Differential rates of glucose metabolism across subregions of the subfornical organ in Brattleboro rats

We applied [14C]deoxyglucose autoradiography and imaging techniques to determine rates of glucose metabolism in distinct subdivisions of the subfornical organ (SFO) of conscious Brattleboro rats. Seven anatomically-defineD SFO subregions were discerned having metabolic activities that differed from one another by as much as 29% in water-sated Brattleboro rats. The highest metabolic activity was found in the ventromedial zone of central and caudal subregions where previous studies identified the greatest densities of neurons, capillaries, putative angiotensin receptors, and angiotensin-immunoreactive fibers. Homozygous Brattleboro rats had rates of glucose metabolism that were 39-68% greater than those in corresponding SFO subregions of Long-Evans rats; these differences were accentuated by about 50% following 18 h of water deprivation. Exogenous treatment of Brattleboro rats with vasopressin uniformly normalized subregional glucose metabolism in the SFO. In Sprague-Dawley rats, water deprivation over 120 h provoked greater increases in metabolism of ventromedial than of dorsolateral SFO zones in amounts similar to the differences between Long-Evans and Brattleboro rats. The findings identify focal areas of high metabolic activity within subregions of the SFO where central responses are likely initiated to defend against homeostatic disturbances. The data represent further evidence for the probability that angiotensin II, as both hormone and neurotransmitter, is a metabolic stimulant of its target cells in the nervous system.     

Competition causes regular spacing of alder in Alaskan shrub tundra

Summary Alders (Alnus crispa) in shrub tundra in northern Alaska showed significant regularity of spacing. Removal of neighboring alder shrubs stimulated nutrient accumulation and growth of remaining alders but did not stimulate nutrient accumulation or growth of any other shrub species. This demonstrates that neighboring alders competed with one another and that, when alders were removed, the resources made available were used preferentially by remaining alders rather than by the community in general. Neither patterns of seedling establishment nor patterns of frostrelated features could explain the regular distribution of alder. We suggest that regular patterns of plant distribution are restricted to sites of low-resource availability, because in these habitats (1) there is strong competition for a scarce resource, and (2) there are only one or a few dominant species to compete for these resources in a given canopy height or rooting depth.     

Reproductive effort in cotton grass tussock tundra

Eriophorum vaginatum ssp. spissum is a dominant plant species of undisturbed cotton grass tussock tundra in Alaska. It also quickly invades and dominates recently disturbed sites. The hypothesis tested in this research was that the success of E. vaginatum on disturbed sites might be achieved through a higher allocation of biomass to reproductive structures relative to other tundra species. Reproductive allocation of tundra plants in general also was compared with plants of the temperate zone. The results indicate that E. vaginatum is about average among the common tundra species in terms of total reproductive allocation, allocation to seeds, and the proportion of total reproductive allocation that is accounted for by viable seeds. Tundra species, on a relative basis, allocate less biomass to all reproductive structures than temperate species but not necessarily less biomass to the output of viable seeds.     

Interannual variability of plant phenology in tussock tundra: modelling interactions of plant productivity, plant phenology, snowmelt and soil thaw

We present a linked model of plant productivity, plant phenology, snowmelt and soil thaw in order to estimate interannual variability of arctic plant phenology and its effects on plant productivity. The model is tested using 8 years of soil temperature data, and three years of bud break data of Betula nana. Because the factors that trigger the end of the growing season of arctic vegetation are less well known than those of the start of the growing season, three hypotheses were formulated and tested for their effects on productivity and its sensitivity to climate change; the hypothesised factors determining the end of the growing season were frost, photoperiod and periodic constraints. The performance of the soil thermal model was good; both the onset of soil thaw in spring and the initiation of freezing in autumn were predicted correctly in most cases. The phenology model predicted the bud break date of Betula nana closely for the three different years. The soil thaw model predicted similar growing season start dates under current climate as the models based on sum of temperatures, but it made significantly different predictions under climate change scenarios, probably because of the non‐linear interactions between snowmelt and soil thaw. The uncertainty about the driving factors for the end of the growing season, in turn, resulted in uncertainty in the interannual variability of the simulated annual gross primary productivity (GPP). The interannual variability ranged from ? 25 to + 26% of the mean annual GPP for the frost hypothesis, from ? 20 to + 20% for the photoperiod hypothesis and only from ? 7 to + 7% for the periodic hypothesis. The different hypotheses also resulted in different sensitivity to climate change, with the frost hypothesis resulting in 30% higher annual GPP values than the periodic hypothesis when air temperatures were increased by 3 °C.     

