Climate and net carbon availability determine temporal patterns of seed production by Nothofagus

We analyzed seed production of mountain beech (Nothofagus solandri var. cliffortioides) forest along an elevational gradient in New Zealand from 1020 to 1370 m (treeline) for the years 1973-2002. We used seed production data from nine elevations and a site- and species-specific net carbon (C) availability model from two elevations (1050 m and 1340 m) to examine how three variables (temperature, soil moisture, and net C availability) during three key periods (resource priming, flowering primordia development, and flowering) influence seed production. These three strongly interrelated variables have all been considered determinants of seed production but have not previously been tested together in a single analysis. Seed production increased over the 30-year period, with the greatest increases at high elevations; this increase was driven by a greater frequency of intermediate-sized seeding years. We then determined how temperature, soil moisture, and net C availability determined seeding, and examined whether temporal trends in the seeding data could be linked to similar temporal trends in temperature, soil moisture, or net C availability. High seed production was related to cool summers with high soil moisture during resource priming, warm summers during flower primordia development, and low net C availability during flowering. Positive temporal trends in temperatures during the period of flower primordia development accounted for the increase in seed production, suggesting that increasing temperatures are promoting more frequent seed production at high elevations.