Are observed rates of productivity compared to model predictions indicating negative climate impacts in perennial plants?

Understanding the apparent discrepancy between observed and modelled primary production is explored in this paper using long-term pasture and radiata pine (Pinus radiata) growth data sets and relevant biophysical models. To better understand the historical trends on primary production from pastoral agriculture and forestry, annual net herbage accumulation (NHA) and total volume yield was modelled (APSIM and PPM88) and compared with long-term pasture and forestry data sets from key regions in New Zealand.

Ballantrae and Poukawa (Southern and Central Hawke’s Bay) showed declining trends in NHA (observed and modelled), over the last 20 years. In contrast, Woodlands and Waikato have shown no changes or a slightly increase in NHA despite large changes in management practices and nutrient inputs, particularly at the Waikato site. In Kaingaroa Forest in the central North Island empirical models have overestimated tree growth and volume production in recent years relative to observations. The minimal changes or decline in vegetation growth challenge the prevailing notion of expected increases in vegetation growth under future climate scenarios.