Assessment report on NRP subtheme “Effects of climate change on terrestrial ecosystems”

Abstract

In the projects fostered by the NRP the effects of changed climate (atmospheric CO 2 concentration, temperature) on different terrestrial ecosystems were studied. For forests it was concluded that the initial stimulation of tree growth in general did not persist after two years, and therefore care must be taken not to overestimate the potential contribution of increased carbon sequestration by forests. On the other hand, shifted patterns of carbon distribution in the tree-soil system may lead to a higher soil organic matter content, which will contribute to an improved soil structure and availability of soil moisture. A sensitivity analysis revealed that, for the poor sandy forest soils, improved rooting depth is however more effective for drought prevention than higher soil organic matter. From the model exercises it was also inferred that with increased precipitation, as predicted under the projected future climate, runoff and recharge of the groundwater will be increased, especially in winter and for deciduous forest stands. Changed seasonal temperatures will change the timing of phenology but will, in Dutch conditions, not lead to a higher risk for spring frost damage in the period of bud burst. However, competition between tree species may change as the duration of the closed and functional canopy is differentially influenced. For instance for Larix and oak the effective growing season is extended, whereas beech and Tilia cordata will have a shortened leaf area duration. Mechanistic forestry production models, adapted to include also the changes in [CO 2 ], (e.g. in a transient climate scenario), showed that after a transient increase in production, a double-C 2 climate from GCM calculations caused a subsequent decline in productivity. A high variability of the growth and production enhancement by rising [CO 2 ] was also detected in the OTC (Open Top Chambers) and Rhizolab experiments for the crop species studied (potato, wheat, faba bean). The physiological parameters (photosynthesis and respiration) and full season canopy and soil gas exchange measurements showed no growth stage or light and temperature dependent CO 2 -enhancement effect. An analysis using crop growth models produced clues as to the origin of the existing confusion about the variability of the C0 2 -enhancement factor for biomass production and yield. Using the growth and weather data of different years it could be shown that interactions between growth stage, light and especially temperature in the early growth stages could explain a large part of the variation. Also about half of the differences in growth enhancement between e.g. (winter) wheat (16 — 34%) and faba bean (35 – 56%) could, according to the model outcome, be ascribed to temperature differences in the early (spring) growth stages. An important and for the carbon cycle very significant finding was that the roots of grass, grown at 700 ppm CO 2 were degraded much slower by the soil organisms than reference root material. It was shown that this change in properties may fully offset the stimulation of the decomposition of soil organic matter by the projected temperature rise. In a pilot study the potential distribution within Europe of mosquitoes that can act as a vector for malaria transfer was investigated. The combined effects in the various growth stages of the mosquito, as brought together in a simulation model, indeed point to a highly increased risk for infectious individuals. The probability of an epidemic is considered low, as the European health care system is expected to be sufficiently effective in picking up disease incidence. The concept of “infection potential”, developed in this project, offers excellent possibilities to quantify risks also for other, more vulnerable, areas or world wide studies. The methodology developed in this project can be used for other (also agricultural) pests and diseases.