| Changes of vegetation cover not only directly affect the formation and evolution of terrestrial ecosystems, but also may give rise to different scales of atmospheric circulation and climate change. At the same time, climate change has a direct impact on changes of land surface vegetation. Thus the interaction and feedback mechanism between vegetation and atmosphere becomes one of the important scientific issues in current global change research. There are poor climatic conditions, fragile ecological environment and backwardness economic in arid and semiarid climate regions in West China. The Ecological Reforestation Project had been implemented since 1999 in there. This land use change is bound to cause great changes of local climate and to affect agriculture. How to assess the possibility climatic and agricultural effects of returning farmland to forest and grassland is the major issue involved in local sustainable development of socio-economic and ecological environment.In the study, based on the region with less precipitation, poor forest site, converting farmland into forest or grass land in ShanGanNing, the relationship between local vegetation change and regional climate was firstly analyzed by using some observations. Then, it was investigated that coupling method and its effect between regional climate model and crop / pasture growth model. Finally, interactions between vegetation and climate were researched under the conditions of gradient changes of land use in the actual progress. Climate and agricultural effects produced by Alone Returning Farmland to Forest and Grassland (AReFFG) were discussed through many numerical simulations by using the coupling model. The main conclusions were as follows:1. Analysis of observational data showed that (1) Distribution of major land use types in ShanGanNing region had produced significant changes since 2000. Planting area of winter wheat reduced by about 1568 km2 y-1 due to returning farmland to forest or grassland. As a result of degradation or converting to farm land or forest land, the area of grass land reduced by 1776 km2 y-1. There were reductions of 1776 km2 y-1 in desert area since the implementation of desertification control projects. Deciduous broad-leaved forest area increased by 3312 km2 y-1 based on afforestation. Because of conversion from farmland or degradation of grassland, semi-desert / sparse grass area increased by 2300 km2 y-1. (2) Increasment slowed down in July temperature while accelerated in precipitation with time in Typical region of Ecological Reforestation in Central and Southern study Area (TERCS region). (3) Summer temperature was significantly associated with changes of deciduous broad-leaved forest, semi-desert and desert area. (4) Spring temperature and precipitation had a significant effect on the yields of winter wheat, while autumn temperature and annual precipitation affected the amount of forage grass. These facts from observations were a prerequisite to carry out simulation studies on climate and agricultural effect of converting farmland into forest or grass land.2. Calibration and validation of models showed that (1) the performance of WOFOST in local scale was improved by adding re-operation process of the dry matter. Regional application of WOFOST was achieved based on regionalization of winter wheat parameters. (2) LINGRA could not simulate the growth of natural grass until including stem growth process and making the water stress coefficient insensitive. (3) RegCM3 could reasonably reproduce the seasonal changes and regional distribution of climate in ShanGanNing region under both grid and semi-grid scale. However, there were slightly lower in simulated temperature and higher in simulated precipitation than observations.3. Coupling design and test of models showed that (1) RegCM3 and RegWOFOST\LINGRA could match in days and semi-grid scale. Both models were coupled by means of that RegCM3 outputted the maximum, minimum temperature, precipitation, radiation, vapor pressure and wind speed to RegWOFOST\LINGRA, and vegetation LAI, which was calculated by using leaf dry weight and specific leaf area in the latter, was then fed back the former. (2) RegCM3\WOFOST\LINGRA slightly improved the situation of high in precipitation and low in temperature simulated by RegCM3 and achieved to simulate simultaneously the climate, crops and pasture.4. Numerical simulation by using RegCM3\WOFOST\LINGRA during 1999 to 2008 showed that (1) As a result of AReFFG, there were a decrease in the maximum, minimum and average temperature, a slight decrease in global solar radiation in most parts of the study area, the region of increase in precipitation, evapotranspiration and soil moisture was wider than the one of decrease, and the ratio of surface runoff and precipitation had a little change. (2) AReFFG had inhibitory effect on continuous increase of the maximum temperature in study area and promoted effect on the increase of precipitation in TERCS region. These situations led to noticeable delay in winter wheat maturity and increase of dry weight of storage organ (WSO). Temperature change was mainly corresponding with conversion of farmland or grassland into forest land.5. Numerical simulation results by using RegCM3\WOFOST\LINGRA during 1990 to 2015 showed that (1) As a result of AReFFG, there were a decrease of 0.05℃-0.27℃in the maximum temperature, a decrease of 0.1-0.3℃in the minimum temperature, a reduction of less than 80mj m-2 in the global solar radiation, a increase of less than 25mm in precipitation, a advancement of about 2 days in winter wheat maturity, a increase of less than 0.2 in LAI in anthesis, an enhance of less than 614 kg hm-2 in WSO in maturity in average of study area. (2) The proportion of average temperature and precipitation changes caused by AReFFG were 3%-7% in the total climate changes, while other climate and agricultural elements account for the larger proportion. Climate and agricultural effect of AReFFG could not be neglected in probing regional climate change. However, it still could not reverse the large background of climate change.In this paper, there were innovation in revealing the observed facts, coupling regional climate model and crop / pasture growth model, trying to separate the effects of AReFFG, considering the actual change situation of land use, higher spatial resolution and long-time numerical integration and many numerical simulation test as well. We had a useful attempt in study methodology against scientific issues of the interaction and feedback mechanism between vegetation and climate. This research not only promotes the interdisciplinary study of climate and agricultural, but also has social benefits. It can be further investigated to improve model performance, add more entry point of models coupling, and consider the situation of larger area and effects of space weather variables in the next step. |
