The distribution of regeneration makes it possible to assess whether populations of tree species will maintain or change their distributions. For Neotropical dry forests there is little information on the potential changes in the distribution of tree species. Here, we evaluate the potential distributions of adults and seedlings of eight timber tree species of the Piedmont Forest of north-western Argentina by recording the presence of seedlings and adults in plots and modelling with MaxEnt software using three bioclimatic variables. The potential distribution areas of seedlings and adults and the percentage of overlap of seedlings with respect to adults were calculated. The potential distribution for adults was 694 457 ± 62 535 ha, and this figure was 656 564 ± 194 769 ha for seedlings. The potential distribution of seedlings of Calycophyllum multiflorum covered the smallest area (184 496 ha) and had the least overlap with the adults (18%). The difference in the overlap of the potential distribution areas between adults and seedlings suggests that there could be changes in the future distribution of this tree species and C. multiflorum should therefore be the focus of conservation strategies so that the species can follow its bioclimatic niche as the climate changes.

Tropical dry forests (TDF) are one of the most threatened and poorly protected ecosystems in the Americas. Although there are international efforts for the restoration of TDF, how stress factors such as herbivory or water limitation due to changes in precipitation, impact the regeneration dynamics of these forests is poorly understood. Specifically, how seedlings of key tree species for TDF restoration cope with current abiotic pressures such as the intensification of climatic events, and biotic factors like herbivory, is not yet fully understood. Here, we compared seedling performance, and allocation of biomass, and water to roots vs. shoots for three legume, and one non-legume TDF tree species, as a response to water limitation and herbivory in an 8-month greenhouse experiment. Contrary to our expectations, we found that the non-legume species, G. ulmifolia, had the best performance compared to legumes, while N-fixing and non-fixing legumes showed similar performance. Based on our findings, we suggest the use of G. ulmifolia in TDF restoration projects due to its high performance despite abiotic and biotic stress factors, its allocation of biomass and water to belowground structures. We also recommend the use of N-fixing legume species owing to their ability to fix nitrogen, which guarantees an N input to the soil, important in the first stages of succession. However, the legume species used in this experiment do not appear to resist the abiotic and biotic stressors studied. Thus, more studies exploring the response of dry forest plant species to stress factors are key for informing and assuring more effective TDF restoration efforts.

Invasive plants may dramatically impact forest ecosystems by establishing dense populations and suppressing the recruitment of native tree species. One invasive shrub currently spreading throughout eastern deciduous forests of North America, Japanese barberry (Berberis thunbergii DC), may be limiting tree recruitment in stands where it invades. Once established, B. thunbergii becomes densely populated within forest understories and suppresses native plants by competing for limited resources, altering soil conditions, and changing the understory microclimate. To quantify native seedling inhibition caused by B. thunbergii invasion, we conducted an observational study on seedling abundance within forest plots that were either invaded or not invaded by B. thunbergii and used survey data to generate Bayesian models of native seedling densities along gradients of increasing B. thunbergii stem counts and aboveground plant dry mass. Model outputs predicted that B. thunbergii–invaded plots had 82% lower seedling densities compared with uninvaded plots. Native tree seedling densities were very low even in areas with moderate B. thunbergii density, suggesting that reduced tree seedling densities are observed even at low densities of invasion. Our findings indicate that forests invaded with B. thunbergii harbor substantially lower densities of native tree seedlings, with potentially significant long-term consequences for forest ecological integrity, biodiversity, and ecosystem services.

In laboratory experiments, we studied barnyardgrass, common lambsquarters, and redroot pigweed seed dormancy release by alternating temperatures under different soil water regimes, to determine whether that response serves as a mechanism for assuring seedling establishment. Alternating temperatures were important in breaking seed dormancy in the three weeds, but the species' response to alternating temperatures differed with soil water content. For redroot pigweed and newly dispersed seeds of barnyardgrass, dormancy breaking by alternating temperatures occurred only when soil water content was high enough for germination. Moreover, adverse incubation conditions (constant temperature and low water content) induced secondary dormancy in these seeds. Seventy percent of seeds incubated in open boxes emerged before the soil reached wilting point, when seeds were incubated at temperatures high enough for a high germination rate. A dormancy breaking mechanism that triggers a high rate of germination may be important in assuring success of seedling establishment where rainfall patterns result in wetting and drying cycles of the soil, especially in soil types that lose water rapidly from the surface layers.

