Seed rain and soil seed bank compensatory roles on Nassella tenuis(Phil.)Barkworth seedling recruitment in ungrazed and grazed sites

In semi-arid lands,vegetation is distributed in shrub patches immersed in a less vegetated interpatch matrix.Grazing affects perennial grass seed bank through a decrease in seed rain and an increase in seed predation and soil compaction.Nevertheless,some species with anchorage mechanisms in their seeds might overcome this,such as Nassella tenuis(Phil.)Barkworth.This is an important species in grazing paddocks because it has an intermediate palatability and its relatively tolerant to grazing.These characteristics allow N.tenuis to increase its abundance in grazed sites.Our objective was to assess how grazing affects the key palatable species from seeds to seedlings:i.e.,seed rain,soil seed bank,and seedling recruitment in different microsites along a windward-leeward transect across shrub canopy.We hypothesized that:(1)the negative effects of grazing on N.tenuis fructification are reflected in its seed rain,soil seed bank,and seedling recruitment,especially in interpatches;(2)Nassella tenuis seed rain reduction,soil compaction by cattle in grazed sites,and removal of seeds by wind decrease its soil seed bank,especially in microsites exposed to the predominant wind;and(3)the decrease in N.tenuis soil seed bank and cover increase in annual species in grazed sites have negative effects on its seedling recruitment,especially in microsites exposed to predominant wind.We placed seed traps,collected soil samples,and monitored seedling recruitment in different locations around shrub canopy to address our hypotheses.Also,we established a manipulative experiment in which we sow N.tenuis seeds and followed its recruitment in different microsites.We compared the seed rain,soil seed bank,natural seedling recruitment,and sown seeds recruitment of N.tenuis between grazed and ungrazed sites.We analyzed differences between microsites along a windward-leeward transect across shrubs patches.Seed rain and soil seed bank had the same density in patches and interpatches both in ungrazed and grazed sites.But seed rain was higher,and soil seed bank was lower in ungrazed sites than in grazed sites.Almost all under-canopy microsites showed greater soil seed bank abundance and natural seedling recruitment in ungrazed sites.Sown seeds recruitment was the same between grazed and ungrazed sites,but it showed protective effects of shrubs in leeward microsites under grazed sites.As a conclusion,seed rain and soil seed bank are complementary under grazed sites.

Precipitation amount and frequency affect seedling emergence and growth of Reaumuria soongarica in northwestern China

Climate change is shifting the amount and frequency of precipitation in many regions, which is expected to affect seedling recruitment across ecosystems. However, the combined effects of precipitation amount and frequency on seedling recruitment remain largely unknown. An understanding of the effects of precipitation amount and frequency and their interaction on seedling emergence and growth of typical desert plants is vital for managing populations of desert plants. We conducted two experiments to study the effects of variation in precipitation on Reaumuria soongarica （Pall.） Maxim. First, greenhouse experiments were conducted to examine the effects of three precipitation amount treatments （ambient, ＋30%, and -30%） and two precipitation frequency treatments （ambient and -50%） on seedling emergence. Second, the morphological responses of R. soongarica to changes in precipitation amount and frequency were tested in a controlled field experiment. Stage-specific changes in growth were monitored by sampling in different growth seasons. Our results showed that precipitation amount significantly affected germination, seedling emergence, and growth of R. soongarica, and there was a larger effect with decreased precipitation frequency compared with ambient. Germination and seedling emergence increased as precipitation increased under the same frequency of precipitation. The highest emergence percentage was obtained with a 30% increase in precipitation amount and a 50% reduction in precipitation frequency. Compared with ambient precipitation, a 30% increase in precipitation amount increased above- and below-ground biomass accumulation of R. soongarica during the growth season. A decrease of 30% in precipitation amount also resulted in an increase in below-ground biomass and root/shoot ratio in the early stages of the growth season, however, above- and below-ground biomass showed the opposite results at the end of the growth season, with larger effects on above-ground than below-ground biomass under decreased precipitation frequency. When precipitation frequency decreased by 50%, values of all growth traits increased for a given amount of precipitation. We concluded that precipitation frequency may be as important as precipitation amount to seedling emergence and growth of R. soongarica, and that understanding the effects of precipitation variability on seedling recruitment requires the integration of both precipitation amount and frequency. In particular, the combination of a 30% increase in precipitation amount and 50% reduction in precipitation frequency increased the emergence and growth of seedlings, suggesting that alteration of amount and frequency of precipitation caused by climate change may have significant effects on seedling recruitment of R. soongarica.

