Nitrogen fixation by Scotch broom and red alder planted under precommercially thinned Douglas-fir

Red alder (Alnus rubra Bong.), scotch broom (Cytisus scoparius L.) and snowbrush (Ceanothus velutinus Dougl.) were planted underprecommercially thinned Douglas—fir (Pseudotsuga menziesii Mirb.Franco) in the central Oregon Cascades. Plug grown red alder hadsignificantly greater survival than all wildling propagules except forthe second broom planting. Browsing was strongly positivelycorrelated with mortality for snowbrush. Survival was somewhatcorrelated with Douglas—fir basal area (BA) and cover but not withavailable light.
Nitrogen fixation in underplanted alder and broom and naturallyoccurring alder, broom and snowbrush was measured by acetylenereduction. Plant moisture stress (PMS) appeared to exert a strongercontrol on nitrogen fixation through the season than soil temperature.'Zitrogen fixation did not appear to be related to available light or7-ouglas—fir BA or cover. On single dates neither PMS nor soiltemperature appeared to be related to nitrogen fixation.
Averaged over the season, broom had significantly greater
nitrogen fixation per unit nodule weight than alder but on a per plantbasis the species were nearly equal. For underplanted alder, leafarea may have a higher immediate priority for photosynthate thannitrogen fixation as indicated by negative correlations betweenpercent foliar nitrogen and leaf/top and leaf/nodule dry weightratios. Nitrogen fixation ability keeps apace with leaf developmentas seen by the positive correlations between leaf and stem totalnitrogen values with nodule weights; leaf and nodule dry weights; andleaf weights and nitrogen fixation. For all species, total foliarnitrogen (leaf dry weight x percent nitrogen) was better correlated to^itrogen fixation than percent foliar nitrogen. A greater leaf/noduleratio for underplanted alder compared to naturally occurring may haveresulted from lowered light levels in the thinned stands. In lateFebruary, all broom had strong nitrogenase activity compared to spora-lic weak activity for alder and no activity for snowbrush. This maysuggest that fixation on broom is much less controlled by dormancy.7ed alder plugs offer the best potential for nitrogen fixation because3f their higher survival, potentially greater growth and fixation,ease of planting, and lesser weed potential.
Given seasonal estimates of nodule fixation rates and the propor-tion of alder mass as active nodules, nitrogen fixation for alder canbe estimated over time from its expected growth under the Douglas—fir."."7e termination of alder growth is likely set by crown closure of the7,11121as—fir. This in turn depends on the number of Douglas—fir left
after precommercial thinning and the radial growth rate of theircrowns.
A probability tree can organize and display the uncertainties offuture growth and nitrogen fixation from underplanted alder. Itdisplays possible paths of growth that individual alder may follow andallows calculation of an expected or mean value for cumulative nitro-gen fixation. For an example 15 year period, the expected cumulativefixation for an individual underplanted alder is about 0.668 kg. Forcomparison, a fully stocked alder stand with this rate of accretionaverages 121 kg halyr1 of nitrogen. The initial cost of 224 kg Nha-1 from underplanted alder accrued over a 15 year period is aboutdouble the current cost of applying an equivalent amount of urea. Thelonger recovery period for the underplanting costs further magnify thedifference