Selection for fertility in mice using different methods of litter size manipulation

Summary A short-term selection experiment for increasing the first-day litter size (LS1) and 28-day litter weight (LW28) was conducted with three populations of mice over 8 generations. Different methods of litter size manipulation were used for the populations — in S the litter size was standardized to 8 (4 , 4 ) on the first day, in LA it was adjusted to the average size of all litters born on the same day and NL had the natural litter size. To eliminate temporary environmental effects, a control population was kept in each case. The selection results per generation were, for LS 1 b=0.30 (S, NL) and 0.20 (LA), and for LW28 b=5.62 g (S), 5.26 g (NL), and 4.32 g (LA). The heritability obtained was between 0.18 and 0.13 for LS 1 and from 0.42 to 0.12 for LW28. The populations differed in the correlated responses for body weight parameters (litter weight gain). The implantation rate increased in populations S and NL (b=0.19, 0.37), but not in population LA. Postnatal mortality went down (b=-0.07) and the dam's milk production rose (b=1.11 g) only in population LA. The estimated partial regression coefficient linking body weight at mating (BWM) for the dam and the daughter's litter size showed an effect on the litter size.     

Chiasma frequency in strains of mice selected for litter size and for high body weight

Summary Chiasma frequency was measured in male mice of three outbred lines: FZt:DU (control); DU:6, selected for increased body weight; and DU:C, selected for high fertility. Chiasma frequency was seen to increase in the high body weight line, but decrease in the high fertility line. In both selected lines the intragroup variance in chiasma frequency increased while in DU:C the intracell variance was lower than in the control.     

Selection response for litter size at birth and litter weight at weaning in the first parity in mice

Summary Five 60-pair lines of mice were selected for seven generations for the following criteria: number born alive (LSO), random selection of litters (LC), number born alive divided by the weight of the dam at 9 weeks (LSO/DWT), total litter weight weaned divided by the weight of the dam at 9 weeks (LWT/DWT), and weight of litter weaned (LWT). All traits were measured in the first parity only and litters were not standardized. Realized heritabilities for LSO, LWT, LSO/DWT, and LWT/ DWT were 0.10± 0.06, 0.11±0.07, 0.22 ± 0.04, and 0.22 ± 0.08, respectively. Selection response for the two ratio lines was due to correlated responses in the respective numerators, LSO and LWT, as DWT did not decrease. In terms of improving LWT, selection for LWT/ DWT was three times as effective as selection for LSO/ DWT.     

Influence of early pregnancy on reproductive rate in lines of mice selected for litter size

Summary The influence of male-induced early puberty on female reproductive rate was determined in three lines of mice differing in litter size and body weight. The lines originated from a single base population and had undergone 20 generations of selection for the following criteria: large litter size at birth (L⁺), large litter size and small 6-week body weight (L⁺W^–), or small litter size and large 6-week body weight (L^–W⁺). Females were paired with a mature intact male of the same line at 3, 5 or 7 weeks of age. Mean mating age, averaged over lines, was 26.5 ± .3, 38.3 ± .3 and 52.7 ± .3 days. Exposure to a mature male accelerated female sexual maturation in each line. When contrasted with their sibs mated at a later age, early-pregnant females from each line exhibited a decline in one or more component of reproductive performance, suggesting that the physiological state of the very young female was not optimum for normal pregnancy. In comparisons of early and later mating ages, all three lines showed a decreased littering rate at first mating, number born alive, and individual birth weight of progeny adjusted for litter size; L⁺ and L⁺W^– mice showed an increased perinatal mortality rate; L⁺ and L^–W⁺ had a reduction in litter size at birth. When the L⁺, L⁺W^– and L^–W⁺ lines were compared with an unselected strain and a line selected for high postweaning gain in similar experiments, a genotype by environment interaction was apparent since all lines did not respond in a similar manner to early mating. The line ranking for litter size at birth for each age at male-exposure was L⁺>L⁺W^–>L^–W⁺, despite the significant line by age interaction. When litter size was adjusted by covariance for body weight at mating, the significant effects of age at male-exposure and line by age interaction were eliminated. All fertile females were remated after they had weaned their first litter to obtain information on litter size in parity two. Line differences in litter size at birth and number born alive were uniform across parities. An age by parity interaction was evident since the decreased fecundity at younger ages of male exposure in the L⁺ and L^–W⁺ litters of parity one was not evident in parity two. Litter feed efficiency during first parity gestation was defined as litter birth weight divided by either cumulative feed intake of the dam from mating to parturition (GEI) or cumulative feed intake from weaning to parturition (GEII). The ranking of lines for GEI and GEH was L⁺> L⁺W^–>L^–W⁺, but when feed efficiency was adjusted for littering rate, L⁺W^– and L^–W⁺ were not significantly different. With regard to age at mating, the ranking for GEI (7 wk > 5 wk > 3 wk) was reversed from GEII (3 wk >5 wk > 7 wk) and these significant differences were maintained after adjustment for littering rate.Paper No. 6302 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentionedOn leave at the Institute of Animal Genetics, West Mains Road, Edinburgh EH9 3JN, Scotland. Supported, in part, by an Underwood Fund Fellowship, Agricultural Research Council, and an Edinburgh University Biological Fellowship in the Department of Genetics     

