Drivers of vegetative dormancy across herbaceous perennial plant species.

Vegetative dormancy, that is the temporary absence of aboveground growth for ≥ 1 year, is paradoxical, because plants cannot photosynthesise or flower during dormant periods. We test ecological and evolutionary hypotheses for its widespread persistence. We show that dormancy has evolved numerous times. Most species displaying dormancy exhibit life-history costs of sprouting, and of dormancy. Short-lived and mycoheterotrophic species have higher proportions of dormant plants than long-lived species and species with other nutritional modes. Foliage loss is associated with higher future dormancy levels, suggesting that carbon limitation promotes dormancy. Maximum dormancy duration is shorter under higher precipitation and at higher latitudes, the latter suggesting an important role for competition or herbivory. Study length affects estimates of some demographic parameters. Our results identify life historical and environmental drivers of dormancy. We also highlight the evolutionary importance of the little understood costs of sprouting and growth, latitudinal stress gradients and mixed nutritional modes.

Mixotrophy in Pyroleae (Ericaceae) from Estonian boreal forests does not vary with light or tissue age.

Background and Aims: In temperate forests, some green plants, namely pyroloids (Pyroleae, Ericaceae) and some orchids, independently evolved a mode of nutrition mixing photosynthates and carbon gained from their mycorrhizal fungi (mixotrophy). Fungal carbon is more enriched in 13C than photosynthates, allowing estimation of the proportion of carbon acquired heterotrophically from fungi in plant biomass. Based on 13C enrichment, mixotrophic orchids have previously been shown to increase shoot autotrophy level over the growth season and with environmental light availability. But little is known about the plasticity of use of photosynthetic versus fungal carbon in pyroloids. Methods: Plasticity of mixotrophy with leaf age or light level (estimated from canopy openness) was investigated in pyroloids from three Estonian boreal forests. Bulk leaf 13C enrichment of five pyroloid species was compared with that of control autotrophic plants along temporal series (over one growth season) and environmental light gradients (n=405 samples). Key Results: Mixotrophic 13C enrichment was detected at studied sites for Pyrola chlorantha and Orthilia secunda (except at one site for the latter), but not for Chimaphila umbellata, Pyrola rotundifolia and Moneses uniflora. Enrichment with 13C did not vary over the growth season or between leaves from current and previous years. Finally, although one co-occurring mixotrophic orchid showed 13C depletion with increasing light availability, as expected for mixotrophs, all pyroloids responded identically to autotrophic control plants along light gradients. Conclusions: A phylogenetic trend previously observed is further supported: mixotrophy is rarely supported by 13C enrichment in the Chimaphila + Moneses clade, whereas it is frequent in the Pyrola + Orthilia clade. Moreover, pyroloid mixotrophy does not respond plastically to ageing or to light level. This contrasts with the usual view of a convergent evolution with orchids, and casts doubt on the way pyroloids use the carbon gained from their mycorrhizal fungi, especially to replace photosynthetic carbon.

Demographic shifts related to mycoheterotrophy and their fitness impacts in two Cephalanthera species.

Evolutionary losses of photosynthesis in terrestrial plants all originate in photosynthetic ancestors. The adaptive context under which this transition happens has remained elusive because of the rarity of plants in which both photosynthetic and non-photosynthetic forms exist as a polymorphism. Here, we report on demographic patterns in photosynthetic ("green") and nonphotosynthetic ("albino") individuals within populations of two such species, Cephalanthera damasonium and C. longifolia, which also acquire nutrition from their mycorrhizal hosts (partial mycoheterotrophy). We hypothesized that demographic shifts in albinos relative to greens would include compensatory patterns with respect to fitness, such that maladaptive changes to survival or reproduction would be adaptively countered by changes to other parameters, such as growth probabilities. We tracked individuals in two populations of C. damasonium for 3 yr, and in one population of C. longifolia for 14 yr. We then analyzed vital rates for both phenotypes using general linear mixed models (GLMMs) and multi-state capture mark-recapture models (CMR), and used these models to develop size-classified, function-based population projection matrices. We estimated fitness as the deterministic population growth rate (λ) for each phenotype, and explored the impact of shifts in demographic patterns to albinism via life table response experiments (LTREs). Mortality differed between greens and albinos, but not similarly across species. Albinos generally sprouted less than greens, and flowered more when small but less at other times. Albinos typically had a higher probability of fruiting, although their lower flower numbers yielded lower numbers of fruits overall. Fitness did not differ significantly among phenotypes. Thus, we did not find significant evidence that albinism is adaptive or maladaptive; however, if in fact it is the latter, then we did find evidence of incomplete compensation for declines in survival and reproduction from growth transitions, particularly to small flowering size classes in C. damasonium, and to large vegetative size classes in C. longifolia. These patterns indicate some support for the idea that albinism may lead to the speciation of mycoheterotrophic plants.

Ecosystem-scale biosphere-atmosphere interactions of a hemiboreal mixed forest stand at Järvselja, Estonia.

During two measurement campaigns, from August to September 2008 and 2009, we quantified the major ecosystem fluxes in a hemiboreal forest ecosystem in Järvselja, Estonia. The main aim of this study was to separate the ecosystem flux components and gain insight into the performance of a multi-species multi-layered tree stand. Carbon dioxide and water vapor fluxes were measured using the eddy covariance method above and below the canopy in conjunction with the microclimate. Leaf and soil contributions were quantified separately by cuvette and chamber measurements, including fluxes of carbon dioxide, water vapor, nitrogen oxides, nitrous oxide, methane, ozone, sulfur dioxide, and biogenic volatile organic compounds (isoprene and monoterpenes). The latter have been as well characterized for monoterpenes in detail. Based on measured atmospheric trace gas concentrations, the flux tower site can be characterized as remote and rural with low anthropogenic disturbances. Our results presented here encourage future experimental efforts to be directed towards year round integrated biosphere-atmosphere measurements and development of process-oriented models of forest-atmosphere exchange taking the special case of a multi-layered and multi-species tree stand into account. As climate change likely leads to spatial extension of hemiboreal forest ecosystems a deep understanding of the processes and interactions therein is needed to foster management and mitigation strategies.