Shucun Sun

Shifting phenology and abundance under experimental warming alters trophic relationships and plant reproductive capacity.

Phenological mismatches due to climate change may have important ecological consequences. In a three-year study, phenological shifts due to experimental warming markedly altered trophic relationships between plants and insect herbivores, causing a dramatic decline of reproductive capacity for one of the plant species. In a Tibetan meadow, the gentian (Gentiana formosa) typically flowers after the peak larva density of a noctuid moth (Melanchra pisi) that primarily feeds on a dominant forb (anemone, Anemone trullifolia var. linearis). However, artificial warming of approximately 1.5 degrees C advanced gentian flower phenology and anemone vegetative phenology by a week, but delayed moth larvae emergence by two weeks. The warming increased larval density 10-fold, but decreased anemone density by 30%. The phenological and density shifts under warmed conditions resulted in the insect larvae feeding substantially on the gentian flowers and ovules; there was approximately 100-fold more damage in warmed than in unwarmed chambers. This radically increased trophic connection reduced gentian plant reproduction and likely contributed to its reduced abundance in the warmed chambers.

THE GENERALITY OF LEAF SIZE VERSUS NUMBER TRADE-OFF IN TEMPERATE WOODY SPECIES

BACKGROUND AND AIMS Trade-offs are fundamental to life-history theory, and the leaf size vs. number trade-off has recently been suggested to be of importance to our understanding leaf size evolution. The purpose of the present study was to test whether the isometric, negative relationship between leaf size and number found by Kleiman and Aarssen is conserved between plant functional types and between habitats. METHODS Leaf mass, area and number, and stem mass and volume of current-year shoots were measured for 107 temperate broadleaved woody species at two altitudes on Gongga Mountain, south-west China. The scaling relationships of leaf size (leaf area and mass) vs. (mass- and volume-based) leafing intensity were analysed in relation to leaf habit, leaf form and habitat type. Trait relationships were determined with both a standardized major axis method and a phylogenetically independent comparative method. KEY RESULTS Significant negative, isometric scaling relationships between leaf size and leafing intensity were found to be consistently conserved across species independent of leaf habit, leaf form and habitat type. In particular, about 99 % of the variation in leaf mass across species could be accounted for by proportional variation in mass-based leafing intensity. The negative correlations between leaf size and leafing intensity were also observed across correlated evolutionary divergences. However, evergreen species had a lower y-intercept in the scaling relationships of leaf area vs. leafing intensity than deciduous species. This indicated that leaf area was smaller in the evergreen species at a given leafing intensity than in the deciduous species. The compound-leaved deciduous species were observed usually to have significant upper shifts along the common slopes relative to the simple-leaved species, which suggested that the compound-leaved species were larger in leaf size but smaller in leafing intensity than their simple counterparts. No significant difference was found in the scaling relationships between altitudes. CONCLUSIONS The negative, isometric scaling relationship between leaf size and number is largely conserved in plants, while the leaf size vs. number trade-off can be mediated by leaf properties. The isometry of the leaf size vs. number relationship may simply result from a biomass allocation trade-off, although a twig size constraint may provide an alternative mechanism.

Global warming reduces plant reproductive output for temperate multi-inflorescence species on the Tibetan plateau

• Temperature is projected to increase more during the winter than during the summer in cold regions. The effects of winter warming on reproductive effort have not been examined for temperate plant species. • Here, we report the results of experimentally induced seasonal winter warming (0.4 and 2.4°C increases in growing and nongrowing seasons, respectively, using warmed and ambient open-top chambers in a Tibetan Plateau alpine meadow) for nine indeterminate-growing species producing multiple (single-flowered or multi-flowered) inflorescences and three determinate-growing species producing single inflorescences after a 3-yr period of warming. • Warming reduced significantly flower number and seed production per plant for all nine multi-inflorescence species, but not for the three single-inflorescence species. Warming had an insignificant effect on the fruit to flower number ratio, seed size and seed number per fruit among species. The reduction in seed production was largely attributable to the decline in flower number per plant. The flowering onset time was unaffected for nine of the 12 species. Therefore, the decline in flower production and seed production in response to winter warming probably reflects a physiological response (e.g. metabolic changes associated with flower production). • Collectively, the data indicate that global warming may reduce flower and seed production for temperate herbaceous species and will probably have a differential effect on single- vs multi-inflorescence species.

