Research in our lab focuses on ecological responses to global change, particularly in temperate forests. Approaches include long-term field experiments where forest plots are exposed to elevated CO2 levels or increased soil temperatures. Recent studies have shown that increased carbon dioxide affects forest community composition and succession, that warmer soil temperatures increase nitrogen availability and CO2 flux, and that the biomass and toxicity of poison ivy increases in response to elevated atmospheric CO2. Collectively, this work is important for predicting future responses of forest ecosystems to global change, including the ability of forests to mitigate human impacts on the global carbon budget.
(1) Effects of elevated CO2 on forest community composition and succession
My work included investigating effects of elevated atmospheric CO2 on juvenile trees and other woody vegetation growing in the Duke University Free-Air Carbon Dioxide Enrichment (FACE) experiment. I focused on early life-history stages of forest trees because they play an essential role in explaining diverse assemblages of tree species. These plants, growing at ambient (~370 μl/l) and elevated (~570 μl/l) CO2 concentrations, represent twenty-eight species comprising tree, vine, and shrub functional groups as well as several shade-tolerance classes. A portion of this work is a Mohan et all. 2007 “Long-term CO2 enrichment of a forest ecosystem: implications for forest regeneration and succession”.
Although shade-intolerant trees, particularly nitrogen-fixing Robinia pseudoacacia, often grew faster under high CO2 they also had very low survivorship rates. I found that elevated CO2 greatly enhanced the population biomass of tree species with high survivorship under low light conditions. These species were shade-tolerant, slow-growing taxa at maturity, suggesting that future temperate forests may exhibit a regeneration bias towards less-productive tree species. However, the effect of elevated CO2 on individual tree growth was small relative to the random variability within populations.
(2) I also investigated the potential for inherent genetic variation to affect vegetation response to global change. Such variation bears consequences for ecosystem productivity and evolutionary responses, and for extrapolating predictions based on experimental data, which necessarily document the dynamics of a limited number of individuals. To determine how levels of relatedness affect responses to CO2, I collected seeds from red maple (Acer rubrum) trees representing six populations and three provenances in North Carolina; this work is described in Mohan et al. (2004) published in Global Change Biology. I germinated these seeds and grew the resulting trees for two years under four CO2 levels: 180 μl/l, 270 μl/l, 360 μl/l, and 600 μl/l - representing Pleistocene, pre-Industrial Revolution, ambient, and potential future conditions. I found that all levels of relatedness interacted with CO2 to explain variation in plant response and that, in particular, red maple trees from different regions responded independently to CO2 enrichment. This suggests that species' responses to future atmospheric CO2 and the accompanying effects on forest composition will vary significantly among families, populations, and provenances.
By growing red maple trees under very different CO2 conditions, I produced foliar samples of widely varying qualities (i.e., [carbon:nitrogen] ratios). In a second phase of the above experiment, I asked whether genetic variation in foliage production and nutrient uptake would translate into intraspecific differences in CO2 effects on decomposition and nutrient turnover. I found a large degree of genetic variation in foliage production, plant uptake of nitrogen and phosphorus, and eventual tissue decomposition responses to atmospheric CO2. Due to lower [carbon:nitrogen] ratios and enhanced tissue quality, foliage produced under subambient CO2 conditions decomposed much more rapidly than that grown under present-day concentrations. This implies historical and paleo-rates of ecosystem nutrient turnover may have been faster than current rates would suggest. However, due to genetic variation in responses, future decomposition responses to increasing CO2 will vary even within mono-specific stands, and among same-species forests of different regions.(3) Effects of rising temperatures on forest composition and biogeochemical cycles.
At Harvard Forest, I am investigating whether juvenile trees exhibit species-specific responses to warming (5C), and what the potential long-term implications are for forest composition and ecosystem functioning. Thus far, similar to my work on CO2 impacts at Duke Forest, I am finding that the tree species benefiting most from warming are the least-productive taxa. This work is described in a Mohan, Melillo, et al. manuscript, in review with Nature. In addition, recruitment of red oak – a major dominant tree in the eastern and central U.S. – is negatively affected by increased soil temperatures, suggesting both a decline in future abundance and a northward range shift for this species relative to maple competitors.
In collaboration with Jerry Melillo I am currently examining the impacts of a 5C increase in temperature on carbon and nitrogen dynamics of temperate forest soils and phenological, growth and survivorship responses of canopy trees. One of the most important findings from the original soil warming experiment at Harvard Forest, initiated in 1991, was that warming caused a large increase in the flux of CO2 from soils to the atmosphere. But after 10 years, respiration rates in heated plots were equivalent to those of un-heated control plots, indicating enhanced soil respiration was a transient response. However, that the rate is still holding at control levels suggests that although an initial, highly-labile pool of carbon was respired with warming, microbes in the warmed plots are now able to decompose more recalcitrant carbon at the same rate that more labile carbon is utilized under control conditions. Further, due to an unexpected lightening-strike in 2005, I have been able to determine that heated plots respire 34% less carbon when they are maintained at the same temperature as control plots. This suggests that a direct temperature effect, such as increased rates of biogeochemical cycling and/or acclimation by soil microbes, is at least partially responsible for the equivalent rates of respiration in warmed plots. I am working with Mark Bradford (Univ. Georgia), Kathleen Treseder (Univ. California, Irvine), Matthew Wallenstein (Univ. California, Santa Barbara), and Serita Frey (Univ. Hew Hampshire) investigating the possibility that shifts in microbial communities are also responsible for altered carbon metabolism in the heated soils.
