Factors other than temperature may influence northern hardwood tree phenology

Background/Question/Methods

The seasonal change seen in temperate hardwood species produces a physiological response at the leaf level. Through this response, the canopy compensates for changes in light, temperature, humidity, & vapor pressure (Bassow & Bazzaz, 1998). Photosynthesis may decline in response to such changes and is indicated by the onset and progression of leaf senescence and leaf abscission. These fall phenophases may be advanced or delayed due to such environmental influences and serve as effective markers of climate change.

The hardwood species evaluated in this research study were Betula alleghaniensis (yellow birch), Betula cordifolia (Mt. paper birch), Betula papyrifera (paper birch), Acer saccharum, (sugar maple), and Fagus grandifolia (American beech). The environmental factors studied were thermal time or summations of average daily temperatures over an established temperature threshold (ADD), photoperiod (PH), incoming solar radiation (IR), and day of year (DOY). Two elevational transects, Mt. Tecumseh (MTE), Waterville (central NH) and Mount Starr King (MSK), Jefferson (northern NH) were evaluated using phenological timing and microclimate. Forty deciduous trees were selected and monitored for fall senescence and abscission.

Nonlinear mixed effects models using logistic growth curves and repeated measures were created for each environmental factor and each species. Using Akaike’s Information Criterion (AIC), models were ranked based on their predictive capability. Models having the lowest AIC ranking were assumed to have the strongest relationship with phenological progression.

Results/Conclusions

Leaf senescence (LS) was controlled primarily by DOY, PH, and IR among all species on both research sites. Leaf abscission (LA) was primarily controlled by DOY, ADD, and IR.  Specifically, LS models for DOY best represented BEAL, BECO, and FAGR. LS models for PH best represented ACSA and BEPA, and IR models of LS best represented BEAL and FAGR. Leaf abscission models of DOY yielded a best fit for BEAL, BECO, FAGR, and ACSA. ADD models for LA best represented ACSA, BEPA, BEAL, and BECO. LA for FAGR best represented IR.

A complex interaction between hardwood species and their environment exists; seasonality and topography affect the individual and the species as a whole. These results indicate that hardwood species track environmental cues other than temperature. Given this, the effects of a warming climate will still be evident, however the degree in which warming temperatures affect phenology may not be as strong.