Abstract: Hydraulic testing of isolated sapwood from mature tree trunks is time-consuming and prone to errors, whereas the measurement of compression strength is a standardized and rapid wood technological application. In this study, we aimed to analyze if compression stress perpendicular to the grain relates to hydraulic vulnerability of mature Norway spruce (Picea abies) trunk wood with an expected narrow vulnerability range. The sample-set comprised 52 specimens originating from 34 trees harvested in Sweden. Before mechanical testing, the P 50, i.e., the water potential resulting in 50 % of hydraulic conductivity loss, was estimated on small sapwood beams employing the air injection method. Compression strength perpendicular to the grain was defined as the first peak of a stress-strain curve (peak stress) when the wood is subjected to radial compression. Peak stress ranged between 1,65 MPa and 5,07 MPa, P 50 between -2,98 MPa and -1,98 MPa. We found a good correlation between the peak stress and P 50 (r = 0,80; P < 0,0001). This provides further evidence that peak stress in radial compression and P 50 are both extremely dependent on the characteristics of the “weakest” wood part, i.e., the highly conductive earlywood. We conclude that the radial compression strength is a good proxy for P 50 of mature Norway spruce trunk wood.

RESUMO Processos hidrológicos de povoamentos florestais são predominantemente influenciados pela composição de espécies arbóreas e suas características morfo-fisiológicas. No entanto, existem poucos estudos sobre os padrões anatômicos que determinam o sistema hidráulico de plantas em ecossistemas tropicais. Por isso, nosso objetivo foi o de analisar os padrões da área do xilema ativo em 34 árvores de 26 espécies de uma floresta de terra firme na Amazônia central por meio de imersão em solução de corante. A área do xilema ativo foi relacionada a características autoecológicas das espécies, anatomia da madeira e parâmetros de crescimento (diametro à altura do peito - DAP, altura total e volume total). Equações alométricas exponenciais foram utilizadas para ajustar a área do xilema às variáveis medidas. Características do alburno (área transversal não-uniforme e visibilidade do cerne) variaram significativamente entre e dentro de espécies, apesar de que todas as espécies apresentaram vasos difusos. DAP foi fortemente e não-linearmente correlacionado à área do alburno (R 2 = 0,46; P < 0,001), enquanto diâmetro (P = 0,94) e frequência (P = 0.58) de vasos não apresentaram nenhum grau de relacionamento. O tamanho e forma do alburno foram afetados pela ocorrência de obstrução de poros (tilose) e troncos ocos. Estes padrões sugerem que a área do xilema é influenciada por características intrínsicas de cada espécie, microclima e estágio sucessional dentro do povoamento. Nossos resultados implicam que características individuais de árvores podem fortemente influenciar o transporte de água e, consequentemente, os processos hidrológicos e a quantificação de biomassa do povoamento. Essas caracteristicas deveriam ser consideradas (por exemplo, por meio da coleta de amostras da área do xilema ativo ao longo da área transversal) ao estimar-se a transpiração de uma floresta altamente biodiversa.

ABSTRACT Hydrological processes in forest stands are mainly influenced by tree species composition and morpho-physiological characteristics. Few studies on anatomical patterns that govern plant hydraulics were conducted in tropical forest ecosystems. Thus, we used dye immersion to analyze sapwood area patterns of 34 trees belonging to 26 species from a terra firme forest in the central Brazilian Amazon. The sapwood area was related with wood anatomy and tree size parameters (diameter-at-breast-height - DBH, total height and estimated whole-tree volume). Exponential allometric equations were used to model sapwood area using the biometrical variables measured. Sapwood area traits (cross-section non-uniformity and heartwood visibility) varied significantly among and within species even though all were classified as diffuse porous. DBH was strongly and non-linearly correlated with sapwood area (R 2 = 0.46, P < 0.001), while no correlation was observed with vessel-lumen diameter (P = 0.94) and frequency (P = 0.58). Sapwood area and shape were also affected by the occurrence of vessel obstruction (i.e., tyloses), hollow stems and diseases. Our results suggest that sapwood area patterns and correlated variables are driven by intrinsic species characteristics, microclimate and ecological succession within the stand. We believe that individual tree sapwood characteristics have strong implications over water use, hydrological stand upsaling and biomass quantification. These characteristics should be taken into account (e.g., through a multi-point sampling approach) when estimating forest stand transpiration in a highly biodiverse ecosystem.

We assessed the effect of xylem sap flux on radial CO2 efflux of woody tissue of a tropical trees species growing in the Center of Manaus (Amazonas State, Brazil). An open chamber system was used to constantly measure diurnal changes in CO2 efflux over several days. Xylem sap flux was monitored additionally. We found a strong relationship between temperature and woody tissue respiration rates during night time. CO2 efflux rates were reduced up to 35% during daytime most probably due to vertical water uptake within the tree trunks. The results suggest a distinct daytime depression of the CO2 efflux compared with a night time temperature relationship. Xylem sap flux appears to be a major influence on CO2 efflux rates. The reductions in CO2 efflux will become most distinct during periods with a high evaporative demand and predictions of CO2 efflux by the temperature/flux relation are critical during these periods.

Evaluamos el efecto del flujo de la savia del xilema en la afluencia del CO2 radial en tejidos leñosos de una especie de árbol tropical en el centro de Manaos (Amazonas, Brasil). Un sistema de cámara abierta fue usado constantemente para medir cambios diarios en la afluencia del CO2 durante varios días. Al mismo tiempo, se midió el flujo de la savia del xilema. Encontramos una fuerte relación entre las tazas de temperatura y la respiración del tejido leñoso durante el periodo nocturno. Las tazas de afluencia de CO2 se redujeron hasta un 35% durante el día, probablemente debido a la absorción vertical del agua dentro de los troncos. Los resultados sugieren una depresión de la afluencia del CO2 específica durante el día comparada con la relación de la temperatura durante la noche. El flujo de la savia del dilema parece tener una influencia más fuerte en las tazas de afluencia del CO2. Las reducciones en la afluencia del CO2 serán más marcadas durante periodos con una alta demanda de evaporación y las predicciones de la afluencia del CO2 dadas por la relación de temperatura/flujo son críticas durante dichos periodos.