Growth and mortality of planted interior Douglas-fir and western larch seedlings during the establishment phase in Idaho, USA

Abstract

Planting for reforestation is increasingly important for the establishment of forests for future timber production and enhanced carbon sequestration. Planting also requires significant investment to ensure successful seedling establishment. However, initial seedling growth and survival is a common missing link in forest growth and yield models as they are predominantly designed to project established trees often post canopy closure. The multitude of both biotic and abiotic factors that influence growth and mortality of planted seedlings during the establishment phase is less examined. This is likely due to the high variability and complexity of this type of analysis, as well as the general lack of available data. The current study utilized annual measurements of 13,701 seedlings beginning immediately after planting to evaluate growth and mortality of planted interior Douglas-fir (Pseudotsuga menziesii var. glauca [Mirb.] Franco) and western larch (Larix occidentalis Nutt.) seedlings across multiple contrasting sites in northern Idaho and eastern Oregon for the first two years. Our results indicate that growth and mortality of seedlings were strongly influenced by their initial diameter and height such that slimmer and taller seedlings had higher mortality, while diameter and height growth was positively related to initial diameter and height, respectively. Weather conditions, especially measures of temperature and precipitation, had a similarly important role on seedling growth and mortality. The effects of root growth potential and competing vegetation were generally insignificant as pre-planting chemical preparation was applied to all sites. Improved western larch seedlings from seed orchards had higher growth rates with small increases in mortality compared to wild collected sources, but improved Douglas-fir had minimal gains in growth and much higher mortality. The developed seedling models readily fit into the architecture of current individual tree growth and yield models, which extend these models' ability to forecast seedlings during and after establishment. The identified influential factors and specific differences in seedling growth and mortality during the establishment phase provide critical information and guidance to the increasingly important reforestation practice of planting, and the overall analytical framework presented in this analysis is generally applicable to species beyond Douglas-fir and western larch.