Plasticity in Meristem Allocation as an Adaptive Strategy of a Desert Shrub under Contrasting Environments

The pattern of resource allocation to reproduction vs. vegetative growth is a core component of a plant’s life-history strategy. Plants can modify their biomass allocation patterns to adapt to contrasting environments. Meristems can have alternative fates to commit to vegetative growth, reproduction, or remaining inactive (dormant or senescent/dead). However, knowledge about whether meristem fates can interpret adaptive changes in biomass allocation remains largely unknown. We measured aboveground plant biomass (a proxy of plant size) and meristem number of a dominant shrub Artemisia ordosica in three populations occupying different habitats in the Mu Us Desert of northern China. Size-dependent biomass allocation and meristem allocation among habitats were compared. The size-dependent biomass allocation and meristem allocation of A. ordosica strongly varied across habitats. There were significant positive linear relationships between meristem allocation and biomass allocation in all habitats, indicating that meristem allocation is an indicator of the estimated resource allocation to reproductive and vegetative organs in this species. Plasticity in meristem allocation was more likely caused by larger individuals having less active meristems due to environmental stress. Vegetative meristems (VM) were likely more vulnerable to environmental limitation than reproductive ones, resulting in the ratio of resource investment between vegetative and reproductive functions exhibiting plasticity in different habitats. A. ordosica invested a higher fraction of its resource to reproduction in the adverse habitat, while more resource to vegetative growth in the favorable habitat. A. ordosica adopts different resource allocation patterns to adapt to contrasting habitat conditions through altering its meristem fates. Our results suggest that the arid-adapted shrub A. ordosica deactivates more VM than reproductive ones to hedge against environmental stress, representing an important adaptive strategy. This information contributes to understand the life-history strategies of long-lived plants under stressful environments.

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title: The Shape of the Trade-off Function between Reproduction and Growth
abstract: Publisher Summary Although, there is a general agreement that there is indeed a trade-off between current and future reproduction in plants that is mediated through the trade-off between reproduction and growth, there is very little empirical data available on what this trade-off function actually looks like. Most researchers, either implicitly or explicitly assume that it is a linear function such that an increase in current reproduction results in a proportional decrease in growth. It is possible, however, to develop the hypotheses that predict curvilinear trade-off functions. A concave curvilinear function would result if there were “economies of scale” associated with increasing the level of current reproduction to a higher level. This chapter summarizes recent research from the laboratory using a model plant species, Plantago major, which addresses the question of what this trade-off function actually looks like. It is not intended to be a review of the literature on this topic. Rather, it represents a particular perspective on this issue and how this perspective has evolved.

title: Interpreting reproductive allometry: Individual strategies of allocation explain size-dependent reproduction in plant populations
abstract: Size-dependent or allometric relationships between reproductive and vegetative size are extremely common in plant populations. Reproductive allometry where plant size differences are due to environmental variability has been interpreted both as an adaptive strategy of plant growth and allocation, and as the product of fixed developmental constraints. Patterns of development are crucial in defining reproductive allometry but development is not fixed across individuals. For example, environmental adversity (e.g. resource impoverishment) tends to favor reproduction at relatively small sizes – an adaptive response to environmental adversity. While small individuals may have lower reproductive output than large individuals, all plants should maximize their reproductive output and relative allocation to reproduction may be constant across sizes. Thus, where individual plants within a population initiate reproduction at different sizes, no significant reproductive allometry is an appropriate null expectation. Reproductive allometry occurs in plant populations where initiating reproduction at small sizes yields relatively high or low reproductive size at final development. Both of these outcomes are common in plant populations. Our interpretation of reproductive allometry combines previous adaptive and developmental constraint interpretations, and is the first to successfully explain the range of relationships in plant populations where relative allocation has been observed to increase, decrease or remain constant will increasing plant size.

