Drought and stress tolerance

Comparison of photosynthetic rates for seedlings of dry- and wet-habitat tropical tree species. On average, photosynthetic rates were ~1/3rd higher for dry-habitat species. 

I wrote a bit on this just the other day, but here is a new paper that raises questions about whether low-water species should be considered "stress-tolerators". Pineda-Garcia et al. grew seedlings of 10 pairs of closely related tropical tree species and measured a suite of traits. Dry-habitat species had higher photosynthetic rates than wet-habitat species. In addition, dry-habitat species retained their leaves longer after watering was ceased.

There are always a number of ways plasticity can alter relationships. For example, I once showed that high resource species can have lower N concentrations and longer leaf longevity than low-nutrient species due to patterns of feedbacks to N cycling after establishment. Here, there are a number of mechanisms that could generate the higher photosynthetic rates and longer leaf longevity in this particular experiment that could be reversed in another. Parsimony accrues slowly.

Yet, overall, this is another example where drought-tolerant species are not necessarily following the general "stress-tolerator" syndrome. It will be interesting to begin to officially tally the evidence to see whether there is much support for the two to be linked.

Pineda-Garcia, F., H. Paz, and C. Tinoco-Ojanguren. 2011. Morphological and physiological differentiation of seedlings between dry and wet habitats in a tropical dry forest. Plant, Cell & Environment 34:1536-1547.

Leaf architecture and physiological drought tolerance

Patterns of physiological drought tolerance and leaf venation architecture among 10 woody species.

Quick note on a new paper.


Scoffoni et al. determined the physiological drought tolerance and architecture of 10 woody species. The authors test key components of leaf venation architecture to understand the underlying leaf structural mechanisms for drought tolerance. Most work on drought tolerance focuses on stems and highlight xylem geometries, but the authors show that the density of veins in a leaf are the best correlate with its physiological tolerance of drought. High vein density provides insurance against embolism and allows water to continue to be supplied to areas adjacent to veins that have experienced embolisms that necessarily accompany low water potentials. 


The authors highlight the need to separate leaf size and vein density, which were correlated in the study. But, the research raises an interesting question as to whether the need for higher vein densities serves as a constraint on leaf size and ultimately contributes to one of the major biogeographic patterns of plant form.


I also think their figure, shown above, is pretty stunning. 

Scoffoni, C., M. Rawls, A. McKown, H. Cochard, and L. Sack. 2011. Decline of leaf hydraulic conductance with dehydration: relationship to leaf size and venation architecture. Plant Physiology 156:832-843.

Independence of leaf and stem traits

Multivariate relationships between leaf and stem traits for 600+ Neotropical tree species. 

Before reading Baraloto et al. in Ecology Letters, I think the best assumption for how leaves and stems correspond is that plants with high-activity leaves (low tissue density, high N concentrations, low leaf longevity) would be associated with low density wood. Pioneer species are typically thought of this way--think Cecropia. Essentially cheap leaves come with cheap stems. Late-successional species typically have low-activity leaves and high wood density.


Yet, if you think about it a bit more, pines and firs have low wood density, yet their leaves live a long time. So what is the pattern? Do leaf and stem traits correlate or are they independent.


Baraloto et al. compared key leaf and stem functional traits for over 600 tropical tree species. The authors convincingly show that leaf and stem economic axes are orthogonal. Species with low leaf tissue density and high foliar nitrogen concentrations are equally as likely to be associated with high stem density as low. In short, cheap leaves can be born on expensive stems. 


The factorial ecology of leaf and stem economics has still to be worked out and the obvious next question is to reexamine patterns with roots, but the paper is an excellent example of the power of sampling large numbers of species and distilling data to a clear, simple message.



Baraloto, C., C. E. Timothy Paine, L. Poorter, J. Beauchene, D. Bonal, A. M. Domenach, B. Herault, S. Patino, J. C. Roggy, and J. Chave. 2010. Decoupled leaf and stem economics in rain forest trees. Ecology Letters 13:1338-1347.

Comparing two measures of leaf tissue density

Relationship between leaf tissue density (RhoL) and leaf dry matter content (DMC) across 42 Konza grassland species.

There has been some debate on how best to represent plant investment into leaves. Specific leaf area, the ratio of area to mass, is at best an imperfect measure. Plants with high SLA certainly produce a lot of leaf area for minimum investment. Yet, high SLA can come as a result of being thin or low density. And it seems that many of the ecological conditions associated with high SLA are really associated with low tissue density rather than thin leaves.

How to measure tissue density is one of the current debates. On the one hand, tissue density (mass per unit volume) can be derived by measuring the thickness of leaves in addition to SLA. Deriving leaf tissue density (LTD) from thickness measurements provide a direct covariate (thickness) and are relatively simple to do. Yet, for some leaves, measuring the average thickness can be problematic. On the other hand, an approximation of tissue density can be derived from the leaf dry matter content (LDMC). Leaves are weighed in a hydrated state and then again dry. The ratio of dry mass to wet mass is LDMC. There are a number of assumptions to equate this ratio to leaf tissue density, but it has been favored.

