Roger W. Ruess

Large Mammals and Process Dynamics in African EcosystemsHerbivorous mammals affect primary productivity and regulate recycling balances

emphasized the balances among trees, shrubs, and grasses as factors both influencing and influenced by large mammals (Lamprey 1963, McNaughton and Georgiadis 1986). Estimates of energy flow through food webs were an early contribution of this research (Lamprey 1964), but only recently have investigators begun to consider how large mammals can control such ecosystem processes as energy flow and nutrient cycling (Botkin et al. 1981, Frost 1985, McNaughton 1976, 1979a, 1985). Research reveals that African ecosystems are highly interactive; they cannot be understood without close

COUPLING FINE ROOT DYNAMICS WITH ECOSYSTEM CARBON CYCLING IN BLACK SPRUCE FORESTS OF INTERIOR ALASKA

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2 fluxes. Fine root production, measured over a two-year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm–2·d–1 over winter, to 0.051 ± 0.015 mm·cm–2·d–1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. M...

Root respiration in North American forests: effects of nitrogen concentration and temperature across biomes

Root respiration rates have been shown to be correlated with temperature and root N concentration in studies of individual forest types or species, but it is not known how universal these relationships are across forest species adapted to widely different climatic and edaphic conditions. In order to test for broad, cross-species relationships, we measured fine root respiration, as O2 consumption, over a range of temperatures on excised root samples from ten forested study sites across North America in 1997. Significant differences existed among study sites in root respiration rates, with patterns among sites in respiration rate at a given temperature corresponding to differences among sites in fine root N concentrations. Root respiration rates were highly correlated with root N concentrations at all measurement temperatures (r2>0.81, P<0.001, for 6, 18 and 24°C). Lower root respiration rates in gymnosperms than in angiosperms were largely explained by lower fine root N concentrations in gymnosperms, and root N concentrations and respiration rates (at a given temperature) tended to be lower at warm sites (New Mexico, Florida, and Georgia) than at cool sites with short growing seasons (Michigan and Alaska). Root respiration rates increased exponentially with temperature at all sites. The Q10 for root respiration ranged from 2.4 to 3.1, but there were no significant differences among the forest types. The average Q10s for gymnosperms (Q10=2.7) and angiosperms (Q10=2.6) were almost identical, as were the average Q10s for roots of ectomycorrhizal species (Q10=2.7) and arbuscular mycorrhizal species (Q10=2.6). In 1998, fine root respiration at the study sites was measured in the field as CO2 production at ambient soil temperature. Respiration rates under field conditions were dependent on both ambient soil temperature and root N concentration. Relationships between respiration (adjusted for temperature) and root N concentration for the field measurements were similar to those observed in the 1997 laboratory experiments. For root respiration in tree species, it appears that basic relationships with temperature and nitrogen exist across species and biomes.

The role of balsam poplar secondary chemicals in controlling soil nutrient dynamics through succession in the Alaskan taiga.

The vegetation mosaic of the Alaskan taiga is produced by patterns of disturbance coupled to well-defined successional patterns. In primary succession on river floodplains, one of the critical transitions in succession is that from thinleaf alder (Alnus tenuifolia) to balsam poplar (Populus balsamifera). This is the shift from a N2-fixing shrub to a deciduous tree. Through this transition there are major changes in N cycling including a decrease in N2-fixation, mineralization, and nitrification. Most models of plant effects on soil processes assume that these changes are caused by shifts in litter quality and C/N ratio. This paper reviews several studies examining the effects of balsam poplar secondary chemicals on soil nutrient cycling. Balsam poplar tannins inhibited both N2-fixation in alder, and decomposition and N-mineralization in alder soils. Other poplar compounds, including low-molecular-weight phenolics, were microbial substrates and increased microbial growth and immobilization, thereby reducing net soil N availability. Thus, substantial changes in soil N cycling through succession appear to have been mediated by balsam poplar secondary chemicals.

