Lodgepole Pine Seed Germination Following Tree Death from Mountain Pine Beetle Attack in Colorado, USA

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ABSTRACT. -

Cones of lodgepole pine (Pinus contorta var. latifolia) are often serotinous, releasing their seeds from closed cones under heat from fire. Stand-replacing fires in predominantly serotinous stands can thus be expected to result in a strong regeneration response. After large-scale mortality caused by mountain pine beetle (Dendroctonus ponderosae), however, the seeds in serotinous cones may remain on the dead trees for a number of years, impacting germination and viability. We tested seeds collected from living and beetle-killed serotinous stands to determine whether they remain viable after tree death, and whether germination rates were affected by cone age. There was no significant difference in percent germination from the living stand vs. the dead stand. While there was a significant relationship between cone age and percent germination, cones that were 21-25 y still had >30% germination. We conclude that post-beetle regeneration likely will not be limited by viable seed availability in stands with serotinous cone-bearing trees.

INTRODUCTION

The mountain pine beetle (Dendroctonus ponderosae) has recently reached epidemic population levels in the United States and Canada (Rafia et al, 2008). In Colorado, the State Forest Service estimates that over 600,000 ha have been affected during the current outbreak, a scale unprecedented in recorded history. Lodgepole pine (Pinus contorta var. latifolia), the tree most affected in the outbreak, usually regenerates in large numbers following stand-replacing fire. An important regeneration mechanism is the production of serotinous cones, which remain closed until stimulated by the heat of the fire to open and release their seeds (Tower, 1909; Clements, 1910). Lodgepole pine vary greaüy in the proportion of serotinous and non-serotinous cones (e.g., Schoennagel et al, 2003), but many of the stands affected by the current outbreak in Colorado are composed of predominantly serotinous trees; e.g., a recent survey of lodgepole pine stands on the west side of Rocky Mountain National Park found a mean serotiny per stand of 63% (C. Aoki, pers. obs.). hi the absence of fire, large numbers of beetle-killed trees will remain on the landscape, with their seeds still tightly held within the cones. What will this mean for the future regeneration of these stands across the landscape?

Unlike a fire, in which seeds are released by heal in a single event, a large-scale mortality event caused by beetles will result in a slower release of the seeds. The cones witl open over a number of years, either through radiant heat in the canopy or by absorbing heat near the ground as limbs break off and fall (Tower, 1909; Lotan, 1964). A key question, then, is whether or not seeds held in serotinous cones remain viable for years after the tree has died. Early studies indicated that in some instances, seeds could survive a number of years on a dead tree, or even separated from the tree (Sargent, 1880; Tower, 1909; Mills, 1915) . Mirov (1946) showed that lodgepole pine seeds kept in cold storage for over nine years still maintained high germination. However this has not been evaluated in a controlled experiment utilizing seeds remaining in the canopy of standing dead trees. Will stands dominated by serotinous lodgepole pine have the viable seed needed for regeneration over the years following the beetle outbreak?

To test this question, we conducted an experiment using serotinous trees from the current beetle outbreak in Rocky Mountain National Park, Colorado, comparing germination in serotinous cone seeds between living trees and trees killed in the current outbreak, as well as between cones located on younger vs. older portions of branches. Knowing whether seed germination declines over time in dead trees will help us to understand the future regeneration possibilities for these stands with extensive overstory mortality.

Before we began our study, we also needed to determine an effective method of determining cone age from sampled branches. …