Fine-scale edaphic control of leaf senescence in winter-deciduous trees

Background/Question/Methods

A longstanding paradigm holds that photoperiod and autumn temperature regulate leaf senescence in winter-deciduous trees, but recent evidence suggests carbon and nutrient status also have a strong influence. Trees that are flush with carbohydrates but limited by nutrients may adopt a nutrient-conservative strategy of earlier leaf senescence. This strategy could reduce the risk of poor nutrient resorption due to freezing and photoinhibition, but also curtail the season for photosynthesis. We would thus expect trees on less fertile soils to show: (1) earlier leaf senescence, (2) higher nutrient resorption, and (3) greater production of photoprotective pigments like anthocyanins. We tested these hypotheses at two nearby sites in southern Québec. The sites were close enough (< 2 km) to share a common macroclimate but separated by a sharp boundary in soil properties: One was on fertile, marine sediment-derived soil, the other on infertile peat soil. We studied two species (Acer rubrum and Betula populifolia) found at both sites. During senescence, A. rubrum produces anthocyanins (red leaves) but B. populifolia cannot (yellow leaves). We tagged sunlit leaves of each species at each site and repeatedly measured their reflectance spectra to track chemical trait changes from the peak of the growing season until abscission.

Results/Conclusions

In accordance with Hypothesis 1, both species had advanced timelines of senescence at the infertile site. On average, tagged A. rubrum leaves fell about 12 days earlier at the infertile site, and B. populifolia leaves fell about 20 days earlier. Spectral indices revealed that chlorophyll breakdown in both species and anthocyanin production in A. rubrum were also earlier at the infertile site. In accordance with Hypothesis 3, anthocyanin content in A. rubrum reached higher levels at the infertile site. Measurements of initial and final nutrient content to test Hypothesis 2 are pending. We show that soil properties can exert strong control over senescence timing in both anthocyanic and non-anthocyanic winter-deciduous trees. This finding supports the idea that senescence and fall colors are dependent on carbon and nutrient status, especially since moderately N-limited plants often have high foliar carbohydrate concentrations. The upregulation of anthocyanins in the less fertile site is also consistent with the idea that they help trees acclimate to photoinhibitory conditions during senescence.