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The number and intensity of heat waves has increased, and this trend is likely to continue throughout the 21st century. Often, heat waves are accompanied by drought conditions. It is projected that the global land area experiencing heat waves will double by 2020, and quadruple by 2040. Extreme heat events can impact a wide variety of tree functions. At the leaf level, photosynthesis is reduced, photooxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. In some species, stomatal conductance increases at high temperatures, which may be a mechanism for leaf cooling. At the whole plant level, heat stress can decrease growth and shift biomass allocation. When drought stress accompanies heat waves, the negative effects of heat stress are exacerbated and can lead to tree mortality. However, some species exhibit remarkable tolerance to thermal stress. Responses include changes that minimize stress on photosynthesis and reductions in dark respiration. Although there have been few studies to date, there is evidence of within-species genetic variation in thermal tolerance, which could be important to exploit in production forestry systems. Understanding the mechanisms of differing tree responses to extreme temperature events may be critically important for understanding how tree species will be affected by climate change.

HighlightStomatal conductance of two species (a broadleaf and a conifer) increased with increasing temperature. This response was independent of carbon metabolism, plant water status, or vapour pressure difference.

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SummaryUpward transport of CO 2 via the transpiration stream from belowground to aboveground tissues occurs in tree stems. Despite potentially important implications for our understanding of plant physiology, the fate of internally transported CO 2 derived from autotrophic respiratory processes remains unclear.We infused a 13 CO 2 -labeled aqueous solution into the base of 7-yr-old field-grown eastern cottonwood (Populus deltoides) trees to investigate the effect of xylem-transported CO 2 derived from the root system on aboveground carbon assimilation and CO 2 efflux. The 13 C label was transported internally and detected throughout the tree. Up to 17% of the infused label was assimilated, while the remainder diffused to the atmosphere via stem and branch efflux. The largest amount of assimilated 13 C was found in branch woody tissues, while only a small quantity was assimilated in the foliage. Petioles were more highly enriched in 13 C than other leaf tissues.Our results confirm a recycling pathway for respired CO 2 and indicate that internal transport of CO 2 from the root system may confound the interpretation of efflux-based estimates of woody tissue respiration and patterns of carbohydrate allocation.

We monitored sap flux density (v) diurnally in nine mature southeastern pine (Pinus spp.) trees with a thermal dissipation probe that spanned the sapwood radius. We found the expected pattern of high v near the cambium and decreasing v with depth toward the center of the tree; however, the pattern was not constant within a day or between trees. Radial profiles of trees were steeper earlier in the day and became less steep later in the day. As a result, time-dependent changes in the shape of the radial profile of v were sometimes correlated with daily changes in evaporative demand. As the radial profile became less steep, the inner xylem contributed relatively more to total tree sap flow than it did earlier in the day. We present a 3-parameter Gaussian function that can be used to describe the radial distribution of v in trees. Parameters in the function represent depth in the xylem from the cambium, maximum v, depth in the xylem where maximum v occurs, and the rate of radial change in v with radial depth (beta). Values of beta varied significantly between trees and with time, and were sometimes correlated with air vapor pressure deficit (D). We hypothesize that this occurred during periods of high transpiration when the water potential gradient became great enough to move water in the inner sapwood despite its probable high hydraulic resistance. We examined discrepancies among estimates of daily water use based on single-point, two-point and multi-point (i.e., every 20 mm in the sapwood) measurements. When radial distribution of v was not considered, a single-point measurement resulted in errors as large as 154% in the estimate of daily water use relative to the estimate obtained from a multi-point measurement. Measuring v at two close sample points (10 and 30 mm) did not improve the estimate; however, estimates derived from v measured at two distant sample points (10 and 70 mm) significantly improved the estimate of daily water use, although errors were as great as 32% in individual trees. The variability in v with depth in the xylem, over time, and between trees indicates that measurements of the radial distribution of v are necessary to accurately estimate water flow in trees with large sapwood areas.

Summary• Respiration consumes a large portion of annual gross primary productivity in forest ecosystems and is dominated by belowground metabolism. Here, we present evidence of a previously unaccounted for internal CO 2 flux of large magnitude from tree roots through stems. If this pattern is shown to persist over time and in other forests, it suggests that belowground respiration has been grossly underestimated.• Using an experimental Populus deltoides plantation as a model system, we tested the hypothesis that a substantial portion of the CO 2 released from belowground autotrophic respiration remains within tree root systems and is transported aboveground through the xylem stream rather than diffusing into the soil atmosphere.• On a daily basis, the amount of CO 2 that moved upward from the root system into the stem via the xylem stream (0.26 mol CO 2 m) rivalled that which diffused from the soil surface to the atmosphere (0.27 mol CO 2 m −2 d −1). We estimated that twice the amount of CO 2 derived from belowground autotrophic respiration entered the xylem stream as diffused into the soil environment.• Our observations indicate that belowground autotrophic respiration consumes substantially more carbohydrates than previously recognized and challenge the paradigm that all root-respired CO 2 diffuses into the soil atmosphere.

Complementary laboratory and field experiments showed that the internal transport of carbon dioxide (CO 2 ) in the xylem of trees is an important pathway for carbon movement. Carbon dioxide released by respiration dissolves in sap and moves upward in the transpirational stream. The concentration of CO 2 in xylem sap can be up to three orders of magnitude greater than that found in the atmosphere. In the present experiments, diffusion outward of a portion of xylem-transported CO 2 caused a substantial overestimation of the apparent rate of stem and branch respiration. Rates of CO 2 efflux were linearly related to sap CO 2 concentration. Direct manipulations of xylem sap CO 2 concentration produced rapid and reversible changes in CO 2 efflux from stems and branches, in some cases quadrupling the rate of efflux. These results demonstrated that apparent rates of stem and branch respiration of trees are in large part a byproduct of the rate of CO 2 diffusion from xylem sap.