Propagule sources, disturbance characteristics and the initial establishment of riparian vegetation after debris flows

Debris flows are a major cause of disturbance toriparian vegetation. Both observational and experimentalstudies were used to examine the initial recovery ofriparian vegetation after debris flows on headwater streamsin the western Cascades of Oregon. My goal was todetermine the roles of seedlings and vegetative sprouts,propagule sources, and disturbance characteristics in earlysuccession.
I sampled three debris flows at the end of the firstgrowing season after disturbance. Vegetative regrowthdominated early succession, providing 74% of the totalcover. Early successional patterns reflected the type(scour and deposit) and intensity of disturbance.Vegetative regrowth (13.7% cover, 28 shoots/m2) and totalcover (14.8%) were highest on low intensity scour.
Seedling establishment was highest on fine and graveldeposits (2.1%-3.0% cover, 38-46 seedlings/m2). Theobservational results also showed that revegetation ofdebris flows by sources other than dispersed seed isimportant. These additional sources are the residualpropagule bank and the disturbance-transported propagulebank.
I hypothesized that the differences in total cover,seedling establishment, and vegetative regrowth resultedfrom differences in how scour and deposition affectedpropagule availability. I used this hypothesis to developpredictions of the effects of increasing intensity of scourand deposition on these three vegetation attributes andconstructed experimental debris flow treatments to test thepredictions. Six treatments provided three depths ofdeposition (5, 10 and 19 cm), two depths of scour (2.5 and5 cm) and a control (no substrate modification). As in theobservational study, the experimental results were recordedat the end of the first growing season after disturbance.The potential contributions of all three propagule sourcesto revegetation were also assessed.
Total cover and vegetative regrowth decreased withincreasing intensity of both types of disturbance, butseedling establishment did not differ with disturbance typeor intensity. The predictions for total cover, vegetativeregrowth, and seedling establishment on deposits wereconsistent with the experimental results, but the
predictions for seedling establishment on scour were not.
Most individuals establishing on the experimental sitein the first growing season came from seeds and vegetativepropagules in the transported and residual propagule banks(4731 and 8538 propagules/m2 respectively). Dispersed seedcontributed 31 seeds/m2.
Fifteen species, comprising 59.2% of the overallcover, showed significant responses to the experimentaltreatments. These species responded individualistically tothe disturbances and their responses could not be explainedsolely on the basis of propagule availability. The highcover of six species (for example Madia gracilis andEpilobium paniculatum) on deposits clearly resulted fromdifferences in propagule availability between the residualand transported propagule banks. But the responses of mostother species (for example Oxalis oregana and trilliifolia,Petasites frigidus and Galium triflorum) were determined bypropagule availability, propagule removal by scour,physiological tolerances to disturbance and morphologicalcharacteristics.
This thesis demonstrates that both seedlings andvegetative sprouts are important in early debris flowsuccession, that debris flow revegetation can occur fromthree propagule sources and that one mechanism throughwhich early successional patterns develop is throughdifferences in how scour and deposition affect propaguleavailability. This thesis also demonstrates that
understanding early succession requires a consideration notonly of disturbance characteristics and propaguleavailability, but of individualistic species responses todisturbance.