Aboveground Wood Production Is Sustained in the First Growing Season after Phloem-Disrupting Disturbance

Changes in the Differentiation Program of Birch Cambial Derivatives following Trunk Girdling

The mechanisms regulating the tree trunk radial growth can be studied in original experiments. One technique for studying cambium activity (the meristem involved in radial growth) under conditions of an increased photoassimilate level is trunk girdling. We girdled the trunks of 17- to 22-year-old silver birch plants (Betula pendula Roth var. pendula) during the active growth period and collected xylem and phloem samples at two height levels (1 cm and 35 cm) above girdle, 10, 20, and 30 days after girdling. We investigated the changes that occurred at the anatomical level, as well as the activities of sucrose-metabolizing enzymes and antioxidant-system enzymes and the expression of genes that encode proteins involved in sucrose and auxin transport and metabolism. A moderate increase in photoassimilates (35 cm above the girdle) resulted in a change in the ratio of phloem to xylem increments and an increase in the proportion of parenchyma in the conducting tissues. The increase of photoassimilates above the level at which they can be used in the processes of normal tissue growth and development (1 cm above the girdle) led to xylogenesis suppression and the stimulation of phloem formation, a significant increase in the parenchyma proportion in the conducting tissues, and formation of large sclereid complexes. The differentiation of parenchyma and sclereid cells coincided with biochemical and molecular markers of abnormal conducting tissue formation in Karelian birch, which are also characterized by high proportions of parenchyma and sclereid near the cambium. The results obtained are important in understanding the cambium responses to the photoassimilate distribution changes and estimating tree productivity and survival under changing environmental conditions.

Climate Drives Modeled Forest Carbon Cycling Resistance and Resilience in the Upper Great Lakes Region, USA

Forests dominate the global terrestrial carbon budget, but their ability to continue doing so in the face of a changing climate is uncertain. A key uncertainty is how forests will respond to (resistance) and recover from (resilience) rising levels of disturbance of varying intensities. This knowledge gap can optimally be addressed by integrating manipulative field experiments with ecophysiological modeling. We used the Ecosystem Demography-2.2 (ED-2.2) model to project carbon fluxes for a northern temperate deciduous forest subjected to a real-world disturbance severity manipulation experiment. ED-2.2 was run for 150 years, starting from near bare ground in 1900 (approximating the clear-cut conditions at the time), and subjected to three disturbance treatments under an ensemble of climate conditions. Both disturbance severity and climate strongly affected carbon fluxes such as gross primary production (GPP), and interacted with one another. We then calculated resistance and resilience, two dimensions of ecosystem stability. Modeled GPP exhibited a two-fold decrease in mean resistance across disturbance severities of 45%, 65%, and 85% mortality; conversely, resilience increased by a factor of two with increasing disturbance severity. This pattern held for net primary production and net ecosystem production, indicating a trade-off in which greater initial declines were followed by faster recovery. Notably, however, heterotrophic respiration responded more slowly to disturbance, and it's highly variable response was affected by different drivers. This work provides insight into how future conditions might affect the functional stability of mature forests in this region under ongoing climate change and changing disturbance regimes.

Disturbance-accelerated succession increases the production of a temperate forest

Many secondary deciduous forests of eastern North America are approaching a transition in which mature early-successional trees are declining, resulting in an uncertain future for this century-long carbon (C) sink. We initiated the Forest Accelerated Succession Experiment (FASET) at the University of Michigan Biological Station to examine the patterns and mechanisms underlying forest C cycling following the stem girdling-induced mortality of >6,700 early-successional Populus spp. (aspen) and Betula papyrifera (paper birch). Meteorological flux tower-based C cycling observations from the 33-ha treatment forest have been paired with those from a nearby unmanipulated forest since 2008. Following over a decade of observations, we revisit our core hypothesis: that net ecosystem production (NEP) would increase following the transition to mid-late-successional species dominance due to increased canopy structural complexity. Supporting our hypothesis, NEP was stable, briefly declined, and then increased relative to the control in the decade following disturbance; however, increasing NEP was not associated with rising structural complexity but rather with a rapid 1-yr recovery of total leaf area index as mid-late-successional Acer, Quercus, and Pinus assumed canopy dominance. The transition to mid-late-successional species dominance improved carbon-use efficiency (CUE = NEP/gross primary production) as ecosystem respiration declined. Similar soil respiration rates in control and treatment forests, along with species differences in leaf physiology and the rising relative growth rates of mid-late-successional species in the treatment forest, suggest changes in aboveground plant respiration and growth were primarily responsible for increases in NEP. We conclude that deciduous forests transitioning from early to middle succession are capable of sustained or increased NEP, even when experiencing extensive tree mortality. This adds to mounting evidence that aging deciduous forests in the region will function as C sinks for decades to come.