Forest patches and the upward migration of timberline in the southern Peruvian Andes

• Climate change and above-timberline forest patches at a tropical timberline.
• Seed rain and diversity decreases sharply outside the forest–puna ecotone.
• Microclimate severity increases in the puna but is buffered by patches.
• Seedling abundance decreases with distance into the puna.
• Timberline shifts are facilitated by forest patches but hindered by human activity.

Montane plant species around the globe are predicted to shift their distributions upslope in response to increasing temperatures associated with climate change. In the tropical Andes, which are one of the most diverse and threatened of all biodiversity hotspots, many plant and animal species have already started to shift their distributions to higher elevations in response to warming. However, a variety of biotic and abiotic factors may stabilize Andean timberlines and halt further upslope migrations into the high elevation grasslands (referred to as “puna” in southern Peru) above the forest. One possibility is that small forest patches that occur above-timberline throughout the Andes may facilitate forest expansion into newly suitable areas in the puna, yet little is known about the ecology or function of these patches. In this study, we examined seedling recruitment patterns, seed dispersal, and microclimate at the timberline, in and around above-timberline forest patches, and in the puna. The above-timberline forest patches had similar patterns of seed dispersal as the timberline but overal <1% of captured seeds were dispersed 10 m into the puna. At both the patch edges and the forest timberline, seedling abundances were lower in the puna relative to the adjacent forest and forest–puna ecotone. This reduction may be a result of reduced seed dispersal across the forest–puna ecotone and/or decreased germination of dispersed seeds due to the harsh microclimatic conditions occurring in the puna (daily temperature fluctuations are greatly elevated in the puna relative to the forest and frost events are more frequent and severe). While increasing temperatures associated with climate change may ameliorate some of the severe climatic conditions occurring in the puna, it will not directly affect other potential recruitment limitations such as reduced seed dispersal, high levels of UV radiation, and anthropogenic activities (cattle grazing and fires) in the puna. With the reduction of anthropogenic activities, above-timberline forest patches may serve as nucleating foci for future forest expansion into the puna. However, our results indicate that any upslope migration of the timberline into the puna will likely occur at a rate that is slower than what is required to keep pace with warming because recruitment is restricted to a narrow strip along the forest–puna borders. Slowed forest expansion into the puna could act as a barrier to the upslope migration of Andean cloud forest species leading to extreme losses of Andean biodiversity.