The roles of beetles and flies in yak dung removal in an alpine meadow of eastern Qinghai-Tibetan Plateau.

Abstract: Flies and beetles are 2 major functional invertebrate groups responsible for dung removal and nutrient cycling in many alpine pastures. In order to determine the effect of the 2 groups on dung removal rate, as well as the associated mechanisms, we conducted a two-factor, two-level factorial-designed experiment in an alpine meadow of eastern Qinghai-Tibetan Plateau. Four treatments were included: (A) flies excluded, beetles included; (B) beetles excluded, flies included; (C) both flies and beetles excluded; and (D) both flies and beetles included, each having 45 replicates of artificial dung pats (17 cm in diameter; ca. 248 g in dry weight). We examined dung quality, removal rates, and abundance and biomass of macro-soil faunas within the dung 9 times (5 replicates sampled each time) during the experimental period of 32 d. Both beetles and flies (maggots), as well as the interaction between the 2 groups, markedly affected the dung removal rates. The total dry weight loss of the dung during the experiment was 167.6 g and 127.5 g on average for the beetle and fly groups, respectively. The removal rate in terms of both dung weight and organic matter loss was significantly greater for the beetles than the flies, which was principally because the consumption rate was lower and the resident time was shorter for fly maggots than for the beetles. The coexistence of the 2 species increased dung removal synergistically on the first sampling day and additively between the 2nd and the 5th day. However, after the 8th day of the experiment, the coexistence effect was negative, such that the dung loss of treatment D was comparable to treatment A but significantly greater than treatment B. The synergistic control might be due to the mediating effect of predator beetles on the relative abundance of coprophagous beetles and maggots, and the negative effect of the functional group diversity was because of the overwhelming competitive advantage of the beetles over fly maggots. In conclusion, although both fly and beetle species may significantly contribute to dung removal, the effect of the interaction between flies and beetles appears to depend on dung age (the sampling timing) in the alpine meadow.

Contrasting leaf trait scaling relationships in tropical and temperate wet forest species

Summary 1. We investigated whether plants adapted to thermally contrasting environments (e.g. tropical-temperate habitats) exhibit inherent differences in leaf trait scaling relationships. 2. Thirteen tropical and 12 temperate species (all characteristic of wet forests) were grown in a glasshouse (25/20 °C day/night). A range of leaf traits were quantified, including mass-based leaf nitrogen [N], mass per unit area (LMA), light-saturated photosynthesis (A) and respiration (Rdark). 3. Average area- and mass-based rates of net CO2 exchange were higher in the temperate species, compared to their tropical counterparts. Average leaf [N] and LMA values were also higher in temperate species than in their tropical counterparts. 4. The higher LMA in the metabolically more active temperate species was the most striking contrast to the patterns and predictions of the GLOPNET leaf trait data base, and was associated with different elevations (i.e. y-axis intercepts) but not slopes of bivariate trait scaling relationships. As expected, mass-based rates of A and Rdark scaled positively with increasing [N] and negatively with increasing LMA in both tropical and temperate species. No differences were found between temperate and tropical species groups in terms of log-log scaling relationships linking A and Rdark to N. However, at any given LMA, mass-based values of [N], A and Rdark were all higher in the temperate species than in their tropical counterparts. 5. Underpinning higher A in temperate species was a higher capacity for carboxylation (Vcmax) and RuBP regeneration (Jmax), with Jmax:Vcmax being greater in temperate species. 6. In conclusion, our results suggest that as a consequence of greater overall N investment as well as greater proportional N investment in metabolic capacity, cool-adapted temperate wet forest species exhibit higher photosynthetic and respiration rates than their warm-adapted tropical counterparts when compared in a common environment.

