Ecology, 88(2), 2007, pp. 381–390 Ó 2007 by the Ecological Society of America EARLY ONSET OF VEGETATION GROWTH VS. RAPID GREEN-UP: IMPACTS ON JUVENILE MOUNTAIN UNGULATES NATHALIE PETTORELLI, 1 FANIE PELLETIER, 2,3 ACHAZ VON HARDENBERG, 4 MARCO FESTA-BIANCHET, 2 AND STEEVE D. CO ˆ TE ´ 1,5 1 De´partement de Biologie and Centre d’e´tudes nordiques, Universite´ Laval, Que´bec G1K 7P4 Canada 2 De´partement de Biologie, Universite´ de Sherbrooke, 2500 Boul. de l’Universite´, Sherbrooke, QC J1K 2R1 Canada 3 Division of Biology, Faculty of Life Sciences, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY UK 4 Alpine Wildlife Research Centre, Parco Nazionale Gran Paradiso, via della Rocca 47, 10123 Torino, Italy Abstract. Seasonal patterns of climate and vegetation growth are expected to be altered by global warming. In alpine environments, the reproduction of birds and mammals is tightly linked to seasonality; therefore such alterations may have strong repercussions on recruitment. We used the normalized difference vegetation index (NDVI), a satellite-based measurement that correlates strongly with aboveground net primary productivity, to explore how annual variations in the timing of vegetation onset and in the rate of change in primary production during green-up affected juvenile growth and survival of bighorn sheep (Ovis canadensis), Alpine ibex (Capra ibex), and mountain goats (Oreamnos americanus) in four different populations in two continents. We indexed timing of onset of vegetation growth by the integrated NDVI (INDVI) in May. The rate of change in primary production during green-up (early May to early July) was estimated as (1) the maximal slope between any two successive bimonthly NDVI values during this period and (2) the slope in NDVI between early May and early July. The maximal slope in NDVI was negatively correlated with lamb growth and survival in both populations of bighorn sheep, growth of mountain goat kids, and survival of Alpine ibex kids, but not with survival of mountain goat kids. There was no effect of INDVI in May and of the slope in NDVI between early May and early July on juvenile growth and survival for any species. Although rapid changes in NDVI during the green-up period could translate into higher plant productivity, they may also lead to a shorter period of availability of high-quality forage over a large spatial scale, decreasing the opportunity for mountain ungulates to exploit high-quality forage. Our results suggest that attempts to forecast how warmer winters and springs will affect animal population dynamics and life histories in alpine environments should consider factors influencing the rate of changes in primary production during green-up and the timing of vegetation onset. Key words: body mass; green-up; NDVI; plant phenology; population dynamics; resource–animal interactions; survival. INTRODUCTION Predicting the effects of global warming on organisms of different ecosystems is a major challenge for ecologists (Walther et al. 2002). In recent decades, climate change has affected many biological systems (Crick and Sparks 1999, Post and Stenseth 1999, Inouye et al. 2000), and much effort is devoted to understand the consequences of such changes (Hughes 2000, Hulme 2005). Global climate change is altering seasonal patterns: for example, the average start of the growing season shifted by eight days from 1989 to 1998 in Europe (Chmielewski and Ro¨ tzer 2002) and by 5–6 days from 1959 to 1993 in North America (Schwartz and Reiter 2000). The life history strategies of species experiencing seasonal environments have been selected to match the best environmental conditions. With seasonal patterns altered, however, the reproduction of these species may become out of phase with the period of highest environmental productivity (Thomas et al. 2001, Berteaux et al. 2004). The ultimate consequences of the timing of birth are expected to depend to a large extent on the phenology of organisms at other trophic levels (Visser et al. 2004). In seasonal environments, large herbivores typically give birth in late spring or early summer to match the vegetation green-up period and allow offspring to benefit from the entire vegetation growing season (Rutberg 1987). By shifting plant phenology toward an earlier vegetation onset, global warming could affect juvenile growth and survival of many species (Inouye et al. 2000, Visser et al. 2004, Pettorelli et al. 2005a, c). Highly seasonal environments such as those in arctic or alpine areas are expected to be strongly affected by climate change (Oechel et al. 1997). There is much interest in the influence of global warming in mountainous regions (Diaz and Bradley Manuscript received 25 May 2006; revised 3 August 2006; accepted 24 August 2006. Corresponding Editor: C. M. Herrera. 5 Corresponding author. E-mail: steeve.cote@bio.ulaval.ca 381