Print PageAndean plant species are being displaced towards higher elevations as they track suitable climatic conditions. With ongoing warming, cloud forest species are predicted to shift their ranges into high-elevation grasslands (puna) above current timberline. However, timberline can act as a barrier to migration if forest seeds do not disperse into the puna or if dispersed seeds are subsequently killed by harsh environmental conditions. If cloud forest species are unable to cross timberline and invade the puna they will suffer rapid range contractions and elevated extinction risks.
Small forest patches found above primary timberlines may act as “nucleating” areas and facilitate the establishment of forest into the puna. These patches may directly increase seed dispersal by acting as seed sources and/or increase seedling establishment by creating amiable microclimates. My objective was to determine if above-timberline forest patches can facilitate the upslope migration of Andean tree species in response to climate change. I quantified seed rain in 392 seed traps established along 14 transects extending from above-timberline forest patches, across puna, and into the forest below timberline. Microclimate data loggers were placed at 5 locations along 3 transects to determine differences in climate between forest, puna, patches and forest ecotones.
Results/Conclusions
Seed rain declined from forest, across timberline and into the puna (label permutation test, p < 0.05) but seed rain within forests did not vary with distance from puna edge (p > 0.05). Seed rain within above-timberline forest patches was not different than in forest below timberline (p > 0.05), suggesting that seed production in small forest patches may be comparable to that in continuous forest. The dramatic reduction of seed rain just 2m outside of forest indicates that seed dispersal may limit the pace of species migrations.
Microclimate data indicate that forest patches have similar temperature profiles as in continuous forest, mean daily temperatures are lower in forest than in puna (6.8°C vs. 7.5°C), diurnal temperature range is drastically reduced in the forest relative to the puna (11.1°C vs. 22.0°C), and nightly frost occurrence is greatly reduced in the forest relative to puna (10% vs. 59%).
These results suggest that above-timberline forest patches may play a significant role in forest expansion into the puna, by acting as seed sources and creating microclimatic conditions conducive to forest establishment. The nucleating role of forest patches will be incorporated into future models predicting species migration rates and forest expansion due to climate change.
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