Bioeconomic weed management models help growers achieve appropriate weed management with less herbicide by matching management to the weed population in a field. Growers, however, will not use bioeconomic models unless cost-effective methods to sample their weed populations are identified. Counting and identifying seeds and seedlings is the most time-consuming and costly part of sampling weed populations. The time required for this process was investigated and modeled as a first step in developing sampling plans for growers using WEEDCAM, a bioeconomic model for weed management in Zea mays L. in Colorado. The time required to count and identify seeds or seedlings was recorded for 9,405 soil cores (5 cm in diameter and 10 cm deep) and 9,726 quadrats (18-cm band over 1.52 m of crop row) collected or examined in eight corn fields in eastern Colorado. The time required to count and identify seeds was best described using a log-linear regression with time increasing with the number of seeds and species and the amount of sand in the core. The average cost of determining there are no seeds in a core is $1.07 for a core from a field with 37% sand and $4.32 if the field has 88% sand. The average cost of counting and identifying 36 seeds of four species is $2.70 and $10.88 for cores with 37 and 88% sand, respectively. The time required to count and identify seedlings was best described using a log-linear regression with time increasing with the number of seedlings and species. Classifying seedlings as grass or broadleaf did not improve the model. The average cost of determining that a quadrat is weed-free is $0.02. The average cost of counting and identifying 37 seedlings is $0.05, $0.07, $0.19, and $0.26 per quadrat for 1, 2, 4, and 6 species, respectively. The cost of identifying seeds and seedlings in eastern Colorado Z. mays fields to use WEEDCAM is estimated as $2.71 per core for the seed bank and $0.08 per quadrat for seedlings.

Plants started with seed, 'seedlings', and established plants of silverleaf nightshade (Solanum elaeagnifolium Cav.) were grown in the field under shade levels of 0, 47, 63, and 92% of full sunlight to determine vegetative, reproductive, and physiological responses to shade. Dry-matter production of both 'seedling’ and established plants declined markedly with increasing shade levels. Established plants did not bear fruit under 92% shade, and 63% shade prevented fruit production by 'seedlings'. Taproots of plants grown in full sunlight contained 16% more total non-structural carbohydrate (TNC) per gram dry weight than taproots of plants grown under 92% shade. Leaves of established plants grown under moderate shade had significantly more chlorophyll per unit leaf fresh weight than plants grown in full sunlight; however, plants under heavy (92%) shade had 35% less chlorophyll per unit leaf area than unshaded plants. The chlorophyll a/b ratio of the 92%-shaded plants was significantly less than with other treatments. Leaf area increased, with increasing shade; however, leaf weight per unit area decreased because of thinner leaves. Photosynthetic rates of recently expanded leaves were 10.4, 4.6, 3.3, and 0.9 mg CO2 · dm−2 · h−1 for the 0, 47, 63, and 92% shade treatments, respectively.

Different growth rates of young seedlings (genets) and plants grown from root pieces (ramets) of yellow toadflax could influence their respective competitive ability and their susceptibility to management techniques. Shoot production was similar for genets and ramets (approximately 10 shoots were produced 12 or 13 wk after transplanting or cotyledon appearance, respectively), but the rate of shoot biomass accumulation was higher for genets than for ramets. Genets consistently produced more underground shoots than ramets. Replanted underground shoots separated from their roots were able to produce new shoots and roots. Rate of elongation for roots 0.5 to 1.5 mm in diameter was higher for ramets than for genets, but their shoot production potential was the same. Root pieces from genets did not have the ability to produce daughter shoots until 3 wk after cotyledon appearance. This indicates that very young genets would be more susceptible than older genets or ramets to management control systems.

