Climatic Influences on Seedlings in Eastern Nort America
Unprecedented rates of warming and an inability to curtail greenhouse gas production
has fueled the discussion of how to mitigate climatic impacts. Climate impacts
on forests are coarsely understood. A large number of interacting environmental
variables affect every aspect of forest life cycles, and studies incorporating local
factors are scarce. For this study, a large manipulative climate experiment in both
northern(Massachusetts) and southern(North Carolina) sites was used to examine the
effect of climate change on demographic and physiological rates of 11 tree species
representative of the Eastern Deciduous Forest.
First, to clarify how environmental conditions of the next century will alter
seedling carbon assimilation, a hierarchical multivariate model that synthesizes over
16,000 instantaneous carbon exchange measurements from 285 trees of four species
was developed. Estimates of species-level light response curve parameters were used
to predict individual-level seasonal carbon budgets. This model revealed how the
balance between respiration and photosynthesis shifts with temperature, moisture,
and overstory canopy status. Furthermore, it showed that elevated temperatures
(3C and 5C) shift the seasonal species carbon budget allowing some Lirodendron
tulipifera to grow as much a 5.5 times more massive in elevated temperatures.
In addition, for certain species, demographic rates of seedlings can be scaled to
above-ground growth using short-term physiological responses.
Second, the relationship between seedling size and water-use was examined.
Trees grow from environmentally sensitive seedlings to large canopy individuals
capable of buffering environmental stress. At some intermediate size, a threshold is reached
where greater resilience results when increased resource gain overcomes the added
costs of size. For two seasons of this manipulative warming experiment, 123 external
heat pulse sapflow sensors were applied to 4 species (Acer rubrum, Lirodendron tulipifera,
Quercus alba, and Quercus rubra). The experimental seedlings, which vary in
mass from less than a tenth of a gram to 41 grams, were included to demonstrate
that both size and species influence water use. While larger size leads to increased
transpiration, reduced soil moisture paired with larger size leads to unpredictable
reductions in sapflow. Small seedlings were predictably reactive, but large seedlings
were both the least and most reactive to soil moisture reductions. When soil moisture
improved, after periods of moisture limitations, small individuals quickly recovered
and large individuals to a lesser extent. These results suggest that the seedlings of
this experiment were not consistently big enough to gain an advantage in water-use
like their larger canopy counter parts.
Lastly, this climate-warming experiment was expanded to include 4000 seedlings
of 11 total species across both the northern and southern sites. High-resolution subannual
growth measurements and a hierarchal Bayesian state-space model provided
a more accurate picture of seedling growth responses to sub-annual climatic variation,
by focusing on determinate vs. indeterminate growers and the interactions
between growth phenology, temperature tolerances, and climatic shifts. Determinate
species enhanced annual growth by shifting growth earlier in the season when
temperatures were more suitable, avoiding hot and dry conditions of summer. Indeterminate
species annual growth, which is focused within the summer, is dependent
on their ability to maintain growth during increasingly warmer and dryer summers.
Co-occurring species may respond differently and competitive regimes are shifted by
how growth phenology aligns with the seasonal patterns of climate change.
Questions of forest responses to climate change are multi-scaled. Responses depend
on local conditions, life-stage, natural history, and the scale of inference. Climate
change does not occur in isolation all factors must be weighed when evaluating
a response. For this study a manipulative climate experiment was used to address
these questions and investigate the effect of climate change on trees of the Eastern
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