Ørjan Totland

Plant community responses to experimental warming across the tundra biome

Recent observations of changes in some tundra ecosystems appear to be responses to a warming climate. Several experimental studies have shown that tundra plants and ecosystems can respond strongly to environmental change, including warming; however, most studies were limited to a single location and were of short duration and based on a variety of experimental designs. In addition, comparisons among studies are difficult because a variety of techniques have been used to achieve experimental warming and different measurements have been used to assess responses. We used metaanalysis on plant community measurements from standardized warming experiments at 11 locations across the tundra biome involved in the International Tundra Experiment. The passive warming treatment increased plant-level air temperature by 1-3°C, which is in the range of predicted and observed warming for tundra regions. Responses were rapid and detected in whole plant communities after only two growing seasons. Overall, warming increased height and cover of deciduous shrubs and graminoids, decreased cover of mosses and lichens, and decreased species diversity and evenness. These results predict that warming will cause a decline in biodiversity across a wide variety of tundra, at least in the short term. They also provide rigorous experimental evidence that recently observed increases in shrub cover in many tundra regions are in response to climate warming. These changes have important implications for processes and interactions within tundra ecosystems and between tundra and the atmosphere.

Responses of tundra plants to experimental warming : Meta-analysis of the international tundra experiment

The International Tundra Experiment (ITEX) is a collaborative, multisite experiment using a common temperature manipulation to examine variability in species response across climatic and geographic gradients of tundra ecosystems. ITEX was designed specifically to examine variability in arctic and alpine species response to increased temperature. We compiled from one to four years of experimental data from 13 different ITEX sites and used meta-analysis to analyze responses of plant phenology, growth, and reproduction to experimental warming. Results indicate that key phenological events such as leaf bud burst and flowering occurred earlier in warmed plots throughout the study period; however, there was little impact on growth cessation at the end of the season. Quantitative measures of vegetative growth were greatest in warmed plots in the early years of the experiment, whereas reproductive effort and success increased in later years. A shift away from vegetative growth and toward reproductive effort and success in the fourth treatment year suggests a shift from the initial response to a secondary response. The change in vegetative response may be due to depletion of stored plant reserves, whereas the lag in reproductive response may be due to the formation of flower buds one to several seasons prior to flowering. Both vegetative and reproductive responses varied among life-forms; herbaceous forms had stronger and more consistent vegetative growth responses than did woody forms. The greater responsiveness of the herbaceous forms may be attributed to their more flexible morphology and to their relatively greater proportion of stored plant reserves. Finally, warmer, low arctic sites produced the strongest growth responses, but colder sites produced a greater reproductive response. Greater resource investment in vegetative growth may be a conservative strategy in the Low Arctic, where there is more competition for light, nutrients, or water, and there may be little opportunity for successful germination or seedling development. In contrast, in the High Arctic, heavy investment in producing seed under a higher temperature scenario may provide an opportunity for species to colonize patches of unvegetated ground. The observed differential response to warming suggests that the primary forces driving the response vary across climatic zones, functional groups, and through time.

How does climate warming affect plant‐pollinator interactions?

Climate warming affects the phenology, local abundance and large-scale distribution of plants and pollinators. Despite this, there is still limited knowledge of how elevated temperatures affect plant-pollinator mutualisms and how changed availability of mutualistic partners influences the persistence of interacting species. Here we review the evidence of climate warming effects on plants and pollinators and discuss how their interactions may be affected by increased temperatures. The onset of flowering in plants and first appearance dates of pollinators in several cases appear to advance linearly in response to recent temperature increases. Phenological responses to climate warming may therefore occur at parallel magnitudes in plants and pollinators, although considerable variation in responses across species should be expected. Despite the overall similarities in responses, a few studies have shown that climate warming may generate temporal mismatches among the mutualistic partners. Mismatches in pollination interactions are still rarely explored and their demographic consequences are largely unknown. Studies on multi-species plant-pollinator assemblages indicate that the overall structure of pollination networks probably are robust against perturbations caused by climate warming. We suggest potential ways of studying warming-caused mismatches and their consequences for plant-pollinator interactions, and highlight the strengths and limitations of such approaches.

