Abstract
Research linking soil moisture availability to nonstructural carbohydrate (NSC) storage suggests greater NSC reserves promote survival under acute water stress, but little is known about how NSC allocation responds to long-term differences in water availabilty. We hypothesized populations experiencing chronic or frequent water stress shift carbon allocation to build greater NSC reserves for increased survival probability during drought relative to populations rarely experiencing water stress. Over a year, we measured soluble sugar and starch concentrations from branches, stems, and coarse roots of mature Pinus palustris trees at two sites differing in long-term soil moisture availability. Xeric and mesic populations exhibited a cycle of summer depletion-winter accumulation in root starch. Xeric populations reached a maximum root starch concentration approximately 1–2 months later than mesic populations, indicating delayed summer depletion. Xeric and mesic populations reached the same minimum root starch at similar times, suggesting extended winter accumulation for xeric populations. These results suggest seasonal mobilization from root starch is compressed into a shorter interval for xeric populations instead of consistently greater reserves as hypothesized. Seasonal trends differed little between xeric and mesic populations for starch and sugars, suggesting the importance of roots in seasonal carbon dynamics and the primacy of starch for storage. If roots are the primary organ for longterm storage, then our results suggest that whole-plant mobilization and allocation respond to chronic differences in water availability.
Highlights
The survival of many tree species depends on stored carbohydrates or nonstructural carbohydrates (NSCs; e.g., sugars, starch) [1,2]
We present here observations of NSC differences between mature, wild Pinus palustris experiencing long-term, chronic differences in soil moisture availability due to soil drainage differences
We observed significant temporal dynamics for starch concentrations in all organs, but we only observed a seasonal cycle of winter accumulation and summer depletion—with a clear maximum and minimum—in roots (Figure 1a)
Summary
The survival of many tree species depends on stored carbohydrates or nonstructural carbohydrates (NSCs; e.g., sugars, starch) [1,2]. A number of studies observe evidence suggestive of carbon allocation to storage preferentially over growth [3,4]. Several studies have observed changes in NSCs in response to disturbances such as fire [5,6,7] and frost [8]. Plants respond to water stress by ceasing growth [13,14,15] and reducing stomatal conductance to prevent hydraulic conductance loss via cavitation [16,17].
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