https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2009.03167.x

Physiological mechanisms of drought-induced tree mortality are far from being resolved.

The Tropical Rain Forest

Summary
Plant survival during drought requires adequate hydration in living tissues and carbohydrate reserves for maintenance and recovery. We hypothesized that tree growth and hydraulic strategy determines the intensity and duration of the ‘physiological drought’, thereby affecting the relative contributions of loss of hydraulic function and carbohydrate depletion during mortality.
We compared patterns in growth rate, water relations, gas exchange and carbohydrate dynamics in three tree species subjected to prolonged drought.
Two Eucalyptus species (E. globulus, E. smithii) exhibited high growth rates and water-use resulting in rapid declines in water status and hydraulic conductance. In contrast, conservative growth and water relations in Pinus radiata resulted in longer periods of negative carbon balance and significant depletion of stored carbohydrates in all organs. The ongoing demand for carbohydrates from sustained respiration highlighted the role that duration of drought plays in facilitating carbohydrate consumption.
Two drought strategies were revealed, differentiated by plant regulation of water status: plants maximized gas exchange, but were exposed to low water potentials and rapid hydraulic dysfunction; and tight regulation of gas exchange at the cost of carbohydrate depletion. These findings provide evidence for a relationship between hydraulic regulation of water status and carbohydrate depletion during terminal drought.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.12064

Drought response strategies define the relative contributions of hydraulic dysfunction and carbohydrate depletion during tree mortality.

The pool size of mobile, i.e. non-structural carbohydrates (NSC) in trees reflects the balance between net photosynthetic carbon uptake (source) and irreversible investments in structures or loss of carbon (sink). The seasonal variation of NSC concentration should reflect the sink/source relationship, provided all tissues from root to crown tops are considered. Using the Smithsonian canopy crane in Panama we studied NSC concentrations in a semi-deciduous tropical forest over 22 months. In the 9 most intensively studied species (out of the 17 investigated), we found higher NSC concentrations (starch, glucose, fructose, sucrose) across all species and organs in the dry season than in the wet season (NSC 7.2% vs 5.8% of dry matter in leaves, 8.8/6.0 in branches, 9.7/8.5 in stems, 8.3/6.4 in coarse and 3.9/2.2 in fine roots). Since this increase was due to starch only, we attribute this to drought-constrained growth (photosynthesis less affected by drought than sink activity). Species-specific phenological rhythms (leafing or fruiting) did not overturn these seasonal trends. Most of the stem volume (diameter at breast height around 40 cm) stores NSC. We present the first whole forest estimate of NSC pool size, assuming a 200 t ha−1 forest biomass: 8% of this i.e. ca. 16 t ha−1 is NSC, with ca. 13 t ha−1 in stems and branches, ca. 0.5 and 2.8 t ha−1 in leaves and roots. Starch alone (ca. 10.5 t ha−1) accounts for far more C than would be needed to replace the total leaf canopy without additional photosynthesis. NSC never passed through a period of significant depletion. Leaf flushing did not draw heavily upon NSC pools. Overall, the data imply a high carbon supply status of this forest and that growth during the dry season is not carbon limited. Rather, water shortage seems to limit carbon investment (new tissue formation) directly, leaving little leeway for a direct CO2 fertilization effects.

Non-structural carbohydrate pools in a tropical forest

The study of phenological aspects of plants involves the observation, recording and interpretation of the timing of their life history events. This review considers the phenology of leafing, flowering and fruit production in a range of species and communities. The selective forces (both abiotic and biotic) that influence the timing of these events are discussed. Within the limits imposed by phylogenetic constraints, the phenological patterns (timing, frequency, duration, degree of synchrony, etc.) of each phase are probably the result of a compromise between a variety of selective pressures, such as seasonal climatic changes, resource availability, and the presence of pollinators, predators and seed dispersers. Many studies on flowering times stress the role of interactions between plant species which share pollinators or predators. The timing of fruiting plays a key role in controlling the abundance and variety of obligate frugivores in many tropical communities. The importance of long-term recording is stressed, particularly in species which fruit irregularly. An understanding of the phenology of plants is crucial to the understanding of community function and diversity.

The phenology of growth and reproduction in plants

Summary 1. The effect of seasonal water availability on phenology and shoot total non-structural carbohydrate (TNC) of three Psychotria species was investigated by irrigating two large plots of mature tropical forest in Panama during the dry season for five consecutive years. This study reports results for the fifth and final year of irrigation. 2. Irrigation did not affect total production of leaves, inflorescences or infructescenes but did advance the timing of leaf production in all Psychotria, and advanced the timing of inflorescence production in l? furcata and P. marginata and infructescence production in P. furcata. Irrigation also reduced shoot TNC levels. 3. Shoot TNC levels in all three species of Psychotria showed similar depressions at the end of the dry season and maximum levels near the end of the wet season. Depletion of shoot TNC occurred just before or during peak leaf production but did not coincide with production of inflorescences or infructescences. Therefore, we hypothesize that shoot TNC supported part of the cost of new leaf production but production of inflorescences and infructescences was most likely supported by current photosynthate. 4. Tropical plants generally show less seasonal variation in TNC than plants from more variable climates but stored carbon still plays an important role in the growth of Psychotria, although its direct role in reproduction appears limited.

http://www.researchgate.net/profile/David_Tissue/publication/273077475_Effect_of_Seasonal_Water_Availability_on_Phenology_and_the_Annual_Shoot_Carbohydrate_Cycle_of_Tropical_Forest_Shrubs/links/550736420cf27e990e0528ab.pdf

S.J. Wright

Papers

Effect of seasonal water availability on phenology and the annual shoot carbohydrate cycle of tropical forest shrubs