SIMULATED CLIMATE CHANGE ALTERED DOMINANCE HIERARCHIES AND DIVERSITY OF AN ALPINE BIODIVERSITY HOTSPOT

Alpine and arctic ecosystems may be particularly vulnerable to climate change. We know little about alpine plant community responses to the predicted abiotic changes, or to possible changes in the biotic environment caused by climate change. Four years of experimental warming and nutrient addition altered dominance hierarchies, community structure, and diversity of an alpine biodiversity hotspot in south Norway. The previously dominant dwarf shrub Dryas octopetala was replaced by graminoids and forbs under nutrient addition and warming with nutrients. Community diversity declined due to decreased bryophyte and lichen richness and abundances, and dwarf shrub abundances. In controls and in plots with only warming, where Dryas maintained dominance, the relationships between changes in Dryas cover and changes in community parameters were negative, suggesting that Dryas controls community processes. Under nutrient addition, bryophyte and lichen diversity decreased with decreasing Dryas cover, probably due to increased competition from graminoids and forbs. The shift in dominance hierarchies changed community structure and dynamics through increased biomass, vegetation height, and competition for light. Community diversity dropped primarily because changes in the abiotic environment modified biotic interactions, highlighting that species interaction must be considered in climate change experiments and in models predicting climate change effects.

Environment-dependent pollen limitation and selection on floral traits in an alpine species

Phenotypic selection on flower size in animal-pollinated plants can only occur when fitness is limited by pollinator visitation and when there is a relationship between flower size and pollinator attraction. The purpose of this study was (a) to examine how abiotic environmental conditions affect pollen limitation on female reproductive success and pollinator flower visitation and (b) to reveal whether phenotypic selection on floral traits in an alpine flowering species varied across an abiotic environmental gradient. In alpine populations of the perennial herb Ranunculus acris, I found that female reproductive success in a low-altitude population was significantly increased by experimental pollen addition, while plants at a higher altitude population did not respond to this treatment, presumably because low temperatures limited their seed production. At the low altitude, pollinators (muscoid flies) preferred to visit relatively large flowers, while no sign of flower discrimination occurred at the high altitude. Phenotypic selection regression analysis found that selection on flower size traits was more intense and frequent at the low-altitude compared to the high-altitude population. This result is consistent with the findings of stronger pollen limitation on seed set and the existence of pollinator discrimination on flower size at the low altitude compared to the high altitude. Pollinator visitation rates to flowers were nearly three times higher at the low altitude, compared to the high altitude. Thus, this study shows that the commonly assumed pathway from low pollinator availability to strong pollen limitation is not always correct but supports the view that selection on floral traits is positively related to the degree of pollen limitation. In R. acris environmental factors interact with pollen availability in determining the reproductive success of plants, and this interrupts the pathway from low pollinator availability via significant pollen limitation to selection on floral traits.

Relationships between species’ floral traits and pollinator visitation in a temperate grassland

Knowledge about plant–plant interactions for pollinator service at the plant community level is still scarce, although such interactions may be important to seed production and hence the population dynamics of individual plant species and the species compositions of communities. An important step towards a better understanding of pollination interactions at the community level is to assess if the variation in floral traits among plant species explain the variation in flower visitation frequency among those species. We investigated the relative importance of various floral traits for the visitation frequency of all insects, and bumblebees and flies separately, to plant species by measuring the visitation frequency to all insect-pollinated species in a community during an entire flowering season. Visitation frequency was identified to be strongly positive related to the visual display area and the date of peak flowering of plant species. Categorical variables, such as flower form and symmetry, were important to the visitation frequency of flies only. We constructed floral similarity measures based on the species’ floral traits and found that the floral similarity for all species’ traits combined and the continuous traits separately were positively related to individual visitation frequency. On the other hand, plant species with similar categorical floral traits did not have similar visitation frequencies. In conclusion, our results show that continuous traits, such as flower size and/or density, are more important for the variation in visitation frequency among plant species than thought earlier. Furthermore, differences in visitation frequency among pollinator groups give a poor support to the expectations derived from the classical pollination syndromes.