A first comprehensive census of fungi in soil reveals both hyperdiversity and fine‐scale niche partitioning

Fungi play key roles in ecosystems as mutualists, pathogens, and decomposers. Current estimates of global species richness are highly uncertain, and the importance of stochastic vs. deterministic forces in the assembly of fungal communities is unknown. Molecular studies have so far failed to reach saturated, comprehensive estimates of fungal diversity. To obtain a more accurate estimate of global fungal diversity, we used a direct molecular approach to census diversity in a boreal ecosystem with precisely known plant diversity, and we carefully evaluated adequacy of sampling and accuracy of species delineation. We achieved the first exhaustive enumeration of fungi in soil, recording 1002 taxa in this system. We show that the fungus : plant ratio in Picea mariana forest soils from interior Alaska is at least 17:1 and is regionally stable. A global extrapolation of this ratio would suggest 6 million species of fungi, as opposed to leading estimates ranging from 616 000 to 1.5 million. We also find that clos...

Ecological conditions that determine when grazing stimulates grass production

SummaryWe report the results of a pot experiment that examined the effects of three ecologically important factors controlling plant growth rates in savanna grasslands: defoliation, soil nitrogen and soil water availability. The experiment was conducted in the Amboseli region in east Africa, and was designed to simulate natural conditions as far as possible, using local soils and a grass species that is heavily grazed by abundant large herbivores. Productivity by different plant components was reduced, stimulated or unchanged by defoliation, depending on specific watering and fertilization treatments. Total above-ground production was stimulated by defoliation and was maximized at moderate clipping intensities, but this was statistically significant only when plants were watered infrequently (every 8 days), and most important, periods between clipping events were extended (at least 24 days). Under these conditions, plant growth rates were limited by water availability at the time of clipping, and soil water conserved in clipped, compared to unclipped plants. Within a given fertilization treatment, whole-plant production was never stimulated by defoliation because root growth was unaffected or inhibited by clipping. However, when fertilization was coupled to defoliation, as they are in the field, whole-plant production by fertilized and moderately clipped plants exceeded production by infertilized, unclipped plants. Under this interpretation, maximum whole-plant production coincided with optimum conditions for herbivores (maximum nitrogen concentration in grass leaves) when watering was frequent, and plants were moderately defoliated. However, these conditions were not the same as those that maximized relative above-ground stimulation of growth (infrequent watering and clipping).The results indicate that above-ground grass production can be stimulated by grazing, and when that is likely to occur. However, the results emphasize that plant production responses to defoliation can vary widely, contigent upon a complex interaction of ecological factors.

REGULATION OF FINE ROOT DYNAMICS BY MAMMALIAN BROWSERS IN EARLY SUCCESSIONAL ALASKAN TAIGA FORESTS

The effects of browsing by moose and snowshoe hares on fine root production, mortality, and decomposition in early successional forest ecosystems along the Tanana River floodplain in interior Alaska were studied over a 3-yr period using minirhizotrons placed inside and outside large permanent exclosures. Fine root production and mortality varied seasonally, with greatest rates of production occurring during June each year, and greatest rates of mortality occurring in fall and over winter. Annual production and mortality during 1993, a year of unusually low precipitation, were significantly higher than during either 1992 or 1994. Aboveground herbivory significantly reduced monthly rates of fine root production, and on an annual basis, fine root production of browsed plots (311.4 ± 31.7 mm·tube−1·yr−1) was significantly less than that of unbrowsed plots (453.8 ± 49.8 mm·tube−1·yr−1) when averaged over 3 yr. Because herbivory had less of an effect on monthly or annual rates of fine root mortality, fine root ...

