Environmental virtues of rubber

Dr N Yogaratnam

“Climate change has become an issue of serious concern on the supply potential of natural rubber. Apart from fall in yield, even the rubber growing regions in major rubber producing countries are gradually rendered unsuitable for growing rubber,” says a recent ANRPC report.

Of the seven countries that account for 93 percent of the global rubber production, five have recorded sharp falls. Production fell by 18 percent in Thailand, 25 percent in Malaysia, 6 percent in Indonesia, percent in Vietnam and 9.4 percent in India in 2009. Only China and Sri Lanka were able to stave off the falling trends.

Sri Lanka recorded a production of 117, 129 and 133 (‘000 tonnes) in years 2007, 2008 and 2009 (+6.09 percent) respectively and a further increase of 10.6 percent between January - September, 2010 in comparison with the same period in 2009.

Drought

Drought being the most important manifestation of climate change, the strategies should be aimed at improving tolerance to drought which is a useful trait not only in the hot and dry non-traditional regions, but also in the traditional regions.

In field surveys in India, it has been observed that casualty immediately after planting was higher in years which experienced unexpected dry spells and bright sunny days with warm temperature during the monsoon season, even though soil moisture was sufficient.

These changes in weather pattern may or may not be related to global climate change.

Sri Lanka’s weather pattern

Vagaries in local weather rather than global climate patterns are more relevant in determining the impact of weather events on crop production. One of the widely predicted and observed effects of climate change, which has direct impact on rubber productivity, is the changes in precipitation regimes with more precipitation deficit during summer season.

Rising temperature and uncertainties in weather such as unexpected dry spell during rainy season with hot and bright sunshine hours are some of the other manifestation of climate change.

Rainfall

The ideal annual rainfall for rubber is known to fall within the range of 1650 - 3000mm and be reasonably distributed throughout the year. It was reported that, in general, tree performance is severely affected if rainfall over a six-month period is less than 500 mm, especially when it is not uniformly distributed.

In some areas, long-term annual rainfall values show significant declining trends. Annual rainfall values at Agalawatta, Badulla, Kalutara, Ratnapura, Kurunegala and Hanwella had no significant declining trends.

However, Peradeniya, Aranayake, Matara, Nalanda, Ambanpitiya, Galle, Avissawella, Matale and Okkampitiya indicated significant negative trends.

At Agalawatta, it was not possible to identify the end of rains in the 1st rain spell in most of the years, but there was no indication of any significant shift between years 1941 and 2000.

However, at Avissawella after 1994, a late start and an early end were observed. Ratnapura had no considerable variation throughout the period reported. Marked shift was not identified in any of the locations.

In the Ambanpitiya area, the start of the first rain spell was found to be fairly late during the latter part of 1990. Moreover, the length of the first spell seemed to be narrowing towards the year 2000. A similar pattern was observed for the 2nd spell also.

An understanding on the probability of receiving a wet week (greater=10mm) is very useful in planning cultural operations. It was observed that in all the areas the probability lines indicating the change with standard weeks for the period; 1971-2000 lies below the 1941-1970 lines for most of the occasions.

This indicates that there was a decrease in probability of receiving a wet week during the period 1971-2000 when compared to the period; 1941-1970.

In other words this indicates an increase in risk of receiving dry spells in future

No significant departures were observed in wet zone areas in occurrence of extreme rainfall events. At Okkampitiya, the reduction in number of days in the category; 50-100 mm/day was compensated by rains in the category; 100-150 mm/day. In Kurunegala, no events were observed in the category; 150-200 mm/day after 1981. The occurrence of rains in the categories; 150-200 and 200-250 mm/day for Ratnapura was more after 1968. Badulla had five events under the category; 100-150 mm, in year 1986

In all the areas for most of the months, the variability in maximum dry run has increased during 1971 to 2000 when compared to the period 1941 to 1970. The average length of the maximum dry run has also increased in most locations.

Environmental factor

In the wet traditional rubber-growing areas of the Southwest region of Sri Lanka, moisture deficits are relatively absent while in the dry marginal areas, moisture deficits are severe and prolongs for a period of four to five months.

Recovery of rubber plants at the time of planting indicates that the establishment success under dry weather conditions is nearly 30 percent lower and the girth at six months after planting is 35 percent lower than in the wet region.

