|A picture of the Everglades from my recent trip.|
So you first need to think of how phosphorus naturally cycles through a system. An important part of this cycle involves the redistribution of nutrients and detritus at the landscape level. Basically plant and animal litter, dissolved compounds, soil particles, dry fallout (dust), and feces can be moved in and out of a system by various transport mechanisms - think wind, water, etc. The transfer of these compounds through a system and into or out of a food web can have consequences in terms of enhancing or limiting biological productivity. You can also end up with nutrient gradients that lead to landscape heterogeneity and geomorphological changes (large changes in a particular area that may impact movements of a material within the landscape - like the growth of certain plants or the attraction of groups of animals).
The Florida Everglades is a phosphorus limited, oligotrophic (lacking nutrients and having large amounts of oxygen) freshwater ecosystem. As such, it is composed of vast amounts of land that is covered by shallow water interspersed with tree islands and some other non-forested areas (usually sawgrass areas) that this paper refers to as marshes. Tree islands can be relatively uncommon and are usually about 10-70ha and stand about 0.2-2.5m above the surrounding area. These tree islands can be divided into zones based on the direction of water flow and elevation. Once trees become established in these areas the redistribution of nutrients, including phosphorus, begins. This paper takes a look at the redistribution of phosphorus from marshes to tree islands, hypothesizing that phosphorus levels should be higher on tree islands, particularly because of their elevation.
|Figure 1 from the article showing a logitudinal cross section of a tree island.|
Alright. So how to go about measuring this? Well, first, like all good scientists, they did a literature search, gathering all of the existing data on phosphorus measurements from fixed tree islands. Then soil cores were collected from the head, near tail, and far tail along the island. These cores were then analyzed for TP (total soil phosphorus) and bulk density.
For higher soil depths (10cm), on the heads of the tree islands TP was found to be higher than in marshes, but it was slightly variable. The near tail TP was about 20 times lower than that of the heads, and the far tail TP were similar to the near tail means. For lower soil depths (10-20 and 20-30cm) the TP concentrations were higher for heads and near tails than the 10cm cores, but were lower on far tails. The head TP ratios were found to be positively correlated with elevation, but as there is little data for high elevation spots the authors point out that this could be an artifact. They also point out that high elevation areas could simply be older and as such have larger trees. Older equals more time and more accumulation of phosphorus, but as the age of these tree islands can only be speculated at it is hard to know. Or maybe it is that higher elevation equals drier conditions. Dryer equals no washing away and more accumulation. Anyway, you get the point right?
The researchers took a look at this accumulation rate specifically. They found that the rate of annual phosphorus accumulation was higher in the heads. When comparing the TP of the heads to the TP ascribed to mean annual inputs associated with wet and dry fallout (remember the constituents of the cycle?) they found that less than 10% of the annual TP input in the heads can be associated with the fallout. This means that just 1m^2 of a head is sequestering the same amount of phosphorus as 10m^2 of the marsh.
Think about that in terms of distribution and heterogeneity. Also think about it in terms of Everglades restoration. Looking at the history of the Everglades you see a drastic decline in the number of tree islands over time - up to 60% since 1940! Now lets throw some more numbers into your thinking. If tree islands sequestered around 67% of the TP entering the Everglades and you factor in the decline of tree islands (up to 90% locally), the decline of wading birds (up to 90% over the last 100 years), and various draining practices by agricultural, mining, and urban developers, what do you get? Not necessarily less phosphorus (some models indicate a possible increase), but rather a more homogeneous ecosystem, phosporically (and likely biotically) speaking. This homogeneous effect can be seen in areas where agricultural runoff (that has bunches and bunches of phosphorus in it) are high - the landscape patterns have disappeared. You get less tree islands and more marshes and wet prairies dominated by sawgrass.
Is there a take-home message? Well, considering the Everglades restoration is a multi-million dollar investment, I would hope so. Perhaps the message is to use tree islands not as just a performance measure but to actually designate them as part of the restoration itself in order to ensure diversity and long-term survival in the system.
Here's the paper:
Wetzel, Paul R. et al. (2009) Heterogeneity of phosphorus distribution in a patterned landscape, the Florida Everglades. Plant Ecology: 200(1), 83-90. (DOI: 10.1007/s11258-008-9449-3)