Food resource allocation patterns in lactating females in a long-term selection experiment for litter size in mice

Resource allocation patterns, as quantified by residual food intake (RFI), and the consequences for offspring development were investigated during lactation in 96 females of a mouse line selected for 104 generations for high litter size at birth (S-line) and in 87 females of a non-selected control line (C-line). Litters of 45 C-line dams (Cs) and 48 S-line dams (Ss) were standardised (s) at birth; other dams (ns) supported total number of pups born (Cns and Sns, respectively). RFI during lactation was significantly lower in Sns-dams than in C-line dams and Sns-dams. After weaning Sns-dams seemed to be able to restore the negative resource situation. Sns-pups were about 25% less mature than Cns-pups at all times. Maturity was similar for Cs- and Ss-pups from 2 d in lactation on, and about 18% and 53% higher than Cns- and Sns-pups. The pre-weaning mortality rate was significantly higher in Sns-litters (35.6 ± 2.76) than in Cns-litters (4.95 ± 2.23). The results suggest that S-line dams allocated considerably more resources to maintenance of offspring than C-line dams. This was insufficient to provide the offspring with an adequate amount of resources, resulting in reduced pup development and increased pre-weaning mortality rates.     

Genetic parameters for canalisation analysis of litter size and litter weight traits at birth in mice

The aim of this research was to explore the genetic parameters associated with environmental variability for litter size (LS), litter weight (LW) and mean individual birth weight (IW) in mice before canalisation. The analyses were conducted on an experimental mice population designed to reduce environmental variability for LS. The analysed database included 1976 records for LW and IW and 4129 records for LS. The total number of individuals included in the analysed pedigree was 3997. Heritabilities estimated for the traits under an initial exploratory approach varied from 0.099 to 0.101 for LS, from 0.112 to 0.148 for LW and from 0.028 to 0.033 for IW. The means of the posterior distribution of the heritability under a Bayesian approach were the following: 0.10 (LS), 0.13 (LW) and 0.03 (IW). In general, the heritabilities estimated under the initial exploratory approach for the environmental variability of the analysed traits were low. Genetic correlations estimated between the trait and its variability reached values of -0.929 (LS), -0.815 (LW) and 0.969 (IW). The results presented here for the first time in mice may suggest a genetic basis for variability of the evaluated traits, thus opening the possibility to be implemented in selection schemes.     

Increases in plasma corticosterone and stretched-attend postures in rats naive and previously exposed to the elevated plus-maze are sensitive to the anxiolytic-like effects of midazolam