Swallowwort, a twining perennial of the Asclepiadaceae family, has become increasingly invasive in cultivated fields, orchards, fence rows, natural areas, and roadsides of some provinces in Iran. The ability of this weed to establish and develop from seed was studied in laboratory and greenhouse experiments. Optimum germination temperature was 25 C. Germination did not occur at ≤ 15 C either in dark or light. In temperature fluctuation experiments, maximum germination occurred at 30/22 C, but light fluctuations had no effect on swallowwort seed germination. At pH 4 to 9 seed germination was 44 to 66%. Osmotic potential up to −0.2 MPa had little effect on germination. Seed germination was less than 5% at −0.8 Mpa. Swallowwort seeds were not able to germinate at 300 mM NaCl; however, there was 12% germination at the 200 mM concentration. Maximum emergence occurred when the seeds were planted at 1.5 cm depth. Seedlings did not emerge when planted at 6.5 cm depth. Germination percentage of the seeds kept under immersed (water) conditions was similar to those of nonimmersing periods; however, seedling growth was greater in treatments without immersing periods.

At locations in Argentina and the United States, solaria (miniature, portable, plastic greenhouses or a plastic sheet approximately 1 m2) were placed on field soils in autumn or late winter in an attempt to predict summer annual weed densities. Initial emergence of summer annual weeds covered by solaria commenced weeks before that of weeds in exposed seedbeds. Cumulative emergence of many species in solaria reached asymptotes before crops were sown. At asymptotic cumulative emergence, densities of dominant weeds in solaria (common lambsquarters, green foxtail, and large crabgrass) were correlated with weed densities occurring 4 wk after sowing, the typical time for making postemergence weed control decisions. These results indicate that solaria may supplement seedbank-sampling techniques for predicting weed densities in crops.

We studied the response of naturalized kudzu plants to simulated herbivory at three locations: Delaware (DE), Pennsylvania (PA), and North Carolina (NC). At the DE and PA sites, plant mortality after the first yr was 14 and 50%, respectively, and was highest for plants that had a small starting root crown size. At both sites, 50 and 75% leaf and shoot clipping, and drilling one or two large holes from the root crown into roots, had no effect on aboveground biomass. In NC, all plants survived for 3 yr. Plants subject to 50% vine removal at this site showed significant decrease in aboveground biomass compared to the control, but 50% leaf cutting and root drilling had no effect. In the greenhouse, kudzu seedlings grown in 60 and 100% light compensated for 50% leaf removal, but 75% damage reduced aboveground biomass. Plants survived for 1 to 2 mo in 0% direct light, but only one of 53 plants survived to the end of the experiment. Results suggest that established kudzu plants are able to compensate for biomass removal, seedlings can survive for several weeks without light, and that effective biocontrol might require more than 2 to 3 yr of continuous damage.

Introduction. Many fruit trees with a hard seed coat exhibit seed dormancy, posing problems for their regeneration. Vitex doniana, an indigenous multipurpose but threatened fruit and vegetable tree that supports the livelihoods of many households in West Africa, is a typical example. Materials and methods. In our research, we evaluated five dormancy-breaking treatments. We tested the effects of sulphuric acid at 95% concentration (T1); 3 d sun-drying + 48 h soaking in tap water (T2); 3 d alternation of 8 h sun-drying + 1 h soaking in tap water (T3); 2 weeks sun-drying with regular watering in the daytime (T4); and physical shock (T5). These treatments were compared with two controls (T0 and Tc), with seeds from two different sources. The germination percentage, mean germination time, time to first germination and time to threshold germination (20%) were compared; the seedling height, diameter and biomass produced were monitored for 15 weeks. We used generalised linear models and correlation tests to compare the effects of the various treatments on germination and seedling growth. Results and discussion. T3 significantly enhanced seed germination in V. doniana (72% after 12 months). T4 best promoted homogeneity in germination (p < 0.01), followed by T3. The best seedling growth was obtained with T4 and T3. Alternation of sun-drying followed by soaking of seeds, a technique with almost no cost, improved seed germination in V. doniana and, in 33 d, just over 1 month, 20% germination can be achieved. Vitex doniana is a fast-growing species (at the nursery stage), in contrast to the common opinion. Conclusion. Our method should be further investigated to assess the adequate soaking and drying length so as to speed up germination and reach homogenous cohorts.