The effect of twig architecture and seed number on seed size variation in subtropical woody species

This study aims to determine the effect of twig (current year shoot) size on seed size variation and to test whether a seed size vs number tradeoff occurs for the twigs of subtropical broad-leaved species. Fruit-bearing twigs were sampled for 55 woody species (including 33 evergreen and 22 deciduous dicot species) from a southwest Chinese forest. Twig size, fruit size and number, and seed size and number were measured for each species. The relationships among these functional traits were determined both across species and across correlated phyletic divergences. Total fruit mass and total seed mass were positively correlated with twig size. Seed size was positively associated with fruit size, which was, in turn, positively correlated with twig diameter, but negatively correlated with the ratio of twig length to diameter. The effect of twig size on seed size variation was not significant, possibly as a result of the large variation in seed number per fruit. Cross-species and across-phyletic divergence analyses revealed that seed size was negatively and isometrically correlated with seed number per twig mass in both the evergreen and deciduous species groupings, demonstrating the existence of tradeoff between seed size and number. A seed size vs number tradeoff is detectable in the twigs of woody species. Comparatively little of the variance in seed size was attributable to twig size variation.

Domesticated honey bees evolutionarily reduce flower nectar volume in a Tibetan lotus

Plants have evolved costly flowering traits, including the provisioning of rich nectar, to attract and reward their pollinators. Beekeeping (apiculture) locally increases densities of honey bees, which might drive economization of pollinator-attracting traits, but the potential evolutionary consequences of beekeeping on plant–pollinator interactions remain unknown. Here, we present evidence suggesting that intensive apiculture has driven the rapid evolution of plant traits in the alpine lotus (Saussurea nigrescens) on the Tibetan Plateau by allowing reduced nectar volume provisioning without compromising pollination success. This conclusion is supported by measurements of reproductive and vegetative traits, including nectar, at sites of varying distance from apiaries that have housed introduced honey bees (Apis mellifera) since the early 1980s. Nectar volume was more than 60% lower at sites close to apiaries than at more distant sites, while nectar concentration remained consistent. When seedlings from field sites were grown under common garden conditions, trends in nectar volume identical to those in the field were observed, indicating that recently evolved genetic differences likely underlie patterns observed in the field. The adaptive advantage of reduced nectar volume under high pollinator density was clear in both the field and in the common garden. Specifically, plants from sites close to apiaries were taller, had more aboveground biomass, and produced more flowers and seeds compared to those at distant sites, which is consistent with the tradeoffs between nectar volume per flower and flower number per inflorescence within sites. The evolution of reduced nectar volume suggested by our results shows that the widespread practice of beekeeping might be a strong selective agent acting on wild plant populations and illustrates that human activities may indirectly affect evolution by changing critical species interactions.

Petal Color, Flower Temperature, and Behavior in an Alpine Annual Herb, Gentiana leucomelaena (Gentianaceae)

Abstract Gentiana leucomelaena (Gentianaceae), an alpine herbaceous species of the Qinghai-Tibetan Plateau, has two colors of flowers (blue and white) that bloom in early spring. In order to determine the effect of petal color on flower interior temperature and behavior in the gentian, we investigated the differences in timing of flower opening and closure and the interior temperature of blooming flowers between the two colors, while recording the ambient temperature, light intensity, and relative humidity over the flowering season from March to May of 2009. When the flowers were open, the anther temperature was higher in the white flowers than in the blue flowers in various weather patterns; in particular it was about 2 °C higher on sunny days. Relative to the ambient temperature, the anther temperature was 1.27 °C higher in the white flowers, but was 0.41 °C lower in the blue flowers. Compared to blue flowers, white flowers opened later but closed earlier in the day at a higher ambient temperature. The two-factor two-level experiment (10 °C vs. 20 °C and 10,000 lux vs. darkness) indicates that temperature is the factor eventually determining the timing of flower opening and opening rates, but light may accelerate flower opening at the same temperature. The dye experiment, in which blue flowers were painted with red and purple food coloring, showed that the purple flowers had higher anther temperature, opened later but closed earlier, relative to the red ones. These results suggest that flower interior temperature is affected by both flower color and behavior in the species. In addition, we surveyed the percentages of the two flower colors in the field during the flowering season and also experimentally placed individuals with flower buds into growth chambers with contrasting day/night temperatures (12 °C/2 °C, 15 °C/2 °C, and 20 °C/2 °C), so as to examine the temperature effect on flower color frequency. A greater proportion of white flowers emerged in the early stage of the flowering season and in the low-temperature chambers, but blue ones dominated the late season and in the high-temperature chambers. This suggests that the color differentiation in the species is associated with temperature. The different strategies of adaptation to temperature might have allowed for flower color polymorphism.