title: The allometry of reproduction within plant populations
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title: Meristem allocation : a new classification theory for adaptive strategies in herbaceous plants
abstract: A major goal in studies of evolutionary ecology has been to define a classification of adaptive strategies that can be used for interpreting variation in fitness within species and patterns of diversity and species coexistence within and between habitats. An adaptive strategy is often associated with a particular pattern of allocation (e.g. of a limiting resource) that maximizes fitness in a given environment (Solbrig 1994). Different allocation patterns are assumed to be a consequence of tradeoffs between life history traits resulting from certain constraints (Stearns 1992). Hence, organisms may exhibit sets of traits that are predictably related to their ecology (Grime et al. 1988). Virtually all adaptive strategy theories based on allocation patterns in plants define strategies in terms of biomass allocation. A number of life history traits have been quantified in this way. The most common of these involves biomass allocation to reproductive versus vegetative functions (i.e. reproductive effort) (Hancock and Pritts 1987, Wilson and Thompson 1989, Bazzaz and Ackerly 1992). Biomass allocation to above versus below ground structures (Tilman 1988) and to growth, reproduction, structure, and defence (Taylor et al. 1990) have also been used to identify adaptive strategies in plants. Watson (1984) traces the roots of biomass allocation in life history strategies to the animal ecology literature (e.g. Cody 1966). Extending the use of biomass allocation to plant strategy theories seems to have arisen from convenience and established precedent in animal studies (Watson 1984). However, the use of biomass in allocation measures has a number of limitations. Problems have been most widely noted in studies of reproductive effort in plants. Partitioning biomass to reproductive and vegetative functions is difficult when support structures that are essential to, but not directly involved in reproduction, are considered. This is especially difficult when these structures have photosynthetic capability (a vegetative function) (Reekie and Bazzaz 1987a, Bazzaz and Ackerly 1992). Studies of biomass allocation also assume that there is one resource (i.e. carbon) that limits both growth and reproduction. However, different resources may limit growth in different environments and at different times in a plant's life (see Bazzaz and Ackerly 1992 for a review). A critical feature of life history strategy theories is that increased allocation to one function (e.g. reproduction) must tradeoff with a decreased allocation to another function (e.g. growth). To be of evolutionary significance, cost must be measured in terms of a true limiting resource (Watson 1984, Watson and Casper 1984, Reekie and Bazzaz 1987b, Bazzaz and Ackerly 1992). Another major criticism of biomass allocation is that it does not reflect internal constraints that may control plant growth and life history (Salomonson et al. 1994). Growth in grasses, for example, is constrained by tiller production (Law 1979). Any function that reduces tiller survival (e.g. flowering) decreases growth as only those tillers that survive can produce new tillers. Developmental "choices" of a plant can affect fitness through overall plant form (architecture). Biomass does not determine plant form (Smith 1984). In this paper we develop an alternative to biomassbased strategy theories based on allocation of above ground meristems in herbaceous flowering plants. Previous studies of meristems in ecology have focussed on several issues. Meristem demography (the births and deaths of modules within a plant) has been used to study growth patterns within a plant (Bazzaz and Harper 1977, Maillette 1982, 1985, 1987, 1990, Lehtila et al. 1994). Fitness may be defined at the level of the module (Tuomi and Vuorisalo 1989a, b). Modules are the units of reproduction, growth and survival and so the cycle of meristems producing new meristems may be used to estimate fitness in clonal plants (Fagerstr6m 1992, Wikberg 1995). Adaptive architecture has been studied via tradeoffs between growth (i.e. branching) and reproduction (Smith 1984, Watson 1984, Geber

title: Allocation, plasticity and allometry in plants
abstract: Allocation is one of the central concepts in modern ecology, providing the basis for different strategies. Allocation in plants has been conceptualized as a proportional or ratio-driven process (‘partitioning’). In this view, a plant has a given amount of resources at any point in time and it allocates these resources to different structures. But many plant ecological processes are better understood in terms of growth and size than in terms of time. In an allometric perspective, allocation is seen as a size-dependent process: allometry is the quantitative relationship between growth and allocation. Therefore most questions of allocation should be posed allometrically, not as ratios or proportions. Plants evolve allometric patterns in response to numerous selection pressures and constraints, and these patterns explain many behaviours of plant populations. In the allometric view, plasticity in allocation can be understood as a change in a plant's allometric trajectory in response to the environment. Some allocation patterns show relatively fixed allometric trajectories, varying in different environments primarily in the speed at which the trajectory is travelled, whereas other allocation patterns show great flexibility in their behaviour at a given size. Because plant growth is often indeterminate and its rate highly influenced by environmental conditions, ‘plasticity in size’ is not a meaningful concept. We need a new way to classify, describe and analyze plant allocation and plasticity because the concepts ‘trait’ and ‘plasticity’ are too broad. Three degrees of plasticity can be distinguished: (1) allometric growth (‘apparent plasticity’), (2) modular proliferation and local physiological adaptation, and (3) integrated plastic responses. Plasticity, which has evolved because it increases individual fitness, can be a disadvantage in plant production systems, where we want to optimize population, not individual, performance.

title: Reproductive allometry in Pedicularis species changes with elevation
abstract: 1. Plants produce biomass and then allocate some of it to reproductive structures, so the relationship between reproductive (R) and vegetative (V) mass is a fundamental aspect of a plant’s reproductive strategy. 
 
 
 
2. Differences among populations or taxa in the allometric relationship between R and V have been attributed to environmental conditions. We hypothesize that populations and taxa living at high elevations should exhibit a lower log R vs. log V slope than those in more benign environments, because the environmental limits on size in alpine environments should favour a relatively large reproductive allocation at smaller sizes and a smaller investment in reproduction per additional unit of biomass accumulated. 
 
 
 
3. We investigated variation in the allometric relationship between R and V among 44 naturally occurring populations representing 24 species of Pedicularis in the Tibetan Plateau, to test the hypothesis that the slope of the relationship declines with increasing elevation. 
 
 
 
4. There was a significant negative relationship between the slope of the log R vs. log V relationship and elevation among populations, although the relationship among populations within species varied. We interpret this in terms of abiotic limitations on size and decreasing efficiency of resource allocation to reproduction with increasing plant size (measured as vegetative biomass) at high elevations. We also found a significant positive relationship between the y-intercept of the regression of log R on log V and elevation across species, but the relationship was not significant among populations within species. The combination of the lower slopes and the higher y-intercepts for high-elevation populations means that plants growing at high elevations allocate proportionately more biomass to reproduction at smaller sizes and less at larger sizes than plants growing at lower elevations. 
 