Vertical stratification of vascular epiphytes is frequently attributed to niche partitioning along microclimatic gradients but experimental confirmations of this notion are rare. This study investigates the role of the regeneration phase for the stratification of five bromeliad (Catopsis sessiliflora, Guzmania subcorymbosa, Tillandsia anceps, T. bulbosa and Werauhia gladioliflora) and seven aroid species (Anthurium acutangulum, A. brownii, A. clavigerum, A. durandii, A. friedrichsthalii, A. hacumense and A. scandens) in a Panamanian rain forest. We documented gradients of temperature, vapour pressure deficit and light (n = 10 d) as well as species height distributions (n = 11–120). Microclimatic gradients were substantial (maximal T and RH differences between strata: 5 °C and 18%, respectively) and mean attachment heights of the study species (range = 4–21 m) differed significantly. We tested sensitivity to recurrent drought (four treatments) during germination (all species, cumulative germination of 20 seeds, n = 3) and seedling growth (four aroid species, n = 25). Seedling survival of six aroid species transplanted to three heights (n = 27) was monitored in situ. Some species did not germinate under severe recurrent drought while others germinated at the same rate in all treatments. Seedlings of the most exposed species grew fastest under intermediate recurrent drought while those of the other three species grew fastest when kept constantly wet. Survival of transplanted seedlings did not depend on species attachment height, but this may be attributable to insufficient statistical power. Taken together, the results suggest that the stratification can be explained to a large degree by differential sensitivity to the vertical moisture gradient during the regeneration phase.

Forest fragmentation can alter herbivory on tree recruits with possible consequences for regeneration. We assessed effects of forest-fragment quality (tree diversity, vegetation complexity, relative abundance of pioneer trees) and matrix habitat on arthropods and herbivory in KwaZulu-Natal, South Africa. We compared arthropod abundances and herbivory on woody seedlings and saplings among four forest-fragment types differing in size and matrix (large fragments and small fragments surrounded by natural grassland, eucalypt and sugarcane plantations; nplots = 24) using analyses of covariance. We recorded 3385 arthropods and inspected 897 seedlings (71 species) and 876 saplings (91 species). Relative abundance of predators increased with fragment quality; that of herbivores decreased. Herbivory responses to fragment quality varied: seedling herbivory decreased with relative abundance of pioneers and sapling herbivory increased with vegetation complexity. Matrix effects were low with little variation in relative abundance of predators (0.39–0.53) and herbivores (0.22–0.32), proportion of seedling (8.3–11.0%) and sapling herbivory (12.4–14.3%) among the forest-fragment types. These findings indicate that herbivory on tree recruits is mediated by forest-fragment quality rather than matrix habitat. Future studies should evaluate whether contrasting effects of fragment quality on arthropods and herbivory are caused by weak trophic interactions and variable herbivore compositions.

Research has increasingly established that mesoherbivores influence the regeneration of woody plants. However the relationship between mesoherbivore density and degree of impact, and the spatial component of this impact, has not been well established. Using a novel sampling design, we assessed in iMfolozi Park, South Africa, the impact of impala (Aepyceros melampus) across the full complement of woody species within the home range, evaluating its spatial component and relationship to impala density. We used four GPS collars, in separate breeding herds, and a GIS to detect zones of different density of impala in the landscape, thus defining a fine-grain browsing gradient. We assessed impact on woody recruits (≤ 0.5 m height) across this gradient by means of 1600 random 1 × 1-m quadrats. Densities of woody seedlings, and mean percentage of remaining canopy, were significantly less in areas of high impala density versus low-density areas. There was a significant correlation between increasing impala density and decreasing density of favoured woody recruits. We propose a hypothesis of impala-induced patch dynamics. It seems likely that the ubiquitous impala may create and sustain a shifting mosaic of patches, and thus function as a key determinant of landscape heterogeneity.

