individuals to commonly experienced variations in habitat conditions.     

Grassland composition,structure, and diversity patterns along major environmental gradients in the Central Tien Shan

What species and traits signal vegetation types along prominent environmental gradients in the Central Tien Shan and what are the corresponding diversity patterns? Vegetation was sampled at 41 sites throughout the Kyrgyz Republic using quadrats stratified throughout a 1,000-m² sample area. Relationships among major environmental gradients, vegetation structure, and species composition were explored with nonmetric multidimensional scaling. Species distributions were examined to characterize phytogeographic patterns. Seven vegetation types ranging from desert grassland to meadow steppe were identified with cluster analysis, ordered primarily along elevation/mean annual temperature gradients. Four arid grassland types were distinguished, ranging mainly from 900 to 1,700 m elevation, and characterized by co-dominance of grasses and forbs with secondary dominance by shrubs. Annual and biennial forbs equaled perennial forbs in total importance. Grasses include C3 and C4 species. Three montane grassland types were recognized and characterized by co-dominance of perennial C3 grasses and forbs. Transition to montane steppe occurred from 1,500 to 1,900 m and is correlated with absence of C4 grasses and dominance of Festuca valesiaca. Highest diversity was found at intermediate elevations, from 1,800 to 2,600 m, in meadow steppe habitats. Forty-six percent of 580 identified species are Middle Asian endemics and remaining species primarily have distributions including Eastern Europe, the Caucasus, and western Siberia. Although grassland degradation from overgrazing has been chronic throughout the region, grasslands are widespread throughout the Kyrgyz Republic and many, particularly mid-elevation meadow steppes, retain high levels of native species diversity.     

Age-specific responses to climate identified in the growth of Quercus alba

The objective of this research was to determine whether the dendroclimatic responses of young Quercus alba (aged 29–126 years) differ from those of old Q. alba (149–312 years). We collected Q. alba increment cores across a range of size classes from Buffalo Mountain Natural Area Preserve, an oak-hickory forest in southcentral Virginia, USA. Tree cores were crossdated and raw ring widths were detrended to remove the influence of increasing circumference with age, microsite, and local stand dynamics. Standardized ring widths were averaged to develop two master chronologies from the 20 oldest and youngest trees. Ring-width indices were correlated with temperature, precipitation, and Palmer Drought Severity Index (PDSI). Annual tree-ring growth in old and young Q. alba was significantly correlated with precipitation from the previous growing season, but was not significantly correlated with temperature. Only the old trees showed a significant correlation between annual ring width and PDSI. These results may indicate that growth in old trees is more sensitive to drought than in young trees. If future climate change includes the predicted increase in mid-growing season droughts, tree-level responses are likely to be age-dependent with older trees experiencing relatively greater reductions in growth.     

Reducing Competitive Suppression of a Rare Annual Forb by Restoring Native California Perennial Grasslands

Populations of the rare annual forb Amsinckia grandiflora may be declining because of competitive suppression by exotic annual grasses, and may perform better in a matrix of native perennial bunchgrasses. We conducted a field competition experiment in which Amsinckia seedlings were transplanted into forty 0.64‐m² experimental plots of exotic annual grassland or restored perennial grassland. The perennial grassland plots were restored using mature 3 cm‐diameter plants of the native perennial bunchgrass Poa secunda planted in three densities. The exotic annual grassland plots were established in four densities through manual removal of existing plants. Both grass types reduced soil water potential with increasing biomass, but this reduction was not significantly different between grass types. Both grass types significantly reduced the production of Amsinckia inflorescences. At low and intermediate densities (dry biomass per unit area of 20–80 g/m²), the exotic annual grasses reduced Amsinckia inflorescence number to a greater extent than did Poa, although at high densities (>90 g/m²) both grass types reduced the number of Amsinckia inflorescences to the same extent. The response of Amsinckia inflorescence number to Poa biomass was linear, whereas the same response to the annual grass biomass is logarithmic, and appeared to be related to graminoid cover. This may be because of the different growth forms exhibited by the two grass types. Results of this research suggest that restored native perennial grasslands at intermediate densities have a high habitat value for the potential establishment of the native annual A. grandiflora.     

