Seed Dispersal Phenology and Germination Characteristics of a Drought-Prone Vegetation in Southeastern Brazil

Seed germination is determined by the environmental conditions typical of a habitat and also by the geographical origin of the source species pool. During the Quaternary, Brazilian Atlantic Rain Forest species expanded their distribution into the sandy coastal plains (restingas). Periods of water shortage, however, are frequent in the sandy substrate of the restinga. We investigated whether the germination characteristics of restinga species are more related to their biogeographical origin in the humid forest or to water shortage on sandy substrates. We characterized the seed dispersal phenology of a restinga community and conducted experiments to determine the water requirements for seed germination and the short-term seed dehydration sensitivity of different species. Species shed seeds throughout the year in the restinga. When subjected to Ψ=−0.37 MPa, seed germination percentage decreased and germination time increased in six of ten species when compared with Ψ=0 MPa. Most species showed high seed moisture content (MC>40 %) at seed dispersal. Seeds took 3–17 d to dehydrate when subjected to relative humidity≤76 percent and only two of eight species had seeds sensitive to short-term dehydration. Thus, rather than a specific set of germination characteristics related to humid or dry habitats, we gathered evidence to show that the germination characteristics of restinga species represent a multiplicity of responses that may be found in both kinds of habitat.     

Photosynthetic activity of US biomes: responses to the spatial variability and seasonality of precipitation and temperature

We examined the response of the normalized difference vegetation index, integrated over the growing season (gNDVI) to mean precipitation, maximum temperature (T_max), and minimum temperature (T_min) over an 11‐year period (1990–2000) for six biomes in the conterminous United States. We focused on within‐ and across‐biome variance in long‐term average gNDVI, emphasizing the degree to which this variance is explained by spatial gradients in long‐term average seasonal climate. Since direct measurements of ecosystem function are unavailable at the spatial and temporal scales studied, we used the satellite‐based gNDVI as a proxy for net photosynthetic activity. Forested and nonforested biomes differed sharply in their response to spatial gradients in temperature and precipitation. Gradients in mean spring and fall precipitation totals explained much of the variance in mean annual gNDVI within arid biomes. For forested biomes, mean annual gNDVI was positively associated with mean annual and seasonal T_min and T_max. These trends highlight the importance of the seasonal components of precipitation and temperature regimes in controlling productivity, and reflect the influence of these climatic components on water balance and growing‐season length. According to the International Panel on Climate Change (IPCC) (2001) increases in temperature minima and fall precipitation have contributed the dominant components of US increases in temperature and precipitation, respectively. Within the range of conditions observed over the study region, our results suggest that these trends have particularly significant consequences for above‐ground plant productivity, especially for Grassland, Open Shrubland, and Evergreen Needleleaf Forest. If historical climatic trends and the biotic responses suggested in this analysis continue to hold, we can anticipate further increases in productivity for both forested and nonforested ecoregions in the conterminous US, with associated implications for carbon budgets and woody proliferation.     

Modelling respiration of vegetation: evidence for a general temperature‐dependent Q10

Temperature responses of rates of respiratory CO₂ efflux from plants, soils, and ecosystems are frequently modelled using exponential functions with a constant Q₁₀ near 2.0 (fractional change in rate with a 10 °C increase in temperature). However, we present evidence that Q₁₀ declines with short‐term increases in temperature in a predictable manner across diverse plant taxa. Thus, models using a constant Q₁₀ are biased, and use of a temperature‐corrected Q₁₀ may improve the accuracy of modelled respiratory CO₂ efflux in plants and ecosystems in response to temperature and predicted global climate changes.     

Ant assemblages in the taiga biome: testing the role of territorial wood ants

Summary Ants were collected with sets of pitfall traps in four coniferous-forest habitats in southern Finland. A three-level competition hierarchy concept was used to generate predictions on ant community structure. The levels of the hierarchy, and the respective predictions, from top to bottom were: (1) The dominant territorial wood ants (Formica rufa-group species), expected to exclude each other. (2) The other aggressive species, likely to be excluded by the F. rufa-group. (3) The submissive species, non-aggressive and defending only their nest, and thus likely to coexist with the dominants but in reduced numbers. As expected, the species of the F. rufa-group excluded each other, and the species number of the other aggressive ants was significantly cut down in the presence of the F. rufa-group. The aggressive species F. sanguinea and Camponotus herculeanus showed complementary occurrences with the F. rufa-group, and Lasius niger reduced occurrences. The number of the submissive species was not significantly affected by the F. rufa-group. However, pairwise correlation coefficients were significantly more often negative than positive between presence of the F. rufa-group and average proportion of pitfalls per set with a submissive species, each analyzed in turn. The result indicates that the F. rufa-group also reduced the colony densities of the submissive species. We conclude that in the taiga biome territorial wood ants are, after adjusting for physical vicissitudes of the environment, the major structuring force of ant species assemblages.     

Constraining future terrestrial carbon cycle projections using observation‐based water and carbon flux estimates

The terrestrial biosphere is currently acting as a sink for about a third of the total anthropogenic CO₂ emissions. However, the future fate of this sink in the coming decades is very uncertain, as current earth system models (ESMs) simulate diverging responses of the terrestrial carbon cycle to upcoming climate change. Here, we use observation‐based constraints of water and carbon fluxes to reduce uncertainties in the projected terrestrial carbon cycle response derived from simulations of ESMs conducted as part of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). We find in the ESMs a clear linear relationship between present‐day evapotranspiration (ET) and gross primary productivity (GPP), as well as between these present‐day fluxes and projected changes in GPP, thus providing an emergent constraint on projected GPP. Constraining the ESMs based on their ability to simulate present‐day ET and GPP leads to a substantial decrease in the projected GPP and to a ca. 50% reduction in the associated model spread in GPP by the end of the century. Given the strong correlation between projected changes in GPP and in NBP in the ESMs, applying the constraints on net biome productivity (NBP) reduces the model spread in the projected land sink by more than 30% by 2100. Moreover, the projected decline in the land sink is at least doubled in the constrained ensembles and the probability that the terrestrial biosphere is turned into a net carbon source by the end of the century is strongly increased. This indicates that the decline in the future land carbon uptake might be stronger than previously thought, which would have important implications for the rate of increase in the atmospheric CO₂ concentration and for future climate change.     

