Drought tolerance increases with elevation in tropical understory plant communities

Background/Question/Methods

Tropical Andes are undergoing shifts in temperatures and precipitation due to climate change, and such changes are especially measurable along tropical elevation gradients. Understory plants are a key component of forest structure in montane forests. However, most studies of this region have neglected understory species and focusing mainly on the ecophysiological relationships of canopy species with temperature. Along a 2.6 km elevational gradient in the eastern slopes of Manu National Park, Peru, plants are exposed to different water regimes, mostly related to the presence of clouds and rainfall. Rainfall decreases with elevation and there is a distinct cloud forest habitat at mid elevation.

We studied the physiological climatic boundaries of understory plants in relation to water availability. We were interested to what degree, leaf drought tolerance could explain the distribution of species along the elevation gradient. We hypothesized that species at higher elevations, with reduced precipitation and outside the cloud forest envelope, will have higher drought tolerance.

Fifty-three 5x5m plots were sampled across the gradient from 1000 – 3600m asl. Five mature leaves from all woody understory plants with basal diameter > 1cm, and height < 3m were collected from the plots. Leaf discs were submerged into a hypertonic solution to simulate the low water potentials experienced by plant cells during drought. The solute leakage after the treatment was used to calculate a relative index for drought tolerance (%) per plot.

Results/Conclusions

We found that drought tolerance increased significantly with elevation. Plots above and at the upper-limit cloud forest had on average the highest percentage of drought tolerance (mean±SD: 19.7 ±6.20, 39.93±10.73, respectively). Plots below and at the lower-limit cloud forest showed the lowest percentage drought tolerance (67.52%±8.99, 54.22%±9.05). Our results indicate that understory species growing in lower elevations with higher rainfall (~5000mm yr-1) are less tolerant to drought. Additionally, cloud forest plots (2000-3200m asl) showed similar percentage drought tolerance across the elevation gradient (50.61%±5.93). Our results suggests that changes in water availability, not only temperature, can likely affect plant distribution in montane forests into the future.