Another response to warming we are observing in both the original and new soil warming experiments is increased net nitrogen mineralization rates, representing nitrogen available to plants. I am examining the growth and survivorship of canopy trees in the large-scale warming experiment, initiated in 2003, and calculating net carbon accumulation rates in woody biomass, to determine if increased growth of trees will offset the increased carbon flux from the heated soils. I initiated phenological studies in 2004, and am finding that, contrary to my prediction, canopy trees in the warmed plot are leafing-out about seven days earlier in the spring relative to those in the control plot. Beginning in 2004 I began documenting increased canopy tree growth, but I am also observing, in accordance with ecological theory, increased mortality rates of canopy trees due to self-thinning in the heated, “fertilized” plot. The result thus far is that live canopy trees are growing fast enough with warming to accumulate about 50% of the “extra” carbon currently respired from the warmed soils. The future, however, is less certain. The additional soil nitrogen availability, which is sustained over time, may result in future forests growing faster and becoming greater carbon sinks (increased soil respiration is a transient response, diminishing after about 10 years). However, increased tree mortality under warmed conditions suggests the possibility that over time, decomposition of dead woody debris will increase the amount, and perhaps extend the duration, of carbon respired by warmed forest soils, resulting in temperate forests being net sources of atmospheric CO2.
The large-scale warming experiment at Harvard Forest provides a unique opportunity to assess how warming, and accompanying impacts on soil nitrogen and growing season duration, may affect reproductive efforts in forest trees and herbs. In 2004 I initiated a long-term study of reproductive responses of two common forest herbaceous species – Maianthemum canadensis (Canadian mayflower) and Trientalis borealis (starflower). My students and I are finding consistent results in 2004-2006; namely, that soil warming and associated increases in soil nitrogen are diminishing overall fruit production and reproductive probabilities for a given plant size in both species. These results mirror what I am observing for the dominant Quercus rubra (red oak) and Acer rubrum (red maple) tree species. Both trees produce fewer and smaller seeds under warmed conditions, and red maple seeds from heated-plot trees exhibit reduced germination probabilities even after seed size is taken into account. These new data are some of the first from an intact forest experiment, and I look forward to pursuing similar reproductive work in other ecosystem-scale global change experiments.
The Canadian Global Change Model predicts mean temperature increases in the southeastern U.S. of up to 5.5C by year 2100, and predicted increases in maximum summer temperatures are the highest in the nation . Yet it is unclear how these changes will impact plant community composition and ecosystem functioning. I propose to initiate the first ecosystem-scale warming experiment in the southeastern U.S., and in particular to do this in a setting allowing cross terrestrial-riparian-aquatic investigations. Although the northeast now has three soil warming experiments in forests, globally most warming experiments are located in short-statured plant communities such as alpine meadows, bogs, and tundra. The large-scale nature of such an experiment (i.e., the Harvard Forest experiment is 1800 m2) would allow an assessment of basic ecological responses ranging from tree growth and survivorship, reproduction and recruitment, phenology, tissue chemistry and decomposition, to shifts in community structure that are simply not possible to investigate in smaller-scale studies. The large sample sizes of plants and soils in this project would allow hierarchical Bayesian analyses to account for inherent variability between individuals and microenvironments. This research would permit comparisons with northeastern U.S. and Scandinavian experiments that include differences in climate and soils, but also allows examinations of the effects of species identities and species diversity on overall ecosystem response. The aquatic component of the experiment invites novel climate change research opportunities.
Until now, vegetative work at most warming experiments, including those at Harvard Forest, focused primarily on aboveground vegetation processes. I recently initiated an experiment quantifying fine root dynamics in the new, large-scale warming experiment at Harvard Forest, and am currently placing root in-growth cores in the field to assess relative root production and turnover rates in 2006-2007. During the 2006 summer, I found that fine root biomass in the warmed plot was about 40% less than in the control plot, suggesting increased soil nitrogen availability is causing a shift away from belowground production. Root in-growth cores will also be placed at the original warming experiment begun in 1991 to compare root dynamics in forests at different stages of warming-induced soil organic matter depletion. In 2007, I will be collaborating with Lindsey Rustad (Univ. Maine) initiating a minirhizotron study at both sites to further assess dynamics of root growth and turnover. Additionally, I will be working with Mark Bradford (Univ. Georgia), Kathleen Treseder (Univ. California, Irvine), and Matthew Wallenstein (Univ. California, Santa Barbara) performing ∂13C pulse-chase studies at both the new and the original warming experiments. In these investigations 13C-labelled compounds will be used to access abundances of different microbial indicators (Treseder, Wallenstein), and rates and amounts of 13C incorporation into root, foliage, and woody biomass (Mohan, Melillo) and soil organic matter (Bradford) pools.(4) Biomass and toxicity responses of poison ivy to elevated atmospheric CO2
A portion of this work was recently published in the Proceedings of the National Academy of Sciences: “Biomass and toxicological responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2” (Mohan et al. 2006).
