3.2.1 Plot grids and tree selection
In order to obtain a representative sample of the tree individuals occurring on each plot a random sampling method was designed. Based on the estimated trunk density in the different land use types, different plot sizes were defined for reaching a similar sample size on all plots. Thus, the plot size in the natural forest, the forest garden and agroforestry system was 2500 m2, compared to 400 m2 in the secondary forest, which has a much higher stem density.
Plot grids with an x-axis in north-south direction and an east-west oriented y-axis were established using compass, ropes and wooden sticks, marking every 5 m. This resulted in a grid with 121 nodes on 2500 m2. In each plot grid 20 spots were randomly selected. The closest mature or pre-mature tree with fully sunlit leaves to each spot was recorded and sampled (see below). The trunk circumference was measured and height was estimated. The
3 MATERIAL AND METHODS
phenological status of the individual (presence of fruits and flowers) and the name, or other identification, were recorded. Subsequently, the tree was marked with an individual number on a plastic label for future needs. Herbarium specimens of unknown species were collected in parallel.
Table 3.1 List of the plots included in the study, covering four different land use types on altitudes between 700 and 1100 m asl. The study sites were situated in the Palolo and Lore Utara sub-districts in the Indonesian province of Central Sulawesi. (Abbr. – Abbreviation, NF - Natural forest, FG – Forest garden, SF – Secondary forest, AF – Agroforestry system)
Study area Abbr. Land use type Sub-plot Plot size Altitude Sub district [ha] [m a s l ]
Napu 1 NF1 NF 0.25 1100 Lore Utara
Napu 2 NF2 NF 0.25 1100 Lore Utara
Gunung Potong NF3 NF 1 0.25 900 Palolo
2 0.25
3 0.25
4 0.25
Kamarora 1 FG1 FG 0.25 700 Palolo
Kamarora 2 FG2 FG 0.25 700 Palolo
Alitupu 1 A1 SF 1 0.04 1100 Lore Utara
2 0.04
Alitupu 2 A2 SF 1 0.04 1100 Lore Utara
2 0.04
Kaduaa K SF 1 0.04 1100 Lore Utara
2 0.04
Watumaeta W SF 1 0.04 1100 Lore Utara
2 0.04
Dodolo 1 AFD1 AF 0.25 1100 Lore Utara
Dodolo 2 AFD2 AF 0.25 1100 Lore Utara
Kaduaa AFK AF 0.25 1100 Lore Utara
Watumaeta AFW AF 0.25 1100 Lore Utara
In this manner three natural forest plots (Gunung potong, Napu I and II), four secondary forest plots (Watumaeta, Kaduaa, Alitupu I and II), two forest garden plots (Kamarora I and II) and four agroforestry plots (Dodolo I and II, Kaduaa and Watumaeta) were established and sampled. The plots are listed in Table 3.1. They belonged to the jointly selected focal study sites within the STORMA project and were studied by several of the project’s natural science researchers.
3.2.2 Leaf collection
For obtaining data on several leaf traits, e.g. nutrient concentration, carbon isotope ratio (δ13C), nitrogen isotope ratio (δ15N), leaf size, specific leaf area (SLA) and other
morphological characteristics, exclusively fully sunlit mature, vigorous leaves were collected from each of the randomly selected trees. In the secondary forest and the
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agroforestry systems leaves could be accessed with a branch cutter on an extendable stick at heights of 3 – 6 m. For sampling the natural forest trees at 15 – 25 m, local tree climbers and the staff of the Herbarium Celebense in Palu were employed, using climbing equipment with ropes and iron hooks or manual climbing techniques. Due to extreme tree heights and trunks with too many lianas and other epiphytes, which made it impossible to reach the sunlit parts of the crown, a few randomly selected trees could not be sampled. These trees were as far as possible replaced by other randomly sampled individuals, in some cases on neighbouring sub-plots, resulting in slightly different numbers of sampled individuals per sub-plot. In total, sun leaves of 354 trees were randomly sampled. Depending on leaf size, 4 - 30 leaves per tree were collected with their petiole and put in paper bags.
Random sampling of the natural forest and forest garden plots was conducted between February and October 2001. Sampling of secondary forest and agroforestry plots was carried out between March and July the same year.
3.2.3 Definition of random sample and species average
Most comparative studies on tree ecology are focused on certain tree species or families that are common, or have been selected more or less arbitrarily. This allows detection of differences between the taxa, but often cannot give information on the “average tree” in a stand. This study is an attempt to apply a random sampling strategy in very species rich forests. This was achieved by sampling the stems randomly, as described above, in order to get a representative sample of the trees in the stand, irrespective of frequency and species identity. By this means, forest stands as assemblies of trees may be compared with respect to functional traits, instead of comparing species means. To assure comparability of the leaf traits studied, only upper canopy branches were sampled.
Table 3.2 Specification of random sample and species average parameters as referred to throughout this work.
(NF - Natural forest, SF - Secondary forest, AF – Agroforestry system, FG – Forest garden)
Random sample Species average
NF SF AF FG Total NF SF AF FG No. individuals
sampled 119 160 49 26 354
No. species
sampled 69 35 4 18 107 69 35 4 18
Canopy strata
sampled Upper
canopy Upper
canopy Upper
canopy Upper
canopy Upper
canopy Upper
canopy Upper
canopy Upper
canopy Upper canopy
No. of plots 3 4 4 2 13 3 4 4 2
Total plot size
[m2] 15000 3200 10000 5000 33200 15000 3200 10000 5000
3 MATERIAL AND METHODS
Based on the species composition of the random sample, a species average was
additionally calculated for some leaf traits. This is the mean of the 4 to 69 species present in each land use type, with each species being represented by a mean of 1 to 10 individuals.
The sampling parameters for the two sample designs as referred to throughout this work are defined in Table 3.2. Means at the land use level always include all plots studied, and if nothing else is mentioned, always base on the random sample. The only exception to this definition occurs in the sampling design chapter, where only two of the three natural forest plots were included (see chapter 4.2).
3.2.4 Species identification
The collected samples were either immediately identified, or brought to the National Herbarium of the Netherlands for identification. Herbarium Celebense in Palu also contributed with valuable identification work and the preparation of herbaria specimen.
The scientific plant names throughout this work consequently follow the Checklist of woody plants of Sulawesi, Indonesia (Keßler et al. 2002).
3.2.5 Additional sampling for analysis of intraspecific variation
Only a minority of the tree species occurred with a sufficient number of individuals in the random sample to study intraspecific variation in leaf traits. Leaves from additional tree individuals of eight abundant natural forest species, eight secondary forest species and three agroforestry species were sampled to attain a replication number of 10 tree individuals per species for assessment of intraspecific variations of morphological and chemical leaf traits.
3.3 Seasonal variation of foliar nutrient concentrations and leaf δ13C