Factors affecting establishment of Quercus liaotungensis Koidz. under mature mixed oak forest overstory and in shrubland

Abstract

From September 1999 to 2000, field acorn burying experiments were conducted to examine the effects of gaps, litter cover and burying position, and simulated defoliation and cotyledon removal on oak ( Quercus liaotungensis Koidz.) seedling establishment and biomass allocation in the understory of mixed oak forest in Dongling Mountain, Northern China. The effects of shrub and tree overstory were also studied in shrubland simultaneously. We found that acorn germination increased from 43.4 to 61.6%, that seedling recruitment increased from 33 to 47.6%, and that growth improved significantly in gaps relative to that in understory habitats in mature oak forest. Both acorn germination and seedling recruitment showed strong positive correlation with openness of acorn burying sites ( R 2=0.659, P=0.001 and R 2=0.477, P=0.034, respectively). Both acorn germination and seedling recruitment improved in order as follows for different burying treatments: placed on litter< on ground surface < buried at 5 cm depth in soil < buried at 3 cm depth in soil < covered with litter on ground surface. Furthermore, different burying treatments influenced seedling growth and biomass allocation. Seedling grew tallest with thin stems from acorns covered by litter on the ground. Burying acorns too deeply in soil caused seedlings to allocate more dry mass to stem growth, and resulted in later emergence, shoot height decreases and leaf number reduction, which may lead to high mortality due to competition for light in shaded understorys. Simulated defoliation affected seedling dry mass accumulation significantly ( R 2=−0.899, P<0.001) after considering defoliated loss during treatment, and complete defoliation caused mortality to rise to 16.7%. Similarly, cotyledon removal influenced seedling dry mass accumulation ( R 2=−0.746, P<0.001), but had more serious effects on survival, causing 50% death where cotyledon were removed completely 1 month after emergence. The growth decline may increase mortality risks in future in shaded environments. In shrubland, we found that shrub or tree overstorys significantly improved acorn germination (from 4.4 to 26.7%), seedling survival (from 11.1 to 100%) and recruitment (from 0.5 to 26.7%), but had no effects on growth in the first growing season. At open sites in shrubland, seedling death resulted from direct or indirect effects of high light incidence. Overall, less acorn germination, seedling survival and recruitment occurred in shrubland than those beneath the mature forest canopy. In conclusion, in mature forest of Q. liaotungensis, we suggest that selected-cutting or canopy disturbance to permit light access to the understory will help to improve poor natural regeneration. With additional cage protection from browsers on acorns and herbivory free methods, burying acorns at 3 cm depth in soil or placed on ground with litter cover were the most effective methods to produce large number of seedlings. With facilitation of shrub or tree overstorys at early seedling stages in shrubland, direct burying acorns at 3 cm depth in soil was the most effective and economical technique for oak reforestation in such open early successional habitats.