 
 
5. Synthesis. The allometric slope (exponent) of the R–V relationship decreases with increasing elevation among Pedicularis populations and species, reflecting fundamental changes in the costs and benefits of increased vegetative biomass with elevation.

title: When the classical reaction norm is corrected by body size
abstract: For nearly half a century, plasticity has been a controversial issue in ecology and evolution. Observed plasticity was classically quantified as the slope of a reaction norm or quantified in time series, which concealed its relationship with body size. With a theoretical framework and an experiment, here we demonstrated that: (1) body size significantly contributes to nearly all traits’ variability, which produces apparent, but not true plasticity; (2) the classical reaction norm seriously misestimates the origins and levels of plasticity, and a size-correction to the reaction norm can eliminate its size-dependency and leave only the environmental-induced plasticity; and (3) the absence of true plasticity in beneficial traits can be compensated for by true plasticity in compensating traits. This study emphasizes that the role of true plasticity varies throughout ontogeny. It also suggests that the classical reaction norm or function-valued traits needs to be integrated with body size (i.e., size serial analysis) when we evaluate environmental effects on phenotypes.

title: Hierarchical Reproductive Allocation and Allometry within a Perennial Bunchgrass after 11 Years of Nutrient Addition
abstract: Bunchgrasses are one of the most important plant functional groups in grassland ecosystems. Reproductive allocation (RA) for a bunchgrass is a hierarchical process; however, how bunchgrasses adjust their RAs along hierarchical levels in response to nutrient addition has never been addressed. Here, utilizing an 11-year nutrient addition experiment, we examined the patterns and variations in RA of Agropyron cristatum at the individual, tiller and spike levels. We evaluated the reproductive allometric relationship at each level by type II regression analysis to determine size-dependent and size-independent effects on plant RA variations. Our results indicate that the proportion of reproductive individuals in A. cristatum increased significantly after 11 years of nutrient addition. Adjustments in RA in A. cristatum were mainly occurred at the individual and tiller levels but not at the spike level. A size-dependent effect was a dominant mechanism underlying the changes in plant RA at both individual and tiller levels. Likewise, the distribution of plant size was markedly changed with large individuals increasing after nutrient addition. Tiller-level RA may be a limiting factor for the adjustment of RA in A. cristatum. To the best of our knowledge, this study is the first to examine plant responses in terms of reproductive allocation and allometry to nutrient enrichment within a bunchgrass population from a hierarchical view. Our findings have important implications for understanding the mechanisms underlying bunchgrass responses in RA to future eutrophication due to human activities. In addition, we developed a hierarchical analysis method for disentangling the mechanisms that lead to variation in RA for perennial bunchgrasses.

title: Plasticity of reproductive allocation of a woody species (Ulex europaeus) in response to variation in resource availability
abstract: ContextWhile many woody perennials grow and reproduce in varying environments, their ability to modify their reproductive allocation under varying resource availability is unclear.AimsThis study aimed to demonstrate the occurrence of plasticity of reproductive allocation in a pioneer woody species (Ulex europaeus).MethodsWe studied seed production in 144 plants under different irradiance and population densities. We measured their basal area, their whole shoot biomass and photosynthetic biomass, and their reproductive output (number of fruits per individual and number of seeds per fruit).ResultsWe found that newly produced photosynthetic biomass was influenced by environmental variation, which in turn determined its reproductive output. In addition, in low light conditions investment in reproduction was more reduced than investment in growth indicating plasticity of reproductive allocation.ConclusionThese results provide the first evidence for plasticity of reproductive allocation of a woody plant. It may lead to the ability of this pioneer species to survive and to continue to reproduce even at low rates in non-optimal shady habitats and to maintain a non-empty seed bank in forest systems with long unfavourable periods between two disturbances.

title: Plasticity in reproduction and growth among 52 range‐wide populations of a Mediterranean conifer: adaptive responses to environmental stress
abstract: A plastic response towards enhanced reproduction is expected in stressful environments, but it is assumed to trade off against vegetative growth and efficiency in the use of available resources deployed in reproduction [reproductive efficiency (RE)]. Evidence supporting this expectation is scarce for plants, particularly for long‐lived species. Forest trees such as Mediterranean pines provide ideal models to study the adaptive value of allocation to reproduction vs. vegetative growth given their among‐population differentiation for adaptive traits and their remarkable capacity to cope with dry and low‐fertility environments. We studied 52 range‐wide Pinus halepensis populations planted into two environmentally contrasting sites during their initial reproductive stage. We investigated the effect of site, population and their interaction on vegetative growth, threshold size for female reproduction, reproductive–vegetative size relationships and RE. We quantified correlations among traits and environmental variables to identify allocation trade‐offs and ecotypic trends. Genetic variation for plasticity was high for vegetative growth, whereas it was nonsignificant for reproduction. Size‐corrected reproduction was enhanced in the more stressful site supporting the expectation for adverse conditions to elicit plastic responses in reproductive allometry. However, RE was unrelated with early reproductive investment. Our results followed theoretical predictions and support that phenotypic plasticity for reproduction is adaptive under stressful environments. Considering expectations of increased drought in the Mediterranean, we hypothesize that phenotypic plasticity together with natural selection on reproductive traits will play a relevant role in the future adaptation of forest tree species.

title: A hydraulic explanation for size-specific plant shrinkage: developmental hydraulic sectoriality.
abstract: • While great attention has been paid to the mechanisms controlling plant growth, much less is known about why and how plants shrink. The modular design of plants may facilitate the independence of modules if the xylem vasculature is hydraulically sectored. We examined the hydraulic connectivity of modules comprising juveniles and adults of the aridland chamaephyte Cryptantha flava (Boraginaceae), motivated by the observation that rosette mortality is spatially aggregated in adults, but not in juveniles. • We explored spatial patterns of leaf wilting after clipping a single lateral root, tracked physiological dyes taken up by a single root, and measured within-plant variation in leaf water potentials after watering a portion of the root system. We then measured xylem anatomical features related to hydraulic connectivity. • Our approaches revealed hydraulic integration in juveniles but hydraulic sectoriality in adults. We attribute such developmental changes to increasing distances between xylem bundles, and larger xylem lumen and heartwood areas as plants age. • We have demonstrated functional sectoriality in a desert chamaephyte, and report the mechanism by which sectoriality occurs, offering a hydraulic explanation for the death of whole plant portions resulting in shrinkage of large plants, and for the high occurrence of this design in deserts.