LIMITATIONS TO SYMBIOTIC NITROGEN FIXATION IN PRIMARY SUCCESSION ON THE TANANA RIVER FLOODPLAIN

Constraints on nitrogen fixation are the ultimate causes of N limitation of primary production, but hypotheses concerning limitations to N2 fixation remain largely untested in natural terrestrial ecosystems. We examined limitations to N2 fixation by thinleaf alder (Alnus tenuifolia) in two stages of primary forest succession on the Tanana River floodplain (interior Alaska, USA) and focused on the hypothesis that N, fixation was limited by low soil P availability. Paired control and P fertilized plots were established at four replicate early successional alder stands and four later successional poplar (Populus bal- sanzifera) stands (dense alder understories with mature poplar overstories) and N2 fixation was estimated with an acetylene reduction assay. In alder stands, P fertilization increased total nodule dry biomass and increased total ecosystem N inputs, but it had little effect on nitrogenase activity per unit nodule dry mass (specific acetylene reduction activity, ARA). Specific ARA increased only in late July when soil temperature and ARA were at their maximum values. In contrast, fertilization had no effect on these measures in poplar stands where reduced soil moisture may have superseded limitation by P. We detected no differ- ences in specific ARA, total nodule biomass, or N inputs, between alder and poplar stands but all of these measures were highly variable. Leaf area of the alder canopy emerged as the best predictor of ecosystem inputs of fixed N among control plots. Alders resorbed high amounts of P but little N (consistent with low P availability and a high P demand and a high N availability in alder), and P fertilization reduced P resorption but had no effect on N resorption. The timing of N, fixation and N resorption indicate that late-season increases in leaf N, following a midseason reduction in leaf N, were driven by N2 fixation in excess of plant N demands as nodules continued fixing N while alder leaves senesced. These results have shown that P limits N2 fixation in alder stands in this nitrogen-limited sere, but that factors limiting N2 fixation can change over short successional time scales.

Cycling Dynamics of NH4+ and Amino Acid Nitrogen in Soils of a Deciduous Boreal Forest Ecosystem

AbstractConventional studies of nitrogen (N) cycling in forest ecosystems have focused on inorganic N uptake as the primary source of N for plant metabolism. More recently, however, alternative sources of N for plant nutrition, such as free amino acids, have gained attention, particularly in nutrient-limited systems. Using a multiple stable isotope (13C and 15N) design, that allowed us to simultaneously assess root uptake of ammonium (NH4+) and glycine, we compared the cycling dynamics of NH4+ and amino acid N within the soils of several interior Alaskan floodplain balsam poplar stands. Our design included multiple sampling periods extending from 45 min to 14 days, which permitted us to study interpool transfers of our carbon (C) and N isotopes over time. Microbial biomass N was the largest sink of both 15N-ammonium and glycine. Percent recovery of 15N for this pool was an order of magnitude larger than fine-root 15N uptake for most sampling periods. Although recovery of 15N in fine-root biomass was small, amino acid N and NH4+ were assimilated at approximately the same rate irrespective of sampling period, and total recovery was still increasing 2 weeks after application. Recovery of 15N in bulk soil samples did not vary significantly over time for either treatment. However, bulk soil 13C declined steadily during the experiment, measuring less than 30% recovery of added label after 14 days. We suspect that the majority of 13C lost from our soils was respired. Soil microorganisms strongly outcompeted plants in the short term for both NH4+ and amino acid N. However, amino acid N appears to cycle through soil N pools at approximately the same rate as inorganic N forms. The similarity in uptake patterns for inorganic and organic N suggests that these stands are meeting part of their N requirements directly from amino acids.

Landscape Heterogeneity of Shrub Expansion in Arctic Alaska

The expansion of shrubs into tundra areas is a key terrestrial change underway in the Arctic in response to elevated temperatures during the twentieth century. Repeat photography permits a glimpse into greening satellite pixels, and it shows that, since 1950, some shrub patches have increased rapidly (hereafter expanding), while others have increased little or not at all (hereafter stable). We characterized and compared adjacent expanding and stable shrub patches across Arctic Alaska by sampling a wide range of physical and chemical soil and vegetation properties, including shrub growth rings. Expanding patches of Alnus viridis ssp. fruticosa (Siberian alder) contained shrub stems with thicker growth rings than in stable patches. Alder growth in expanding patches also showed strong correlation with spring and summer warming, whereas alder growth in stable patches showed little correlation with temperature. Expanding patches had different vegetation composition, deeper thaw depth, higher mean annual ground temperature, higher mean growing season temperature, lower soil moisture, less carbon in mineral soil, and lower C:N values in soils and shrub leaves. Expanding patches—higher resource environments—were associated with floodplains, stream corridors, and outcrops. Stable patches—lower resource environments—were associated with poorly drained tussock tundra. Collectively, we interpret these differences as implying that preexisting soil conditions predispose parts of the landscape to a rapid response to climate change, and we therefore expect shrub expansion to continue penetrating the landscape via dendritic floodplains, streams, and scattered rock outcrops.