Stand structure dynamic for forest yield regulation based on number of trees : case on a logged over area of a low and dry-land of tropical rain natural forest in Kalimantan

Abstract

Differences in logging intensity, forest encroachment, and fires, caused the variability of natural forests condition, including their horizontal and vertical stand structures. Information on the variability of stand structure and dynamic of secondary forests is essential for projecting the future stand structure, which can be used to develop a forest management plan. The study, which used 101 permanent sample plots data established on a low and dry-land natural forest in Kalimantan, showed that there was an obvious variability of the stands condition after logging in terms of the number of trees ha-1 and horizontal stand structure. The variability of stand condition, time after logging, and environmental factors could result in variability of the components of stand structure dynamics (e.g., proportion of trees upgrowth and staying). This study aimed to develop models for estimating the dynamics of stand structure of logged-over natural forests, which used 75 permanent sample plots data of lowland and dryland of natural forests in Kalimantan. The proportions of trees upgrowth, staying (ai), and recruitment (bi) were obtained from 3-years growth data. Trees were classified into several 5-cm diameter classes and two species groups: dipterocarp and non-dipterocarp. This study tried to predict ai and bi by four methods. It showed that the proportion of trees upgrowth and staying of each species group could not be predicted satisfactory by three regression methods using the number of trees, stand basal area or time after logging and elevation that were used as independent variables in multiple linear regression models. The regression models produced overestimate and unrealistic projections of stand structures. In contrast, the projection of stand structures using the dynamics of stand structure’s components that were calculated using arithmetic mean was better performance than that of the three regression models. The simulation of stand structures also confirmed that the methods for calculating trees recruitment significantly affected the projected stand structures. The simulation of stand structure dynamics by using data from three sample plots showed that the different initial condition of stand (i.e. combination of trees number and stand structure) at the same harvesting intensity produced different cutting cycle as well. It was concluded that yield regulation of secondary forest should be determined specific to stand characteristics.

Intensitas penebangan yang beragam, perambahan hutan, dan kebakaran hutan dapat menyebabkan beragamnya kondisi hutan alam, seperti jumlah pohon dan struktur tegakannya. Informasi tentang keragaman struktur tegakan dan dinamika hutan bekas tebangan sangat diperlukan dalam menduga struktur tegakan hutan pada masa yang akan datang untuk keperluan penyusunan rencana pengelolaan hutan. Kajian dengan menggunakan 101 plot ukur permanen (PUP) pada hutan alam dataran rendah tanah kering di Kalimantan menunjukan beragamnya kondisi tegakan setelah penebangan dalam hal jumlah pohon ha-1 dan struktur tegakannya. Struktur tegakan hutan alam bekas tebangan diketahui beragam dan terbagi menjadi 7 (tujuh) tipe struktur tegakan, yaitu tipe I : Nokecil-kkecil (32%), tipe II : Nokecil-ksedang (17%), tipe III : Nokecilkbesar (1%), tipe V : Nosedang-ksedang (30%), tipe VI : Nosedang-kbesar (4%), tipe VIII : Nobesar-ksedang (4%) dan tipe IX : Nobesar-kbesar (13%). Di mana: No = tetapan yang menunjukan jumlah pohon pada KD terkecil; k = tetapan yang menunjukkan laju penurunan jumlah pohon pada setiap kenaikan diameter pohon. Keragaman kondisi tegakan setelah ditebang, lamanya waktu setelah penebangan dan faktor lingkungan diduga berpengaruh terhadap keragaman komponen-komponen dinamika struktur tegakan, seperti proporsi pohon tambah tumbuh dan tetap. Tujuan penelitian ini adalah untuk mendapatkan model atau cara yang dapat digunakan untuk menduga struktur tegakan hutan alam bekas tebangan pada masa yang akan datang melalui proyeksi struktur tegakan. Penelitian dinamika struktur tegakan ini menggunakan data dari 75 PUP hutan alam tanah kering dataran rendah di Kalimantan, yang diameter setinggi dada semua pohon berdiameter 10 cm ke atas diamati dan diukur ulang secara periodik. Nilai proporsi pohon tambah tumbuh, tetap dan rekrutmen dalam penelitian ini diduga dengan menggunakan 4 metode (3 metode regresi linier berganda dan 1 metode rata-rata aritmatik) berdasarkan data contoh dalam rentang waktu 3 tahun. Kelas diameter dibuat dengan lebar kelas 5 cm dan jenis pohon dikelompokan ke dalam 2 kelompok, yaitu : kelompok jenis dipterokarpa dan nondipterokarpa.