Grassland ecosystem responses to warmer nights.

摘要

Global minimum temperatures (TMIN) are increasing faster than maximum temperatures (TMAX), but the ecological consequences of this are largely unexplored. I used two approaches to investigate the manner in which native North American shortgrass steppe vegetation may respond to increasing TMIN. First, I used long-term data sets to identify correlations between seasonal TMIN and several vegetation variables. Most notably, increased spring TMIN was correlated with decreased aboveground net primary production by the dominant C4 grass ( Bouteloua gracilis) and with increased abundance and production by exotic and native C3 forbs. Second, I conducted a two-year field experiment designed to assess the relationships identified from the correlation analyses. I developed a new method, using automated tents to cover field plots at night, to elevate TMIN over intact shortgrass steppe vegetation. This technique effectively reduced soil nocturnal long-wave re-radiation and raised soil and air TMIN an average of 1--2°C, while leaving TMAX unchanged. Furthermore, surface soil layers in the warmed plots were significantly drier than soils in control plots. After two years of warming, B. gracilis responded with decreased aboveground mass, C:N ratios, and tiller density; a common C3 grass ( Pascopyrum smithii) responded with increased aboveground biomass, but no significant differences in C:N ratios or tiller density; and, a CAM cactus (Opuntia polyacantha) responded with decreased cladode density, but increased flower and fruit number, and no change in C:N ratios. Both grasses exhibited decreased herbivory damage during at least part of both years.; In a climate change scenario in which TMIN continues to increase, more favorable conditions early in the growing season may allow C3 vegetation (both native and exotic species) to more thoroughly exploit important resources prior to the period when maximum temperatures are optimal for growth by C4 grasses. Additionally, by decreasing basal cover of the dominant species, climate change may transform the shortgrass steppe into an ecosystem more vulnerable to invasion by exotic species and less tolerant of drought and grazing.