Guest Post: There are the Subtropics! Response to Richard Corlett

[Editor’s Note: One of the goals of our blog is to provide a forum for members of our community to discuss previously published work. Our first guest post is by Thomas Fickert from the University of Passau, whois providing an alternative view on subtropical ecosystems to that presented by Richard Corlett in his Commentary article in Biotropica 45(3). Please join us in the discussion by commenting below!]

Longleaf Pine savanna at the Ordway Swisher Biological Station in Central Florida.

Longleaf Pine savanna at the Ordway Swisher Biological Station in Central Florida.

Setting boundaries in nature is a tricky issue, not only due to their ecotonal character and the many different attributes available for defining them. Further complicating is a wide array of meanings associated with the term boundary itself (Strayer et al. 2003). Boundaries, or what we consider to be those, differ in their origin and maintenance, their spatial structure, their function, and their temporal dynamics. Boundaries may also vary in sharpness, resolution, complexity and dimension and they exist on many different scales of observation (Strayer et al. 2003, Grüninger 2011). Thus, boundaries between ecological or climatological “units” always represent generalized delineations connecting punctually available threshold values along ecological gradients and sometimes may appear arbitrary and vague (Bailey 2009, Grüninger 2011).  Despite those shortcomings boundaries are nevertheless essential for the study of ecosystems on all scales of inquiry as well as for comparisons between them.

When dealing with obscure topics pragmatic approaches sometimes are helpful to avoid lengthy discussion about pros and cons of suggested solutions. Richard T. Corlett (2013) picks such an issue in his Commentary in Biotropica 45(3) when asking: “Where are the Subtropics?” He shows that the general understanding of the terms subtropic(s) and subtropical varies widely between and even within scientific disciplines and he makes a good point that this is nonsatisfying, not only impeding pan-subtropical comparisons. Corlett therefore proposes a straightforward delineation for the subtropics, which is primarily based on a review about the use of the terms subtropic(s) and subtropical in articles published in high ranked ecological journals since 2000. As most studies in this survey applied the terms to study areas between the astronomical poleward boundary of the tropics at 23.4°N/S and 30°N/S, he advocates the exclusive use of the term subtropic(al) for these two belts in the future.

Just as a Commentary in Biotropica is supposed to be it is a stimulating and thought-provoking paper. I fully agree with Corlett, that terms in science have to be clearly defined and everyone using and/or reading these terms should have a clear understanding what is meant. From a biogeographical point of view, however, the proposed delineation of the subtropical zone is challenging, for several reasons. At first, I see a methodical weakness when employing the use of specific terms (here the terms subtropics and/or subtropical) for particular regions in the literature as criterion for a classification (here the designation of the subtropical zone), if beforehand the inconsistent use in the scientific literature of exactly these terms is criticized. That is coming close to a circular reasoning and the usefulness of that approach must be questioned. Even more so, as several climatical and ecological criteria for a clear outline of the subtropics exist, making arbitrarily drawn boundaries actually unnecessary.

Defining ecological and/or bioclimatological zones and their respective boundaries is one of the core topics in physical geography and in particular the German-speaking geography has contributed important findings since early pioneers such as Alexander v. Humboldt (1805, see also Berghaus 1845). Later contributions include the climate classification schemes of Köppen (1884, 1900, with revisions by Geiger (1961), Trewartha & Horn (1980), Kottek et al. (2006), or Peel at al. (2007)), Troll & Paffen (1964) or Lauer et al. (1996) as well as attempts of global ecological zonations  for example by Schimper (1898), Passarge (1929), Müller-Hohenstein (1981), Walter & Breckle (1999), Schultz (2000), or Richter (2001). In progressively higher resolution and accuracy all these classification efforts conducted during the last century sought to delineate climate or ecological zones objectively by means of ecological as well as bioclimatical evidence and/or particular threshold values. Even if the boundaries of the various attempts differ in detail, the overall picture is one of broad analogy between the different classifications as well as between the distribution of particular climatic conditions and vegetation formation types. Due to English-language editions of at least some of the above mentioned volumes (e.g. Schimper et al. 1903, Walter & Breckle 1985, Schultz 2005) these findings were actually broadly recognized by the international scientific community and made it into numerous international geography and ecology textbooks. These ideas provide a very useful framework in the search for non-arbitrary, universally agreed definitions supported by data (Bailey 2009).

In general, there is consensus that the subtropics are transitional between the tropical and the cool-temperate zone. Even if etymology might imply, the prefix “sub” (in an equal manner as in “subalpine”, “subnival” or “subpolar”) does not mean that the subtropics are a subdivision of the tropics. Rather, the subtropics represent a discrete zone with particular characteristics regarding climate and vegetation. For our discussion of potential ways to delineate the subtropical belt, let’s start with the putatively easy side, the boundary between the tropics and the subtropics. For convenience the tropics are often defined as area between the Tropics of Cancer and Capricorn, where the sun is directly overhead at least once per year. Latitude is reflected by the spatial variation in solar radiation input which, in turn, is expressed in the variation of day-length during the course of the year. Between the Tropic of Cancer and the Tropic of Capricorn day-length variation equals 3 hours or less (Lauer et al. 1996). As a direct consequence of the high solar angle, within the tropics the oscillation of temperatures between day and night is wider than between the coldest and the warmest month (ΔTd > ΔTa, “Tageszeitenklima” according to Troll 1959). That provides an important initial criterion for clearly defining the tropics, which, by the way, easily distinguishes tropical from extratropical mountain regions and thus bypasses the “problem” – which in fact does not exist – of an altitudinal boundary of the tropics based on thermal threshold values (Fickert 2011). Even if night time temperatures can be well below freezing almost daily at higher elevations of mountain ranges close to the equator (“summer every day and winter every night”, as Hedberg (1948) enunciated), they are still tropical and the particular altitudinal belt may be called “tropical alpine” or better yet “altotropical”.

