Effects of Pre-sowing Seed Treatments and Mycorrhizae on Germination and Seedling Growth of Native Tree Species for Forest Restoration  

ABSTRACT: Forests in Thailand have declined over the past 30 years, due to illegal logging and agricultural expansion. Deforestation causes depletion of soil fertility, soil erosion, flooding in the rainy season and streams drying up in the dry season. Forest restoration, by planting native tree species, can help protect biodiversity, but many native tree species have long periods of seed dormancy or low germination rates. Knowledge about how to propagate them from seeds is often lacking. For successful forest restoration, vigorous seedlings are needed. Suitable seed-germination methods must be developed, by testing various pre-sowing seed treatments to optimize germination. To produce high quality seedlings for forest restoration, seedling roots may be inoculated with mycorrhizae, to accelerate seedling growth in the nursery, before planting out in deforested sites.

This research was conducted at the Forest Restoration Research Unit (FORRU) at about 1,000 meters elevation. Seeds were collected from 6 native tree species: Careya arborea Roxb. (Lecythidaceae), Ficus auriculata Lour. (Moraceae), Holigarna kurzii King (Anarcardiaceae), Michelia baillonii Pierre (Magnoliaceae), Xantolis burmanica (Coll. & Hemsl.) P. Royen (Sapotaceae), and Quercus vestita Rehd. & Wils. (Fagaceae). Five pre-sowing treatments were applied to the seeds with three replications 1. control, 2. soaking in water for 24 hours, 3. heating in water at 60-70° C for 20 minutes, 4. scarification by hand (cutting the seed coats to make small holes about 1- 2mm wide for each species) and 5. scarification with H2SO4 for about 3-10 minutes. After the seeds germinated and seedlings had developed 2 pairs of leaves, and were vigorous, seedlings were transferred into plastic bags (23 x 6 cm), filled with a mixture of forest soil, coconut husk, and peanut husks(2:1:1). Seedlings were divided into three groups: one received 3 ml of TRITON (mycorrhizal innoculum) per bag, one 6 ml of TRITON per bag and the control group received no TRITON. TRITON is a commercially produced mixture of the fungal spores of Glomus etunicatum, G. intradices and G. fasciculatum, adsorbed on to clay particles. Seedlings height, stem diameter and mortality were measured at interval. Finally shoots and roots were separated and the shoot:root dry weight values were calculated.

For Careya arborea, the best treatment was water soaking for 24 hours, which raised the germination percent from 55.1% to 79.6%. Almost all seeds were killed when treated with H2SO4. For Ficus auriculata, heating in water at 60-70° C was the best treatment - germination (42.1%). For Holigarna kurzii and Michelia baillonii, water soaking for 24 hours increased germination from 22.7% and 2.8% to 54.2% and 9.3%, respectively, but seed germination percentage of Michelia baillonii remained unacceptably low. For Xantolis burmanica, the control had the highest percentage seed germination, but it was still unacceptably low, at about 12.9%. Seedlings of three species (Careya arborea, Ficus auriculata and Holigarna kurzii) were unaffected by TRITON. However, for Xantolis burmanica, the 6-ml TRITON treatment produced better results than did 3 ml of TRITON and the control treatment. Observations at the Laboratory of Applied Microbiology Research Unit, Chiang Mai University found fungi of Glomus sp. in the roots of Xantolis burmanica seedlings, but no infection of Careya arborea, Ficus auriculata and Holigarna kurzii seedlings was observed. Xantolis burmanica is recommended for TRITON treatment, to increase the growth rate of seedlings and improve their vigour in the nursery.