title: THE COST OF MERISTEM LIMITATION IN POLYGONUM ARENASTRUM: NEGATIVE GENETIC CORRELATIONS BETWEEN FECUNDITY AND GROWTH
abstract: Growth and reproduction in higher plants depend on meristems, which have three developmental fates. A meristem can become reproductive, but doing so terminates its activity, it can differentiate vegetatively, or it can remain quiescent for extended periods. The first two fates are mutually exclusive, and only the second leads to the production of additional meristems for subsequent growth and reproduction. In Polygonum arenastrum (frequently referred to as P. aviculare in North American Floras), an annual species lacking quiescent meristems, a quantitative genetic analysis of inbred full‐sibling families revealed genetic variation in the developmental pattern of axillary meristem commitment to vegetative growth versus reproduction. Developmental variation resulted in family differences in the age of first reproduction, in age‐specific fecundity and growth, and in final plant size and reproductive output. Furthermore, there were strong negative genetic correlations between age‐specific growth and fecundity. Early commitment of meristems to reproduction favors high early fecundity, but reduces the number of meristems available for vegetative differentiation, and leads to lowered growth rates and fecundity later in life, when meristems are limiting. Conversely, meristem commitment to vegetative growth early in life results in low early fecundity but high late fecundity and growth. Meristem limitation, like resource limitation, is a proximate mechanism that generates trade‐offs between life history traits. Differences between meristem limitation and resource limitation are discussed. Meristem limitation leads automatically to a senescent life history because of the determinate fate of reproductive meristems.

title: Allometry and plasticity of meristem allocation throughout development in Arabidopsis thaliana
abstract: The three principal fates of axillary meristems in flowering plants are branching or vegetative growth, reproduction, and inactivity. We examined the relationship between allocation to each of these fates and plant size in five genotypes of Arabidopsis thaliana. Nutrient availability was used to obtain a range of plant size. 
 
2 
Allocation to each meristem fate was recorded at weekly intervals throughout development and at final developmental stage. 
 
3 
 There were significant allometric relationships between meristem allocation and plant size for each meristem fate throughout development in each genotype. Allometry and plasticity tended to be strongest early in development. In addition, although there were differences in allometry between genotypes, meristem allocation through development was generally consistent across genotypes. 
 
4 
 We interpret the results of this study as an adaptive response to nutrient availability. Increased allocation to growth meristems in high nutrient treatments early in development should maximize both size and the total number of meristems that can be committed to reproduction later in development. This suggests that the pattern of meristem allocation is a component of an adaptive strategy through development in Arabidopsis thaliana.

title: Meristem Allocation as a Means of Assessing Reproductive Allocation
abstract: Publisher Summary
The principle of resource allocation states that resources are not sufficient to fully supply the demands of all plant functions. If resources are limited, allocation to reproduction must decrease the allocation to some other plant functions. Allocation of resources to reproduction (reproductive allocation) has implications for several study fields in biology. Early studies discussed allocation patterns in light of the evolution of life-history traits and interaction between life-history traits and population dynamics. Initial studies considered animals, but the idea was soon applied to plant ecology to explain how allocation affects plant life-history strategies and plant community dynamics. To test the principle of allocation, biologists have tried to measure the resource use of different plant functions. This task has turned out to be difficult. It is not clear which currency should be used to measure reproductive allocation. Meristems have been proposed as a currency to measure reproductive allocation. Meristems come as distinct reproductive or vegetative entities. This chapter aims to discuss whether meristem allocation can be used as a supplement or a substitute when studying reproductive allocation. It first describes the structural, developmental, and physiological background of the meristem system. It then uses a modeling approach to show theoretically how meristem allocation may affect fitness. It discusses model assumptions and reviews empirical studies that have tested allocation models and their assumptions.

title: Developmental Constraints: Effect on Population Growth and Patterns of Resource Allocation in a Clonal Plant
abstract: A comparison of ramet population growth rates in a flowering and a nonflowering population of Eichhornia crassipes Solms., the water hyacinth, indicates that inflorescence production is inversely correlated with ramet population growth rate. The differences in patterns of resource allocation exhibited by the two populations are explained in terms of developmental constraints acting on the differentiation and growth of a limited meristem population. The meristem population is limited in size by the developmental morphology of the ramets. This interpretation contrasts with more traditional ones in which fixed carbon, measured as dry weight, is viewed as the limiting currency. The inadequacy of fixed carbon as a limiting currency for plant resource allocation studies is discussed in relation to the origins of its use in the zoological literature. While most animals must acquire all usable carbon through a single mouth and then apportion it among competing needs, herbaceous plants generate new "mouths" (= carbon acquiring organs) as they build aboveground structure.

title: Plant reproductive strategies and resource allocation
abstract: A major theme in plant evolutionary ecology concerns the allocation of resources to fitness-enhancing activities. Patterns of allocation have been regarded as reflecting evolved reproductive strategies. Here I review recent studies that focus on problems of resource allocation. These studies concern resource currencies, plant ontogeny and bud dynamics, reproductive costs, and clonal growth. While there is a certain amount of tension between the descriptive utility and the evolutionary validity of the reproductive strategy concept, the idea of strategy remains a robust one.