Responses of plants to herbivory are dependent on the type of damage and the ontogenetic stage of the plant. We compared the effects of stem pruning and defoliation on seedlings of Colophospermum mopane, an ecologically important tree species widely distributed in southern Africa. The growth of 160 greenhouse-grown juveniles were measured for 6-mo after germination and then 6-mo after treatments including 50% defoliation, 100% defoliation, 50% stem pruning and controls. Pruning resulted in 30% reductions in total leaf area, height and biomass. Partial defoliation resulted in 30% reductions in total leaf area and plant biomass. However, complete defoliation resulted in a 30% increase in biomass production, a doubling in leaf and lateral branch number, a 45% reduction in leaf size, and no change in total leaf area. Thus, completely defoliated seedlings showed greater performance than those that were only partially defoliated, indicating that C. mopane has become adapted to the chronic and severe defoliation inflicted by Imbrasia belina caterpillars. Comparison of our results with other studies indicates that C. mopane seedlings are less herbivory-tolerant than adults and that pruning has more negative effects than defoliation. Thus, seedling browsers may constrain recruitment in C. mopane, influencing its population dynamics and abundance.

Germination and early seedling development are critical for successful stand establishment of plants. Following germination, the cotyledons, which are derived from embryonic tissue, emerge from the seed. Arabidopsis seedlings at post-germinative stages are supported mainly by the supply of nutrition from the cotyledons until vegetative leaves emerge and initiate photosynthesis. The switch to autotrophic growth is a significant transition at the post-germinative stage. Here, we provide evidence that down-regulation of SQUAMOSA PROMOTER-BINDING PROTEIN LIKE13 (SPL13) by microRNA156 (miR156) plays an important role in the regulation of the post-germinative switch from the cotyledon stage to the vegetative-leaf stage. Silent mutations created in the SPL13 sequence in the region that is complementary to the miR156 sequence caused the deregulation of the mutant form of SPL13 (mSPL13) mRNA from miR156. Mutant seedlings over-accumulated miRNA-resistant messages and exhibited a delay in the emergence of vegetative leaves compared to wild-type seedlings. The delay was not observed in control transgenic plants expressing non-mutated SPL13, indicating that the phenotype was caused specifically by the silent mutations and deregulation of SPL13 from miR156. Characterization of the SPL13 promoter indicated that this gene is expressed mainly in the hypocotyl and affects leaf primordium development. These results suggest that the repression of SPL13 by miR156 is essential for normal post-germinative growth in Arabidopsis.

Introduction. Drought stress is the major factor affecting growth, development and production of walnut trees. In Iran, approximately 33 Mha of land is affected by salinization and drought stress. Finding genetic resources tolerant to drought stress at different growth stages is important for such semi-arid regions. Our aim was to understand better the adaptive mechanisms that enable different genotypes of walnut population to survive under drought stress, and to provide some useful clues for walnut tree breeding toward improved drought tolerance with utilization of existing drought-tolerant genetic resources. Materials and methods. To study the mechanism(s) involved in drought tolerance of some Persian walnut genotypes, drought stress was induced using polyethylene glycol-6000 to produce water potentials of 0 Mpa (control), –0.10 MPa, –0.50 MPa, –0.75 MPa, –1.00 MPa, –1.50 MPa and –2.00 MPa. The amount of proline and soluble sugar accumulation in four walnut genotypes (‘Panegine20’, ‘Lara’, ‘Serr’ and ‘Chandler’) were determined after being exposed to the various water potential levels. Results. The rates of seed germination in all genotypes were significantly reduced by low external water potentials. Plants exposed to water stress had a higher amount of soluble sugars in roots and shoots of tolerant genotypes (‘Panegine20’ and ‘Chandler’) and a lower amount of starch in their tissues. These results imply the important roles of soluble sugars as solutes conferring resistance to drought in these genotypes. The free proline levels were also increased in response to drought stress. They were higher in drought-tolerant genotypes than in sensitive ones (‘Lara’ and ‘Serr’). Proline increased more in shoots than in roots. However, the soluble sugar and starch fluctuations were higher in the roots. Conclusion. Our results support a direct correlation between the degree of drought stress and proline content. As a consequence, proline concentrations could be used as a biochemical marker of drought stress level in walnut plants.

