Drought-induced tree death has become a serious problem in global forest ecosystems. Two nonexclusive hypotheses, hydraulic failure and carbon starvation, have been proposed to explain tree die-offs. To clarify the mechanisms, we investigated the physiological processes of drought-induced tree death in saplings with contrasting Huber values (sapwood area/total leaf area). First, hydraulic failure and reduced respiration were found in the initial process of tree decline, and in the last stage carbon starvation led to tree death. The carbohydrate reserves at the stem bases, low in healthy trees, accumulated at the beginning of the declining process due to phloem transport failure, and then decreased just before dying. The concentrations of non-structural carbohydrates at the stem bases are a good indicator of tree damage. The physiological processes and carbon sink-source dynamics that occur during lethal drought provide important insights into the adaptive measures underlying forest die-offs under global warming conditions.

Old-growth forests consist of various types of small patches that reflect their own gap-forming process, which includes changes in environmental conditions occurring over several decades. We reconstructed the gap-forming processes that had occurred during a 40-year period for eight representative patches of an oldgrowth evergreen broad-leaved forest in Japan, and examined the current community structure. The selected patches were based on (1) changes in canopy heights estimated from aerial photographs taken in four different years, (2) long-term ecological research (LTER) monitoring records, and (3) a recent field survey, so that they sufficiently covered characteristic gap-forming processes such as a new gap, an old gap and consistently closed canopy. Height and diameter at breast height (DBH) were measured on all living trees taller than 1.3 m. In their height distributions, currently almost closed patches that were open in 1966 show a rotated sigmoid, whereas their DBH distributions are an inverse J-shape. In contrast, patches that have been consistently under a closed canopy exhibit gentle inverse J-shapes for both distributions. For species composition, there are no clear contrasts associated with the past gap-forming processes except for the existence of fast-growing deciduous species in large currently open patches. Our results suggested that the variation in several decades of gap-forming processes played a central role in the high patch diversity and the complex patch mosaic of the forest. Diverse gap-forming processes created microenvironmental heterogeneity both vertically and horizontally, and contributed to the maintenance of the species-rich, warm-temperate old-growth forest.

Matrix models have been widely used to investigate the population dynamics of plant species. To make use of this method, we first divide individuals into groups and estimate transition probabilities per pair of groups. When a continuous variable, such as plant size, is used for grouping, there is often a trade-off: if the class intervals are narrow each group will only include a small number of samples, but if the intervals are wider, this may obscure some changes. This paper introduces a new matrix model in which we no longer have to divide individuals into arbitrarily defined size classes. The methodology is based on the Bayesian non-parametric binary regression. We first divide the data into 'very fine' intervals. For estimating transition probabilities in a 'large' matrix, we do not use the observed transition rate per class directly, but we smooth neighboring observed rates and select the most appropriate degree of smoothing using an information criterion called the Akaike Bayesian Information Criterion (ABIC). Our approach is illustrated using longterm forest monitoring data from an old-growth, warm-temperate evergreen forest, in which we examined the population dynamics of four evergreen subcanopy tree species. Transition probabilities allowed us to represent d.b.h.-related growth and mortality patterns graphically, and matrix analysis provided stable size distributions, reproductive values and elasticity that vary smoothly for trees of different sizes. The quantitative approach makes it possible to determine characteristic patterns of population dynamics for qualitatively similar species.

The frequency of extreme weather has been rising in recent years. A 3-year study of street trees was undertaken in Tokyo to determine whether: (i) street trees suffer from severe water stress in unusually hot summer; (ii) species respond differently to such climatic fluctuations; and (iii) street trees are also affected by nitrogen (N) deficiency, photoinhibition and aerosol pollution. During the study period (2010-12), midsummers of 2010 and 2012 were unusually hot (2.4-2.8 °C higher maximum temperature than the long-term mean) and dry (6-56% precipitation of the mean). In all species, street trees exhibited substantially decreased photosynthetic rate in the extremely hot summer in 2012 compared with the average summer in 2011. However, because of a more conservative stomatal regulation (stomatal closure at higher leaf water potential) in the hot summer, apparent symptoms of hydraulic failure were not observed in street trees even in 2012. Compared with Prunus × yedoensis and Zelkova serrata, Ginkgo biloba, a gymnosperm, was high in stomatal conductance and midday leaf water potential even under street conditions in the unusually hot summer, suggesting that the species had higher drought resistance than the other species and was less susceptible to urban street conditions. This lower susceptibility might be ascribed to the combination of higher soil-to-leaf hydraulic conductance and more conservative water use. Aside from meteorological conditions, N deficiency affected street trees significantly, whereas photoinhibition and aerosol pollution had little effect. The internal CO2 and δ(13)C suggested that both water and N limited the net photosynthetic rate of street trees simultaneously, but water was more limiting. From these results, we concluded that the potential risk of hydraulic failure caused by climatic extremes could be low in urban street trees in temperate regions. However, the size of the safety margin might be different between species.

Developmental processes and biomass allocation of a multi-stemmed shrub, Stephanandra incisa , were measured to reveal the advantages of a multi-stemmed growth form on matter production. Stephanandra incisa is a low shrub species that mainly grows in the forest understory. The current-year shoots become shorter by yearly branching that effectively increases the allocation ratio to photosynthetic organs. The frequency distribution of relative current-year shoot length (the ratio of current-year shoot length to stem length) in each ramet (a stem and its crown) of a clump (genet) were skewed to larger length classes in younger ramets and gradually skewed to shorter length classes in older ramets. The biomass allocation ratio of non-photosynthetic organs to photosynthetic organs in ramets changed from a declining trend to a rising trend as the age of the ramets increased. The biomass allocation ratios of ramets and clumps increased with their size, but the ratio of clumps showed lower values than that of ramets of the same size. The age and size structures of ramets in clumps contributed to the narrow variability of the biomass allocation ratio. Stephanandra incisa showed a high biomass allocation ratio to leaves and maintained the ratio with a multi-stemmed growth form.