Net Ecosystem Production NEP
67 precipitation. Year-to-year changes in spatial pattern of NEP were most probably
caused by changes in the spatial pattern of precipitation, which can be changed dramatically by the El Nino events Vorosmarty et al., 1996.
Ecosystem maintenance involves two tasks: 1 keeping the system‟s carbon and nutrient stocks organized; and 2 minimizing net losses of carbon and
nutrients from the system. Both tasks require energy investments in the system. A useful relative index of the magnitude of this investment in maintenance appears
to be net ecosystem production NEP. Net ecosystem production refers to the net change in organic matter stocks
in the system for some defined period of time. For agricultural systems NEP is the sum of NPP and organic matter inputs associated with maturing minus the sum
of heterotrophic respiration plus organic matter outputs associated with harvest and erosion. If NEP is negative, then the system is probably losing nutrients. The
optimum rate of NPP in agricultural systems is that where a sufficient fraction of the NPP is invested to maintain NEP equal tu or greater than zero. Agriculturalists
should be “caretakers” or “builders” and not “miners”.
Refer to the previous research in various areas of tropical forests Potter et al., 2003 showed that the average Rh total respiration of vegetation about
850 g C m
-2
yr
-1
. Results of this measurement, also have done by Chamber et al., 2004 which measures the risk of canopy cover, tree trunks and forest soil
surface, with estimation result about 900 g C m
-2
yr
-1
, with an annual average of Rh g C 850 m
-2
yr
-1
, with a range between 846 to 857 g C m
-2
yr
-1
. Based on results of from previous research, the assumption of estimated annual average of
Rh total respiration on island of Sumatra is 850 g C m
-2
yr
-1
. Estimation of annual Net Ecosystem Productivity NEP obtained from the
calculation of total annual NPP subtracted with total respiration. Furthermore, result of estimation average annual carbon fluxes Net Ecosystem Productivity,
NEP of Sumatra terrestrial as show in Figure 4.19.
68
Figure 4.19 Annual Net Ecosystem Productions in Sumatra
Figure 4.19 shows the pattern of NEP patterns over Sumatra terrestrial during normal climate condition 2005 and during abnormal climate condition in
year 2006 El Nino event and 2007 La Nina event. These NEP patterns reflect the complex patterns of precipitation that change among El Nino years.
Precipitation has its greatest effect on NPP during the drier part of the year.
69 During normal year or non ENSO year in year 2005, most of the
northwest, northeast and western part of Sumatra island acted as a carbon sink positive value meanwhile in the central and southern part of Sumatra island
acted as carbon source negative value. During El Nino years much of the Sumatra region releases carbon to the
atmosphere, i.e. negative annual NEP. The magnitude and location of this negative annual NEP varied among El Nino events. During El Nino event in year
2006, NEP with positive value indicated as carbon sink in Sumatra Island has occurred in as same as positive value during normal climate event. Increased
NEP has occurred in west part of the Island. However, during La Nina event in year 2007, NEP with positive value indicated as carbon sink in Sumatra island
has occurred in same area as positive value during normal climate condition or El Nino event.
This condition of annual NPP pattern affected with precipitation. Locations with large positive annual NEP are often those that receive a high
amount of precipitation. In contrast, locations with negative NEP are often those that receive little precipitation.
Understanding the responses of ecosystem processes to climate variability is essential for reducing the uncertainty in the estimates of CO
2
exchange between the biosphere and atmosphere. Plant growth is often limited by sub-optimum
climatic conditions such as low temperature, water shortage and light deficiency covered by cloud. Therefore, the distribution of ecosystems and their
productivity show predictable relationships to climatic variables. On an annual basis, result analyses have indicated that precipitation,
especially the amount falling in the drier part of the year, is the primary factor influencing annual carbon storage in the island. The net exchange of carbon
dioxide between tropical land ecosystems and the atmosphere could be influenced by interannual variations in climate that could cause this region to function as a
carbon sink in some years and a carbon source in other years. Changes in precipitation combine with changes in temperature to affect soil moisture, a factor
70 we have identified as an important controller of carbon storage in the Amazon
Basin. Reduction in soil moisture can lead to a decrease in net ecosystem production through influencing the availability of nitrogen Raich et al., 1991.
Braswell et al. 1997 suggested that the terrestrial response to changes in temperature results in either enhanced plant production, reduced heterotrophic
respiration, or both, such that global NEP is positive about 2 years after an El Nino event. These observations and analyses indicated that interactions among
ecosystem processes are important in controlling the carbon cycle. In addition to the effects of increasing atmospheric CO2 and climate
variability, annual carbon storage can also be influenced by nitrogen deposition re-growth on abandoned land, tropical deforestation, and fire Nepstad et al.,
1999. Although climate variability mainly determines the signal of terrestrial carbon fluxes on seasonal and interannual time scales, on time scales of decades
or centuries, vegetation dynamics following disturbances might play an important role in controlling carbon storage. Managing the global carbon cycle is now firmly
on the world‟s environmental agenda. We have a compelling need to understand the components of the global carbon budget and how natural climate variability
and human-induced climate changes affect them.