Story highlights Officials estimate 100 million to 500 million trees have died in 2011 drought

Preliminary estimates are part of a long-term program by state and federal experts

It could take years to obtain enough data to complete the study, analyst says

In the spring, satellite imagery, aerial photography will be used for a more in-depth analysis

As many as a half-billion trees may have died across Texas from the effects of the 2011 drought, the state's forest service says.

A survey released Monday by the Texas Forest Service estimates 100 million to 500 million trees, or 2% to 10% of the state's 4.9 billion trees, have been killed by the severe drought, which began last year.

Although it will take several years to obtain enough data to determine the full effect of the drought, "ultimately, Mother Nature is going to dictate what will happen," said Chris Edgar, a forest resource analyst.

So far, early estimates show the effects of the drought are numerous and widespread.

"Large numbers of trees in both urban communities and rural forests have died or are struggling to survive," said Burl Carraway, head of the forest service's Sustainable Forestry Department.

The survey was the first of a three-part long-term program that industry experts are using to gather scientific data that will help determine how many trees died in the drought.

"During this time of year, it's difficult to tell in some cases if a tree is truly dead," Carraway said.

But in the spring, foresters plan to use satellite imagery and aerial photography for a more in-depth analysis that will check for leafing.

Edgar said that measuring the trees that produce new leaves for the season will help determine how they responded to the drought and show which ones survived.

And a more scientific, long-term study will be completed as the agency collects data through its Forestry Inventory and Analysis, a federally funded program that is considered a census for trees.

The agency's main purpose is to keep a close watch on trees -- and how they're growing and changing -- across the state.

"Trees grow over many years and decades," Edgar said. But the combination of heat, lack of rain and wind has taken its toll, he said.

When you couple a tree's complex and extensive life cycle with the known variables that come with a drought, the process could take a long time. Edgar said that during this time, he expects more trees to succumb to the ongoing drought.

According to the National Oceanic and Atmospheric Administration, severe to extreme drought affected about 20% of the contiguous United States as of the end of November.