Similarly, data on girthing pattern of rubber trees in the two regions indicated that the harvesting age in the dry regions is more than two years longer than in the wet region.

There are significant differences between different soil moisture regimes on plant diameter, plant height and leaf area at the end of 12 months after planting, indicating a decline in all growth parameters with the increase of soil moisture stress.

Root length, root spread and root dry weight data obtained also showed significant differences between different soil moisture levels on root length, root spread and root dry weight.

Organic manure

The recovery percentage observed after field planting of young budding plants was 98 percent with application of organic matter (OM) compared to 85 percent recovery without OM. Similarly, when OM is applied to the planting hole when planting, the recovery was 98 percent compared to 80 percent without OM application. Girth after 12 months of planting was also higher in both instances with the application of OM.

Nutrients

The role of Potassium (K) on plant water status of rubber indicates that under soil moisture stress conditions, Transpiration Rate (TR) decreased in rubber plants supplied with sufficient K.

However, in the absence of K, TR is greater under dry conditions, which results in more water loss from the tree. It is known that rubber trees grown under sufficient K, reduce TR more readily than K deficient trees.

Some studies suggested that water stress in plants low in K status develops due to sluggish opening and closing of stomata. Under low K conditions, stomata have low capacity to respond to rapid changes in physiological conditions of the tree.

Effect of K on Relative Water Content (RWC) in different situations also revealed better water status of leaf tissues even under dry situations when a higher dose of K is applied.

It was also noted that the plant diameter at adequate soil water status with recommended level of K is almost similar to the diameter of rubber plants under soil moisture stress condition when applied with double the recommended level of K.

Moreover, this is further confirmed by the higher girth and tappability recorded with the increase in the level of K, under field condition, in dry areas.

It has also been reported that under short dry spell both Silicon (Si) and K were effective in reducing the adverse effects of drought stress. However, under prolonged water stress, K was more effective than Si.

Ground covers

Mucuna bracteata, is a leguminous creeper, introduced in Sri Lanka recently. Some data indicate that green matter and litter production of Mucuna was three times higher than Pueraria. Similar results were observed with regard to thickness of the green matter and litter layers and the Mucuna exhibited 45 cm and 106 cm thick layers compared to 15 cm and 36 cm thick layers of Pueraria, for green matter and litter, respectively.

Soil moisture content in soils under Mucuna was higher with a moisture content of 20 percent and 18 percent for the 0-15 cm and 15-30 cm depths respectively, in comparison to soils under Pueraria.

Among the two species, the highest moisture profile storage capacity of 25.8 cm was observed under Mucuna for 90 cm profile depth. There was an increase of 41 percent in the moisture storage capacity as compared to the soils under Pueraria .

Thick organic matter layer influences the moisture content of the soil by their effect on water intake through the immediate surface layer and also by decreasing losses due to soil evaporation and transpiration of weeds by suppressing weed growth.

Also, an improved soil structure from accumulation of organic matter decreases crusting and surface sealing and permits greater infiltration, thereby increasing the water holding capacity.

Moreover, the transpiration rate of Mucuna (1.4 ug/cm2/sec2) is nearly ten times lower than Pueraria (11.6 ug/cm2/sec2), allowing more moisture to remain in the soil for a longer period of time.

Further, it is known that Mucuna has a deep root system (70 cm) than Pueraria (20 cm), which allows uptake of water from deeper layers of soil reducing the competition for water with young rubber plants which have a 30 cm deep root system.

Therefore, it appears possible to minimize the adverse effects of moisture stress by growing Mucuna. The higher soil moisture content may increase the water uptake by young plants thereby improving their growth especially during dry periods.

Mulching

Among the different soil management practices that are considered for their effects on moisture conservation, dead mulch is known to exhibit the highest soil moisture storage capacity of 27.6 cm in comparison with other practices such as growing leguminous covers or naturals.

Mulches also would influence the moisture content of the soil by their effect on water intake through the immediate surface layer and due to improved soil structure by higher organic matter content, which decreases crusting and surface sealing and permits greater infiltration, thereby increasing the water holding capacity.

Moreover, root density is known to be higher under mulching than under legume cover or natural cover.

Such differences in feeder root development of Hevea under different soil management practices could be attributed to the higher organic matter content of the soil under mulching.

It was reported that mulch induced changes in hydrothermal regime of soil increased the density of rooting and caused greater lateral spread of roots.