The survival, height and leaf production of seedlings and juveniles of Aspidosperma fendleri and Richena grandis were measured monthly for three years after germination. During the first year, some seedlings and juveniles of Aspidosperma fendleri were collected and the number of rootlets, the primary root length and the shoot: root ratio were determined.

Both species show periodicity in growth but their relative growth rate differs between species. For Aspidosperma fendleri, the highest relative growth rate (0.313 y-1) was found for individuals grown under greenhouse conditions followed by individuals growing in an old forest gap (0.143 y-1) and finally individuals under the forest canopy (0.137 y-1). For Richeria grandis, the relative growth rate under the forest canopy was 0.261 y-1. Leaf production for Aspidosperma fendleri in the forest (natural conditions) was 4.39 total mean leaf number for five years and 5.46 total mean leaf number under greenhouse conditions. For Richeria grandis it was 5.34 mean leaf production for four years. The root: shoot ratio for Aspidosperma fendleri was constant during the observation year. Aspidosperma fendleri showed a lower number of rootlets than did Richeria grandis. There was an inverse relationship between growth and survival during the dry season. During this period, mortality was higher and the highest mortality occurred during the first year when the total relative growth rate was highest.

Slow growth is evident in both species. This mechanism may represent an adaptive advantage to remain dormant until there is an opening in the forest canopy. Since the seeds of Aspidosperma fendleri and Richeria grandis are highly vulnerable to predators and pathogens, slow growth as seedlings and juveniles allows them to survive, and contributes to regeneration in cloud forest.

Succession was monitored over ten years in a 10×10m plot in forest with mor humus at 1550 m in the Blue Mountains of Jamaica, following the removal in January 1975 of all aerial plant parts and some of the root material. In April 1977, August–January 1980/1 and August 1984, all woody recruits in the plot were tagged and numbered, identified and measured (height), and mapped on a 1 m grid. The height of coppice was recorded. Ten of the eleven tree species present before felling produced coppice shoots. Two individuals almost attained canopy height by 1984. Twenty tree species and three shrub species were recruited from seed; six of the tree species were normally absent from the forest. Species composition changed very little with time.

The rate of seedling recruitment was greatest immediately after clearing. The overall den-sity of individuals changed little after 1977. Overall mortality of recruits was about 10% per annum. Mortality of the earliest recruits declined with time since establishment. Later recruits and individuals with poor growth had higher mortality than other plants. Mortality was not density-dependent. Growth rates of recruits were relatively slow. Only ten individuals exceeded 4 m by 1984.

Recruitment rates, density, growth and species diversity were greatest in the parts of the plot where the mor humus had been removed or piled up during the initial clearance. Recruit-ment, growth and density were least, and mortality was greatest, at the edge. There was no relationship between any of these parameters and the presence of coppicing stumps.

Tree species showed a clear spectrum from obligate gap-demanders to obligate shade-bearers. The persistence of gap-demanding species in this forest (in which gaps are normally rare) may be due to infrequent hurricanes, and also to a natural ability to produce basal sprouts. The succession conforms to an initial floristic composition model; it is slow, and we suggest that at least 50 years will elapse before the plot begins to resemble the undisturbed forest.