Weather Conditions Drive the Damage Area Caused by Armillaria Root Disease in Coniferous Forests across Poland.

Armillaria root disease affects forests around the world. It occurs in many habitats and causes losses in the infested stands. Weather conditions are important factors for growth and development of Armillaria species. Yet, the relation between occurrence of damage caused by Armillaria disease and weather variables are still poorly understood. Thus, we used generalized linear mixed models to determine the relationship between weather conditions of current and previous year (temperature, precipitation and their deviation from long-term averages, air humidity and soil temperature) and the incidence of Armillaria-induced damage in young (up to 20 years old) and older (over 20 years old) coniferous stands in selected forest districts across Poland. We used unique data, gathered over the course of 23 years (1987-2009) on tree damage incidence from Armillaria root disease and meteorological parameters from the 24-year period (1986-2009) to reflect the dynamics of damage occurrence and weather conditions. Weather parameters were better predictors of damage caused by Armillaria disease in younger stands than in older ones. The strongest predictor was soil temperature, especially that of the previous year growing season and the current year spring. We found that temperature and precipitation of different seasons in previous year had more pronounced effect on the young stand area affected by Armillaria. Each stand's age class was characterized by a different set of meteorological parameters that explained the area of disease occurrence. Moreover, forest district was included in all models and thus, was an important variable in explaining the stand area affected by Armillaria.

When and why do trees begin to decrease their resource allocation to apical growth? The importance of the reproductive onset.

Trees reduce their allocation to apical growth compared with that to radial growth during later life stages. This has often been attributed to hydraulic and mechanical limitations; however, a growth-reproduction tradeoff might also play an important role. To test whether the height-diameter (H-D) relationship in trees changes with the timing of the onset of reproduction, we analyzed the H-D relationship of 13 deciduous broad-leaved tree species in a Japanese old-growth temperate forest using linear and segmented-linear regression models. These models showed a better fit than common continuous models (simple allometry and saturating curve). For 11 out of 13 species having break points on the H-D lines, we assessed whether the height at the breakpoint (BPH) was related to the height at the onset of reproduction determined by multiyear observatory records on several trees. Although BPH estimates for these 11 species were highly variable, most were within 3.1 m of the stable flowering height (staFLH), or the height at which trees achieve the ability to flower regardless of their growth conditions. The interspecies mean value of the difference between staFLH and BPH was only 45 cm, which suggested a causal relationship between these parameters. BPHs of nine out of 11 species were within the canopy layer and, for the two remaining species, were within the subcanopy layer. These results suggest that several species in this forest begin to reduce their allocation to apical growth around the canopy layer at the onset of stable reproduction.