Climatic Influences on Seedlings in Eastern Nort America

Abstract

Unprecedented rates of warming and an inability to curtail greenhouse gas productionhas fueled the discussion of how to mitigate climatic impacts. Climate impactson forests are coarsely understood. A large number of interacting environmentalvariables affect every aspect of forest life cycles, and studies incorporating localfactors are scarce. For this study, a large manipulative climate experiment in bothnorthern(Massachusetts) and southern(North Carolina) sites was used to examine theeffect of climate change on demographic and physiological rates of 11 tree speciesrepresentative of the Eastern Deciduous Forest.First, to clarify how environmental conditions of the next century will alterseedling carbon assimilation, a hierarchical multivariate model that synthesizes over16,000 instantaneous carbon exchange measurements from 285 trees of four specieswas developed. Estimates of species-level light response curve parameters were usedto predict individual-level seasonal carbon budgets. This model revealed how thebalance 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 Lirodendrontulipifera 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 toabove-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 reachedwhere greater resilience results when increased resource gain overcomes the addedcosts of size. For two seasons of this manipulative warming experiment, 123 externalheat pulse sapflow sensors were applied to 4 species (Acer rubrum, Lirodendron tulipifera,Quercus alba, and Quercus rubra). The experimental seedlings, which vary inmass from less than a tenth of a gram to 41 grams, were included to demonstratethat both size and species influence water use. While larger size leads to increasedtranspiration, reduced soil moisture paired with larger size leads to unpredictablereductions in sapflow. Small seedlings were predictably reactive, but large seedlingswere both the least and most reactive to soil moisture reductions. When soil moistureimproved, after periods of moisture limitations, small individuals quickly recoveredand large individuals to a lesser extent. These results suggest that the seedlings ofthis experiment were not consistently big enough to gain an advantage in water-uselike their larger canopy counter parts.Lastly, this climate-warming experiment was expanded to include 4000 seedlingsof 11 total species across both the northern and southern sites. High-resolution subannualgrowth measurements and a hierarchal Bayesian state-space model provideda more accurate picture of seedling growth responses to sub-annual climatic variation,by focusing on determinate vs. indeterminate growers and the interactionsbetween growth phenology, temperature tolerances, and climatic shifts. Determinatespecies enhanced annual growth by shifting growth earlier in the season whentemperatures were more suitable, avoiding hot and dry conditions of summer. Indeterminatespecies annual growth, which is focused within the summer, is dependenton their ability to maintain growth during increasingly warmer and dryer summers.Co-occurring species may respond differently and competitive regimes are shifted byhow growth phenology aligns with the seasonal patterns of climate change.Questions of forest responses to climate change are multi-scaled. Responses dependon local conditions, life-stage, natural history, and the scale of inference. Climatechange does not occur in isolation all factors must be weighed when evaluatinga response. For this study a manipulative climate experiment was used to addressthese questions and investigate the effect of climate change on trees of the EasternDeciduous Forest.