Ecology

Peatlands comprise a globally important carbon pool whose input–output budgets may be significantly altered by climate change. To experimentally determine the sensitivity of the carbon stored in peatlands to climate change, we constructed a mesocosm facility with 54 peat monoliths from a bog and fen in northern Minnesota, USA. These mesocosms were subjected to nine combinations of heat and water‐table levels over eight years. Bog mesocosms initially accumulated soil carbon, with greater gains in wetter mesocosms, but after three years no further water‐table effects occurred. In contrast, fen mesocosms lost or had no change in soil carbon, with the greatest losses in drier and warmer mesocosms. Changes in soil‐carbon storage resulted in concomitant changes in water‐table depth, so that water‐table depths were similar to those in the natural source sites by the end of the experiment regardless of the initial treatment. These results were primarily due to water‐table effects onSphagnummoss production in the bog mesocosms and to a more complicated suite of warming and water‐table effects on production and decomposition in the fen mesocosms. We show that different kinds of peatlands will rapidly gain or lose carbon following hydrological disturbance until they return to their characteristic (“equilibrium”) water‐table levels. Our results illustrate the potential for a rapid homeostatic response of these ecosystems to future climate change at small spatial scales. Climate change will likely also interact with other carbon cycle–hydrological feedbacks at the scale of the entire peatland over longer time frames and larger spatial scales.

The interactive effects of harvest intensity, site preparation, and herbicide treatment were evaluated in a clear—cut Piedmont site in North Carolina. Forest harvesting caused increased nitrogen mineralization and nitrification in all treatments, but harvesting without additional treatment had little effect on nitrate—nitrogen pool sizes and losses. The removal of most surface organic material during intensive site preparation led to greatly increased nitrate pool sizes and losses. Treatment with herbicide accentuated this effect, and in combination such removals and herbicide applications led to accumulations of nitrate—nitrogen in surface soil of 27 and 24 kg/ha in the first and second summers following site preparation. Nitrogen losses by leaching, denitrification, and erosion were also greatest in the plots where organic residues had been removed and herbicides applied. These results suggest that microbial immobilization controlled nitrogen pool sizes and losses, and this suggestion was confirmed using ¹⁵N. Soils from the plots without residue removal or herbicide treatment immobilized >90% of added ¹⁵N in 28 d, while those from the residue removal/herbicide treated plots immobilized <70%. Microbial activity was the predominant process regulating nitrogen availability and losses following disturbance in this site; site—preparation practices that removed important substrates for microbial metabolism caused elevated nitrogen losses.

Photosynthetic rates were measured for five species of intertidal marine algae, in the air and submerged. Ulva expansa and Prionitis lanceolata from the lower intertidal show reduced photosynthetic capacity air in air compared to submergedrates. In contrast, species from the middle and upper littoral (Iridaea flaccida, Porphyra perforata, Fucus distichus, and Endocladia muricata) reach maximum photosynthesis after some degree of drying. For these latter species, photosynthetic rates can be 1.6 to 6.6 times greater in air than in water at the same illumination and temperature. Desiccation rates under natural conditions are slow enough that these algae are capable of continuing a high rate of photosynthetic activity for extended periods while exposed and may fix the bulk of their carbon at this time. The capacity of these algae for sustained photosynthesis in air vary according to their intertidal zonation. It is suggested that these relationships may be partially responsible for the vertical distribution of intertidal marine algae.

Sporophytes of the sea palm Postelsia normally occur on a variety of intertidal surfaces. An experimental technique was developed permitting spores to be sown onto natural substrata whose relative suitabilities were evaluated using three criteria. All major surface categories (bare rock, animal, plant) were equivalent if judged by the appearance the following spring of a descendant sporophyte. However, the proportion of these plants surviving to maturity and continuing into the following year was highly substratum dependent. Bare rock surfaces are the most suitable for Postelsia as judged by probability of successful occupancy, sporophyte density, and inter—annual persistence. Single fertile plants can establish or maintain a population. The probability of local extinction is a function of population size: only 36% of small populations (1—30 plants) tended to continue to the next year, whereas all those >120 individuals did. Postelsia seems incapable of persisting in the presence of the turf—like algae Corallina or Holosaccion. Thus sea palms may require the presence of competitively superior mussels: mussels out—compete algae of low stature; waves remove mussels, thereby generating Postelsia's most suitable substratum.