Ecological connectivity in fragmented agricultural landscapes and the importance of scattered trees and small patches

Background:Fragmentation and habitat loss can restrict species movement and reduce connectivity,negatively impacting biodiversity.Characterising the overall connectivity of an area can inform better management of human modified landscapes.Contemporary connectivity modelling methods seldom incorporate fine-scale movement patterns associated with movement between fine-scaled structural connectivity elements such as scattered trees,roadside corridors and small patches of habitat.This study aims to characterise connectivity within the Karuah-Myall catchments,a typical woodland ecosystem that is fragmented by agriculture,using least-cost path analysis and a graph-theoretic approach;it focuses on how fine-scaled vegetation such as scattered trees support connectivity.We mapped scattered(and paddock)trees within this agricultural landscape where the main human modified land use was pasture.We modelled connectivity for a general representative woodland species using an interpatch dispersal distance and gap crossing threshold,and resistance from different land cover types.The gap crossing distance threshold was used to model movement between fine-scaled vegetation features.We compared the least-cost paths modelled with and without scattered trees.Results:Our results show that by excluding scattered trees,least-cost paths across the cleared pasture landscape did not reflect the types of movement patterns typically observed from field studies,such as those associated with a foray-search strategy used by small and medium mammals and birds.The modelling also shows that the KaruahMyall catchments are well connected and provide value to biodiversity beyond the catchment borders,by connecting coastal vegetation to the Great Eastern Ranges national wildlife corridor initiative.Conclusion:Connectivity models that exclude fine-scale landscape features such as scattered trees and small,linear patches risk misrepresenting connectivity patterns.Models of regional-scale connectivity can be influenced by the presence or absence of even the smallest features,such as scattered trees.

Dispersed trees on smallholder farms enhance soil fertility in semi-arid Ethiopia

Introduction:Dispersed trees such as Oxytenanthera abyssinica(A.Rich.)and Dalbergia melanoxylon(Guill.&Perr.)which are objectively maintained or planted on farmland provide a significant contribution to soil fertility improvement.However,there was no quantitative information on the level of soil nutrient additions of these trees to the soil system.Methods:This study was conducted on the farmers’fields in Kafta Humera district,Tigray region(northern Ethiopia),where mature stands of O.abyssinica and D.melanoxylon trees exist.Radial distance-based soil sampling(under the canopy,near to canopy,and far from canopy)was adopted to quantify the role of these trees on soil fertility improvement.Soil parameters tested were soil reaction(pH),total nitrogen(TN),available phosphorus(AvP),electrical conductivity(EC),cation exchange capacity(CEC),and organic carbon(OC).Results:There was a negative linear relationship between the radial distance of the O.abyssinica tree trunk and soil TN,OC,CEC,and AvP contents but not for pH.Similarly,negative linear relationship between distance from D.melanoxylon and TN,OC,and AvP was obtained.The average total nitrogen(0.26%and 0.13%),available phosphorus(7.21 ppm and 6.37 ppm),and organic carbon(1.73%and 1.02%)contents were respectively higher under the tree canopies of O.abyssinica and D.melanoxylon compared with the adjacent open canopies.The amount of soil OC,TN,AvP,and CEC under O.abyssinica tree species was also significantly higher by 69%,100%,13%,and 42%compared to that of D.melanoxylon tree species.However,the amount of EC and soil pH was significantly lower by 57%and 19%,respectively.Conclusion:In general,O.abyssinica and D.melanoxylon added a significant amount of nutrients to the soil.Thus,retaining these important tree species on farmland played a positive role in replenishing soil fertility for resource-constrained households so as to reduce chemical fertilizer amendments.