Dendrochronology in Coniferous forests around Lake Rara,West Nepal

In two forest stands, one domonated byAbies spectabilis, and the other byPinus wallichiana-Picea smithiana, 198 cores were taken from 105 conifers in May 1983 and the annual ring widths were measured. The annual ring widths usually had significant similarities between cores taken from the same tree and with cores taken from different trees. these similarities increased with tree size. The climatic change affected the large trees more strongly than it did the small trees. Micro environmental changes, such as canopy gap affected the small trees more strongly. Annual ring widths were also correlated with the annual precipitations at Jumla 30 km south of the plots for a recent 20 year period. A multiple regression analysis between ring width and seasonal precipitation showed that the growth ofA. spectabilis was correlated primarily with the rain from May to August and secondarily with that from September to December in the previous year.     

Restoration of Native Perennials in a California Annual Grassland after Prescribed Spring Burning and Solarization

Grasslands dominated by exotic annual grasses have replaced native perennial vegetation types in vast areas of California. Prescribed spring fires can cause a temporary replacement of exotic annual grasses by native and non‐native forbs, but generally do not lead to recovery of native perennials, especially where these have been entirely displaced for many years. Successful reintroduction of perennial species after fire depends on establishment in the postfire environment. We studied the effects of vegetation changes after an April fire on competition for soil moisture, a key factor in exotic annual grass dominance. As an alternative to fire, solarization effectively kills seeds of most plant species but with a high labor investment per area. We compared the burn to solarization in a study of establishment and growth of seeds and transplants of the native perennial grass Purple needlegrass (Nassella pulchra) and coastal sage species California sagebrush (Artemisia californica). After the fire, initial seed bank and seedling densities and regular percent cover and soil moisture (0–20 cm) data were collected in burned and unburned areas. Burned areas had 96% fewer viable seeds of the dominant annual grass, Ripgut brome (Bromus diandrus), leading to replacement by forbs from the seed bank, especially non‐native Black mustard (Brassica nigra). In the early growing season, B. diandrus dominating unburned areas consistently depleted soil moisture to a greater extent between rains than forbs in burned areas. However, B. diandrus senesced early, leaving more moisture available in unburned areas after late‐season rains. Nassella pulchra and A. californica established better on plots treated with fire and/or solarization than on untreated plots. We conclude that both spring burns and solarization can produce conditions where native perennials can establish in annual grasslands. However, the relative contribution of these treatments to restoration appears to depend on the native species being reintroduced, and the long‐term success of these initial restoration experiments remains to be determined.     

Assessment of woody species encroachment in the grasslands of Nechisar National Park,Ethiopia

Ecological survey was executed to assess woody species encroachment into the grassland plain of Nechisar National Park (NNP). Forty‐one woody species were recorded. Dichrostachys cinerea Wight & Arn., Acacia mellifera (Vahl) Benth., Acacia nilotica (L) Willd., Acacia senegal (L.) Willd., Acacia seyal Del. and Acacia tortilis (Forssk.) Hayne were among the major encroaching woody species. The majority of the woody species were found to be highly aggregated in their pattern of distribution, while only few species showed some degree of randomness. The mean woody species density was ca. 1995 woody plants ha^?1. Mean cover of woody, grass, unpalatable forbs and total herbaceous species were 31%, 58%, 68% and 121%, respectively. The woody species density and cover, unpalatable forbs and bare land cover were significantly higher in the highly grazed and fire‐suppressed part of the grassland plain. Pearson correlation coefficient matrix indicated that woody species cover and density were negatively correlated with total herbaceous and grass cover. The high woody, unpalatable forbs and bare land cover indicated the progressively increasing perennial grass species diversity deterioration in the grass plain of the Park. Decline in the grassland condition, unless reversed, will jeopardize the biological diversity as well as the aesthetic value of the NNP.     

