Research Symposium: How Forests Adapt to Higher Temperatures
Angelica Patterson presented her work on broadleaf and conifer physiological responses to climate change at the 10th biennieal Research Symposium hosted by Black Rock Forest Consortium on June 26 and jointly sponsored by the Consortium and the Palisades Interstate Park Commission.
Patterson’s Ph.D. dissertation at Columbia University explores how conifers and broadleaf trees respond to increasing temperatures, and what that may mean for the future of Eastern forests. She investigated respiration rates (or the release of CO2 from leaves) by sampling leaves from the canopies of trees at Black Rock Forest over several years and temperature ranges. Ten broadleaved and 6 conifer species were sampled and placed in airtight devices, where the released CO2 was measured and the respiration rate calculated. Patterson also examined the Respiratory Quotient (RQ, or ratio of CO2 released from the sample leaf to O2 consumed) and compared the ratios across 9 broadleaf species.
Results reveal that Northern broadleaved species (that is, species of broadleaf trees whose natural range lies mainly north of the Hudson Highland area) show higher rates of respiration in temperatures from 5-35 degrees Celsius than broadleaved species from central or southern ranges, whereas conifers with more Northern ranges show little change to lower respiration rates across 5-35 degree range from their central counterparts. The differing results of respiration response to temperature suggest that the northern conifers may have physiologically acclimated to this region's changing climate, whereas the northern broadleaved species may not be able to adjust.  This is consistent with these species’ observed migration northward towards cooler climates.  
"In the context of a tree exposed to temperatures outside of their optimal growth temperatures, higher respiration rates (carbon loss), paired with low photosynthetic rates (carbon gain), minimize the net carbon gain of the tree (ie. slow growth)," Patterson said. "Scaling up to the forest level, a lowered net carbon gain of northern broadleaved trees, coupled with species replacement due to climate-induced species migration, may affect the carbon storage potential of the forest."
The RQ of each species gives a more in-depth picture of the processes underlying these respiratory changes. In balanced conditions during respiration, the ratio of oxygen atoms used to carbon is 1:1, as plants use carbohydrates (sugars) to fuel the respiration process. In an unbalanced condition, however, proteins or organic acids may be used instead of sugars, resulting in less or more oxygen used per carbon. Indeed, where RQ values increased across all broadleaved species as temperatures increased, this increase was lower for those with Northern ranges than those with Central or Southern ranges. This suggests that the Northern species are running out of carbohydrates faster, perhaps as a result of higher respiration or a re-use of CO2 by a temperature-dependent enzyme, PEPcase, involved in the Krebs cycle, a metabolic pathway involved in respiration that utilizes molecules for energy and releases CO2. The Krebs cycle can be more or less active depending on the species.
Patterson's results reveal mechanisms behind observed shifts in species ranges in a warming climate, as well as potential trajectories of the future forests of the Hudson Highlands. Since respiration is not reduced in these Northern broadleaved species, local forests may become a carbon source instead of a carbon sink as these species migrate northward. Conifers, which only comprise 2% of the forests within this area, have suppressed their respiration rates, although this may not influence the amount of carbon released from the trees in this system. With the information from this study we can better predict the carbon potential of our Northeastern forests, as well as the future of species composition in a climate that may resemble Georgia more than current New York climate.