Summary1. The long-standing view that biomass growth in trees typically follows a rise-and-fall unimodal pattern has been challenged by studies concluding that biomass growth increases with size even among the largest stems in both closed forests and in open competition-free environments. We highlight challenges and pitfalls that influence such interpretations. 2. The ability to observe and calibrate biomass change in large stems requires adequate data regarding these specific stems. 3. Data checking and control procedures can bias estimates of biomass growth and generate false increases with stem size. 4. It is important to distinguish aggregate and individual-level trends: a failure to do so results in flawed interpretations. 5. Our assessment of biomass growth in 706 tropical forest stems indicates that individual biomass growth patterns often plateau for extended periods, with no significant difference in the number of stems indicating positive and negative trends in all but one of the 14 species. Nonetheless, when comparing aggregate growth during the most recent five years, 13 out of our 14 species indicate that biomass growth increases with size even among the largest sizes. Thus, individual and aggregate patterns of biomass growth with size are distinct. 6. Claims concerning general biomass growth patterns for large trees remain unconvincing. We suggest how future studies can improve our knowledge of growth patterns in and among large trees.

The interpretation and communication of fire danger warning levels based on fire weather index values are critical for fire management activities. A number of different indices have been developed for various environmental conditions, and many of them are currently applied in operational warning systems. To select an appropriate combination of such indices to work in different ecoregions in mountainous, hilly and flat terrain is challenging. This study analyses the performance of a total of 22 fire weather indices and two raw meteorological variables to predict wildfire occurrence for different ecological regions of Austria with respect to the different characteristics in climate and fire regimes. A median-based linear model was built based on percentile results on fire days and non-fire days to get quantifiable measures of index performance using slope and intercept of an index on fire days. We highlight the finding that one single index is not optimal for all Austrian regions in both summer and winter fire seasons. The summer season (May–November) shows that the Canadian build-up index, the Keetch Byram Drought Index and the mean daily temperature have the best performance; in the winter season (December–April), the M68dwd is the best performing index. It is shown that the index performance on fire days where larger fires appeared is better and that the uncertainties related to the location of the meteorological station can influence the overall results. A proposal for the selection of the best performing fire weather indices for each Austrian ecoregion is made.

Abstract. Over the past decade, several methods have been used to compare the performance of fire danger indices in an effort to find the most appropriate indices for particular regions or circumstances. Various authors have proposed comparators and demonstrated different responses of indices to their tests, but rarely has much effort been put into demonstrating the validity of the comparators themselves. We present a demonstration that many of the published comparators are sensitive to the different frequency distributions, that may be inherent in the performance of the different indices, and outline a non-parametric method that may be useful for future work. We compare four hypothetical fire danger indices, three of which are simple mathematical transformations of each other. The hypothesis tested is that the comparators often used in such studies may indicate spurious performance differences between these indices, which is found to be the case. Non-parametric methods are robust to differences in index value frequency distribution and may allow more valid comparisons of fire danger indices. The new comparison method is shown to have advantages over other non-parametric comparators.

The aim of this paper is to determine whether a detectable impact of climate change is apparent in Austrian forests. In regions of complex terrain such as most of Austria, climatic trends over the past 50 years show marked geographic variability. As climate is one of the key drivers of forest growth, a comparison of growth characteristics between regions with different trends in temperature and precipitation can give insights into the impact of climatic change on forests. This study uses data from several hundred climate recording stations, interpolated to measurement sites of the Austrian National Forest Inventory (NFI). Austria as a whole shows a warming trend over the past 50 years and little overall change in precipitation. The warming trends, however, vary considerably across certain regions and regional precipitation trends vary widely in both directions, which cancel out on the national scale These differences allow the delineation of 'climatic change zones' with internally consistent climatic trends that differ from other zones. This study applies the species-specific adaptation of the biogeochemical model BIOME-BGC to Norway spruce (Picea abies (L.) Karst) across a range of Austrian climatic change zones, using input data from a number of national databases. The relative influence of extant climate change on forest growth is quantified, and compared with the far greater impact of non-climatic factors. At the national scale, climate change is found to have negligible effect on Norway spruce productivity, due in part to opposing effects at the regional level. The magnitudes of the modeled non-climatic influences on aboveground woody biomass increment increases are consistent with previously reported values of 20-40 kg of added stem carbon sequestration per kilogram of additional nitrogen deposition, while climate responses are of a magnitude difficult to detect in NFI data.