The effect of experimental warming and precipitation change on proteolytic enzyme activity: positive feedbacks to nitrogen availability are not universal

Nitrogen regulates the Earth's climate system by constraining the terrestrial sink for atmospheric CO₂. Proteolytic enzymes are a principal driver of the within‐system cycle of soil nitrogen, yet there is little to no understanding of their response to climate change. Here, we use a single methodology to investigate potential proteolytic enzyme activity in soils from 16 global change experiments. We show that regardless of geographical location or experimental manipulation (i.e., temperature, precipitation, or both), all sites plotted along a single line relating the response ratio of potential proteolytic activity to soil moisture deficit, the difference between precipitation and evapotranspiration. In particular, warming and reductions in precipitation stimulated potential proteolytic activity in mesic sites – temperate and boreal forests, arctic tundra – whereas these manipulations suppressed potential activity in dry grasslands. This study provides a foundation for a simple representation of the impacts of climate change on a central component of the nitrogen cycle.     

Mineral nutrition and leaf longevity in Ledum palustre: the role of individual nutrients and the timing of leaf mortality

The effects of fertilization on leaf longevity and leaf mortality in the Alaskan evergreen shrub, Ledum palustre (Ait.) Hult., were investigated in a field experiment. The fertilization treatments included N alone, P alone, N plus P, and N plus P plus K. After 5 years all treatments had the same effect on leaf longevity, decreasing life expectancy from about 2 years in controls to 1–1.5 years in the fertilized plants. In the NPK-fertilized plants, most of the decrease in leaf longevity was due to increased winter leaf mortality; fertilization actually decreased leaf losses during the growing season. The results are consistent with previous research suggesting that one function of overwintering evergreen leaves is to serve as nutrient storage organs, a function that is superfluous when nutrient supplies for new growth can be obtained from current uptake.     

Nitrogen dynamics in arctic tundra soils of varying age: differential responses to fertilization and warming

In the foothills of the Brooks Range, Alaska, different glaciation histories have created landscapes with varying soil age. Productivity of most of these landscapes is generally N limited, but varies widely, as do plant species composition and soil properties (e.g., pH). We hypothesized that the projected changes in productivity and vegetation composition under a warmer climate might be mediated through differential changes in N availability across soil age. We compared readily available [water-soluble NH₄ ⁺, NO₃ ^?, and amino acids (AA)], moderately available (soluble proteins), hydrolyzable, and total N pools across three tussock-tundra landscapes with soil ages ranging from 11.5k to 300k years. The effects of fertilization and warming on these N pools were also compared for the two younger sites. Readily available N was highest at the oldest site, and AA accounted for 80–89 % of this N. At the youngest site, inorganic N constituted the majority (80–97 %) of total readily available N. This variation reflected the large differences in plant functional group composition and soil chemical properties. Long-term (8–16 years) fertilization increased the soluble inorganic N by 20- to 100-fold at the intermediate-age site, but only by twofold to threefold at the youngest site. Warming caused small and inconsistent changes in the soil C:N ratio and AA, but only in soils beneath Eriophorum vaginatum, the dominant tussock-forming sedge. These differential responses suggest that the ecological consequences of warmer climates on these tundra ecosystems are more complex than simply elevated N-mineralization rates, and that the responses of landscapes might be impacted by soil age, or time since deglaciation.