In early 70’s New Territories was a countryside and most of the population were living in villages; farming was their primary work so that agriculture was vigorous in the period and many farms were cultivated around the foothill. Since the poor transportation with less urbanization, it has a lower population than both urban places “Kowloon” and “Hong Kong Island”. When the transportation network was not yet developed completely to connect with those urban cities, some of the hillsides or slopes were away from the human activities and remained for future development. The selected location in this project (Tai Wai area) was one of the places where away from high population density and without human disturbance.
The research project aim:
To evaluate the Old and Valuable Tree (OVT) from past, current and future;
To discuss soil science of OVT;
To discuss the plant physiology of OVT.
Tree information and Location:
The Botanical name of the tree is Ficus microcarpa with 1.3m DBH and 14m of crown spread. The tree is registered as Old and Valuable Tree (OVT) in 08 September 2004 and under managed by Highways Department. The tree is located at Chik Chuen Street in Sha tin. The slope number is known as 7SW-D/FR406.
(Photo 1.1 Source: Google map)
The past tree evaluation:
In that early period, the target tree was still a seedling and grew in an open slope area in Tai Wai, where provided a fertile soil, enough spaces and sufficient sunlight to support the seedling grows.
Function groups of soil biota:
Less human activities would keep the fertility of the soil since all soil biota groups can function and change the formation of the soil, such as the wild animal group can provide more organic matters to enrich the soil. For instance, earthworm feed on plant debris (dead roots, leaves, grasses, manure) and soil. Their digestive system concentrates the organic and mineral constituents in the food they eat, so their casts are richer in available nutrients than the soil around them. Another soil biota groups, such as bacteria and fungi, helped to change the formation and the structure of the soil. It provided more rooms in the soil structure to reserve more water and air available for the tree root growth.
Soil structure & pH value:
According to the lecture 1, it has mentioned the ideal soil structure is composed with appropriate soil component with particles and pores and the ratio of the ideal soil is 50% of solid component plus 50% of space (25% for water/ 25% for air). As our on-site investigation, the soil structure of the slope is mostly composed with sandy clay which provided enough pores space to allow the water and air penetrated from the top soil layer to the bottom soil layer. Once the bottom soil is deep soaking, this would encourage trees’ roots to grow deeper to seek more water. Besides the structure of soil, pH value also influences nutrient uptake and tree growth. Soils with a pH of 6.0-7.0 typically have high concentrations of available nutrients (Williston and LaFayette 1978). Extremes in soil pH (8.5) can make some nutrients toxic and others unavailable to plants. At low pH levels (7.5), calcium and potassium are overabundant. So we could assume the soil structure and pH value (between 6.0-6.5 pH) which had delivered a sufficient of nutrients for seedling needs. That can explain the OVT can glow large and dominated on the slope area in the past. However, the site condition has changed, since the current slope modification work disturbed the tree grow by limited the soil volume and space.
In current location, we could find a driveway, sidewalk, fence and subway were built around the tree. Those facilities are not only limited the tree root expansion and also reduced the availability of current soil volume for OVT. According to the government information, the crown spread of the OVT is 14m and the DBH is 1.3 and is classified as a larger tree. We use the crown projection to calculate the ideal soil volume for the OVT. Through the calculation of the soil volume, we can more understand how much spaces and soils have been reduced by human activities.
The calculation is shown as follow:
Soil Volume (m3) = 3.14 x 72 (m2) x 0.6(m) (we assume tree roots utilized the top 0.6m of soil).
Soil Volume (m3) =93m3 = 3249 cu ft.
The result showed the size of soil volume which requires for OVT is much more than the current location has. Since the slope area is limited and we assume the size of the slope only has 1500-2000cu ft. according to our site observation. When the soil volume and space are not enough to support the tree size, this affected the root development and damage the root, since tree roots cannot freely to expend and blocked the channel to gain more water and nutrients from those above barriers such as, the concrete material of the driveway and the erosion mat on the slope surface.
