Global vegetation models: incorporating transient changes to structure and composition

Abstract. We describe an approach for developing a Dynamic Global Vegetation Model (DGVM) that accounts for transient changes in vegetation distribution over a decadal time scale. The DGVM structure is based on a linkage between an equilibrium global vegetation model and smaller scale ecosystem dynamics modules that simulate the rate of vegetation change. Vegetation change is classified into four basic types, based largely on the projected change in above-ground biomass of the vegetation. These four types of change are: (1) dieback of forest, shrubland or grassland; (2) successional replacement within forest, shrubland or grassland; (3) invasion of forest, shrubland or grassland; (4) change in tree/grass ratio. We then propose an approach in which the appropriate ecosystem dynamics module for each type of change is applied and the grid cells of the global model updated accordingly. An approach for accounting for fire, as an example of a disturbance which may strongly influence the rate and spatial pattern of forest dieback, is incorporated. We also discuss data needs for the development, calibration and validation of the model.     

Stomatal responses to increased CO2: implications from the plant to the global scale

Increased atmospheric CO₂ often but not always leads to large decreases in leaf conductance. Decreased leaf conductance has important implications for a number of components of CO₂ responses, from the plant to the global scale. All of the factors that are sensitive to a change in soil moisture, either amount or timing, may be affected by increased CO₂. The list of potentially sensitive processes includes soil evaporation, run-off, decomposition, and physiological adjustments of plants, as well as factors such as canopy development and the composition of the plant and microbial communities. Experimental evidence concerning ecosystem-scale consequences of the effects of CO₂ on water use is only beginning to accumulate, but the initial indication is that, in water-limited areas, the effects of CO₂-induced changes in leaf conductance are comparable in importance to those of CO,₂-induced changes in photosynthesis. Above the leaf scale, a number of processes interact to modulate the response of canopy or regional evapotran-spiration to increased CO₂. While some components of these processes tend to amplify the sensitivity of evapo-transpiration to altered leaf conductance, the most likely overall pattern is one in which the responses of canopy and regional evapotranspiration are substantially smaller than the responses of canopy conductance. The effects of increased CO₂ on canopy evapotranspiration are likely to be smallest in aerodynamically smooth canopies with high leaf conductances. Under these circumstances, which are largely restricted to agriculture, decreases in evapotranspiration may be only one-fourth as large as decreases in canopy conductance. Decreased canopy conductances over large regions may lead to altered climate, including increased temperature and decreased precipitation. The simulation experiments to date predict small effects globally, but these could be important regionally, especially in combination with radiative (greenhouse) effects of increased CO₂.     

Nitrogen supply effects on productivity and potential leaf litter decay of Carex species from peatlands differing in nutrient limitation

We investigated the effect of increased N-supply on productivity and potential litter decay rates of Carex species, which are the dominant vascular plant species in peatlands in the Netherlands. We hypothesized that: (1) under conditions of N-limited plant growth, increased N-supply will lead to increased productivity but will not affect C:N ratios of plant litter and potential decay rates of that litter; and (2) under conditions of P-limited plant growth, increased N-supply will not affect productivity but it will lead to lower C:N ratios in plant litter and thereby to a higher potential decay rate of that litter. These hypotheses were tested by fertilization experiments (addition of 10 g N m^-2 year^-1) in peatlands in which plant growth was N-limited and P-limited, respectively. We investigated the effects of fertilization on net C-fixation by plant biomass, N uptake, leaf litter chemistry and potential leaf litter decay. In a P-limited peatland, dominated by Carex lasiocarpa, there was no significant increase of net C-fixation by plant biomass upon enhanced N-supply, although N-uptake had increased significantly compared with the unfertilized control. Due to the N-fertilization the C:N ratio in the plant biomass decreased significantly. Similarly, the C:N ratio of leaf litter produced at the end of the experiment showed a significant decrease upon enhanced N-supply. The potential decay rate of that litter, measured as CO₂-evolution from the litter under aerobic conditions, was significantly increase upon enhanced N-supply. In a N-limited peatland, dominated by C. acutiformis, the net C-fixation by plant biomass increased with increasing N-supply, whereas the increase in N-uptake was not significant. The C:N ratio of both living plant material and of dead leaves did not change in response to N-fertilization. The potential decay rate of the leaf litter was not affected by N-supply. The results agree with our hypotheses. This implies that atmospheric N-deposition may affect the CO₂-sink function of peatlands, but the effect is dependent on the nature of nutrient limitation. In peatlands where plant growth is N-limited, increased N-supply leads to an increase in the net accumulation of C. Under conditions of P-limited plant growth, however, the net C-accumulation will decrease, because productivity is not further increased, whereas the amount of C lost through decomposition of dead organic matter is increased. As plant growth in most terrestrial ecosystems is N-limited, increased N-supply will in most peatlands lead to an increase of net C-accumulation.     

Chaperone SecB: Conformational changes demonstrated by circular dichroism

The chaperone SecB, which is involved in protein export inEscherichia coli, is shown by circular dichroism measurements to contain a high content of-pleated sheets. Prediction of the secondary structure of SecB is in good agreement with the observed content of-sheet. In accordance with the previous studies in which changes in conformation were assessed indirectly [Randall (1992),Science 257, 241–245], here we show that the conformation of SecB changes with the concentration of salt in the milieu and also when SecB interacts with a peptide ligand.Abbreviations ANS 1-anilino-naphthalene-8-sulfonate - CD circular dichroism - NMR nuclear magnetic resonance - CCA convex constraint analysis     