Bioclimatological and ecological observations, however, indicate that a schematic astronomical definition of the tropics by the Tropics of Cancer and Capricorn does not meet reality and that the delineation of the tropics does not simply follow a mathematically set boundary of solar radiation input. Tropical conditions can be found outside and extratropical conditions inside the so defined area, as also Richard T. Corlett (2013) states. The actual limit of the tropics is modified by many factors, amongst others by the distribution of landmasses and oceans, the presence of warm or cold ocean currents, the degree of continentality, as well as topography causing windward and leeward effects. These variables entail more or less pronounced deviations of the real tropic-subtropic transition from the solar boundary of the tropics at 23.4°N or S (Fig. 1). This is reflected by the occurrence respectively the lack of megatherm tropical organisms requiring particular thermal conditions at low elevations, strikingly portrayed by the asymmetric distribution of frost sensitive mangrove forests on the continents western and eastern coasts (see e.g. Spalding et al. 2010). Hence, two additional thermal parameters at sea level clearly identify the tropics, the lack of freezing temperatures and a mean temperature of the coldest month not below 18°C (Köppen 1900).

From a biogeographical perspective even more of a mismatch exists between an arbitrarily drawn poleward limit of the subtropics at 30°N and S and delineations based on climatical and/or ecological data. There might be differences between the various existing classifications and the underlying climatic threshold values but there is general agreement that areas with a “mediterranean” climate (hot and dry summers, mild and humid winters) and vegetation on the continents western margins are part of the subtropics (amongst others Trewartha & Horn 1980, Alekseev & Golubev 2000, McKnight & Hess 2000, Schultz 2000, Bailey 2009, Barry & Chorley 2010). These areas are almost completely disregarded when setting a poleward boundary of the subtropics at 30° and also a significant portion of the humid subtropics along the continents eastern margins – on which Richard T. Corlett obviously focuses without mentioning that explicitly –  are not included (Fig. 1). Only the subtropical drylands and the equatorward parts of the humid subtropics remain, making such a delineation of the subtropics highly questionable.

FIGURE 1. Spatial distribution of Köppen climate types (a, after BARRY & CHORLEY 2010) and of vegetation types (b, after TROLL 1948) displayed on a generalized continent (modified from FICKERT 2011).

FIGURE 1. Spatial distribution of Köppen climate types (a, after BARRY & CHORLEY 2010) and of vegetation types (b, after TROLL 1948) displayed on a generalized continent (modified from FICKERT 2011).

Radiation-wise the subtropics are characterized by a still high solar angle and pronounced differences in solar radiation input between northern and southern exposures. The day length variation is distinct with values of four to seven hours during the course of the year, so thermal seasons – in contrast to the tropics – are already present (Lauer et al. 1996). Temperatures below freezing may occur in the subtropics (at sea level, in subtropical mountains anyway), but are rare. Megatherm tropical organisms, however, sensitive to freezing temperatures are missing (Box 1996). From an ecological point of view the (low elevation) subtropics should encompass all areas without an interruption of the growing period by low temperatures, which occurs within the adjacent cool-temperate zone to the North and South on the respective hemispheres. As temperatures at sea level allow for year-round productivity – an interruption of the growing period within the subtropics, if at all, is due to drought rather than low temperatures – woody taxa are primarily evergreen. According to differences in the seasonal distribution of precipitation, sclerophyllous leaves dominate in the mediterranean subtropics with pronounced drought during the summer months, and laurophyllous (i.e. thin coriaceous, mesomorphic) leaves in the humid subtropics. In the arid subtropics with dry and warm conditions co-occurring drought-deciduous and/or microphyllous species are widespread to avoid lethal water loss by transpiration.

Thus, both climate characteristics and plant adaptations exist offering opportunities to clearly define the subtropics and their subzones. Arbitrarily drawn boundaries doesn’t seem helpful, neither for the designation of zones and even less for comparisons, as two sites at, let’s say, around 26°N could be entirely different (e.g. subtropical (semi)deserts in Central Baja California and mesic subtropical pine forests (Pinus elliottii) in Central Florida (Fig. 1.) while two sites at 26° and 32°N  (e.g.  subtropical moist forests in Southeastern China or in Eastern Australia) might be ecologically similar and worth comparing. It is not a particular latitude that makes sites classifiable and comparable but the environment. Simply using certain latitudes as selection criterion for study sites in pan-continental comparisons we run the risk of comparing apples and oranges. If the ecological and bioclimatological classifications at hand exhibit deficiencies, revisions are necessary to achieve a wider acceptance. Those revisions have to be based on (1) multiple (bioitic and abiotic) factors and (2) on the causes for differences between classes rather than the effects they generate (Bailey 2009). For sure, it is not an easy task and there will be continuing debates within and between disciplines on particular features. Nevertheless we should adhere to the search for universally accepted definitions of zones and their respective boundaries supported by physical and biological evidence. That is, however, only achievable if joint efforts between cognate disciplines are carried out and a much broader awareness of findings already provided by neighboring disciplines emerges than it is the case today.

Thomas Fickert
Physical Geography, University of Passau, Germany

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