Climate-influenced ponderosa pine (Pinus ponderosa) seed masting trends in western Montana, USA

Abstract

Aim of study: The aim of this study was to analyze 10-year records of ponderosa pine (Pinus ponderosa) seed production, in order to confirm synchronic seed production and to evaluate cyclical masting trends, masting depletion effect, and climate-masting relationships. Area of study: The study area was located in a P. ponderosa stand in the northern Rocky Mountains (western Montana, USA). Material and methods: The study was conducted in one stand that had been subjected to a silvicultural study of uneven-aged management techniques that was carried out in 1984, and which resulted in three separate units consisting of one control, one cut/ no-burn treatment, and one cut/burn treatment. Seeds were collected during the 10 years following treatment in 15 traps systematically deployed within each of the stand’s three units. The total numbers of seeds collected in each unit were plotted over time to analyze crop synchrony, with Spearman rank correlation coefficient used to test for masting cycles and crop depletion after a mast year. Meteorological records over the period 1983-1994 were related to the occurrence of a mast event (defined as crops exceeding 50,000 viable seeds/ha). Main results: The seed production pattern was non-cyclical, synchronous, and independent of silvicultural treatment history. A mast-depletion effect was evident but was not statistically significant. Mast events seem to be promoted by the occurrence of optimum mean temperatures at the beginning of spring during both the first (11 °C) and second (9 °C) years of cone maturation. The probability of a mast year was also affected by summer temperature (number of late frost days; negative effect) and precipitation amount (positive effect). All these factors would seemingly explain the observed synchronous pattern in cone production. Research highlights: The non-cyclical trend of ponderosa pine seed mast years is influenced by specific climate determinants. Fluctuations in mean early spring temperatures, late frost and water availability are likely to affect P. ponderosa seed production, with implications for natural regeneration in this region.