Islands: stability, diversity, conservation

Islands present both a diversity and a stability paradox. They are often highly species-poor but have considerable biological interest in terms of extraordinary endemic genera and taxonomically isolated groups. They appear to be stable, as in some cases these organisms have persisted for many millions of years, and having an oceanic climate, extreme climatic events may be comparatively rare. However, when subject to extrinsic (anthropogenic) disturbance they do not appear to be stable, but often suffer catastrophic ecological change. These apparent paradoxes are resolved when it is realized that all these features are consequences of the same island characteristics: biotic isolation and oceanicity. As a result of these two characteristics, far oceanic islands are quantitatively different from continental systems in the nature of their ecological processes, which appear to give rise to an extreme punctuated equilibrium model of evolutionary change. Endemics may be ancient relict endemics displaying prolonged stasis and persistence, or products of adaptive radiation representing rapid punctuational events. A process-based definition of a relict endemic (palaeoendemic) is one whose founding lineage (i.e. the original continental source taxon) has not left any descendents. A corollary of this definition is that the time of divergence between an endemic and its continental sister-group should predate the colonization of the island by the now endemic lineage. An example is Dicksonia arborescens which has been on St Helena for at least 9 Myrs and no longer occurs in the likely source area of Africa. These relict endemics, frequent on islands, are important as the last remnants of tranches of biodiversity that have vanished elsewhere. Island conservation strategies require an integrated understanding of both sides of the diversity and stability paradox so that both island processes and island organisms can be conserved.     

Effects of water level fluctuations on phytoplankton in a river-floodplain lake system (Paraná River,Argentina)

Several aspects of community organization wereanalyzed comparatively in a small side-arm of theParaná River (Correntoso) and a shallowfloodplain lake (El Tigre) (31° 41 S and60° 42 W), in relation to the hydrology of thesystem. Taxonomic and morphological composition inthe river differed from that in the lake: the riverhad lower species richness (151 vs 218),different contributions of some Classes to totalspecies number (higher Cyano-, Zygo- andDiatomophyceae vs higher Chlorophyceae), anddiffent proportions of nannoplanktonic algae (67.5%vs 80.7%) and netplanktonic filamentousspecies (18.2% vs 4.2%). Phytoplanktonbiomass, higher in the lake than in the river due tothe retention time, was mostly dominated bynannoplankton and netplankton. Loticphytoplankton was dominated by typical fluvialspecies of Diatomophyceae (R-strategists). Riverconditions seem to maintain a subclimacticcommunity, which was little impacted by the flushingof populations from floodplain lakes. Water levelwas the main factor controlling phytoplanktonbiomass, species diversity (H), evenness (E) andcommunity change rate () in the river. Inthe lake, phytoplankton had an autogenicsuccessional sequence during the isolation phase (C-to S-strategists) and other responses todisturbance, mainly during the flood(R-strategists). Frequent changes in phytoplanktoncomposition, biomass, H, E and , revealed aenvironmental instability in the lake, which may beexplained by interactions of external factors(hydrology and climatology) and those of internalorigin, such as nutrients and grazing.     

Effects of temperature on phenological synchrony and altitudinal distribution of jumping plant lice (Hemiptera: Psylloidea) on dwarf willow (Salix lapponum) in Norway

Summary.

• 1 The geographical distributions of three species of jumping plant lice (psyllids) along an altitudinal transect (988–1300 m a.s.l.) in southern Norway were restricted within the range of their host plant Salix lapponum. One species, Cacopsylla propinqua, occurred at all sampling locations between 988 and 1222 m, whereas C.palmeni was confined to higher altitudes (1153–1222 m) and C.brunneipennis was more abundant at lower altitudes (988–1101 m).
• 2 C.brunneipennis and C.palmeni developed only on female catkins. Development times of catkins and psyllids were similar (approximately 50 days) and successful psyllid development depended on close phenological synchrony with catkins.
• 3 Thermal requirements for development of female catkins were greater at low altitude (988 m) compared with higher altitude (1222 m), showing local adaptation of S.lapponum to altitude. In general, thermal requirements of psyllids were less than those of catkins at the same location. C.brunneipennis had higher thermal requirements than C.palmeni.
• 4 Field experiments, using polythene enclosures to elevate temperatures at two sites at different altitudes (by 0.6–1.4 deg. C), showed that insects had an enhanced relative rate of development under elevated temperatures compared with their host plants.
• 5 Indices of phenological synchrony were calculated from thermal requirements of psyllids and catkins. Under elevated temperatures, phenological synchrony decreased at both sites. This resulted in the subsequent development of smaller adult insects at low altitude, although at higher altitude, insects developing under elevated temperatures were larger and had a higher survival rate compared with controls.
• 6 Effects of temperature on phenological synchrony may explain the limits to the geographical range of psyllids. The consequences of climate change on psyllid populations will depend on the effects of decreased phenological synchrony on insect development and this may differ within the insect's geographical range.

     

Seasonal ectomycorrhizal fungal biomass development on loblolly pine (Pinus taeda L.) seedlings

Ergosterol, a membrane sterol found in fungi but not in plants, was used to estimate live mycelial biomass in ectomycorrhizae. Loblolly pine (Pinus taeda L.) seeds were sown in April 1993 and grown with standard nursery culture practices. Correlations between total seedling ergosterol and visual assessment of mycorrhizal colonization were high during July and August but low as ectomycorrhizal development continued into the growing season. Percentages of mycelial dry weight over lateral roots decreased from 9% in July to 2.5% in November because seedling lateral root dry weight accumulated faster than mycelial dry weight. Total ergosterol per seedling increased from July through February. As lateral root dry weight ceased to increase during winter months, ectomycorrhizal mycelia became the major carbohydrate sink of pine seedlings. No distinctive seasonal pattern of soil ergosterol content was observed. The impact of ectomycorrhizal fungi on plant carbohydrate source-sink dynamics can be quantitatively estimated with ergosterol analysis but not with conventional visual determination.