Correlations between Tree/Shrub Diversity and Herbaceous Biomass with Soil Physico-chemical Properties under Acacia saligna Canopy
Authors: Nigusse Redae , Gebretsion Kahsu 
Published: October 2024

Acacia saligna (A. saligna) is originated from South Western Australia, and it was brought to the Tigray region of Ethiopia in 1972 with the intention of restoring the ecosystem and conserving soil and water. This study aims to evaluate the correlations between tree/shrub diversity and herbaceous biomass with some soil physico-chemical properties under Acacia saligna canopy and away from the canopy in Atsibi Wemberta district, Tigray, Ethiopia. For collecting of vegetation data, herbaceous cover and soil sample, twelve A.saligna tree stands were used as a replication.Woody species identification, herbaceous cover and biomass production were collected under the canopy of A.saligna and 50 m away from the canopy at 1 m2 for woody species, and 0.25 m2 for herbaceous cover, and biomass production. Soil samples were taken using auger at 20 cm soil depth under the canopy and in the canopy gap A.saligna. The species richness under the canopy of A.saligna was positively insignificant correlated with pH (r = 0.48, p = 0.11), availability of phosphorus (Av.P) (r = 0.16, p = 0.63), total nitrogen (TN (r = 0.12, p = 0.7) and Silt (r = 0.71, p = 0.009). However, it is also negatively correlated with soil organic carbon (SOC (r = -0.06, p = 0.85), and sand (r = -0.77, p = 0.003). The Pearson’s correlation matrix in the canopy gap of A.saligna showed that, Shannon diversity index is insignificantly positively correlated with SOC (r = 0.19, p = 0.53), Av.P (r = 0.46, p = 0.12), availability of potassium (Av.K) (r = 0.43, p = 0.16) and TN (r = 0.13, p = 0.69). However, species richness and Shannon diversity index have insignificantly negatively correlated with pH(r = -0.13, p = 0.68, r = -0.15, p = 0.63) and sand (r = -0.23, p = 0.51, r = -0.44, p = 0.14), respectively.  In the canopy gap of A.saligna, the herbaceous biomass was positively correlated with Av.K (r = 0.47, p = 0.12), silt (r = 0.39, p = 0.20), and clay( r = 0.37, p = 0.24), but the Pearson’s correlation matrix between herbaceous biomass and some soil parameters in the canopy gap  is observed to be insignificantly negatively correlated with pH (r = -0.24, p = 0.44), SOC (r = -0.04, p = 0.9), Av.P (r = -0.34, p = 0.28), and TN (r = -0.33, p = 0.31).The species richness, Shannon diversity indexes and herbaceous biomass were insignificantly positively correlated with pH under the canopy of A.saligna, but for similar parameters in the canopy gap was observed negatively insignificant correlated with pH. Overall, the small positive correlation found in the current study between same soil physico chemical and diversity indexes under the A. saligna canopy suggests that the presence of these large A. saligna trees may increase soil nutrient availability through litterfall. The study found clear relationships between plant density and Shannon diversity with herbaceous biomass in Acacia saligna's canopy gap and beneath its canopy. In particular, both settings showed a substantial negative connection between herbaceous biomass and plant density, suggesting that increased plant density tends to diminish herbaceous biomass. On the other hand, a strong positive association was seen between Shannon diversity and herbaceous biomass, indicating that higher species diversity is linked to higher herbaceous biomass. Therefore, it is imperative to manage and conserve A. saligna and the woody species growing beneath the canopy in order to preserve and strengthen the favorable connections that exist between diversity indexes and the physical and chemical characteristics of soil.