ROXAS – an efficient and accurate tool to detect vessels in diffuse-porous species

Wood-anatomical parameters form a valuable archive to study past limitations on tree growth and act as a link between dendrochronology and ecophysiology. Yet, analysing these parameters is a time-consuming procedure and only few long chronologies exist. To increase measurement efficiency of wood-anatomical parameters, novel tools like the automated image-analysis system ROXAS were developed. So far, ROXAS has only been applied to measure large earlywood vessels in ring-porous species. In this study, we evaluate if ROXAS is also suitable for efficient and accurate detection and measurement of vessels in diffuse-porous European beech. To do so, we compared the outcome of ROXAS with that of the established measurement programme Image-Pro Plus in terms of efficiency and accuracy. The two methods differed substantially in efficiency with automatic measurements using ROXAS being 19 times faster than with Image-Pro Plus. Although the procedures led to similar patterns in annual variation of mean vessel area and vessel density, the absolute values differed. Image-Pro Plus measured systematically lower mean vessel areas and higher vessel densities than ROXAS. This was attributed to the species-specific technical settings in ROXAS, leading to more realistic results than those obtained using the default settings in Image-Pro Plus. A shortcoming of ROXAS was, however, that small vessels

Driving factors of a vegetation shift from Scots pine to pubescent oak in dry Alpine forests

An increasing number of studies have reported on forest declines and vegetation shifts triggered by drought. In the Swiss Rhone valley (Valais), one of the driest inner-Alpine regions, the species composition in low elevation forests is changing: The sub-boreal Scots pine (Pinus sylvestris L.) dominating the dry forests is showing high mortality rates. Concurrently the sub-Mediterranean pubescent oak (Quercus pubescens Willd.) has locally increased in abundance. However, it remains unclear whether this local change in species composition is part of a larger-scale vegetation shift. To study variability in mortality and regeneration in these dry forests we analysed data from the Swiss national forest inventory (NFI) on a regular grid between 1983 and 2003, and combined it with annual mortality data from a monitoring site. Pine mortality was found to be highest at low elevation (below 1000 m a.s.l.). Annual variation in pine mortality was correlated with a drought index computed for the summer months prior to observed tree death. A generalized linear mixed-effects model indicated for the NFI data increased pine mortality on dryer sites with high stand competition, particularly for small-diameter trees. Pine regeneration was low in comparison to its occurrence in the overstorey, whereas oak regeneration was comparably abundant. Although both species regenerated well at dry sites, pine regeneration was favoured at cooler sites at higher altitude and oak regeneration was more frequent at warmer sites, indicating a higher adaptation potential of oaks under future warming. Our results thus suggest that an extended shift in species composition is actually occurring in the pine forests in the Valais. The main driving factors are found to be climatic variability, particularly drought, and variability in stand structure and topography. Thus, pine forests at low elevations are developing into oak forests with unknown consequences for these ecosystems and their goods and services

Responses of wood anatomy and carbon isotope composition of Quercus pubescens saplings subjected to two consecutive years of summer drought

International audienceTo withstand and to recover from severe summer drought is crucial for trees, as dry periods are predicted to occur more frequently over the coming decades.* In order to better understand growth-related tree responses to drought, wood formation, vessel characteristics and stable carbon isotope composition (δ13C) in tree rings of Quercus pubescens saplings imposed to two consecutive summer droughts were compared with regularly watered control trees.* In both years, photosynthetic activity was strongly inhibited during the drought periods of five to seven weeks but quickly restored after re-watering, reinitiating wood formation. Stress caused more than a 20% reduction in ring width, a 0.5‰ increase in latewood δ13C and changes in vessels characteristics in both the current year latewood and the next year earlywood. The latewood displayed up to 90% increased hydraulic conductivity than control trees, likely to compensate for a cavitation-induced reduction of water transport.* The earlywood after the first drought year was characterized by more but smaller vessels suggesting the attempt of restoring conductivity while minimizing the risk of hydraulic failure. However, after the second year, the reduction of hydraulic conductivity and the increased δ13C values indicate a structural adjustment towards a reduced growth induced by exhaustion of carbon reserves

