Influence of age on growth variables of dryobalanops beccarii dyer in a rehabilitated forest

Dryobalanops beccarii is one of the vulnerable indigenous tree species in Malaysia. In order to conserve and protect this species, growth response over their lifespan needed to be studied. However information on D. beccarii growth rate at different age is limited. Hence, the ontogenetic development study of D. beccarii was conducted in Universiti Putra Malaysia (UPM) – Mitsubishi Forest Restoration area at UPM Bintulu Sarawak Campus. The aims of the study were to observe the growth performance of D. beccarii and to understand the changes in physiological and morphological of leaves as well as its stem with different tree ontogenetic development stages. Stands of 1, 9, 14 and 20 year-old were selected for this study as where at least 4 stems of D. beccarii could be found in each of the stand. The size of the established stand was 30 × 10 m. The parameters observed were tree morphology, leaves morphology and physiology, and selected soil properties. The tree morphology study involved heights (total and merchantable), stem diameter, canopy size (depth and diameter), and Leaf Area Index (LAI) of D. beccarii. Total height growth rate increased rapidly during the early stages and decreased in 20- year-old stand due to competition for light and space from the surrounding trees. The change in total height was accompanied by rapid merchantable height growth rate and apparent bulking up in stem diameter in 9-year-old stand. Upon reaching certain height, the young tree focused on the stem growth in order to support the massive tree height. The high tree density in 14-year-old stand contributed to high LAI value. The expansion of canopy size was rapid in 20-year-old stand in order to fill the empty gaps between trees. The tree height increased sigmoidally with time as the growth was rapid from year 1 to year 9 and slowing down after reaching 14-year-old. Meanwhile, stem diameter, canopy size, and LAI still showed continuous growth up to 20 years. The leaf area, width, length, thickness, and stomata density were determined in the leaf morphology study. The leaf morphology was significantly correlated with the tree morphology parameters, except for leaf width and canopy diameter. Compared to other leaf morphology parameters, the leaf width and stomata density was found to decline significantly (P<0.05) during yearly assessment in each stand age due to the response to the drought season. The 20-year-old stand recorded highest leaf area, thickness, width, and length. Due to leaf adaptation to overlapping canopy layer, the leaf size was larger in mature tree than juvenile tree. The photosynthetic rate, transpiration rate, stomatal conductance, and chlorophyll content were included in the leaf physiology study. Similarity was found between the photosynthetic rates, transpiration rate, and stomatal conductance. These parameters rate was low in the freshly emerged saplings, increased rapidly until D. beccarii age between 9-14 years, and decreased upon reaching the mature phase. The young tree is not efficient in physiological process as the mature tree that already went through series of environment adaptation in many years. The chlorophyll content decreased as the leaf nitrogen (N) decreased. The chlorophyll content recorded positive correlation (P<0.05) with the tree morphology parameters, except canopy size. The rate of photosynthesis, transpiration rate, and stomatal conductance increased from 1-year-old and began to decreased once reached age of 14, while chlorophyll content continuously decrease from 1-year-old to 20-year-old. The leaf N, phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca) were included in the leaf nutrient analysis. The leaf N was decline significantly (P<0.05) in 1-year-old stand while leaf Ca was increase significantly (P<0.05) in the 20 years old stand during the yearly assessment. Only leaf P showed significant correlation (P<0.01) with LAI. The leaf N, P, and Mg were decreased with stand age, while leaf K and Ca presented the opposite trend. The soil total N, available P, exchangeable K, Mg, and Ca as well as pH, moisture and bulk density were analyzed in the soil property studies. No obvious changes in soil pH, moisture, bulk density and exchangeable Mg between stands. Significant reduction (P<0.05) in soil total N, available P and exchangeable K were observed in the older stands. The soil total N was negatively correlated with the tree morphology while soil available P and exchangeable Ca recorded the opposite result. The soil total N, available P, and exchangeable K were reduced as these nutrients were greatly consumed for the tree growth compared to exchangeable Mg and Ca. The current study indicated that forest rehabilitation using D. beccarii as one of the tree species can restore a good forest within 20 years.

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