Studi Mutu Kayu Jati di Hutan Rakyat Gunungkidul II. Pengukuran Tegangan Pertumbuhan

Pohon dengan tegangan pertumbuhan yang tinggi dikhawatirkan akan mudah mengalami cacat seperti pecah, retak, dan pelengkungan. Penelitian ini bertujuan untuk mengetahui variasi nilai tegangan pertumbuhan pohon jati yang tumbuh di tiga tempat hutan rakyat Gunungkidul. Nilai pelepasan regangan juga dibahas dari hubungannya dengan kecepatan tumbuh dan berat jenis. Pelepasan regangan pada arah longitudinal dan tangensial ditentukan melalui metode strain-gauge. Nilai pelepasan regangan di permukaan batang berkisar antara -130,5 sampai -999,5 µå sedangkan nilai pelepasan regangan tangensial antara -103 to 1411,5 µå. Beberapa nilai pelepasan regangan longitudinal yang cukup tinggi mengindikasikan adanya kayu tarik. Selanjutnya, variasi dalam pohon untuk tegangan pertumbuhan menunjukkan tidak ada kecenderungan tertentu. Perbedaan nyata diamati dimana sampel dari Nglipar memberikan nilai paling tinggi (-628,25 + -223,73 µå). Meskipun demikian, tidak ada hubungan nyata yang diukur antara nilai pelepasan regangan dihubungkan dengan laju pertumbuhan dan berat jenis. Penyebaran nilai tegangan sisa internal dalam arah radial bervariasi diantara pohon satu dengan lainnya dimana beberapa sampel menunjukkan adanya perbedaan nilai pelepasan regangan yang drastis. Untuk itu, perlu dilakukan usaha untuk mengurangi perbedaan yang mencolok di nilai pelepasan regangan dari pusat ke permukaan batang untuk mencegah cacat yang berkaitan dengan tegangan pertumbuhan.Katakunci: Tectona grandis, tegangan pertumbuhan, pelepasan regangan, sifat kayu, GunungkidulA study of teak wood quality from community forests in Gunungkidul II. Growth-stress measurementAbstractTrees containing large growth stresses, leads to significant losses due to split, checked and also warped. The variation of growth-stress in teak trees grown in the three community forest sites of Gunungkidul regency was observed. The released strain levels were also discussed in relation to the growth-rate and specific gravity. The strains released in the longitudinal and tangential directions were measured by the strain-gauge method. The values of longitudinal released strain at the periphery of the stem were ranged from -130.5 to -999.5 µå whereas tangential released strain were from -103 to 1411.5 µå. Some high values of longitudinal released strain indicated the presence of tension wood. Further, intra-tree variation of growth stress showed no particular tendencies among the samples. There were significant differences in the longitudinal strain as samples from Nglipar site showed the highest amounts (-628.25 + -223.73 µå). However, no significant correlation was found between the values of released strains with the growth-rate and specific gravity. The radial distributions of internal residual-stress were varied among the individuals which some trees exhibited steeper released strain gradients. Thus, it is important to reduce the gradient from pith to periphery of released-strain patterns to prevent the defect related to the growth stresses.