A single exposure to the elevated plus-maze test (EPM) reduces or abolishes the anxiolytic efficacy of benzodiazepines on a second trial. This phenomenon known as one-trial tolerance (OTT) is considered to be due to a shift in the emotional state of the animals across the test/retest sessions. Activation of the hypothalamic-pituitary-adrenocortical (HPA) axis has been considered to be an adaptive response to stressful or challenging situations such as height and openness of the EPM. This work looks at the phenomenon of OTT to the benzodiazepine compound midazolam through the conjoint examination of the novel ethological measures of the EPM and adrenocortical response of rats exposed to single and repeated sessions of the EPM. The results obtained confirmed that the approach/avoidance conflict on the first trial of the EPM is very sensitive to the anxiolytic effects of benzodiazepines. Moreover, stressful stimuli present upon initial exposure to the EPM render the standard measures of the EPM resistant to the anxiolytic effects of benzodiazepines on retest. However, the increases in plasma corticosterone and in risk assessment behavior observed in rats submitted to single or repeated sessions in the EPM were reversed by pretreatment with midazolam. The administration of metyrapone, a glucocorticoid synthesis blocker, decreased risk assessment but did not affect locomotion and anxiety-like behaviors. It is suggested that the detection of the dangerous environment through the stretched-attend postures in the second trial leads to the known low level of exploration and the consequent OTT upon retest. Moreover, in view of the similarity between the risk assessment and plasma corticosterone patterns in both naive and experienced rats, this hormone-behavior profile may be crucial for the expression of OTT to benzodiazepines in rodents exposed to the EPM.     

Direct and correlated responses to selection for weaning weight,post-weaning weight gain and six-week weight in mice

Summary Four lines of mice were formed from a common base population and selected for 37 generations for either increased 3-week weight (weaning weight), 6-week weight, 3–6 week gain, or maintained as a randomly bred control line. Realised heritability estimates for short-term (long-term) responses were 0.33±0.20 (0.07±0.10), 0.46±0.14 (0.26±0.09), 0.36±0.14 (0.24±0.11) for 3-week weight, 6-week weight and 3–6 week gain, respectively. Realised genetic correlations estimated from short-term (long-term) responses were 0.23±0.08 (0.35±0.10) between 3-week weight and 3–6 week gain; 0.82±0.04 (0.58±0.08) between 3-week weight and 6-week weight; and 0.81±0.04 (0.97±0.04) between 3–6 week gain and 6-week weight. The genetic correlation between 3-week weight and 6-week weight was asymmetric with a greater correlated response for 3-week weight when selecting for 6-week weight (1.06) than vice versa (0.63).     

Restricted selection index in mice designed to change body fat without changing body weight: correlated responses

Summary Correlated responses were studied in lines of mice selected for eight generations based on the criterion of a restricted selection index. Two replicate lines were selected in each treatment as follows: HE, high epididymal fat pad weight (EF) with zero change in body weight (BW) at 12 weeks of age; LE; low EF with zero change in BW; and RS, randomly. Correlated responses showed considerable variation between replicates, suggesting that genetic drift was important. Further, correlated responses for most traits were relatively small, probably because of low selection intensity. The HE line responded as expected in component traits of the restricted index. Associated compositional traits in HE responded as predicted since traits correlated with adiposity increased and hind carcass weight did not change significantly. Feed intake increased and feed efficiency (weight gain/feed intake) decreased in HE, as predicted. In contrast, the LE line did not respond in component traits as predicted since EF did not decrease and BW increased. Consequently, LE exhibited little change in traits associated with adiposity, but hind carcass weight, feed intake and feed efficiency increased. Of the correlated responses scored for fitness traits (littering rate, number of days from pairing of mate to littering, litter size and preweaning pup survival rate), significant effects were found for decreased littering rate in LE and increased prenatal survival rate in HE. In summary, correlated responses to restricted index selection generally agreed with expectation when responses in component traits of the index were considered.The research reported in this publication was funded by the North Carolina Agricultural Research Service (NCARS), Raleigh, NC 27695-7643, USA. The use of trade names in this publication does not imply endorsement by the NCARS, nor criticism of similar ones not mentioned     

Genome-enabled methods for predicting litter size in pigs: a comparison

Predictive ability of models for litter size in swine on the basis of different sources of genetic information was investigated. Data represented average litter size on 2598, 1604 and 1897 60K genotyped sows from two purebred and one crossbred line, respectively. The average correlation (r) between observed and predicted phenotypes in a 10-fold cross-validation was used to assess predictive ability. Models were: pedigree-based mixed-effects model (PED), Bayesian ridge regression (BRR), Bayesian LASSO (BL), genomic BLUP (GBLUP), reproducing kernel Hilbert spaces regression (RKHS), Bayesian regularized neural networks (BRNN) and radial basis function neural networks (RBFNN). BRR and BL used the marker matrix or its principal component scores matrix (UD) as covariates; RKHS employed a Gaussian kernel with additive codes for markers whereas neural networks employed the additive genomic relationship matrix (G) or UD as inputs. The non-parametric models (RKHS, BRNN, RNFNN) gave similar predictions to the parametric counterparts (average r ranged from 0.15 to 0.23); most of the genome-based models outperformed PED (r = 0.16). Predictive abilities of linear models and RKHS were similar over lines, but BRNN varied markedly, giving the best prediction (r = 0.31) when G was used in crossbreds, but the worst (r = 0.02) when the G matrix was used in one of the purebred lines. The r values for RBFNN ranged from 0.16 to 0.23. Predictive ability was better in crossbreds (0.26) than in purebreds (0.15 to 0.22). This may be related to family structure in the purebred lines.     

