Drought impacts on tree phloem: from cell-level responses to ecological significance

Abstract

On-going climate change is increasing the risk of drought stress across large areas worldwide. Such drought events decrease ecosystem productivity and have been increasingly linked to tree mortality. Understanding how trees respond to water shortage is key to predicting the future of ecosystem functions. Phloem is at the core of the tree functions, moving resources such as non-structural carbohydrates, nutrients, and defence and information molecules across the whole plant. Phloem function and ability to transport resources is tightly controlled by the balance of carbon and water fluxes within the tree. As such, drought is expected to impact phloem function by decreasing the amount of available water and new photoassimilates. Yet, the effect of drought on the phloem has received surprisingly little attention in the last decades. Here we review existing knowledge on drought impacts on phloem transport from loading and unloading processes at cellular level to possible effects on long-distance transport and consequences to ecosystems via ecophysiological feedbacks. We also point to new research frontiers that need to be explored to improve our understanding of phloem function under drought. In particular, we show how phloem transport is affected differently by increasing drought intensity, from no response to a slowdown, and explore how severe drought might actually disrupt the phloem transport enough to threaten tree survival. Because transport of resources affects other organisms interacting with the tree, we also review the ecological consequences of phloem response to drought and especially predatory, mutualistic and competitive relations. Finally, as phloem is the main path for carbon from sources to sink, we show how drought can affect biogeochemical cycles through changes in phloem transport. Overall, existing knowledge is consistent with the hypotheses that phloem response to drought matters for understanding tree and ecosystem function. However, future research on a large range of species and ecosystems is urgently needed to gain a comprehensive understanding of the question.

Potential Solar Radiation as a Driver for Bark Beetle Infestation on a Landscape Scale

“…It has been assumed that a stressed physiological state of trees leads to outbreaks of phloem-feeding insects, since impaired protein synthesis can lead to increased nitrogen compounds in phloem and the attractiveness of phloem increases. If phloem transport ceases, the plant might die, because carbon cannot be delivered to tissues of infested trees [18,30,56]. Forest decline events are particularly sensitive to temperature [57] and topographic exposure and south-and west-facing trees encounter an enhanced probability of bark beetle damage [58].…”

“…European spruce bark beetles mostly colonize trees with impaired defenses and their larvae feed on the phloem and cambium cells, thereby increasing the disruption of phloem tissues [18]. I. typographus creates infestation spots ranging from several neighboring infested trees to large continuous areas of dead mature trees ( Figure 1).…”

Combined effects of drought stress and psyllid herbivory on the invasive weed Scotch broom, Cytisus scoparius

“…Stress‐related increases in plant nitrogen may only be accessible to sap feeders when turgor pressure rebounds during periods of reduced stress (Huberty & Denno, 2004), and the periods of severe stress (i.e., wilting) that low‐water‐input plants experienced in our greenhouse experiment may have had more damaging effects on turgor pressure than the more persistent stress that plants likely endured in the field. The effects of drought on phloem feeders in general may depend on the balance of beneficial and detrimental changes in pressure and phloem nutrient content (Salmon et al, 2019).…”

Seasonality, drivers, and isotopic composition of soil CO<sub>2</sub> fluxes from tropical forests of the Congo Basin

“…The strongest 13 C enrichment of soil CO 2 was observed at the end of the dry season (September) in the montane site, likely caused by lower photosynthetic 13 C-CO 2 discrimination conveyed to soil respiration ( Figure A2 c). Indeed, the enrichment of 13 C of autotrophic soil respiration resulting from stomatal closure during periods of drought has been widely documented McDowell et al, 2004;Blessing et al, 2016;Salmon et al, 2019). Such distinct enrichment was presumably not detected in the lowland sites due to the absence of a prolonged dry season (Figure A2 a).…”