Does species diversity affect the function of residential greening in the Yangtze River Delta

Abstract

The purpose of this study was to investigate whether high plant species diversity could increase the landscape and ecological function of residential greening in a random selection of nine cities, Ningbo, Shaoxing, Jiaxing, Hangzhou, Nanjing, Zhenjiang, Changzhou, Wuxi and Suzhou, in the Yangtze River delta. A quantitative evaluation system was used to evaluate the landscape and ecological function of green space. The relationship between landscape/ecological function and species diversity was analyzed by linear regression. The results indicated that the average Simpson’s index of the 27 surveyed residential green spaces was 8.74 ± 0.60, while the score for landscape effect/ecological function was 6.25 ± 1.42 and 2.70 ± 0.90 respectively. There was a significant correlation between vegetation species diversity and landscape function, and further investigation indicated that more tree species could have a greater effect than more shrubs and herbs. In contrast, there was no significant correlation between vegetation species diversity and ecological function. Thus, the results suggested that high plant species diversity in the majority of residential areas would only improve landscape, and not ecological, function.

Appendix: Quantitative evaluation system of outdoor greening

The quantitative evaluation system of outdoor greening is built to assess and compare the greening level of certain open space landscapes of residential areas. The system has two hierarchy levels based on the theory of the comprehensive evaluation method, which combined the Analytic Hierarchy Process (abbrev. AHP; Saaty 2003) and the Delphi method (Okoli and Pawlowski 2004). The first level has five indicators of ecological function, landscape function, adaptability, green ratio and greening cost. The former two indicators have different weights and the latter three indicators are adjusted coefficients. The second level has eight indicators belonging to ecological function, and another eight indicators belonging to landscape function.

Then the evaluation model is built up as:

$$ K \, = \, (\beta F \, + \gamma H)NML $$

$$ F = \sum\limits_{{i = 1}}^{8} {A_{i} V_{i} } $$

$$ H = \sum\limits_{j = 1}^{8} {B_{j} W_{j} } $$

in which: K is the index of comprehensive evaluation; β is the weight of ecological function; F is the index value of evaluation function; γ is the weight of landscape function; H is the index value of landscape function; N, M, L are adjusted coefficient values of adaptability, green ratio (GR) and greening cost, respectively; i is the sequence number of second level of ecological function; A _i is the index value of the i-th indicator of ecological function; V _i is the weight of the i-th indicator; j is the sequence number of second level of landscape function; B _i is the index value of the j-th indicator of landscape function; and W _i is the weight of the j-th indicator.

The index value of ecological function of plant community follows the following steps: (1) to build the plant resource information bank that has been built up by Shanghai Botanical Garden. (2) To grade qualitatively ecological functions of individual plants based on the bank. (3) To quantify the qualitative indicators on a 10-point system with 6 grades: 10 (better), 8 (good), 6 (general), 4 (bad), 2 (worse) and 0 (zero). (4) To convert the individual plant values into community value by the following formula:

$$ S = \sum\limits_{i = 1}^{3} {EiGi} = E_{1} G_{1} + E_{2} G_{2} + E_{3} G_{3} $$

$$ E_{i} = \sum\limits_{{j = 1}}^{n} {E_{{ij}} Q_{{ij}} } = E_{{i1}} Q_{{i1}} + E_{{i2}} Q_{{i2}} + \cdots + E_{{in}} Q_{{in}} $$

in which S is the value of community ecological function; i is the sequence number of community layers (1: tree layer, 2: shrub layer, 3: herb layer); E _i is the means of ecological function of i-layer; G _i is the cover value of i-layer; n is the number of plant species of i-layer; j is the sequence number of plant species of i-layer; E _ij is the value of ecological function of j-species at i-layer; G _ij is the relative cover value of j-species at i-layer.

Among the second level of landscape function, the former four indicators are quantitative, assigned with Simpson index and then multiplied by 10, while the latter four are qualitative to be assigned by the 10-point system by experts.

The three adjusted coefficient indicators are all assigned as 0–1. Specifically, N = 1 when all surveyed plants show fine adaptability; N = 1 − percentage of general plants when partly plants show general adaptability; N = 1 − 3 × percentage of bad plants when partly plants show bad adaptability. M = 1 when GR ≥ 35 %; M = GR/0.35 when GR 30–35 %; M = 0 when GR <30 %. L is ratio of benchmark cost/actual cost, except it is 1 when the ratio >1. Benchmark cost means construction cost and annual maintenance cost of local standard green space, while actual cost means construction cost and annual maintenance cost of the green space of the object being evaluated.