© iForest -Biogeosciences and Forestry IntroductionFuture impacts of global climate change such as predicted increases in annual air and surface temperatures and variations in pre cipitation will cause significant alterations in forest ecosystems (Eastaugh 2008). Impacts of these changes will be proportionally more perceptible at high elevations (Bensiton et al. 1997).In timberline ecotones near the upper limit of closed forests tree growth, forest structure and forest dynamics are mainly temperaturedriven (Tranquillini 1979, Innes 1991, Körner 1998. The sensitivity of these biomes to climate variability is high and thus of special interest for understanding the ef fects of global change.Swiss stone pine (Pinus cembra L.) is dis tributed in timberline ecotones across Europe from the Carpathian Mountains to the French Alps (Polunin & Walters 1986, Ulber et al. 2004. During several hundred years of hu man activities such as alpine farming or tim ber extraction Swiss stone pine was often eliminated and therefore restricted to stands on inaccessible slopes exposed to the North (Holtmeier 1966, Motta & Nola 2001, Höhn et al. 2009). In recent decades, socio-eco nomic and silvicultural changes have fa voured the establishment of Swiss stone pine (Motta et al. 2006). This five-needled conifer tree is well adapted to the harsh subalpine climate conditions in the Central European Alps (Ulber et al. 2004) and is often asso ciated with mountain pine (Pinus montana Miller), Scots pine (Pinus sylvestris L.), European larch (Larix decidua Miller) and Norway spruce (Picea abies L. Karst). In the continental subalpine forests of the Central Alps with relatively low rainfall and mean annual temperatures below 1.5 °C (Ellenberg 1996), stands develop from early-successio nal stages dominated by mountain pine to a late-successional stage dominated by Swiss stone pine and European Larch (Risch et al. 2003, Höhn et al. 2009).This review will summarize the evidence of Swiss stone pine responses to climate change at the timberline ecotone. The review will consider all life stages, and possible dis tribution shifts of Swiss stone pine popula tions in the future will be discussed. Seedling establishmentSwiss stone pine is a monoecious, wind pollinated species which reaches reproduc tive maturity at 40-60 years of age (Ulber et al. 2004) with good seed production years occurring on average twice in ten years (Mattes 1982). Seed production is especially sensitive to climate because important deve lopmental processes such as the initiation of flower and cone primordia, meiosis and the release of pollen depend to a large degree on climatic variables (Pigott 1992). The tempo ral dynamics of seed production and the in fluence of climate change on seed produc tion of Swiss stone pine have however not been comprehensively investigated to date.The wingless Swiss stone pine seeds are mainly dispersed by the European nutcracker bird (Nucifraga caryocatactes L.). Swiss stone pine seeds are the main food source for European nutcrackers, and those bird...

Scaling is widely recognized as a central issue in ecology. The associated cross-scale interactions and process transmutations make scaling (i.e., a change in spatial or temporal grain and extent) an important issue in understanding ecosystem structure and functioning. Moreover, current concepts of ecosystem stewardship, such as sustainability and resilience, are inherently scale-dependent. The importance of scale and scaling in the context of forest management is likely to further increase in the future because of the growing relevance of ecosystem services beyond timber production. As a result, a consideration of processes both below (e.g., leaf-level carbon uptake in the context of climate change mitigation) and above (e.g., managing for biodiversity conservation at the landscape scale) the traditional focus on the stand level is required in forest ecosystem management. Furthermore, climate change will affect a variety of ecosystem processes across scales, ranging from photosynthesis (tree organs) to disturbance regimes (landscape scale). Assessing potential climate change impacts on ecosystem services thus requires a multi-scale perspective. However, scaling issues have received comparatively little attention in the forest management community to date. Our objectives here are thus first, to synthesize scaling issues relevant to forest management, and second, to elucidate ways of dealing with such complex scaling problems by highlighting examples of how they can be addressed with ecosystem models. We have focused on three current management issues of particular importance in European forestry: (i) climate change mitigation through carbon sequestration, (ii) multi-functional stand management for biodiversity

Extreme temperature events are known to favor large wildland fires. It is expected that fire activity will increase with changing climate. This work analyzes the effects of hightemperature days on medium and large fires (those larger than 50 ha) from 1978 to 2010 in Spain. A high-temperature day was defined as being when air temperature at 850 hPa was higher than the 95th percentile of air temperature at that elevation from June to September across the years 1978-2010. Temperature at 850 hPa was chosen because it properly characterizes the state of the lower troposphere. The effects of high temperature on forest fires were remarkable and significant in terms of fire number (15 % of total large fires occurred under high-temperature days) and burned area (25 % of the total burned area occurred under high-temperature days). Fire size was also significantly higher under the 95th percentile air temperature at 850 hPa, and a large part of the largest fires in the past 20 years were under these extreme conditions. Additionally, both burned area and fire number only decreased under non-high-temperature days in the study period and not under high-temperature conditions.

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