Limitations on the recruitment of the rare sand shrubby legume Eremosparton songoricum(Fabaceae) in Gurbantunggut Desert,China

Eremosparton songoricum,a rare and endemic sand dune plant,appears to be experiencing recruitment failure.The structure of five populations from the Gurbantunggut Desert,China,was investigated for recruitment patterns,and two of them were examined for flowering,pollination and seed germination limits on regeneration.The results showed that total 150 seedlings only occurred on line transects in riverside Dure population,but they all died half a month later;no seedlings occurred in other four hinterland desert populations indicating recruitment was a failure at all populations although flowers were plentiful.Reproductive success depends on pollinators.Nectar is ＇reward＇ for pollinators,with 0.06 μL-0.12 μL and 0.15 μL-0.35 μL per flower in Dure and Kabu populations,respectively,in continuously two secreting days.Spontaneous self pollination is rare with nearly zero fruit production.Geitonogamous self pollination is predominant with 14.47% fruit set.Seed mass in the riverside Dure population was significantly greater than that in other hinterland desert populations.Consequently,the Dure population exhibited a significantly higher germination rate（about 90%） than those in other populations（about 30%）.Results suggested that recruitment failure of E.songoricum is not due to flower shortage,pollination limitation,or poor seed germination but environmental pressure and/or human disturbance.

Importance of microhabitat selection by birds for the early recruitment of endangered trees in a fragmented forest

Background:Remnant microhabitats are important for bird habitat selection and plant regeneration in the fragmented habitat.However,empirical information on the consequences of how microhabitat use by birds affects the early recruitment of plants is lacking.Methods:In this study,we evaluated whether microhabitat selection by the Black Bulbul(Hypsipetes leucocephalus)(J.F.Gmelin,1789)impacts the early recruitment of the endangered tree species,the Chinese Yew(Taxus chinensis(Pilger)Rehd),in a fragmented forest over a 4-year period(2011–2012,2018–2019).Results:Our results showed the main factors affecting H.leucocephalus microhabitat selection were distance to the nearest T.chinensis mature tree,herb cover and density,leaf litter cover,and vegetation type.Moreover,the results of logistic regression also highlighted the importance of elevation,distance to light gap and roads,tree cover in bird microhabitat selection.Furthermore,the seed emergence rate in microhabitats used by birds did not differ from the natural forest,which was related to five factors of bird microhabitat.The Random Forest model showed that seedling emergence rate was increased with leaf litter cover and distance to fallen dead trees,but decreased in relation to herb cover,slope,and elevation.Conclusion:Our results highlight the importance of remnant microhabitats in fragmented forests for sustaining forest ecology and optimal management.The contribution of microhabitats used by birds to plant recruitment provides insights into how frugivore species contribute to plant regeneration,which should be incorporated in future conservation and management practices of fragmented forests.

Spatial structure and species composition of soil seed banks in moving sand dune systems of northeast China

Soil seed banks can provide a mechanistic for understanding the recruitment dynamics and can inform conservation management of ecosystems. To investigate the contribution of soil seed banks to vegetation restoration in moving sand dune systems, we compared seed structure and species similarity between soil seed banks and standing vegetation among moving sand dunes, ecotones and dune slacks in northeast China. Average seed density in dune slacks was greater than in ecotones or moving sand dunes. Seed density in the soil layer of 0-10 cm was greater than at 10-20 cm both in the moving sand dunes and the eco- tones, but seed densities were similar at depths of 10-20 and 20-30 cm in moving sand dunes. Moreover, the spatial autocorrelation of seed density on moving sand dunes was weak but was strong on the ecotones and dune slacks. The species in the soil seed bank of moving sand dune systems were nearly all annuals, and the low similarity was mainly due to the lack of perennial species that were common in standing vegetation. Consequently, vegetation restoration cannot mainly rely on the soil seed banks in the movingsand dunes and more attention should be paid to protection of the dune slacks because they are the main source of seed disperse and seedling recruitment in moving sand dune systems.