Correlated responses of growth traits to selection for high and low plasma alkaline phosphatase in mice

Summary The effectiveness of two way selection for plasma alkaline phosphatase (ALP) was investigated in order to determine its influences on growth traits through thirteen generations. The responses of the two lines selected for high (HP) and low (LP) ALP at 45 days of age were compared to that of the mice selected for large (L) and small (SM) body size. The selection responses of plasma ALP were very effective for both HP and LP lines, with average responses per generation calculated from linear regressions of 0.227±0.037 and –0.088±0.022 respectively. The final levels of ALP in HP and LP were 5.54±0.71 and 1.27±0.20 in the thirtheenth generation, while the SM, L and base population had levels of 3.49±0.08, 0.86±0.55 and 2.77±0.56 respectively. The body weight at 45 days of age in LP (31.4±1.4 g) as a correlated response was significantly higher than HP (23.4±1.8 g) at generation 10. The correlated response of milk yield, measured by weight gain up to 12 days of age, was significantly greater in the LP line than in HP, but the correlated response of gains after weaning was not so different as the response of milk yield. The response of litter size and weight in LP showed significant higher levels than that of HP, but pups' birth weight did not differ between LP and HP. It is suggested that the correlated response of milk yield contributed more to the divergence of body size between HP and LP than the gain after weaning.Realized heritabilities of ALP were 0.335±0.059 (HP) and 0.279±0.051 (LP). Realized genetic correlations between ALP and 45 days' body weight were –0.27±0.13 (HP with SM) and –0.52±0.19 (LP with L). Realized genetic correlations between ALP and milk yield were –0.95±0.03 (HP) and –0.37±0.29 (LP). Correlations between ALP and postweaning gains were fairly low.     

Direct and correlated responses to selection for post-weaning weight gain on ad libitum or restricted feeding in mice

Summary Selection for post-weaning weight gain in mice from 21 to 42 days, on either a full or restricted feeding level during this period was carried out for seven generations. Control lines were maintained for each feeding level. The rate of selection response was higher on full feeding due to a higher heritability and a larger phenotypic variance. Realised heritabilities of 0.29±0.05 and 0.19±0.04 for selection on full and restricted feeding respectively, were in close agreement with base population estimates.Selection on full feeding led to positive correlated responses in 21 day weight, 42 day weight, food intake and efficiency between 21 and 42 days, and 42 day tail length, but with little change in reproductive performance.Correlated responses to selection on restricted feeding were reduced 21 day weight, but an increase in 42 day weight and increased efficiency from 21 to 42 days. However, overall reproductive performance fell.     

Selection for litter size in pigs. II. Efficiency of closed and open selection lines*