Pollen Limitation in the Alpine: A Meta-Analysis

Abstract Pollen limitation (PL) is thought to be an important factor driving the evolution of floral traits in alpine plants. However, results of pollen supplementation experiments in alpine plants do not always show high levels of PL, and a general review of the importance of PL on alpine-plant reproduction is needed. We assessed to what extent alpine plants are pollen limited, and whether the intensity of PL differs between alpine and lowland species. Moreover, we assessed whether or not PL of alpine species depends on their reproductive system and, finally, whether the outcome of PL studies depends on whether supplemental-pollination was done at the whole-plant level or within a subset of available flowers. We performed both classical and phylogenetic meta-analysis. Our results show that alpine plants are pollen limited. However, PL did not differ significantly between alpine and lowland species. In the alpine, self-incompatible and self-compatible species show similar levels of PL. We did not find differences in PL between different manipulation levels. These results will help understand the real importance of PL of seed production in the alpine. We identify gaps in our knowledge of PL in the alpine that could serve to guide future directions of research in this field.

Plant functional traits mediate reproductive phenology and success in response to experimental warming and snow addition in Tibet

Global climate change is predicted to have large impacts on the phenology and reproduction of alpine plants, which will have important implications for plant demography and community interactions, trophic dynamics, ecosystem energy balance, and human livelihoods. In this article we report results of a 3-year, fully factorial experimental study exploring how warming, snow addition, and their combination affect reproductive phenology, effort, and success of four alpine plant species belonging to three different life forms in a semiarid, alpine meadow ecosystem on the central Tibetan Plateau. Our results indicate that warming and snow addition change reproductive phenology and success, but responses are not uniform across species. Moreover, traits associated with resource acquisition, such as rooting depth and life history (early vs. late flowering), mediate plant phenology, and reproductive responses to changing climatic conditions. Specifically, we found that warming delayed the reproductive phenology and decreased number of inflorescences of Kobresia pygmaea C. B. Clarke, a shallow-rooted, early-flowering plant, which may be mainly constrained by upper-soil moisture availability. Because K. pygmaea is the dominant species in the alpine meadow ecosystem, these results may have important implications for ecosystem dynamics and for pastoralists and wildlife in the region.

Effects of an exotic plant and habitat disturbance on pollinator visitation and reproduction in a boreal forest herb

The invasion of exotic species into natural habitats is considered to be a major threat to biodiversity, and many studies have examined how exotic plants directly affect native plant species through competitive interactions for abiotic resources. However, although exotics can have potentially great ecological and evolutionary consequences, very few researchers have studied the effect of exotics on the interactions between plants and their mutualistic partners, such as pollinators, and none have reported on such impacts in logged and undisturbed boreal forest ecosystems. Here we show how experimental introductions of an exotic plant species (Phacelia tanacetifolia Bentham) affect pollinator visitation and female reproductive success of a native plant (Melampyrum pratense L.) in recently disturbed (i.e., logged) and in undisturbed boreal forest habitats. The presence of Phacelia significantly increased the number of bumble bees entering plots in both habitat types. However, the exotic species had a strong negative impact on the visitation rate to the native species in both habitat types. Despite this negative impact on pollinator visitation, the exotic had no effect on female reproductive success of the native species in any habitat. Our results show that seed production may be more robust than pollinator visitation to exotic invasion, irrespective of habitat disturbance history.

Pollen limitation of reproductive success in two sympatric alpine willows (Salicaceae) with contrasting pollination strategies.

We compared the extent of pollen limitation on female reproductive success of Salix lanata L., an entirely insect-pollinated willow, and S. lapponum L., which is 50 : 50% insect : wind pollinated (ambophilous). Supplemental hand-pollination significantly increased seed number per fruit by nearly 50% in the insect-pollinated willow, but had no significant impact on seed number in the dually pollinated species. Fruit set was not affected by the treatment in either of the species. These results demonstrate that pollen limitation on reproductive success is most pronounced in the species that depends entirely on insects for pollination. In general, pollinator visitation was highest to S. lapponum, but bumble bees were only observed on S. lanata, suggesting that the quantity and quality of pollinator visitation differed between the species. Our results empirically support the hypothesis that a dual pollination strategy is most effective in alpine environments with low and infrequent pollinator activity and high wind speeds.