Dieback intensity but not functional and taxonomic diversity indices predict forest productivity in different management conditions: Evidence from a semi-arid oak forest ecosystem

Abstract

The relationships between different aspects of diversity (taxonomic, structural and functional) and the aboveground biomass (AGB) as a major component of global carbon balance have been studied extensively but rarely under the simultaneous influence of forest dieback and management. In this study, we investigate the relationships between taxonomic, functional and structural diversity of woody species (trees and shrubs) and AGB along a gradient of dieback intensity (low, moderate, high and no dieback as control) under two contrasted management conditions (protection by central government vs. traditional management by natives) in a semi-arid oak (Quercus brantii Lindl.) forest ecosystem. AGB was estimated and taxonomic diversity, community weighted average (CWM) and functional divergence indices were produced. We found that the aerial biomass was significantly higher in the intensively used area (14.57 (±1.60) t/hm 2 ) than in the protected area (8.70 (±1.05) t/hm 2 ) due to persistence of some large trees but with decreasing values along the dieback intensity gradient in both areas. CWM of height (H), leaf nitrogen content (LNC) and leaf dry matter content (LDMC) were also higher in the traditional managed area than in the protected area. In contrast, in the protected area, the woody species diversity was higher and the inter-specific competition was more intense, explaining a reduced H, biomass and LDMC. Contrary to the results of CWM, none of the functional diversity traits (FDvar) was affected by dieback intensity and only FDvar values of LNC, leaf phosphorus content (LPC) and LDMC were influenced by management. We also found significantly positive linear relationships of AGB with CWM and FDvar indices in the protected area, and with taxonomic and structural diversity indices in the traditional managed area. These results emphasize that along a dieback intensity gradient, the leaf functional traits are efficient predictors in estimating the AGB in protected forests, while taxonomic and structural indices provide better results in forests under a high human pressure. Finally, species identity of the dominant species (i.e., Brant's oak) proves to be the main driver of AGB, supporting the selection effect hypothesis.

Tree dieback, woody plant diversity, and ecosystem driven by topography in semi-arid mountain forests: Implication for ecosystem management

“…Our observations are in line with Karami et al (2022) who reported the highest rate of oak tree dieback in forest stands with a higher admixture index, as spatial diversity or dissimilarity of tree species in semi-arid areas. Moreover, crown size (Fan et al, 2008), forest structure, and ecological conditions such as soil properties (Turner and Lambert, 2005;Kabrick et al 2008;Gazol et al, 2018;Cunningham et al, 2010) have also proven to be effective in the occurrence of tree dieback.…”

“…The dieback severity was estimated based on visual criteria of dieback in each plot according to the field method developed by Karami et al (2022), such as, were recorded the important signs of decline, including the separation of the trunk bark and the sap outflow, the color change of leaves, and the partial or complete drying of the crown.…”

“…Forest dieback is often more severe in high-density stands due to between trees competition for resources (Mensforth et al, 1994;Fensham and Fairfax, 2007). Along a gradient of dieback severity, taxonomic diversity and stand structure are highly correlated (Karami et al 2022). However, tree dieback could be not related to stand characteristics, but to the location in floodplain, where dieback decreased with increasing water availability (Cunningham et al, 2010) underscoring the role of site quality.…”

Modeling Climate Change Effects on the Distribution of Oak Forests with Machine Learning

“…Climate change has negatively affected these forests by increasing temperature and decreasing rainfall [35]. The occurrence of oak decline and secondary pests and diseases in this region in recent decades, according to many researchers, has been associated with the phenomenon of climate change [36][37][38]. In addition, human activities such as land use change, wood harvesting and livestock grazing have destroyed large parts of these forest ecosystems [34,39,40].…”