Within-twig biomass allocation in subtropical evergreen broad-leaved species along an altitudinal gradient: allometric scaling analysis

We studied the effects of twig size and altitude on biomass allocation within plant twigs (terminal branches of current-year shoots), to determine whether species with large twigs/leaves or living at low altitude allocate a higher proportion of biomass to laminas than their counterparts with small twigs/leaves or living at high altitude. Stem mass, lamina mass and area, and petiole mass were measured for terminal branches of current-year shoots in 80 subtropical evergreen broad-leaved species belonging to 38 genera from 24 families along an altitudinal gradient of Mt. Emei, Southwest China. The scaling relationships between the biomass allocations of within-twig components were determined using model type II regression method. Isometric relationships were found between leaf mass and twig mass and between lamina mass and twig mass, suggesting that the biomass allocation to either leaves or laminas was independent of twig mass. Petiole mass disproportionally increased with both lamina mass and twig mass, indicating the importance of leaf petioles to the within-twig biomass allocation. These cross-species correlations were consistent with those among evolutionary divergences. In addition, species at low altitude tended to have a greater leaf and lamina mass but a smaller stem mass at a given twig mass than at middle and high altitudes. This is possibly due to the high requirement in physical support and the low efficiency of eco-physiological transport for the species living at high altitude. In general, within-twig biomass allocation pattern was not significantly affected by twig size but was greatly modulated by altitude.

Size-dependent leaf area ratio in plant twigs: implication for leaf size optimization

BACKGROUND AND AIMS Although many hypotheses have been proposed to explain variation in leaf size, the mechanism underlying the variation remains not fully understood. To help understand leaf size variation, the cost/benefit of twig size was analysed, since, according to Corners rule, twig size is positively correlated with the size of appendages the twig bears. METHODS An extensive survey of twig functional traits, including twig (current-year shoots including one stem and few leaves) and leaf size (individual leaf area and mass), was conducted for 234 species from four broadleaved forests. The scaling relationship between twig mass and leaf area was determined using standardized major axis regression and phylogenetic independent comparative analyses. KEY RESULTS Leaf area was found to scale positively and allometrically with both stem and twig mass (stem mass plus leaf mass) with slopes significantly smaller than 1.0, independent of life form and habitat type. Thus, the leaf area ratio (the ratio of total leaf area to stem or twig mass) decreases with increasing twig size. Moreover, the leaf area ratio correlated negatively with individual leaf mass. The results of phylogenetic independent comparativeanalyses were consistent with the correlations. Based on the above results, a simple model for twig size optimization was constructed, from which it is postulated that large leaf size-twig size may be favoured when leaf photosynthetic capacity is high and/or when leaf life span and/or stem longevity are long. The models predictions are consistent with leaf size variation among habitats, in which leaf size tends to be small in poor habitats with a low primary productivity. The model also explains large variations in leaf size within habitats for which leaf longevity and stem longevity serve as important determinants. CONCLUSIONS The diminishing returns in the scaling of total leaf area with twig size can be explained in terms of a very simple model on twig siz