Happy Earth Day! We would like to share a few recent headlines for you to peruse to ponder and protect our planet…

Pollution
From high to low, all around the world, pollution affects our world. Recent headlines show that “Toxic chemicals are accumulating in the ecosystems of the Himalayas and the Tibetan plateau,” according to Nature. Tiny plastic particles aren’t just trouble in the oceans; the Great Lakes contain millions of microplastics, too. The New York Times’ Dot Earth blog has a short post about the importance of not littering. And National Geographic has an article about how pollution on land can affect marine life like dolphins and local sea otters.

Colorado River
While many U.S. rivers have problems with pollution, the Colorado River’s mismanagement, overuse and drought put it atop the list of Endangered Rivers of 2013. National Geographic has an entire series on the Colorado River delta, and the New York Times has offered both an article and video last week on the region’s hopeful revival.

Drought
Speaking of drought… Do drought-stressed trees cry for help? French scientists are listening for clues. Climate change was not responsible for last summer’s Midwestern drought, according to NOAA, but then what was? And how might we be able to predict future droughts?

Climate Change
Climate change may not have caused of the recent drought, but it is responsible for other devastating events and looming disasters: bumpier flights, more storms, bark beetle plagues, drowned islands, failures in agriculture systems and more extinctions. Researchers are also getting a better handle on tracking climate change through mapping ocean eddies and looking at historic ocean temperatures and air pressure.

Ecology
How do species react to environmental changes? Rapid evolution, according to one study. Another study suggests that endangered species are already doomed. And Nature offers an update on a decades-long study of habitat fragmentation in the Amazon.

Energy
How has energy usage in our country changed over the past two hundred years? NPR has a graph (or four) for that. In response, Scientific American presents a diagram illustrating our potential for future alternative energy use and resources accompanying an article titled, “How to Power the World without Fossil Fuels.” Germany seems to have taken notice—the European country has ambitious renewable plans. But it’s not the only one. The U.S. had a huge year in 2012 for wind power. And, heading across the country soon? How about a solar-powered flight?

Earth Day
Finally, let’s truly celebrate the planet’s holiday with history, maps, jokes about Earth Day, and facts and photos of our beautiful home.

Image: Terra/ASTER/NASA and NASA Earth Observatory

Just like people, leaves do not grow indefinitely. Once they reach a given size, they stop growing. But why? Researchers at Harvard and University of California Davis, published in the journal Physical Review Letters, have recently determined the cause.

After comparing 1,925 tree species, with leaves ranging in length from a few millimeters to over one meter, it turns out that the height of the tree is the determining factor. The reason lies in the plant’s circulatory system and the flow of energy-rich sugars along it.

All sugar in a tree is produced within its leaves. Through a network of tube-shaped cells called the phloem, it flows out to the trunk and the roots. But when a tree transports sugar through its leaves and trunk, resistance comes into play. The trunk’s resistance is directly proportional to the height, while the leaf’s resistance is inversely proportional to its length. This means that the taller the tree, the more energy is needed to transport the sugar around. For the leaf, the longer it gets, the less the resistance and the faster it can transport the sugar down to the trunk.

Scientists used these physical factors to create a mathematical model that relates the leaf size to a given tree size. As trees get taller, trunk resistance slows the flow. Increasing leaf size might partly offset this, but eventually the trunk resistance becomes so large and dominant that leaf size has little effect on the flow rate. Beyond a certain leaf size, there’s no advantage in making the leaves any bigger. For trees that are 100 meters tall, it doesn’t make sense to produce leaves longer than 20 cm. Meanwhile, if the leaf is too small, the sugar travels too slowly around the tree—setting the minimum leaf size.

The mathematical curves for maximum and minimum leaf size converge as trees grow taller. This could also be the reason why trees cannot grow beyond a certain height. The scientists’ formula predicts that at exactly 106 meters, close to the height of the tallest trees, these curves cross. According to the theory, if a tree were taller than this, no leaf size could meet its vascular requirements.

Some of the tallest trees are eucalyptus. They can grow up to 100-110 meters high with leaves between 10 and 30 cm.

Learn more about this research on You Tube: Trees are Freaking Awesome!

Zuberoa Marcos is a former biologist and current science writer based in Barcelona. She writes articles regularly for Science Today.

¿Por qué los árboles no pueden alcanzar más de 100 metros de alto?

Por Zuberoa Marcos

De igual manera que las personas, las hojas no crecen indefinidamente. Una vez que alcanzan un tamaño dado, éstas paran de desarrollarse. ¿Pero por qué? Recientemente, investigadores de Harvard y de la Universidad California Davis, han determinado la causa en un artículo publicado en la revista Physical Review Letters.

Tras comparar 1,925 especies de árboles, cuyas hojas oscilan desde unos pocos milímetros a más de un metro de longitud, han concluido que la altura del árbol es el factor clave. La explicación reside en el sistema circulatorio de la planta y en el flujo de azúcares a lo largo de él.

Todo el azúcar de un árbol se produce en sus hojas. A través de una red de tubos llamada floema, el azúcar transporta hacia el tronco y las raíces. Sin embargo, cuando un árbol transporta el azúcar a través de las hojas y del tronco, entra en juego la resistencia. En el tronco la resistencia es directamente proporcional a su altura, mientras que en la hoja es inversamente proporcional a su longitud. Esto significa que cuanto más alto sea el árbol, más energía se necesita para transportar el azúcar a través de él. En el caso de la hoja, cuanto más grande sea ésta menor es la resistencia y más rápido se puede transportar el azúcar.

Los científicos utilizaron estos factores para crear un modelo matemático que relaciona el tamaño de la hoja con la altura de un árbol en particular. Cuanto más crece un árbol, más lento es el flujo de azúcares a través de su tronco. El aumento de tamaño de la hoja podría compensar en parte este proceso, pero llegaría un punto en el que la resistencia del tronco sería tan grande y tan dominante que el tamaño de la hoja tendría poco o ningún efecto sobre la velocidad de transporte de los azúcares. Es decir, más allá de un cierto tamaño, producir hojas más grandes no proporciona ninguna ventaja. Para los árboles de 100 metros de altura, no tiene sentido producir hojas de más de 20 cm de longitud. Por otro lado, si la hoja es demasiado pequeña, el azúcar viaja muy lentamente a través del árbol, lo que determina el tamaño mínimo de la hoja.

Conforme los árboles crecen, las curvas matemáticas que determinan el tamaño de hoja máximo y mínimo convergen. Esto podría explicar por qué los árboles no pueden crecer más allá de cierta altura. La fórmula elaborada por los científicos predice que exactamente a 106 m, un valor cercano a la altura de los árboles más altos, ambas curvas se cruzan. De acuerdo a esta teoría, si un árbol supera esta altura no hay tamaño de hoja que pueda satisfacer sus necesidades vasculares.

Los eucaliptos son algunos de los árboles más altos. Pueden alcanzar hasta 100-110 metros y sus hojas miden entre 10 y 30 cm.

Zuberoa Marcos es bióloga retirada y actualmente trabaja como periodista científica desde Barcelona. Escribe de forma regular para Science Today.

Image: TTaylor/Wikipedia