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Results

Technical highlight
Old-field communities respond to warming


Species abundance
  • In 2003, elevated temperatures reduced cover of the dominant C3 legume Trifolium pratense, but did not affect the dominant C3 bunchgrass Dactylis glomerata. In 2004, these responses were reversed: warming reduced cover of Dactylis but not Trifolium.

  • This pattern was likely driven by the increasing dominance of Lespedeza between 2003 and 2004: high cover of Lespedeza by autumn 2004 may have effectively suppressed Trifolium, but not Dactylis which completes much of its vegetative growth by late spring.

  • In both 2003 and 2004, late-season perennials such as Solidago canadensis (C3 dicot) and Andropogon virginicus (C4 grass) had higher foliar cover, and taller and longer shoots, under elevated temperatures.

Production
  • Total aboveground biomass for 2003 tended to be greatest under ambient temperatures, elevated [CO2], and wetter treatment conditions.

  • In 2004 warming increased production of Plantago 3-fold, but reduced production of Dactylis by 50%. Production of Trifolium was little affected by any one treatment, or combination of treatments.

Phenology
  • NDVI measurements in spring 2004 suggested that elevated [CO2] appeared to ameliorate a negative effect of elevated temperatures on community greenness.

  • Throughout the growing season NDVI was consistently greater in wet than dry plots; NDVI was positively correlated with the sum of all species' cover values, and NDVI was most highly correlated with cover of Lespedeza, particularly late in the growing season.

  • Species that flower later in the season (e.g., Lespedeza, Solidago, Andropogon) all bolted or flowered earlier in warmed plots. In contrast, cool-season grasses and forbs that flower in early spring showed little response to any treatment in terms of timing and number of inflorescences.

Belowground responses
  • Soil respiration responded immediately to CO2 enrichment and has remained consistently higher in elevated [CO2].

  • Responses to elevated temperature are much more complicated, varying through time and in interaction with other variables. Soil moisture is an important modifying influence on the temperature effect on soil respiration.

  • Soil nitrogen dynamics have not been responsive to the treatments.


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Last revised: January 3, 2005