Brent E. Ewers

Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere.

Northern mid-latitude forests are a large terrestrial carbon sink. Ignoring nutrient limitations, large increases in carbon sequestration from carbon dioxide (CO2) fertilization are expected in these forests. Yet, forests are usually relegated to sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen. Here we present evidence that estimates of increases in carbon sequestration of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic. In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilizing at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus, fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2. Assessment of future carbon sequestration should consider the limitations imposed by soil fertility, as well as interactions with nitrogen deposition.

WATER BALANCE DELINEATES THE SOIL LAYER IN WHICH MOISTURE AFFECTS CANOPY CONDUCTANCE

To link variation in canopy conductance to soil moisture in the rooting zone, measurements of throughfall (PT), volumetric soil moisture (θ) to 0.7 m, transpiration from trees >10 mm in diameter (EC), and vapor pressure deficit (D) were made in a forest dominated by Pinus taeda. Total evapotranspiration (ET) was estimated from PT, changes in volumetric soil water content within a defined soil volume (ΔS), and drainage out of that volume (Q), calculated from unsaturated soil hydraulic conductivity and θ. Our calculations suggest that over 145 growing-season days, Q was negligible, and most of PT, averaging ∼2 mm/d, was partitioned between soil moisture recharge (0.4 mm/d) and ET (1.6 mm/d, not including ∼0.4 mm/d of interception, I, by canopy trees), of which EC was estimated from direct measurements at nearly 1.2 mm/d. Evapotranspiration by the subcanopy component accounted for slightly over 0.4 mm/d, about a third of ET (a fourth if I is included). Most of the water used for ET (>90%) was taken from the ...

Carry-Over Effects of Water and Nutrient Supply on Water Use of Pinus Taeda

A study of the effects of nutrients and water supply (2 × 2 factorial experiment) was conducted in a 12-yr-old stand of loblolly pine (Pinus taeda L.) during a period in which soil moisture was not augmented by irrigation because of frequent rain events. Information on the responses of sapwood-to-leaf area ratio and early-to-late wood ratio, to four years of treatments led to the hypothesis that the combination of increased nutrient and water supply (IF treatment) will increase tree transpiration rate per unit leaf area (EC,1) above EC,1 in the control (C), as well as increasing EC,1 above that when either the supply of water (I) or of nutrients (F) is increased. We further hypothesized that canopy transpiration (EC) will rank IF > F > I = C, based on the ranking of leaf area index (L) and assuming that the ranking of EC,1 is as first hypothesized. We rejected our first hypothesis, because F had lower EC,1 than the other treatments, rather than IF having higher values. We could not reject the second hypothesis; the ranking of average daily EC was 1.8 mm for IF, 1.2 mm for F, and 0.7 mm for both C and I (se < 0.1 mm for all treatments). Thus, it was the lower EC,1 of the F treatment, relative to IF, that resulted in ranking of EC similar to that hypothesized. Lower EC,1 in F trees was found to relate to lower canopy stomatal conductance, even though soil moisture conditions during the time of the study were similar in all treatments. Only trees in the F treatment absorbed a substantial amount of water (25%) below 1 m in the soil. These results indicate a “carry-over” effect of irrigation when combined with fertilization that increases EC in irrigated trees, relative to unirrigated trees, even under conditions when soil moisture is high and similar in all treatments.