title: Plasticity and allometry of meristem allocation in response to density in three annual plants with different architectures
abstract: An experiment was conducted to investigate plasticity of meristem allocation under intraspecific competition in three annual species (Xanthium sibiricum Patrin., Datura stramonium L., and Fagopyrum...

title: Ecophysiological behavior ofArtemisia ordosica on the process of sand dune fixation
abstract: The population structure and water relations ofArtemisia ordosica were studied at different stages of the sand dune fixation process. Vegetation coverage and biomass increased as the sand dune fixation process progressed. In contrast, individual growth rate decreased in the late fixation stage. On fixed sand dunes the modal age ofA. ordosica plants was higher and seedlings or saplings were rearely observed. On active sand dunes, settlement of seedlings was regulated by sand mobility; that is, seedlings were observed only when sand mobility was below 10 cm per year. Leaf transpiration was highest in active sand dunes. Stand transpiration in fixed sand dunes was lower than in semi-fixed sand dunes. These differences in transpiration were related to a decrease in soil water availability, which was affected by the increase in the aeolian fine soil component during the course of sand dune fixation. The reduction in soil water status and the cessation of sand movement were considered to be important factors in the decline of theArtemisia ordosica community.

title: Soil N retention and nitrate leaching in three types of dunes in the Mu Us desert of China
abstract: A large reservoir of soil nitrate in desert subsoil zones has been demonstrated in previous studies; however, information on the subsoil nitrate reservoir and its distribution characteristics in the deserts of China is still limited. This study investigated the distribution patterns of soil total nitrogen (N), nitrate, ammonium, and stable isotopic ratios of 15N (δ15N) in shallow (1 m) and subsoil (5 m) profiles in three types of dunes in the Mu Us desert of China. We found that soil N retention of the fixed and semi-fixed dunes followed a progressive nutrient depletion pattern in shallow soil profiles, whereas the subsoil nitrate of the fixed, semi-fixed and mobile dunes maintained a conservative accumulation pattern. The results indicate that the subsoil of the Mu Us desert may act as a reservoir of available nitrate. Furthermore, a soil δ15N analysis indicate that the nitrate content of the fixed dune is likely derived from soil nitrification, whereas the nitrate content in the mobile dune is derived from atmospheric nitrate deposition. Within the context of looming climate change and intensifying human activities, the subsoil nitrate content in the deserts of northern China could become mobilized and increase environmental risks to groundwater.

title: Habitat-specific demography across dune fixation stages in a semi-arid sandland: understanding the expansion, stabilization and decline of a dominant shrub
abstract: 1. Maintaining viable populations in different habitats requires physiological, morphological and demographic adaptations of plants. In sandland environments, plants experience substantial variation in growing conditions during the dune fixation process, with high sand mobility in early stages and denser vegetation cover in later stages.



2. We studied the changes in demography of a dominant shrub, Artemisia ordosica, at three stages of dune fixation: semi-fixed dunes, fixed dunes and fixed dunes covered with microbiotic crust. Demographic data from three annual censuses were used to parameterize integral projection models (IPMs) to conduct comparative demographic analyses.



3. Plant growth and reproduction decreased strongly as dunes became more fixed. Shrinkage in plant height occurred very frequently, particularly in the fixed dunes with microbiotic crust. Population growth rate (λ) declined substantially with dune fixation: from rapid expansion in semi-fixed dunes (λ = 1.35–1.09) to moderate decline in fixed dunes with microbiotic crust (λ = 0.94–0.89).



4. Elasticity analysis revealed that survival was a key vital rate for population growth in all habitats. Growth and fecundity were of higher importance in the semi-fixed habitat than in the other two habitats where shrinkage became an important factor determining λ. Seedlings and small plants were critical for population growth in semi-fixed dunes, whereas moderate to large-sized plants were most important in the other habitats.



5. Results of life table response experiments showed that the observed strong decrease in λ during dune fixation was mainly caused by reduction in fecundity, but with additional and considerable contributions from reduced plant growth and increased occurrence of shrinkage. Thus, populations in semi-fixed dunes are able to expand rapidly due to a much higher fecundity compared to those in other habitats.



6. Synthesis. Artemisia ordosica adopts different life history strategies along the dune fixation process. Fast expansion in semi-fixed dunes is enabled by high seed production and effective recruitment, while populations at later dune fixation stages are maintained through frequent plant shrinkage. Integral projection models are highly appropriate tools for analysing such life history changes as they are based on statistical comparisons of vital rates across habitats.

title: Effects of denudation and burial on growth and reproduction of Artemisia ordosica in Mu Us sandland
abstract: Plants in a dune environment have a high risk of being denudated by wind or buried by sand. We conducted a field experiment to assess growth and reproductive performance as well as biomass allocation of Artemisia ordosica after denudation and burial. Height growth decreased after denudation, but remained constant after burial; biomass was reduced by severe burial, but not by moderate ones. Vegetative growth measured by current-year generated vegetative branches declined with increasing severity of burial. Reproductive maturity was not affected by either denudation or burial, and reproduction decreased only after severe denudation. After denudation, biomass allocated to reproduction was greatly reduced, as shown by strongly reduced biomass allocation to fruits and whole reproductive branches, but that allocated to current-year vegetative growth was maintained. Biomass allocation to reproduction and vegetative growth remained rather stable after burial. Current-year reproductive-to-vegetative-biomass ratio was lower after denudation than after burial, indicating that vegetative growth was more favored by A. ordosica after denudation. These modifications are adaptive in the fluctuating environment of mobile sand dunes where denudation and burial frequently occur.

