Constraints to tree invasion on a nutrient-rich site during old-field succession

Surveys and experiments addressed the factors controlling tree seedling establishment during old-field succession in east central Illinois. Seed bank density and its concentration beneath woody plants matched patterns observed for seed rain in fields last cultivated in 1968 and 1984. 736 seed traps, monitored monthly for 3$1\over2$ years, collected over 54000 seeds of over 30 plant species. Bird dispersed species were more strongly clustered than wind dispersed species. Seed inputs per unit area are predictable at a coarse scale but not at a fine scale. Densities of naturally established seedlings reflected estimates of seed input, but also varied with vegetation.Seeds of several deciduous tree species were germinated in growth rooms over several temperature and moisture combinations. Total emergence and emergence rate of most species exhibited approximately bivariate Gaussian response surfaces, but with greater sensitivity to temperature than to moisture. Tree seeds were also sown into old-field vegetation patches dominated by eight different plant species. Emergence by some species was associated with low maximum temperatures, or with sparse litter cover. Subcanopy temperatures and soil moisture levels in an old-field mosaic did not vary enough to explain vegetation-induced differences in field emergence.Performance of seedlings sown into old-field patches was monitored for three growing seasons. Tree seedling gas exchange responded primarily to subcanopy irradiance, which was determined by neighborhood foliage density. High-light photosynthesis in all tree species was highly correlated with photosynthesis under ambient light. The relationship of photosynthesis to leaf conductance is altered in some competitive environments, possibly due to altered leaf morphology or reduced nitrogen availability.Height growth and biomass production generally followed trends exhibited by net photosynthesis in different patch types. Growth without competition was exponential, while growth with any competitor was additive. Optimal neighborhoods for tree seedling growth differed among tree species, from year to year, and with seedling age. Acer saccharum was the most competition-tolerant species tested; Prunus serotina was the least. Setaria faberi fields were most conducive to tree seedling establishment; Prunus canopies were most inhibitory. Tree seedling mortality was associated with herbivory and frost-heaving more than low carbon gain.U of I OnlyETDs are only available to UIUC Users without author permissio