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

Deciduous Forest.