Disentangling the Effect of Management Decisions and Emergent Forest Structure on Forest Structure Heterogeneity

Abstract

Contemporary ecological silviculture aims to conserve old-growth elements and improve forest heterogeneity, increase forest resilience and conserve biodiversity and other forest-related ecosystem services. Forest resilience and ecosystem services are influenced by forest structure resulting from management and from emerging forest structures. In turn, forest structure is affected by the interaction of management and environmental conditions. However, effects of direct management versus emergent structures on forest heterogeneity remain difficult to disentangle.  In this study, we compared the effect of stand-level characteristics under direct management to emergent forest structure variables (i.e. variables that result from indirect management) with respect to their impact on forest heterogeneity in the Black Forest (southwest Germany). We investigated parameters targeted by management, such as the number and composition of tree species, their density, deadwood accumulation and tree-related micro-habitats. The number of canopy layers, light conditions, and tree diameter at breast height are emerging from forest management. Forest heterogeneity was measured as the standard deviation of all these measured variables. We analyzed the three underlying factor complexes (direct forest management, emergent forest structures and forest heterogeneity) and their relationships using latent variables in a structural equation model. Then, we split the data to quantify the effects of forest management  for younger forest stands and more mature forest stands  (based on tree DBH) seperately . Our results show that overall direct management had a stronger influence on forest heterogeneity than emergent forest structures. The selection of broad leaf tree species and tree-species diversity resulted in a higher overall forest heterogeneity. In contrast, we found a stronger indirect management effect on heterogeneity for the younger forests stands: effective number of layers variation was quite strong, but originated from the last harvesting intervention.We also found that selection of broad-leaved tree species due to their growth patterns have a more substantial effect on forest heterogeneity in the mature forest subset. Our study highlights the importance of long-term influences of forest management as tree species selection plays a major role in developing a high level of forest heterogeneity in temperate managed forests.