Isohydric and anisohydric tree strategies result in differing carbon allocation patterns in drought-stressed tree saplings
A recent conceptual framework for understanding how trees respond to drought characterizes species along a continuum from isohydry to anisohydry. Isohydric trees exhibit tight stomatal regulation and close stomates in response to declining soil water potential, keeping leaf water potential constant. Anisohydric trees, on the other hand, continue gas exchange under water stress, drastically reducing leaf water potential. As tree hydraulics, gas exchange, and C fixation are intimately coupled, there is reason to believe that drought may alter C allocation patterns differently for trees depending on their degree of isohydry. Specifically, we hypothesized that drought-stressed isohydric trees would decrease allocation to roots while anisohydric trees would increase allocation. In order to investigate how C allocation patterns differ among trees with varying levels of isohydry, four deciduous hardwood tree species (Acer saccharum, Carya ovata, Liriodendron tulipifera, and Quercus alba) were subjected to a five week drought in the greenhouse. These species were classified along an isohydric spectrum using semi-continuous measurements of C fixation, leaf water potential, and gas exchange. Finally, a 13CO2 pulse-labeling was performed before and after the drought to determine C allocation patterns, and the label was chased into leaves, stems, fine roots, and coarse roots.
Two complementary metrics of isohydry revealed significant differences among species in their level of stomatal regulation (p<0.05). Q. alba proved the most anisohydric, followed by L. tulipifera and C. ovata. A. saccharum was highly isohydric. However, the degree of isohydry was not related to declines in photosynthesis under drought, as both the anisohydric Q. alba and the isohydric A. saccharum kept photosynthesizing as soil moisture declined. Proportional C allocation shifted in stems and fine roots in response to drought, with Q. alba allocating more 13C to stems and A. saccharum and L. tulipifera allocating less to fine roots. These shifts in C allocation could be a result of non-structural carbohydrate accumulation in Q. alba stems and a decreased need for water uptake by the fine roots of A. saccharum and L. tulipifera. No C allocation shifts were detected in leaves and coarse roots. These results demonstrate that the physiological strategies for dealing with water stress can be coupled to unique allocation patterns that arise from the metabolic requirements and limitations of these tree strategies under drought.