COS 37-1
Hitting the ‘grass ceiling’: Recruitment limitation inhibits climate change shifts of tropical treelines
As global temperatures increase, climatic conditions suitable to tropical montane plant species will be displaced towards higher elevations. Plants are predicted to experience range contractions and elevated extinction risks unless they shift distributions upslope to remain within their thermal niches. In the Andes, grasslands (puna) above high-elevation treelines are one possible area where montane plant species may expand their ranges with warming. However, there is strong evidence that low temperatures and intense UV solar radiation limit tree recruitment at tropical treelines. In addition, rare freezing events may limit seedling survival outside of the temperature-buffering effects of a closed-canopy forest. Such freezing events are predicted to stay relatively common despite increasing mean temperatures. To better understand the potential limits to tree establishment above treeline, we experimentally tested the effects of low temperatures and shading on tree seedling survival in the puna.
In the Peruvian Andes, over 1700 seedlings of 4 treeline species were translocated to 5 locations under the following treatments; control, open puna, shading, warming, and shading + warming during the winter and spring of 2012. In addition, freezing trials of leaf material of the 4 species was conducted from July to November 2013.
Results/Conclusions
For the most-common treeline-forming species, transplant experiments showed that warming results in higher mortality. For a second common treeline-forming species shading had a large effect on seedling survival while warming only marginally increased survival. Freezing trials revealed that the freezing tolerance of all species were between -7 and -10°C. The lowest temperature recorded at the treeline study sites over the past 4 years was -8.9°C indicating that rare freezing events may also limit forest recruitment in the puna.
Average temperatures are increasing, which is predicted to increase tree seedling recruitment above treeline. Yet our data show that warming actually decreases seedling survival for some high-elevation species. For other species, it appears that shade may be necessary for successful establishment beyond the treeline and average temperatures play only a minor role.
In addition, seedlings in the open puna experience more severe and frequent frost occurrences relative to the forest. Freezing trials revealed that recruitment may also be limited outside of the forest due to rare freezing events. The frequency of these freezing events is not expected to decrease in the future.
These results indicate that Andean treelines may create a ‘grass ceiling’ that cannot be crossed by many forest species despite rising temperatures.