Patterns and controls of primary production in the Patagonian steppe: A remote sensing approach

EG JOBBÁGY; OE SALA; JM PARUELO

ECOLOGY

Ecological Society of America

Lugar: Washington DC; Año: 2002 vol. 83 p. 307 - 319

0012-9658

We took advantage of regional gradients to study the spatial relationshipsbetween aboveground net primary production (ANPP) and climate in the Patagonian steppeof South America. We explored the same relationships through time, considering the naturalvariations of ANPP and climate for 11 yr. Based on NOAA/AVHRR satellite normalizeddifference vegetation index (NDVI) data, we evaluated the effects of climate on annualand seasonal ANPP across regional gradients of precipitation (100–500 mm/yr) and temperature(218–98C of annual mean). We studied ANPP climatic controls through time atfour sites using NDVI and meteorological data.We used annual NDVI integral as a surrogate of annual ANPP. Annual NDVI integralincreased linearly along regional gradients of precipitation, and its annual variability decreasedexponentially. Annual NDVI integral was, in most cases, unrelated to precipitationthrough time. We described the seasonality of ANPP using four variables derived fromseasonal NDVI curves: the dates of growing season start and end, the date of maximumNDVI, and the length of the growing season. The growing season started later toward thecold extreme of the regional temperature gradients and, within a given site through time,during the coldest years. The dates of maximum NDVI and end of the growing seasonoccurred later toward the humid or cold extremes of the regional gradients, whereas thelength of the growing season was positively affected by precipitation and temperature alongthese gradients. These variables were not associated with climate through time. The responseof the start of the growing season to temperature was greater in time, following the naturalclimatic fluctuations, than in space, accompanying regional temperature gradients. Thisdifference probably resulted because the time required for shifts in community compositionand plant adaptation is longer than one year. Climatic determinants of ANPP shifted fromprecipitation alone to precipitation plus temperature when the temporal scale of analysischanged from annual to seasonal. Our results indicate the feasibility of forecasting forageavailability a few months prior to the beginning of the growing season, but not during thewhole year. Longer term data sets and manipulative experiments are required to forecastannual ANPP and predict its response to climate change.