What makes Haloxylon persicum grow on sand dunes while H. ammodendron grow on interdune lowlands: a proof from reciprocal transplant experiments

Determining the mechanisms underlying the spatial distribution of plant species is one of the central themes in biogeography and ecology. However, we are still far from gaining a full understanding of the autecological processes needed to unravel species distribution patterns. In the current study, by comparing seedling recruitment, seedling morphological performance and biomass allocation of two Haloxylon species, we try to identify the causes of the dune/interdune distribution pattern of these two species. Our results show the soil on the dune had less nutrients but was less saline than that of the interdune; with prolonged summer drought, soil water availability was lower on the dune than on the interdune. Both species had higher densities of seedlings at every stage of recruitment in their native habitat than the adjacent habitat. The contrasting different adaptation to nutrients, salinity and soil water conditions in the seedling recruitment stage strongly determined the distribution patterns of the two species on the dune/interdune. Haloxylon persicum on the dunes had lower total dry biomass, shoot and root dry biomass, but allocated a higher percentage of its biomass to roots and possessed a higher specific root length and specific root area by phenotypic traits specialization than that of Haloxylon ammodendron on the interdune. All of these allowed H. persicum to be more adapted to water stress and nutrient shortage. The differences in morphology and allocation facilitated the ability of these two species to persist in their own environments.

Bulbuls and crows provide complementary seed dispersal for China's endangered trees

Background: Different functional frugivores generally exhibit unequal contributions,both in terms of quantity(seed removal) and quality(seedling recruitment),to effective seed dispersal of plant species.However,variations in this dispersal pattern generated by frugivores across different regions are still unknown.Methods: In our study,we evaluated the contributions of two functional frugivore bird groups,the bulbuls(Pycnon?otidae) and crows(Corvidae),in both the seed removal and seedling recruitment of the endangered Chinese yew tree(Taxus chinensis) across three different geographical regions in eastern China.Results: The dominant crow species,Urocissa erythrorhyncha,was the most common disperser crow species at all sites,while the dominant bulbul species varied across the three sites.Furthermore,the two functional groups of dispersers diverged in the aspects of seed removal(quantity) and seedling recruitment pattern(quality).While bulbuls outperformed crows in seed removal(quantity),crows took more seeds to a safer site for seedlings(quality).Conclusions: Our results highlight the importance of variations in the effective seed dispersal patterns of different functional disperser groups across different regions in the conservation and management of endangered tree species.

Tree mortality and regeneration of Euphrates poplar riparian forests along the Tarim River,Northwest China

Background:Tree mortality and regeneration(seedling and sapling recruitment)are essential components of forest dynamics in arid regions,especially where subjected to serious eco-hydrological problems.In recent decades,the mortality of the Euphrates poplar(Populus euphratica)along the Tarim River in Northwest China has increased.However,few studies have quantified the causes of mortality and regeneration in this azonal riparian forest type.Methods:The present study describes the annual hydrological response of tree mortality and regeneration in forest gaps.A total of 60 canopy gaps were investigated in six replicate grid plots(50m×50 m)and the annual runoff and water consumption data during the period of 1955–2016 were collected from hydrological stations in the middle reaches of the Tarim River.We compared the regeneration density of seedlings and saplings within the canopy gap areas(CGAs),undercanopy areas(UCAs),and uncovered riverbank areas(RBAs)through detailed field investigation.Results:Our study found that the mortality of young and middle-aged gap makers has increased remarkably over recent decades,particularly since the year 1996.The main results indicated that regional water scarcity was the primary limiting factor for long-term changes in tree mortality,as shown by a significant correlation between the diameter at breast height(DBH)of dead trees and the annual surface water.The average density(or regeneration rate)of seedlings and saplings was highest in the RBAs,intermediate in the CGAs,and lowest in the UCAs.Compared with the UCAs,the CGAs promote tree regeneration to some extent by providing favorable conditions for the survival and growth of seedlings and saplings,which would otherwise be suppressed in the understory.Furthermore,although the density of seedlings and saplings in the CGAs was not as high as in the RBAs,the survival rate was higher in the CGAs than in the RBAs.Conclusion:Forest canopy gaps in floodplain areas can play a decisive role in the long-term germination and regeneration of plant species.However,as a typical phreatophyte in this hyper-arid region,the ecosystem structure,functions and services of this fragile P.euphratica floodplain forests are threatened by a continuous decrease of water resources,due to excessive water use for agricultural irrigation,which has resulted in a severe reduction of intact poplar forests.Furthermore,the survival of seedlings and saplings is influenced by light availability and soil water at the regional scale.Our findings suggest that policymakers may need to reconsider the restoration and regeneration measures implemented in riparian P.euphratica forests to improve flood water efficiency and create canopy gaps.Our results provide with valuable reference information for the conservation and sustainable development of floodplain forest ecosystems.

