Seasonality in the rainfall and its effect on Net primary PRODUCTIVITY (NPP) in tropical rainforest ecosystem with high rainfall regime
Rainforests are characterized by a monthly mean temperature greater than 18°C and a mean annual rainfall not less than 1500 mm1. Tropical rainforests are distributed in North and South America, Central- West Africa and South and South-east Asia. It accounts for one third of the terrestrial net primary productivity (NPP) 2. Pan et al(2011) had estimated that tropical forests serve as a sink for 1±0.5 Pg. C/ year3–5.
Various factors can influence stand and community structure and function of tropical rainforest over local and regional scales. These differences are due to several biotic (e.g. presence and absence of certain species) and abiotic (e.g. physical property of soil, elevation water and temperature)factors6. Precipitation acts as one of the major drivers of annual primary production due to its vital role in affecting photosynthesis and nutrient availability7. Precipitation is not evenly distributed across the globe and also intra annually.8–10. Intra annual variation in rainfall lead to decrease in water content of environment during dry season relative to wet season9,11. However during wet season if rainfall is very high it may lead to nutrient runoff and leaching. So understanding the role of this variation in rainfall annually and it temporal and long term effect on ecosystem is important. A
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precipitation to gain a maximum value at approx. 2250 mm per year after which it drops(fig:1)12. The reason for such drop in NPP is could be:
(a) Nutrient runoff and leaching: With increase in rainfall, the soil water content saturates. The water that remains above the top layer of soil then flows off as runoff, carrying away the dissolved nutrients. Dissolved nutrient also move downward in soil profile with percolating water. This phenomenon is called leaching which remove nutrient away from rooting zone.13
b) Low oxygen level in the soil: With increase in rainfall oxygen diffusion rate in the soil decreases leading to slowing down the decomposition rate of soil organic matter due to anaerobic conditions. This might lead to temporal deficient environment during peak wet season leading to decrease in NPP
A study conducted at two long term 1-ha plots in Peru with a mean annual rainfall of approximately 2500mm suggests that wet season above ground biomass productivity is higher than the dry season14. However region with extreme mean annual rain fall (>3000mm) might see different trend due to anoxic soil condition, soil nutrient loss by leaching and runoff during heavy rainfall. Also during wet season the cloud cover decrease the soil irradiation received by plant relative to dry season when could cover is less15. So it is important to study effect of seasonal variation in rainfall on biomass productivity. Based on above reported findings I ask following questions.
Question 1:- How does change in precipitation seasonality affect net primary productivity in tropical rainforests?
In order to understand the relationship between NPP, mean annual rainfall and seasonal variation in precipitation in tropical rainforest ecosystem I will synthesizing the data.
- Net primary productivity and global position of tropical rainforest plots will be collected from published articles.
- 10’ resolution data of mean monthly precipitation of land areas along with its coefficient of variation form 1960-91 will be downloaded from climatic research unit database16.
Dry season will defined as period when region get less than 100 meter of rainfall17. Extend of dry period and wet period will be defined as seasonality in precipitation.
- Plot a 3D curve with precipitation and seasonality as independent variables and net primary productivity as dependent.
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- Places with rainfall greater than 2000-3000 mm of rainfall and high seasonality will have higher NPP compared to places with similar amounts of rainfall but less seasonal precipitation. Region with high rain fall are not limited by water during wet season14,17. Due to presence of cloud these forest might be limited by solar irradiation leading to decrease in photosynthesis which will decrease gross primary productivity.15. Also heavy rainfall during wet season could lead to nutrient limited environment due to nutrient leaching, runoff and anoxic soil condition.
- Places with rainfall lower than 2000-3000mm but higher seasonality will have lower NPP than less seasonal areas with similar rainfall amounts. A possible explanation could be the fact that in seasonal forests, trees receive water only for short period of time in an year, therefore in the dry season the system becomes water limited either due to loss of water by evapotranspiration or because plant shut down their stomata in order to save the loss of water leading to decrease in carbon dioxide diffusion in cells15.
Question 2:- How does biomass productivity change during wet season relative to dry season in region with extreme mean annual rainfall and high seasonality in precipitation?
As discussed above, areas with 2500mm (approx.) MAP have increased productivity in wet season relative to dry season because increase in water availability increases photosynthesis rate, suggesting that productivity is limited by water14. In regions with extreme annual rainfall (>5000 mm) photosynthesis might not be limited by water but by solar irradiance which may result in decrease the productivity during wet season. Hence I hypothesize that region with extreme rainfall will show decrease in biomass productivity during wet season compared to dry season.
To test the above hypothesis, the following observational set-up can be used:-
Three 1-ha long term measurement plot need to set-up across precipitation gradient in seasonal forest with MAP greater than 5000 mm according to GEM protocol. (Sirsi will be chosen as the model forest for this study as mean annual rainfall vary from 5000mm to 9000mm, and majority of precipitation occurs during the months of June to September).
- Metrological data and seasonal change in biomass will be measured during pre-monsoon, monsoon and post monsoon period.
- To assess the seasonality of relative biomass accumulation seasonality index will be calculated according to Rowland et al,2014
- Within the site, biomass productivity should decrease during wet season, due to low availability of solar irradiance and decrease in soil oxygen level lead to slowing in soil organic matter decomposition.
- Biomass production across the rainfall gradient will decrease with increase in precipitation.
In future, I would like to study variation in NPP allocation during wet and dry season in these ecosystem. One could expect during wet season when solar irradiation is low plant will allocate NPP in canopy in order to capture the maximum light. Hence this might lead relative increase allocation to canopy NPP compared to total NPP resulting in increase in NPP canopy to NPP total ratio during wet season.
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