Growth characteristics of Rhododendron campanulatum above treeline in the Himalaya

Background/Question/Methods

Many studies have suggested that some attribute of temperature limits tree distribution at high elevations. In the central Himalaya, Rhododendron campanulatum has attained a species limit higher (>150m) than the treeline. This elevational gradient corresponds to a temperature difference of ~1°C. I hypothesized that the plants at species limit should have slower growth rate, smaller crown size, and more leaf damage than those at treeline. Since our interest was in understanding the growth of recently established saplings, we avoided small sized but older plants. After avoiding such stunted individuals (we encountered four of them at species limit), forty saplings between 10 and 50 cm in height were randomly selected at species limit and another forty right above treeline (the two groups did not have significantly different heights). For each sapling, we measured the following: plant height, shoot expansion in the last and current year, length and width at the widest part of current year’s leaves, crown spread at an angular interval of 45 degrees, and the number of healthy and damaged leaves. Additionally, we measured a range of potential covariates: (1) vegetation cover within a 1-m diameter plot around each sapling, (2) height of surrounding vegetation at four equally spaced points on the circumference of the plot, (3) slope, and (4) aspect. 

Results/Conclusions

Shoot expansion in the current year and last year was not different between the two sites. Crown spread was wider at species limit than at treeline, after controlling for plant height and aspect. In contrast, a higher fraction of total leaves had visible damage at species limit (72% of the total) than at treeline (57%). When healthy leaves were studied separately, they were bigger (larger area) and wider (smaller length to width ratio) at the species limit than at treeline. Bigger crown size and the lack of slower shoot expansion at species limit could be the result of less intense biotic competition at species limit (less surrounding vegetation cover and shorter vegetation) than at treeline or adequate adaptation of population at species limit to the environmental extremes.