A selection experiment on litter size in the pig was carried on for seventeen generations in an Inra experimental herd. The founder population was made up of 10 males and 120 females from the Large White breed. Selection was first performed for ten generations in a closed line, compared to a control line derived from the same founder population. Selection was carried on within sire family on the total number of piglets born in the first two litters of the dam (TB1 + TB2). After ten generations, the selection criterion became dam TB1 only. The control line was then discontinued and a panel of frozen semen from the 11th generation boars was created for later comparisons. The selected line was opened to gilt daughters of hyperprolific boars and sows, at a rate of 1/8 per generation, and the same selection procedure was applied irrespective of the origin of the gilt. During the whole experiment, the number of ova shed (OS) and the number of live embryos (LE) at 30 days in the 3rd pregnancy were recorded. These two parts of the experiment were analysed using REML estimation of genetic parameters and a BLUP-Animal Model in order to estimate the responses to selection. Significant heritabilities for TB1, TB2, OS and LE were obtained, i.e. 0.10, 0.05, 0.43 and 0.19, respectively. Significant common environment variances and covariances were estimated for nearly all traits. Significantly positive BLUP responses per generation were observed from G0 to G17 for TB1 (+0.086), TB2 (+0.078), OS (+0.197) and LE (+0.157). However, the responses were 3- to 4-fold higher in the G12–G17 interval compared to G0–G11, and they were also in fair agreement with previous estimates based on standard least-squares procedures, using the control line and the control frozen semen panel. Since G11, the selection intensity was increased by nearly 80 p. cent compared to the previous generations, and the proportion of hyperprolific ancestry increased up to 65 p. cent in the sows of the last generation. The total genetic gain of about 1.4 piglets at birth per litter could be shared between a gain due to immigration, of about 0.8 piglets per litter, and a within-line selection gain of about 0.6 piglets. Thus by combining selection and immigration in the second part of the experiment, advantage could be taken from both the genetic superiority of the immigrants and the higher internal selection intensity made possible by immigration.     

The effects of selection for gain in mice on the direct-maternal genetic correlation

Components of genetic variation for postweaning growth traits were estimated for both control and growth stocks of mice. The effect of phenotypic selection for gain, which genetically combines selection for additive direct and maternal effects, on additive genetic variance components, heritability, and additive genetic correlationsis discussed. Quantitative genetic theory predicts that simultaneous selection for two metric traits in the same direction will cause the genetic correlation between the two traits to become more negative. The results presented in this paper conflict with this theory. The direct-maternal additive genetic correlation was more negative in the control line (with 356 mice) than in the growth-selected line (with 320 mice) for the three traits analyzed (0.310 vs 0.999 for 21-day weight, 0.316 vs 1.000 for 42-day weight, and 0.506 vs 1.000 for gain from 21–42 days). Estimates were obtained by restricted maximum likelihood (REML) computed under a derivative free algorithm (DFREML).     

Effect of selection for spot size on reproduction and body weight in mice

Summary Spot size in descendants from the Goodale white-spotted stock of mice responded to selection for increased spot size. The realized heritability estimate was 0.52. However, no correlated response of reproduction to spot size selection was found in the present study, nor was there any correlated response among body weight variables.Joint project of Purdue University and USDA-SEA-ARS, North Central Region. Journal paper number 8279 from the Purdue Agricultural Experiment Station     

Response to selection for litter size in Danish Landrace pigs: a Bayesian analysis

A replicated selection experiment aimed at increasing litter size (total number of pigs born per litter) in Danish Landrace pigs was conducted from 1984 to 1991. The experiment included two selection and two control lines. In each generation, 30 and 14 first litters were produced in selection and control lines, respectively, and dams produced two litters. Each replicate, consisting of one selection and one control line, was founded from 60 families chosen randomly from the population at large. Family selection was practiced, and the criterion was the predicted breeding value for litter size computed using a repeatability animal model, and taking into account all available information. The data consisted of 947 records from 523 dams (424 dams had two litters) representing five cycles of selection of increased litter size. Data were analyzed from a Bayesian perspective, based on marginal posterior distributions of genetic parameters of interest. Marginalization was achieved using Gibbs sampling, with a single chain length of 1 205 000. After discarding the first 5 000 iterations, a sample was drawn every ten iterations, so 120 000 samples in total were saved. Densities were estimated and plotted, and summary statistics were computed from the estimated densities. The posterior means (± standard error) of heritability and repeatability were 0.22 ± 0.06 and 0.32 ± 0.05, respectively. These point estimates of genetic parameters were within the range of literature values, although on the high side. The posterior mean (± standard error) of genetic response to selection, defined as the difference between the mean breeding values of the selected lines and that of the base population, was 1.37 ± 0.43 pigs after five cycles of selection. The regression (through the origin) of breeding values in the selected lines on generation was 0.25 ± 0.08 pigs. Several informative priors constructed from information obtained with field data in this population were used to examine their influence on inferences. The priors were influential because of the relatively small scale of the experiment. An analysis excluding data from one of the control lines gave smaller genetic variance and heritability, and a smaller response to selection. However, it appears that selection for litter size is effective, but that the true rate of response is probably smaller than data from this experiment suggest.     

