Net Primary Productivity (NPP) is the net amount of organic matter produced by live plants over a specific time interval, and is often based on tree biomass calculations in a given area of forest. However, it remains a challenge to determine how a certain climate parameter drives tropical forest productivity. 

In a study published in Forest Ecology & Management, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) estimated NPP at three-month intervals from 2009 to 2017 for a tropical seasonal rainforest in Xishuangbanna, SW China. 

The researchers used data from over 2,000 dendrometer bands and litter fall traps within a 20-ha permanent forest dynamics plot. They asked which climatic factor had the greatest effect on forest NPP at the sub annual scale, and how the relationships varied with seasonality. 

In the study plot, all local climate variables and NPP didn’t show statistically significant temporal variation from 2009 to 2017, but the NPP increased over time. Thus, it is a sink of carbon. 

They further found that there was no single climatic variable influencing the NPP in rainy season. However, during dry season, precipitation was the most important factor that facilitated NPP. The precipitation showed a significant threshold effect at the rainfall change point of 229 mm/three months. 

The observed clear variation of NPP between rainy and dry season demonstrated that forests productivity was highly seasonal in tropical rainforest of Xishuangbanna, China. 

"Our finding showed that climate is a key to NPP dynamics at the sub annual scale in the tropical seasonal tropical forests of southwestern China, which strengthens our understanding of the dynamics of NPP", said Dr. HU Yuehua, correspondence author of the s tudy.