Conifer seedling recruitment across a gradient from forest to alpine tundra: effects of species, provenance, and site

Abstract

Background: Seedling germination and survival is a critical control on forest ecosystem boundaries, such as at the alpine–treeline ecotone. In addition, while it is known that species respond individualistically to the same suite of environmental drivers, the potential additional effect of local adaptation on seedling success has not been evaluated.

Aims: To determine whether local adaptation may influence the position and movement of forest ecosystem boundaries, we quantified conifer seedling recruitment in common gardens across a subalpine forest to alpine tundra gradient at Niwot Ridge, Colorado, USA.

Methods: We studied Pinus flexilis and Picea engelmannii grown from seed collected locally at High (3400 m a.s.l.) and Low (3060 m a.s.l.) elevations. We monitored emergence and survival of seeds sown directly into plots and survival of seedlings germinated indoors and transplanted after snowmelt.

Results: Emergence and survival through the first growing season was greater for P. flexilis than P. engelmannii and for Low compared with High provenances. Yet survival through the second growing season was similar for both species and provenances. Seedling emergence and survival tended to be greatest in the subalpine forest and lowest in the alpine tundra. Survival was greater for transplants than for field-germinated seedlings.

Conclusions: These results suggest that survival through the first few weeks is critical to the establishment of natural germinants. In addition, even small distances between seed sources can have a significant effect on early demographic performance – a factor that has rarely been considered in previous studies of tree recruitment and species range shifts.

Keywords:

• common garden
• climate gradient
• local adaptation
• Picea engelmannii
• Pinus flexilis
• seedling emergence
• seedling recruitment
• species range boundaries
• subalpine forest
• treeline

Acknowledgements

This research was supported by the Office of Science (BER), US Department of Energy, Grant No. DE-FG02-07ER64457. We thank the Mountain Research Station and Niwot Ridge LTER at the University of Colorado, Boulder for logistical support. Many thanks to S. Ferrenberg for logistical support and to M. Redmond, A. Faist, S. Taylor Smith, S. Love Stowell, A. Allen, M. Anantharaman, W. Baird, S. Barlerin, R. Butz, P. Cuartero, K. Darrow, H. Dole, A. Farnham, H. Finkel, D. Haller, M. Harte, T. Lemieux (CU Boulder Greenhouse), M. McLaughlin (USFS CDA Nursery), A. Peterson, K. Riddel, L. Senkyr, J. Wilkening and X. Zhai, for their valuable assistance constructing exclosures, monitoring cone maturity, collecting, processing and sowing seed, transplanting and surveying seedlings, and entering survey data. F. Zust made the map in Figure 1. J. Harte was instrumental in the initial proposal, experimental design and moral support as well as helping with field work. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.