Masting promotes transformation from predation to mutualism in an oak-weevil-rodent system

The significance of ecological non-monotonicity(a function whose first derivative changes signs)in shaping the structure and functions of the ecosystem has recently been recognized,but such studies involving high-order interactions are rare.Here,we have proposed a threetrophic conceptual diagram on interactions among trees,rodents,and insects in mast and non-mast years and tested the hypothesis that oak(Quercus wutaishanica)masting could result in increased mutualism and less predation in an oak-weevil-rodent system in a warm temperate forest of China.Our 14-year dataset revealed that mast years coincided with a relatively low rodent abundance but a high weevil abundance.Masting not only benefited seedling recruitment of oaks through increased dispersal by rodents but also a decrease in predation by rodents and weevils,as well as an increase in the overwintering survival of rodents.Masting appeared to have increased weevil survival by reducing predation of infested acorns by rodents.These results suggest that masting benefits all participants in the plant-insect-rodent system by increasing mutualism and reducing predation behavior(i.e.,a non-monotonic function).Our study highlights the significance of masting in maintaining the diversity and function of the forest ecosystem by facilitating the transformation from predation to mutualism among trophic species.

Scatter-hoarding rodents as secondary seed dispersers of a frugivore-dispersed tree Scleropyrum wallichianum in a defaunated Xishuangbanna tropical forest, China

Local extinction or population decline of large frugivorous vertebrates as primary seed dispersers,caused by hu-man disturbance and habitat change,might lead to dispersal limitation of many large-seeded fruit trees.However,it is not known whether or not scatter-hoarding rodents as secondary seed dispersers can help maintain natural regeneration(e.g.seed dispersal)of these frugivore-dispersed trees in the face of the functional reduction or loss of primary seed dispersers.In the present study,we investigated how scatter-hoarding rodents affect the fate of tagged seeds of a large-seeded fruit tree(Scleropyrum wallichianum Arnott,1838,Santalaceae)from seed fall to seedling establishment in a heavily defaunated tropical forest in the Xishuangbanna region of Yunnan Province,in south-west China,in 2007 and 2008.Our results show that:(i)rodents removed nearly all S.wallichianum seeds in both years;(ii)a large proportion(2007,75%;2008,67.5%)of the tagged seeds were cached individually in the surface soil or under leaf litters;(iii)dispersal distance of primary caches was further in 2007(19.6±14.6 m)than that in 2008(14.1±11.6 m),and distance increased as rodents recovered and moved seeds from primary caches into subsequent caching sites;and(iv)part of the cached seeds(2007,3.2%;2008,2%)survived to the seedling stage each year.Our study suggests that by taking roles of both primary and secondary seed dispersers,scatter-hoarding rodents can play a significant role in maintaining seedling establishment of S.wallichianum,and are able to at least partly compensate for the loss of large frugivorous vertebrates in seed dispersal.

Effects of logging on the trade-off between seed and sprout regeneration of dominant woody species in secondary forests of the Natural Forest Protection Project of China

Background:Promoting natural regeneration(including seed and sprout regeneration)of dominant woody species is essential for restoring secondary forests.However,such restoration processes have been decelerated by the enclosure under Natural Forest Protection Project of China(NFPP).It remains unclear how to implement appropriate management measures(e.g.,whether to apply logging and the suitable intensity)to facilitate natural regeneration according to the responses of two regeneration modes to management measures.We monitored the early stages of seed regeneration(seed rain,soil seed bank,and 1–3-year-old seedlings)and sprout regeneration(stump sprout rate,stump survival rate,probability of sprouting,and number of sprouts per stump)over the first 3 years(2017–2019)after logging under three intensity regimes(control[0%],25%,and 50%logging intensity)in secondary forests.Results:The seed rain density decreased markedly,seedling density increased insignificantly after logging,and logging promoted seedling survival at an increasing conversion rate of 3-year-old seedlings(37.5%under 0%,100%under 25%,and 80.95%under 50%logging)compared to those of the control.The proportion of 3-year-old seedlings increased with logging intensity and was the highest(16.2%)at 50%logging intensity.Sprout density was not affected by logging intensity,however,under 25%and 50%logging,it decreased by 27%and 6%in 2018,and by 37%and 33%in 2019,respectively.Seedling density was 41.65-and 15.31-fold higher than that of sprouts in the 50%and 25%logging treatments,respectively.Based on the relative contributions of the two regeneration modes after logging,three groups of natural regeneration patterns were classified for dominant woody species in temperate secondary forests,i.e.,seed regeneration preference(Betula dahurica,Carpinus cordata and Fraxinus mandshurica),sprout regeneration preference(Acer mono and Acer pseudosieboldianum)and no preference(Quercus mongolica,Fraxinus rhynchophylla,and Juglans mandshurica).Conclusion:In addition to enclosure,appropriate logging can be applied according to the responses of various natural regeneration patterns of dominant woody species to logging in temperate secondary forests under the NFPP.