Decline In Net Primary Productivity Research Articles

Assessment of global forest change between 1986 and 1993 using satellite-derived terrestrial net primary productivity

SummaryAlthough forest removal has been well documented at a global level, knowledge of how major forest processes such as photosynthesis have been affected remains poor. Global forest change between 1986 and 1993 was assessed using the NOAA/AVHRR satellite data converted to terrestrial net primary productivity (NPP). Forest loss was a dominant feature in tropical regions, with the most severe destruction in Latin America followed by southeast Asia and Africa. Loss of high-productivity forests over wide areas was observed for countries such as Brazil and Bolivia. Further analysis showed that approximately 12% (9100999 km2) and 19% (2 600000 km2) of the low-NPP regions (<500 g m−2yr−1, e.g., deserts, tundra) and the high-NPP regions (> 2000 g m−2yr−1, e.g., tropical rain forests), respectively, were transformed to intermediate-NPP regions (500–1500 g m−2yr−1, e.g., savanna, grassland, or cultivated land), between 1986 and 1993. The extent of global forest degradation or fragmentation may be more severe than the deforestation itself. Low-latitude ecosystems were more prone to decline in NPP than mid- and high-latitude ecosystems. The NPP method offers insight into global forest change in a timely, practical and consistent manner.

Declining forest productivity in aging forest stands: a modeling analysis of alternative hypotheses.

Several explanations have been advanced to account for the decline in forest net primary productivity (NPP) with age in closed-canopy stands including the hypotheses that: (1) sapwood maintenance respiration rate increases, reducing the availability of carbon to support new growth; (2) stomatal conductance and hence photosynthetic efficiency decline; and (3) soil nutrient availability declines. To evaluate these hypotheses we applied the ecosystem model G'DAY to a 40- and a 245-year-old stand of lodgepole pine (Pinus contorta Dougl. ex Loud.), growing on infertile soils. Net primary productivity estimated from biomass data was 0.47 and 0.25 kg C m(-2) year(-1) and foliar nitrogen/carbon ratio (N/C) was 0.0175 and 0.017 for the 40- and 245-year-old stands, respectively. Productivities of the young and old stands were derived from a graphical analysis of the G'DAY model. The graphical analysis also indicated that the observed age-related decline in NPP can be explained in terms of interacting processes associated with Hypotheses 2 and 3. However, the relative importance of these two hypotheses differed depending on key model assumptions, in particular those relating to variation in soil N/C ratio. Thus, if we assumed that soil N/C ratio can vary significantly during stand development, then Hypotheses 2 and 3 jointly explain the decline in NPP, whereas if we assumed that soil N/C ratios are constant, then Hypothesis 3 alone explains the decline in NPP. The analysis revealed that only a small fraction of the decline of NPP can be explained in terms of increasing sapwood respiration.