title: Habitat Effect on Allometry of a Xeric Shrub (Artemisia ordosica Krasch) in the Mu Us Desert of Northern China
abstract: Allometric models are useful for assessment of aboveground net primary productivity (ANPP) and aboveground biomass (AGB) of forests and shrubs, and are widely implemented in forest inventory and management. Multiple forms of allometric models have been used to estimate vegetation carbon storage for desert shrubland, but their validity for biomass estimation has not been tested at a region scale with different habitats. To verify the validity of habitat-specific models, general models (combining data from all habitats/sites), and previously developed models for biomass prediction, we developed both general models and habitat-specific models for aboveground biomass and ANPP of Artemisia ordosica Krasch, a dominant shrub of the Mu Us Desert. Our results showed that models based on crown area or canopy volume consistently explained large parts of the variations in aboveground biomass and ANPP. Model fitting highlighted that general allometric models were inadequate across different habitats, and habitat-specific models were useful for that specific habitat. Previous models might be inappropriate for other sites because of site quality differences. There was a strong habitat effect on the allometric relationships of A. ordosica. Although our study is a case in point, the results indicate that allometric models for desert shrubs should be used with caution and require robust validation if adopted from other studies or applied to different sites/habitats.

title: Increased Precipitation and Nitrogen Alter Shrub Architecture in a Desert Shrubland: Implications for Primary Production
abstract: Shrublands are one of the major types of ecosystems in the desert regions of northern China, which is expected to be substantially more sensitive to global environmental changes, such as widespread nitrogen enrichment and precipitation changes, than other ecosystem types. However, the interactive effects of nitrogen and precipitation on them remain poorly understood. We conducted a fully factorial field experiment simulating three levels of precipitation (ambient, +20%, +40%) and with two levels of nitrogen deposition (ambient, 60 kg N ha-1 yr-1) in a desert shrubland in the Mu Us Desert of northern China. We used plant architectural traits (plant cover, volume, twig size and number) as proxies to predict aboveground net primary productivity (ANPP) of the dominant shrub (Artemisia ordosica Krasch), and assessed the responses of plant productivity and architectural traits to water and nitrogen addition. We found significant differences in twig size and number of A. ordosica under water and nitrogen treatments but not in shrub cover/volume, which suggest that twig size and number of the shrub species were more sensitive to environmental changes. The productivity of the overall community was sensitive to increased precipitation and nitrogen, and shrubs played a more important role than herbaceous plants in driving productivity in this ecosystem. Precipitation- and nitrogen-induced increases in vegetation production were positively associated with increases in twig size and number of the dominant shrub. Water addition enhanced the twig length of A. ordosica, while nitrogen addition resulted in increased twig density (the number of twigs per square meter). Water and nitrogen interacted to affect twig length, but not twig number and shrub ANPP. The trade-off, defined as negative covariance between twig size and number, was likely the mechanism underlying the responses of twig length and shrub ANPP to water and nitrogen interactions. Our results highlight the sensitivity of twig size and number as indicators to estimate shrub production and the mechanism underpinning desert shrub ANPP response to global environmental changes.

title: Fractal approach to estimating changes in soil properties following the establishment of Caragana korshinskii shelterbelts in Ningxia, NW China
abstract: To identify the effects of shelterbelt establishment on soil recovery, we compared soil properties at depths of 0–5, 5–10, and 10–20 cm as affected by Caragana korshinskii shelterbelts across a chronosequence of soil undergoing restoration for 7, 11, and 26 years, as well as reference grasslands in Ningxia, NW China. Fractal dimension of soil particle size distribution (PSD) was also integrated to describe variations in soil properties. The results indicated that (1) fine particles, total porosity, total nitrogen, phosphorous, and available nitrogen, phosphorous, and potassium increased significantly with plantation age (p

title: Study on classification progress and cataloging system of sandy land in China
abstract: As unique landforms, sandy lands are mostly found in northern China, the QinghaiTibet Plateau, and along the edges of seas, lakes, and rivers. Sandy lands are generally in ecologically fragile zones and are important in maintaining regional sustainable development.Researchers in China have focused on sandy lands since the 1950 s and have proposed many classification methods for these lands. However, there is no widely accepted classification system, owing to a lack of integration of sandy land resources. Therefore, confusion has resulted from too many classification methods. Furthermore, the absence of a sandy land resource database might have restricted appropriate management of sandy land resources by the government and affected proper implementation of ecological management policies. In this paper, progress on sandy land research in China is comprehensively summarized. Sandy land classification methods and a cataloging system are also discussed. Results show that:(1)Existing sandy land classification methods are mainly based on sandy land mobility,desertification degree and processes, and geographic location. This may have resulted in the omission of some sandy land areas, a smaller quantitative classification index, degraded ability to express spatial information, a chaotic classification index system, and others. Moreover,systematicness and comparability of sandy land classification results were inadequate.(2)Vegetation coverage has been widely used for sandy land classification and its index can quantitatively express the mobility, desertification degree and processes, and other features of such land. According to climatic differences, using vegetation coverage to classify sandy land by sand mobility(fixed, semi- fixed and mobile sand) and desertification degree(mild,moderate, severe or very severe) could highlight the most important information.(3) The cataloging system here includes different code levels for identification, geographic location,mobility, desertification degree, and other sandy land features. The sandy land identification code can effectively differentiate desert, Gobi, and other desertification types. Based on an ecogeographical regional system of China, geographic location codes include information on 11 temperature zones, 21 wet and dry areas, and 49 natural regions, covering the entire sandy land area in the country. Those codes also effectively identify differences of temperature, humidity,vegetation, landscape, and others. Codes of mobility, desertification degree, area, and sandy land use can highlight its natural and social characteristics. Thus, the cataloging system of sandy land in this study is comprehensive, well-integrated, and easy to use.

title: Optimal balance of water use efficiency and leaf construction cost with a link to the drought threshold of the desert steppe ecotone in northern China.
abstract: In arid environments, a high nitrogen content per leaf area (Narea) induced by drought can enhance water use efficiency (WUE) of photosynthesis, but may also lead to high leaf construction cost (CC). Our aim was to investigate how maximizing Narea could balance WUE and CC in an arid-adapted, widespread species along a rainfall gradient, and how such a process may be related to the drought threshold of the desert-steppe ecotone in northern China.



