The indirect effect of drought on plants

The direct effects of stresses on plants are often fatal (making them disturbances, by definition). For example, drought can cause cavitation in a plant's xylem, which leads to tissue desiccation and ultimately death. But, the indirect effects of stresses can cause mortality, too. Stresses can reduce the defense systems of plants allowing pests and pathogens to kill plants before the direct effects of drought ever do. Direct tests of the generality of this principle are uncommon though.

Jactel et al. recently published a meta-analysis of the effects of drought on damage to trees by insects and pathogens. The results were neat. They found that agents that attack plant leaves were enhanced by water stress to plants. Yet, agents that attack the plant through its wood caused less damage to water-stressed plants than to unstressed plants.

The best part of the paper was linking the degree of water stress to the severity of damage (shown above). Their metric was the reduction in plant water potential relative to the water potential at which conductance is reduced by 50%. The greater the severity of water stress, the greater the damage.

Well done.

Jactel, H., J. Petit, M.-L. Desprez-Loustau, S. Delzon, D. Piou, A. Battisti, and J. Koricheva. 2012. Drought effects on damage by forest insects and pathogens: a meta-analysis. Global Change Biology 18:267-276.

Submissions to NSF

David Inouye posted this to Ecolog. I'm not sure where he got it, but it looks pretty real.

The key here is that the number of proposals to DEB has been going up while award numbers have been flat, leading to a decline in success rate.

NSF knows there is pain out there and has worked to respond to the pain on reviewers.

More proposals means more reviews.

NSF has the power to reduce the burden on reviewers, so they instituted a pre-proposal stage with 4-page pre-proposals and a limit on the number of proposals a person can submit as a pi or co-pi. Some have argued that this reduces this stage of evaluation to a raffle that can harm early-career and soft-money scientists. 

The key here seems to be what the funding rate should be. Or even better, what the total level of funding should be. Congress determines this. 

My guess is that the policies of NSF now are less of a burden to good science than funding levels, but NSF is more proximal. It will be interesting to see if more effective arguments can be made to raise the level of funding.

Why trees die: case example

Understanding mortality in plants is a tangle of proximal and distal as well as competing hypotheses. A recent paper in PNAS tried to disentangle a number of issues for understanding mortality in trembling aspen (Populus tremuloides).

The authors use a mix of gradients and experiments to examine patterns of carbohydrate reserves and hydraulic properties for droughted and non-droughted aspen plants. Plants that were droughted and non-healthy did not have reduced carbohydrate levels in their tissues (leaves or roots). In contrast, dying plants consistently were experiencing loss of hydraulic conductance and cavitation.

What is interesting here is that aspen is the lettuce of trees. It is an isohydric, physiologically drought-intolerant species. The research shows that pot experiments should be pretty good at determining the drought tolerance characteristics of species. Screening experiments (and rated, more involved detailed studies like these) should allow for the type and degree of drought tolerance to be assessed for other  species. hence, models of future mortality could be generated for forests across the world.

Anderegg, W. R., J. A. Berry, D. D. Smith, J. S. Sperry, L. D. Anderegg, and C. B. Field. 2012. The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off. Proceedings of the National Academy of Sciences of the United States of America 109:233-237.