Low Non-structural Carbon Accumulation in Spring Reduces Growth and Increases Mortality in Conifers Defoliated by Spruce Budworm

Abstract

Spruce budworm (Choristoneura fumiferana) outbreaks are important disturbance events in the boreal forests of northeastern North America, causing major growth loss and widespread tree mortality. The physiological mechanisms leading to tree mortality remain poorly understood and two important functional traits, tree-ring width and concentration of stored carbohydrate, can serve as indicators of tree vitality during defoliation. This study aims to test the hypothesis that storage starch is an indicator of tree vitality by (1) exploring the link among reductions in storage, growth and mortality, and (2) identifying starch or sugar threshold to predict the risk of mortality. We use balsam fir and black spruce, two main host species of spruce budworm. We sampled 81 trees across seven experimental sites in eastern Quebec, Canada, and assessed defoliation intensity, tree-ring growth, and tree vitality. Soluble sugar and starch concentrations in needles, twigs, and roots were measured from spring to autumn. Under conditions of increased defoliation, carbon allocation to reserves and radial growth decreased in a similar manner for both species. Starch concentration within twigs and needles in May and June was the best indicator of carbon status in defoliated trees. We observed the highest reductions in growth two to 3 years prior to mortality concurrently with reductions in starch in May and June. When starch concentrations were lower than 28 mg•g −1 dw in needles, the probability of balsam fir mortality exceeded 50%. At this level of starch, reserves and newly produced carbon are insufficient to support tree growth and vitality.

Sprouts and Needles of Norway Spruce (Picea abies (L.) Karst.) as Nordic Specialty—Consumer Acceptance, Stability of Nutrients, and Bioactivities during Storage

“…In contrast, SS show the highest amounts in winter and early spring (to protect the tissues from freezing), and the lowest values during late spring–mid-summer during the active period of annual growth [ 27 ]. The concentrations of needle starch and SS observed in our study are within the range of values reported in the literature for boreal and temperate conifers during the same phenological phase of the annual cycle [ 28 , 29 ].…”

Plant respiration: Controlled by photosynthesis or biomass?

“…Palacio, Hernández, Maestro‐Martínez, and Camarero () found that black pine trees ( Pinus nigra Arnold) that were repeatedly defoliated for 11 years, and left to recover for another 6 years, showed reduced growth but similar stem NSC concentration when compared to control trees. Fierravanti et al () found that low NSC accumulation in conifers defoliated by spruce budworm led to a reduction in growth and an increase in mortality.…”

“…A minimum NSC level, which has been found to proportionally increase with biomass, may also be required to maintain a safety margin and a proper internal functioning of trees (including osmoregulation), regardless of whether growth is limited by carbon supply (Huang et al, 2019;Martínez-Vilalta et al, 2016;Sala, Fouts, & Hoch, 2011;Sala et al, 2012;Woodruff & Meinzer, 2011 (2012) found that black pine trees (Pinus nigra Arnold) that were repeatedly defoliated for 11 years, and left to recover for another 6 years, showed reduced growth but similar stem NSC concentration when compared to control trees. Fierravanti et al (2019) found that low NSC accumulation in conifers defoliated by spruce budworm led to a reduction in growth and an increase in mortality.…”

Manipulating phloem transport affects wood formation but not nonstructural carbon concentrations in an evergreen conifer

“…Dynamic soluble sugar concentrations have already been argued to drive the early-to-latewood transition (Cartenì et al ., 2018), which is supported by the observed decline in mean cell-wall area at low carbon supply here. While limitation in carbon-supply due to natural defoliation can reduce growth (Fierravanti et al ., 2019) and cell numbers substantially (Castagneri et al ., 2020), we found that the cumulative number of cells formed, thus cell division, and to a smaller degree cell-wall deposition, seem to be regulated by carbon supply more generally (including at elevated carbon supply). Interestingly, cell size was not affected, despite the marked increase in cell number above all phloem transport manipulations, resulting in much higher cumulative cell-wall area, thus biomass.…”

“…Experiments manipulating carbon assimilation by artificially increasing atmospheric CO 2 (Norby and Zak, 2011), defoliating (Hoch, 2005) or shading trees (O’Brien et al ., 2014) have often been undertaken to describe and quantify the relation between sources and sinks. Growth sinks and carbon status have been shown to be coupled under low atmospheric CO 2 (Hartmann et al ., 2013), severe shading (Weber et al ., 2019), and defoliation (Piper et al ., 2015; Wiley et al ., 2017; Fierravanti et al ., 2019). However, a meta-analysis found that under increased assimilation due to elevated atmospheric CO 2 plant growth does not increase proportionally to carbon uptake (Ainsworth and Long, 2005), although most included studies used herbaceous plants.…”