Across 40+ species at Konza, I measured LTD and LDMC. The two metrics correlated pretty well (r = ~0.8). Some species seemed to have higher LTD than one would expect based on LDMC. In species with a high proportion of veins, thickness is probably underestimated, since it is generally measured between major veins. The Ambrosia artemisiifolia I selected was deeply lobed and did not have much lamina relative to veins. Its LTD was probably too high. On the other hand, both Bothriochloa and Schizachyrium species had higher LTD than expected from LDMC, but this likely would not have been caused by underestimating thickness or area. Instead, these species likely have high silica concentrations that add more mass per unit volume than other species. This is something I still need to confirm.

As to whether LTD or LDMC does a better job of predicting abundance, they both were about the same. Using long-term abundance data, they both had equal predictive power on average.

Whether one metric is better than another is likely equivocal. It depends on the situations as both have their limitations. I’d probably use both for awhile until better consensus can be reached.

I’m not sure I’ll get around to publishing these data, so I thought I’d put so of the results up here. 

Leaf dry matter content: ecologically relevant?

There is still some contention about whether leaf tissue density (mass per unit volume) leaf dry matter content (LDMC; dry mass per unit wet mass) are equivalent and whether past work has shown LDMC to be ecologically relevant, no less more relevant than specific leaf area (SLA).

I've looked through the literature pretty hard. Here's about all I can find:


1. LDMC and leaf tissue density should be positively correlated and there has been some excellent work investigating the underlying causes of variation in LDMC that are relevant for understanding leaf tissue density (Vile et al. 2005, Roderick et al. 1999, Shipley 1995). I still haven't found the perfect test of the two methods, but they should be pretty strongly related.
2. LDMC can predict plant strategies (Vendramini et al. 2002, Wilson et al. 1999). LDMC does a better job than SLA in predicting CSR placement for example.
3. LDMC can predict relative growth rates within species (Ryser and Aeschlimann 1999) and digestibility (Pontes et al. 2007, Ansquer et al. 2009, Duru et al. 2008).
4. LDMC was not correlated with competitive effect or response (Liancourt et al. 2009).
5. LDMC correlated better with soil fertility and sheep grazing intensity than SLA in Norwegian alpine ecosystems (Rusch et al.2009). [Note I still haven't read this paper--it's on order.]

That's about it.

The use of SLA still outnumbers tissue density or LDMC 50 to 1 and there still are essentially no published tests of the utility of either tissue density or LDMC in explaining abundance.

When will SLA R.I.P.?

Relationship between leaf tissue density and the abundance of grassland species in uplands at Konza Prairie. Each point is a different species with its abundance measured over 14 years.

For almost two decades, SLA (or its inverse alter-ego, LMA) has reigned supreme as the central functional trait of plants. SLA, i.e. specific leaf area--the ratio of leaf area to mass, has stood to represent the amount of investment into light acquisition. Entire pyramids of approaches to traits are built on the fundamental supremacy of SLA. The only thing more important than SLA in these pyramids is relative growth rate (RGR).

But why SLA? Why the ratio of area to mass? The thinking is that plants that grow fast need to absorb as much light as possible with the least amount of investment. Hence, selection favors plants that produce a lot of leaf area with little carbon investment, i.e. a high SLA. Plants in stressful or low-resource areas have low SLA, which presumably aids plants in resisting stress or maximizing the utilization of a limiting resource. Consistently, there are good correlations between SLA and RGR as well as other leaf characteristics such as photosynthetic rates, which have reinforced the primacy of SLA.

For almost all of the 20 years, there has always been a countervailing opinion of SLA that has never been rectified. If it ever was squared, SLA would likely never be measured again.

A leaf can high SLA either because it is thin or because it has low tissue density—thickness and density are the two components of SLA. In 1991, Witkowski and Lamont examined thickness and density across a series of ecological contrasts for sclerophyllous species. In short, from the patterns they observed, the authors concluded that “leaf density and thickness may respond to independently to resource and other gradients, and thus are more appropriate measures than [SLA] which confounds them.” Because thickness is so easy to measure—a quick squeezing of calipers—there is no good reason to not break down SLA to density and thickness every time.

Thickness and density have different functional roles in a leaf. They often vary independently across ecological contrasts. A thick, low density leaf and a thin, high density leaf would have the same SLA, but very different performances in most environments. By extension, SLA might be important to plant ecologists, but not to selection.

But maybe this is a bit hasty. SLA is supposed to be ecologically important and help explain the abundance of species across contrasts. Maybe SLA explains abundance better than thickness or tissue density. Surprisingly, the relative explanatory of SLA and its components have rarely been tested quantitatively. In general, this is probably the Achilles heel of most traits work. We spend more time examining relationships among traits than rigorously testing their relative predictive capacity.

Refuting the ecological importance of SLA or either of its components will not be a simple affair. It’ll take a number of studies before we understand their relative empirical importance. I’ve now done two. The first was at Cedar Creek along fertilization and disturbance gradients. The second is at Konza where I measured leaf traits for 130 grassland herbaceous species and tested their predictive capacity for species abundance across topographic, burning, and grazing contrasts. The results for Konza? SLA explained no variation in the abundance of species. Yet, tissue density did. Consistently across gradients it was tissue density not SLA that explained the abundance of species. The Cedar Creek work largely concluded the same thing.

SLA should not be buried yet, but at some point, we are going to have to fundamentally reexamine the hierarchy of traits in the ecology of plants. A dichotomous world of high SLA and low SLA (if not high RGR and low RGR) plant species might have to be replaced. Until then, at the very least, measure thickness.