THE WATER RELATIONS OF RUBBER (HEVEA BRASILIENSIS): A REVIEW

Abstract

SUMMARYThe results of research done on water relations of rubber are collated and summarised in an attempt to link fundamental studies on crop physiology to crop management practices. Background information is given on the centres of origin (Amazon Basin) and production of rubber (humid tropics; south-east Asia), but the crop is now being grown in drier regions. The effects of water stress on the development processes of the crop are summarised, followed by reviews of its water relations, water requirements and water productivity. The majority of the recent research published in the international literature has been conducted in south-east Asia. The rubber tree has a single straight trunk, the growth of which is restricted by ‘tapping’ for latex. Increase in stem height is discontinuous, a period of elongation being followed by a ‘rest’ period during which emergence of leaves takes place. Leaves are produced in tiers separated by lengths of bare stem. Trees older than three to four years shed senescent leaves (a process known as ‘wintering’). ‘Wintering’ is induced by dry, or less wet, weather; trees may remain (nearly) leafless for up to four weeks. The more pronounced the dry season the shorter the period of defoliation. Re-foliation begins before the rains start. The supply of latex is dependent on the pressure potential in the latex vessels, whereas the rate of flow is negatively correlated with the saturation deficit of the air. Radial growth of the stem declines in tapped trees relative to untapped trees within two weeks of the start of tapping. Roots can extend in depth to more than 4 m and laterally more than 9 m from the trunk. The majority of roots are found within 0.3 m of the soil surface. Root elongation is depressed during leaf growth, while root branching is enhanced. Stomata are only found on the lower surface of the leaf, at densities from 280 to 700 mm−2. The xylem vessels of rubber trees under drought stress are vulnerable to cavitation, particularly in the leaf petiole. By closing, the stomata play an essential role in limiting cavitation. Clones differ in their susceptibility to cavitation, which occurs at xylem water potentials in the range of −1.8 to −2.0 MPa. Clone RRII 105 is capable of maintaining higher leaf water potentials than other clones because of stomatal closure, supporting its reputation for drought tolerance. Clones differ in their photosynthetic rates. Light inhibition of photosynthesis can occur, particularly in young plants, when shade can be beneficial. Girth measurements have been used to identify drought-tolerant clones. Very little research on the water requirements of rubber has been reported, and it is difficult to judge the validity of the assumptions made in some of the methodologies described. The actual evapotranspiration rates reported are generally lower than might be expected for a tree crop growing in the tropics (<3 mm d−1). Virtually no research on the yield responses to water has been reported and, with the crop now being grown in drier regions, this is surprising. In these areas, irrigation can reduce the immaturity period from more than 10 years to six years. The important role that rubber plays in the livelihoods of smallholders, and in the integrated farming systems practised in south-east Asia, is summarized.

Converting forests into rubber plantations weakened the soil CH4 sink in tropical uplands

“…Our study, however, does not support this observation: the top layer of rubber plantation soils was wetter compared with natural forest soils. The transpiration rates of rubber trees growing under tropical humid or monsoon climates are actually not very high (<3 mm day −1 ; Carr, 2011;Kobayashi et al, 2014;Niu, Röll, Meijide, Hendrayanto, & Hölscher, 2017). To explain the difference in soil water usage between natural forest and rubber plantation, further studies need to cover both land uses and all components of the water cycle.…”

How do rubber (Hevea brasiliensis) plantations behave under seasonal water stress in northeastern Thailand and central Cambodia?

“…8). SON, 2010, SON, 2011and DJF, 2011-2012 at CRRI because of their original narrow range of D. The theoretical ratio 0.60 (Oren et al, 1999) is shown by a solid line for each site. -Claramonte et al (2008) surmised that the introduction of rubber plantations may reduce stream flow leading to drier conditions throughout a catchment area, mainly because of high water use by rubber trees at the very peak of the dry season (cf., Fox et al, 2014).…”

“…On the other hand, some studies reported that rubber plantations replacing non-forested lands could act as a carbon sinks (Wauters et al, 2008). Further, while eddy covariance measurements in rubber revealed high plantation water use and a large proportion of energy being used to drive evapotranspiration for most of the year (Tan et al, 2011;Giambelluca et al, submitted for publication, hereinafter referred to as submitted manuscript, 2015), the results of sapflow measurements of individual rubber trees showed water use of rubber plantations may not be as high as anticipated (Isarangkool Na Ayutthaya et al, 2011;Carr, 2012;Kobayashi et al, 2014), but these findings might be related to differences in regional climate, geography, physiological adaptations, and measurement method. Of concern to our research, large-scale land use changes to rubber plantations may have important implications for local-to-regional carbon and water balances, but they still remain unclear.…”

“…December-February: DJF, March-May: MAM, June-August: JJA, and September-November: SON. Note that the narrower D range estimations for g sref and m were not made in JJA, 2009 at Som Sanuk and inSON, 2010, SON, 2011 and DJF, 2011-2012 at CRRI because of their original narrow range of D. Shaded columns represent periods when actual water balance (P − ET) was negative (seeFig. 4).…”

Rubber Trees Demonstrate a Clear Retranslocation Under Seasonal Drought and Cold Stresses

“…Rapid conversion of tropical forests to agriculture, timber production or other uses has generated vast, human-dominated landscapes with negative consequences for tropical forest ecosystems (Gardner et al, 2009; Gibson et al, 2011). Rubber tree ( Hevea brasiliensis ) is an evergreen tree species in the rainforests of Amazon (Carr, 2012; Chen and Cao, 2015) and introduced to tropical Asia at the end of 19th century (Limkaisang et al, 2005). Global rubber plantations reached 10.06 million ha in 2010 (FAO, 2010) with 81% in South and Southeastern Asia (FAO, 2010).…”