At the same time higher soil moisture content and higher root density increases the water uptake and along with the water, nutrients are also taken up thereby increasing the growth of rubber plants under dry climatic conditions.

Mitigation and adaptation

Among the terrestrial ecosystems, natural forests sustain the most efficient moisture conservation system. However, the water use by rubber plantations is estimated to be 500mm to 600mm lower than typical tropical rain forest ecosystem.

The rate of evapo-transpiration is lower in rubber plantation compared to the forest ecosystem; mean daily evapo-transpiration of rubber is about 4.5 mm per day.

The crop coefficient values of rubber tree are reported to be lower during dry seasons (0.179) and even under wet conditions (1.058), the transpiration rate is lower compared to many other forest species.

These findings highlight the ability of rubber plantations to conserve soil moisture more efficiently compared to most of the forest species.

High levels of epicuticular wax observed in the rubber leaves are associated with high reflectance of heat energy and probably reduce transpirational water loss.

With the increase in soil moisture stress, stomatal conductance and transpiration rate of rubber plants decrease resulting in the closing of stomata and reduced transpiration with the onset of drought conditions .

Moreover, when rains fall, about 19 percent of the rain is intercepted and stays on trees and most of this water either re-evaporates or may be directly absorbed by the plant tissues.

This intercepted water probably suppresses water loss (transpiration) from the rubber leaves. Also, efficiency of water use by rubber is better at low soil moisture conditions. This indicates that rubber plants are capable of growing and producing latex even in drought conditions.

The capability of plants to retain more water and more efficient or sensitive turgor regulation is of importance as explained that plant growth responds more sensitively to water stress than to other physiological processes such as photosynthesis.

High latex vessel turgor and low latex solute potential in rubber in the dry season indicate osmotic adjustment.

The proportion of water from rainfall reaching the ground as stem-flow and through-fall, which also called ‘net rainfall’ is of great importance to rubber tree, which totally depends on rainfall for its moisture requirement.

This amount of rainfall that percolates through and accumulates in soil is used by rubber plants for their growth and productivity.

Though, a five to six years old rubber stand is capable of providing an excellent thick tree cover to the land, the rainfall interception by the canopy is only about 19 percent. 81 percent of the rainfall reaches the ground as through-fall and stem-flow.

CO2 sequestration

A monoculture of rubber has been reported to be a relatively efficient converter of solar energy into dry matter production. The total biomass produced by a rubber tree in Sri Lanka at the age of 33 years is 1.8 tonnes which extrapolates to 903 tonnes per hectare.

This value includes, the weights of fruits, leaves and branches fallen and trees uprooted during this period of 33 years. The amount of carbon sequestered in one hectare of 33 year old stand of rubber trees is 596 tonnes, the major portion of which is sequestered in the trunks and branches.

Around 25 million tonnes of carbon can be sequestered in the total biomass of rubber trees under Sri Lankan conditions on the basis of extrapolation from the figures available for one hectare.

The total amount of carbon sequestered in one hectare of rubber plantation made up of tree biomass, latex produced and cover crop biomass is computed to be 680 tonne.

Although CO2 sequestered by the existing plantations are currently not eligible to receive carbon credits, the global stakeholders should strongly push agenda for existing NR plantations to qualify under the CDM, for the environmental services provided by such plantations.

Rubber growing in Sri Lanka therefore, besides being a profitable business, contributes to the sustenance of an environmentally friendly, ecologically sustainable crop with dual economic potentials of both rubber (latex) and timber production, while simultaneously contributing to maintenance of the carbon balance in the atmosphere.

Also, there had not been any marked changes in annual or monthly rainfall in the traditional rubber growing areas in the distant past. Nevertheless, negative trends have been reported in many locations with regard to total annual rainfall. In several locations changes have been observed in monthly rainfall in March and December.

There is therefore a risk of dry spells (reduction in the probability of receiving greater=10mm of rain) when compared to the period 1941-1970.

Although these changes are of little magnitude, yet it is important to adopt suitable adaptation and mitigation measures such as, timely planting and other acceptable agronomic practices.

Tillage as well as life saving irrigation has been found to be effective in India. Yet, tillage is a more economic and feasible management practice in Sri Lankan plantations.

Tillage is expected to provide additional soil conservation by blocking capillary pores in the soil.