Effects of Established Perennial Grasses on Introduction of Native Forbs in California

Prairie restoration is not complete without the establishment of both grasses and forbs. However, if desirable forbs and grasses are seeded simultaneously, control of broadleaf weeds is problematic. If possible, a two‐step process of introducing forbs after establishing grasses would allow use of broadleaf‐specific herbicides at the critical early stages of grass growth. We conducted experiments to investigate methods for introducing forbs into previously restored native perennial grasslands on rural roadsides in the Sacramento Valley of California. In one experiment, we studied the effects of background vegetation (established perennial grasses or tilled ground) on seven native forb species planted from seed. In a second experiment, we evaluated the effects of background vegetation (existing perennial grasses or tilled ground) and container size (36 ml or 105 ml) with excavation technique (excavation by core removal [core] or by creating an impression [dibble]) on the growth of transplants of the native perennial forbs Asclepias fascicularis and Sisyrinchium bellum. The presence of established perennial grasses reduced the growth of seeded forbs, but did not affect transplants, indicating the vulnerability of seedling forbs to interference. When compared to control plots that had been tilled in the autumn, weed canopy cover was significantly lower in the presence of perennial grasses if seeded with forbs, but not in the presence of perennial grasses alone. Both transplanted species grew better in the large container/core treatment than the small container/dibble treatment; however, existing grasses eliminated these positive effects. Asclepias fascicularis performed better when grown in large containers than in small containers, but its growth was not affected by excavation method; S. bellum performed better when planted with the core method than the dibble method of excavation, but container size made no difference. We attribute differences in the responses of the species to interactions between phenological differences and expansive clay soils that naturally de‐compact upon drying.     

Disturbance: a double-edged sword for restoration in a changing climate

Ecological restoration often relies on disturbance as a tool for establishing target plant communities, but disturbance can be a double-edged sword, at times initiating invasion and unintended outcomes. Here we test how fire disturbance, designed to enhance restoration seeding success, combines with climate and initial vegetation conditions to shift perennial versus annual grass dominance and overall community diversity in Pacific Northwest grasslands. We seeded both native and introduced perennial grasses and native forbs in paired, replicated burned-unburned plots in three sites along a latitudinal climate gradient from southern Oregon to central-western Washington. Past restoration and climate manipulations at each site had increased the variation of starting conditions between plots. Burning promoted the expansion of extant forbs and perennial grasses across all sites. Burning also enhanced the seeding success of native perennial grass and native forbs at the northern and central site, and the success of introduced perennial grasses across all three sites. Annual grass dominance was driven more by latitude than burning, with annuals maintaining their dominance in the south and perennials in the north. At the same time, unrestored grasslands surrounding all sites remained dominated by perennial grasses, suggesting that initial plot clearing may have allowed for annual grass invasion in the southern site. When paired with disturbance, further warming may increase the risk of annual grass dominance, a potentially persistent state.     

The type of competition modulates the ecophysiological response of grassland species to elevated CO2 and drought

The effects of elevated CO₂ and drought on ecophysiological parameters in grassland species have been examined, but few studies have investigated the effect of competition on those parameters under climate change conditions. The objective of this study was to determine the effect of elevated CO₂ and drought on the response of plant water relations, gas exchange, chlorophyll a fluorescence and aboveground biomass in four grassland species, as well as to assess whether the type of competition modulates that response. Elevated CO₂ in well‐watered conditions increased aboveground biomass by augmenting CO₂ assimilation. Drought reduced biomass by reducing CO₂ assimilation rate via stomatal limitation and, when drought was more severe, also non‐stomatal limitation. When plants were grown under the combined conditions of elevated CO₂ and drought, drought limitation observed under ambient CO₂ was reduced, permitting higher CO₂ assimilation and consequently reducing the observed decrease in aboveground biomass. The response to climate change was species‐specific and dependent on the type of competition. Thus, the response to elevated CO₂ in well‐watered grasses was higher in monoculture than in mixture, while it was higher in mixture compared to monoculture for forbs. On the other hand, forbs were more affected than grasses by drought in monoculture, while in mixture the negative effect of drought was higher in grasses than in forbs, due to a lower capacity to acquire water and mineral nutrients. These differences in species‐level growth responses to CO₂ and drought may lead to changes in the composition and biodiversity of the grassland plant community in future climate conditions.     