Along rainfall gradients with a moisture index (MI) of 0·17-0·41 in northern China and the northern Tibetan Plateau, we measured leaf traits and stand variables including speciﬁc leaf area (SLA), nitrogen content relative to leaf mass and area (Nmass, Narea) and construction cost (CCmass, CCarea), δ(13)C (indicator of WUE), leaf area index (LAI) and foliage N-pool across populations of Artemisia ordosica



In samples from northern China, a continuous increase of Narea with decreasing MI was achieved by a higher Nmass and constant SLA (reduced LAI and constant N-pool) in high-rainfall areas (MI > 0·29), but by a lower SLA and Nmass (reduced LAI and N-pool) in low-rainfall areas (MI ≤ 0·29). While δ(13)C, CCmass and CCarea continuously increased with decreasing MI, the low-rainfall group had higher Narea and δ(13)C at a given CCarea, compared with the high-rainfall group. Similar patterns were also found in additional data for the same species in the northern Tibetan Plateau. The observed drought threshold where MI = 0·29 corresponded well to the zonal boundary between typical and desert steppes in northern China.



Our data indicated that below a climatic drought threshold, drought-resistant plants tend to maximize their intrinsic WUE through increased Narea at a given CCarea, which suggests a linkage between leaf functional traits and arid vegetation zonation.

title: Pitfalls and Possibilities in the Analysis of Biomass Allocation Patterns in Plants
abstract: Plants can differentially allocate biomass to leaves, stems, roots, and reproduction, and follow ontogenetic trajectories that interact with the prevailing climate. Various methodological tools exist to analyze the resulting allocation patterns, based either on the calculation of biomass ratios or fractions of different organs at a given point in time, or on a so-called allometric analysis of biomass data sampled across species or over an experimental growth period. We discuss the weak and strong points of each of these methods. Although both approaches have useful features, we suggest that often a plot of biomass fractions against total plant size, either across species or in the comparison of treatment effects, combines the best of both worlds.

title: The allometry of reproductive allocation in a Chloris virgata population in response to simulated atmospheric nitrogen deposition
abstract: The quantitative relationship between the size of a plant and its reproductive output is a central aspect of its reproductive strategy. Resource availability influences plant size and the allocation of biomass to different structures. Here we investigate how variation in nitrogen (N) addition affects the allocation of biomass to vegetative and reproductive structures in a population of Chloris virgata . We used four levels of N addition to simulate atmospheric N deposition in the field, and evaluated size-dependent and size-independent effects on reproductive allocation. Total biomass of C. virgata increased with increasing N levels but decreased at the highest level (20 g m −2  N). Reproductive output (mass of seeds produced) was higher when plants were fertilized with 2.5 g m −2  N than in the control treatment (no N addition), but further increases in N did not result in a significant further increase in seed production. The relationship between size and reproduction in C. virgata in response to different N levels was largely allometric (size dependent), but there were also size-independent effects. C. virgata allocated absolutely more, but proportionally less, biomass to reproduction in response to increased N availability, except at the very highest level of N addition, where biomass production was lower. C. virgata is adapted to low nutrient levels, and this limits its ability to utilize high N levels to produce more offspring.

title: SMATR 3 - an R package for estimation and inference about allometric lines
abstract: 1. The Standardised Major Axis Tests and Routines (SMATR) software provides tools for estimation and inference about allometric lines, currently widely used in ecology and evolution. 
 
2. This paper describes some significant improvements to the functionality of the package, now available on R in smatr version 3. 
 
3. New inclusions in the package include sma and ma functions that accept formula input and perform the key inference tasks; multiple comparisons; graphical methods for visualising data and checking (S)MA assumptions; robust (S)MA estimation and inference tools.

title: Allometry and development in herbaceous plants: functional responses of meristem allocation to light and nutrient availability.
abstract: We examined the relationship between meristem allocation and plant size for four annual plant species: Arabidopsis thaliana, Arenaria serphyllifolia, Brassica rapa, and Chaenorrhinum minus. Gradients of light and nutrient availability were used to obtain a range of plant sizes for each of these species. Relative allocation to reproductive, inactive, and growth meristems were used to measure reproductive effort, apical dominance, and branching intensity, respectively. We measured allocation to each of these three meristem fates at weekly intervals throughout development and at final developmental stage. At all developmental stages reproductive effort and branching intensity tended to increase with increasing plant size (i.e., due to increasing resource availability) and apical dominance tended to decrease with increasing plant size. We interpret these responses as a strategy for plants to maximize fitness across a range of environments. In addition, significant differences in meristem response among species may be important in defining the range of habitats in which a species can exist and may help explain patterns of species competition and coexistence in habitats with variable resource availability.

title: The evolutionary ecology of vegetative dormancy in mature herbaceous perennial plants
abstract: This kind of vegetative dormancy has been noted for at least 40 years, but has only recently become a topic of study. 
 