Meta-analysis shows forest soil CO2 effluxes are dependent on the disturbance regime and biome type

Forest soil CO₂ efflux (FCO₂) is a crucial process in global carbon cycling; however, how FCO₂ responds to disturbance regimes in different forest biomes is poorly understood. We quantified the effects of disturbance regimes on FCO₂ across boreal, temperate, tropical and Mediterranean forests based on 1240 observations from 380 studies. Globally, climatic perturbations such as elevated CO₂ concentration, warming and increased precipitation increase FCO₂ by 13% to 25%. FCO₂ is increased by forest conversion to grassland and elevated carbon input by forest management practices but reduced by decreased carbon input, fire and acid rain. Disturbance also changes soil temperature and water content, which in turn affect the direction and magnitude of disturbance influences on FCO₂. FCO₂ is disturbance- and biome-type dependent and such effects should be incorporated into earth system models to improve the projection of the feedback between the terrestrial C cycle and climate change.     

The climate,the fuel and the land use: Long‐term regional variability of biomass burning in boreal forests

The influence of different drivers on changes in North American and European boreal forests biomass burning (BB) during the Holocene was investigated based on the following hypotheses: land use was important only in the southernmost regions, while elsewhere climate was the main driver modulated by changes in fuel type. BB was reconstructed by means of 88 sedimentary charcoal records divided into six different site clusters. A statistical approach was used to explore the relative contribution of (a) pollen‐based mean July/summer temperature and mean annual precipitation reconstructions, (b) an independent model‐based scenario of past land use (LU), and (c) pollen‐based reconstructions of plant functional types (PFTs) on BB. Our hypotheses were tested with: (a) a west‐east northern boreal sector with changing climatic conditions and a homogeneous vegetation, and (b) a north‐south European boreal sector characterized by gradual variation in both climate and vegetation composition. The processes driving BB in boreal forests varied from one region to another during the Holocene. However, general trends in boreal biomass burning were primarily controlled by changes in climate (mean annual precipitation in Alaska, northern Quebec, and northern Fennoscandia, and mean July/summer temperature in central Canada and central Fennoscandia) and, secondarily, by fuel composition (BB positively correlated with the presence of boreal needleleaf evergreen trees in Alaska and in central and southern Fennoscandia). Land use played only a marginal role. A modification towards less flammable tree species (by promoting deciduous stands over fire‐prone conifers) could contribute to reduce circumboreal wildfire risk in future warmer periods.     

The diversification of the northern temperate woody flora – A case study of the Elm family (Ulmaceae) based on phylogenomic and paleobotanical evidence

Ulmaceae is a woody family widespread in northern temperate forests. Despite the ecological importance of this family, its phylogeny and biogeographic history are poorly understood. In this study, we reconstruct phylogenetic relationships within the family and infer spatio-temporal diversification patterns based on chloroplast genome (complete cpDNA) and nuclear ribosomal DNA sequences (nrDNA). The seven Ulmaceae genera are resolved in two main clades (temperate vs. tropical) by both cpDNA and nrDNA sequences. The temperate clade includes four genera, Hemiptelea, Zelkova, Planera, and Ulmus. The relationships among Planera and other genera are controversial because of inconsistent topologies between plastid and nuclear data. The tropical clade includes three genera ((Ampelocera, Phyllostylon), Holoptelea). Molecular dating and diversification analyses show that Ulmaceae originated in the Early Cretaceous (ca. 110–125 Ma) with the main lineages establishing from the Late Cretaceous to the early Eocene. The diversification rate slowed during the middle to the late Paleogene (ca. 23–45 Ma), followed by a rapid diversification of the East Asian temperate group in the Neogene, congruent with a global cooling event. The ancestral state optimization analysis suggests an East Asian origin of the temperate Ulmaceae clade during the Paleocene, which is consistent with the fossil record. Both phylogenomic and fossil evidence support East Asia as a center of origin and diversification for the temperate woody lineages.     

Implications of a large global root biomass for carbon sink estimates and for soil carbon dynamics

Recent evidence suggests that significantly more plant carbon (C) is stored below ground than existing estimates indicate. This study explores the implications for biome C pool sizes and global C fluxes. It predicts a root C pool of at least 268 Pg, 68% larger than previously thought. Although still a low-precision estimate (owing to the uncertainties of biome-scale measurements), a global root C pool this large implies stronger land C sinks, particularly in tropical and temperate forests, shrubland and savanna. The land sink predicted from revised C inventories is 2.7 Pg yr(-1). This is 0.1 Pg yr(-1) larger than current estimates, within the uncertainties associated with global C fluxes, but conflicting with a smaller sink (2.4 Pg yr(-1)) estimated from C balance. Sink estimates derived from C inventories and C balance match, however, if global soil C is assumed to be declining by 0.4-0.7% yr(-1), rates that agree with long-term regional rates of soil C loss. Either possibility, a stronger land C sink or widespread soil C loss, argues that these features of the global C cycle should be reassessed to improve the accuracy and precision of C flux and pool estimates at both global and biome scales.