Understory plant recovery and initial conditions: Thirty years after burial by tephra from Mount St. Helens

Background/Question/Methods

During recovery from disturbance, vegetation may reflect the pattern of initial environmental conditions. We studied the response of understory plants in old-growth subalpine conifer forest after the 1980 tephra deposit from Mount St. Helens, Washington. Tephra 15 cm deep that fell on snow almost destroyed the understory; in contrast, 4.5 cm tephra on snow-free sites affected primarily low evergreen herbs and bryophytes. Soon after tephra emplacement, we recorded tephra depth, snow distribution beneath tephra, plot micro-topography, cover of surface wood, light intensity, and tree canopy composition for each of 100 1-m² plots at each of four sites. We measured plant cover periodically from 1980 to 2010. We used logistic regression to relate presence of major growth forms and taxa in 1981 and 2010 to initial levels of environmental factors at the 1 m² plot scale. For each significant relationship, we used Spearman rank correlation, or means that differed using a Kruskal-Wallis test, to determine its sign. 

Results/Conclusions

Plant presence was significantly related to ten factors.  Those most often significant were plot micro-topography, presence of snow beneath the tephra, light intensity, tephra depth, and presence of Chamaecyparis nootkatensis, Tsuga heterophylla, T. mertensiana, and gaps in the canopy.  Deep tephra and presence of T. heterophylla, T. mertensiana, and Pseudotsuga menziesii usually reduced plant presence; light intensity and presence of Abies amabilis had both + and – relationships; and other factors were usually associated with increased presence.  In many cases, not summarized here, relationships were significant only at a single site. Many of the 42 significant relationships changed between 1981 and 2010, with 9 gaining and 8 losing significance, while 2 relationships reversed sign. Thirty years after disturbance, there were still pronounced relationships to conditions at the time tephra fell.  In 2010, herb presence increased with light intensity and declined with tephra depth.  Five common herb species differed in their individual response patterns.  Shrub presence declined in deep tephra and the two major species differed, with Vaccinium membranaceum less frequent in deep tephra, but V. ovalifolium, more frequent.  Tree seedlings were more frequent in deep tephra, with snow, and, for the youngest, under canopy gaps.  Bryophyte presence was greater in shallow tephra but taxa differed, with greater frequency related to snow and surface wood for Dicranum spp.; snow for Mnium and its relatives; and lack of P. menziesii in the canopy for Rhytidiopsis robusta.