 
 
2. Vegetative dormancy may be considered in a life-history context. Both vegetative dormancy and mortality typically decrease with increasing size. Vegetative dormancy’s relationship to reproduction is more complex, because some species increase flowering and fruiting after dormancy while others do the opposite. 
 
 
 
3. If vegetative dormancy is adaptive, then it is most likely a bet-hedging trait. Dormancy-prone plants are often long-lived, and in such organisms, bet-hedging traits should counter the effects of environmental stochasticity on adult survival. This adaptive context may vary by life span, because in shorter-lived plants, fitness is most sensitive to changes in reproduction rather than survival. 
 
 
 
4. Vegetative dormancy could evolve if the costs of sprouting ever outweigh the benefits. The benefits of sprouting include: (i) photosynthesis and (ii) the opportunity to flower and reproduce. The costs include: (i) greater chance of herbivory, (ii) greater need for limiting nutrients, and (iii) greater maintenance costs. The many losses of photosynthesis among plants suggest that these benefits may not always outweigh the costs. 
 
 
 
5. Vegetative dormancy may be an evolutionary step towards the loss of photosynthesis. Many non-photosynthetic plants acquire carbon from their mycorrhizal fungi. Many autotrophic, dormancy-prone plants also acquire some carbon from their mycorrhizal fungi. Further, non-photosynthetic plants often become dormant to an even greater extent than autotrophic, dormancy-prone plants. 
 
 
 
6.Synthesis Vegetative dormancy often occurs in clades with non-photosynthetic, myco-heterotrophic plants, with implications for the evolution of traits involved in carbon nutrition. The links between vegetative dormancy, other life-history traits, mycorrhizas and the loss of photosynthesis should provide exciting directions for further research in plant evolutionary ecology. Particularly needed is an assessment of the physiology of vegetative dormancy, including whether the mycorrhiza is a carbon source in all dormancy-prone plant species. Equally important is a better understanding of the genetic relationships among photosynthesis, myco-heterotrophy and dormancy.

title: Are winter and summer dormancy symmetrical seasonal adaptive strategies? The case of temperate herbaceous perennials
abstract: Dormancy in higher plants is an adaptive response enabling plant survival during the harshest seasons and has been more explored in woody species than in herbaceous species. Nevertheless, winter and summer shoot meristem dormancy are adaptive strategies that could play a major role in enhancing seasonal stress tolerance and resilience of widespread herbaceous plant communities.  This review outlines the symmetrical aspects of winter and summer dormancy in order to better understand plant adaptation to severe stress, and highlight research priorities in a changing climate. Seasonal dormancy is a good model to explore the growth–stress survival trade-off and unravel the relationships between growth potential and stress hardiness. Although photoperiod and temperature are known to play a crucial, though reversed, role in the induction and release of both types of dormancy, the thresholds and combined effects of these environmental factors remain to be identified. The biochemical compounds involved in induction or release in winter dormancy (abscisic acid, ethylene, sugars, cytokinins and gibberellins) could be a priority research focus for summer dormancy. To address these research priorities, herbaceous species, being more tractable than woody species, are excellent model plants for which both summer and winter dormancy have been clearly identified.  Summer and winter dormancy, although responding to inverse conditions, share many characteristics. This analogous nature can facilitate research as well as lead to insight into plant adaptations to extreme conditions and the evolution of phenological patterns of species and communities under climate change. The development of phenotypes showing reduced winter and/or enhanced summer dormancy may be expected and could improve adaptation to less predictable environmental stresses correlated with future climates. To this end, it is suggested to explore the inter- and intraspecific genotypic variability of dormancy and its plasticity according to environmental conditions to contribute to predicting and mitigating global warming.

title: Carbon and water exchange over a temperate semi-arid shrubland during three years of contrasting precipitation and soil moisture patterns
abstract: Water is considered a key factor affecting ecosystem carbon exchange in dryland regions. However, the relationship between inter-annual variations in precipitation and ecosystem productivity remains to be clarified for arid and semi-arid areas. Based on eddy-covariance measurements, we examined how ecosystem production in a shrubland of northern China varied over three years (2012–2014) with contrasting precipitation and soil moisture patterns. Net ecosystem production ( NEP ) was 77 ± 10 (± standard deviation), −4 ± 10 and −22 ± 5 g C m −2 in 2012–2014, respectively, indicating a rapid shift from an annual sink to a source of carbon. Gross ecosystem production ( GEP ), total ecosystem respiration ( TER ) and evapotranspiration ( ET ) also declined over the three years. Annual carbon and water fluxes appeared to be suppressed in years with low spring soil moisture, which declined dramatically from 2012 to 2014. GEP declined more than TER and ET , leading to reduced carbon sequestration capacity and water use efficiency ( WUE  =  GEP / ET ). Neither annual nor growing-season precipitation could explain the year-to-year variations in carbon fluxes, whereas at our site ET was a better proxy for water available to ecosystem carbon exchange on an annual basis. Autumn soil moisture levels were carried over winter to the following spring, and, thus, may affect the rates of leafout, plant growth and carbon uptake in the early- to mid-growing season. Our conclusions were drawn from only three years of measurements and are therefore preliminary. Longer timeseries encompassing a wider range of precipitation and soil moisture conditions are needed to confirm or refine these conclusions. Our findings highlight the importance of precipitation timing and soil moisture carry-over in controlling ecosystem productivity. Winter warming and decreases in autumn and winter precipitation may induce spring drought and thus impair the carbon sequestration potential of shrubland and steppe ecosystems in semi-arid and arid Eurasia.