Crossbreeding effects after long-term selection for purebred performance: a model experiment with mice

Summary Results are presented from two replicated three-breed cross diallels that were conducted after 20 generations of selection for purebred performance in mice. The selection criteria for the different lines were: litter size at birth (LS), weaning weight at 4 weeks (WW), weight gain from week 4 to week 6 (WG), and body fat content at week 6 (FT). Additionally, a random-mating control line (C) was kept. Significant maternal heterosis was found in litter size and weaning weight. Estimates of maternal heterosis in litter size were very high, ranging from 17 to 50% of the mean of the corresponding single crosses. Maternal heterosis in weaning weight usually was negative and ranged from +9 to -11%. Significant maternal heterosis in feed efficiency and weaning weight could only be found in a few cases. Total performance of three-breed crosses was highly superior to that of single crosses and purebreds. Means of the corresponding purebreds or single crosses were of little use in predicting three-breed cross performance.     

Experimental litter size reduction reveals costs of gestation and delayed effects on offspring in a viviparous lizard

Experimental studies have often been employed to study costs of reproduction, but rarely to study costs of gestation. Disentangling the relative importance of each stage of the reproductive cycle should help to assess the costs and benefits of different reproductive strategies. To that end, we experimentally reduced litter size during gestation in a viviparous lizard. We measured physiological and behavioural parameters during gestation and shortly after parturition, as well as survival and growth of females and their offspring. This study showed four major results. First, the experimental litter size reduction did not significantly affect the cellular immune response, the metabolism and the survival of adult females. Second, females with reduced litter size decreased their basking time. Third, these females also had an increased postpartum body condition. As postpartum body condition is positively related to future reproduction, this result indicates a gestation cost. Fourth, even though offspring from experimentally reduced litters had similar weight and size at birth as other offspring, their growth rate after birth was significantly increased. This shows the existence of a maternal effect during gestation with delayed consequences. This experimental study demonstrates that there are some costs to gestation, but it also suggests that some classical trade-offs associated with reproduction may not be explained by gestation costs.     

Furnished cages for laying hens: study of the effects of group size and litter provision on laying location, zootechnical performance and egg quality

The furnished cage is a new housing system for layers. A current trend in furnished cage design is to increase group size and replace the litter box with a mat provided with litter. An experiment was set up to determine the effects of group size and litter provision on laying performance and egg quality of beak-trimmed ISA Brown hens housed in large furnished cages with more than 12 hens. Six treatments, each of 18 furnished cages (768 cm(2)/hen including nest and litter area) were compared in a 3 × 2 experimental trial: three group sizes (S20 (20 hens per cage), S40 (40) and S60 (60)), with or without feed as litter distributed on the mat of the litter area. The provision of facilities per hen was equal in all treatments. Mortality, laying rate, mean egg weight, feed intake and feed conversion ratio were unaffected by group size over the 53-week laying period, and performance exceeded the ISA production standards. The overall percentage of eggs laid in the nest exceeded 95% except that it was slightly lower in group S20 (92.0% ± 6.4% v. S40: 96.0% ± 3.3% and S60: 96.2% ± 2.7%) leading to a higher proportion of dirty eggs (S20: 1.6% ± 2.2%, S40: 1.4% ± 1.5%, S60: 1.0% ± 1.0%). At 66 to 70 weeks, eggs laid outside the nest had a slightly higher count of mesophilic bacteria on the eggshell (5.0 log CFU/egg ± 0.4) than those laid in the nest (4.8 log CFU/egg ± 0.5) but no difference in contamination was observed between group sizes. Litter provision had no effect on mortality, egg weight or egg quality traits except for a higher proportion of broken eggs in cages with litter (5.3% ± 6.2% v. 4.6% ± 5.7%). Providing hens with feed for litter was associated with a higher laying rate (97.3% ± 3.2% v. 94.8% ± 4.4% at 23 weeks) and an apparent improvement in feed efficiency at the beginning of the laying period (feed conversion ratio based on feed consumption at the trough: 2.18 ± 0.06 with litter v. 2.28 ± 0.09 without litter at 25 weeks). The results of this study showed that a high level of productivity and good egg quality could be obtained in large furnished cages. Further research is needed to assess the impact on hens' welfare and performance of using more economically competitive substrates than feed for litter.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.