Comparative physiology of canopy tree leaves in evergreen and deciduous forests in lowland Thailand

The typical seasonally dry forests in Southeast Asia are the mixed deciduous forest (MDF), dry dipterocarp (deciduous) forest (DDF), and dry evergreen forest (DEF). We obtained 21 physiological traits in the top/sunlit leaves of 107, 65 and 51 tree species in MDF, DEF and DDF, respectively. Approximately 70%, 95% and 95% of canopy tree species which consist of MDF, DEF and DDF are sampled, respectively. Light-saturated photosynthetic rates (Asat) exhibit a positive correlation with foliar nitrogen (N) and phosphorus (P) on leaf mass and area bases across tree species. Decreased leaf mass-based P reduces the positive slope of the mass-based N and Asat relationship across species and habitats. The differences in nutrient and water use and leaf habits are well matched to the variation in soil properties among the forest types, highlighting the reliability of this comprehensive database for revealing the mechanism of niche segregation based on edaphic factors.

Parenchyma underlies the interspecific variation of xylem hydraulics and carbon storage across 15 woody species on a subtropical island in Japan

Parenchyma is an important component of the secondary xylem. It has multiple functions and its fraction is known to vary substantially across angiosperm species. However, the physiological significance of this variation is not yet fully understood. Here, we examined how different types of parenchyma (ray parenchyma [RP], axial parenchyma [AP] and AP in direct contact with vessels [APV]) are coordinated with three essential xylem functions: water conduction, storage of non-structural carbohydrate (NSC) and mechanical support. Using branch sapwood of 15 co-occurring drought-adapted woody species from the subtropical Bonin Islands, Japan, we quantified 10 xylem anatomical traits and examined their linkages to hydraulic properties, storage of soluble sugars and starch and sapwood density. The fractions of APV and AP in the xylem transverse sections were positively correlated with the percentage loss of conductivity in the native condition, whereas that of RP was negatively correlated with the maximum conductivity across species. Axial and ray parenchyma fractions were positively associated with concentrations of starch and NSC. The fraction of parenchyma was independent of sapwood density, regardless of parenchyma type. We also identified a negative relationship between hydraulic conductivity and NSC storage and sapwood density, mirroring the negative relationship between the fractions of parenchyma and vessels. These results suggest that parenchyma fraction underlies species variation in xylem hydraulic and carbon use strategies, wherein xylem with a high fraction of AP may adopt an embolism repair strategy through an increased starch storage with low cavitation resistance.

Evolutionary loss of thermal acclimation accompanied by periodic monocarpic mass flowering in Strobilanthes flexicaulis

While life history, physiology and molecular phylogeny in plants have been widely studied, understanding how physiology changes with the evolution of life history change remains largely unknown. In two closely related understory Strobilanthes plants, the molecular phylogeny has previously shown that the monocarpic 6-year masting S. flexicaulis have evolved from a polycarpic perennial, represented by the basal clade S. tashiroi. The polycarpic S. tashiroi exhibited seasonal thermal acclimation with increased leaf respiratory and photosynthetic metabolism in winter, whereas the monocarpic S. flexicaulis showed no thermal acclimation. The monocarpic S. flexicaulis required rapid height growth after germination under high intraspecific competition, and the respiration and N allocation were biased toward nonphotosynthetic tissues. By contrast, in the long-lived polycarpic S. tashiroi, these allocations were biased toward photosynthetic tissues. The life-history differences between the monocarpic S. flexicaulis and the polycarpic S. tashiroi are represented by the “height growth” and “assimilation” paradigms, respectively, which are controlled by different patterns of respiration and nitrogen regulation in leaves. The obtained data indicate that the monocarpic S. flexicaulis with the evolutionary loss of thermal acclimation may exhibit increased vulnerability to global warming.

Initial hydraulic failure followed by late-stage carbon starvation leads to drought-induced death in the tree Trema orientalis

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.

Physiological mechanisms of drought-induced tree die-off in relation to carbon, hydraulic and respiratory stress in a drought-tolerant woody plant

Drought-induced tree die-off related to climate change is occurring worldwide and affects the carbon stocks and biodiversity in forest ecosystems. Hydraulic failure and carbon starvation are two commonly proposed mechanisms for drought-induced tree die-off. Here, we show that inhibited branchlet respiration and soil-to-leaf hydraulic conductance, likely caused by cell damage, occur prior to hydraulic failure (xylem embolism) and carbon starvation (exhaustion of stored carbon in sapwood) in a drought-tolerant woody species, Rhaphiolepis wrightiana Maxim. The ratio of the total leaf area to the twig sap area was used as a health indicator after drought damage. Six adult trees with different levels of tree health and one dead adult tree were selected. Two individuals having the worst and second worst health among the six live trees died three months after our study was conducted. Soil-to-leaf hydraulic conductance and leaf gas exchange rates decreased linearly as tree health declined, whereas xylem cavitation and total non-structural carbon remained unchanged in the branchlets except in the dead and most unhealthy trees. Respiration rates and the number of living cells in the sapwood decreased linearly as tree health declined. This study is the first report on the importance of dehydration tolerance and respiration maintenance in living cells.

The dynamics of carbon stored in xylem sapwood to drought-induced hydraulic stress in mature trees

Climate-induced forest die-off is widespread in multiple biomes, strongly affecting the species composition, function and primary production in forest ecosystems. Hydraulic failure and carbon starvation in xylem sapwood are major hypotheses to explain drought-induced tree mortality. Because it is difficult to obtain enough field observations on drought-induced mortality in adult trees, the current understanding of the physiological mechanisms for tree die-offs is still controversial. However, the simultaneous examination of water and carbon uses throughout dehydration and rehydration processes in adult trees will contribute to clarify the roles of hydraulic failure and carbon starvation in tree wilting. Here we show the processes of the percent loss of hydraulic conductivity (PLC) and the content of nonstructural carbohydrates (NSCs) of distal branches in woody plants with contrasting water use strategy. Starch was converted to soluble sugar during PLC progression under drought, and the hydraulic conductivity recovered following water supply. The conversion of NSCs is strongly associated with PLC variations during dehydration and rehydration processes, indicating that stored carbon contributes to tree survival under drought; further carbon starvation can advance hydraulic failure. We predict that even slow-progressing drought degrades forest ecosystems via carbon starvation, causing more frequent catastrophic forest die-offs than the present projection.