Effectiveness of repeated autumn and spring fires for understorey restoration in weed‐invaded temperate eucalypt woodlands

Question. Can strategic burning, targeting differing ecological characteristics of native and exotic species, facilitate restoration of native understorey in weed‐invaded temperate grassy eucalypt woodlands? Location. Gippsland Plains, eastern Victoria, Australia. Methods. In a replicated, 5‐year experimental trial, the effects of repeated spring or autumn burning were evaluated for native and exotic plants in a representative, degraded Eucalyptus tereticornis grassy woodland. Treatments aimed to reduce seed banks and modify establishment conditions of exotic annual grasses, and to exhaust vegetative reserves of exotic perennial grasses. Treatments were applied to three grassland patch types, dominated by the native grass Austrodanthonia caespitosa, ubiquitous exotic annuals, or the common exotic perennial grass Paspalum dilatatum. Results. The dominant native grass Austrodanthonia caespitosa and native forbs were resilient to repeated fires, and target exotic annuals and perennials were suppressed differentially by autumn and spring fires. Exotic annuals were also suppressed by drought, reducing the overall treatment effects but indicating important opportunities for restoration. The initially sparse exotic geophyte Romulea rosea increased in cover with fire and the impact of this species on native forbs requires further investigation. There was minimal increase in diversity of subsidiary natives with fire, probably owing to lack of propagules. Conclusions. While fire is often considered to increase ecosystem invasibility, our study showed that strategic use of fire, informed by the relative responses of available native and exotic taxa, is potentially an effective step towards restoration of weed‐invaded temperate eucalypt woodlands.     

Climate and the evolution of annual/perennial life-histories in Nemesia (Scrophulariaceae)

The Cape Floristic Region and the Succulent Karoo in southwestern Africa are both noted for their plant species richness and high levels of endemism. The southwestern tip of Africa is one of the world's five Mediterranean-type climate regions. The biodiversity in the Cape Floristic Region and Succulent Karoo is thought to be at least partly due to changes to the climate of these regions that have occurred since the middle Miocene. Annual species are usually a significant proportion of local flora in Mediterranean-type climate regions. Previous studies of species radiations in the Cape Floristic Region have concentrated on genera that predominantly contain perennial species. Nemesia (Scrophulariaceae) comprises c. 65 species of annual and perennial herbs and sub-shrubs that are native to southern and tropical Africa. Annuals make up a significant proportion (~75%) of Nemesia species. We have reconstructed a phylogeny of 23 Nemesia species using nucleotide sequences of the ITS, ETS and trnL-spacer regions. Species were grouped into five clades, two composed of annual species, one that contained two annual and one perennial species, one that contained one annual and two perennial species, and one that was predominantly composed of perennial species. Phylogenetic dating of the ITS + ETS based phylogenetic tree using penalised likelihood suggested the genus evolved during the Miocene, and that the majority of extant Nemesia species studied radiated during the Pliocene. Ancestral state reconstruction supports at least three separate origins of the annual habit from plants with a perennial life history. One origin can be traced to the late Miocene while the other two transitions occurred more recently during the Pliocene. The transition from perennial to annual life-histories in Nemesia may have been a response to climate change.     

The Role of Light and Soil Moisture in Plant Community Resistance to Invasion by Yellow Starthistle (Centaurea solstitialis)

To resist establishment by an invasive plant, a community may require one or more species functionally similar to the invader in their resource acquisition pattern. In this study, communities consisting of native winter annual forbs, non‐native annual grasses, native perennials, or a combination of the two native communities were established with and without Centaurea solstitialis to determine the effect of soil moisture and light availability on plant community invasion resistance. The annual plant communities were unable to resist invasion by C. solstitialis. In the native winter annual forb community, senescence in late spring increased light penetration (>75%) to the soil surface, allowing seeded C. solstitialis to quickly establish and dominate the plots. In addition, native annual forbs utilized only shallow soil moisture, whereas C. solstitialis used shallow and deep soil moisture. In communities containing native perennials, only Elymus glaucus established well and eventually dominated the plots. During the first 2 years of establishment, water use pattern of perennial communities was similar to native annual forbs and resistance to invasion was associated with reduced light availability during the critical stages of C. solstitialis establishment. In later years, however, water use pattern of perennial grass communities was similar or greater than C. solstitialis‐dominated plots. These results show that Central Valley grasslands that include E. glaucus resist C. solstitialis invasion by a combination of light suppression and soil water competition. Spatiotemporal resource utilization patterns, and not just functional similarity, should be considered when developing restoration strategies to resist invasion by many non‐native species.