Propagule limitation and competition with nitrogen fixers limit conifer colonization during primary succession

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Abstract

Questions:

Is conifer colonization on primary successional sites limited by propagule availability or environmental filters? How do nitrogen-fixing pioneer species (Lupinus lepidus and Alnus viridis) affect conifer emergence and establishment?

Location:

Mount St. Helens, Washington State, USA.

Methods:

We planted seeds of Pseudotsuga menziesii in four habitat types – barrens, young and old Lupinus patches and Alnus thickets – in a partial factorial cross with shade and soil disturbance treatments in 2002, 2003 and 2004, and monitored survival through 2006. We compared experimental results with surveys of natural establishment in 2007 and 2010.

Results:

Pseudotsuga menziesii emergence and survival were highest in barren and young Lupinus sites and low in old Lupinus and Alnus patches, but varied greatly among cohorts. In barrens, nearly 50% of germinated seedlings of the 2002 cohort survived year 1 and 25% through year 4, compared to 10% (year 1) and ˜0% (to year 4) survival in other habitats. Survival rates were lower in subsequent cohorts. Emergence and establishment were enhanced by shade and initial soil disturbance in barren sites and young Lupinus patches. Soil moisture was higher in barren and young Lupinus habitats. Post-experiment surveys indicated older trees were six times more abundant on sites with old Lupinus patches and Alnus thickets, consistent with earlier colonization at those sites driven by seed availability, but total tree density increased more rapidly in sites with barren and young Lupinus habitats, surpassing older sites by 2010, as predicted from experimental results.

Conclusion:

Increased seedling survival in barren and young Lupinus sites and under shade was attributable to decreased competition for soil moisture in those sites, possibly combined with apparent competition in old Lupinus and Alnus sites via seed and seedling predators. Overall, high emergence and survival rates in low-density habitats suggest that P. menziesii colonization is strongly propagule limited, and that survival of seeds that successfully arrive and germinate is more strongly affected by competition in early succession than by soil N. This conclusion is supported by evidence for decreased recruitment over time in old Lupinus patches and Alnus thickets and accelerating recruitment in areas with less developed vegetation.

Conclusion:

On Mount St. Helens many primary successional surfaces were bare while others were colonized by nitrogen-fixing plants which facilitate community development. Dense N-fixing plants inhibit Pseudotsuga menziesii establishment via moisture competition. However, in sparsely vegetated areas, conifer seeds had high establishment probability, demonstrating strong propagule limitation. Conifer establishment has decreased in densely vegetated areas but accelerated in sparsely vegetated areas.

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