Willow Sawfly Management Trials - Weeds Australia

22Plant growth is limited under these dense willow canopies because the low light levels mean thatmany species are not able to photosynthesise at a rate which provides sufficient carbohydrates tomaintain growth. The light level at which photosynthesis (production of carbohydrates) balancesplant respiration (use of carbohydrates) is termed the light compensation point.The light levels measured under untreated willows at a number of the planting positions and inthe direct-seeded plots in this trial were below the expected light compensation points for manyspecies. Thus the shade cast by the trees in these plots was sufficient to inhibit plant growth.Treating the willows by stem injection removed the canopy, allowing greater levels of lightthrough to the understorey (Table 5). In most plots, the amount of light reaching seedlings wouldhave been sufficient to allow growth. Defoliation by willow sawfly also removed willow canopyincreasing understorey light levels as seen in the untreated trees at Kiewa River in Table 5.The results from Kiewa River, which was the only site in the trial to be affected by willow sawflydefoliation in both seasons, illustrate the impact of this defoliation on establishment of nativeseedlings. There was no significant difference between seedling survival, seedling heights orrelative growth rates between those plots under stem-injected willows and those under untreatedwillows. If the untreated trees had not been defoliated by willow sawfly, then seedling growthwould have been expected to be significantly lower under the intact canopies of these trees.Unfortunately it was not possible to thoroughly test this hypothesis from the data collected in thistrial as seedling growth was so limited by browsing at the two remaining sites. The CampaspeRiver site would have provided an ideal control for the Kiewa River as there was no defoliationof willows at this site during the trial, so growth rates under untreated trees would be expected tobe lower than those under stem-injected trees.The development of high willow sawfly populations at Happy Valley in the second season of thetrial may also have provided an interesting insight into the direct impact of tree defoliation onseedling growth, with contrasting growth rates in the two seasons, had there been adequategrowth at this site prior to the defoliation.Key Points: It is possible to re-establish native species under willows which have been defoliated; Barriers to successful establishment of tube-stock include browsing by herbivores andweed competition; Barriers to successful establishment of seedlings using direct seeding include drought andweed competition; In the riparian areas studied, weed competition had a lower impact on seedlingestablishment and growth than either browsing or drought; Very low light levels under intact willow canopies limit understorey plant growth; Defoliation, through stem injection or willow sawfly activity, increases available light tolevels which allow understorey growth.