Tag Archives: Puerto Rico

The history beneath your feet

I think about dirt a lot more than most people. Probably this is the result of my background in ecosystem science, the study of how nutrients and energy flow throughout the biosphere. The soil represents a huge sink for all major nutrients that sustain life on earth, and an important source of nutrients such as carbon and nitrogen that are naturally recycled to the atmosphere.

But it’s not economic, or really even environmental value that makes soil so fascinating to me. It’s history. In the forests of northeastern Puerto Rico where I’m currently conducting research, the historic record deposited in soil is up to 8, 10 or 15 meters thick and stretches back 300,000 years into the past, to a time when the island itself was shooting up out of the Gulf of Mexico due to rising magmatic rock deep beneath the ocean floor.

If you start looking closely at the composition of soil, you will quickly discover a wealth of information recorded within it. Tiny grains of minerals, produced from thousands to millions of years of water dissolving rock, form the physical matrix on which life has developed. As these clean, crystalline minerals slowly rot, they are chemically transformed into new compounds such as clays. Clays and other secondary minerals add stickiness to the soil, allowing decomposing organic materials to adhere. Slowly, a strange collection of organic materials that were once living and the inorganic ingredients that supported their existence begins to accumulate. As this assortment of the dead and rotting grows, so does the living biomass that it sustains. Most of the soil microfauna is involved in feeding off dead (or other living) organic materials and ultimately recycling nutrients that would otherwise be locked away forever. Embracing death is a way of life in the world’s most biologically diverse ecosystem.

But wait- I was talking about history. Yes, living organisms, dead organic materials, clays and minerals are all important components of the soil, but how do we piece together a history (and of what? The geology that the soil formed over? The forest that once stood atop it? The long-dead animals whose traces still linger within it?) from such a complex and dynamic system?

The answer is not entirely clear. But I am convinced that history is sitting in the dirt, rotting away, waiting patiently for someone to find a way to unwrap the stories contained within.


Applying equations to forests- carbon storage across environmental gradients in northeastern Puerto Rico

If you’ve ever read about carbon storage or sequestration (and if you’ve read my blog before, there’s a good chance you have), have you ever wondered how scientists come up with the numbers they love to throw around? How do we “know” that there are 684-724 petagrams of carbon to a depth of 30 cm in all the soils across the whole wide world? How could anyone possibly know that?

We don’t. And we can’t. But there are those crazy enough to try, and trying essentially boils down to  sampling, sampling, sampling, then a whole lot of extrapolating. If you imagine a landscape in which there is a continuous but highly variable distribution of carbon, the only way we’re gong to get any sort of meaningful estimate of the total is to dig a lot of holes. The points you see below represent a lot of holes that I actually helped dig! (I’m one of those crazies). I’ve thrown them up over this topographical surface to give you a sense of just how variable carbon can be across space.

Rather than subject you to a discussion of a ecosystem carbon storage, I’d like to share some pretty pictures I’ve been putting together. I’m attempting to create a model for carbon storage across a roughly 10 km area of forest in northeastern Puerto Rico that is underlain by two bedrock types, contains three distinct forests dominated by different tree species, and has broad gradients in temperature and rainfall across the topographically varied landscape.

Bluer regions represent areas of greater carbon storage while yellower regions store less carbon. Essentially I’ve created an image made of a series of pixels -here, the resolution is coarse enough that you can distinguish individual pixels around the edges. For each pixel, a predicted carbon value has been calculated from a very simple equation that takes bedrock, forest type and elevation into account.

Breaking it down by forest type…



That’s all for now, hopefully I’ll have more and more interesting pictures to show in the future.

In an unpredictable environment, trees network for stability

In the highly variable ridge, slope and valley mosaic that forms the Luquillo Mountains of northeastern Puerto Rico, Dacroydes excelsa, commonly known as Tabnuco, dominates the landscape. Though tropical forests are generally quite diverse and seen as ideal environments for plant growth, life in this rainforest can actually be quite challenging. Powerful hurricanes pass through the Caribbean annually and hit the Puerto Rican mainland every few years. Large swatches of the Luquillo forest were flattened several years back when hurricane Hugo struck in 1989.  Aside from directly damaging or wiping out forest stands, hurricanes cause landslides that severly erode the already shallow, nutrient depauperate soils.

On steep, harsh slopes that experience such frequent disturbance, what allows one tree species to gain a competitive advantage over the hundreds of others struggling to survive? Rather than compete fiercely for limited resources only to be at the mercy of the next devastating hurricane, Tabunuco trees have adopted an alternative strategy- cooperation and resource sharing through root grafting.

Root grafting, the joining of neighboring tree roots to produce a network, is a phenomenon that scientists have been aware of for decades, though the extent of its occurrence and the benefits that it provides trees are largely unknown. In Tabunuco forests, however, root grafting is widespread and many of its benefits obvious.

Tabunuco trees grow in dense stands and will graft roots with neighboring trees as they mature, forming unions that comprise anywhere from two to over a dozen trees. A clear advantage of this strategy in an environment that experiences powerful storms is structural stability. Trees that have entered unions increase their base of support and are less likely to be uprooted during a wind event or landslide. In increasing their wind-firmness, individual trees boost their survival chances during a storm. Fewer uprooting events also reduces the probability of a major landslide and helps ensure the retention of the surface organic matter that contains most of the forest’s available nutrients.

Root networks can also improve soil conditions during the off-season. Densely packed surface roots form “organic benches” which trap leaves and other decaying plant matter rather than allowing these important nutrient sources be washed downslope. Roots aerate the soil, facilitating decomposition and nutrient flow. They also “prime” the surrounding soil for productivity by releasing sugary compounds that stimulate beneficial microbial activity (the interaction between plants and microbes in the root zone known as the “rhizosphere” is another fascinating topic entirely, which I will do attempt to do justice to in the future).

Scientists are now discovering previously undetectable advantages of Tabunuco grafting that underscore the high degree of sophistication and evolutionary purpose in the development of these networks. It is now known that root networks can actually serve as conduits for the transfer of carbon and essential nutrients between trees. This can provide an immense competitive advantage over non-networked trees. Tabunuco trees that receive the most sunlight and produce the most carbon through photosynthesis can transfer carbon to neighboring Tabunucos to ensure the long-term health and survival of the community. Individuals of less common species, such as the Caribbean palm and Colorado tree are excluded from Tabunuco networks and must compete for growth given only the resources available in the vicinity of their roots.

Though in Tabunucos root grafting precludes the need for inter-tree competition, it is theoretically possible that trees could use grafting for more selfish purposes. Ecologists have speculated whether trees can gain a competitive advantage over their neighbors by leeching a neighbor’s nutrients, much as the fungal organisms that associate symbiotically with plant roots can become greedy and actually sap nutrients from their host under stressful conditions. Root networks may even serve as a conduit for disease or herbicide transfer, allowing trees that produce or tolerate a harmful compound to efficiently clear out their competitors.

Basnet, K., F.N. Scatena, G.E. Likens, and A.E. Lugo. 1992. Ecological consequences of root grafting in tabonuco (Dacryodes excelsa) trees in the Luquillo Experimental Forest, Puerto Rico. Biotropica 25:28-35.