Vegetation trends over eleven years on mountain summits in NW Argentina

Abstract

As global climate change leads to warmer and dryer conditions in the central Andes, alpine plant communities are forced to upward displacements following their climatic niche. Species range shifts are predicted to have major impacts on alpine communities by reshuffling species composition and abundances. Using a standardized protocol, we surveyed alpine plant communities in permanent plots on four high Andean summits in NW Argentina, which range from 4,040 to 4,740 m a.s.l. After a baseline survey in 2006-2008, we resurvey the same plots in 2012, and again in 2017. We found a significant decrease in plant cover, species richness, and diversity across the elevation gradient in the three censuses and a strong decrease in soil temperature along the elevation gradient. We found a high plant community turnover (37%-49%) among censuses, differentiating according to summits and aspects; major changes of community turnover were observed in the lowest summit (49%) and on the northern (47%) and western (46%) aspects. Temporal patterns in community changes were represented by increases in plant cover in the highest summit, in species richness in the lower summit, and in diversity (Shannon index) in the four summits, over time, together with increase in small herbs and non-tussock grasses. We suggest that the observed trend in plant community dynamics responds to short-term temperature and precipitation variability, which is influenced by El Niño Southern Oscillation (ENSO), and due to time lags in plant community response, it may take much longer than one decade for the observed trends to become stables and statistically significant. Our study provides an important foundation for documenting more profound changes in these subtropical alpine plant communities as global climate change continues.

Keywords: Andes; GLORIA initiative; climate change; community turnover; elevation gradient; plant diversity.

Figure 1

Plant community known as cryptofruticetum, (a) composed by flattened vegetation, with great diversity of tiny colorful flowers in summer. (b) Adesmia crassicaulis (in the hand) one of the species of the cryptofruticetum community observed in the ground, in Cumbres Calchaquíes, Tucumán, Argentina

Figure 2

Study area. GLORIA summits indicated with a red point in a Landsat image. Tucuman, Argentina map (left corner). Sampling design scheme showing 1 m² plot, the position of data loggers and summit lower limit (right corner, from Pauli et al., 2015). Linear regression between mean soil temperature and elevation is superposed with the image (represent the main hypothesis). The main spatial and temporal trends in vegetation variables are represented by graphs in the right; symbols indicate increasing trends over time

Figure 3

Scatterplot of (a) plant cover, (b) species richness, and (c) Shannon diversity index at plot scale in relation to mean monthly soil temperature for baseline, first, and second resurveys. Each point represents and individual 1 m² plot (four units per aspect except 1‐ALZ north and 3‐SIN east‐ 56 per census). *** p < 0.001, **p < 0.01

Figure 4

Non‐metric multidimensional scaling (NMDS) based on a 48‐sampling units x 95‐vascular species cover matrix. Each point represents an individual summit/aspect/year combination, where the species data from the 4 1 m² plots in each summit‐aspect were pooled. Axis scale is proportional to variance contribution explained. Black arrows represent successional trajectories over time (Baseline: BL ‐> Resurvey1: RS1 ‐> Resurvey 2: RS2) by aspect: north (N), east (E), south (S), west (W). Main environmental and topographic factors are indicated with blue arrows (obtained from correlation with secondary matrix)

Figure 5

Box plot of relative change in (a) plant cover, (b) species richness, and (c) Shannon diversity index in the four summits in 11 years (Resurvey 2‐ baseline/baseline). **p < 0.001, ***p < 0.0001