Compaction of soil space & Shallow root:
Not enough space and soil compaction in the current slope, plus some of the tree branches are grow leaning toward the driveway because of the tropism (we will discuss in later paragraph), it makes the tree root grow upward to the land surface (shallow root occur) and also increase the tension roots to respond the tension from the extra weight of the leaning branches. (Photo 1.2 & 1.3) Shallow rooting occurred when roots survive in a steadily diminishing aerobic layer, and as the anaerobic layer expands toward the surface, the physical space available for living roots declines. The consequences of having smaller volumes of colonizable space at the surface of the soil mean roots and their resources are subject to much greater fluctuation in water, heat loading, and mechanical damage. The soil compaction associated physiological dysfunctions cause systemic damage and decline, as well as failures in dealing with additional environmental changes. Physical / mechanical constraints negatively modify responses in the tree resulting in inefficient use of essential resources. Growth regulators are destroyed prematurely or allowed to build up, causing wild changes in tissue reactions. Carbon allocation patterns, following highly modified growth regulation patterns, change food production, storage, use, and transport processes.
(Photo 1.2 &1.3 Source: Denni Chan
The current tree evaluation & Future prediction:
In 90’s the New Territories Development Project started and Tsing Sha Highway Project began in 2003. New Territories is being urbanized, it moved a huge population from Kowloon and Hong Kong Island to Sha Tin and Tai Wai areas (OVT’s area), and many of hillside and slopes had been modified or damaged by the construction works. With the development of the new community and resolve the housing need, many buildings, utilities and infrastructure have been built to satisfy the population need. Those activities would bring negative impact and change the planting environment to the local tree. During the site visit, we found that there is the gas station was built near the tree, the bridge of Tsing Sha highway has built on top of the tree, the modification work of the slope has disturbed tree grow and many solid wastes have surrounded the tree. These all factors are affecting the future tree’s condition and become worse.
When the traffic becomes convenience, more vehicles were used in daily life, so many of gas stations were built to fulfil the drivers need. Just like our tree location, the gas station is next to the slope it brings up pollution matters and affects the tree. The emitted gasoline and the exhaust from cars is one of the causes which polluted the air and land and affected the tree condition.
According to the article, it stated ‘the effects of air pollution on plants develop over time and can’t be undone. Some plants are more susceptible to pollution damage than others according to Fred Davis, a chemist from Kent State University. Chemicals such as sulfur dioxide, ozone, fluorides and peroxyacetyl nitrate damage the leaves of plants. If enough leaves are damaged, the entire plant will die. Sulfur dioxide, a by-product of burning fossil fuels such as oil, coal and gasoline, causes changes in the colours of leaf tissue, which may turn white, brown or yellow.’
Dr Kent reports that nitrogen dioxide, a byproduct of combustion from car engines or open fires, can slow the growth of plants. Moreover, lead pollutant from car exhaust would stay into the air and stored accumulate in the soil. Plus the soil is always disturbed by human activities and mixed with other materials such as solid wastes. Those litters are mixed in the soil profile; it affects all soil properties and creates barriers for the penetration of tree root. The poor soil condition is affecting the current tree’s condition and it makes tree becomes unhealthy in foreseeable future. Once the soil cannot provide enough water and nutrients to support the tree needs and it will expand the branches or increase the water sprout to gain light energy to produce food for survival.
In 2003, Tsing Sha highway project has begun, the construction went through from Tsing Yi to Sha Tin and affected around areas. Tai Wai Tunnel has been built on top of the selected slope where the sunlight cannot directly deliver to OVT. As mentioned above the current ground soil is not enough to maintain the tree needs and it expands the branches or increases the water sprout to gain light energy to form food. Therefore, sunlight becomes an important element for the current tree to form sugars through the function of photosynthesis.
