EFFECTS OF NURSE-PLANT CANOPY TEMPERATURES ON SHRUB SEED GEMINATION AND SEEDLING GROWTH

Conversion of upland grasslands to thorn woodland in southern Texas ha s occurred in part through initial colonization by Prosopis glandulosa followed by establishment of Celtis pallida and other shrubs beneath P. glandulosa. We tested the hypothesis that temperatures characterist ic of the microenvironment under P. glandulosa enhance seed germinatio n and seedling growth of C. pallida and inhibit germination and seedli ng growth of Acacia smallii. Soil temperatures beneath P. glandulosa i ndividuals and in adjacent herb-dominated interspaces between canopies and effects of interspace and canopy temperatures on germination and seedling growth of the 2 shrub species were determined. Greatest germi nation of C. pallida seeds occurred at temperatures (25 and 30 degrees C) within the range of mean monthly soil temperatures (25 +/- 0.3 to 31 +/- 5 degrees C, (x) over bar +/- SD) under P, glandulosa during Ma y and June, the peak period of seedling emergence in the field, Exposi ng unimbibed C. pallida seeds to 50 degrees C, similar to maximum temp eratures (49 +/- 6 degrees C) beneath canopies during the peak period of C. pallida seed dispersal by birds during July and August, increase d germination 62%. Aggregation of C. pallida beneath P. glandulosa may occur in part because birds disperse seeds beneath P, glandulosa wher e maximum summer temperatures reduce seed dormancy and mean May-June t emperatures favor germination. Absence of C. pallida in interspaces ma y result in parr from lack of dispersal by birds and inhibition of ger mination by high temperatures characteristic of interspaces.