Categorized analysis of forest ecological values in the China’s conversion cropland to forest program

Background The China’s Conversion Cropland to Forest Program (CCFP) is one of the large state ecological construction programs. Up to now, the program has effectively improved the ecological environment and produced large ecological benefit. However, there were also some problems in its implementation process, the program has been sometimes less effective than the expected. Methods Based on the data and the methods of ‘State report on monitoring ecological effects in CCFP’ and the Chinese Forest Ecosystem Research Network (CFERN) in 2013, we analyzed the categorized ‘forest ecological benefit value’ (B-V) s in the three forest restoration ways in different regions in China to provide references for CCFP construction. Results The unit area B-Vs in CCFP varied between 35 000 RMBs.hm−2.a−1 and 100 000 RMBs.hm−2.a−1. Water conservation B-V and species conservation B-V were the two largest constituents, nutrient accumulation B-V was the least in all the categorized B-Vs on regional and unit area scale. The rank of restoration ways on average unit area total B-Vs was—‘hillside forest conservation’ > ‘returning cropland to forest’ > ‘afforestation on suitable barren hills and wasteland’ in CCFP. Among the categorized B-Vs, some pairs were positively correlated with each other and some were negatively correlative. The correlation coefficients and some regression equations were given in the text and the attached Fig.s. Conclusions Water conservation B-V was the highest and nutrient accumulation B-V was the lowest whether on regional or unit area scale in CCFP. Forest ecological B-Vs varied in different forest restoration ways and different regions in CCFP. The ‘hillside forest conservation’ restoration way and the water conservation B-V should be paid more attention in China’s future forest restoration. We suggest that suitable forest restoration ways should be selective according to the regional specific and ecological targets. There were correlations among the categorized B-Vs, and the correlations varied with different forest restoration ways in CCFP. Knowing about the correlations could clarify the targeted restoration ways according to the goal of ecological benefit.

barren hills and wasteland' in CCFP. Among the categorized B-Vs, some pairs were positively correlated with each ways on a large scale and in long-term in CCFP were rarely reported. the categorized analysis on B-Vs in CCFP. The objective of the study is to find the differences, features and the 94 relationships of the categorized B-Vs among the three forest restoration ways-'hillside forest conservation'

95
(H-f-c), 'afforestation on suitable barren hills and wasteland' (A-b-w), 'returning cropland to forest' (R-c-f). We 96 hope it will be able to provide references for the construction of forest restoration and be an interesting issue for us 97 to communicate with the peers.

187
There were also different performances between regional and unit area scale. The rank of regional total B-V was water conservation B-V and the unit area atmosphere purification B-V had significantly positive correlations with 197 their total B-Vs (r=0.906, p<0.01; r=0.914, p<0.01), so were the water conservation B-V with atmosphere 198 purification B-V (r=0.722, p<0.01) and water conservation B-V with nutrient accumulation B-V (r=0.633, p<0.01).

199
Meanwhile, the unit area nutrient accumulation B-V had significantly negative correlation with the unit area 200 species conservation B-V (r=-0.532, p<0.05). We adopted 77 data lines of categorized regional B-Vs to analyze 201 their correlations. The result showed that regional total B-V had significantly positive correlations with all the 202 relevant categorized regional B-Vs (Table 1). Several regressions among categorized B-Vs of 'hillside forest 203 conservation' were shown in Fig. 4. The low yield regions with crucial ecological function; The regions at river source or river side and the croplands 210 with ecological importance which were seriously damaged by the wind and sand.

219
The rank of regional total B-V was HU-N > HE-B > G-S > Y-N > HU-B > L-N, and the rank of unit area total conservation B-V with 19 228.00 million RMBs, but its unit area water conservation B-V ranked the second with 222 31 811.40 RMBs·hm -2 .a -1 . In addition, HE-B produced the highest regional annual B-V of 'carbon fixation and 'returning cropland to forest' way were analyzed. The result showed that some of the correlations among the 228 categorized B-Vs were positive and others were negative whether on regional scale or on unit area scale. However, 229 the consistent result showed that the B-V pairs of water conservation with its total, 'carbon fixation and oxygen   including 'carbon fixation and oxygen release' with its relevant species conservation, 'carbon fixation and oxygen 263 release' with its relevant atmosphere purification, nutrient accumulation with its relevant species conservation, and 264 nutrient accumulation with its relevant atmosphere purification (Table 3). Several regressions were given in Fig. 8.

Table 3 Correlation coefficients among the annual B-Vs of afforestation on suitable barren hills and wasteland
Wa-co So-co Cf-Or Nu-ac At-pu Sp-co Total Wa-co So-co Cf-Or Nu-ac At-pu Sp-co Total  (Table 4). 284

296
The unit area B-Vs of species conservation and water conservation in southern regions (e.g. HU-N, Y-N) were 297 higher than the northern regions ( Fig. 3; Fig. 5; Fig. 7). The abundant water and heat resources in southern regions 298 make the forest and other spices easy to make use of the resources for growth and reproduction, and hence the 299 diversity and water-holding ability of these species increased. Only one exception is in HE-B where produced the 300 highest unit area water conservation B-V especially in the 'hillside forest conservation' way. The more plateau 301 sandy land and more shrubs in the northern parts in HE-B, and the more raining in its hot season, make the shrubs 302 easy to develop their stronger function on water conservation in the sandy land that suffers from serious water loss.

303
The larger area of fast-growing forest seems to be the main reason for the highest unit area B-V of nutrient According to Yin (2010), the 'carbon fixation and oxygen release' B-V of mixed forest is significantly higher 307 than that of coniferous forest (e.g. Pinus massoniana Lamb.), and we have the consistent result that HU-N had the 308 lowest 'carbon fixation and oxygen release' B-V but produced the highest atmosphere purification B-V with its 309 more coniferous forest. Probably, the low growth rate of coniferous forests resulted in the lowest 'carbon fixation The rank of classified regional B-Vs in the six monitoring provinces was-'afforestation on suitable barren hills 318 and wasteland' > 'returning cropland to forest' > 'hillside forest conservation'. Whereas, the rank of average unit 319 area total B-Vs was opposite-'hillside forest conservation' > 'returning cropland to forest' > 'afforestation on 'afforestation on suitable barren hills and wasteland' was larger than the other two ways. In fact, this was partly 322 because of its larger area (Fig. 2), and the rank of unit area total B-Vs reflected the reality of the ecosystem service. should be taken into account to achieve joint-win of ecological, social and economic benefits in the process of 328 forest restoration.  showed that water conservation B-V was the main part in total B-V in CCFP, and hence, it makes the water more 359 sensitive to the total B-V.

360
Nutrient accumulation B-V was the least in total B-V and varied in different regions in CCFP. The higher nutrient accumulation B-Vs occurred in the North and Northeast China (Fig. 3, Fig. 5 and Fig. 7). This result might 362 be related to the differences of climate, soil and afforestation species. sequence: water conservation B-V > species conservation B-V > carbon fixation and oxygen release B-V > soil not only in regional but also in unit area (Table 4). ecological service values for the forest should be taken into the analysis. Such as noise reduction, landscape value 370 and so on. We also did not involve them for lack of the detailed materials of these aspects. Duo to the limitations of 371 the level of cognition and study, some points of the discussion are speculative. We are willing to communicate with 372 our peers to improve our research.