When the plant’s cell react to the sunlight and it converts the light energy into chemical energy and store in carbohydrate molecules, plus the components of carbon dioxide and water it turns in to sugars to fuel up the plant and release oxygen as the by-product. (Co2+H2O–>Ch2o+O2). Photosynthesis occurs in two stages. In the first stage, light-dependent reactions capture light energy and use it to make energy storage molecules ATP and NADPH. In the second stage, light-independent reactions using these products to capture and reduce carbon dioxide. In photosynthesis, light-dependent reactions occur in the thylakoid membrane. The inside of the thylakoid membrane is Lumen, and outside of the membrane is called stroma. There are four major protein complexes in the thylakoid membrane are photosystem II, cytochromes, photosystem I and ATP synthase. After the processing by these complexes which produce ATP and NADPH. When ATP and NADPH formed during light-dependent reactions are used in stroma to fuel the Calvin cycle. The Calvin cycle consists of four phases: carbon fixation, reduction, ribulose 1,5-bisphosphate and regeneration of RuBP, in order to produce one molecule of glucose and form the sucrose and starch for plants use. This process occurs only when the light is available; no Calvin cycle will be processed at the nighttime or in dark.
Since the sunlight has been reduced by the barrier of the bridge, the tree branches are leaning toward the driveway because of the tropism and gain light energy from the sun and get away from unfavourable environmental conditions. Tropism is the growth response of plant that results in curving of whole plant organs toward or away from external stimuli due to differential rates of cell elongation.
Phototropic responses involve the bending of growing stem and other plant parts toward the source of light. Auxin is a plant hormone, which reacts when phototropism occurs, this causes the plant to have elongated cells on the farthest side of the light. Phototropism influences the plant movement and responds to external stimuli. There are two types of phototropism: positive and negative phototropism. The positive phototropism is mean the plant will grow towards the light source and the negative phototropism is growth away from light. Most of the plant shoots are positive phototropism, it stimulates growth by obtaining photosynthetic energy. Roots are usually negative phototropism and gravitropism will also affect the root development.
Unfortunately, the soil area in the location is limited and compressed by human activities, no more space in soil that reduced the nutrient (N+ P) and living elements (water and air) for the tree. In the end, the tree will be destructed or withered by the over length of branches and overgrowth of water sprouts.
To resolve the problem which stated above, some management works can be applied to improve the site condition and save a tree.
· By using mulch to retain the moisture for the soil and it also provided additional carbon source or organic matters. If using suppressing weeds for mulch it is recommended to have 3 to 5 cm depths and away the root flare, because too much mulch will put extra weight to damage the root and compress the soil.
· Remove the erosion mat from slope, if the slope condition is not allowed to take out the mat, then the routine maintenance work should be arranged to cut the mat in the routine period to allow unconstrained growth of the tree.
· Use fertilizer to provide Nitrogen (N) and other macro-elements such as phosphors (P), potassium (K) and sulfur (S), etc. The water management like water sprinkler is also recommended to install on-site to supply enough water for the tree roots. But the most useful method removes the driveway and turn it to soil area for the tree future grow. (Photo 1.4)
(Photo 1.4 Source: Denni Chan)
In that early period, when less disturbance from human, the selected location has provided a fertile soil, enough spaces and sufficient sunlight to support the seedling (OVT) grows. However, with the New Territories development, a huge population has moved to Sha Tin and Tai Wai area (OVT Location), many of hillside and slopes had been modified or damaged by the construction works. Many buildings, utilities and infrastructure have been built to satisfy the population need. Those activities would bring negative impact to the planting environment of OVT. Such as, Pollution from the gas station, the sunlight blockage by the bridge, the lack of soil space and volume on slope and solid wastes are the negative factors to the current tree. In the future, the tree will be destructed or withered by the over length of branches and overgrowth of water sprouts.
To resolve the problem which stated above, management works are necessary to improve the site condition and save the tree.