Tree-growth analyses to estimate tree species' drought tolerance

Climate change is challenging forestry management and practices. Among other things, tree species with the ability to cope with more extreme climate conditions have to be identified. However, while environmental factors may severely limit tree growth or even cause tree death, assessing a tree species' potential for surviving future aggravated environmental conditions is rather demanding. The aim of this study was to find a tree-ring-based method suitable for identifying very drought-tolerant species, particularly potential substitute species for Scots pine (Pinus sylvestris L.) in Valais. In this inner-Alpine valley, Scots pine used to be the dominating species for dry forests, but today it suffers from high drought-induced mortality. We investigate the growth response of two native tree species, Scots pine and European larch (Larix decidua Mill.), and two non-native species, black pine (Pinus nigra Arnold) and Douglas fir (Pseudotsuga menziesii Mirb. var. menziesii), to drought. This involved analysing how the radial increment of these species responded to increasing water shortage (abandonment of irrigation) and to increasingly frequent drought years. Black pine and Douglas fir are able to cope with drought better than Scots pine and larch, as they show relatively high radial growth even after irrigation has been stopped and a plastic growth response to drought years. European larch does not seem to be able to cope with these dry conditions as it lacks the ability to recover from drought years. The analysis of trees' short-term response to extreme climate events seems to be the most promising and suitable method for detecting how tolerant a tree species is towards drought. However, combining all the methods used in this study provides a complete picture of how water shortage could limit species

Anatomical characteristics and hydrologic signals in tree-rings of oaks (Quercus robur L.)

Q. robur decline in European floodplain forests in recent years seems to be strongly associated with the deteriorating hydrological regime. We investigated the influence of the Krka River flow on tree-ring patterns of Q. robur from the Krakovo floodplain forests (Slovenia) to assess the effect of micro-location conditions on hydrological signals in wood-anatomical characteristics. We selected two groups of Q. robur trees growing at nearby locations with different hydrological conditions, resulting in frequent autumn and spring flooding at the wetter site (=W oaks) but no flooding at the other, drier site (=D oaks). We found differences between the two groups in the anatomical structure of tree-rings; however, ring width proved to be the main variable determining the anatomical structure of oak wood. D and W oaks responded differently to the Krka River flow in the studied period. Radial growth of D oaks was negatively influenced by spring flow, but positively influenced by minimum summer flow. In W oaks, ring width was positively correlated with mean summer flow. Thus, environmental information stored in wood-anatomical features may vary, even within the same forest stand, and largely depends on the micro-environment. Reduced wood increments of D oaks suggest that growth conditions are less favourable, implying a link between the health state of oaks from lowland forest and hydrological conditions. Trees intended for hydrological reconstruction must therefore be carefully selected to avoid the possibility of error and potential loss of information. Anatomical characteristics and hydrological signals in tree-rings of oaks from areas with regular flooding may vary, even within the same forest stand, and largely depends on the micro-environmental conditions

Growth response of Scots pine with different crown transparency status to drought release

Context - One short-term adjustment of trees to drought is the reduction of photosynthetic tissues via leaf shedding. But in conifers, it usually takes several years to fully restore needle mass and assimilation capacity. Aims - This study aims to evaluate whether leaf shedding sustainably damages conifers or if these trees still have the ability to recover from drought with respect to their foliage and wood formation. Methods - An irrigation experiment was established in a mature dry forest to test the growth reactions of Scots pine (Pinus sylvestris L.) differing in crown transparency (low, medium, high) to a drought release by irrigation in comparison with equivalent control trees growing under naturally dry conditions on the same site. Results - Drought and high crown transparency had a combined negative effect on radial tree growth: Control trees with medium to high crown transparencies showed a substantially shorter growth period and a long-lasting growth depression in response to the severe summer drought in 2003. However, all trees benefited from irrigation, irrespective of their crown status, and immediately increased growth in response to irrigation. Conclusion - The progressed drought-induced defoliation seemed to be a weakening factor for trees suffering from drought, but this can be reversed if the water supply is improved