Hydraulic architecture and photoinhibition influence spatial distribution of the arborescent palm Euterpe edulis in subtropical forests

Palm functional trait responses to local environmental factors in the Colombian Amazon

Functional traits play a key role in driving plant community effects on ecosystem function. We examined nine functional traits in various palm (Arecaceae) species and their relationships with moisture, tree-fall gaps, slope, and forest type at 29 transects (500×5 m) in the northeastern region of the Colombian Amazon. Redundancy analysis of mean trait values of species within a plot weighted by their abundance and Pearson correlations were used to evaluate the relationships between traits and environmental factors. The community trait composition was correlated with local environmental factors, which explained 23% of the trait variance. We detected functional dominance of the tallest palms in soils with high moisture and in floodplain forests (p ≤0.05). Palms with relatively long leaves were dominant in the flooded forests. Acaulescent and small palms were dominant on high slopes, and in terra firme forests, long-petioled palms were dominant in forest gaps. The number of seeds per fruit was not correlated with any environmental variable. Thus, hydrology is one of the main drivers of the functional composition of neotropical palm communities at the local scale, segregating tall palms with competitive and evasive strategies from small understory palms, which are mainly stress tolerant.

The hydraulic architecture of an arborescent monocot: ontogeny-related adjustments in vessel size and leaf area compensate for increased resistance

Bamboos are arborescent monocotyledons that have no secondary growth, but can continually produce conduits with diameters appropriate to the current size of the plant. Here, we studied bamboo hydraulic architecture to address the mechanisms involved in compensating for the increase in hydraulic resistance during ontogeny. We measured the hydraulic weighted vessel diameters (D_h ) at different distances from the apex along the stem of Bambusa textilis. The hydraulic resistance of different components and individuals of different heights were quantified using the high-pressure flowmeter method. The D_h showed tip-to-base widening with a scaling exponent in the range of those reported for trees. Although theoretical hydraulic conductivity decreased from base-to-tip, leaf-specific conductivity did not change. Leaves contributed the most to the whole-shoot hydraulic resistance, followed by the leaf-bearing branches. Roots contributed c. 13% to whole-plant resistance. Interestingly, taller individuals showed lower whole-shoot resistance owing to an increased number of resistances in parallel (side-branches), while leaf-specific resistance was independent of plant size. Tip-to-base vessel widening and height-independent constant leaf-specific conductance could be mechanisms for hydraulic optimization in B. textilis. Similar patterns have also been found in woody plants with secondary growth, but this bamboo exhibits them without secondary growth.

Habitat loss and canopy openness mediate leaf trait plasticity of an endangered palm in the Brazilian Atlantic Forest

Forest cover and light availability comprise key factors for plant establishment in tropical forests. In the Brazilian Atlantic Forest (AF), Euterpe edulis (Areacaceae) is an endangered and keystone food resource contributing to forest functionality. We investigated the influence of forest loss and light availability on leaf traits and acclimatization of young individuals of E. edulis in AF fragments. We aimed to understand (i) how canopy openness and transmitted light are affected by forest cover at the landscape scale and the individual palm level; and (ii) how local and landscape features, combined and separately, affect key leaf traits widely known to be related to plant growth. The study was carried out in 15 forest fragments, ranging from 16 to 97% of surrounding forest cover. In each fragment, we sampled 10-20 individuals of E. edulis and analyzed nine leaf traits related to morphological, biochemical and chemical aspects. We also took hemispherical photographs to estimate canopy openness on the top of each E. edulis and also within fragment plots. We found that young plants predominantly occurred in more shaded environments. Additionally, E. edulis succeeded to acclimate in six of the nine traits analyzed, with most traits being affected by local and landscape features. It is likely that the lack of variation in traits related to protection against herbivory are limiting the species establishment in highly deforested landscapes. Our results provide novel evidence that both landscape and local contexts affect the leaf traits of E. edulis young plants leading to biochemical, chemical and morphological adjustments.

Acclimation responses of macaw palm seedlings to contrasting light environments

The photosynthetic adjustments of macaw palm (Acrocomia aculeata) were evaluated in 30-day-old seedlings exposed to high and low light environments, and sudden transference from low to high light and comparisons were made with the hardening protocol used in nurseries. Furthermore, we evaluated the responses to long-term exposure (265 days) to high and low light environments. Macaw palm seedlings exhibited an efficient mechanism that maximized light capture under scarce conditions, and dissipated excess energy to avoid damaging to the photosystem II under high light. The seedlings showed low saturation irradiance but no photoinhibition when exposed to excess light. When grown under low light intensities, seedlings presented higher photochemical efficiency and minimized the respiratory costs with positive carbon balance at lower irradiance than hardened seedlings did. The hardening procedure did not appear to be an advantageous method during seedling production. Long-term exposure to either low or high light did not cause significant leaf anatomical adjustments. However, the low light seedlings showed higher leaf area and chlorophyll content than those exposed to higher light intensity did, which enabled shaded seedlings to maximize the captured light. Furthermore, the high non-photochemical dissipation allowed rapid acclimation to excessive light exposure. These responses allow macaw palm cultivation and establishment in very different light environments.

A possible link between hydraulic properties and leaf habits in Hevea brasiliensis

A link exists between hydraulic traits and leaf habit. However, few attempts have addressed a possible link between hydraulic traits and altered leaf habit in introduced ranges. Within its native range, the Amazon rainforest, Hevea brasiliensis (Willd. ex A. Juss) Muell. Arg. is an evergreen but it becomes drought-deciduous in non-native ranges. The reason for this change remains poorly understood. The hydraulic-related traits, gas exchange rates and water status of H. brasiliensis and the co-occurring evergreen Drypetes indica (Muell. Arg.) Pax et Hoffm. were examined in Xishuangbanna, China. The water potential at turgor loss point in both species almost overlapped, but the water potential at which leaf relative water content reached 70% was more negative in D. indica. The water loss rate from excised leaves was quicker in H. brasiliensis. Leaf and stem hydraulic conductivity were more susceptible to drought-induced embolisms in H. brasiliensis than in D. indica. Vessels were significantly wider in H. brasiliensis but D. indica had more vessels. H. brasiliensis displayed higher rain-season gas exchange rates than D. indica. During the dry season, low soil water potential rendered water transport inefficient in H. brasiliensis; this effect was less pronounced in D. indica. D. indica has traits that help prevent hydraulic failure but has a low photosynthetic capacity. The opposite was found for H. brasiliensis. The results suggest that a combination of hydraulic traits, gas exchange characteristics and water status during the dry season might trigger a change in the leaf habits of H. brasiliensis in introduced ranges.